tin Tin Alloys, And Tin Compounds

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c 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim 10.1002/14356007.a27 049 Tin, Tin Alloys, and Tin Compounds 1 Tin, Tin Alloys, and Tin Compounds unter G. Graf, Freiberg, Federal Republic of Germany 1. History ................. 2 2. Properties ................ 2 3. Occurrence; Ore Extraction and Beneficiation .............. 5 3.1. Minerals ................ 5 3.2. Deposits ................. 5 3.3. Mining .................. 6 3.4. Ore Beneficiation ........... 7 4. Smelting ................. 8 4.1. Fundamental Theory of Smelting 8 4.2. Special Aspects of the Winning of Tin from its Ores ........... 10 4.3. Production of Crude Tin ...... 11 4.3.1. General Aspects ............ 11 4.3.2. Ore Preparation prior to Reduction 11 Pyrometallurgical Enrichment of Low-Grade Concentrates ..... 11 Roasting ................. 12 Leaching ................. 13 4.3.3. Reduction ................ 14 Reduction in a Shaft Kiln ...... 14 Reduction in a Reverberatory Furnace ................. 15 Reduction in Rotary Kilns ...... 16 Reduction in an Electric Furnace .. 17 Other Reduction Processes ..... 18 4.3.4. Slag Processing ............ 18 5. Refining ................. 20 5.1. Pyrometallurgical Refining .... 20 5.1.1. Removal of Iron ............ 20 5.1.2. Removal of Copper .......... 21 5.1.3. Removal of Arsenic .......... 21 5.1.4. Removal of Lead ............ 22 5.1.5. Removal of Bismuth ......... 22 5.2. Electrorefining ............ 22 5.2.1. Electrorefining in Acid Medium .. 22 5.2.2. Electrorefining in an Alkaline Medium ................. 23 5.2.3. Other Methods of Electrorefining . 23 6. Recovery of Tin from Scrap Materials and Residues ....... 23 7. Analysis ................. 24 7.1. Analysis of Ores and Concentrates 24 7.2. Analysis of Metallic Tin ...... 25 8. Economic Aspects .......... 25 9. Tin Alloys and Coatings ...... 26 10. Inorganic Tin Compounds ..... 28 10.1. Tin(II) Compounds ......... 28 10.2. Tin(IV) Compounds ......... 29 11. Organic Compounds of Tin .... 30 11.1. Properties of Organotin Compounds .............. 31 11.2. Production of Organotin Compounds .............. 31 11.3. Industrially Important Compounds .............. 32 11.4. Analysis of Organotin Compounds .............. 33 11.5. Storage and Shipping of Organotin Compounds ....... 33 11.6. Pattern of Production and Consumption .......... 33 12. Toxicology ............... 33 13. References ............... 34 1. History [1–4], [6], [11], [15] Because of its luster and softness, tin was usu- ally assigned to the planet Jupiter, more rarely to Venus. The name of the element is derived from the Old High German zin and the Norse tin. The symbol Sn from the Latin stannum was proposed by Berzelius. Historically tin is of major cul- tural importance, being an essential component of the copper alloy bronze which gave its name to the Bronze Age. The first bronze objects ap- peared in Egyptian tombs dating from the end of the 4th millennium b.c. Pure tin was first produced in China and Japan around 1800 b.c. Around 600 b.c., the ancient Egyptians occasionally placed pure tin artifacts in mummies’ tombs. Tin is not only an essen- tial constituent of tin bronze, but is also a con- stituent of lead alloys for solders and tin plating. Tin and especially its alloys have shaped the de- velopment of many geographical regions, e.g., China, Indochina, Indonesia, India, the Near East, North Africa, and Europe. The cultural and historical importance of tin from the Middle Ages to early modern times lay

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c© 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim10.1002/14356007.a27 049

Tin, Tin Alloys, and Tin Compounds 1

Tin, Tin Alloys, and Tin Compounds

Gunter G. Graf, Freiberg, Federal Republic of Germany

1. History . . . . . . . . . . . . . . . . . 22. Properties . . . . . . . . . . . . . . . . 23. Occurrence; Ore Extraction and

Beneficiation . . . . . . . . . . . . . . 53.1. Minerals . . . . . . . . . . . . . . . . 53.2. Deposits . . . . . . . . . . . . . . . . . 53.3. Mining . . . . . . . . . . . . . . . . . . 63.4. Ore Beneficiation . . . . . . . . . . . 74. Smelting . . . . . . . . . . . . . . . . . 84.1. Fundamental Theory of Smelting 84.2. Special Aspects of the Winning of

Tin from its Ores . . . . . . . . . . . 104.3. Production of Crude Tin . . . . . . 114.3.1. General Aspects . . . . . . . . . . . . 114.3.2. Ore Preparation prior to Reduction Pyrometallurgical Enrichment

of Low-Grade Concentrates . . . . . Roasting . . . . . . . . . . . . . . . . . Leaching . . . . . . . . . . . . . . . . . 134.3.3. Reduction . . . . . . . . . . . . . . . . Reduction in a Shaft Kiln . . . . . . Reduction in a Reverberatory

Furnace . . . . . . . . . . . . . . . . . Reduction in Rotary Kilns . . . . . . Reduction in an Electric Furnace . . Other Reduction Processes . . . . . 184.3.4. Slag Processing . . . . . . . . . . . . 185. Refining . . . . . . . . . . . . . . . . . 205.1. Pyrometallurgical Refining . . . . 205.1.1. Removal of Iron . . . . . . . . . . . . 205.1.2. Removal of Copper . . . . . . . . . . 215.1.3. Removal of Arsenic . . . . . . . . . . 21

5.1.4. Removal of Lead . . . . . . . . . . . . 225.1.5. Removal of Bismuth . . . . . . . . . 225.2. Electrorefining . . . . . . . . . . . . 225.2.1. Electrorefining in Acid Medium . . 225.2.2. Electrorefining in an Alkaline

Medium . . . . . . . . . . . . . . . . . 235.2.3. Other Methods of Electrorefining . 236. Recovery of Tin from Scrap

Materials and Residues . . . . . . . 237. Analysis . . . . . . . . . . . . . . . . . 247.1. Analysis of Ores and Concentrates 247.2. Analysis of Metallic Tin . . . . . . 258. Economic Aspects . . . . . . . . . . 259. Tin Alloys and Coatings . . . . . . 2610. Inorganic Tin Compounds . . . . . 2810.1. Tin(II) Compounds . . . . . . . . . 2810.2. Tin(IV) Compounds . . . . . . . . . 2911. Organic Compounds of Tin . . . . 3011.1. Properties of Organotin

Compounds . . . . . . . . . . . . . . 3111.2. Production of Organotin

Compounds . . . . . . . . . . . . . . 3111.3. Industrially Important

Compounds . . . . . . . . . . . . . . 3211.4. Analysis of Organotin

Compounds . . . . . . . . . . . . . . 3311.5. Storage and Shipping of

Organotin Compounds . . . . . . . 3311.6. Pattern of Production

and Consumption . . . . . . . . . . 3312. Toxicology . . . . . . . . . . . . . . . 3313. References . . . . . . . . . . . . . . . 34

1. History [1–4], [6], [11], [15]

Because of its luster and softness, tin was usu-ally assigned to the planet Jupiter, more rarely toVenus. The name of the element is derived fromthe Old High German zin and the Norse tin. Thesymbol Sn from theLatin stannumwas proposedby Berzelius. Historically tin is of major cul-tural importance, being an essential componentof the copper alloy bronze which gave its nameto the Bronze Age. The first bronze objects ap-peared in Egyptian tombs dating from the endof the 4th millennium b.c.

Pure tinwasfirst produced inChina and Japanaround 1800 b.c. Around 600 b.c., the ancientEgyptians occasionally placed pure tin artifactsin mummies’ tombs. Tin is not only an essen-tial constituent of tin bronze, but is also a con-stituent of lead alloys for solders and tin plating.Tin and especially its alloys have shaped the de-velopment of many geographical regions, e.g.,China, Indochina, Indonesia, India, the NearEast, North Africa, and Europe.

The cultural and historical importance of tinfrom the Middle Ages to early modern times lay

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in its use for sacred objects, articles of daily use,and jewelry.

There is no historical evidence concerningthe oldest methods of tin extraction. It is fairlycertain that in 100 b.c. in Cornwall, England,tin was smelted from very pure ore over woodfires in pits and later in small furnaces. Up to the1200s, Cornwall provided most of Europe’s tin.Today, these deposits are virtually exhausted.Tin was probably produced in Bohemia around1150. Also, the first tin mines were opened inSaxony at this time, and these supplied Euro-pean requirements until they were destroyed inthe Thirty Years War. Then, as these various de-posits gradually became exhausted and as oceantransport developed, tin from overseas becamedominant.

The largest tinmines are inAsia, themost im-portant ore-supplying countries in the world be-ing Malaysia and Indonesia, followed by China.The second largest tin-producing region in-cludes Brazil and Bolivia. The countries export-ing the largest quantities of tin ores also producethe most tin metal.

World annual production has developed asfollows:

ca. 1800 9 100 tca. 1850 19 000 tca. 1900 91 900 tca. 1950 172 100 t

1980 243 600 t1990 225 600 t

The principal consumer countries are theUnited States, Japan, China, and Russia.

2. Properties

Physical Properties [1], [2], [4], [16–19],[20]. Tin, Sn, exists in two crystalline modifica-tions, the α- and β-forms. A third modificationmay also exist. Some physical properties of α-and β-tin are listed in the following. (see rightcolumn)

In the periodic table, tin lies on the bound-ary between metals and nonmetals. The trans-formation of the α- to the β-modification isaccompanied by a complete change of latticestructure, affecting the physical, chemical, andmechanical properties. Also, at 170 C there isa secondorder transformation accompanied by adiscontinuous change in the lattice parameters

and thermomechanical properties. A tetragonalhighpressure modification of tin, stable between3500 and 11 000MPa, is described in the litera-ture, the lattice constants being a = 381 pm andb =348 pm.

Natural isotopes 10Relative atomic mass 118.69Crystal structure

α-Sn (gray tin) fcc (A4) diamond typeβ-Sn (white tin) tetragonal (A5)

Transformation temperatureα-Sn↔ β-Sn


Enthalpy of transformation 1966 J/molLattice constants at 25 C

α-Sn a = 648.92 pmβ-Sn a = 583.16 pm

c = 318.13 pmDensity of β-tin

20 C 7.286 g/cm3

100 C 7.32 g/cm3

230 C 7.40 g/cm3

Density of α-tin 5.765 g/cm3

Density of liquid tin240 C 6.992 g/cm3

400 C 6.879 g/cm3

800 C 6.611 g/cm3

1000 C 6.484 g/cm3

Molar heat capacity of β-tin25 C 27.0 Jmol−1 K−1

230 C 30.7 Jmol−1 K−1

Liquid 28.5 Jmol−1 K−1

Melting point 505.06KEnthalpy of fusion 7029 J/molBoiling point 2876KEnthalpy of vaporization 295 763 J/molVapor pressure1000K 9.8×10−4 Pa1800K 750 Pa2100K 8390 Pa2400K 51 200 Pa

Cubic coefficient of expansionα-tin at −130 C to +10 C 14.1×10−6 K−1

to 4.7×10−6 K−1

β-tin at 0 C 59.8×10−6 K−1

β-tin at 50 C 69.2×10−6 K−1

β-tin at 100 C 71.4×10−6 K−1

β-tin at 150 C 80.2×10−6 K−1

Molten tin at 700 C 105.0×10−6 K−1

Coefficient of thermal conductivity ofβ-tin at 0 C

0.63Wcm−1 K−1

Surface tension232 C 0.53 – 0.62N/m400 C 0.52 – 0.59N/m800 C 0.51 – 0.52N/m1000 C 0.49N/m

Dynamic viscosity232 C 2.71×10−3 Pa · s400 C 1.32×10−3 Pa · s

1000 C 0.80×10−3 Pa · sSpecific electrical resistivity

α-tin at 0 C 5×10−6 Ωmβ-tin at 25 C 11.15×10−6 Ωm

Transition temperature forsuperconductivity


Magnetic susceptibility of β-Sn 2.6×10−11 m3/kg

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Tin, Tin Alloys, and Tin Compounds 3

The transformationofβ-tin (white tin) intoα-tin (gray tin) is of practical importance, as it in-volves a volume increase of 21%. The transfor-mation process requires a high energy of activa-tion, and can be very strongly hindered.Whiteβ-tin can therefore exist for many years at−30 C.The presence of α-tin seed crystals is impor-tant for the transformation process, and theseare formed by repeated phase transitions. For-eign “elements” also affect the transformationtemperature and rate. These can consist of impu-rities and deformations. The effect of impuritieson transformation behavior is described in [21].Tin vapor consists of Sn2 molecules.

Mechanical Properties [1], [2], [4], [20–23]. Mechanical properties are not of great rele-vance to most applications of pure tin. The mostimportant are listed in the following:

Yield strength at 25 C 2.55N/mm2

Ultimate tensile strength−120 C 87.6N/mm2

15 C 14.5N/mm2

200 C 4.5N/mm2

Brinell hardness (10mm,3000N, 10 s)

0 C 4.12100 C 2.26200 C 0.88

Modulus of elasticity E−170 C 65 000N/mm2

−20 C 50 000N/mm2

0 C 52 000N/mm2

40 C 49 300N/mm2

100 C 44 700N/mm2

200C 26 000N/mm2

Chemical Properties [1], [2], [4], [20]. Tinhas the atomic number 50 and is a member ofgroup 14 of the periodic table. The electronicconfiguration is 1s22s2p63s2p6d104s2p4d10

5s2p2. Tin can be di- or tetravalent. It is sta-ble in dry air, but is considerably more rapidlyoxidized at a relative humidity of 80%. Brightmetallic tin becomes dull within 100 d evenin indoor atmospheres. Oxygen is rapidly andirreversibly chemisorbed, and the oxide layerformed grows at an exponentially increasingrate. Typical impurities present after metallurgi-cal production (e.g., Sb, Tl, Bi, and Fe) promoteoxidation. Treatment with carbonate or chro-mate solutions leads to passivation.

Molten tin at temperatures up to ca. 500 Cpicks up oxygen from the air at a rate that obeys

a parabolic law, a result of the compact layer ofoxide formed.

Gaseous water and nitrogen do not dissolvein solid tin. Dissolution in molten tin only oc-curs at high temperatures (ca. > 1000 C). Un-der the conditions of electrochemical reductionin hydrochloric acid solutions, atomic hydro-gen forms SnH4, and elemental nitrogen formsSn3N4.

Tin is stable towards fluorine at room tem-perature, but SnF2 or SnF4 are formed at highertemperatures.

Rapid and vigorous reactions occur withchlorine, bromine, and iodine, these reactionsbeing accelerated by moisture and elevated tem-peratures. The reaction products are SnCl4,SnBr4, SnI2, and SnI4.

Sulfur reacts rapidly with molten tin at> 600 C to form the sulfides SnS, Sn2S3, andSnS2. The reaction rate is lower above 900 C,and only SnS is formed.

Reaction with hydrogen sulfide is slow andonly occurs in the presence of oxygen and mois-ture. Sulfur dioxide reacts with molten tin toform SnO2 and S, and a molten solution of tinin copper reacts with SO2 to form SnO2 andCu2S (an important reaction in pyrometallurgy).Tin is stable towards pure hot water, steam, anddry ammonia. Nitrogen oxides only react withmolten tin.

