25

Sulfur and Sulfuric Acid

Gerard E. d'Aquin* and Robert C. Fell**

SULFUR

Sulfur is one of the few elements that is foundin its elemental form in nature. Typical sulfurdeposits occur in sedimentary limestone/gypsum formations , in limestone/anhydriteformations associated with salt domes or in

. I 'volcanic rock. A yellow solid at normal tem-~eratur~s , sulfur becomes progressivelylighter III color at lower temperatures and isalmost white at the temperature of liquid air.It ~elts at 114-119°C (depending on crys­talline form) to a transparent light yellowliquid as the temperature is increased. Thelow viscosity of the liquid begins to risesharply above 160°C, peaking at 93 Pa s at188°C, and then falling as the temperaturecontinues to rise to its boiling point of 445°C.This and other anomalous properties of theliquid state are due to equilibria between the

"Presldent, Con-Sui (consuljnc@aol.com) ProfessionalConsulting Services. SULFUR.* *Process Consultant . Monsanto Enviro-Chem SystemsSULFURICACID. •The authors wish to acknowledge that major portionsof th is chapter are taken from the ninth edition version(1992) which was written by Dr Robin W. Str ickland .

various molecular species of sulfur whichincludes small chains and rings. '

Sulfur also is found as sulfide minerals incombination with iron or base metals(e.g., pyrites) and as sulfates in combinationwith alkali metals and alkaline earths(e.g., gypsum). Hydrogen sulfide, with its "rot­ten egg" odor, is the primary sour componentof sour gas. Crude oil and coal contain a vari­ety of complex sulfur-containing organicspecies. These sulfur compounds are removedfrom the liquid fuels by treatment with hydro­gen to convert the sulfur to hydrogen sulfidewhich is taken off in the gas stream. Th~recovery of sulfur from sour fuels for envi­ronmental reasons is the largest source ofsulfur today.

World elemental sulfur production in 2003was almost 45 million metric tons.' Over 99percent of the sulfur that is marketed is sold ascrude sulfur. The two primary grades are"bright ," which is bright yellow and at least99.8 percent pure (typically 99.9+% pure witha maximum of 0.02% carbonaceous material),and "dark," which at the time of productioncan.contain in excess of0.25 percent carbon, istypically sold as 99.5 percent (min) sulfur with

1157

1158 KENTANDRIEGEL'S HANDBOOK OF INDUSTRIAL CHEMISTRY AND BIOTECHNOLOGY

a carbon content not to exceed 0.25 percentand ash of less than 0.25 percent. To achievethose specifications certain types of minedsulfur must be both filtered and blended with"bright" sulfur. Bright sulfur, which todayrepresents more than 95 percent of world pro­duction of elemental sulfur, is almost exclu­sively obtained as a derivative of oil and gasprocessing. Certain deposits of mined sulfur,such as those still being produced in Polandalso produce bright sulfur. However, mostsulfur mines are associated with oil depositsfound in conjunction with geologic forma­tions known as salt domes. In situ cross­contamination (as at the Mishraq mine inIraq) normally leads to the production of darksulfur with varying degrees of hydrocarboncomponents. Volcanic sulfur deposits yieldbright sulfur, but it is often contaminated withtoxic metal oxides. Sulfuric acid productionaccounts for 93 percent of the elemental sul­fur used in the United States. Small quantitiesof several specialty sulfurs are produced for avariety other applications, including bleach­ing, fumigation, pharmaceuticals, pyrotech­nics, rubber manufacture, and cutting oils.Applications of elemental sulfur as a fertilizerrepresents a growing use with significantpotential and much commercialization left tobe done.

