Laval University

From the SelectedWorks of Fathi Habashi

June, 1969

Processes For Sulfur Recovery From OresFathi Habashi

Available at: https://works.bepress.com/fathi_habashi/437/

Processes for~ Sulfur Recovery

from Ores

By FATHI HABASHl, Senior Research EngineerExtracl.ive Metallurgiccll Research DivisionAnaconda Co.

Reprinted f ramMINING CONGRESS JOURNAL

June 1969

American Mining CongressRING BUILDING . WASHINGTON, D.C. 20036 . TELEPHONE 202 -338-2900

PROCESSES FOR SULFUR RECOVERY FROM ORES

Paper to be presented at the 1968Mining Show, Las Vegas, Nevada,October 7-10 .

Fathi Habashi, Senior Research EngineerExtractive Metallurgy DivisionThe Anaconda Company, Tucson, Arizona

While the sulfuric acid production is continuously rising, sulfur

stocks, on the other hand, are gradually decreasing. It would be, therefore,

timely to review the possibilities of sulfur recovery from sources other than

brimstone, the primary source for sulfuric acid production. Calcium sulfate

deposits (in the form of gypsum or anhydrite) , and sulfide ore deposits constitute

the most abundant reserve which is not yet extensively exploited as a source of

sulfur. The present paper is an attempt to outline the processes known for sulfur

recovery from these two sources .

For sulfate ores, thermal, wet, and microbiological methods are

discussed. For sulfide ores, pyrometallurgical, hydrometallurgical, and

electrolytic methods are outlined. Pyrometallurgical methods involve thermal

dissociation, reaction of sulfide ores with S02 gas, production of sulfur via

S02, reduction smelting, chlorination, and production of sulfur via H2S.

Hydrometallurgical methods involve the production of sulfur via H2S, and the

aqueous oxidation of sulfide ores. Electrolytic methods involve electrolysis

of molten sulfides, and the anodic dissolution of sulfide anodes in aqueous

phase .

Fig.1. Sulfur reserves

Processes forSulfur Recovery

from Ores

By FATHI HABA§HI, Senior Resear_ch EngineerExtractive Metallurgiccll Research Div.Is.IonAnac:onda Co.

W HILE SULFURIC ACID PRODUCTION is con-tinuously rising, sulfur stocks, on the other hand,are gradually decreasing (table 1) . In 1967, U.S. sulfurprices were raised $4.00 per ton and in Mexico $10.00per ton, making it $32.50 and $50.00 per ton, respec-tively. World prices run up to $80.00 per ton. Conse-quently, there has been great interest in studying thepossibilities of sulfur recovery from sources other thanbrimstone. Fig. 1 shows a schematic diagram of sulfurreserves as Cas04 in anhydrite and gypsum deposits;as Fes, Fes2, Pbs, Zns, etc. in sulfide ore deposits; asH2S (mainly) in natural gas and refinery gas, and aselemental sulfur in brimstone.

This paper is mainly concerned with the first twosources since they are the largest reserves known and,at the same time, the least exploited for su_1£ur re-covery.

SuliFUR RECOVERY FROM ANIIYI)RITE AND GYPSUM

There are at least three methods known for recover-ing sulfur from anhydrite and gypsum:

Table 1. Sulfur-sulfuric acid balance in U.S.U.S. SulfurProducer's

stck, Iiec. 3 1(rmllion

long tons )

U.S. SulfurENuction

(Mlllionlongtons)

*Preliminary

U.S. SulfuricAcid

Hoduction(Million

short tons )20;922.924.828.528.828.4*

1. Thermal method. Decomposition of anhydrite orgypsum by heating to yield sulfur dioxide according tothe reaction:

caso4 - coo + SOQ + yz oQrequires a high temperature, about 1300°C. The tem-perature of decomposition, however, can be consider-ably decreased if the gypsum is mixed with an additive.For example, when coal is added to gypsum, sulfurdioxide can be readily liberated at about 900°C sincean intermediate product, Cas, is formed which en-hances the decomposition:

Ca,S04 + 4 C - Cos + 4 COCos + 3 das04 - 4 Coo + 4 Soe

Overall reaction:caso4 + a - coo + co + SOQ

Table 2. Plants producing sulfuric acid from calciumsulfate (Hull et al,1957)

