SULFUR

1. Introduction

Sulfur [7704-34-9], S, a nonmetallic element, is the second element of Group 16(VIA) of the Periodic Table, coming below oxygen and above selenium. In massiveelemental form, sulfur is often referred to as brimstone. Sulfur is one of the mostimportant raw materials of the chemical industry. It is of prime importance tothe fertilizer industry (see FERTILIZERS) and its consumption is generally regardedas one of the best measures of a nation’s industrial development and economicactivity (see SULFUR COMPOUNDS; SULFUR REMOVAL AND RECOVERY; SULFURIC ACID AND

SULFUR TRIOXIDE).Sulfur has been known since antiquity. Early humans used sulfur to color

cave drawings, employed sulfur fumes to kill insects and to fumigate, and knewabout sulfur’s color-removing or bleaching action. Mystical powers were attribu-ted to the ethereal blue flame and pungent odor given off by burning the yellowrock. Medicinal use of sulfur was known to the Egyptians and Greeks. One con-temporary use was developed as early as 500 BC, when the Chinese used sulfur asan ingredient of gunpowder. Although the modern history of sulfur may havebegun with Lavoisier’s proof in the late-eighteenth century that sulfur is an ele-ment, the first commercial sulfur was produced in Italy early in the fifteenth cen-tury. Sulfur production become Italy’s main industry when, in 1735,development of a process to make sulfuric acid from sulfur was commercialized.When a French company gained an effective monopoly on the Sicilian deposits in1839 and tripled the price, other countries, particularly England and the UnitedStates, developed internal sources of sulfur and sulfuric acid. Consumers inmany countries quickly learned that sulfuric acid could be made from sulfurdioxide obtained from the roasting of iron pyrites, obviating the need for Siciliansulfur.

The United States continued to depend on foreign sources of elemental sul-fur even after the mineral was discovered in the United States in 1867 by oil pro-spectors investigating a salt dome in Calcasieu Parish, Louisiana. Variousattempts were made to sink mine shafts. Realizing that conventional mining pro-cesses would be too uneconomical to compete with Sicilian sulfur, H. Frasch con-ceived of melting the sulfur underground by injecting superheated water into theformation and then lifting the melted sulfur to the surface using a sucker-rodpump. In 1894, the first flow of molten sulfur was pumped from the CalcasieuParish deposit. In 1902, the Frasch process was successfully commercialized.This mining method later became important in the development and productionof sulfur not only from the Texas–Louisiana salt dome area, but also from areasin western Texas, Mexico, Poland, and Iraq. The Frasch process is no longer usedin the U.S.

Today sulfur recovered as a by-product, involuntary sulfur, accounts for alarger portion of world supply than does mined or voluntary material. Sulfur isobtained from hydrogen sulfide, which evolves when natural gas (see GAS, NAT-

URAL), crude petroleum (qv), tar sands (qv), oil shales (qv), coal (qv), and geother-mal brines (see GEOTHERMAL ENERGY) are desulfurized (see SULFUR REMOVAL AND

RECOVERY). Other sources of sulfur include metal sulfides such as pyrites; sulfate

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Kirk-Othmer Encyclopedia of Chemical Technology. Copyright John Wiley & Sons, Inc. All rights reserved.

materials, including gypsum (see CALCIUM COMPOUNDS); and elemental sulfur innative and volcanic deposits mined in the traditional manner.

Sulfur constitutes about 0.052 wt % of the earth’s crust. The forms in whichit is ordinarily found include elemental or native sulfur in unconsolidated volca-nic rocks, in anhydrite over salt-dome structures, and in bedded anhydrite orgypsum evaporate basin formations; combined sulfur in metal sulfide ores andmineral sulfates; hydrogen sulfide in natural gas; organic sulfur compounds inpetroleum and tar sands; and a combination of both pyritic and organic sulfurcompounds in coal (qv).

2. Properties

2.1. Allotropy. Sulfur occurs in a number of different allotropic modifi-cations, that is, in various molecular aggregations which differ in solubility, spe-cific gravity, crystalline form, etc. Like many other substances, sulfur alsoexhibits dynamic allotropy, ie, the various allotropes exist together in equili-brium in definite proportions, depending on the temperature and pressure.The molecular formulas for the various allotropes are S–Sn, where n is a largebut unidentified number, such as n � 106. The particular allotropes that may bepresent in a given sample of sulfur depend to a large extent on its thermal his-tory, the amount and type of foreign substance present, and the length of timethat has passed for equilibrium to be attained.

In the solid and liquid states, the principal allotropes are designated tradi-tionally as Sl, Sm, and Sp. Of these, only Sl is stable in the solid state. Upon soli-dification of molten sulfur, Sp rapidly changes into Sm, which is converted intoSl, although at a much slower rate. The molecular structure of Sp is that ofan octatomic sulfur chain (1,2). The symbol Sm designates long, polymerizedchains of elemental sulfur. Sl is perhaps the most characteristic molecularform of sulfur, namely, that of a crown-shaped, octatomic sulfur ring designatedin more recent literature as S(r/8) (3). The allotropes have different solubility incarbon disulfide. Sp and Sl are soluble in carbon disulfide, whereas Sm does notdissolve in this solvent.

Sulfur crystallizes in at least two distinct systems: the rhombic and themonoclinic forms. Rhombic sulfur, Sa, is stable at atmospheric pressures up to95.58C, at which transition to monoclinic sulfur, Sb, takes place. Monoclinic sul-fur is then stable up to its natural melting point of 114.58C. The basic molecularunit of both of these crystalline forms of sulfur is the octatomic sulfur ring S(r/8).Other forms of solid sulfur include hexatomic sulfur as well as numerous modi-fications of catenapolysulfur (2,4).

The molecular constitution of liquid sulfur undergoes significant and rever-sible changes with temperature variations. These changes are evidenced by thecharacteristic temperature dependence of the physical properties of sulfur. Inmost studies of liquid sulfur, some striking changes in its physical propertiesare observed at about 1608C. For example, the viscosity of purified sulfur,which at 1208C is about 11 mPa�s (¼cP), drops to a minimum of 6.7 mPa�s atabout 1578C, and then begins to rise. At 159–1608C, the viscosity of liquid sulfurrises sharply, increasing to 30 mPa�s at 1608C and reaching a maximum of about

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93 Pa�s (930 P) at 1878C. Above this temperature, the viscosity gradually dropsoff again to about 2 Pa�s (20 P) at 3068C. A qualitative exploration of these visc-osity changes in terms of the allotropy of sulfur implies that below 1598C, sulfurconsists mainly of S8 rings. A normal decrease of viscosity with rising tempera-ture is observed. The sudden increase in the viscosity of sulfur above 1598C isattributed to the formation of polymeric sulfur chain molecules. Then, as thetemperature rises further, the concentration of polymeric sulfur continues toincrease, but the opposing effect of decreasing chain length resulting from ther-mal sulfur–sulfur bond scission causes a gradual decrease in viscosity in thetemperature range between 1878C and the boiling point of sulfur. The chemicalequilibria between the various forms in molten sulfur have been extensivelyinvestigated (2,3,5). A critical review of the literature concerning the molecularcomposition of molten sulfur is also available (6). Experiments that added muchto the knowledge of the species present under different time–temperature para-meters have been described (7,8). Previous theories concerning the polymeriza-tion of S8 were shown to be in disagreement with well-established experimentalfacts and are considered unsatisfactory.

The molecular composition of sulfur vapor is a complex function of tempera-ture and pressure. Vapor pressure measurements have been interpreted in termsof an equilibrium between several molecular species (9,10). Mass spectrometricdata for sulfur vapor indicate the presence of all possible Sn molecules from S2 toS8 and negligible concentrations of S9 and S10 (11). In general, octatomic sulfur isthe predominant molecular constituent of sulfur vapor at low temperatures, butthe equilibrium shifts toward smaller molecular species with increasing tem-perature and decreasing pressure.

2.2. Constants and Chemical Properties. The constants of sulfur arepresented in Table 1. Two freezing points are given for each of the two crystallinemodifications. When the liquid phase consists solely of octatomic sulfur rings, thetemperature ranges at which the various modifications form are called the idealfreezing points. The temperatures at which the crystalline forms are in equili-brium with liquid sulfur containing equilibrium amounts of Sp and Sm are callednatural freezing points.

There are four stable isotopes of sulfur: 32S, 33S, 34S, and 36S, which haverelative abundances of 95.1, 0.74, 4.2, and 0.016%, respectively. The relativeabundance of the various isotopes varies slightly, depending on the source ofthe sulfur; the ratio of 32S to 34S is 21.61–22.60. Three radioactive isotopes ofmasses 31, 35, and 37 having half-lives of 2.6 s, 87 d, and 5 min, respectively,have been generated artificially.

Sulfur falls between oxygen and selenium in Group 16 and resembles oxy-gen in its chemical reactions with most of the elements. The normal orbital elec-tron structure (17) is of the arrangement 1s2 2s2 2p1 3s2 3p4. Sulfur has valencesof �2, þ2, þ3, þ4, and þ6. Selenium is a closely related element having a similargroup of valences and analogous allotropy. Sulfur is between phosphorus andchlorine in the third Period of the Periodic Table. Although the properties of sul-fur are generally those to be expected from its position in the Table, an exceptionis that its melting point is higher than expected, probably because of its complexmolecular structure (17).

Vol. 23 SULFUR 3

Sulfur is insoluble in water but soluble to varying degrees in many organicsolvents, such as carbon disulfide, benzene, warm aniline, warm carbon tetra-chloride, and liquid ammonia (18). Carbon disulfide is the most commonly usedsolvent for sulfur.

Sulfur combines directly and usually energetically with almost all of theelements. Exceptions include gold, platinum, iridium, and the helium-groupgases (19). In the presence of oxygen or dry air, sulfur is very slowly oxidizedto sulfur dioxide. When burned in air, it forms predominantly sulfur dioxidewith small amounts of sulfur trioxide. When burned in the presence of moistair, sulfurous acid and sulfuric acids are slowly generated.

Hydrochloric acid reacts with sulfur only in the presence of iron to formhydrogen sulfide. Sulfur dioxide forms when sulfur is heated with concentratedsulfuric acid at 2008C. Dilute nitric acid up to 40% concentration has little effect,but sulfur is oxidized by concentrated nitric acid in the presence of bromine witha strongly exothermic reaction (19).

Sulfur combines directly with hydrogen at 150–2008C to form hydrogen sul-fide. Molten sulfur reacts with hydrogen to form hydrogen polysulfides. At redheat, sulfur and carbon unite to form carbon disulfide. This is a commerciallyimportant reaction in Europe, although natural gas is used to produce carbondisulfide in the United States. In aqueous solutions of alkali carbonates andalkali and alkaline-earth hydroxides, sulfur reacts to form sulfides, polysulfides,thiosulfates, and sulfites.

At room temperature, sulfur unites readily with copper, silver, and mercuryand vigorously with sodium, potassium, calcium, strontium, and barium to formsulfides. Iron, chromium, tungsten, nickel, and cobalt react much less readily. Ina finely divided state, zinc, tin, iron, and aluminum react with sulfur on heating(19).

Various sulfides of the halogens are formed by direct combination of sulfurwith fluorine, bromine, and chlorine. No evident reaction occurs with iodine;instead, the elements remain as components of a mixture. Mixtures of sulfurand potassium chlorate, or sulfur and powdered zinc, are highly explosive.

Sulfur is involved in numerous organic reactions (20). When dissolved inamines, chemical interaction between sulfur and the solvent results in the for-mation of colored species ranging from deep yellow to orange and green (see SUL-

FUR DYES). Many organic reactions involving sulfur are commercially significant.Sulfur is important in the manufacture of lubricants, plastics, pharmaceuticals(qv), dyes, and rubber goods (see DYES AND DYE INTERMEDIATES; LUBRICATION AND

LUBRICANTS; PLASTICS PROCESSING; RUBBER CHEMICALS).Sulfur is not considered corrosive to the usual construction materials. Dry,

molten sulfur is handled satisfactorily in mild steel or cast-iron equipment. How-ever, acid-generating impurities, which may be introduced in handling and sto-rage, create corrosive conditions. The exposure of sulfur to moisture and aircauses the formation of acids which attack many metals. To combat such corro-sion difficulties, protective coatings of organic compounds, cement, or sprayedresistant metals are often applied to exposed steel surfaces, including pipe andequipment used in handling liquid sulfur, and to structural members that comein contact with solid sulfur. Also practical in some applications is the use of resis-tant metal alloys, particularly those of aluminum and stainless steel. Naturali-

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zation of the generated acids by the addition of basic chemicals is sometimesemployed.