Tin is amphoteric, reacting with both strongbases and strong acids with evolution of hy-drogen. Having a normal electrode potential of−0.136V, tin lies between nickel and lead in theelectrochemical series.

With sodium hydroxide solution, tin formsNa2[Sn(OH)6], and with potassium hydroxidesolution K2[Sn(OH)6].

Tin reacts slowly with acids in the absenceof oxygen. The high hydrogen overvoltage iscausedby a layer of atomic hydrogen at themetalsurface preventing further attack. Vigorous re-actions occur with nitric acid, the rate depend-ing on the acid concentration. The reactions arevery vigorous with 35% acid, but complete pas-sivation can occur at concentrations> 80%. Tinis stable towards fuming nitric acid. While hy-drogen fluoride does not attack tin, hydrochloricacid reacts even at a concentration of 0.05% andtemperatures below 0 C. Tin is not attacked bysulfurous acid or by < 80% sulfuric acid.

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The most important use of tin and tin-platedmaterials is in the preserved food industry. Forthis reason, the possibility of reactions of tinwithcertain organic acids is important. Lactic, malic,citric, tartaric, and acetic acids either do not re-act at all at normal temperatures or do so to anegligible extent, especially in the absence ofatmospheric oxygen. This is also true of alco-hols and hydrocarbons.

3. Occurrence; Ore Extraction andBeneficiation

The average concentration of tin in the earth’scrust is estimated to be 2 – 3 ppm, comparable tocerium and yttrium. Owing to the high atomicmass of tin and the high density of its impor-tant minerals, its volume concentration is verylow. However, it occurs in only a small num-ber of locations, where consequently its relativeabundance is high. In general, 1000-fold enrich-ment is necessary to give workable tin deposits,i.e., with a tin content of at least 0.2%. Thequestion of whether a deposit can be econom-ically extracted, for a given world market pricelevel, depends on the mining conditions. For ex-ample, there are deposits in Bolivia containing1%Snwhich cannot be economically extracted,whereas in South East Asia placer deposits con-taining 0.02% Sn are successfully mined.

3.1. Minerals

Native tin occurs only very rarely and has onlybeen identified with certainty in Canada.

Cassiterite, SnO2, is the most economicallyimportant tin mineral. It forms tetragonal crys-tals, has a Mohs hardness of 6 – 7, a density of6.8 – 7.1 g/cm3, and a tin content of up to 79%.The color is usually brown to brownish black.The presence of Ti, Fe, Nb, Ta, or Mn can leadto colors varying from gray towhite. Contact de-posits of cassiterite can be combined with, e.g.,magnetite, arsenical iron pyrites, or zinc blende.Placer deposits of cassiterite are of major im-portance. “Wood tin” consists of gel-like or veryfine grained aggregates of cassiterite. Cassiteriteis an oxidic mineral and is chemically very re-sistant, in particular towards weathering.

Stannite (bell metal ore), Cu2(Fe,Zn)SnS4,forms tetragonal crystals, has a Mohs hardnessof 4, a density of 4.4 g/cm3, and a tin contentof up to 27.6%. Its color is steel gray with anolive green tinge. It is chemically less resistantthan cassiterite, seldom occurs in hydrothermaldeposits, and is of little economic importance.

Hydrocassiterite (varlamoffite), H2SnO3, isa tetragonal gel-like stannic acid. It occurs inBolivia, usually accompanying cassiterite.

Other tin minerals include teallite,(Sn,Sb)S; herzenbergite, SnS; franckeite,Pb5Sn3Sb2S14; cylindrite, Pb3Sn4Sb2S14;thoreaulite, SnTa2O7; hulsite, (iron tin borate);and stokesite, CaSn(Si3O9) · 2H2O. None areof economic importance.

3.2. Deposits

The economically important tin deposits areclosely associated with acidic to intermediatemagmatic rocks which were formed in the oro-genic phases of the earth’s history. Tin, a volatilemetal, was deposited primarily during the peg-matitic, pneumatolytic, or hydrothermal phasein the region of the exo- or endocontact of theintrusive bodies. The economically much moreimportant secondary tin deposits exist as eluvial,alluvial, or marine placer deposits which can benear to or remote from these acidic magmaticrock complexes. An overview of the ore reservesand of the amounts extracted in 1990 is given inTable 1.

Table 1. Tin ore reserves and mine outputs for various countries in1990

Country Ore reserves (t metalcontent)

Output (t metalcontent)

Malaysia 1 200 000 28 500Thailand 1 200 000 14 600Indonesia 1 550 000 31 700Bolivia 980 000 17 300Russia 1 000 000 13 000China 1 500 000 35 800Australia 330 000 7 400Brazil 400 000 35 100Zaire 200 000 1 600United Kingdom 260 000 3 200South Africa 50 000 1 100Nigeria 280 000 200Others 765 000 17 000World total 9 715 000 210 700

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Tin, Tin Alloys, and Tin Compounds 5

Primary deposits originate from the peg-matitic formation of cassiterite in contact withgranites or their secondary rocks. Cassiterite oc-curs there in idiomorphic pyramidal crystals of> 2mmdiameter. These tin-bearinggranites andpegmatites typically include theminerals quartz,albite, potassium feldspar, muscovite, and cas-siterite. Columbite is an important accompany-ing mineral. Deposits of this type are found incentral and southern Africa, Brazil, and Russia(Siberia). They account for < 5% of world pro-duction.

Cassiterite quartzes of the pneumatolyticcatathermal phase were formed in vein fissuresof granites and their secondary rocks. Individ-ual veins or lodes can have a tin content of up to3%. They can be 0.2 – 1m thick and up to 200mdeep.

There are various types of paragenesisof the granites, leading to the greisen type(mica – feldspar – quartz formed by pneumatol-ysis with fluorspar, lepidolite, and tourmaline),the topaz – quartz type, the feldspar – quartztype, and the quartz type. Such deposits con-tain ca. 20% of the world’s tin reserves, and arefound in Malaysia, Russia (Siberia), and Ger-many (Erzgebirge in Saxony).

Cassiterite – sulfide deposits of the hy-drothermal phase are formed as vein ores as-sociated with intrusions of granodioritic rocks,or are formed by remigration of the metal con-tent of older pegmatitic tin deposits caused byyounger acidic subvolcanoes. Characteristic ofthese types of deposit is the paragenesis ofcassiterite with stannite, iron pyrites, arseni-cal iron pyrites, galena, zinc blende, magneticiron pyrites, and copper sulfides. These deposits,which can be very large, are mined in Bolivia,Russia, southern China, Thailand, Burma, Aus-tralia, SouthAfrica, and alsoEngland. They con-stitute ca. 15% of the world’s tin reserves.

Because cassiterite is resistant to weatheringand is hard and dense, the weathering of the pri-mary tin-bearing rocks enables it to become con-centrated as it is transported to form secondarydeposits, i.e., eluvial, deluvial, or marine plac-ers.

Eluvial placers are formed by intense weath-ering and breakdown of cassiterite-containinggranites or granodiorites, especially under trop-ical climate conditions. The lighter minerals arewashed out or carried away by wind, while the

cassiterite and other heavy minerals remain be-hind and can become concentrated in deposits ofconsiderable thickness, as inMalaysia andZaire.Very coarse and deluvial placers are also formedby gravitational enrichment due to landslips andeluviation at the bottom of mountainsides.

Alluvial or fluviatile placers are formed bythe transport of weathered tin-bearing rocks byflowing water derived from atmospheric precip-itation. The softer and lighter components ofthe rock are more extensively size reduced andtherefore transported further than the hard, re-sistant cassiterite minerals, which sink due totheir high density and are deposited at pointswhere flow rates are low. The most important al-luvial placer deposits are in centralAfrica (Zaire,Ruanda), western Africa (Nigeria, Niger), andBrazil.

Marine placers are formed where primarytin-bearing rock complexes have been directlytransported by surf, or where rivers have car-ried the cassiterite-containing sediment into thesea, where it is then deposited in coastal strips.These are the most important deposits, and re-present ca. 60% of the world’s workable re-serves. The largest of these deposits are in south-east Asia, the coast of Thailand, the Thai Islandof Phuket,in Malaysia, and on the IndonesianIslands of Bangka and Billiton.

Secondary deposits usually have tin contentsof 0.05 – 0.5%, reaching 3% in some cases.Ma-rine placers have tin contents of 0.01 – 0.03%.At present, deposits containing 0.1% Sn areworkable by the open-pit method. For under-ground mining, deposits should contain 0.3%Sn.

3.3. Mining

Primary tin ores are extracted by undergroundmining. Depths can reach 1000m in exceptionalcases. Most of the technologies used in nonfer-rous metal mining are used, the method in agiven situation being determined by the thick-ness, shape, and orientation of the ore body, andgeological factors.

In secondary deposits, the loosely packedweathered hard mineral rock which contains thecassiterite together with associated deposits ofsand and gravel is extracted by high-productionloading techniques which also perform prelimi-

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nary classification. General local conditions, in-cluding the state of economic development inthe region, have a great influence on the min-ing conditions. For example, in Zaire, surfaceeluvial deposits (weathered pegmatites) with tincontents of up to 0.15%are extracted by conven-tional open-pit methods. In Thailand, Malaysia,and Indonesia, loose alluvial and marine de-posits in river valleys and in the undersea re-gions just off the coast are extracted by dredgingshovels, dragline excavators, chain and bucketexcavators, and similar equipment specially de-signed for local conditions. The initial separa-tion of gangue and other foreign materials (e.g.,wood) is performed by this equipment. Cassi-terite in thick deposits of loose sediments andcoarse detritus in Southern China and Thailandis treated with powerful water jets operating atpressures of up to 1.5MPa. These generate amixture of water and heavy sand which is thenfed to the treatment plant.

Off the coasts of Indonesia, Thailand, andMalaysia, chain and bucket excavators are usedto extract cassiterite from alluvial deposits un-der water at depths of up to 40m. This also givesa preliminary beneficiation.

3.4. Ore Beneficiation [11], [24–28]

The beneficiation of primary tin ores is diffi-cult. The principal mineral, cassiterite, is non-magnetic and is not suitable for flotation, so thatmainly gravimetric sorting processes must beused. Furthermore, cassiterite is often stronglyintergrown, and the accompanying minerals be-have similarly to cassiterite during processing.

Current technologies are characterized bycontrolled multistage size reduction of the oresand separation of the cassiterite released aftereach size reduction stage using sorting meth-ods based on density. Screen jigs and shakingtables of various designs are used. However,very small particles (< 30µm) cannot be pro-cessed to give satisfactory yields and productionrates. If the degree of intergrowth of the ores re-quires finer grinding, as is increasingly the casewith ores fromRussia, theUnitedKingdom, Bo-livia, South Africa, and Portugal, the flotationmethod for sorting particles < 100µm is some-times used. This technique was also used forthe tin ore from the Altenberg region of Saxony,

whose mines were closed in 1990. The follow-ing stages of beneficiation of primary tin oresare used:

Ores with an average degree of intergrowthare concentrated mainly by processes based ondensity. Flotation is increasingly used to sortfine-grained material and ground middlings ob-tained by the density-based sorting process, andhas now become the preferred method for treat-ing themost finely intergrown, complex tin ores.Flotation of cassiterite with particle sizes bet-ween 40 and 10µm is mainly carried out witharsonic acids.

The flow diagram (Fig. 1) shows the flotationof primary tin concentrates to remove sulfides ofsimilar paragenesis, followed by flotation of cas-siterite from the preconcentrate, and magneticseparation of paramagnetic minerals from theflotation product.

Composition ranges for complex tin concen-trates are:

Sn 5.6 – 60wt%S 1.0 – 15wt%As 0.1 – 3wt%Sb 0.1 – 2wt%Bi 0.1 – 0.5wt%Cu 0.1 – 0.7wt%Pb 0.1 – 3wt%Zn 0.1 – 4wt%Ag up to 500 ppmW 0.1 – 5wt% WO3

Nb/Ta 1 – 3wt% Nb2O3 +Ta2O3

The ores from placer deposits are thoroughlybroken down by natural weathering processeswhich separate thematerial roughly according tothe rate at which it settles out of suspension. Thefine-grained cassiterite is mixed with coarsersand or gravel. On board the floating dredges,which operate in artificial dredging ponds ornatural surface waters, there is ore beneficia-tion equipmentwhich produces a preconcentratefor further processing on shore (Fig. 2). The orepasses through a drum screen which removescoarse gravel (10 – 20mm), wood, and other for-eign bodies. The material passing through thescreen is desludged in a hydrocyclone, and treat-ment on a three-stage screen jig then producesheavy metal concentrate for further processingon shore.

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Tin, Tin Alloys, and Tin Compounds 7

The ore obtained fromplacer deposits on landusing water cannons or from the seabed usingspecial ships with suction pumps is processedusing conventional ore beneficiation methodssuch as screen jigs or screen troughs.

Figure 1. Flotation of primary tin ores

Concentrates from placer deposits are rela-tively pure; a typical analysis follows:

Sn 70wt% Ni 0.01wt%As 0.1wt% Ta2O5 0.2wt%Sb 0.05wt% Nb2O5 0.1wt%Pb 0.008wt% WO3 0.05wt%Cu 0.005wt% SiO2 5.0wt%Zn 0.01wt% CaO 0.1wt%Bi 0.015wt% TiO2 0.1wt%Fe 0.3wt% Al2O3 1.0wt%

Figure 2.Preparation of secondary tin ores (placer deposits)on floating dredgers

4. Smelting

4.1. Fundamental Theory of Smelting[1–3], [5], [29–34]

Because the most important tin-bearing mineralin tin ore is cassiterite (SnO2), the carbothermicreaction

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8 Tin, Tin Alloys, and Tin Compounds

SnO2 + 2CO Sn + 2CO2

is of fundamental importance.A theoretical consideration of tin smelting

must include the temperature dependence of thisequilibrium and the behavior of the importantreactants (Sn, O, and C) and accompanying el-ements and impurities in the concentrate, e.g.,Fe, Cu, Sb, Bi, Pb, Ag, Si, Ca, Al, Mg, Nb, Ta,etc., ofwhich Fe is themost important. The equi-librium diagrams for Sn –O –C and Fe –O –Care of crucial importance in the reduction of tin.At temperatures above 1100 C, not only the Snbut also the Fe present in the oxidic precursorsis reduced, so that selective winning from indus-trial tin-containing raw materials, which alwaysalso contain iron, is impossible. Furthermore,in this temperature range, up to 20% tin dis-solves in iron, and iron – tin compounds (knownas hard head) are formed at lower temperatures,so that iron-free tin cannot be obtained underthese conditions. This restricts the advantage ofusing a high process temperature to give a fasterreaction rate in carbothermic cassiterite reduc-tion. Also, the technique of stably binding ironin fayalite, to enable tin reduction to be carriedout selectively, though realized on a laboratoryscale by Wright [29], could not be scaled up toproduction conditions.