Location Co¥e££on T%£7C+9ar RemarksLeverkusen, 1916 12 ,00 0 Experimenta I

Germany

Wolfen,Germany

Miramas,France

Billingham,England

Widnes,England

Whitehaven,England

Breslew,Poland

Busko, PolandZdrio, PolandLinz, Austria

later dis-mantled

938 85,000955 115,000

93 7 25,000 Shut down

92.9 100,000955 75'000

955 150'000

955 100'000

950 rranananla rla

953. 50,000

A process in operation in Europe (table 2), known as\the Miiller-Kiihne process, is based on this principle.Gypsum, coal, and clay are mixed together and heatedin a rotary kiln. Sulfur dioxide formed is used for mak-ing acid, while Cao reacts with the clay to form cement-a by-product.

2. Wet method. This method is in commercial opera-tion in many countries (table 3) and it utilizes gypsum,phosphogypsum (i.e., gypsum produced as a wasteproduct of phosphoric acid manufacture), and an-hydrite, instead of sulfuric acid, for making ammoniumsulfate fertilizer. Finely ground gypsum reacts at roomtemperature with an aqueous solution of ammoniumcarbonate (made by absorbing C02 in aqueous am-monia), as follows:

(NH4)ec03 + Cas04 -CCLC03 + (NH4)QS04After filtering off calcium carbonate, the solution isevaporated to crystallize ammonium sulfate which issold as fertilizer.

3. Microbiological method. This process involves theformation of hydrogen sulfide gas by the action of thereducing bacteria Desulforibrio Desulfuricans on gyp-sum in the presence of organic matter on which thisbacteria lives. The reaction taking place may be repre-sented as follows:

Bacteria,

Caso4+a[Organ,ieMatter ]

-CC.S + 2 COB

Ca,S + 2 HQO -Ca(OH ) Q + H2Sca(oil ) a + coo - caco3 + HQo

Hydrogen sulfide liberated can be collected and con-verted to elemental sulfur if required, by controlled

oxidation:

CatalystH QS + 1/2 0 Q - H 80 + S

In fact, this is the theory advocated by geochemists tointerpret the occurrence of sulfur deposits in nature,e.g., in Gulf Coast region. Such deposits are alwaysassociated with gypsum, organic matter, and limestone.

SULFUR RECOVERY FROM SULFIDE ORES

During the smelting of copper, nickel, lead, and zincsulfide ores, large amounts of sulfur dioxide are pro-duced. Because the release of this gas in the atmo-sphere causes pollution problems, many attempts wei.emade in the past for its recovery. In one instance, pureS02 was obtairied from flue gases by an absorption-desorption process, and in another, the smelting opera-tion was conducted with oxygen instead Of air to get arelatively concentrated S02. In both cases S02 recov-ered was either liquified or processed to sulfuric acid.These two forms of sulfur are the usual forms that areconsumed by the chemical and metallurgical indus-tries, yet their storage and transport usually raise eco-nomic problems. Unless they are used in the smelteritself, or a market`exists near the smelter, the processwill be uneconomical.

It was realized, long ago, that a process by whichsulfur can be recovered directly in the elemental formwould be most attractive. Elemental sulfur is easilystored and transported, weighs only one-half an equiv-lent of S02 and one-third as much as the correspondingquantity of sulfuric acid, and is readily converted toany of these forms when needed. The following pro-cesses are aimed at this goal:

Table 3. Plants manufacturing ammonium sulfate from calcium Sulfate (Gopinath 1968)

Operating company1. BASF (IG Farben Industrie A/G)