3. Elemental Sulfur

Sources of sulfur are called voluntary if sulfur is considered to be the principal,and often the only, product. Sulfur has also been recovered as a by-product fromvarious process operations. Such sulfur is termed involuntary sulfur andaccounts for the largest portion of world sulfur production (see SULFUR REMOVAL

AND RECOVERY).3.1. Occurrence. Salt-Dome Sulfur Deposits. The sulfur deposits

associated with salt domes in the Gulf Coast regions of the southern UnitedStates and Mexico have historically been the primary sources of U.S. sulfur.These remain an important segment of both U.S. and world sulfur supply.Although the reserves are finite, many are large and voluntary productive capa-city ensures the importance of these sources for some time to come.

Salt domes of the U.S. Gulf Coast are vertical structures, usually circular inoutline, with steeply dipping flanks, and composed of coarsely crystalline halite,ie, NaCl, interspersed with anhydrite, CaSO4. The cap rock that surmounts thesalt dome consists of anhydrite in contact with the salt and gypsum,CaSO4�2H2O, derived from the anhydrite. Limestone in the form of fine gray car-bonate interspersed with vugs, seams, fissures, and cavities is frequently asso-ciated with the gypsum and anhydrite formations. It may be present as astratum overlying these formations, as lenticular beds covering part of them,or as disseminated lenses and nodules included in the upper part of the cap rock.

The sulfur occurs as well-developed crystal aggregates in veins and vugs oras disseminated particles in the porous limestone and gypsum section of the caprock. Several theories have been proposed for the occurrence of sulfur in saltdomes. One theory suggests the formation of limestone and hydrogen sulfidefrom anhydrite in the presence of reducing agents. This reaction, however,requires temperatures of about 6508C and, although oil or other hydrocarbonsmay be present to act as reducing agents, the temperature actually attained isnot sufficient to support this theory. In 1946, the presence of anaerobic,sulfate-reducing bacteria was discovered in cap rock. The ability of these bacteriato promote reaction at normal temperatures is recognized as the more likely ori-gin of sulfur. Anaerobic bacteria consume hydrocarbons as a source of energy,but combine sulfur instead of oxygen with the hydrogen. The hydrocarbon-fueledbacteria reduce anhydrite to hydrogen sulfide, calcium carbonate, and water.The hydrogen sulfide remains dissolved in the formation waters until it precipi-tates as crystalline sulfur through various oxidation reactions, possibly initiatedby oxygen and carbon dioxide dissolved in water percolating from the upper sedi-ments. In 1966, 329 salt-dome structures were identified by the U.S. Bureau ofMines in the U.S. Gulf Coast area and offshore tidelands. Of these, 27 have beencommercial sulfur producers. In 2000, one of the salt-dome sulfur deposits, MainPass operated by Freeport McMoRan Sulphur Inc. in Louisiana, that was minedby the Frasch Process was the last to close. Some could be reactivated if justifiedby economic circumstances.

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Evaporite Basin Sulfur Deposits. Elemental sulfur occurs in anothertype of subsurface deposit similar to the salt-dome structures in that the sulfuris associated with anhydrite or gypsum. The deposits are sedimentary, however,and occur in huge evaporite basins. It is believed that the sulfur in these depos-its, like that in the Gulf Coast salt domes, was derived by hydrocarbon reductionof the sulfate material and assisted by anaerobic bacteria. The sulfur deposits inItaly (Sicily), Poland, Iraq, the CIS, and the United States (western Texas) areincluded in this category.

Mining techniques similar to the Frasch salt-dome mining systems havebeen applied successfully. These developments and particularly those in westernTexas and Poland have significantly contributed to world sulfur production andreserves. Hot-water mining of the Polish deposits began in 1966 at Gryzbow. In1979, production at Gryzbow and at another deposit, Jeziorko, was nearly fivemillion metric tons. By 1995, production declined to � 2 million metric tons.The new Osiek Mine officially opened near Grzybow in September, 1993 afterseveral months of test production. The cost of production there was reported tohave been cut about 50% by using a system to recycle hot water from nearbypower stations, cutting labor costs, and closing nonproductive facilities. Produc-tion ceased at the Machow Mine in 1992. However, final transfer of the operationto local authorities has been delayed. Environmental problems continue atMachow, and the cost of remediation, including the elimination of hydrogen sul-fide emissions from the pit has been estimated at more than $200 million. Polandis the only country using the Frasch process today (21). Salt domes and similarsulfur-bearing structures occur in regions other than the United States, Mexico,Poland, Iraq, and the CIS, but sulfur deposits that could be economically produc-tive have not been discovered in any of them.

Frasch production has not been very successful because of the low porosityof the sulfur-bearing ore. There are problems also with product quality owing tobitumen contamination. In addition, significant environmental problems havealso constrained production. In Russia, there are some small operations miningsulfur from underground volcanic deposits.

Volcanic and Native Sulfur Deposits. Elemental sulfur occurs in othertypes of surface or underground deposits throughout the world, but seldom insufficient concentration to be commercially important. Sulfur of volcanic originoccurs in many parts of the world. These deposits originated from gases emittedfrom active craters, solfataras, or hot springs, which contain deposited sulfur infractures of rock, by replacement in the rock itself or in the sediments of lakebeds. Volcanic deposits usually occur in tufas, lava flows, and similar volcanicrocks but also in sedimentary and intrusive formations. Scattered deposits ofthis type have been discovered throughout the mountain ranges bordering thePacific Ocean, particularly in Japan, the Philippine Islands, and Central andSouth America. Some volcanic deposits are worked profitably and are importantin the countries in which this type of deposit is found. The Japanese deposits areamong this group and have had a long and productive history of considerable ton-nages. Most volcanic deposits, however, are in isolated regions and at high eleva-tions where production and transportation costs are prohibitive.

3.2. Extraction. Frasch Process. In 2004, 1% of total sulfur was pro-duced by the Frasch process. This process can be reactivated if necessary (21).

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In the Frasch process, large quantities of hot water are introduced throughwells drilled into buried deposits of native sulfur. The heat from the water meltsthe sulfur in the vicinity of the wells; the melted sulfur is then removed to thesurface as molten elemental sulfur of high purity. Economical operation of asalt dome or a subsurface sulfur deposit by the Frasch process requires a poroussulfur-bearing limestone, a large and dependable supply of water, and a source ofinexpensive fuel. A power plant is required, in which the necessary volume of hotwater is produced, as is compressed air for pumping molten sulfur from the wellsand electric power for drilling, lighting, operating maintenance equipment, load-ing sulfur for shipment, and similar operations.

A typical setting of equipment for a sulfur well and the principles of miningare illustrated schematically in Figure 1. First, a hole is drilled to the bottomlayer of the salt-dome cap rock with equipment of the same type as that usedin oil fields. Three concentric pipes within a protective casing are placed in thehole. A 20-cm pipe inside an outer casing is sunk through the cap rock to the bot-tom of the sulfur deposit. Its lower end is perforated with small holes. Then, a 10-cm pipe is lowered to within a short distance of the bottom. Last and innermost isa 2.5-cm pipe, which is lowered more than halfway to the bottom of the well.

Water-heated under pressure to 1608C is pumped down the space betweenthe 20-cm and 10-cm pipes and, during the initial heating period, also down the10-cm pipe. The superheated water flows out the holes at the bottom into the por-ous sulfur-bearing formation (Fig. 1a). When the temperature of the sulfur-bear-ing formation exceeds the melting point of sulfur, the liquid sulfur, beingapproximately twice as heavy as water, percolates downward through the porouslimestone to form a pool at the bottom of the well. A heating period of 24 h orlonger is required to accumulate a liquid sulfur pool of sufficient size, and thenpumping of hot water down the 10-cm pipeline is stopped. Static pressure of thehot water pumped into the formation then forces the liquid more than 100-m upinto the 10-cm pipe (Fig. 1b). Compressed air forced down the 2.5-cm pipe aeratesand lightens the liquid sulfur so that it rises to the surface (Fig. 1c). Injection ofhot water is continued down the 20-cm pipe to maintain the sulfur melting pro-cess, and the compressed-air volume is adjusted to equalize the sulfur pumpingrate with the sulfur melting rate. If the pumping rate exceeds the melting rate,the sulfur pool is depleted and the well produces water. At this point, thecompressed-air flow is stopped, and hot water is again injected until the liquidsulfur pool is reestablished.

The sulfur-bearing cap rock, being an enclosed formation, is essentially theequivalent of a pressure vessel. Hot water, pumped into the formation to meltsulfur, must be withdrawn after cooling at approximately the same rate as itis put in, otherwise the pressure in the formation would increase to the pointwhere further water injection would be impossible. Bleedwater wells, used toextract water from the formations, usually are located on the flanks of thedome away from the mining area where the water temperature is lowest. Thewater is treated to remove soluble sulfides and other impurities before being dis-charged to disposal ditches or canals.

On the surface, the liquid sulfur moves through steam-heated lines to aseparator where the air is removed. Depending on the mine location, the liquidsulfur may be pumped to storage vats to be solidified, to tanks for storage as a

Vol. 23 SULFUR 7

liquid, to pipelines, or to thermally insulated barges for transport to a centralshipping terminal.

Sulfur wells that are favorably located produce continuously over long per-iods. Some may last a year or more; other may be abandoned within a few weeks,because denseness of the rock formation may retard circulation of hot water andmolten sulfur. The extraction of sulfur weakens the rock formation and subsi-dence may follow. This may break the pipes in the well, ending productivity.Although subsidence is desirable in mining, wells may be lost as a result. Theadvantage of subsidence is that the volume of exhausted formation throughwhich hot water can circulate is reduced. After caving, the crushed exhaustedformation is relatively impervious and therefore confines the circulation of hotwater to the more porous sulfur-bearing parts of the deposit.

Directional drilling techniques were an important advance in sulfur miningmethods. Casings are placed in the hole in such a manner as to extend into thesulfur formation somewhat horizontally. Thus, substantial amounts of sulfur inthe deposit overlie the hole. Also, the casings in the subterranean volume likelyto be affected by subsidence are parallel to the expected earth movement andtherefore are less affected by shear. The result is better utilization of heatingwater and longer well life. Directional drilling has also permitted efficientreworking of areas exhausted to vertical mining techniques.

Another advance in sulfur mining technology has been the development of amethod involving seawater in the Frasch process, making it feasible to minedeposits distant from freshwater supplies. In such a plant, seawater is first deox-ygenated by bringing it in direct contact with combustion gases in a packedtower. The seawater is preheated by these gases and its temperature raised to1068C in indirect heat exchangers by means of steam (qv) furnished by high pres-sure boilers. Condensate from the heat exchangers is recycled to the boilers; thislimits freshwater requirements to leaks and other small losses in the system.Production from several sulfur mines involves seawater from both stationarysystems and portable, barge-mounted power plants (see also MINERALS RECOVERY

AND PROCESSING).Hydrodynamic Process. The hydrodynamic process is similar to the

Frasch process in that superheated water is used to melt the underground sulfur.However, the techniques involved are different. The process was developed inPoland to exploit sulfur deposits, which, because of thin bedding, wide disper-sion, and frequent impermeability, did not appear to be amenable to productionby the Frasch method, which requires some degree of deposit isolation. The tech-niques employed include the use of explosives to control permeability or to createsealed-off gases and the calculated manipulation of underground water pressure,temperature, and flow conditions by control at injection and breakwater points.The system requires a constant rate of fuel consumption throughout the life ofthe mine and improves the rate of sulfur recovery. Hydrodynamic mining isused in Poland and was used in Iraq where the mining area was developedwith Polish assistance.