The elements involved in tin reduction can bedivided into the following groups:

1) Elements more noble than tin are reduced atlower temperatures than tin, and dissolve inmolten tin (e.g., Cu, Pb, and Sb)

2) Elements that are much less noble than tinand which are not reduced under the reduc-tion conditions, but which act as importantslag formers, such as Ca, Al, and Si in theform of their oxides

3) Iron, the most important accompanying ele-ment, which behaves similarly to tin

4) Gaseous compounds produced in the reduc-tion process

5) Sulfur, which has an important role in thereduction and volatilization process of py-rometallurgical tin production

Both the iron and the slag formersmust be re-moved in a liquid slag; this determines the min-imum process temperature. The iron content ofthe metal product depends on the Fe/Sn ratio inthe slag. This relationship, represented in Fig-

ure 3, illustrates the principal problem of tin pro-duction from oxidic raw materials. If a high tinyield is to be obtained, i.e., small losses of tin inthe slag (e.g., < 10%), high reduction tempera-tures must be used, giving an iron content in thetin of > 8%, so that subsequent refining is moredifficult. If a purer metal is desired (e.g., 0.5%iron in the tin), there will be high losses of tinin the slag, whose tin content can be 10 – 25%.These slags are starting materials for a secondprocess stage.

Figure 3. Calculated Fe/Sn ratios in the metal and slag atequilibrium as a function of temperature

The reactions in the slag phase are of ma-jor importance for selective reduction, wherebySnO is an important component. Experiments onpure substances have shown that although SnOmelts at 980 C it is unstable below 1100 C,decomposing into SnO2 and Sn. The activity ofSnO in SnO–SiO2 melts obeys Raoult’s Lawbetween 1000 and 1250 C. The negative freeenergies of mixing SiO2 with FeO, ZnO, PbO,MnO, and MgO increase in this order, and in-crease the SnO activities in the silicate melts.The position of the metal – slag equilibrium inthe reaction

Fe + SnO Sn + FeO

is expressed by the distribution coefficient

K =(






K should be as large as possible, and at the usualreaction temperature of 1000 – 1100 C used in

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Tin, Tin Alloys, and Tin Compounds 9

tin metallurgy, should be ca. 300 to give an ironcontent of ca. 1% in the tin.

Binary and ternary slag systems containingtin oxides have been thoroughly investigated[33] and give useful guidance for carrying outthe tin reduction. However, practical results de-pend very much on the viscosity of the moltenproducts, the density differences, the surfacetension, and colloidal dispersion and chemisorp-tion of the slags. Thus, under production condi-tions stronger bases displace SnO bound in sili-cate, and FeO increases the fluidity of the slags.

The difference between the kinetics of tinoxide reduction and iron oxide reduction affectsthe selectivity of the reduction process and hencethe iron content of the tin and the performanceparameters of the furnace systems. Other impor-tant parameters are the pore structure and parti-cle size of the rawmaterials, the partial pressuresof the reduction gases, removal of the reactiongases, formation of seed crystals and coatings,and heat transfer. Thus, from the point of viewof reaction rate, the reverberatory furnace is notthe best equipment for carrying out reduction asit contains a large slow moving mass of materialwith a large bed thickness where heat is suppliedonly from above. The poor heat transfer leads toan extremely low smelting capacity, i.e., < 0.7 tmetal per m3 furnace volume per day. Becauseslag formation is very slow, some of the tin re-duced at the start of the process can be tappedoff as a relatively pure, low-iron product beforethe entire charge is smelted.

In contrast, highly turbulent reaction sys-tems lead to process rates orders of magni-tude higher. Under practical reaction conditionsabove 900 C, the reaction

SnO2 + 2CO Sn + 2CO2

is rapid, and the reaction


becomes rate determining. This is why oxygenhas to be added to the mixture of solid chargeand reducing carbon in the furnace.

World production of tin is two-thirds fromoxidic and one-third from sulfidic raw materi-als. The main problem in treating sulfidic tinconcentrates is their complex composition. Asmany impurities as possible are vaporized in aninitial roasting stage.

Arsenic requires special treatment, as it is ox-idized to As2O3 and As2O5, which combineswith Fe2O3, formed by roasting, to give non-volatile iron(III) arsenate. In practical operation,a somewhat reducing atmosphere is thereforeproduced by adding charcoal to the charge. An-other possibility is to vaporize heavymetal chlo-rides by adding NaCl.

4.2. Special Aspects of the Winning ofTin from its Ores

Problems in ore beneficiation often lead to con-centrates with low tin contents, as unaccept-ably high losses of material would occur if oreconcentrates with higher tin contents were pro-duced. Therefore, a pyrometallurgical “thermalore beneficiation” stage is necessary prior to theactual reduction process. This is a volatilizationprocess that exploits the fact that the iron com-pounds and other slag formers have low vaporpressures at 1000 – 1500 C, while SnO and SnSvolatilize very readily. This technique can alsobe used to treat tin-containing slags.

The vapor pressure of SnS is considerablyhigher than that of SnO. Therefore, in prac-tice, SnS is vaporized and then oxidized to SnOand SO2. Pyrites (FeS2) added as sulfur sourcecauses problems in later stages of the process,as the FeO formed must be slagged, and theSO2 evolved makes waste-gas cleaning neces-sary. However, this is outweighed by the advan-tages of a high yield of tin at relatively low pro-cess temperatures.

Economic advantages can be achieved by us-ing cheap sulfur-containing grades of heating oilas fuel in the roasting process.

Thenaturally occurringore beneficiationpro-cess that takes place in cassiterite deposits insurface waters and the increasing use of ore con-centration processes with finely intergrown oreslead to extremely fine grained materials. Pro-cesses of agglomeration or compaction wouldbe very costly. For this reason, these types ofraw material are usually treated in an ore rever-beratory furnace.

The main problem in pyrometallurgical tinproduction processes is separating tin from iron.Under production conditions, simultaneous re-duction of SnO and FeO cannot be prevented.Molten tin can dissolve large amounts of iron,

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10 Tin, Tin Alloys, and Tin Compounds

and intermetallic compounds, which are verydifficult to separate, can be formed on solidifica-tion. Tominimize this problem, tin production iscarried out in two stages. In the first stage, undermild reduction conditions, a relatively pure tinand a rich slag are produced. The latter is treatedunder strongly reducing conditions in the secondstage, giving a discardable slag and a very im-pure tin – iron compound. The metallic phase isreturned to the first stage, where the iron is re-oxidized. The two-stage process must be carriedout such that the iron initially in the concentrateis eventually removed from the process in thewaste slags.

4.3. Production of Crude Tin

4.3.1. General Aspects

The choice of a crude-tin production process in-volves consideration of factors associated withboth raw materials and location, and the follow-ing questions must be posed:

1) Whether the raw materials are highly en-riched concentrates with low levels of im-purities

2) Whether the raw materials used are low-grade concentrates whose principal impuri-ties are slag components

3) Whether complex raw materials containingat least one other valuable element (e.g., W,Nb, or Ta) are used

Other important factors include the availabil-ity of rawmaterials, energy costs, environmentalconsiderations, and personnel costs.

4.3.2. Ore Preparation prior to Reduction

As a result of the problems described in Sec-tion 3.4 and also of the efforts to recover asmuchof the tin from the ore as possible, the tin contentof the ore concentrate can range from 8 to 60%.Hence in most cases, pretreatment is necessary,e.g., pyrometallurgical enrichment of low-graderaw materials, a roasting stage (sometimes withaddition of fluxes), or a leaching operation. Pyrometallurgical Enrichment ofLow-Grade Concentrates [1–5], [28–30],[34–41]

Low-grade tin concentrates are subjected to py-rometallurgical enrichment. The tin is vaporizedas sulfide, and then oxidized in the gas phaseby atmospheric oxygen to form SnO2. The non-volatile components of the raw material are se-lectively removed, andwith careful process con-trol, 90 – 95% of the tin can be recovered as anoxidized product containing 40 – 60% Sn.

Optimumresults are only obtained if the reac-tion of the sulfidewith oxygen takes place exclu-sively in the gas phase; therefore, reducing con-ditions must be maintained in the furnace. Al-though gaseous sulfur is the best sulfiding agentfroma thermodynamicviewpoint, pyrites is usedalmost exclusively under production conditions.If calcium sulfate is used, the disadvantage thatenergy is required for its dissociation must bebalanced against the advantage that theCaOpro-duced is a useful slag former.

Although the vaporization of tin as SnS isused in all production plants, there is no gen-erally accepted model of the reactions that takeplace. However, it is probable that in view ofthe S/Sn ratios required, the available polysul-fidic sulfur in the pyrites does not take part in theformation of tin sulfide. Furthermore, it can bededuced from the importance of the added car-bon to the reaction product obtained and fromthe CO content or oxygen demand of the reac-tion gases that SnO2 is reduced first to SnO in anintermediate stage and SnS is then formed by thereaction of SnO with FeS. This is supported bythe fact that the pyrites or the FeS formed there-from forms SnS more readily than does sulfurvapor, contrary to what would be expected fromthermodynamic considerations.

These results, especially when supported bypractical experience, show that the overall equa-tion and the individual reactions are probably asfollows [28]:

SnO2 +FeS2 + 1/2 SiO2 +C+ 3/2O2 SnS + 1/2 (2 FeO · SiO2) + SO2 +CO2

FeS2 FeS + S

S+O2 SO2

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Tin, Tin Alloys, and Tin Compounds 11

C+1/2O2 CO

SnO2 +CO SnO+CO2

SnO+FeS SnS + FeO

2 FeO+SiO2 2 FeO · SiO2

Pyrometallurgical enrichment of low-grade tinconcentrates can be carried out in various typesof furnace.

Initially, rotary and shaft kilns were used forthe vaporization process. Although the opera-tion of both types of equipment was technicallysophisticated, therewere considerable disadvan-tages, which eventually led to their abandon-ment. Apart from the high energy and fuel re-quirements typical of both systems, the shaft kilnprocess led to a relatively low direct yield of tinin the flue dust, mainly due to the production ofmatte, which required separate processing. Therotary kiln method led to a low tin concentrationin the flue dust, and, for rawmaterial of high ironcontent, to the formation of matte, and hence toa drum clinker, which had then to be processedin a shaft kiln.

High-capacity thermal concentration pro-cesses, already established on a large scale inthe nonferrous metal industry, were thereforeadapted to tin enrichment. The fluidized bedand the cyclone smelting processes were notused. Also, the flash smelting (levitation smelt-ing) process has severe limitations because ofthe raw materials used. Good results could beobtained with concentrates in which most of thematerial had a particle size of 200 – 300µm. Us-ing such concentrates containing 10 – 12% Sn,discardable slags containing 0.2 – 0.4% Sn andflue dusts containing 50 – 60% Sn could be ob-tained.Onusing low-grade concentrates that hadnot been desludged and which contained signif-icant amounts of material with a particle size of50 –60µm, the amount of primary flue dust in-creases significantly, and the tin concentrationin the flue dust sometimes decreases to < 40%.Also, the tin content of the slags is unacceptablyhigh (1 – 2%).

The main disadvantage of the flash smeltingprocess is that it imposes strict requirements onthe physical form of the concentrate.

The slag blowing process, originally used fordetinning the slags from the reduction process,

has been increasingly used for enrichment oftin in low-grade concentrates and exploitablegangue from ore processing. These productswere at first added to the initial smelting process,but theywere later added in solid formdirectly tothe blowing furnace. Smelting and blowing canbe carried out in a single furnace, so that it isnot necessary to build a special smelting plant.This enables capital and operating costs to bereduced.

An important part of the practical operationis the maintenance of the correct fuel – air mix-ture with uniform distribution of the fuel andair to the individual jets. Heating oil and naturalgas are preferred because they are more easilymetered than solid fuels. If pyrites is used as thesulfiding agent, an S/Sn ratio of 0.8 is necessary,which places high demands on the equipment forremoving sulfur dioxide. When pyrites is used,only FeS is effective in the slag phase, so thatonly 50% of the sulfur is used for sulfiding. Ifpyrrhotite (magnetic pyrites) or calcium sulfideis used, the S/Sn ratio need only be 0.4, and theemission of sulfur dioxide decreases by 50%.

The good metallurgical results of the slagblowing process, in which flue dusts containing65 – 70% Sn and final slags containing 0.1%Sn are obtained, must be balanced against thedisadvantage of a batch process. This leads tononuniform loading of the downstream plant,e.g., the waste heat recovery and gas cleaningequipment. Roasting [1], [2], [4], [5], [11], [17],[32], [42–50]

The roasting process, not only converts sulfidesto oxides, but also volatilizes major oxidic im-purities (e.g., arsenic). Roasting can be an inde-pendent process, or a pretreatment prior to hy-drometallurgical leaching. It is significant thatthe level of impurities As, Sb, Pb, and Bi in tinconcentrates has increased in spite of great ef-forts to improve ore beneficiation technology. Ifthe levels of As and Pb are > 0.1%, and of Biand Sb > 0.03%, a roasting process, sometimeswith the addition of a leaching stage, is both use-ful and necessary for the benefit of the final tinreduction and refining processes.

The important reactions in the roasting pro-cess are as follows (M=metallic impurity):

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12 Tin, Tin Alloys, and Tin Compounds

1) DissociationFeS2 FeS + 1/2 S24 FeAsS 4 FeS +As4

2) Roasting reactionsMS+ 3/2O2 MO+SO2

3) OxidationMO+1/2O2 MO2

Although the roasting reactions and espe-cially the oxidation reactions are exothermic,addition of fuel is necessary in industrial-scaleroasting processes. The pore structure of thema-terial must be maintained to enable the gaseousmetallic and nonmetallic impurities to be vapor-ized. The upper temperature limit for the roast-ing process is imposed by the melting point ofthe low-melting sulfide eutectic. However, thetemperature should be kept as high as possibleto prevent the formation of sulfates, e.g., leadand calcium sulfate.

A mildly reducing atmosphere is necessaryto suppress sulfate formation and also preventformation of higher nonvolatile oxides of the im-purities (e.g., As2O5).

The roasting processes are carried out inmul-tideck or rotary kilns. The use of fluidized-bedfurnaces with carefully controlled operating pa-rameters has been reported [48].

Chloridizing roasting is also suitable for thepretreatment of tin concentrates, owing to thehigh affinity of the main impurities for chlorine.However, problems can be caused by chlorida-tion of tin, which itself then volatilizes as SnCl4and SnCl2.

Chloridizing roasting of tin concentrates inrotary kilns is carried out in Thailand. The con-tents of lead and bismuth are lowered from2.0%to 0.04%, and from 0.1% to 0.02%, respec-tively. The flue dust contains 10% As, 3% Sn,20% Pb, and 4% Bi. Treatment of this productpresents serious problems [2].

The processes for removing impurities bychloridation differ fundamentally from thosefor the volatilization of tin from concentrates,being based on the fact that a large phasefield exists in the phase diagrams Sn –HCl –H2and Fe –HCl –H2 between 900 and 1000 C inwhich selective chloridation and volatilizationof tin is possible without chloridizing iron.

The overall reactions are as follows (M=Mg,Ca, or Na):

SnO2 +MCl2 +CO SnCl2 +MO+CO2

SnO2 +Cl2 +C SnCl2 +CO2

SnO2 + 2HCl +CO SnCl2 +H2O+CO2

The presence of iron oxide increases yield andreaction rate, which can only be explained byintermediate formation of FeCl2:

4 SnO2 + 6 FeCl2 2 Fe3O4 + 2 SnCl2 + 2 SnCl4

Here, the instability of iron chloride in the pres-ence of tin oxide is the reason for the good sepa-ration of tin from iron. In a reducing atmosphere,i.e., in the presence of C or CO, only SnCl2 isformed.