Transportation and Storage

Although all sulfur is produced-and most isconsumed-in a molten state, the preponder­ance of international commerce takes place insolid form. Vancouver, Canada, is the world'sleading sulfur-exporting port, with 2004 vol­umes of 6.3 millions tons, all solid.* Sulfur issolidified into a variety of "forms," whichwill be discussed in the following section."Formed" solid sulfur is easily transported bytruck or railcar but caution must be exercisedin order to avoid fugitive dust and spillage.Sulfur dust is highly visible, corrosive, and,under certain circumstances, explosive. In the

*Consul, Inc., North American Quarterly Sulfur Review,Jan. 5,2005

case of marine transport, which may last sev­eral weeks, the threat of corrosion duringtransit becomes a significant concern. To thatend it is imperative that the steel ofthe vessel's holds be coated to precludedirect sulfur-steel contact. The need for suchpreparation to avoid a range of potentiallysevere consequences cannot be underesti­mated. Water accumulation in the hold mustalso be eliminated throughout the voyage toavoid the creation of a water-sulfur-steelinterface.

Incontrast, sulfur destined for internal usein the United States, Canada, and Europe isalmost exclusively transported in liquidform. This has led to the establishment of anextensive sulfur infrastructure consisting ofmolten sulfur terminals, tanker vessels,barges, rail tank-cars, and tank-trucks. Inthe cases of marine transport, most vesselsrange from 23,000 to 9500 tons and bargesfrom 1000 to 2500 tons. Sulfur is main­tained in its molten state during transit withthe use of heating coils. But, when trans­ported by tank-car, sulfur is allowed tosolidify in transit and must be re-liquefiedprior to discharge. That process, which cantake up to three days, is accomplished bypassing steam through specially built coilsbuilt into each tank-car. Trucks operatewithin close enough ranges as to precludethe need for steaming. In Tampa, FL, manytrailers carry sulfur within an inner core onthe way out of the port and phosphoric acid,solid fertilizers, or phosphate rock for thereturn trip. Japan exercises yet another pointof view by prohibiting solid sulfur, even inthe case of export activities. That has led toa fleet of coastal vessels, some as small as1000 tons, for exporting molten sulfur tousers in China and formerly Korea. Certaincountries have regulations that requiremolten sulfur to be treated (de-gassed) at theproduction point in order to reduce the liq­uid's hydrogen sulfide level to under 10 ppmweight. The City of Vancouver requiresformed sulfur transshipped at the port to testbelow 30 ppm weight.

Storage of molten sulfur requires insulatedtanks equipped with heating coils through

which steam or glycol is constantly circulated.Exposed tank surfaces, particularly the roof,must be protected against corrosion and care­fully monitored. A means of injecting steaminto the tank void should be provided for firecontrol. Finally, depending on environmentalregulations, tank vents may require scrubbingsystems. In the case of marine terminals,these must be sized to accommodate signifi­cant inflows of product. "Formed" solid sul­fur, on the other hand, can be stored in a pileexposed to the open air. Runoff water con­tainment and neutralization is required aroundsuch sites. In certain localities it must con­form to maximum H2S standards. Anothermeans of storage, employed to minimize costand maximize volume in any given area, is to"vat" or "block" the sulfur. In this case moltensulfur is poured onto a specially prepared areawhich is surrounded by a rectangular metalbarrier (forms). Once the layer of sulfur solid­ifies, the forms are raised and the process isrepeated. This leads to the establishment of a"block" of sulfur which resembles a box orinverted bathtub (vat). Vats containing severalmillion tons of sulfur can be up to 20 m highand of almost any length. Volumes stored inthis manner have fluctuated considerably. Asof December 2004, 15 million tons of sulfurwere present at vats located within theCanadian province of Alberta. This is wellbelow the record of 22 million metric tons inthe late 1970s, but up 10 million metric tonsfrom the early 1990s.

One final consideration when storing solidsulfur is the almost inevitable presence of sul­furic acid. Sulfur can become naturally con­taminated with sulfuric acid through thepresence of thiobacilli thiooxidans' or contin­uous exposure to direct sunlight." Recentresearch has demonstrated the short-termeffectiveness of certain bactericides in delay­ing bacterial colonization. Nevertheless, dis­crete pockets of weak (highly corrosive)sulfuric acid should always be presumed toexist within a sulfur storage pile. Hydrochloricacid, which may also be present when solidsulfur has been transported by vessel,' must beneutralized to avoid potentially disastrous cor­rosion of downstream equipment.