Volcanic and Other Surface Deposits. Sulfur is recovered from volcanicand other surface deposits by a number of different processes, including distilla-tion, flotation, autoclaving, filtration, solvent extraction, or a combination of sev-eral of these processes. The Japanese sulfur deposits are reached by tunnel, and

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mining is done by the room-and-pillar, chamber-and-pillar with filling, and cut-and-fill systems. Sulfur was historically extracted from the ore by a distillationprocess performed in rows of cast-iron pots, each containing about 180 kg of ore.Each row of pots is connected to a condensation chamber outside the furnace. Ashort length of pipe connects each pot with a condenser. Brick flues connect com-bustion gases under the pots. Sulfur vapor flows from the pots to the condensa-tion chamber where the liquid sulfur is collected. The Japanese ore contains 25–35 wt % sulfur. This method has been superseded by other sources of sulfur pro-duction.

The sulfur deposits in Italy have been worked since ancient times. Origin-ally, sulfur was removed by piling the ore in central heaps, covering it withearth, and then igniting the pile. By this method, 30–50 wt % of the sulfurwas burned to provide heat for melting the remainder of the sulfur in the ore.Less than 50 wt % of the sulfur originally contained in the ore was recovered.In about 1880, the first Gill gas furnace was installed. The original furnacehad two chambers arranged so that the heat from burning ore in one chamberpassed through ore in the other chamber to melt a considerable portion of thesulfur. When the sulfur in the first chamber burned, the chamber was refilled.When the partially extracted ore in the first chamber burned, it was refilledand the partially extracted ore in the other chamber was ignited. This methodinvolved better utilization of the heat of combustion. Later, furnaces containedas many as six chambers and permitted up to 80% sulfur recovery (22).

Extensive experimentation has led to numerous patents for various thermalprocesses for extracting sulfur from ores, either as elemental sulfur or as SO2,but very few of these processes have been operated commercially. The proposedprocesses involve shaft furnaces, multiple-hearth furnaces, rotary kilns, andfluidized-bed roasters. In all of these, ground ore is heated with oxygen-free,hot combustion gases to distill elemental sulfur; or the sulfur in the ore is burnedwith air to yield SO2 for sulfuric acid production. In 1953, a commercial plantwas brought on-stream at the Yerrington, Nevada, copper mine for recoveringthe sulfur as SO2 from the Leviathan deposit of low grade sulfur ore in AlpineCountry, California. The process consisted of four fluid-bed reactors, in whichthe ore was roasted in air to produce SO2 for a contact sulfuric acid plant (23).For the production of elemental sulfur, the use of oxygen-free, hot combustiongases in a fluidized bed has been proposed to distill sulfur from the ore as avapor, which is then condensed to liquid sulfur.

Various processes have been proposed and tested for the recovery of sulfurfrom native ores by solvent extraction, and many patents have been issued. Car-bon disulfide, the best solvent for sulfur, has often been suggested for extractionof sulfur from ore. Some plants in Italy, Germany, South America, and the Uni-ted States have used carbon disulfide for this purpose, but the cost of the solvent,the high losses, and its flammability detract from low operating costs. Manyother solvents have been tried, including hot caustic solution, chlorinated hydro-carbons, ammonium sulfide, xylene, kerosene, and various high boiling oils. Thesulfur is recovered either by volatilizing the solvent or by crystallizing the sulfur.

Various combinations of autoclaving, filtration, and centrifuging are used insome processes to recover sulfur from ore. One such process, involving continu-ous autoclaving, flotation (qv), and filtration (qv), was first used commercially at

Vol. 23 SULFUR 9

a plant in Columbia (24). The ore is finely ground to less than 625-mm (28-mesh)and suspended in water to form a slurry of about 30% solids, which is pumpedcontinuously to a three-compartment, agitated autoclave. In the autoclave, theslurry is heated and the sulfur melts by steam injection into the bottom ofeach compartment. Agitation causes the sulfur to coalesce into globules thatseparate from the gangue. Hot slurry from the autoclave flows into a quenchpot to be cooled by water injection, and the sudden cooling solidifies the sepa-rated sulfur particles. The cooled slurry is throttled to atmospheric pressureand flows into a 625-mm (28-mesh) vibrating screen. The oversize material passesdirectly to a sulfur melter, whereas the underflow from the screen passes to aflotation circuit for separation of the smaller sulfur particles. This gives a concen-trate of 90–95 wt % sulfur which then passes to the sulfur melter. Melted sulfuris pumped through a filter for removal of gangue.

One more variation to the many methods proposed for sulfur extraction isthe fire-flood method. It is a modern version of the Sicilian method, by which aportion of the sulfur is burned to melt the remainder. It would be done in situ andis said to offer cost advantages, to work in almost any type of zone formation, andto produce better sweep efficiency than other systems. The recovery streamwould be about 20 wt % sulfur as SO2 and 80 wt % elemental sulfur. The methodwas laboratory-tested in the late 1960s and patents were issued. However, it wasnot commercially exploited because sulfur prices dropped.

4. Sulfide Ores

4.1. Occurrence. The metal sulfides, which are scattered throughoutmost of the world, have been an important source of elemental sulfur. The poten-tial for recovery from metal sulfides exists, although these sources are lessattractive economically and technologically than other sources of sulfur. Never-theless sulfide ores are an important source of sulfur in other forms, such as sul-fur dioxide and sulfuric acid.

Some of the most important metal sulfides are pyrite [1309-36-0],FeS2; chalcopyrite [1308-56-1], CuFeS2; pyrrhotite [1310-50-5], Fen�1Sn; sphaler-ite [12169-28-7], ZnS; galena [12179-39-4], PbS; arsenopyrite [1303-18-0],FeS2�FeAs2; and pentlandite [53809-86-2], (Fe,Ni)9S8. Sulfide deposits oftenoccur in massive lenses, but may occur in tabular shape, in veins, or in a disse-minated state. The deposits may be of igneous, metamorphic, or sedimentaryorigin.

Pyrite is the most abundant of the metal sulfides. For many years, until theFrasch process was developed, pyrite was the main source of sulfur and, for muchof the first half of the twentieth century, comprised over 50% of world sulfur pro-duction. Pyrite reserves are distributed throughout the world and known depos-its have been mined in about 30 countries. Possibly the largest pyrite reserves inthe world are located in southern Spain, Portugal, and the CIS. Large depositsare also in Canada, Cyprus, Finland, Italy, Japan, Norway, South Africa, Swe-den, Turkey, the United States, and Yugoslavia. However, the three main regio-nal producers of pyrites continue to be Western Europe; Eastern Europe,including the CIS; and China.

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Pyrites production is the main source of sulfuric acid for both fertilizer andnonfertilizer uses in China and has been increasing steadily. However, produc-tion has been declining steadily in all other regions. This trend seems likely tocontinue.

In the 1980s, pyrites, as a percentage of total sulfur produced, comprisednearly 18%; in the 1990s, however, pyrites comprised only 13% and fell to around7% in 2004. China was the world’s leading producer of pyrites with 56.3% of itssulfur coming from this source (21).

4.2. Pyrometallurgical Processes. Orkla Process. A process forrecovering sulfur from cuprous pyrite was developed by the Orkla Mining Com-pany in Norway (25). The sulfur output of 80,000–100,000 t/yr furnished animportant part of European requirements until 1962, when the smelter wasshut down. The process was once used on a much smaller scale in Portugaland Spain and probably in the CIS (26). The Orkla process involves recoveryof about 80% of the sulfur contents of a pyritic copper ore by direct smelting inthe presence of a carbonaceous reducing agent.

4.3. Noranda Process. When pyrites are heated to about 5408C in theabsence of oxygen, about half of the sulfur content in the pyrites evolves in theelemental form. Noranda Mines Ltd. and Battelle Memorial Institute developeda process based on this property to recover elemental sulfur from pyrite (27). Thefirst commercial plant was built at Welland, Ontario, in 1954 but operated on anexperimental basis for only a few years before being closed for economic reasons.

4.4. Outokumpu Process. Outokumpu Base Metals Oy, Finland’s lar-gest mining and metallurgical company, discovered a complex ore body at Pyha-salmi, Finland, containing pyrite, sphalerite, chalcopyrite, barite, and smallamounts of pyrrhotite, arsenopyrite, and molybdenite. The ore can be benefi-ciated by flotation to obtain pyrite concentrate as well as copper and zinc concen-trates. A process was developed to treat the pyrite concentrate in a flash smelterfor recovery of elemental sulfur and iron cinder. The commercial smelter locatedat Kokkolla began operating in 1962 (28). In 1977, production of elemental sulfurwas stopped, although sulfur dioxide is still produced and sold for sulfuric acidproduction. Similarly, the Outokumpu process was used to recover elemental sul-fur at a plant in Botswana, but as of the 1990s, the sulfur is recovered asSO2–H2SO4.

4.5. Hydrometallurgical Processes. Recovery of sulfur in the proces-sing of nonferrous metal sulfides has been in the form of SO2 and/or H2SO4 whensmelter (pyrometallurgical) operations are employed. However, there have beenaccounts of processes, mainly hydrometallurgical, in which sulfur is recovered inthe elemental form (see METALLURGY, EXTRACTIVE).

One, the CLEAR process, was investigated by Duval Corporation near Tuc-son, Arizona (29). It involves leaching copper concentrated with a metal chloridesolution, separation of the copper by electrolysis, and regeneration of the leachsolution in a continuous process carried out in a closed system. Elemental sulfuris recovered. Not far from the Duval plant, Cyprus Mines Corporation operated aprocess known as Cymet. Sulfide concentrates undergo a two-step chloride solu-tion leaching and are crystallized to obtain cuprous chloride crystals. Elementalsulfur is removed during this stage of the process.

Vol. 23 SULFUR 11

Another process, which also generates elemental sulfur as a by-product, hasbeen patented by Envirotech Research Center in Salt Lake City (29). In the Elec-troslurry process, a ball mill finely grinds a chalcopyrite concentrate, whichreacts with an acidic copper sulfate solution for iron removal. The liquor is elec-trolyzed and the iron is oxidized to the ferric form. This latter step leaches copperfrom the copper sulfide for deposition on the cathode. Elemental sulfur is recov-ered at the same time.

A pressure leaching system to handle copper sulfide called the Sherritt-Cominco (SC) copper process was developed by these two Canadian firms.Pilot-plant testing was completed in 1976 (29), but commercial application ofthis technology has not been achieved.

5. Sulfates

5.1. Occurrence. The largest untapped source of sulfur occurs in theocean as dissolved sulfates of calcium, magnesium, and potassium (see OCEAN

RAW MATERIALS). The average sulfur concentration in seawater is 880 ppm.Thus, 1 km3 of seawater contains about 0.86 � 106 t of elemental sulfur in theform of sulfates. Natural and by-product gypsum, CaSO4�2H2O, and anhydrite,CaSO4, rank second only to the oceans as potential sources of sulfur. Mineraldeposits of gypsum and anhydrite are widely distributed in extremely largequantities. Gypsum is a by-product waste material from several manufacturingprocesses; most notable is the waste gypsum produced in manufacturing phos-phoric acid from phosphate rock and sulfuric acid (see PHOSPHORIC ACID AND THE

PHOSPHATES).5.2. Extraction. Although many processes have been developed to

recover elemental sulfur from gypsum or anhydrite, high capital and operatingcosts have precluded widespread use of these processes and are expected to con-tinue to do so while less expensive sources remain available. Obtaining sulfurfrom gypsum processes has been attractive during periods when sulfur hasbeen in short supply and energy costs remained relatively low. Because theseprocesses require large amounts of energy when energy costs are high, sulfurextraction is unlikely to be competitive. However, gypsum and anhydrite remaineconomical sources of sulfur in other forms, including sulfuric acid, cement, andammonium sulfate, in areas such as India where sulfur must otherwise beimported.