In the Warren Spring process [50], CaCl2 isused as chloridizing agent, and the CaO formedis used in the reductive smelting by adding lime-stone in accordance with the following equa-tions:

SnO2 +CaCl2 +C SnCl2 +CaO+CO

SnCl2 +CaCO3 +C Sn +CaCl2 +CO+CO2

The industrial-scale reaction is difficult to carryout, as moisture resulting from the hygroscopicproperties of the tin chloride and the quicklimemust be avoided. The corrosive properties of thereaction gases make very high demands on theconstruction materials used.

A special form of roasting is the heating ofhigh-tungsten tin concentrates with NaOH orNa2CO3 to give soluble Na2WO4, which canbe leached out. Leaching [1–4], [32], [47], [51–55]

In tin metallurgy, the main use of leaching pro-cesses is to remove typical impurities from theconcentrate. Tin, which is nearly always presentin the rawmaterials asSnO2, canonly bebroughtinto aqueous solution if the SnO2 is reduced toSnO in a precisely controlled CO/CO2 atmo-sphere. It can then be extracted in acid or al-kaline media. This procedure has not yet beenoperated on an industrial scale.

Only hydrochloric acid is used on an indus-trial scale to remove impurities by leaching, thetypical impurities in the concentrate, Fe, Pb, Cu,Sb, or As, going into solution in the form of their

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Tin, Tin Alloys, and Tin Compounds 13

chlorides.Best results are obtained using> 20%hydrochloric acid at 100 – 110 C. Suitable re-action vessels are high-pressure, acid-resistantspherical boilers with a capacity of 20 t. Thisbatch process must sometimes be repeated sev-eral times. The solids are removed by thickenersand vacuum filters, and the dissolved impuritiesare precipitated from the liquor by cementationon scrap iron.

The high costs of these special reactors, thebatch mode of operation, and the expense ofthe process for recovering the hydrochloric acidhave restricted the use of hydrochloric acidleaching to some special cases.

It is preferable to carry out chloridizing roast-ing before leaching out the impurities. This thenonly requires a dilute acid solution.

Leaching of tungsten-containing tin concen-trates to recover tungsten is important. After di-gesting the ore with sodium carbonate in spher-ical boilers, the tungsten is converted to its hex-avalent form, which is soluble in hot water. It isthen precipitated from the neutral solution withCaCl2:

Na2WO4 +CaCl2 CaWO4 + 2NaCl2

An easily filtered precipitate containing up to60wt% WO3 (on dry basis) is obtained.

Tungsten can also be extracted from tin con-centrates by leaching with an aqueous solutionof ammonia:

H2WO4 + 2NH4OH (NH4)2WO4 + 2H2O

The concentration of WO3 in the filtrate canreach 50 g/L, and this can be precipitated as arti-ficial scheelite (Ca[WO4]). The tungsten contentof the tin concentrate can be reduced to 0.5%.

Most of the tin can be leached from a tin con-centrate containing 2wt% bismuth with 5% hy-drochloric acid at 80 C [46].

Treatment with sulfuric acid is used to re-move iron present as carbonate in Australian tinconcentrates. By removing gangue material inthis way, the tin content of the concentrate isincreased from 37.2% to 47.1%.

A process for the removal of arsenic fromtin concentrates by bacterial leaching withThiobacillus ferrooxidans has been tested inRussian research institutes [54].

The tin can be leached out of ores that are dif-ficult to treat if these are first reductively smelted

with CuCl2 and HCl, in accordance with the fol-lowing series of reactions [42]:

Sn +CuCl2 Cu+SnCl2

Cu+CuCl2 Cu2Cl2

Cu2Cl2 +HCl 2CuCl2 +H2

Tin is precipitated from solution by adding zinc,and copper by adding iron.

4.3.3. Reduction

As explained in Section 4.1, it is not possible toobtain high yield and high metal purity at thesame time. Reduction is therefore carried out intwo stages, the first stage giving a relatively puremetal (up to 97% Sn) and a rich slag (8 – 35%Sn). This slag is treated in a second stage, andsometimes in a third. Slag treatment processesare described in Section 4.3.4.

Various types of furnace are used for reduc-tion. Very low grade ore in lump form can betreated in a shaft kiln. However, as most con-centrates obtained in ore beneficiation are veryfinely divided, and agglomeration, e.g., by sin-tering, is impossible, other types of furnacemustbe used in this case. Reverberatory or rotary airfurnaces are often used for reduction, and elec-tric furnaces are also employed. Each furnacetype has its own advantages and disadvantages. Reduction in a Shaft Kiln [1–3], [5],[30], [56], [57]

Shaft kilns are historically the oldest type usedfor tin reduction. They have their origin in theold Chinese natural draught furnaces made oframmed clay held together with wooden postsand operated on mountainsides. The hearth wasusually sloped so that the molten product ranoff continuously. Modern shaft kilns are water-jacketed, and have a melting capacity of 5.5 –8 tm−2 d−1. The good heat transfer and the con-tinuous method of operation give a higher melt-ing capacity than is obtained in comparable re-verberatory furnaces. The air flow rate must below to prevent volatilization. Shaft kilns requireraw material in lump form. The iron contentmust be as low as possible in order to limit theformation of hard head in the reducing atmo-sphere.

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14 Tin, Tin Alloys, and Tin Compounds Reduction in a ReverberatoryFurnace [1–3], [5], [30], [32], [58–61]

The necessity for treating fine-grained concen-trates from ore beneficiation has led to the re-placement of the shaft furnace by the reverber-atory furnace, and this is today the most im-portant type of reduction equipment used in tinmetallurgy.Modern reverberatory furnaces haveinternal dimensions of 3 – 4m width, 10 – 13mlength, and 1 – 1.5m height. In a freshly linedhearth the height available for the molten chargeis no more than 0.5m. This increases with in-creasing wear of the furnace bottom. The bathvolume ofmodern tin ore reverberatory furnacesis 20 – 50m3. As the tin is very fluid at the re-action temperatures (up to 1400 C), high pres-sures canbeproduced at the bottomof thehearth.To prevent this, either holes must be provided inthe steel sheet bottom to allow the tin to drip intoa lower chamber where it can solidify in stalac-tite form, or the reverberatory furnace must havea water-cooled bottom so that the tin solidifiesin the gaps between the bricks.

The refractory bricks and mortar used to linethe furnacemust bemade of high-grade chrome-magnesite. This must be very pure because ofpossible reactions of SnO or Sn with iron ox-ides or silica. Chamotte can only be used abovethe slag zone. To avoid disturbance of the brick-work in the melting zone, charging is carried outthrough the roof.

The burners, which use heavy fuel oil, aresituated on the narrow sides of the furnace. Thefurnaces operate discontinuously using the re-generative principle, the duration of a heat being16 – 20 h. Specific melting capacities are in therange 1.2 – 2.0 tm−2 d−1. Charge batches con-sisting of concentrate, carbon, and fluxes weighbetween 40 and 70 t. Optimum results can onlybe achieved by extremely careful operation.

Thus, at the start of a new heat, the amount ofmaterial charged to the furnace should be lim-ited, e.g., by adding the charge in twoportions, toprevent a large drop in temperature. The amountof carbon added in the form of coke, coal, pe-troleum coke, or charcoal is determined by thetin and iron contents and also by the incidenceof hard head. The carbon addition is adjusted sothat coreduction of the iron is prevented as faras possible.

The furnace draught must be kept low to pre-vent entry of cold air and to maintain the oxygencontent in the waste gas below 5 vol%, therebyminimizing oxidation of the tin.

The first tin can be tapped off after 4 – 5 hof a 24 h cycle. As this is comparatively pure,it is preferable to treat it separately. The mate-rial tapped off later separates into metal and slagin a settler outside the smelting furnace. Somefurnaces have facilities for tapping off the metaland the slag separately.

Up to 6% of the weight of charge materialis converted to flue dust, which consists of en-trained charge, SnO, and some SnS. Its tin con-tent is therefore considerably higher than that inthe charge material.

Compositions of products obtained whena relatively pure concentrate (> 70% Sn) istreated in a reverberatory furnace are listed inTable 2. The amount of slag is relatively small(14wt% of the charge), but the slag must un-dergo a further stage of processing because ofits high tin content.

Table 2. Typical compositions of products from a reverberatory fur-nace (in wt%)

Component Crude tin Slag Flue dust

Sn 97 – 99 8 – 25 40 – 70Fe 0.02 – 2.0 15 – 40 0.8 – 4.0As 0.01 – 2.0 0.1 – 0.7Pb 0.01 – 0.1 0.1 – 0.5 0.2 – 1.6Bi 0.003 – 0.02 0.01 – 0.1Sb 0.005 – 0.2 0.01 – 0.02 0.1 – 0.4Cu 0.001 – 0.1 0.02 – 0.05Al2O3 7 – 12 ≤1.0SiO2 10 – 30 ≤2.0CaO 4 – 14 ≤0.6MgO 1 – 4 ≤0.6S 0.01 – 0.05 ≤1.0

A simplified flow diagram for the treatmentof high-grade concentrates is shown in Figure 4.

The process flow diagram for low-grade, im-pure, and complex concentrates is considerablymore complex. With high-grade concentrates, atin yield, including that obtained from slag treat-ment, of 99.6% can be achieved, as actual lossesof tin can only occur in thewaste slag and by lossof airborne dust. In the treatment of low-gradeconcentrates, the yields of tin may only be 95%,and in exceptional cases 92%, because of thenumerous tin-containing waste products.

Control of the CO partial pressure, which de-termines the reduction potential, is more diffi-

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Tin, Tin Alloys, and Tin Compounds 15

Figure 4. Process for treating high grade concentrates (simplified)

cult in a stationary reverberatory furnace, whichcontains a slow-moving mass of charge, than ina shaft furnace, in which the coke and air movein countercurrent flow. This is also the reason forthe poor heat transfer and hence the low rates ofreduction andmelting in a reverberatory furnace.

Under operating conditions, care must betaken that the slag formers do not melt toorapidly, as this impairs the contact between thefurnace atmosphere and the unmelted charge. Atthe same time, the melting temperature of theslag must be as low as possible, so that the useof such slag formers as CaO and SiO2 should belimited. Added lime is of special significance,because it displaces tin from silicate, can lead tocalcium stannate formation if present in excess,and increases the melting point of the slag.

The effect of the slag constituents, especiallyon melting point and viscosity, is described indetail in [3]. In a process optimization/cost min-imization exercise, it is always necessary to in-clude the effect on costs of the amount of tin tiedup in materials being recycled [59]. Reduction in Rotary Kilns [1–3], [5],[30], [32], [57–62]

Rotary kilns (rotary air kilns) are horizontalsmelting units that operate batchwise. They havea higher melting capacity than stationary rever-beratory furnaces, but lead to considerably moresevere stress on the refractory lining. Operatingprocedures in two tin smelting works in Indone-sia and Bolivia in which the reduction process isbased on the rotary kiln principle are reported inthe specialist literature [63]. The furnaces have alength of 8m, a diameter of 3.6 m, a surface areaof reacting material of ca. 22m2, and a specificmelting capacity of 1.36 – 1.5 tm−2 d−1.

The furnace availability (300 d/a) is superiorto that of reverberatory furnaces (260 d/a). Also,there is less requirement for mixing operationsor agitation of the melt by stirring (rabbling).The metallurgical results of both types of fur-nace are very similar. However, the consider-ably poorer stability of the refractory lining, thehigher energy requirement, and the significantlylarger quantities of flue dust are all disadvan-

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16 Tin, Tin Alloys, and Tin Compounds

tages. Separation of the tin from the slag has tobe carried out outside the furnace in a settler.

Tin concentrate can also be reduced in shortdrum kilns in which the ratio of the length to thecross-section is 1. The metallurgical function isbasically similar to that of the rotary kilns.

The compositions of the smelting products inan Indonesian tin smelting works in which high-grade concentrates are reduced in a rotary kilnare given in Table 3.

Table 3.Typical compositions of products from a rotary kiln atMen-tok, Bangka (in wt%)

Component Crude tin Slag Flue dust

Sn 99.78 – 99.83 14 – 25 60 – 72Fe 0.089 – 0.144 15 – 26 1 – 4Pb 0.010 – 0.031As 0.010 – 0.188Sb 0.005 – 0.010Bi 0.0025 – 0.003Cu 0.002 – 0.025SiO2 8 – 24 0.2 – 2.0CaO 2 – 10 1 – 1.2MgO 2 – 4S 0.2 Reduction in an Electric Furnace[1–3], [5], [30], [32], [58], [64–68]

Electric resistance and arc furnaces used in met-allurgy are characterized by high reaction tem-peratures and lowwaste-gas volumes.Disadvan-tages are the necessity for thorough premixingof the rawmaterials and the batch operation. Tinsmelting is often carried out in regions whereelectrical energy is less readily available than en-ergy from other sources such as gas, coal, or oil.Wherever electric furnaces are used in tin metal-lurgy, the object is to utilize their advantages ofhigh reaction temperature and the production ofheat by the Joule effect directly in the smeltingbath.

Because their reducing action is so effective,electric furnaces are particularly suitable for ex-tracting tin from slag (see Section 4.3.4). In con-centrates whose iron content is significantly lessthan 5%, it is even possible to produce crude tinin a single stage, with tin levels in the slag of< 0.7%.

Electric furnaces are used in many tin smelt-ing works for primary tin production fromconcentrates. These concentrates are often im-

ported, e.g., in Germany, France, Italy, Canada,and Japan, although tin concentrate producingcountries, e.g., Brazil, Zaire, South Africa, Rus-sia, Thailand, and China also use electric fur-naces. The raw materials for the electric furnaceprocess must be intensively mixed. Very fine-grainedmaterials such as flue dust are pelletized.A moisture content of ca. 2% must not be ex-ceeded.

Typically, circular furnace vessels with anoutside diameter of up to 4.5 m and a height of1.5 – 3m are used, or oval furnace vessels withdimensions 2.5×1.8×1.8m. Heating is carriedout with three-phase electric arcs using graphiteelectrodes. In single-phase furnaces, the furnacebottom acts as the counterelectrode for the im-mersed graphite electrodes.

Both stationary and tilting furnaces are used.Linings of carbon bricks give a service life of upto three years. Electric currents of 6 – 20 kA at50 – 150V are used. Depending on the charac-teristics of the raw materials, energy consump-tion is 750 – 1400 kW · h/t concentrate (1300 –1860 kW · h/t tin). Precise control of the elec-trode immersion depth is essential for good con-trol of the process.

Electric furnace technology enables a widerange of process parameters to be used. For ex-ample, when low-iron concentrates are treated,a tin quality suitable for normal refining can beproduced. The high-tin slags produced (up to30% tin) are treated in a second stage to recoverthe tin. If the iron contents are much greater than3%, the tin obtained has a high iron content(3 – 10%).

It is also possible to operate at ca. 1400 C byusing strongly reducing conditions. A tin-con-taining slag is then obtained together with hardhead containing ca. 40% tin and 50% iron. Theiron can be removed by smelting with ferrosili-con in a second pass, and a crude tin containingca. 1% iron is obtained.

Great efforts have been made to overcomethe disadvantage of batch operation of the elec-tric furnace. For example, a continuously oper-ated lengthened double chamber electric furnacehas been reported in Russia. Each chamber hasa hearth area of 1m2 and two electrodes. Thereduced tin flows out of the first chamber overan air-cooled overflow, and the tin is extractedfrom the high-tin slag in a second chamber [68].

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Tin, Tin Alloys, and Tin Compounds 17

The process is still at the pilot stage. The furnacecapacity is reported to be less than 10 t/d. Other Reduction Processes [1–3],[5], [30], [58], [60], [69]

All the processes described above have disad-vantages, and many other methods and types ofequipment have therefore been suggested for thereductive treatment of tin concentrates, but fewof these proposals have led to an industrial plant.