Oppau, Germany

2. BASF (IG Farben Industrie A/G)Leuna, Germany

3. Wasag-Chemie StickstoffwerkeKrefeld, Germany

4. ICI, Billingham, U.K.5. ONIA, Toulouse, France6. Kuhlman, Selzaete, France

7. Sindri Fertilizers and ChemicalsSindri, India

8. Pakistan Industrial DevelopmentDaud Khel, Pakistan

9. Fertilizers arid ChemicalsTravanc ore Li mitedKerala, India

10. Gujarat State FertilizersIndia

11. Tohoku FertilizersAkita, Japan

12. OSAG ( Oesterriche Stickstoffwerke )Linz, Austria

13. ANIC, Ravenna, Italy14. Azot sanayii

Turkey

Date ofinstallation1913-1914

1918

1958

1923

1942

1947

1957

1947

1966Under construction,

19661956

1957

1958Under construction,

1960

Rawrmterial§

Gypsum andanhydrite

Gypsum

Phosphogypsum

AnhydriteAnhydritePhosphog]psum

Gypsum

Gypsum

Gypsum plusNH3 and H2S04

PhosphogypsumPhosphogypsum

Phosphogypsum

Gypsum

GypsumGypsum

Armualcapacity, tons

Up to 500,000 by 1928;later reduced to110,000

Up to 600,000 by 1943;damaged by bombsin 1944; believedrebuilt

45'000

600'000130,000 in 1951130,000 in 1951200,000 in 1958300,000

100,000

50'000

100,00033'000

10,000

45,000150'000300,000100,000

2

PYROMETALLURGICAL METII0I)S

1. Themial dissociation. To reduce the cost of ship-ping pyrite, Noranda Co. in Canada has designed aplant in which pyrite is first heated in absence Of airto distill off one atom Of sulfur in the elemental formand the residue is then roasted to S02 for sulfuric acidmanufacture, i.e. :

FesB-Fes + S

2 Fes + 7/2 0@-FeQ03 + 2 SOB0utokumpu Oy is operating a plant at Kokkola, Fin-

land, in which pyrite is decomposed in a specially de-signed furna6e, at 1800°C by burning fuel oil with pre-heated air `3-5 b6`rcent abo`ve the stoichiometric amount.The temperature is high enough to melt Fes formedwhich can then be easily separated from the sulfurvapor. Residence tim-a in the.furnace is only 1.5 sec-onds. The molten Fes is tapped from the bottom of thechamber, granulated with water and used for makingH2so4 via so2.

Pyr;te

i so2 ts

ing must be supplied, and the simplest method is byintroducing oxygen to utilize the exothermic combus-tion of carbon.

Carbon monoxide formed according to the equationlast above reacts with S02 at 600°C to form elementalsulfur:

SOB + 2 CO -2 COB + SCarbon oxysulfide is formed by a side reaction:

co + s - cosHowever, it can be converted to elemental sulfur easilywhen passed over bauxite at 425-450°C:

2 COS + Soe -2 COB + 3 SCoke consumption, which is the major factor in theprocess, is about 0.4 tons per ton of sulfur. The processoperated by the Consolidated Mining and Smelting Co.at Trail, British Columbia. The plant was shut downin 1943 because S02 was needed for making H2S04locally to meet the fertilizer demand. Before the shut-

Fe304Fig. 2. Recovery of elemental sulfur from pyrite by reaction with S02

gas

2. Reaction of sulfide ores with S02. An improvementin the thermal dissociation method is to utilize sulfurdioxide in distilling off elemental sulfur from pyriteor pyrrhotite at 800-900°C according to:

3 FisB + 2 SOB -Fe304 + 8 S3 Fes + 2 SOB -Fe804+ 5 S

Sulfur dioxide for the reaction can be produced byburning some of the pyrite with air (Fig. 2).

3. Production of sulfur via S02. Sulfur dioxide can bereduced to elemental sulfur by coke, methane, or nat-ural gas at about |20o°C.

(a) Reduction by coke-The reaction:

SOB + a + COB + Sis exothermic. However, the conditions are favorablefor the reaction:

COB + C -2 COto take place, which is endothermic. To achieve therequired temperature in the reduction, external heat-

3

Fig. 3. Reduction of S02 to elemen-tal sulfur by natural gas

down, the production was 150 tpd of sulfur of 99.9 per-cent purity.

(b) Reduction by Methane and Natural Gas-Sulfur dioxide is reduced by methane at 1250°C

to elemental sulfur according to:2 SOB + CH4 -COB + 2 HQO + 2 S

The combustion takes place in a large vertical steelchamber lined with refractory and insulating bricks.In order to obtain surface for complete combustion, thechamber is packed with a checker work of refractorybrick.

Small amounts of hydrogen sulfide and carbon oxy-sulfide are formed by side reactions:

SOB + CH4 -Hes + CO + HQOco + s - cos

Pyri'e. \` ,,, „,f tr;rr,rrL{m6stone

`<:;i.:.j`LLlrond Silica

Fig. 4. Recovery of elemental sulfurand copper from pyrite by reductionsmelting (Orkla process)

upp®' Zone

R®duclionZone

OxidolionZon®

Both can be converted to elemental sulfur by passingthe gas mixture over a suitable catalyst at the righttemperature. A flowsheet is shown in fig. 3.4. Reduction smelting. Elemental sulfur was first suc-cessfully recovered as a by-product from the blast fur-nace smelting of pyritic copper ore by Orkla GrubeAktiebolag at a small plant at L6kken, Norway, in1928. The success of this pilot plant led to the construc-tion in 1932 of a large modern smelter with four blastfurnaces. Similar operations are in R6nnskar (Swe-den), Mina de S. Domingos (Portugal), Rio Tinto(Spain), and U.S.S.R.