SULFUR AND SULFURIC ACID 1159

Solidification and MeltingFor many years, the standard industry practicewas to ship sulfur as a crushed bulk solid.Currently accepted best available technolo­gies for solidifying sulfur create pelletized,prilled, or granulated products. These types of"formed" sulfur were developed in the late1970s to minimize the creation of, andpollution from, sulfur dust during transportoperations. As the product names imply:pelletized sulfur is generated when a discreteamount of sulfur is deposited and cooled on aconveyor belt. The most common processesare offered by Sandvik" and Bemdorf" butother licensors exist. Prilled sulfur is createdwhen sulfur droplets are cooled by a surround­ing fluid, which may be air or water. Air-prillsare created when spraying sulfur from the topof a tower into a strong updraft of cool air. It isan extremely smooth product which resemblesprilled urea or ammonium nitrate. No air­prilling installations have been built recentlydue the cost of the tower and requisite high­volume air scrubbing system. The last installa­tions erected at Jubail, Saudi Arabia, weredestroyed by fire, believed to have been ignitedby static electricity. Wet prills,created when sul­fur droplets enter a water bath, have an irregu­lar texture caused by water's more rapid coolingaction (DEVCO). Recent improvements yield amore homogeneous product. Wet prills resem­ble small nuggets, but, despite their name, donot encapsulate water within the prill's struc­ture. All sulfur-forming operations in theUnited States manufacture "wet prills" sulfur.

Granular sulfur, as the name implies, is cre­ated through the gradual buildup of successivelayers of sulfur around a central core. Asare-ult, the ultimate product size can becon­trolled. Enersul'" possesses the most wide­spread granulation process, with installationsin Canada, the Former Soviet Union, Europe,and the Middle East. Kaltenbach-Thuring II

provides another alternative.Slate, another type of formed sulfur, was the

sulfur industry's first attempt at developing a"formed" product which would generate lessdust than the traditional "crushed bulk" sul­fur commonly used until the 1960s. Slate isproduced by pouring a thin (usually up to

1160 KENT ANDRIEGEL'S HANDBOOK OF INDUSTRIAL CHEMISTRY AND BIOTECHNOLOGY

1em) layer of molten sulfur onto a movingbelt while simultaneously cooling the belt'sunderside. The solidified sulfur breaks off intoirregular shapes when falling off the end ofthebelt, leading it to resemble pieces of "peanutbrittle." Owing to its greater dustiness, slatesulfur is a less desirable form of materialwhich now faces commercial resistance.

"Crushed bulk" is the least acceptable typeof solid sulfur. It is created when using earth­moving equipment to recover sulfur whichhas been allowed to solidify in sheet or vats.Obviously, the product is extremely dusty.Very few producers (Former Soviet Union,Iran) still ship crushed bulk. Most consumersrefuse to accept shiploads even at significantprice discounts due to dustiness, related con­tamination, and product loss. A U.S. patentwas recently issued to d' Aquin for a revolu­tionary solid sulfur unloading system inmarine application which eliminates dust andcontrols acidity with most formed sulfur.12

Despite developments in forming technol­ogy, moisture remains a necessary tool fordust suppression. To that end, a mixture ofwater and surfactant should be applied duringbulk transfer operations. Disposing of thatmoisture during transport, storage, and at thetime of melting is an added cost.

All forming methods require molten sulfur.This is obtained from production, moltenstorage, or by melting solid inventory.Melting block inventory is most often accom­plished using track-mounted Ellethorpemelters (Fig. 25.1). These resemble rectangu­lar ironing-board irons mounted on "caterpil­lar" tracks which are placed vertically againstthe block surface. Sulfur melted by the rectan­gular surface is collected in a trough at the baseof the block and piped to filtration (if appro­priate) and storage. Mechanical reclamation,normally from inside the block to minimizedust emissions, and transfer of the lumps ofsulfur to a static melter is another approach.