One of the few methods tested at full scaleis the top blown rotary converter (TBRC) de-veloped in the United States and based on the“Kaldo” converter used in ferrous metallurgy.Oxygen is blown onto the top of charge as theconverter rotates about its axis, which is set atan angle. The volume of waste gas is very low asthere is no ballast nitrogen. The favorable heattransfer to the charge leads to a high reactionrate. Disadvantages include batch operation, ahigh rate of wear of the refractory lining, andthe complexity of the system for controlling theoxygen lance.

After completionof the reductionprocess, thetin is tapped off. The tin in the remaining slag isvaporized as chloride by adding calcium chlo-ride, and a discardable slag is thereby produced.The tin chloride is scrubbed out of the wastegases and then precipitated as SnO2 by addingCaO, regenerating CaCl2.

The potential for transferring proven pro-cesses used in nonferrous metallurgy to tin met-allurgy is discussed in detail in [69].

Apparently no completely new processes forreducing tin concentrates have become estab-lished in the industry because of the high capi-tal investment required and the hidden risks in-volved.

4.3.4. Slag Processing [1], [2], [5], [32], [38],[47], [62], [63], [70–79]

The slags produced during the reduction of high-tin concentrates can contain 5 – 10% of the tin.This can increase to 20% in the case of low-grade and complex ores. One- or two-stage treat-ment of the slag is then necessary. It is in prin-ciple possible to use strongly reductive smelt-ing (e.g., in a reverberatory or rotary furnace) or

blowing processes to volatilize the zinc from theslag.

On reductive smelting, the tin and iron forman alloy, the so-called hard head, which is recy-cled to the primary tin production process. Thesecondary slag has such a low tin content that itcan be removed from the process.

In the blowing process, the tin is convertedinto a flue dust, which is recycled to the primarysmelting process. The slag, which usually has ahigh iron content, can be discarded.

Under production conditions, the intermedi-ate products, e.g., the tin-containing secondaryslag, the hard head, and the flue dusts, containconsiderable quantities of tin. These are impor-tant for the economic operation of the processbecause of the amount of capital tied up if theyare not immediately treated. The tin and ironbalances in the treatment of a concentrate with avery high tin content and a low iron content areshown in simplified form in Table 4.

Table 4. Tin and iron balance for two-stage smelting of a concen-trate containing 73% Sn and 0.7% Fe based on 1000 kg concentratetreated

kg Sn kg Fe

Smelting of concentrateCharge :1000 kg concentrate 730 795 kg hard head 38 42220 kg coke/coal 240 kg flue dust 1811 kg recycled material 6 3

Total 792 54Product :733 kg crude tin 730 2200 kg primary slag 47 5130 kg flue dust 15 1

Total 792 54Smelting of slagCharge :200 kg primary slag 47 5140 kg coke/coal

Total 47 51Product :100 kg secondary slag 1 895 kg hard head 38 4210 kg flue dust 52 kg Fe alloy 1

Total 44 51

The balance shows that the iron introducedinto the concentrate smelting process with theconcentrate and reducing agent (7 + 2 kg) mustbe removed from the slag smelting process in thesecondary slag and iron – tin alloy (8 + 1 kg). Of

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18 Tin, Tin Alloys, and Tin Compounds

the tin in the concentrate, 6% is in circulation inthe primary slag (47 kg out of 730 kg).

However, the concentrates treated usuallycontain considerably more iron, so that a largeramount of primary slag and hard head are pro-duced, and the amount of tin contained thereincan rise to 20% of the raw material used.

In practical slag smelting, the SnO in silica-rich slags is mainly present as 2 SnO ·SiO2, andthe high activity of the SnO in silica- and lime-containing slags decreases with increasing FeOcontents, so that in practice simultaneous re-duction of tin and iron occurs. Theoretically, itis only possible to produce crude tin and low-tin slag if the FeO activities are extremely low.However, the processes would proceed at hightemperatures with large additions of reducingcarbon.

The binary Fe – Sn phase diagram shows amiscibility gap at > 1100 C between 20 and50wt% Fe. On cooling to room temperature,separation of α-iron first takes place, followedby formation of FeSn and FeSn2. At room tem-perature, the region of the composition of thehard head is always in the α-iron/FeSn two-phase region.

Wright has found by calculation of the distri-bution constant K at equilibrium

K =(






that the balance of the process is improved as theratio in the first quotient increases, i.e., as theamount of iron removed in the slag increases.However, there must then be a higher iron con-centration in the tin. If the tin in the primary slagis present as 2 SnO ·SiO2, the reaction mecha-nism is as follows:


CO+SnO CO2 +Sn

CO+FeO CO2 +Fe

CO+Fe2O3 CO2 + 2 FeO

3 FeO Fe + Fe2O3

Fe + SnO FeO+Sn

2 FeO+SnO Fe2O3 +Sn

Thus the reaction first occurs only at the surfaceof the carbon, and then via the iron oxides as in-termediate phases. A high relative rate of reac-tion of carbon, slag, and reaction gas is thereforeimportant for the reaction kinetics.

The reactions that occur in the detinning ofslag are described in Section (pyrometal-lurgical enrichment of low-grade concentrates),but here they start from SnO. As a matte phasemust be present, the reaction will be:

SnO+FeS SnS + FeO

As the slag is saturated with FeS, the amountof SnS formed is proportional to the activity ofSnO in the slag, but indirectly proportional tothe activity of FeO.

Reverberatory or electric furnaces are usuallyused in the reductive smelting process for theextraction of tin from primary reduction slags,though shaft kilns are occasionally used. Thenecessary intense reducing effect is achieved byadding 10 – 20% reduction carbon and by oper-ating at temperatures up to 1500 C. In the elec-tric furnace process, energy consumptions are500 – 1000 kW · h/t, and 1 – 10 kg electrode isconsumed per tonne of slag. The reaction prod-ucts, i.e., the final slag and the hard head alloy,are separated in settlers and then granulated inwater.

As highly turbulent conditions are favorableto the process, reductive detinning can be car-ried outwith a lance (“submerged combustion”),which produces a high reaction rate by agitationof the bath. Methane or natural gas have beenused as reducing gas, and experiments have alsobeen carried out using hydrogen, carbonmonox-ide, and powdered solid fuels. Heat is producedby partial combustion of the injected gases andtransferred to themolten slag. However, becauseSnO is volatile, 20%of the tin goes into airbornedust, a further undesired tin-containing reactionproduct in addition to the hard head.

An effective method of detinning primary tinreduction slags is by the blowing process, whichcan be carried out in reverberatory furnaces ortrue blowing furnaces. Gypsum or pyrites can beused as the sulfur source. The gypsum is first re-duced to CaS, and then reacts with SnO to formSnS and CaO. If pyrites is used, impurities suchas Pb, Zn, or As are introduced into the met-allurgical process, adding to the difficulties of

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Tin, Tin Alloys, and Tin Compounds 19

treating the tin-containing flue dusts. Blastingtechnology is also often used as the third stageof slag detinning. In the second stage, a ther-mal reduction, Sn contents of 2 – 5% and FeOcontents of ca. 30% are obtained. The tin con-tent of the molten slag in the furnace is reducedto ca. 0.5% by addition of sulfur sources. Theaddition of pyrites can be up to 400% of thetheoretical amount calculated for the formationof SnS. Since smaller additions are possible inthe case of acid slags, CaS is probably formedin basic slags. The treatment of primary slagsin true blowing furnaces consists of blowing thepyrites into the molten slag. However, the pro-cess is discontinuous. The SnS vapor is oxidizedin the furnace atmosphere, and recovered as SnOdust by filtration.

The slag blowing process can be carried outin furnaces of various designs, e.g., in a type ofshaft kiln with a floor area of 0.5 – 6m2 and awater-cooled shaft 7m in height. The charge ofprimary slag can be 5 – 20 t. Cyclone and shortdrum kilns are also used. Process parameters,e.g., ratios of sulfur source to oxygen, fuel toatmospheric oxygen, and tin to sulfur must becarefully controlled to obtain optimum results.

Other proposed methods of detinning pri-mary tin slags, such as vacuum-assistedvolatilization of SnO (which has a considerablylower vapor pressure than SnS), have not beenused on an industrial scale [29].

5. Refining [1–3], [5], [32], [46], [47],[80–84]

The crude tin obtained by the reduction processis insufficiently pure for most applications.

Most national standards specify maximumcontents of typical impurities. However, non-metallic impurities such as oxygen and sulfurand less common impurities such as noble met-als are neglected, and the tin content is simplydetermined by subtracting the total amount ofanalytically determined impurities from 100.

The following three standard grades are ac-cepted internationally: Sn 99.0%, Sn 99.75%,and Sn 99.9%.

There are some variations in these standardsfrom country to country; the German standard isDIN 1704.

The level of impurities in the crude tin deter-mines the extent of the refining operation. Treat-ment of very pure concentrates can give a tincontent of up to 99.0% in the crude metal. Themain impurity is iron (0.8%), the sum of all theother impurities being only 0.2%. In the case oflow-grade and complex concentrates, the situa-tion is very different, the tin content of the crudetin obtained sometimes being only 92%.

The impurities Fe, As, Sb, Cu, Ni, Pb, Bi,and the noble metals affect the amount of workinvolved in the refining process. The metals Zn,Cd, Mg, Si, Ca, Te, Se, and also sulfur and oxy-gen do not require special treatment, as they arepresent in the crude metal in only small concen-trations, and are removed together with the otherimpurities during the various stages of purifica-tion.

The phase diagrams for tin and its typical im-purities lead to the following conclusions:

In the temperature range between 1000 and1300 Cused in pyrometallurgical reduction, theimpurities are completely soluble with the ex-ception of Fe and Cr.

Only Sb, Cd, Bi, Zn, and Pb are significantlysoluble in tin at room temperature. This is thebasis for the removal of insoluble impurity ele-ments by liquation. However, the liquation prod-ucts have high tin contents,making costly recov-ery processes necessary.

In pyrometallurgical tin refining, the individ-ual impurities are removed stepwise in batchprocesses. The use of the time-consuming op-erations is justified by their high selectivity.

Proposed continuous processes have not beenoperated on an industrial scale.

5.1. Pyrometallurgical Refining

5.1.1. Removal of Iron

The process for removing iron is based on thetemperature dependence of the solubility of ironor Sn – Fe mixed crystals in tin. Accurate exper-iments have shown that the solubility of iron intin at 250 C is 0.0058wt% [81]. In industrialpractice, even lower figures are achieved, whichcan only be explained by other impurities, suchas Cu, As, or Sb, causing deviations from idealsolubility behavior. On coolingmolten crude tin,

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20 Tin, Tin Alloys, and Tin Compounds

α-Fe, γ-Fe, FeSn, and FeSn2 precipitate suc-cessively. The density of the precipitated com-pounds is almost the same as that of the moltentin. In practice, “poling”, i.e., passing steam orair into the melt, is used to coagulate the pre-cipitated particles, which rise to the surface ofthe bath and are removed from the molten tinby filtration through graphite, slag wool, or aslabmadeof silica and limestone chippings.Thisshould be carried out just above themelting pointof tin, or in practice at a temperature not less than260 C. The process is sometimes carried outin two stages. Iron contents of 0.003 –0.01wt%can be achieved.

As Ni, Co, Cu, As, and Sb form intermetal-lic compounds with each other as well as withFe, these impurities are also removed to someextent.

The treatment of the recovered intermetalliccompounds is very complex, as these are in theform of a slurry with large amounts of adher-ing molten tin. The iron content is only a fewper cent, i.e., considerably less than that of theintermetallic compound FeSn2.

The metal slurry is treated in small liquationfurnaces.A controlled temperature increase overthe range 230 – 300 C enables pure tin to beremoved, and a residue containing ca. 15% Fesuitable for use in the primary smelting stage tobe obtained.

In high-capacity tin smelting works, high-temperature centrifuges are used. These enablea solid residue containing up to 25% Fe to beobtained [83].

5.1.2. Removal of Copper

After iron has been removed by liquation, thecopper content is up to 0.01%. Elemental sulfur(2 – 5 kg/t) is stirred in at 250 – 300 C, enablingcopper contents as low as 0.001% to be attained.The resulting copper dross can be removed fromthe process after several stages of enrichment.

5.1.3. Removal of Arsenic

After removal of iron by liquation, the arseniccontent is ca. 0.1%, significantly higher thanpermitted levels. For example, the commonlyused Sn 99.75 grade should contain < 0.05%As according to DIN 1704.

Arsenic can be removed from themelts alongwith someCu,Ni, and residual Fe by forming in-termetallic compounds with aluminum. For this,the aluminum must be present as an ideal solu-tion in the tin. For this reason, the melt must beheated to a temperature close to themelting pointof aluminumbefore the aluminum is added. Spe-cial precautions are necessary, e.g., operation ina closed vessel so that the aluminum does notburn on the surface of the bath. The amountadded must be approximately three times thestoichiometric amount. The use of Al – Sn mas-ter alloys enables the operation to be carriedout at a lower temperature. Intense agitation iffollowed by a settling process with cooling to350 – 400 C, and theAl –Asmixed crystals canthen be removed. Separation of the Al mixedcrystals is assisted by poling.

With correct operation, As contents of< 0.02%can be achieved, i.e., below the permit-ted level for Sn 99.90%. This process enables Sbcontents of 0.005%, Cu contents of 0.02%, andNi contents of 0.005% to be attained, and anyremaining Fe to be removed.

Any aluminum remaining in the molten tincan be removed by adding sodium, sodium hy-droxide, chlorine, or steam, and residual sodiumby adding sulfur.

The storage, transport, and treatment of theAl –As product presents problems. Contact withwater must be avoided as this leads to the for-mation of highly toxic arsine and stibine. Thematerial is stored in closed vessels and is con-verted into a safe product as soon as possible byoxidative roasting or by treating with alkali so-lution and collecting and burning the liberatedarsine to formAs2O3 andH2O. The residues ob-tained both from the roasting and the leachingprocesses can be recycled to the process.

The large differences in vapor pressure bet-ween the impurities (arsenic, antimony, bismuth,and lead) and tin enable selective evaporation atreduced pressure to be used. However, numer-ous proposedprocesses have resulted in only twoindustrial applications.

In a system tested in Russia, the impure tinflows from the top of a vertical reactor under avacuum of 1 Pa into heated evaporating dishes.The evaporated impurities are collected in a sep-arate chamber. Barometric valves are used to re-move the purified tin and the condensate.

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Tin, Tin Alloys, and Tin Compounds 21

In the Bergsoe –Redlac system, a cylinder,cooled on the inside, rotates in a horizontal vac-uum chamber above the melt, and the vapor-ized impurities are deposited on this in solidform. In the next stage of the process, they arescraped off and removed. The results of the vac-uum distillation process depend on the reactiontemperature and time. The high energy require-ment for heating and for producing the vacuum,400 – 700 kW · h/t tin, is a disadvantage. More-over, significant quantities of tin are vaporized.The use of selective evaporation of typical impu-rities in crude tin always involves a compromisebetween the purity and tin yield. Practical exper-iments are described in [85].

5.1.4. Removal of Lead

If the lead content is still not low enough afterthe first stages of the refining process, the leadcan be converted into its dichloride by treatmentwith chlorine, tin dichloride, or tin tetrachloride:

SnCl2 +Pb PbCl2 +Sn

The equilibrium is shifted to the right at lowtemperatures, so that the process must be car-ried out at a temperature only a little above themelting point of tin. The process also removesany remaining zinc and aluminum.