This method is only economical for pyrites contain-ing copper. Th.e ore is mixed with coke, quartz, andlimestone and heated in a blast furnace (fig. 4) .. Copperis recovered as a ,molten matte and iron is eliminatedin the slag. In the upper part of the furnace, one atomof sulfur in pyrite is distilled as elemental sulfur. In theoxidizing zone, Fes formed is oxidized to ferrous oxideand S02'. In the middle part of the furnace, the reduc-tion zone, S02 is reduced by, coke to elemental sulfur,which is volatilized as vapor. The reactions taking placecan be represented'by the following equations:

Up'per zone: FesQ-Fes+ SOcaidatto`ro zone: Fes+02-Feo + SOBMiddle zonte: SOB+C-COB+ S

The method has the disadvantage that only copper isrecovered; iron and other metals present in pyrite,e.g., cobalt and zinc, 'are not recovered. The matte pro-duced contains 6-8 percent Cu, is unsuitable for directconverting, .and is usually resmelted with coke, silica,and limestone to 40 percent Cu.5. Chlorination' of sulfide ores. Chlorine reacts withmetal sulfides readily to form metal chlorides and ele-mental sulfur:

MS + C12 -MCLQ + SThe use of chlorine for the treatment of sulfide oresis an old technique, and much work was published inthis connection. However, no plants are at presentoperating, although the process looks promising. Thereaction can be conducted in two ways:

(a) Low temperature chlol.ination at 60-100°C, i.e.,

4

GoSo. ot 450.C

COSConv®rte/ 400.

Sul(ur Condenser``Woste Heol Boiler)

130,

Heo'Eichooqer

CS2

Conve/ler 300.

SulfurCoT`d-enser

Ext`ous: Gases

Dust '0WOs'®

Sul'u,S'O,age

at temperatures below the melting point of sulfur sothat the liberated sulfur does not melt and agglom-erate the `charge. In the past, seemingly, no attemptswere made to recover the sulfur.

(b) High temperature chlorination at 400-700°C.In this process sulfur is volatilized, condensed, andrecovered.The chlorination reaction is strongly exothermic, and

therefore, once started, it would require no externalheat. The required reaction temperature can be con-trolled simply by adjusting the flow of chlorine to thereactor. In both processes, the nonferrous metals areconver/ted to chlorides and are recovered usually by

`:eyacah::f;gT\::n:e:Cx:dse°]tuot£S:e:Sip:tr:;epfuerr[rfi±:dhf;:=oird°:Each metal is then separated by successive cementa-

~tion, e.g., gold and silver by copper, copper by lead, andlead by zinc. The final solution, containing only zinc,is evaporated under vacuum to anhydrous zinc chlo-ride, which is then el`ectrolyzed in the fused state tometal and chlorine. The cathode is molten zinc and theanode is graphite. The chlorine is collected and reusedfor the process.

Practically all sulfide ores contain pyrite, which isalso easily chlorinated, but there is no chlorine lost inthe process as Fec12 formed is readily decomposed byoxygen liberating chlorine as follows:

2 Fec1,2 + 3/2 0Q -FeQ03 + 2 Cte

Also, if any Fec13 is formed it acts as a chlorinatingagent for other sulfides in the ore:

CuS + 2 Fect3 - 2 Fect2 + CuC1,Q + SNis + 2 Fec13 - 2 Fec12 + Nbct2 + S

It also reacts with pyrite according to the equation:Fes8 + 2 Fect3 - 3 FectQ + 2 S

C12

Zn

Fig. 5. Modified Malm process forlow temperature chlorination of non-ferrous metal sulfides

-.-..,-- : -..-.,....- :

Fig. 6. Recovery of elemental sulfur from pyrite by chlorination(Comstock-Wescott process)

Historically, the high temp_erature chlorination pro-cess was developed first. Thus, as early as 1897, Ash-croft patented this process, and a plant was operated atBroken Hill, Australia for the treatment of a lead-zincOre.