5.3. Thermal Reduction of Gypsum. The initial work involving thethermochemical technique was carried out in Germany and later by the U.S.Bureau of Mines (USBM), which did research work on two processes for therecovery of elemental sulfur from gypsum at the Salt Lake City MetallurgyResearch Center in the late 1960s (30). Both processes involved reduction roast-ing of gypsum using coal or reducing gases at 900–9508C to produce calcium sul-fide. Process one involved carbonation of a water slurry of calcium sulfide withCO2-bearing flue gases from the reduction kiln to precipitate calcium carbonateand to evolve hydrogen sulfide. The latter could be converted to sulfur in a stan-dard Claus unit. Process two made use of a countercurrent ion-exchange systemand sodium chloride to produce by-product sodium carbonate and calcium chlor-

12 SULFUR Vol. 23

ide as well as elemental sulfur. Three metric tons each of the by-product wereproduced per metric ton of sulfur.

Neither of these processes has been commercialized, although some aspectsof the methodology were incorporated into a plant operated for a short time bythe Elcor Company (31). This company, which operated briefly in westernTexas in 1968 using natural gypsum, is the only one known to have commerciallyattempted to recover elemental sulfur from this material by a two-step thermalprocess. The Elcor plant was shut down shortly after it began operation.Although most technical problems were said to have been solved, productioncosts were prohibitive.

5.4. Phosphogypsum. Phosphogypsum is produced in tremendousquantities in the manufacture of phosphate fertilizers (qv). A process used byFertilizer India, Ltd. (Planning and Development) involved a shaft kiln (31). Fol-lowing bench-scale tests, tests on a larger scale were conducted at the RegionalResearch Laboratory at Jorhat (Assam Province). The feed to the top of the kilnconsisted of a nodulized mixture of phosphogypsum, pulverized coke, and clayadditives. Air was introduced to the bottom of the kiln such that the temperaturein the hottest zone was maintained at 1100–12008C. Under these conditions, cal-cium sulfate is reduced to sulfur dioxide, which then reacts to yield elementalsulfur. Although the process was technically feasible, it was found to be uneco-nomical. No commercial process existed for economical sulfur recovery fromphosphogypsum.

5.5. Bacteriological Sulfur. Anaerobic, sulfate-reducing bacteria burnhydrocarbons as a source of energy, but combine sulfur instead of oxygen withthe hydrogen to form hydrogen sulfide. Several experimenters have tried to uti-lize this knowledge in a controlled process for producing sulfur from gypsum oranhydrite (32). This process requires a strain of sulfate-reducing bacteria, anorganic substrate whose hydrocarbons provide food for the bacteria, and closecontrol of environmental conditions in order to obtain maximum sulfur yields.

Finely ground gypsum is fed into a stirred reaction tank containing theorganic substrate and the bacteria. The substrate can be a petroleum fraction,although sewage, spent sulfite liquor, molasses, or brewery waste can also beused. The advantage of a petroleum-based substrate is that its composition canbe more closely controlled. Air must be excluded from the system because thebacteria are anaerobic. A hydrogen-purging system keeps air out and at thesame time promotes fermentation.

Carbon dioxide generated by the fermentation process must be removed tohelp maintain the pH of the solution at pH 7.6–8.0. Carbon dioxide also inhibitsthe activity of the bacteria. The oxidation reduction potential is kept at 100–200mV. The ideal temperature in the reactor varies with different strains in the bac-teria but generally is 25–358C.

As the reaction proceeds, a part of the mix is continuously withdrawn fromthe tank and is centrifuged, and the solids removed by centrifuging are resus-pended in the reactor. Filtrate from the centrifuge goes to a stripping towerfor removal of dissolved carbon dioxide and hydrogen sulfide, which is combinedwith the carbon dioxide and hydrogen sulfide gases passing from the top of thereactor. The combined gases are passed through a scrubbing tower for removal ofthe carbon dioxide and recovery of the hydrogen sulfide, which is fed to a conven-

Vol. 23 SULFUR 13

tional recovery unit for conversion to elemental sulfur. There is also the possibi-lity of recovering other organic co-products, such as vitamins (qv) and steroids(qv). The rate at which bacteria reduce gypsum to hydrogen sulfide is quiteslow, necessitating many large reaction tanks. A 300-t/d plant is estimated toneed 10 3785-m3 (106-gal) reactor tanks. An organism of the Desulfovibriogenus has been used to make hydrogen sulfide in early experiments. Subcultur-ing and selectively reisolating active organisms could lead to a strain with higheractivity. By such techniques, a 1000-fold increase in activity after 40 generationswas achieved in the 1960s (32) (see GENETIC ENGINEERING, MICROBES).

6. Production

Sulfur is produced from a variety of sources using many different techniques inmany countries around the world. Worldwide changes have affected not only thesources of sulfur, but also the amounts consumed. Sulfur sources in the UnitedStates underwent significant changes during the 1980s. Voluntary sulfur fromthe Frasch process (mines) supplied 25% of the sulfur in the United States in1995 and none was supplied by the process in 2004. Whereas recovered or invo-luntary sulfur supplied 63% of the sulfur in the United States in 1995, in 2004,it supplied over 92%. About 8% is supplied from other forms, primarily bymetallurgy (21,33).

Recovered elemental sulfur, a nondiscretionary by-product from petroleum(qv) refining, natural gas processing (see GAS, NATURAL), and coking plants, wasproduced primarily to comply with environmental regulations that were applieddirectly to emissions from the processing facility or indirectly by restricting thesulfur content of the fuels sold or used by the facility. Table 2 shows the esti-mated annual world sulfur production capacity in all forms. Recovered elementalsulfur was produced by 59 companies at 150 plants in 26 states, one plant inPuerto Rico, and one plant in the U.S. Virgin Islands. Most of these plantswere relatively small, with only 22 reporting an annual production exceeding100,000 metric tons. By source, 52% was produced at three coking plants and86 refineries or satellite plants treating refinery gases. The remainder was pro-duced by 27 companies at 61 natural gas treatment plants.

7. Economic Aspects

Sulfur is one of the chemical industry’s most important raw materials. It is usedprincipally as the derivative (sulfuric acid) in many chemical and industrial pro-cesses and is particularly important in the manufacture of phosphate fertilizers,the single largest end use for sulfur.

In 2005, elemental sulfur and byproduct sulfuric acid were produced at 115operations in 29 States and the U.S. Virgin Islands. Total shipments were valuedat about $400 � 106. Elemental sulfur production was 8.8 � 106 tons; Louisianaand Texas accounted for about 45% of U.S. production. Elemental sulfur wasrecovered at petroleum refineries, natural-gas-processing plants, and cokingplants by 38 companies at 109 plants in 26 States and the U.S. Virgin Islands.

14 SULFUR Vol. 23

Byproduct sulfuric acid, representing about 8% of production of sulfur in allforms, was recovered at six nonferrous smelters in five States by six companies.Domestic elemental sulfur provided 66% of domestic consumption, and bypro-duct acid accounted for 6%. The remaining 28% of sulfur consumed was providedby imported sulfur and sulfuric acid (34). United States statistics are listed inTable 3.

World sulfur production in 2004 was approximately 68 � 106 metric tons,with an estimated fob value of about $1.6 � 109. World sulfur production (andapparent consumption) was about 61 � 106 metric tons in 1989 and declinedby almost 14% to a level of approximately 53 � 106 metric tons in 1993. In thepast five years, it has been on the increase. It is expected that over half of theworld’s production of elemental sulfur in coming years will come from gas proces-sing. A moderate increase of about 15% with an annual growth rate of about 2.8%is expected during the forecast period. The supply/demand situation is projectedto be relatively tight for the next couple of years with remelts from inventoriesutilized to supply requirements. By 2007, sulfur from oil refining operations andgas processing operations should add considerable inventory levels. The supplywill be even higher if product from Kazakhstan and Qatar is not reinjected asplanned. On the demand side, nonfertilizer use of sulfur is on the increasewith sulfur-based asphalt and concrete gaining significance. World sulfur pro-duction and consumption are projected to exceed the historical high during theforecast period (35).

The United States continued to be a net importer of sulfur in 2004. Importsof elemental sulfur exceeded exports by almost 2 Mt. Recovered elemental sulfurfrom Canada and Mexico delivered to U.S. terminals and consumers in the liquidphase furnished about 89.6% of all U.S. sulfur import requirements. Total ele-mental sulfur imports were slightly lower in quantity, but higher prices resultedin the value being 8.8% higher than it was in 2003. Imports from Canada, mostlyby rail, were 3.4% lower in quantity, and waterborne shipments from Mexicowere slightly higher than those of 2003. Imports from Venezuela were estimatedto account for about 10.4% of all imported elemental sulfur.

In addition to elemental sulfur, the United States also had significant tradein sulfuric acid. Sulfuric acid exports were slightly lower than those of 2003. Sul-furic acid imports were 11.8 times that of exports. Canada and Mexico were thesources of 89.0% of U.S. sulfuric acid imports, most of which were probablybyproduct acid from smelters. Canadian and some Mexican shipments to theUnited States came by rail, and the remainder of imports came primarily byship from Europe. The tonnage of sulfuric acid imports was 2.64 times that of2003, and the value of imported sulfuric acid increased in proportion. Althoughstill a minor portion of sulfur imports, additional imported sulfuric acid wasrequired to meet the increased demand for sulfur in all forms. The most dramaticincrease was in imports from Canada.

Table 4 gives data on sulfur and sulfuric acid sold in the United States byend use.

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8. Sulfur Terminology

Many terms are used to describe the commercial forms of sulfur. The most com-mon of these terms, along with brief descriptions and typical uses or references,are as follows.

Term Description and use

amorphous sulfur see insoluble sulfurbright sulfur crude sulfur free of discoloring impurities; bright

yellowbrimstone see crude sulfurbroken rock sulfur sulfur broken and sold as a mixture of lumps and

fines; see refined sulfurbroken sulfur solid crude sulfur crushed to <2.38 mm (�8 mesh)colloidal sulfur finely divided sulfur typically suspended in water; can

be prepared by physical or chemical means; uses aremainly pharmaceutical

crude sulfur commercial nomenclature for elemental sulfur; maybe bright or dark but is free of arsenic, selenium, andtellurium

dark sulfur crude sulfur discolored by minor quantities of hydro-carbons having up to 0.3 wt % carbon content

dusting sulfur finely divided crude or refined sulfur prepared forpesticidal uses

elemental sulfur processed sulfur in the elemental form produced fromnative sulfur or combined sulfur sources, generallyhaving a minimum sulfur content of 99.5 wt %

flour sulfur crude sulfur ground through 50–74 mm sieves (300–200 mesh), depending on the brand; used in rubbervulcanization, dyes, gun powder, agricultural dusts,dusting and wettable sulfur

flowable sulfur synonymous with colloidal sulfur but used more fre-quently to describe agricultural sulfur; uses aremainly pesticidal

flowers of sulfur(sublimed sulfur)

powdered form of sulfur produced by sublimation;may contain up to 30% of the amorphous allotrope;used in rubber vulcanization, agricultural dusts,pharmaceutical products, stock feeds

formed sulfur sulfur formed to a specific shape, such as prills,granules, pellets, pastilles, or flakes; see prilled sulfur

Frasch sulfur elemental sulfur produced from native sulfur sourcesby the Frasch mining process

ground sulfur solid sulfur ground into different physical size parti-cles to serve various applications; may be combinedwith additives for special properties such as reduceddusting or improved dispersion