The best results are obtained by using two-stage operation, i.e., the product of the secondstage, with a reduced lead content, is returned tothe first stage. Precise control of the operationcan lead to final lead contents of 0.008wt%.

5.1.5. Removal of Bismuth

In analogy to the thermal refining of lead, bis-muth can be precipitated as an intermetalliccompound by adding calcium or magnesium.The molar ratio Ca/Mg should be ca. 2 to ob-tain the best results [46]. A ternary compound isprobably formed at this ratio. Under productionconditions, scrap magnesium is used, as this isthe most economic material.

Final bismuth contents of 0.06 – 0.003wt%have been reported for full-scale plant. In anal-ogy to a technique used when removing arsenic

from aluminum, the calcium andmagnesium re-maining in the tin can be converted to their chlo-rides and removed, e.g., by treatment with am-monium chloride.

5.2. Electrorefining

The theoretical conditions for the electrorefin-ing of tin are favorable. The position of tin in theelectrochemical series of the elements in aque-ous solution show that Au, Ag, Cu, Bi, As, andSb do not go into solution under electrorefiningconditions, but will appear in the anode slime.The elements Ni, Fe, Zn, and Al can be largelyremoved by a preliminary pyrometallurgical re-fining operation. Only lead lies close to tin in theelectrochemical series. The high electrochemi-cal equivalent weight of tin also favors the use ofan electrometallurgical refining process. How-ever, there are considerable problems in the prac-tical realization of the process.

Simple and cheap electrolytes such as solu-tions of sulfate or chloride lead to spongy orneedle-like deposits, and these effects are onlyslightly moderated by extremely large additionsof colloidal materials. The process can only beoperated at low current densities, leading to lowprocess rates and inefficient utilization of energy(e.g., low current yields). Also, the presence oflarge amounts of expensive metal tied up in theprocess is undesirable economically.

These negative aspects mean that electrore-fining is only worthwhile if the tin contains highconcentrations of noble metals.

Electrorefining can be carried out in acid oralkaline medium.

5.2.1. Electrorefining in Acid Medium

When sulfate electrolytes are used, additions ofchloride, fluoride, crude cresol, glue, nicotinesulfide,α- and β-naphthol, diphenylamine, phe-nol, and/or cresolsulfonic acid are made. Thesulfate ions cause the anodically dissolved leadto go into the anode slime in the form of lead sul-fate. Also, sulfides such as nicotine sulfide canlead to the formation of lead sulfide, which is de-posited in the anode slime. The organic sulfonicacids prevent the formation of basic tin salts onthe anodes.

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22 Tin, Tin Alloys, and Tin Compounds

In spite of these precautions, the formation ofcoatings on the anode is the main problem in theelectrolytic refining of tin in an acid medium.The main cause of coating formation is the leadcontent of the anodes, which must be removedmechanically when the bath voltage increases.The following operational data are quoted:

Anode mass: 100 – 200 kgAnode thickness: 30mmCell dimensions: 3.0 – 4.5m long, 1.0 – 1.2m

wide, 1.0 – 1.5m deepCell construction: wooden cells with lead cladding

or concreteCathode replacement: after 6 dAnode replacement: after 10 – 12 dAnode composition: 94 – 96% Sn

0.01 – 0.03% Fe0.3 – 1.3% Pb0.1 – 0.6% Cu0.1 – 3.5% Bi0.02 – 0.35% As0.1 – 0.25% Sb100 – 300 g/t Ag0.3 – 0.7 g/t Au

Cathodes: starter sheets of pure tin

The current yield is largely determined by therate of removal of anode passivation. The energyconsumption is 150 – 200 kW · h/t tin. Becauseiron accumulates in the electrolyte, regenerationof the electrolyte is necessary.

5.2.2. Electrorefining in an Alkaline Medium

In alkaline medium, i.e., in NaOH or Na2S elec-trolytes, less pure anodes (75% Sn) can be usedthan are used in an acid medium. A smooth de-posit can be obtained without addition of col-loids. However, current densities are very low,and the process must be carried out at 90 C.

Detailed information about the possibilitiesand limitations of electrorefining, deposition be-havior, depositionmechanism, and effects of ad-ditives to the electrolyte, pH, and impurities aregiven in [4].

5.2.3. Other Methods of Electrorefining

Many attempts have been made to use moltensalt electrorefining to overcome the disadvan-tages of electrorefining in aqueous solutions.The electrolyte was molten CaCl2 –KCl –NaCl.Various grades of crude tin were used. The

electrodes consisted of graphite crucibles andgraphite rods. The operating temperaturewas ca.650 C, and the current density 50 –200A/dm2.Arsenic was effectively removed (reduced from1.5wt% to 0.005wt%), and the antimony con-tent was reduced from 0.32wt% to 0.01wt%. Itproved impossible to scale up from pilot scale tofull scale operation, mainly because of problemsin the control of the high-temperature process[77].

6. Recovery of Tin from ScrapMaterials and Residues [1–3], [5], [11],[86], [87]

Scrapmaterials and residueswhich are producedduring the processing of metals to give semifin-ished and finished products are usually knownas “new scrap,” while the returned old materialfrom industry, trade, building construction, fac-tories, and consumption is known as “old scrap.”The use of old and new scrap supplements pri-mary production. It is collected by scrap mer-chants who work directly with the smelters.Also, in the neighborhood of metal smeltingworks there are often small scrap metal opera-tions which sort, separate, refine and blend withprimary metals to produce a primary metal thatcorresponds to the standard specification of anoriginal metal [87].

The recovery of tin from tinplate is becom-ing increasingly difficult, as the change to elec-trolytic methods of tinning is leading to verythin coatings which sometimes diffuse into thesteel sheet. The recycling of this material willcontinue to be a technically and economicallydifficult task. Two processes are used for recov-ering tin from tinplate: the alkaline electrolyticmethod and the alkaline chemical method.

In the alkaline electrolytic detinning of tin-plate, baskets of cleaned scrap are immersedin hot 5 – 10% sodium hydroxide solution. Thebaskets form the anodes, and the tinned steelsheet forms the cathodes. The tin is depositedin the form of a sponge. As contact with at-mospheric carbon dioxide cannot be prevented,sodium carbonate is formed, and the electrolytebath must be frequently regenerated. The lac-quer coating on the scrap tinplate is removedby adding solvents to the bath or by a special

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Tin, Tin Alloys, and Tin Compounds 23

pretreatment process. The bath is operated at atemperature of 65 – 75 C, a voltage of 1.5V, anda cathode current density of 300A/m2.

In the alkaline chemical detinning of tinplate,the scrapmaterial in perforated containers is im-mersed in sodium hydroxide solution. Hydratedsodiumstannate is formed according to the equa-tion

Sn + 2NaOH+4H2O→Na2SnO3 · 3H2O+2H2

The hydrogen liberated must be reacted with anoxidizing agent; sodium nitrate is suitable. Thedissolution process is accelerated by motion ofthe container in the liquor. The detinning time is2 – 4 h, depending on the concentration and thetemperature. If sodium nitrite is used instead ofsodium nitrate, the tin goes into solution, the for-eign metals, e.g., lead, iron, and antimony, canbe precipitated, e.g., by hydrogen sulfide, andthe tin can then be recovered by electrowinning.

Processing tin-containing alloys is easier thanrecovering tin from scrap tinplate. However, thetin content of many alloys has decreased overrecent years, and alloys have in some cases beenreplaced by cheaper materials. Both these devel-opments have tended to limit the potential for re-covering secondary tin, and also explainwhy theamount of recovered tin in Europe has decreasedfrom 15 800 t in 1980 to 13 300 t in 1990. In theUnited States, the amounts of recovered tin haveremained almost constant at 16 900 t in 1980 and17 100 t in 1990 [9].

The quantification of tin recovery from sec-ondary rawmaterials is difficult becausemost ofit is obtained from scrap alloy.

In addition to the tin concentrates treatedin the primary smelting process, there are alsoother materials that must be treated, includingthe slags (see Section 4.3.4), oxidic flue dusts,ash, and sweepings containing very variableamounts of tin. Tin-containing processing scrapis also produced during casting, metal formingand cutting, tinning, and alloying. Considerableamounts of recycled scrap also come from themanufacture of cans, tinplate, tubes, foil, pure tinarticles, and alloys such as solders, antifrictionand bearing metals, type metal, etc.

In recycling it is essential to sort tin-con-tainingmaterials into standard grades with exactanalytical specifications so that themetallurgicalprocess can be optimized.

Short drum furnaces are suitable for process-ing oxidic materials, although shaft kilns andreverberatory furnaces are also used. The re-duction is performed by coke/coal, with addedsodium carbonate or fluorspar as flux. The pro-cess is operated in one or two stages, dependingon the material. Scrap alloys containing highproportions of lead, antimony, or copper areremelted to form alloys. Scrap babbitt (bearingmetal) contains zinc, which is either removed inthe slag or selectively volatilized. Scrap solderalloys are refined like crude tin.

7. Analysis [88]

7.1. Analysis of Ores and Concentrates

Determination of Sn. In the determinationof tin in ore concentrates, the choice of methoddepends on the presence or absence of typicalimpurities.

Tungsten-Free Ores and Concentrates. Afterfusion with sodium peroxide and dissolution inwater, the solution obtained is acidified with hy-drochloric acid and partially evaporated to driveoff arsenic. The antimony is then precipitatedwith iron powder (cementation). The tin in thefiltrate can then be “cemented” by adding alu-minum powder, and determined by iodometrictitration.

Tungsten-Containing Ores and Concen-trates. The tungsten can be precipitated from theacidified solution by adding cinchonine. The ex-cess of this reagent in the filtrate is decomposedby fumingwith sulfuric acid. The solution is thentaken up in hydrochloric acid and treated as inthe determination of tin in tungsten-free ores.

Silicate-Containing Ores and Concentrates.Thematerial is boiled to drynesswith nitric acid,and the residue is then strongly heated, fumedwith HF/H2SO4, and fused with sodium perox-ide. The tin determination can then be carriedout as for tungsten-free materials.

Determination of Other Elements. Thedetermination of tungsten is carried out by pre-cipitating with cinchonine after the fusion stage.The precipitate is strongly heated (ca. 750 C)to form WO3, and tungsten is then determinedgravimetrically.

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24 Tin, Tin Alloys, and Tin Compounds

Arsenic is determined by distilling it out ofthe acidified solution of fused product and thencarrying out a bromatometric titration on the dis-tillate.

Antimony is determined by adding iron pow-der to the acidified fusion product to precipitateantimony sponge. This is dissolved in Br2/HCl,and any arsenic still present is driven off by evap-oratingwithHCl. The antimony is purified by re-precipitation, and determined by bromatometrictitration.

For the determination of other elements suchas cadmium, iron, nickel, copper, bismuth, lead,zinc, and the noble metals, it is best to removezinc, arsenic, and antimony from the dissolvedproduct of a sodium peroxide fusion (acidifiedwith HCl) by evaporating with Br2/HBr. Theelements can be determined in the solution byatomic absorption spectrometry.

To determine sulfur, the sample is heated in astream of oxygen, the SO2 formed is collected ina dilute solutionofH2O2, and theH2SO4 formedis determined by titration. The sulfur content ofthe tin is calculated from this.

7.2. Analysis of Metallic Tin

The preferred methods of analysis of pure andcommercial-grade tin are detailed in DIN 1704[88].

Arsenic is determined by dissolving themetalin a strongly acidic solution of FeCl3. TheAsCl3 formed is distilled off, and can be deter-mined photometrically by themolybdenum bluemethod.

Antimony is determined by dissolving themetal in concentrated sulfuric acid. After addinghydrochloric acid, the antimony is oxidizedwithcerium(IV) to antimony(V). This is then ex-tracted with isopropyl ether and can be de-termined photometrically as yellow potassiumtetraiodoantimonate.

Lead is determined by dissolving the metalin a mixture of Br2, HBr, and HClO4, fumingthis to volatilize the zinc, removing any Tl thatmight still be present by extraction with isopro-pyl ether, and determining the lead by polarog-raphy.

In the determination of copper, the initial dis-solution is similar to that used in the determina-tion of lead. The copper in the solution then is

determined photometrically as the diethyldithio-carbamate complex after extraction with CCl4.

The determination of zinc also requires a dis-solution process similar to that for lead. The so-lution is acidified with hydrochloric acid, andheavymetals still present are precipitated as sul-fides. The zinc in the filtrate obtained can be de-termined by polarography.

Iron is determined by first carrying out thedissolution and precipitation of heavy metals asfor zinc determination. The iron in the filtratecan be determined photometrically as the sul-fosalicylic acid complex.

The determination of aluminum also requiresdissolution and removal of heavy metals by pre-cipitation of their sulfides as for iron determina-tion. The aluminum in the filtrate can be deter-mined photometrically as the eriochrome cya-nine complex.

As well as the analytical methods recom-mended in DIN 1704, rapid and efficient meth-ods such as atomic absorption spectrometry(AAS) and atomic emission spectrometry withinductively coupled plasma excitation (ICP-AES) are also being used to an increasing extent[89]. These methods are specified in the Ger-man and international standards for the analysisof water, wastewater, and sewage sludge.

After dissolution of the tin in a mixture ofBr2, HBr, and HClO4, the accompanying anti-mony, arsenic, bismuth, copper, iron, lead, alu-minum, cadmium, and zinc can be determinedwith a high degree of accuracy in the presenceof each other, without first separating them, bymeans of AAS and ICP-AES. In the determina-tion of arsenic and antimony, other dissolutionprocesses must first be performed. The analysisof metallic tin and tin alloys is often carried outby spectral analysis with spark or arc excitation,and the analysis of tin slags and concentrates byX-ray fluorescence analysis.

8. Economic Aspects [2], [9], [11]

The amounts of tin mined, smelted, and con-sumed in the years 1980 – 1990 are given in Ta-ble 5.

Even in 1938, smelting production amountedto 171 200 t [3], and this had increased by only66 500 t or 38.8% in 1990. This is an averageincrease of 9.75% per annum.

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Table 5.World production and consumption of tin (in 103 t)

Year Mining output Smelting output Consumption

1980 235.5 243.6 221.41985 185.3 216.2 214.31990 210.7 225.6 232.7

Typically in the production and consump-tion of tin, the production processes of miningand smelting are geographically remote from theplaces where the metal is consumed.

The main producers, i.e., Brazil, Indonesia,Malaysia, Bolivia, and to some extent China,consume very little tin themselves (see Table 6).On the other hand, theUnited States, Japan, Ger-many, and the United Kingdom produce little orno primary tin themselves and are the main tinconsumers. In theCIS, production and consump-tion are approximately equal.

Table 6.Mining output and consumption of tin in various countries1990 [9]

Country Mining output Consumption

in 103 t as % in 103 t as %

Brazil 39.1 18.6 6.1 2.6Indonesia 37.1 15.1 1.4 0.6Malaysia 28.5 14.3 3.3 1.4Bolivia 17.3 8.2 0.4 0.2China 35.8 17.0 18.0 7.7Russia (CIS) 13.0 6.1 20.0 8.5United States 0.1 0.04 37.3 16.0Japan 33.8 14.5Germany 19.3 8.2United Kingdom 3.4 1.6 10.4 4.3

The continuous fall in the price of tin(> 30 000DM/t in 1985, ca. 10 000DM/t in1990) has led tomany casualtiesworldwide. Theworst effects have been in Malaysia, where over80 suppliers have been closed down. In 1980,Malaysia was the main producer of tin, witha mine output of 61 000 t. This fell to 28 500 tin 1990, and Malaysia now produces less tinthan Brazil, China, or Indonesia. Even the well-known tin smelter Capper Pass in the UnitedKingdom has ceased production, as have the tinmines in the Altenberg region of Saxony.