Fig. 25.1. An Ellethorpe melter. (Courtesyof Con-Sul, Inc.)

Problems related to acidity and Car-Sui cansurface during melting.

Melting or remelting solid sulfur that hasbeen transported by vessel requires properknowledge and equipment. Contaminants­which can include sand, dirt, rocks, and rustedmetal-mixed with solid sulfur during transitand storage must be removed. Sulfuric acid isanother issue: weak acid is almost always pres­ent in solid sulfur stored for some time, letalone stored, transported by vessel, and thenstored again . As a result , customers mayinclude a specification "below 100 ppm freeacid." The currently accepted practice is to addlime prior to melting for acid neutralization.Liming, in tum, requires the introduction of afiltration stage-large installations use steamjacketed pressure leaf filters, containing adiatomaceous earth medium, to remove thelime/ash residue. Finally, carbon containedwithin sulfur tends to solidify into extremelyabrasive particles when the sulfur is re­liquefied. This Car-Sul abrades burner spraynozzles." Users in the sulfuric acid industrytherefore prefer receiving sulfur in its moltenstate rather than incurring the foregoing operat­ing costs and losses of sulfur as filtrate residue.Owing to the volume ofproduction, legislation,and a developed liquid infrastructure, most ofthe sulfur consumed in North America, Europe,Japan, and Korea is never solidified.

Development of the Sulfur Industry

Early humans doubtless found elementalsulfur in volcanic craters, encrusting the edgesof hot sulfur springs, and embedded inlimestone formations. They discovered that itwould bum and used it for medicinal pur­poses, as a bleach, as a fumigant, as a colorant ,and as incense. Its use for these purposes ismentioned in ancient writings. The Romansproduced incendiary weapons from sulfur. Inthe thirteenth century, the Chinese inventedgunpowder using sulfur, nitrate, and charcoal.

The earliest commercial sulfur came fromlimestone deposits, of which those in Sicilyand the Italian mainland developed world mar­kets in the eighteenth and nineteenth centuries.Traditional mining methods were used to pro-

SULFUR AND SULFURIC ACID 1161

duce sulfur ore, which was burned slowly in apile (Calcarone) to yield crude sulfur." Steamsmelting in autoclaves came into use about1859. In 1890 Gill built a multi-chamber fur­nace to improve the process's production rateand efficiency. Italian monopoly of the sulfurmarkets continued until the early 1900s whenthe Frasch process brought previously unrecov­erable sulfur deposits on the North AmericanGulf Coast into production. Oil explorationefforts in Texas and Louisiana in the late 1800suncovered sulfur deposits in limestone atdepths of200-300 m. Mining was complicatedby intervening layers of quicksand and thepresence of hydrogen sulfide gas. Numerousconventional mining attempts at Sulphur, LA,proved disastrous. Finally, in December 1894,Hermann Frasch demonstrated the hot waterprocess for mining underground sulfurdeposits. With its favorable economics, theFrasch process completely displaced the Italiansulfur industry. The ready availability of low­cost sulfur opened the way for commercial sul­furic acid production by burning sulfur. Thisprocess largely supplanted the long-standingiron pyrite combustion process for sulfuric acidproduction by eliminating its extensive gas­cleaning operations.

In 1883, the Claus process for producingsulfur from hydrogen sulfide through partialcombustion over an iron oxide catalyst waspatented. It enjoyed limited success asa method for producing sulfur over the fol­lowing 50 years, despite a number of processimprovements. Its primary use arose with theneed for a means to remove the sour compo­nent of sour gas for processing reasons andfor environmental compatibility.