In the U.S., John L. Malm o£ Western Metal Co. de-veloped the low temperature chlorination process, anda 10 tpd plant was operated in Corbin, Mont. and an-other 50 tpd plant at Georgetown, Colo.

Chlorination was conducted in two stages-in thefirst stage by chlorine gas, and in the second by simpleroasting so that any Fec13 formed in the first stagereacts with the nonferrous metal sulfides. Fig. 5 showsa flowsheet of the pro.cess.

The high temperature process was applied to pyritecontaining small amounts of lead and zinc. Later, thesame process was applied to pivrite for the sole purposeof the recovery of elemental sulfur. Thus, in 1936 aplant was operated in Niagara Falls, Canada by theComstock-Wescott Co., in which pyrite was treatedwith chlorine gas in rotary kilns to volatilize elementalsulfur according to:

2 Fes2 + 2 Ctz -2 FecL2 + 4 SFerrous chloride was oxidized by air to generate thechlorine for recycle:

2 Fec12 + 3/2 0e -Fe203 + 2 Cl8Fig. 6 shows a flowsheet of the process.

Recently, a pilot plant was operated for the treat-ment of pyrite containing nickel and copper. It waspossible to recover sulfur in the elemental form, andthe nonferrous metals were converted to chlorides andleached from the iron oxide residue. Instead of rotary

5

kilns, fluidized beds were used. An ore containing 85.6percent Fes2, 2.7 percent cassiterite, 6.3 percent Znblend, and 5.4 percent gangue was treated by thismethod, and two separate fractions were obtained: (1)pigment grade Fe203, and (2) Sn02 and Znc12 fraction,which after leaching the zinc analyzed 63.8 percentgangue, 33.2 percent Sn02, and 2.9 percent Fe203. Thiswas achieved by admitting only the required amountof ferric chloride vapor to expel the sulfur from theore, and the rest of the ferric chloride was oxidized ina separate chamber to recover chlorine and pigmentgrade ferric oxide:

2 Fec13 + 3/2 0Q -FeQ03 + 3 Cb2Chlorine is then allowed to react with ferrous chloridein a third chamber to volatilize pure Fec13:

Fect9 + ty!2 CtQ - Fect3Under these conditions, cassiterite is not chlorinated,and therefore, can be concentrated in a separate frac-tion. Fig. 7 shows a flowsheet of the process.

6. Production Of sulfur via H2S. Sulfide ores react withhydrogen at about 800°C to yield hydrogen sulfide,e.8.:

FesQ + H2 - Fes + HasFes + He-Fe + H2S

Hydrogen sulfide can be readily converted to elementalsulfur by oxidation with a limited supply of air at240°C on alumina catalyst:

CatalystHQS + 1/2 08 - S + H@O

HYDROMETALLURGICAL METHODS

1. Production of sulfur via H2S. Pyrrhotite or ferroussulfide produced by the thermal dissociation of. pyritereacts readily with dilute acids to liberate hydrogensulfide:

Fes+2H+-Fd2 +HQSThe gas is collected and oxidized in a Claus furnace toelemental sulfur according to:

H 2S + 1/z 0 2 i S + H o~ODuring the decomposition of Fes with acid, the non-ferrous metal sulfides are not dissolved, and thereforecan be recovered from the unreacted residue. Fig. 8shows a flowsheet of the process.

2. Aqueous oxidatiqu of sulfide ores. Ferric sulfate orferric chloride solutions dissolve metal sulfides at room

Fig. 7. Recovery of elemental sulfur, tin, and zinc frompyrite

temperature with the liberation of elemental sulfur:Zns + 2 Fe3+ - Zrvve+ + Fe2+ + S

The ferrous ion formed is Oxidized by air back to ferric,thus allowing the recycle of the leaching agent. Themain difficulty with this process is the hydrolysis offerric ion and precipitation of hydrated ferric oxides,if the pH Of the solution exceeds 3.5. Other oxidizingagents were suggested to replace ferric ion. 'Thus chlo-rine water, sodium hypochlorite, etc., were used, e.g.:

cus + cbQ (aq) - cw8+ + s + 2 crMOSQ + 6 Cto-+ 4 0H--Mo04Q-+ S049-+ 6 CT + 2H80

+SRecently, air or oxygen under pressure was used as theoxidizing agent, at temperatures below 120°C, themelting point of sulfur. Such oxidizing agents have theadvantage of not contaminating the leach solution withforeign ious, e.g.:

Zrvs + 211+ + % OQ -Zrv®+ + S + HQOcus + 2 H+ + irf2 08 .-cwv8+ + s + H8O

Elemental sulfur is recovered from the residue by pel-1etizing, melting, and sieving, or by flotation.