9. Tin Alloys and Coatings [1], [2], [11]

Tin is one of the most important constituentsof low-melting nonferrous alloys. The followingimportant properties of the metal are exploited

1) Low melting point2) Low hardness3) Good wetting properties4) Effective incorporation of foreign particles5) Good compatibility with foodstuffs

Tin utensils have been produced for over5000 years, and well-preserved examples existfrom the various epochs.

Tin utensils are always made of tin alloys, asthe puremetal is too soft. Themost important al-loying elements are antimony, copper, and lead.The tin is usually melted first, and it then readilyforms alloys with pure metals, which have goodsolubility properties. To minimize burning, es-pecially of antimony, master alloys are usuallyused.

As tin has good flow and casting properties,casting is the most important method of pro-ducing tin articles. All the processes used in amodern foundry are suitable. Those most oftenused are sand, shell, centrifugal, and pressure diecasting. The casting methods are sometimes au-tomated, whereby the low casting temperaturesmake small demands on thematerial used for theshells and molds.

As very thin-walled cast tin cannot alwaysbe produced, pressure forming methods are nowalso widely used. For example, a circular sheetcan be spun over a former. This method is espe-cially suitable for simple rotationally symmetri-cal items without asymmetrical surface effects.

Owing to the good forming properties of tin,other methods can also be used. For example,stamping and extrusion present no special prob-lems, and individual items can be produced byforging and hammering processes.

The composition of the metal used to pro-duce tin articles is specified in DIN 17 810 andthe materials regulation RAL-RG 683. The DIN17 810 specification is as follows:

Grade Sn 90-10Material no. 2.3710Tin min. 90%Antimony ≤ 7%Copper ≤ 3%Silver ≤ 4%Lead max. 0.5%Sum of others 0.3%

The lead limit of 0.5% is imposed merelybecause this is technically feasible. It has been

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26 Tin, Tin Alloys, and Tin Compounds

shown that a lead content of 2% leads to in-significant releases of lead even after utensilshave been kept at unusually high temperaturesand for unusually long periods of time.

Solders. Most solders are based on thetin – lead binary system (→Lead Alloys,Chap. 5.), which has a eutectic at ca. 63% tinand 183 C. The solid solubility of 1 – 2% leadin tin and 13% tin in lead is not relevant to pro-duction conditions.

In solder applications, it is of great impor-tance to knowwhat percentage of impurities cancause problems, and, conversely, whether alloy-ing elements canhave adetrimental effect on sol-dered joints under certain conditions. This ques-tion is extremely important in the electronics in-dustry because of the small amounts of solderused in a soldered joint, and the small distancesbetween the soldered joints. The state of knowl-edge is as follows [11, Chap. 5.8]:

Zinc: Visible impairment of thesurface of the solder by oxideformation at 0.005%.Recommended limit: 0.001%.

Aluminum: Impairment of adhesive bond,hot brittleness, and dullappearance at 0.005%.Recommended limit: 0.001%.

Phosphorus: Increased oxidation at 0.001%.Lower concentrations reduceoxidation in unstirred baths.

It is known that other elements, such as ar-senic and sulfur, can have detrimental effects,but precise quantitative experimental results arenot available.

Low friction materials and bronzes havethe following useful properties:

1) High mechanical strength with good electri-cal conductivity

2) Good soldering properties3) Extremely good properties as a bearing (an-

tifriction) metal4) Good machinability at room temperature5) Good general corrosion resistance towards

the atmosphere and seawater, and, in the caseof zinc-free alloys, towards stress corrosion

Apart from cast articles, the most importantforms are wire, rolled profile, sheet, and strip.The material is also used in bearings and in thechemical industry. Its use in domestic items,

e.g., fittings and mountings, is also consider-able. The alloys are classified as casting alloysand wrought alloys, the latter having lower tincontents. Some important copper – tin alloys, asspecified in DIN 1705/1716, are listed in the fol-lowing:

GunmetalG-Cu Sn 12 88% Cu, 12% SnG-Cu Sn 12Ni 86% Cu, 12% Sn, 2% NiG-Cu Sn 12 Pb 86% Cu, 12% Sn, 2% PbRed bronzeG-Cu Sn 10Zn 88% Cu, 10% Sn, 2% ZnG-Cu Sn 7Zn Pb 83% Cu, 7% Sn, 4% Zn, 6% PbLeaded bronzeG-Cu Pb 5 Sn 85% Cu, 5% Pb, 10% SnG-Cu Pb 15 Sn 77% Cu, 15% Pb, 8% SnG-Cu Pb 22 Sn 76% Cu, 22% Pb, 2% SnWrought alloysCuSn 2 98% Cu, 2% SnCu Sn 6 94% Cu, 6% SnCu Sn 6Zn 88% Cu, 6% Sn, 6% Zn

Although the copper – tin alloys are some ofthe oldest materials used by humans, their de-velopment is not yet exhausted even today.

The most important development aims areimprovement of mechanical properties and cor-rosion resistance, and reduction of the tin con-tent.

Sintering Metallurgy of Bronzes. An in-teresting new use for tin is as an addition inpowder form when sintering bronze. Especiallywhen this is for use as a bearingmetal, economicadvantages are obtained by the addition of 4%tin to the copper powder with or without leadaddition.

Low-melting alloys are of great importancein several technical applications. Their meltingpoints usually lie significantly below 150 C.Bismuth is always an essential alloy constituent.

Their most important application areas are inmold making, safety systems for the preventionof fire and overheating, and stepwise soldering.

Melting points, compositions, and areas ofuse of typical low-melting alloys are listed inTable 7.

Amalgams. Tin has been used for dental fill-ings since theMiddle Ages, and amalgams sincethe 1800s. Subsequent developments have led tothe silver – tin amalgams used today (→DentalMaterials, Chap. 2.3.2.). A typical amalgam has

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Table 7.Melting points, compositions, and applications of typical low-melting alloys

Melting point, C Typical composition, wt% Applications

Bi Sn Pb In Cd

47 44.7 8.3 22.6 19.1 5.3 test castings, fixing lenses58 49.0 12.0 18.0 21.0 fixing components70 49.4 12.9 27.7 10.0 supporting components70 – 73 50.0 13.3 26.7 10.0 tube bending, radiation protection, fixing components96 52.0 16.0 32.0 temperature fuses138 – 170 40.0 60.0 accurate molds, cores, gravity die casting

the following composition: 52% Hg, 33% Ag,12.5% Sn, 2% Cu, 0.5% Zn.

When the mixture of metals is ground to-gether, the following hardening reaction takesplace:

AgSn3 + 4Hg −→ Ag2Hg3 +Sn7−8Hg+Ag3Sn

The metallographic structure of a dental amal-gam consists of islands of solid undissolved par-ticles of alloy (Ag3Sn) in a soft matrix of silverand tin amalgams.

Alloy Coatings. Tin alloys are importantin the production of coatings by electro-plating and hot tinning. The most important ofthese are tin – zinc, tin – nickel, tin – cobalt, andtin – copper.

Tin – lead coatings are mainly used for cor-rosion protection and as a preparation for sol-dering. Electrolytically applied coatingsmust betreated with hot palm oil or by infrared heating.This melts the coating and can prevent the for-mation of whiskers.

Tin – zinc coatings are increasingly replacingthe toxic cadmium coatings.

Tin – nickel and tin – cobalt coatings aremainly used in electrical installations, e.g., toproduce electrical connectors.

Additions to Alloys. Tin is increasinglyused as an alloy addition in the steel indus-try. The addition of 0.1 – 0.5% tin causes castiron to solidify with a pure pearlitic structure,making it uniformly hard and wear-resistant.

Sinter Metallurgy. In the sinter metallurgyof iron, addition of 2.5 – 5% of a tin – copper al-loy (2 : 3) gives reduced sintering temperaturesand times, and in particular improves the dimen-sional accuracy of the sintered components.

10. Inorganic Tin Compounds

Consumption of inorganic tin compounds islower than that of organic tin compounds, butthey are often the starting materials used to pro-duce the organic tin compounds.

In minerals, tin is nearly always tetravalentand bonded to oxygen or sulfur. The only excep-tion is herzenbergite, SnS, inwhich it is divalent.

In industry, tin(II) and tin(IV) compounds areproduced from metallic tin. Many tin(II) com-poundswhich are sufficiently stable for practicalpurposes have a strong tendency to change intotin(IV) compounds and are therefore stronglyreducing. For example, SnCl2 precipitates goldand silver in metallic form from their salt solu-tions.

The salts in both valency states hydrolyze inaqueous solution to form insoluble salts. In al-kaline media, stannites (divalent) and stannates(tetravalent) are formed.

10.1. Tin(II) Compounds

Tin(II) chloride, SnCl2, is the most impor-tant inorganic tin(II) compound. It is producedon an industrial scale by reducing tin(IV) chlo-ride with molten tin, or by direct chlorination oftin.

Solutions of tin(II) chloride are obtained bydissolving metallic tin in hydrochloric acid, orby reducing a solution of SnCl4 with metallictin.

The anhydrous substance is white, has agreasy luster, and dissolves readily in water,alcohol, ethyl acetate, acetone, and ether. Theclear, nondeliquescent, monoclinic dihydrate,SnCl2 · 2H2O, crystallizes from aqueous solu-tion and is the commercial product.

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28 Tin, Tin Alloys, and Tin Compounds

On dilution, the aqueous solution becomescloudy as hydrolysis causes precipitation of thebasic salt:

SnCl2 +H2O Sn(OH)Cl +HCl

The cloudiness can be prevented by small addi-tions of hydrochloric acid, tartaric acid, or am-moniumchloride.Because of its strong tendencyto hydrolyze, the dihydrate can only be dehy-drated over concentrated sulfuric acid or by heat-ing in a stream of hydrogen chloride.

Tin(II) chloride is an important industrial re-ducing agent, being used to reduce aromatic ni-tro compounds to amines, aliphatic nitro com-pounds to oximes and hydroxylamines, and ni-triles to aldehydes.

Tin electroplating canbe carriedout in a fusedeutectic salt mixture of 20% SnCl2 and 80%KCl at 200 – 400 C [90], [91].

Tin(II) Oxide Hydrate and Tin(II) Oxide.If aqueous solutions of SnCl2 or other tin(II)salts are reacted with alkali metal carbonate orammonia, an amorphous white precipitate oftin(II) oxide hydrate, 5 SnO · 2H2O, is obtained[92]. Sn(OH)2 does not exist. Tin(II) oxide hy-drate is amphoteric. Dehydration in a stream ofcarbon dioxide gives tin(II) oxide. Tin(II) oxidehydrate and tin(II) oxide are starting materialsfor the production of other tin(II) compounds.

Other Tin(II) Compounds.Tin(II) fluoride, SnF2, is formed from tin(II)

oxide hydrate and hydrofluoric acid, and isadded to toothpastes as an anticaries agent.

Tin(II) fluoroborate hydrate, Sn(BF4)2 ·nH2O, is formed by dissolving the oxide hy-drate or the oxide in aqueous fluoroboric acid.Sulfuric acid reacts with the oxide hydrate orthe oxide to form tin(II) sulfate. Both tin(II)sulfate and tin(II) fluoroborate are importantin the production of metallic tin coatings byelectroplating.

Tin(II) bromide, SnBr2, and tin(II) iodide,SnI2, are produced by reacting metallic tin withthe appropriate hydrogen halide.

Tin(II) cyanide, which is produced from bis-(cyclopentadienyl)tin(II) and hydrogen cyanide,is the only known compound of tin with inor-ganic carbon:

(C2H5)2Sn + 2HCN −→ Sn(CN)2 + 2C2H6

The tin(II) salt of ethylhexanoic acid is an ef-fective catalyst in polyurethane production.

10.2. Tin(IV) Compounds

Tin(IV) Hydride. The toxic, colorless, flam-mable gas, tin(IV) hydride, is formed by the re-duction of tin(IV) chloride by LiAlH4 in diethylether at −30 C. It is stable for several days atroom temperature, and decomposes into its ele-ments at 150 C in the absence of air, forming atin mirror.

Tin(IV) Halides and Halostannates(IV).Anhydrous tin(IV) chloride is a colorless liquidwhich fumes strongly in air. It is a good solventfor sulfur, phosphorus, and iodine, and is misci-ble in all proportions with carbon disulfide, al-cohol, benzene, and other organic solvents. Ithydrolyzes in water, evolving much heat andforming colloidal tin(IV) oxide and hydrochlo-ric acid:

SnCl4 + 2H2O −→ SnO2 + 4HCl

In moist air, the pentahydrate, SnCl4 · 5H2O, isformed, the so-called butter of tin, a white deli-quescent crystallinemasswith amelting point of60 C. In industry, SnCl4 is produced by the re-action of chlorine with tin. The anhydrous prod-uct is obtained if themetal is coveredwithSnCl4.Anhydrous tin(IV) chloride is an important start-ing material for the production of organic tincompounds (see Sections 11.2 and 11.3).

Tin(IV) bromide, SnBr4, and tin(IV) iodide,SnI4, are also obtained by the reaction of metal-lic tin with the halogens. Tin(IV) fluoride is pro-duced by the reaction of tin(IV) chloride withanhydrous hydrogen fluoride:

SnCl4 + 4HF −→ SnF4 + 4HCl

Tin(IV) halides react readily with metal halidesto form the halostannates(IV), the coordinationnumber of the tin increasing from four to six.The reaction proceeds as follows (X= halogen):

SnX4 + 2MX M2SnX6

One of the best known compounds of this type isammonium hexachlorostannate, (NH4)2SnCl6,the so-called pink salt.

Hexachlorostannic acid, H2SnCl6 · 6H2O, isformed by passing HCl into a concentrated so-lution of SnCl4.

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Tin, Tin Alloys, and Tin Compounds 29

Tin(IV) Oxide, Tin(IV) Oxide Hydrate,and Stannates(IV). Tin(IV) oxide decomposesabove 1500 C without melting to form tin(II)oxide:

2 SnO2 −→ 2 SnO+O2

Pure tin(IV) oxide can be obtained by the com-bustion of powdered tin or sprayed molten tin ina hot stream of air. It is insoluble in acids and al-kalis. Specially prepared tin(IV) oxide hasmanyuses, total world consumption of this materialbeing > 4000 t/a. It is used in combination withother pigments to produce ceramic colorants. Ithas a rutile structure, and hence can absorb thecolored ions of transition metals. The productsobtained form the basis of ceramic colors, andinclude tin vanadium yellow, tin antimony gray,and chrome tin pink. The thermal stability of thetin colors enables them to be used both in andunder the glaze.

Electrodes made of SnO2 are used in the pro-duction of lead glass [93]. They are resistant tocorrosion by molten glass, and have good elec-trical conductivity when hot.

Coatings of tin(IV) oxide treatedwith indiumoxide (<100µm thick) give good transparencyproperties to aircraft window systems, increasetheir strength, and give protection from icing.