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insoluble sulfur(Crystex)

produced by extracting flowers of sulfur with CS2 orby fast quenching liquid or vaporized sulfur andextracting with CS2 to remove soluble sulfur allo-tropes; used in rubber vulcanization, rubber cements,cutting oils, high pressure lubricants

lac sulfur precipitated from polysulfide solutions by sulfuricacid; contains up to 45 wt % calcium sulfated liquidsulfur; uses are almost entirely pharmaceutical

liquid sulfur see molten sulfurmolten sulfur crude sulfur in the liquid phasenative sulfur sulfur that occurs in nature in the elemental formprecipitated sulfur precipitated from polysulfide solution by hydrochloric

acid and washed to remove all calcium chloride; usesare almost entirely pharmaceutical

prilled sulfur solid crude sulfur in pellets; produced by coolingmolten sulfur with air or water

recovered sulfur elemental sulfur produced from combined sulfursources by any method

refined sulfur elemental sulfur produced by distilling crude sulfur;purity not less than 99.8%; when burned in smallquantities, refined sulfur provides sulfur dioxide forfumigation, sugar and starch refining, preserving andbleaching

roll sulfur refined sulfur cast into convenient sizes; uses includechemical manufacturing, burned for curing, fumigat-ing, and preserving or bleaching effects; see refinedsulfur

rubbermaker’s sulfur ground sulfur of various fineness having special spe-cifications for low acid ash, and moisture contents

run-of-mine sulfur mined by the hot-water process; solid shipments maycontain 50 wt % fines; lump diameters up to 20 cm ormore

screened commercialsulfur

run-of-mine sulfur having particle size determined byscreening, generally 9.5 and 1.68 mm (2 and 12 mesh),plus associated fines

slated sulfur solid crude sulfur in the form of slate-like lumps;produced by allowing molten sulfur to solidify on amoving belt

specialty sulfur prepared or refined grades of elemental sulfur thatinclude amorphous, colloidal, flower, precipitated,wettable, or paste sulfur

spray sulfur finely divided sulfur combined with various wettingagents and water; prepared for pesticidal uses; seewettable sulfur

wettable sulfur powdered sulfur that has been treated for easy dis-persion in water

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9. Analytical Methods

Elemental sulfur in either its ore or its refined state can generally be recognizedby its characteristic yellow color or by the generation of sulfur dioxide when it isburned in air. Its presence in an elemental state or in a compound can bedetected by heating the material with sodium carbonate and rubbing the fusedproduct on a wet piece of silver metal. A black discoloration of the silver indicatesthe presence of sulfur. The test is quite sensitive. Several other methods fordetecting small amounts of elemental sulfur have also been developed (36).

Quantitatively, sulfur in a free or combined state is generally determinedby oxidizing it to a soluble sulfate, by fusion with an alkali carbonate if neces-sary, and precipitating it as insoluble barium sulfate. Oxidation can be effectedwith such agents as concentrated or fuming nitric acid, bromine, sodium perox-ide, potassium nitrate, or potassium chlorate. Free sulfur is normally determinedby solution in carbon disulfide, the latter being distilled from the extract. Thismethod is not useful if the sample contains polymeric sulfur.

Generally, crude sulfur contains small percentages of carbonaceous matter.The amount of this impurity is usually determined by combustion, whichrequires an exacting technique. The carbonaceous matter is oxidized to carbondioxide and water; the carbon dioxide is subsequently absorbed (18). Automated,on-stream determination of impurities in molten sulfur has been accomplishedby infrared spectrophotometry (37).

The moisture content of crude sulfur is determined by the differentialweight of a known sample before and after drying at about 1108C. Acid contentis determined by volumetric titration with a standard base. Nonvolatile impuri-ties or ash are determined by burning the sulfur from a known sample and ignit-ing the residue to remove the residual carbon and other volatiles.

The National Safety Council, National Fire Protection Association, andother similar organizations publish technical information that describes generalsafety practices for use during the testing, handling, storage, and transport ofsulfur (21,38–42). Each of these publications include a list of references for addi-tional health and safety information.

10. Environmental Concerns

Increasing environmental concerns and subsequent governmental regulationshave had a large impact on the sulfur industry. Even before the U.S. CleanAir Act was enacted in 1977, oil refineries and natural-gas processors realizedthe necessity of removing sulfur from both upstream products and off-gases.The USBM began collecting data on recovered sulfur in 1938. Although refer-ences were made to this type of sulfur prior to that time, no official data are avail-able to quantify recovery. At first, recovered sulfur was considered a wastematerial, not a commercial by-product. As time went on, the importance of recov-ered sulfur increased as sulfur demand increased faster than the supply ofFrasch and other native sulfur. Recovered sulfur became the primary domesticsource of elemental sulfur in 1982.

18 SULFUR Vol. 23

The U.S. Clean Air Act set limits on the quantity of pollutants that could bereleased into the atmosphere. Sulfur dioxide was identified as one of the mostcommon pollutants and also one of the principal contributors of acid rain,known to damage both natural and artificial environments. Gas-cleaning appa-ratus removed as much sulfur dioxide as was technologically possible from off-gases. Petroleum refiners and gas processors have recovered increasingly greaterpercentages of sulfur from the gas stream as elemental sulfur; metal smelters(especially nonferrous metals) have recovered by-product sulfuric acid; andcoal and oil burning electric power plants have recovered by-product gypsum(calcium sulfate). Each industry has cut emissions dramatically. The type ofby-product recovered is determined by the sulfur content of the gas stream; ele-mental sulfur is recovered when the sulfur dioxide content was relatively high.

The 1990 Amendments to the U.S. Clean Air Act required a 50% reductionof sulfur dioxide emissions by the year 2000. Electric power stations are believedto be the source of 70% of all sulfur dioxide emissions (see POWER GENERATION).

Existing technology to recover elemental sulfur from power plant off-gaseshas a cost estimated to be 50% higher than the cost of recovering by-product gyp-sum, much of which is disposed as waste. As landfill costs become higher, ele-mental sulfur recovery is expected to become a more attractive alternative toby-product gypsum production.

Over the years, larger quantities of sulfur have been recovered for a num-ber of reasons. These include increased petroleum refining and natural-gas pro-cessing, more stringent limitations on sulfur dioxide emissions, and higher sulfurcontents of the crude oil refined. Another contributing factor is the lower sulfurcontent limits set on petroleum-based fuels.

Because sulfur supplies, either as elemental sulfur or by-product sulfuricacid, have grown owing to increased environmental awareness, demand for sul-fur has decreased in some consuming industries for the same reason. Industriessuch as titanium dioxide productions, which traditionally utilized sulfuric acid,have concerted to more environmentally friendly processes. In addition, manyconsumers who continue to use sulfuric acid are putting an emphasis on regen-erating or recycling spent acid.

Another area where improved air quality has impacted on sulfur use is inagriculture. As sulfur dioxide emissions have decreased, sulfur content of soilshas also decreased. Sulfur, recognized as the fourth most important plant nutri-ent, is necessary for the most efficient use of other nutrients and optimum plantgrowth.

Because many soils are becoming sulfur-deficient, a demand for sulfur-containing fertilizers has been created. Farmers must therefore apply a nutrientthat previously was freely available through atmospheric deposition and lowgrade fertilizers.

Environmental regulations governing the desulfurization of transportationfuels have also resulted in significant quantities of sulfur being recovered fromrefinery operations and will continue to lead to increased recovery of byproductsulfur. For many years, sulfuric acid recovered at nonferrous smelters has beenthe main concern. Of greater significance today is the recovery of elemental sul-fur from crude oil refineries as the sulfur content of crudes has increased and asregulations on the sulfur content of oil and gasoline products have become

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increasingly stringent. Ultralow-sulfur diesel fuel will soon be the major formsold (35).

One of the few unregulated sources of human pollution is marine shipping.Marine pollution has thus far been left to shippers under the InternationalMarine Organization’s (IMO) Marine Pollution Treaty (MARPOL). An amend-ment to MARPOL that was ratified in October 2004 limited sulfur to 4.5% forships in general operation beginning May 19, 2005. The allowable sulfur contentof fuel burned in ships operating in the most crowded European waters [the NorthSea, the English Channel, and the Baltic Sea, collectively known as the SOx emis-sion control area (SECA)] was reduced to 1.5% effective May 19, 2006 (43).

The European environmental ministers’ final proposal for the sulfur con-tent in fuels for vessels sailing in European waters was a limit of 1.5% sulfurfor all vessels in the SECA starting in 2007. The 1.5% limit applies to passengervessels between ports within the European Union (EU) and the Baltic Sea begin-ning in May 2006 and the North Sea and the English Channel by 2007. Ships atberth in EU ports will be required to burn 0.1% sulfur fuel starting in January2010 (44).

11. Uses

Sulfur is unusual compared to most large mineral commodities in that the lar-gest portion of sulfur is used as a chemical reagent rather than as a componentof a finished product. Its predominant use as a process chemical generallyrequires that it first be converted to an intermediate chemical product priorto use in industry. In most of the ensuing chemical reactions between thesesulfur-containing intermediate products and other minerals and chemicals, thesulfur values are not retained. Rather, the sulfur values are most often discardedas a component of the waste product.

Sulfuric acid is the most important sulfur-containing intermediate product.In 2005, 62% of U.S. sulfur demand was for sulfuric acid, to be used in fertilizerproduction: 29% was used in pertroleum refining, 3% was used in metal mining;6% was used for a multitude of other minor uses (34).

11.1. Agriculture. Sulfur is one of the elements essential for plantgrowth. Its functions within the plant are related closely to those of nitrogenand the two nutrients are synergistic. Sulfur is required for plant growth inquantities equal to, and sometimes exceeding, those of phosphorus. Sulfur hasa variety of vital functions within the plant’s biochemistry. It is a principal con-stituent of amino acids (qv) such as cysteine and methionine. It is also essentialin the formation of enzymes, vitamins (qv) such as biotin and thiamine, anda variety of other important compounds in the plant, including chlorophyll.When sulfur is deficient, both plant yield and quality suffer. Plants that aresulfur-deficient are characteristically small and spindly. The younger leavesare light-green to yellowish, and in the case of legumes, nodulation of the rootsis reduced. The oil content of seeds is diminished and the maturity of fruits isdelayed in the absence of adequate sulfur.

Plant nutrient sulfur has been growing in importance worldwide as foodproduction trends increase while overall incidental sulfur inputs diminish.

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Increasing crop production, reduced sulfur dioxide emissions, and shifts in ferti-lizer sources have led to a global increase of crop nutritional sulfur deficiencies.Despite the vital role of sulfur in crop nutrition, most of the growth in world fer-tilizer consumption has been in sulfur-free nitrogen and phosphorus fertilizers(see FERTILIZERS).

Agriculture is the largest industry for sulfur consumption. Historically, theproduction of phosphate fertilizers has driven the sulfur market. Phosphate fer-tilizers account for approximately 60% of the sulfur consumed globally. Thus,although sulfur is an important plant nutrient in itself, its greatest use in thefertilizer industry is as sulfuric acid, which is needed to break down the chemicaland physical structure of phosphate rock to make the phosphate content moreavailable to plant life. Other mineral acids, as well as high temperatures, alsohave the ability to achieve this result. Because of market price and availability,sulfuric acid is the most economic method. About 90% of sulfur used in the fer-tilizer industry is for the production of phosphate fertilizers. Based on this tech-nology, the phosphate fertilizer industry is expected to continue to depend onsulfur and sulfuric acid as a raw material.

Another fertilizer use is in the production of ammonium sulfate by reactionof sulfuric acid and ammonia. Some of this production is deliberate, but thegreater portion is by-product material resulting from coke-oven operations, syn-thetic fiber manufacture, and, to a lesser extent, utility stack-gas scrubbing (seeAMMONIUM COMPOUNDS). Furthermore, a small amount of sulfuric acid is used,mainly in Europe, to produce potassium sulfate using the Mannheim process.Finally, sulfur has been safely used for centuries in agriculture as a natural fun-gicide and pesticide, and as an amendment to ameliorate soils and irrigationwaters (see FUNGICIDES, AGRICULTURAL; PESTICIDES; SOIL CHEMISTRY OF PESTICIDES).