Pyrrhotite was also treated in a similar way, butowing to the hydrolysis of ferric ion even at low pH,elemental sulfur and hydrated ferric oxide were ob-tained:

4 Fes + 3 0e - 2 Fe808 + 4 SNonferrous metals present in the pyrrhotite go intosolution and can be recovered. Ferric oxide, which isnow free from nonferrous metals, can be readily dried,sintered, and charged to blast furnaces.

ELECTROLYTIC METHODS

1. Electrolysis of molten sulfides. In the process forthe chlorination of sulfide ores described earlier, ele-mental sulfur and the metal chloride were obtained;the latter was electrolyzed in the fused state to metaland chlorine', which was recycled. In 1906, it was found

Fig. 8. Recovery of elemental sulfur from pyrite via H2Sformation

that the sulfide could be electrolyzed directly in themolten state to yield the metal and elemental sulfur.As most metal sulfides have either a hich melting pointor decompose upon melting, electrolysis is usually car-ried out in a metal chloride- alkali chloride bath towhich the metal sulfide is added. The decompositionpotential of the sulfides is usually much less than thatof the chloride components of the bath, thus allowingthe deposition of the metal at the cathode and elemen-tal sulfur at the anode:

]VI8+ + 2 e- + nffSQ- - S + 2 e-

Overall reaction:MS-M+S

Because the solubility of most sulfides in metal chlo-rides at 700-800°C is usually low, electrolysis is con-ducted when the sulfide is either in form of suspensionin the electrolyte, or as a molten layer floating abovethe electrolyte. The deposition of the metal at the cath-ode, and sulfur at the anode, takes place possibly bysecondary reactions as follows:

Na,+ + er - NaMS + 2 Na-M + 2 Na+ + S@-

2 Ct- - CIB + 2 e-S®- - S + cr

Chlorine liberated at the anode does not interfere withthe process, as it will react with the metal sulfide asfollows:

MS + Ct@ i Mcta + SA semi-commercial plant in Wales, recovered lead

of 99.95 percent purity and sulfur Of 99.5 percent purityfrom galena by this method®2. Electrolysis Of sulfide anodes` in aqueous phase. Theprevious method of electrolyzing sulfides in the moltenstate has the disadvantage of operating at temperaturesin the range 500-900°C. A much simpler method is tocast the sulfide in the form Of anodes and electrolyzeit using an aqueous electrolyte at room temperature.The process can be represented by the equation:Anodic reaction:

MS - MQ+ + S + 2 e-Cathodic reaction:

M:f`+ 2 e-+ JVI

Fig. 9. Recovery of nickel and elemental sulfur by elec-trolysis of nickel sulfide anodes (lnco process)

The metal deposits at the cathode, while elementalsulfur remains adhering to the anode in the form of asludge, which can be melted, separated, and recovered.This process has a great advantage over conventional

Fig.10. Anode of white metal

7

methods of metal recovery. Thus, in the case of nickelextraction it would be possible to cast the nickel mattein the form of anodes and recover the metal and thesulfur directly in one step, instead of roasting thematte to oxide, reducing the oxide to metal, and thencasting the metal in the form of anodes for electrolyticrefining. It should also be noted that this direct elec-trolysis of sulfide anodes not only by-passes the roast-ing and reduction step, but it is possible to recover thesulfur directly in the elemental form.

International Nickel Co. of Canada erected a pilotplant in 1951 and a full scale plant at the Thompsonrefinery in Manitoba in 1964, to apply the process tonickel matte. In this method, nickel sulfide matte ismelted at 980°C and cast into anodes. The anodes areallowed to partially cool in the molds and then prompt-1y placed in a controlled cooling box to maintain adesired cooling rate. This step is a critical operation inpreparing the anodes, otherwise the castings crackowing to the phase transformation in Ni3S2 that takesplace at 5o5°C.