If tin(IV) oxide is reacted with a solution ofalkali, or a solution of stannate is reacted withacid, awhite, gel-like precipitate of tin(IV) oxidehydrates is formed which are very soluble in al-kalis and acids. This precipitate was formerlyknown as “α-stannic acid”, and the aged prod-uct, which is sparingly soluble, as “β-stannicacid” (metastannic acid). Today, this product isregarded as tin(IV) oxide hydrate with the for-mula SnO2 · nH2O, where n decreases with ag-ing.

The reaction of powdered or granulated tinwith concentrated nitric acid leads to the forma-tion of the reactive β-tin(IV) oxide hydrate. Thiscan be used as a catalyst for aromatics.

The β-tin oxide hydrate gels precipitatedfrom SnCl4 by ammonia and then dried are sta-ble towards nuclear radiation, and can be usedin chromatographic columns for separating ra-dioactive isotopes [94].

Fusion of tin(IV) oxide with alkali metal hy-droxides leads to formation of alkali metal hexa-hydroxystannates according to the following re-action scheme (M= alkali metal):

SnO2 + 2MOH+2H2 −→ M2 [Sn(OH)6]

The sodium and potassium complexes are usedas alkaline electrolytes in electrolytic tin plating.

Tin(IV) sulfide, SnS2, is formed as goldenyellow flakes with a hexagonal crystal structureby passing hydrogen sulfide through a weaklyacidic solution of a tin(IV) salt. On heating, thecrystals become dark red to almost black, revert-ing to yellow on cooling.

The tin disulfide known as “mosaic gold” isproduced industrially by heating tin amalgamwith sulfur and ammonium chloride. It is usedfor gilding picture frames, and in painting to pro-duce deep yellow to bronze color shades.

11. Organic Compounds of Tin [3],[95], [96]

The organic chemistry of tin has attracted ma-jor interest since 1945. The first organotin com-pounds were prepared in 1849 by Franklandand in 1852 by Lowig [97]. In the first technicalapplication in 1936, the discovery of the stabi-lizing effect of these compounds on poly(vinylchloride) was utilized. The biocidal propertiesof other organotin compounds have been knownsince ca. 1950.

Only the organic compounds of tetravalenttin are used in industry.

Most organotin molecules contain a singletin atom with 4 substituents. They are classi-fied according to the number of direct tin-carbonbonds: R4Sn, R3SnX, R2SnX2, RSnX3.

Here, R denotes any hydrocarbon group andX denotes a group such as a halide, OH, OR, SR,acid group, etc. Most of these compounds arecolorless liquids at room temperature or slightlyabove. They are very soluble in organic solventsbut sparingly soluble in water.

Organotin compounds have a verywide rangeof applications, depending on their type, includ-ing the stabilization of PVC, catalysis, crop pro-tection, and wood preservation. Also, for sometime, various organotin compounds have beenincreasingly used as laboratory chemicals, espe-cially organotin alkoxides and hydrides. Theseare used as synthesis auxiliaries and as mild andselective reducing agents.

Organotin compounds do not occur in nature.

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30 Tin, Tin Alloys, and Tin Compounds

11.1. Properties of OrganotinCompounds

Under the influence of light, atmospheric oxy-gen, or certain microorganisms, organotin com-pounds are degraded in a relatively short time,the hydrocarbon groups being split off to leavebehind nontoxic inorganic products. Althoughboth the tin – carbon bond (average dissociationenergy 209 kJ/mol) and the tin –oxygen bond(average dissociation energy 318 kJ/mol) are re-active, they are sufficiently stable for generalhandling purposes.

The symmetrical tetraalkyltin compoundshave a very slight odor. They are colorless, formmonomolecular solutions, are fairly stable to-wards water and air, and can be distilled with-out decomposition at < 200 C. Their solubili-ties and boiling points are similar to those of thebranched chain paraffins with similar molecu-lar mass; the higher homologues are waxy sub-stances.

The symmetrical tetraaryltin compounds arestable towards air and water and are also color-less. They melt at temperatures above 150 C.

The organotin hydrides, with the exception ofsome aryl tin hydrides which are solid at roomtemperature, are colorless, nonassociated liquidswhich are rapidly attacked by oxygen and there-fore can only be prepared and stored under inertgas. They are important reducing agents.

The organotin fluorides, the diorganotin di-halides, and the aromatic organotin mono-halides are solids at room temperature, while thealiphatic organotin monohalides and trihalidesare liquids. The higher triorganotin derivativeshave a broad biocidal effect on microorganismssuch as fungi, bacteria, and harmful waterborneorganisms such as algae, tube worms, shellfish,etc. Themost active compounds are the tributyl-,trichlorohexyl-, and triphenyltin compounds.

The di- and monoorganotin derivatives inwhich methyl, butyl, and octyl groups arebonded to the tin stabilize polymers sensitiveto light and temperature such as PVC if the tinis bonded via oxygen or sulfur to certain othergroups.

11.2. Production of OrganotinCompounds

Tetraorganotin Compounds from Tin Tetra-chloride and Organometallic Compounds. Tintetrachloride is the key substance for the pro-duction of organotin(IV) compounds. In indus-try, the tin tetrachloride is first alkylated with or-ganic compounds of magnesium, aluminum, orsodium to form tetraorganotin compounds. Theprocess is usually continuous.

Organotin Chlorides from Tetraorgan-otin Compounds and Tin Tetrachloride. Iftetraorganotin compounds are reacted with stoi-chiometric amounts of tin tetrachloride, the cor-responding organotin chlorides are obtained:

SnR4 + 1/3 SnCl4 4/3R3SnCl

SnR4 +SnCl4 2R2SnCl2

SnR4 + 3 SnCl4 4RSnCl3

The production of triorganotin chlorides anddiorganotin dichlorides proceeds smoothly ac-cording to this reaction. Monoorganotin trichlo-rides can only be obtained in a few cases whereR = acryl or vinyl and in special solvents or withcatalysts.

Direct Synthesis of Organotin Chlorides fromTin. Organotin chlorides can be obtained by di-rect reaction of tin with unsaturated organiccompounds and hydrogen halides (ester tin pro-cess) or from organic halides (catalytic directprocess).

Ester Tin Process for the Production of TinCarboxylic Acid Derivatives from Tin. Unsat-urated organic compounds, such as esters ofsubstituted or unsubstituted acrylic acid, acry-lonitrile, or vinylphosphoric diesters, react withmetallic tin and hydrogen chloride in a po-lar medium (ethanol, concentrated hydrochloricacid, or diethyl ether) to form the so-called es-ter tin compounds, e.g., bis(2-methoxycarbon-ylethyl)tin dichloride [98].

Catalytic Production of Organotin Halidesfrom Tin. Organotin halides can be produceddiscontinuously or continuously from metallictin and organic halides with the aid of cata-lysts at elevated temperatures. The most effec-tive of these catalysts are the tetraalkylammoni-um halides, tetraalkylphosphonium halides, andother derivatives of N, P, As, or Sb.

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Production of Organotin Trichlorides by Ad-dition of TinDichloride andHydrogenChloride.The unsaturated starting compounds suitable forthe ester tin process, when reacted with tin(II)chloride and hydrogen chloride, form the corre-sponding organotin trichlorides, e.g., (2-meth-oxycarbonylethyl)tin trichloride [99].

Production of Unsymmetrical OrganotinCompounds by Addition of Organotin Hydrides(Hydrostannation). Unsymmetrical tetraorgan-otin compounds that may contain functional orunsaturated groups in the alkyl groups can besynthesized by addition of organotin hydridesto alkenes and alkynes. The reaction is favoredby radical formers and UV light.

Conversion of Organotin Chlorides to otherHalides or Derivatives with Heteroatoms viaOrganotin Oxides. The organotin chlorides arequantitatively converted into organotin oxides inalkaline medium:

2R3SnCl −→ (R3Sn)2O

R2SnCl2 −→ 1/n (R2SnO)n

RSnCl3 −→ 1/n (RSnO1.5)n

In acidmedia, many organotin compounds, e.g.,fluorides, bromides, iodides, pseudohalides, car-boxylates, thiolates, etc., are obtained from theorganotin compounds:

(R3Sn)2 −→ 2R3SnX

1/n (R2SnO)n −→ R2SnX2

1/n (RSnO1.5)n −→ RSnX3

X=halogen or other functional group

These transformation reactions are of industrialimportance, as they enable the sensitive organ-otin compounds to be converted into organotinoxides that are insoluble in water, stable to air,easily transported, and storable for long peri-ods. These can then be used to produce eitherthe original organotin oxides or other organotincompounds when required.

11.3. Industrially ImportantCompounds

A few typical examples of themany applicationsof organotin compounds are described here.

Methyl Compounds. A legally permittedstabilizer for PVC used with foodstuffs isproduced from dimethyltin dichloride andmethyltin trichloride. It consists of:

(CH3)2Sn[SCH2COOC5H10CH(CH3)2]2 75%CH3Sn[SCH2COOC5H10CH(CH3)2]3 25%

Butyl Compounds. Tetrabutyltin,(C4H9)4Sn, can be converted to tributyltin chlo-ride, (C4H9)3SnCl, and this can be converted tobis(tributyltin) oxide, (C4H9)3SnOSn(C4H9)3,by treatment with NaOH. This product, which isinsoluble in water and miscible with industrialsolvents, is moderately toxic. It is an active bio-cide with many uses. For example, it is used asan antifouling paint for ships, for the preventionof slimes in industrial recirculating water sys-tems, for combating freshwater snails that causebilharzia, as a wood and textile preservative, andas a disinfectant.

The readily interconvertible compoundsdibutyltin chloride (C4H9)2SnCl2 and dibu-tyltin oxide [(C4H9)2SnO]n (a polymericpowder) are the starting substances for theproduction of the most common PVC sta-bilizers, i.e., the liquid dibutyltin dilaurate(C4H9)2Sn(OCOC11H23)2, the polymericsolid dibutyltin maleate [(C4H9)2SnOCOCH=CHCOO]n, the liquid dibutyltinbis(isooctylmaleate) (C4H9)2Sn[OCOCH=CHCOOC5H10CH(CH)3]2, and the liquid di-butyltin bis(thioacetic acid isooctyl ester)(C4H9)2Sn[SCH2COOC5H10CH(CH3)2]2.

Octyl Compounds. The stabilizers used inthe production of PVC film for the foodstuffsindustry, i.e., the liquid octyltin(thio)aceticacid isooctyl esters with the formulas (n-C8H17)2Sn[SCH2COOC5H10CH(CH3)2]2 andn-C8H17Sn[SCH2COOC5H10CH(CH3)2]3 areproduced from isooctyl mercaptoacetate anddioctyltin dichloride (C8H17)2SnCl2 or octyltintrichloride C8H17SnCl3, respectively.

Cyclohexyl Compounds. Tricyclohexyltinhydroxide (cyclo-C6H11)3SnOH is obtained byalkaline hydrolysis of tricyclohexyltin chloride(cyclo-C6H11)3SnCl. It is a colorless, crystallinesubstance, insoluble in water, and has a verymarked acaricidal action, attacking many mitesand acarides, thereby protecting plants and use-ful insects. It is the main component of the

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32 Tin, Tin Alloys, and Tin Compounds

commercial product Plictran (Dow Chemical),which is used in fruit growing, viticulture, andgreenhouses.

Phenyl Compounds. Thephenyl derivativesof tin are used as fungicides, e.g., for the treat-ment of potato rot and leaf spot in root tubers.Typical compounds are triphenyltin hydrox-ide, (C6H5)3SnOH ( Du Ter, Philips-Duphar),and triphenyltin acetate, (C6H5)3SnOCOCH3 (Brestan, Hoechst).

11.4. Analysis of Organotin Compounds

The analysis of organotin compounds is a com-plex field. Methods used are described in detailin [100].

In the determination of individual organotincompounds by gas chromatography, it is firstnecessary to convert the organotin halides oroxides into unsymmetrical tetraorganotin com-pounds by methylation or butylation with Grig-nard reagents [100].

11.5. Storage and Shipping of OrganotinCompounds

Organotin compounds are stored and transportedin drums and vessels of steel sheet coated on theinside with a special paint. In the case of solids,the emission of hazardous dust can be preventedby incorporating these products in paste formu-lations.

11.6. Pattern of Production andConsumption

World production of organotin compounds wasca. 50 t/a in 1950, 35 000 t/a in 1981, and40 000 t/a in the mid-1990s. The tin content ofthese materials is ca. 25%.

In the main producing and consuming areas–United States, Western Europe, and Japan –76%of the organotin compounds are used as sta-bilizers for PVC, 10% as antifouling biocides,8% as agricultural biocides, and 5% as cata-lysts for the production of polyurethanes andsilicones.

The main producers of organotin compoundsare:

United States: M & T Chemicals,Thiokol-Charstal, and Interstab(Akzo)

Japan Hokko Chemical Industries,Yoshitomi PharmaceuticalIndustries, Nitto Kasei, andSomkyo Organic Chemicals

Europe Schering, Akzo Chemie, andCiba-Geigy

12. Toxicology

Metallic tin is generally considered to be non-toxic. As early as the Middle Ages, it was usedin the form of plates, jugs, and drinking ves-sels. Even large amounts of tin salts in the di-gestive system cause negligible harm. Appar-ently, tin can migrate only very slowly throughthe intestinal walls into the blood. Orally in-gested tin is poorly resorbed by animals andhumans. The half-life in the kidneys and liveris 10 – 20 d, and in the bones 40 – 100 d. Casesof poisoning are almost unknown. Massive in-halation of tin or tin oxide dust by exposedindustrial workers can lead to irritation of therespiratory tract. In extreme cases, metal fumefever with similar symptoms to those of “zincfever” or “brass fever” also occurs. As the per-oral ingestion of tin and its inorganic compoundsis comparatively harmless because of the rela-tively low resorption, the limit value for the pre-vention of sickness and diarrhea for fruit con-serves is ca. 250 – 500mg/kg, and for fruit juices500 – 1000mg/kg. In current industrial practice,the tin cans are additionally lacquered, and thetin concentrations in preserved fruit and veg-etables are < 250 ppm. The MAK value, calcu-lated as tin and measured in total dust, has been2mg/m3 for many years [101], [102].

Hydrochloric acid formed by the hydrolysisof tin chloride can cause acid burns. Tin hydrideis very toxic, having a similar effect on the hu-man organism to arsine [103].

Organotin compounds are very toxic; theMAK value is 0.1mg/m3 [102]. These com-pounds can differ widely in their effects, and canalso be slowly converted to other compoundsin the organism, so that toxic symptoms canchange during the induction time. The toxicity ofalkyl and aryl tin compounds decreases in the se-

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Tin, Tin Alloys, and Tin Compounds 33

ries: trialkyl> dialkyl> tetraalkyl>monoalkylcompounds. Whereas tributyl- and triphenyltincompounds are almost as toxic as HCN, themonoalkyl compounds have a toxicity similarto that of the inorganic tin compounds.

The resorption of tin alkyl and tin arylcompounds can have harmful effects on theCNS, such as edema of the brain and spinalcord and damage to the respiratory center. Thevolatile organotin compounds cause persistentheadaches, epileptiform convulsions, narcosis,and respiratory paralysis.

Dibutyltin dichloride, tributyltin chloride,and analogous alkyl tin halides, after a latentperiod, cause irritation and burning of the skinand especially the mucous membranes, and areintensely sternutatory and lachrymatory [104],[105].

In cases of poisoning by organotin com-pounds, long-term observation of the function-ing of the liver and CNS is necessary.

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