11.2. Petroleum Refining. Petroleum refining includes not only therefining of petroleum but associated chemical processes where process streamsmay serve both the refinery and the chemical complex. About 60% of the sulfuricacid used in petroleum refining is returned as spent acid for reclaiming; there-fore, the demand for new sulfuric acid is about 3% of the total sulfur demand.The principal use for sulfuric acid is as a catalyst for alkylation (qv), a processby which liquid high octane gasoline components having very good stabilityare produced from a combination of gaseous streams. Sulfuric acid and hydro-fluoric acid are competing catalysts in this process. Sulfuric acid for refinery pro-cesses is manufactured from recovered sulfur produced at the refinery and fromcontaminated acid (acid sludge) returned to the acid plants for reconstitution(see PETROLEUM). A process for the production of low sulfur, low olefin gasolinehas been reported (45).

11.3. Nonferrous Metal Production. In the case of copper, sulfuricacid is used for the extraction of the metal from deposits, mine dumps, andwastes, in which the copper contents are too low to justify concentration by con-ventional flotation techniques or the recovery of copper from ores containing cop-per carbonate and silicate minerals that cannot be readily treated by flotation(qv) processes. The sulfuric acid required for copper leaching is usually thebyproduct acid produced by copper smelters (see METALLURGY, EXTRACTIVE; MINER-

ALS RECOVERY AND PROCESSING).

Vol. 23 SULFUR 21

Sulfuric acid is the most commonly used reagent for the recovery of ura-nium from ores, and vanadium is often recovered as a coproduct. The sulfuricacid used is either the byproduct sulfuric acid produced at smelters or sulfuricacid produced from elemental sulfur.

11.4. Iron and Steel Production. Consumption of sulfur in the iron(qv) and steel (qv) industry is in the form of sulfuric acid. The sulfuric acid isused as a pickling agent to removal mill scale, rust, dirt, and grease from the sur-face of steel products prior to further processing (see METAL SURFACE TREATMENTS).The sulfuric acid pickling process faces increasing competition from hydrochloricacid pickling, largely because of the problem of disposing of the ferrous sulfatewaste product. Hydrochloric acid is expected to replace sulfuric acid for picklingover the long term. There are no well-defined sources of sulfuric acid for steelpickling. Rather, it is generally obtained from merchant sulfuric acid plantsthat use the cheapest form of sulfur available in the area where it is produced.

11.5. Plastics and Other Synthetic Products. Sulfur is used in theproduction of a wide range of synthetics, including cellulose acetate, cellophane,rayon, viscose products, fibers, and textiles. Sulfur intermediates for these man-ufacturing processes are equally divided between carbon disulfide and sulfuricacid.

11.6. Paper Products. The largest single segment of demand is in themanufacture of wood pulp by the sulfite process (see PULP). In this process, themain sulfur intermediate is sulfur dioxide, which is generally produced at theplant site by burning elemental sulfur. Some sulfur dioxide, however, is pro-duced as a by-product at smelter operations, purified and liquefied, and shippedto the pulp mills. The sulfur dioxide is converted to sulfurous acid, and the salt ofthis acid is a principal component of the cooking liquor for the sulfite process.

11.7. Paints. The main sulfur use is for the production of titanium diox-ide pigment by the sulfate process. Sulfuric acid reacts with ilmenite or titaniumslag and the sulfur remains as a ferrous sulfate waste product. Difficulties withthis process have led to the development of the chloride process (see PIGMENTS,

INORGANIC; TITANIUM COMPOUNDS).11.8. Soil and Water Treatment. Agricultural soils and waters in the

irrigated, arid regions of the world frequently benefit by the application of sulfurand its compounds. These benefits result from improvements in water penetra-tion and movement and from increased availability of phosphorus and certainmicronutrients which are otherwise unavailable owing to high soil pH. Thereare extensive areas of such soils in the western provinces of Canada and inmany areas west of the Mississippi River in the United States. Throughout theother parts of the world, large tracts of arid, irrigated soils exist, notably in theMiddle East, northern Africa, Australia, and many places in both Eastern andWestern Europe (46,47).

11.9. Animal Nutrition. Sulfur in the diets of ruminant animals is ben-eficial to the animals’ growth (see FEEDS AND FEED ADDITIVES). Sulfur increases feedintake, cellulose and dry matter digestion, and the synthesis of microbial protein.This results in increased meat, milk, and wool production (48). A novel sulfur-containing additive for animal feed from fermentation liquor has been reported(49). The special uses for sulfur in agriculture demonstrate a significant and con-tinuing need for increased use of sulfur (50).

22 SULFUR Vol. 23

11.10. Highway Construction. The preparation and use of sulfur–asphalt (SA) paving materials have been reviewed (51,52). In the 1930s, asphalt(qv) was easily available and priced lower than sulfur. This is no longer the case.There are four different types of sulfur paving materials.

During 1974–1985, about 200 sulfur–asphalt roads were constructedworldwide, half of which were in the United States. All U.S. SEA experimentalsections designed and constructed according to standard practices using stan-dard materials are performing as well as the control sections of conventionalasphalt in these experimental projects.

The physical properties of elemental sulfur can be modified by its reactionwith various organic and inorganic compounds. Many of the resulting sulfur pro-ducts tend to have properties similar to paving asphalt (53,54).

Sand–Asphalt–Sulfur. Sulfur can be utilized as a means of upgradingpoorly graded aggregates. Sand–asphalt–sulfur (SAS) does not provide anappreciable reduction of asphalt content of the paving material, but the utiliza-tion of sand rather than crushed stone is economically important. This technol-ogy was developed by Shell Canada Resources, Ltd., under the trade nameThermopave, which is a high quality paving material (55–58). After being testedin various provinces of Canada, SAS roads were constructed in the U.S.

Recycling of Commercial Asphalt Materials. For many years, repair andmaintenance of roads and highways has consisted of putting a layer of asphaltconcrete over the existing pavement. In many areas, the road level is at the max-imum height for proper drainage and to satisfy safety considerations. As a result,these roads must have exposed surfaces removed before additional paving cantake place. The increasing cost of both asphalt and aggregate has presented agreater incentive to reuse rather than to discard the removed material.

Many existing roads fail because the asphalt becomes stiff and brittle. If thematerials are too stiff, additives that lower the viscosity must be used. The fea-sibility of using sulfur to soften or reduce the viscosity of the oxidized binder inrecycled pavements has been successfully demonstrated by the U.S. Bureau ofMines and others (59–61).

Sulfur Concrete. Sulfur concrete (SC) is a mixture of sulfur and fine andcoarse aggregates. These materials are heated to about 1408C, placed, and thenallowed to cool and solidify into a rigid, concrete-like material. Concretes pre-pared with sulfur as the binder have mechanical properties comparable to port-land cement concretes. Methods of preparing sulfur concretes that are notsubject to rapid deterioration have been developed and evaluated in Austria,Canada, Denmark, Poland, and the United States. A modified sulfur bindercalled sulfur polymer cement (SPC), which contains 3–5 wt % organic material,has been developed for mixing with aggregates and the resulting SC productsmaintain substantially all of the initial strength gain, even after 300 freeze–thaw cycles (53).

Sulfur concretes are used in many specialty areas where Portland cementconcretes are not completely satisfactory. Because SC can be formulated to resistdeterioration and failure from mineral acid and salt solutions, it is used for con-struction of tanks, electrolytic cells, thickeners, industrial flooring, pipe, andothers.

Vol. 23 SULFUR 23

Sulfur Polymer Cement. SPC has been proven effective in reducing leachrates of reactive heavy metals to the extent that some wastes can be managedsolely as low level waste (LLW). When SPC is combined with mercury andlead oxides (both toxic metals), it interacts chemically to form mercury sulfide,HgS, and lead sulfide, PbS, both of which are insoluble in water. A dried sulfurresidue from petroleum refining that contained 600-ppm vanadium (a carcino-gen) was chemically modified using dicyclopentadiene and oligomer of cyclopen-tadiene and used to make SC (62). This material was examined by the CaliforniaDepartment of Health Services (Cal EPA) and the leachable level of vanadiumhad been reduced to 8.3 ppm, well below the soluble threshold limit concentra-tion of 24 ppm (63).

Sulfur polymer cement shows promise as an encapsulation and stabilizationagent for use with low level radioactive and mixed wastes. Use of SPC allowsaccommodation of larger percentages of waste than PCC. SPC-treated wasteforms have met requirements of both the Nuclear Regulatory Commission(NRC) and the Environmental Protection Agency (EPA).

11.11. Coatings. Sulfur coating formulations have been used on a con-crete warehouse in which potash is stored (62). Adhesion to this nonporous con-crete wall has been related to the amount and type of plasticizer. Glass and otherfibers, fillers, and fine aggregates affect strength and abrasion resistance but notadhesion. The use of a sulfur spray coating to stabilize a hilltop which was erod-ing and sliding onto a road and work area below has been described (64).Approximately 8100 m2 on top of the hill was spray-coated to stabilize the hilltop.The coating has prevented further erosion.

11.12. Mortar Substitute. In the early 1960s, the idea of using sulfurcoatings to replace mortar in construction was advanced (65). Bricks, blocks,and similar materials are stacked one on the other and, because the blocks aredry, can be moved and adjusted until the desired wall configuration is achieved.The blocks are thus bonded by applying the coating to the wall surfaces only. Thecoating mixture is applied at 120–1508C and contains 90–95 wt % sulfur andsmall percentages of fiber and additives. The coating solidifies almost immedi-ately to form a hard, impervious surface. In 1963, the first building of thistype was constructed at the Southwest Research Institute. The building hasbeen in continuous use and the coating has not presented any problems.

11.13. Foams. Sulfur can be foamed into a lightweight insulation thatcompares favorably with many organic foams and other insulating materialsused in construction. It has been evaluated as thermal insulation for highwaysand other applications to prevent frost damage (66) (see FOAMED PLASTICS; INSULA-

TION, THERMAL).11.14. Sulfur Impregnation. Impregnation of bonded, abrasive grind-

ing wheels using sulfur improves strength and provides both lubricating andcooling qualities during grinding operations. Sulfur-impregnated wheels arewell suited for grinding tough materials, including stainless steel, brass, bronze,and nickel. The impregnated wheels cut faster and prevent the welding of metalchips. In more difficult jobs, including gear grinding and surface grinding, thesulfur-impregnated wheels have four to eight times the life span of nonimpreg-nated wheels (67) (see ABRASIVES).

24 SULFUR Vol. 23

Sulfur can be used to impregnate ceramic tile. Sulfur impregnation reduceswater absorption and makes the tile frost-resistant when used on exteriorsurfaces, including floors, roofs, or entrances to buildings (68). Investigatorsat the Institute of Paper Chemistry have described the production of sulfur-impregnated paperboard for use in temporary housing and techniques for man-ufacturing sulfurized, corrugated container board suitable for self-sustainingshipping containers (69,70). Sulfurized container board is shock-resistant andmaintains its strength under hot, humid conditions which can cause normal box-board to lose over half of its strength (68) (see PAPER).

11.15. Other Uses. Other uses include intermediate chemical products.Overall, these uses account for 16% of sulfur consumption, largely in the form ofsulfuric acid but also some elemental sulfur that is used directly, as in rubbervulcanization. Sulfur is also converted to sulfur trioxide and thiosulfate for usein improving the efficiency of electrostatic precipitators and limestone/lime wetflue-gas desulfurization systems at power stations (71). These miscellaneoususes, especially those involving sulfuric acid, are intimately associated with prac-tically all elements of the industrial and chemical complexes worldwide.Highly efficient, cheap, and environmentally friendly S-Li batteries have beenreported (72).

BIBLIOGRAPHY

‘‘Sulfur’’ in ECT 1st ed., Vol. 13, pp. 358–373, by F. L. Jackson, E. C. Thaete, Jr., L. B.Gittinger, Jr., L. A. Nelson, Jr., and H. Blanchet, Freeport Sulphur Co.; in ECT 2nded., Vol. 19, pp. 337–366, by P. T. Comiskey, L. B. Citlinger, Jr., L. F. Good, F. L. Jackson,L. A. Nelson, Jr., and T. K. Wiewiorowski, Freeport Sulphur Co., and M. D. Barnes, Cove-nant College; in ECT 3rd ed., Vol. 22, pp. 78–106, by D. W. Bixby and H. L. Fike, TheSulphur Institute, J. E. Shelton, U.S. Bureau of Mines, and T. K. Wiewiorowski, FreeportMinerals Co.; in ECT 4th ed., Vol. 23, pp. 232–266, by The Sulphur Institute; ‘‘Sulfur’’ inECT (online), posting date: December 4, 2000, by The Sulphur Institute.