The anodes corrode smoothly and uniformly, and thesludge formed is granular and porous and adheres onthe anode. The anode increases in thickness duringelectrolysis, owing to the formation of elemental sulfur.By the end of electrolysis the thickness is usually dou-bled. The temperature of electrolyte is 55-60°C. Theanodes can be electrolyzed to about 10 percent scrap,and the efficiency during the anode dissolution is 95percent. To facilitate the handling of the scrap anodescontaining the sludge, they are enclosed in bags duringelectrolysis.

During electrolysis, Fe, Cu, Co, As, and Pb originally

Fig.11. White metal anode after test, showing ad-hering slimes of CuS and elemental sulfur

Fig. 12. Kineticsof sulfur formationin slimes, currentdensity I.0 A/sq.din., temperature2o°C (Habc,shi andTorres-Acuna I 968)

present in the anodes go into solution, while the pre-cious metals are collected in the sulfur sludge. Theelectrolyte is regenerated by first oxidation and hy-drolysis of iron, then selective oxidation with chlorineand hydrolysis of cobalt, arsenic, and lead, and finallycementation of copper by nickel powder. The nickelcontent of the electrolyte and the pH are adjusted andit is recycled. (Fig. 9) .

White metal, which is mainly Cu2S, was cast at Mon-tana College o£ Mineral Science and Technology, Butte,in the form of anodes (fig. 10) and immersed in anacidified CuS04 solution containing copper sheet cath-ode. When an electric current was passed, the anodeswere found to corrode smoothly and uniformly, andthe slimes formed were granular and porous and re-mained adhering on the anode (fig. 11). Under themicroscope, it was observed that the slimes arescom-posed Of two distinct substances: black and yellow par-ticles. These were identified as CuS and elemental sul-fur respectively.

The formation of elemental sulfur was found to takeplace in three distinct steps (fig. 12) .

1. During the first three hours, no slimes wereformed although copper was continuously depositedon the cathode. During this period, the surface of theanode showed a marked change in color from grayto blue.

2. Tn the next four hours, elemental sulfur wasrapidly formed.

3. After about 10 hours from starting the test, therate of sulfur formation in the slimes increased slow-Iy but steadily.

Fig.13. Flowsheet of copper smelting and refining, show-ing modification whereby white metcil con be cast directlyfrom the reverberatory furnace

8

Fathi Habashi is associated with theAnaconda Co. as a senior researchengineer. Before his service with thecompany, he was a professor of Metal-lurgy at the Montana College of Min-ercil Science anc] Technology in Butte.Habashi attended the University ofCairo and the University of Technology,Vienna.

After removing the adhering slimes it was found thatthe residual anode changed its color from gray to darkblue. The change in color was restricted only to a thinlayer about one mm thick. This dark blue layer wasexamined by x=rays and found to correspond to themineral digenite Cugs5 (or Cu].8S). The anodic disso-lution is believed to take place in three steps:

5 Cues - Cwgs 5 + CW+ + e~Cugs6 -4 CuS + 5 Cw+ + S + 5 e-

CuS - Cue+ + S + 2 e-

Spectrographic analysis of cathodic _copper was foundto be comparable to commercially available electrolyticcopper. The process may be applied on commercialscales since it has the advantage of by-passing the con-verting and poling steps, decreasing pollution prob-

metal in the elemental fo::t:]d±:echo::%2i::€a:e£:°=er(ifg.the sulfur of the white13).

As a result of this work, it was suggested to producethe white metal directly in the reverberatory furnace,i.e., the matte is processed further in the same furnacebut in an oxidizing atmosphere and by adding theproper flux till all the iron is removed and the bathattains the chemical composition of white metal. Inthis way, it would be possible to eliminate the con-verter completely, and the white metal can be directlycast from the reverberatory furnace.

CONCLUSIONS

These are the methods presently known as sulfurrecovery from ores. A process can only be successfulif it also proves to be economically feasible. This is thechallenging question facing the present-day engineers,namely, transforming chemical reactions into technicalprocesses.

REFERENCES

Habashi, F., "The Recovery of Elemental Sulfur fromSulfide Ores," Montana Bureau o£ Mines and Geol-ogy, Bulletin 51, 1966.

and N. Torres-Acuha, "The Anodic Dissolutionof Copper (1) Sulfide and the Direct Recovery ofCopper and Elemental Sulfur from White Metal,"Trams. Met. Soc. AIME 242, 780-7, 1968.

9