CITED PUBLICATIONS

1. P. W. Shenk and V. Thummler, Z. Elektrochem. 63, 1002 (1959).2. B. Meyer, Chem. Rev. 76, 367 (1976).3. T. K. Wiewiorowski and F. J. Touro, J. Phys. Chem. 70, 3528 (1966).4. M. Schmidt, Angew. Chem. 12, 445 (1973).5. B. Meyer, Sulfur, Energy, and Environment, Elsevier Science Publishing Co., Inc.,

New York, 1977.6. V. R. Steudel, Z. Anorg. Alig. Chem. 478, 139 (1981).7. V. R. Steudel and H. J. Mausle, Z. Anorg. Alig. Chem. 478, 156 (1981).8. H. J. Mausle and V. R. Steudel, Z. Anorg. Alig. Chem. 478, 177 (1981).9. C. Preuner and W. Schupp, Z. Phys. Chem. (Leipzig) 68, 129 (1909).

10. H. Braune, S. Peter, and V. Neveling, Z. Naturforsch Teil A 6, 32 (1951).11. J. Berkowitz and J. R. Marquart, J. Chem. Phys. 39, 275 (1963).12. W. A. West and A. W. C. Menzies, J. Phys. Chem. 33, 1880 (1929).13. G. Fouritier, Compt. Rend. 218, 194 (1944).

Vol. 23 SULFUR 25

14. K. Newmann, Z. Phys. Chem. A171, 416 (1934).15. R. Farelli, J. Am. Chem. Soc. 72, 4016 (1950).16. E. W. Washburn, ed., International Tables of the Numerical Data of Physics, Chem-

istry and Technology (ICT), Vols. 1–7, McGraw-Hill Book Co., Inc., New York, 1926–1930.

17. P. C. L. Thorne and A. M. Ward, Inorganic Chemistry, 3rd ed., Nordeman PublishingCo., Inc., New York, 1939, p. 6.

18. W. N. Tuller, ed., The Sulphur Data Book, McGraw-Hill Book Co., Inc., New York,1954.

19. J. W. Mellor, A Comprehensive Treatise on Inorganic and Theoretical Chemistry,Vol. 10, John Wiley & Sons, Inc., New York, 1930, p. 27.

20. A. Senning, Die Schwefelurg Organischer Verbindungen, Chemisches Institut derUniversitat Aarhus, Denmark (1971).

21. J. A. Ober, ‘‘Sulfur,’’ Minerals Yearbook, U.S. Geological Survey, Reston, Va., 2004.22. D. B. Mason, Ind. Eng. Chem. 30, 740 (1938).23. R. B. Thompson and D. MacAskill, Chem. Eng. Prog. 51, 3691 (1955).24. T. P. Forbath, Trans. AIME 196, 811 (1953).25. T. Kaier, Eng. Min. J. 155(7), 88 (1954).26. H. R. Potts and E. G. Lawford, Trans. AIME 58, 1 (1949).27. Eng. Min. J. 155(9), 142 (1954).28. G. O. Argall, Jr., World Min. 18 (Mar. 1967).29. Chem. Eng. 31 (Dec. 5, 1980).30. Sulphur 80 (1969).31. Sulphur 147 (1980).32. Chem. Eng. News 21 (Mar. 20, 1967).33. Sulphur Outlook, The Sulphur Institute, Washington, D.C., 1995 and 1996.34. J. A. Ober, ‘‘Sulfur,’’ Mineral Commodity Summaries, U.S. Geological Survey,

Reston, Va., Jan. 2006.35. B. Suresh, ‘‘Sulfur,’’ Chemical Economics Handbook, SRI Consulting, Menlo Park,

Calif., March 2006.36. F. Feigl, Spot Tests in Inorganic Analysis, Elsevier Science Publishing Co., Inc.,

New York, 1958, pp. 372–375.37. R. F. Matson, T. K. Wiewiorowski, and D. E. Schof, Jr., Chem. Eng. Prog. 61, 9, 67

(1965).38. Handling and Storage of Solid Sulfur, Data Sheet 612, National Safety Council,

1991.39. Properties and Essential Information for Safe Handling and Use of Sulfur, Chemical

Safety Data Sheet SD-74, Manufacturing Chemists Association (CMA), 1959.40. J. D. Beaton, S. L. Tisdale, and J. Platou, Crop Response to Sulphur in North

America, Technical Bulletin 18, The Sulphur Institute, Washington, D.C., 1971.41. Handling Liquid Sulfur, Data Sheet 592, National Safety Council, 1993.42. Prevention of Sulfur Fires and Explosions, NFPA No. 655, National Fire Protection

Association, 1993.43. Fertilizer Week 18(26), 5 (Oct. 22, 2004).44. Sulphur (294), 13–14 (Sept./Oct. 2004).45. U.S. Pat. Appl. 20060086645 (Apr. 27, 2006), K. L. Rock, Y. G. Xiong, and A.

Judzis, Jr.46. S. L. Tisdale, Sulphur Inst. J. 6(1), 2 (1970).47. L. K. Stromberg and S. L. Tisdale, Treating Irrigated Arid-Land Soils with Acid-

Forming Sulphur Compounds, Technical Bulletin 24, The Sulphur Institute,Washington, D.C., 1979.

26 SULFUR Vol. 23

48. S. L. Tisdale, Sulphur in Forage Quality and Ruminant Nutrition, Technical Bulle-tin 22, The Sulphur Institute, Washington, D.C., 1977.

49. U.S. Pat. Appl. 20050271768 (Dec. 12, 2005), M. Buckholz and co-workers (to Degussa).50. J. D. Beaton and S. L. Tisdale, Potential Plant Nutrient Consumption in North

America, Technical Bulletin 16, The Sulphur Institute, Washington, D.C., 1969.51. H. L. Fike, Some Potential Applications of Sulphur, Sulphur Research Trends,

American Chemical Society, Washington, D.C., 1972.52. D. Y. Lee, Modification of Asphalt and Asphalt Paving Mixtures by Sulphur Addi-

tives, Interim Report, Engineering Research Institute, Iowa State University, Ames,Iowa, 1971.

53. W. C. McBee, T. A. Sullivan, and H. L. Fike, Sulfur Construction Materials, Bulletin678, U.S. Bureau of Mines, Washington, D.C., 1985.

54. H. J. Lentz and E. A. Harrigan, Laboratory Evaluation of Sulphlex-233: BinderProperties and Mix Design, FHWA/RD-80/146, U.S. GPO 1981-0-725-620/1163, 1980.

55. W. C. McBee, T. A. Sullivan, and B. W. Jong, Modified-Sulfur Cements for Use inConcretes, Flexible Pavings, Coatings, and Grouts, RI 8545, U.S. Bureau of Mines,Washington, D.C., 1981.

56. R. A. Burgess and I. Deme, in J. R. West, ed., Sulphur in Asphalt Paving Mixes,American Chemical Society, Washington, D.C., 1975.

57. I. Deme, Proceedings of International Road Federation World Meeting, Munich, Ger-many, 1973.

58. T. A. Sullivan, W. C. McBee, and K. L. Rasmussen, Studies of Sand–Sulphur–Asphalt Paving Materials, RI 8087, U.S. Bureau of Mines, Washington, D.C., 1975.

59. D. Saylak and co-workers, Beneficial Uses of Sulfur in Sulfur–Asphalt Pavements,American Chemical Society, Washington, D.C., 1975.

60. W. C. McBee, T. A. Sullivan, and D. Saylak, Recycling Old Asphalt Pavements withSulfur, ASTM STP622, American Society for Testing Materials, Philadelphia, Pa.,1978.

61. D. L. Strand, Rural Urban Roads 18(7), 42 (1980).62. J. S. Platou, ed., Sulphur Research and Development, The Sulphur Institute, Wa-

shington, D.C., 1981.63. Treatment Standards of Liquid Redox Waste in California, State of California De-

partment of Health Services, Toxic Substances Control Program, Alternative Tech-nology Division, June 1990; Sulphur Polymer Cement Concrete, Design andConstruction Manual, The Sulphur Institute, Washington, D.C., 1994.

64. J. E. Paulson and co-workers, Sulfur Composites as Protective Coatings and Con-struction Materials, American Chemical Society, Washington, D.C., 1978.

65. U.S. Pat. 3,306,000 (1967), M. D. Barnes (to Research Corp.).66. G. L. Woo and co-workers, Sulfur Foam and Commercial-Scale Field Application

Equipment, American Chemical Society, Washington, D.C., 1978, pp. 227–240.67. U.S. Pat. 3,341,355 (Sept. 12, 1967), T. P. Gallager (to Fuller-Merriam Co.).68. U.S. Pat. 3,208,190 (Sept. 25, 1965), M. J. Kakos and J. V. Fitzgerald (to Tile Council

of America).69. J. A. Van der Akker and W. A. Wink, Pap. Ind. Pap. World 30(2), 231 (1948).70. U.S. Pat. 2,568,349 (Sept. 18, 1951), R. C. McKee (to assigned).71. D. R. Owens and co-workers, Power (May 19, 1988).72. U.S. Pat. 7,029,796 (Apr. 18, 2006), S. S. Choi and co-workers (to Samsung SDI Co.,

Ltd.).

Vol. 23 SULFUR 27

GENERAL REFERENCES

E. P. Johnson, Chem. Eng. 4 (Nov. 2, 1981).Chem. Eng. News 20 (May 24, 1982).P. A. Gallagher, Proc. R. Dublin Soc. Ser. B 2(20), (1969).M. Braud, Sulphur Inst. J. 5(4), 3 (1969–1970).A. C. Ludwig, B. B. Cerhardt, and J. M. Dale, Material and Techniques for Improving the

Engineering Properties of Sulphur, FHWA/RD-80-023, U.S. GPO 1980-625-892/2032,1980.

H. L. Fike, Proceedings of Symposium on New Uses for Sulphur and Pyrites, Madrid,Spain, The Sulphur Institute, Washington, D.C., 1976, pp. 215–230.

Updated by Staff

28 SULFUR Vol. 23

Table

1.PhysicalConstants

ofSulfur

Valu

e

Pro

per

tyId

eal

Natu

ral

Ref

eren

ce

free

zin

gp

oin

tof

soli

dp

hase

,8C

rhom

bic

112.8

110.2

mon

ocli

nic

119.3

114.5

boi

lin

gp

oin

t,8C

444.6

12

den

sity

ofso

lid

ph

ase

,208C

,g/c

m3

rhom

bic

2.0

7m

onoc

lin

ic1.9

6am

orp

hou

s1.9

2d

ensi

tyof

liqu

id,g/c

m3

1258C

1.7

988

1308C

1.7

947

1408C

1.7

865

1508C

1.7

784

den

sity

ofvap

or,444.68C

an

d101.3

kP

a(¼

1atm

),g/L

3.6

4re

fract

ive

ind

ex,n

110

D1.9

29

vap

orp

ress

ure

,aP

inP

a,T

inK

rhom

bic

,20

–808C

logP¼

16.5

57�

5166/T

13

mon

ocli

nic

,96

–1168C

logP¼

16.2

57�

5082/T

14

liqu

id120

–3258C

logP¼

19.6

�0.0

062238T�

5405.1

/T12

325

–5508C

logP¼

12.3

256�

3268.2

/Tsu

rface

ten

sion

,m

N/m

(¼d

yn

/cm

)1208C

60.8

315

1508C

57.6

7cr

itic

al

tem

per

atu

re,8C

1040

crit

ical

pre

ssu

re,M

Pab

11.7

5cr

itic

al

vol

um

e,m

L/g

2.4

8sp

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ch

eat,C

p,J/(

kg�K

)c

rhom

bic

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Cp¼

468þ

0.8

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mon

ocli

nic

,�

4.5

to118.98C

Cp¼

465�

0.9

08T

liqu

id,Sl,

118.9

–444.68C

Cp¼

706�

0.6

5T

29

gas,

S,25

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Cp¼

709�

0.0

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3.5

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2

gas,

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Cp¼

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offu

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c

112.88C

Srh

om

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Sl(

l)49.8

118.98C

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l)38.5

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7.4

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5

50

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8.6

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5

78

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10�

5

98

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5

late

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cLd

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2008C

308.6

3008C

289.3

4008C

286.4

278.0

4208C

287.6

276.3

4408C

290.1

274.6

4608C

293.1

273.0

elec

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m�cm

16

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17

1108C

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4008C

8.3�

10

6

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1.5

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1.5

39

stan

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Table

1.ðC

ontinued

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30

Table 2. Sulfur: World Production in All Forms, by Country and Source, � 103 ta

Country and source 2000 2001 2002 2003 2004b

Australia, byproductmetallurgy 654 817 899 863 865petroleum 30 45 60 60 60Total 684 862 959 923 925

Canada, byproductmetallurgy 831 762 703 614 621natural gas, petroleum, tarsands

8,621 8,154 7,671 7,891 8,271

Total 9,452 8,916 8,374 8,505 8,892Chile, byproduct, metallurgy 1,100 1,160 1,275 1,430 1,510China

elemental 290 290 290 290 300pyrites 3,370 3,090 3,240 3,400 3,730byproduct, metallurgy 1,900 2,000 2,200 2,400 2,600

Total 5,560 5,380 5,730 6,090 6,630Finland

pyrites 260 270 359 341 336byproduct:

metallurgy 283 227 308 305 301petroleum 46 46 55 60 65Total 589 543 722 706 702

France, byproductnatural gas and petroleum 887 837 787 816 765unspecified 260 260 229 196 196Total 1,150 1,100 1,020 1,010 961

Germany, byproductpyrites 30 61byproduct:

metallurgy 618 684 754 701 591natural gas and petroleum 1,753 1,749 1,745 1,661 1,560Total 2,401 2,494 2,499 2,362 2,150

Indiapyrites 32 32 32 32 32byproduct:

metallurgy 359 458 458 539 539natural gas and petroleum 376 526 371 451 501Total 767 1,020 861 1,020 1,070

Iran, byproductmetallurgy 50 50 50 50 60natural gas and petroleum 963 880 1,200 1,310 1,400Total 1,010 930 1,250 1,360 1,460

Italy, byproductmetallurgy 203 203 142 127 113petroleum 490 540 560 565 575Total 693 743 702 692 688

Japanpyritesbyproduct:

metallurgy 1,384 1,319 1,326 1,281 1,263petroleum 2,072 2,424 1,865 1,951 1,890Total 3,456 3,743 3,191 3,232 3,150

Kazakhstan, byproductmetallurgy 300 310 260 325 325natural gas and petroleum 1,200 1,400 1,600 1,600 1,650Total 1,500 1,710 1,860 1,930 1,980

Vol. 23 SULFUR 31

Korea, Republic of, byproductmetallurgy 572 665 737 747 796petroleum 679 690 687 757 879Total 1,250 1,360 1,420 1,500 1,680

Kuwait, byproduct, natural gasand petroleum

512 524 634 714 682

Mexico, byproductmetallurgy 474 572 588 539 703natural gas and petroleum 851 878 877 1,052 1,122Total 1,325 1,450 1,465 1,591 1,825

Netherlands, byproductmetallurgy 123 126 124 131 137petroleum 428 384 373 408 410Total 551 510 497 539 547

Polandc

Frasch 1,482 942 760 762b 750byproduct:

metallurgy 279 277 275b 275b 275petroleum 70b 133 180 175b 150Total 1,831 1,352 1,220b 1,210b 1,180

Russianative 50 50 50 50 50pyrites 400 320 350 350 300byproduct:

metallurgy 440 460 500 520 570natural gas 4,900 5,300 5,600 5,800 6,000Total 5,790 6,130 6,500 6,720 6,920

Saudi Arabia, byproduct, allsources

2,101 2,350 2,360 2,180 2,230

Spainpyrites 138 71b

byproduct:coal, lignite, gasification 1 1 1 1 1metallurgy 454 461 544 560 488petroleum 115 135 140 145 145Total 708 668 685 706 634

United Arab Emirates, byproduct, natural gas andpetroleum

1,120 1,490 1,900 1,900 1,930

United StatesFrasch 900b

byproduct:metallurgy 1,030 982 772 683 739natural gas 2,230 2,000 1,760 1,940 1,990petroleum 6,360 6,480 6,750 6,970 7,390Total 10,500 9,470 9,270 9,600 10,100

Otherd;e

Frasch 24 24 23 19 20native 422 457 449 216 161pyrites 284 364 372 375 367byproduct:

metallurgy 1,350 1,530 1,840 1,850 1,820natural gas 196 226 255 305 365

natural gas, petroleum, tarsands, undifferentiated

1,010 1,060 1,360 1,320 1,570

Table 2. ðContinuedÞCountry and source 2000 2001 2002 2003 2004b

32 SULFUR Vol. 23

petroleum 866 785 849 810 837unspecified 1,080 1,120 1,090 1,140 1,150Total 5,220 5,560 6,240 6,030 6,290

Grand total 59,300 59,500 60,600 61,900 64,100of which:

Frasch 2,410 966 783 781 770nativef 762 797 789 556 511pyrites 4,510 4,210 4,350 4,500 4,770byproduct:

coal, lignite, gasification 1 1 1 1 1metallurgy 12,400 13,100 13,800 13,900 14,300natural gas 7,320 7,530 7,610 8,050 8,360natural gas,petroleum, tar sands,undifferentiated

17,300 17,500 18,100 18,700 19,500

petroleum 11,200 11,700 11,500 11,900 12,400unspecified 3,440 3,730 3,680 3,510 3,580

aRef. 21.bEstimated.cGovernment of Poland sources report total Frasch and native mined elemental sulfur output an-nually, undifferentiated; this figure has been divided between Frasch and other native sulfur onthe basis of information obtained from supplementary sources.dSulfur is believed to be produced from Frasch and as a petroleum byproduct; however, information isinadequate to formulate estimates.eExcept for the above mentioned countries, ‘‘Other’’ includes Albania, Algeria, Aruba, Austria, Bah-rain, Belarus, Belgium, Bosnia and Herzegovina, Brazil, Bulgaria, Colombia, Croatia, Cuba, theCzech Republic, Denmark, Ecuador, Egypt, Greece, Hungary, Indonesia, Iraq, Israel, North Korea,Kuwait, Libya, Macedonia, Namibia, the Netherlands Antilles, Norway, Oman, Pakistan, Peru, thePhilippines, Portugal, Qatar, Romania, Serbia and Montenegro, Singapore, Slovakia, South Africa,Sweden, Switzerland, Syria, Taiwan, Trinidad and Tobago, Turkey, Turkmenistan, Ukraine, the Uni-ted Kingdom, Uruguay, Uzbekistan, Venezuela, Zambia, and Zimbabwe.f Includes ‘‘China, elemental.’’

Table 2. ðContinuedÞCountry and source 2000 2001 2002 2003 2004b

Vol. 23 SULFUR 33

Table 3. United States Sulfur Statisticsa

Salient statistics 2001 2002 2003 2004 2005b

production:recovered elemental 8,490 8,500 8,920 9,380 8,840other forms 982 772 683 739 750Total (may be rounded) 9,470 9,270 9,600 10,100 9,600

shipments, all forms 9,450 9,260 9,600 10,100 9,600imports for consumption:

recovered, elementalb 1,730 2,560 2,870 2,850 2,800sulfuric acid, sulfur content 462 346 297 784 700

exports:recovered, elemental 711 709 840 949 650sulfuric acid, sulfur content 69 48 67 67 20

consumption, apparent, all forms 10,900 11,400 12,000 12,800 12,400price, reported average value,

dollars per ton of elemental sulfur,f.o.b., mine and/or plant

10.01 11.84 28.71 32.50 35.00

stocks, producer, yearend 232 181 206 185 170employment, mine and/or plant,

number2,700 2,700 2,700 2,700 2,700

net import reliance as a percentage ofapparent consumption

13 19 20 21 23

aRef. 34.bEstimated.

34 SULFUR Vol. 23

Table

4.SulfurandSulfuricAcid

Sold

orUsedin

theUnitedStates,ByEndUse,�103tSulfurContenta

Ele

men

talsu

lfu

rcS

ulf

uri

caci

d(s

ulf

ur

equ

ivale

nt)

Tot

al

SIO

bE

nd

use

2003

2004

2003

2004

2003

2004

102

cop

per

ores

421

452

421

452

1094

ura

niu

man

dvan

ad

ium

ores

42

42

10

oth

eror

es58

658

626,261

pu

lpm

ills

an

dp

ap

erp

rod

uct

sW

W225

272

225

272

28,285,

286,

2816

inor

gan

icp

igm

ents

,p

ain

ts,an

dall

ied

pro

du

cts;

ind

ust

rial

organ

icch

emic

als

,ot

her

chem

ical

pro

du

ctsd

5W

71

154

76

154

281

oth

erin

organ

icch

emic

als

188

W97

108

285

108

282,2822

syn

thet

icru

bber

an

dot

her

pla

stic

mate

rials

an

dsy

nth

etic

s82

70

82

70

2823

cell

ulo

sic

fiber

sin

clu

din

gra

yon

12

12

283

dru

gs

21

21

284

soap

san

dd

eter

gen

ts2

22

2286

ind

ust

rial

organ

icch

emic

als

22

25

22

25

2873

nit

rogen

ous

fert

iliz

ers

206

209

206

209

2874

ph

osp

hati

cfe

rtil

izer

s6,6

60

6,8

70

6,6

60

6,8

70

2879

pes

tici

des

11

16

11

16

287

oth

eragri

cult

ura

lch

emic

als

1,5

90

1,9

70

46

49

1,6

30

2,0

10

2892

exp

losi

ves

10

10

10

10

2899

wate

r-tr

eati

ng

com

pou

nd

s98

89

98

89

28

oth

erch

emic

al

pro

du

cts

45

105

45

105

29,291

pet

role

um

refi

nin

gan

dot

her

pet

-ro

leu

man

dco

al

pro

du

cts

3,7

00

4,1

00

140

248

3,8

40

4,3

50

30

rubber

an

dm

isce

llan

eou

sp

last

icp

rod

uct

sW

44

331

stee

lp

ick

lin

g58

958

9333

non

ferr

ous

met

als

33

33

33

oth

erp

rim

ary

met

als

96

96

35

3691

stor

age

batt

erie

s(a

cid

)13

29

13

29

exp

orte

dsu

lfu

ric

aci

d1,4

20

26

1,4

20

26

Totaliden

tified

5,480

6,070

9,700

8,770

15,200

14,800

Un

iden

tifi

ed678

801

409

518

1,0

90

1,3

20

Gra

ndtotal

6,160

6,870

10,100

9,290

16,300

16,200

aR

ef.

21,

W¼

wit

hh

eld

.bS

tan

dard

ind

ust

rial

class

ifica

tion

.cD

oes

not

incl

ud

eel

emen

tal

sulf

ur

use

dfo

rp

rod

uct

ion

ofsu

lfu

ric

aci

d.

dN

oel

emen

tal

sulf

ur

was

use

din

inor

gan

icp

igm

ents

,p

ain

ts,

an

dall

ied

pro

du

cts.

Table

4.ðC

ontinued

ÞE

lem

enta

lsu

lfu

rS

ulf

uri

caci

d(s

ulf

ur

equ

ivale

nt)

Tot

al

SIO

En

du

se2003

2004

2003

2004

2003

2004

36

Water

Water Water Water

Air

(a) (b) (c)

Fig. 1. The Frasch process: (a) initial heating; (b) movement of liquid sulfur; and (c)result of pumping compressed air. The thinner arrows indicate the flow of molten sulfur.See text. (Courtesy of Freeport Minerals Company.)

37

Vol. 23 SULFUR 37