The number of Claus installations grew dur­ing the second half of the twentieth century.Technical and environmental requirements ledto lower sulfur content in hydrocarbon fuels.Concurrently, the sulfur content of extractedoil and gas has increased significantly duringthe past 20 years. The trend towards Claus­produced "recovered" sulfur accelerated rap­idly after the 1970s when environmentallegislation fin ally took hold. In 1985, Fraschand recovered sulfur in the United Stateseach accounted for about 5 million tons of

1162 KENTANDRIEGEL'S HANDBOOK OF INDUSTRIAL CHEMISTRY AND BIOTECHNOLOGY

production. By 1995 Frasch had declined to3.1 million tons and recovered represented 7.3million tons. Continuation ofthe trend changedthe industry dramatically: by 2001, faced withoil companies' disposal of recovered sulfur atlow prices, all U.S. sulfur mines had closed."Frasch sulfur technology, with its colorful andinventive history, lasted just over 100 years inthe United States. Sulfur output now exceedsworld demand by more than 3 million tons andthe surplus is increasing. No respite is in sightfrom either production or demand, leading sev­eral companies to consider re-injecting eitherhydrogen sulfide or elemental sulfur into geo­logic strata, or other means, for permanent dis­posal. If such efforts are not successful theamount of sulfur stored in vats will eventuallyexceed the level which local communities arewilling to tolerate, leading possibly to loweruse of sour hydrocarbon reserves.16

Sulfur is also produced from sulfide ores(pyrites) by thermal decomposition in theabsence of air, by roasting/smelting underreducing conditions, or by reaction of the orewith S02' Hydrometallurgical processes haveproduced sulfur from metal pyrites as a by-

product. Except for China, pyrites roasting nolonger accounts for significant quantities ofsulfur values. And even China has embarkedon a rapid conversion to elemental sulfur use inpyrites roasters in order to lower productioncosts and pollution. Canadian sulfur exporters,who initiated this trend with the developmentand introduction of innovative technology,have gained the most. Canadian exports of sul­fur to China rose from 31,000tons in 1995 to1.8 million tons in 2001, then reached 3.75million tons in 2004.17

Sulfur Production Processes

Despite the economically driven closure ofthe Main Pass 299 sulfur mine in 2000, justeight years after initial output, the Fraschprocess remains the most economical methodfor extracting sulfur from native deposits.Certain constraints on the geological forma­tions required for the Frasch process limit itsuse to deposits along the Gulf of Mexico, inPoland, in the Former Soviet Union, and inIraq. Other sulfur deposits may yield to theFrasch process but they have marginal eco­nomics. Figure 25.2 shows the structure

Salt ·

Fig. 25.2. Frasch process for mining sulfur from salt-dome formations. (Courtsey Freeport Sulphur Co.)

needed for Frasch mining from salt domes.The sulfur-bearing limestone must have suffi­cient porosity to allow the sulfur to migrateupon melting. Both the caprock and theunderlying anhydrite formation should beimpervious to prevent the loss ofthe hot waterpumped into the mine. These salt-domedeposits are typically lens shaped and are1-75 m thick with diameters of a few hundredmeters up to several kilometers.

A sulfur well consists of a casing and threeconcentric pipes reaching into the sulfur­bearing strata. The outer 8- to 10-in. pipecarries 165°C water pumped into the forma­tion to melt the sulfur. An inner cement­lined 3- to 6-in. pipe is used to transport themelted sulfur to the surface. Compressed airis passed through the l-in, tube in the centerto air lift the sulfur. Without the air lift themolten sulfur would rise only part way inthe middle pipe. The compressed air pro­duces a low-density sulfur froth that rises tothe surface.

The superheated water melts the sulfur inthe vicinity of the well, forming a molten sul­fur pool at the bottom of the well. As produc­tion continues, the formation fills with water.To continue production, bleed wells are drilledat the periphery of the formation to allow fordischarge of the cooled mine water. In somemine fields, sufficient mine water is lost to thegeological formation to provide for continuedproduction. To limit mine water loss, mud orsynthetic foam sometimes is pumped into theformation to seal major crevices.

Although most U.S. Frasch mines werelocated inland, Freeport Sulfur Company pio­neered offshore sulfur mining in the 1960s.15

The company utilized offshore oil drillingtechniques to access several shallow waterdeposits. Developing a means to utilize salt­water to provide heated mine water presentedthe company with unique challenges incorrosion and scaling control.This now defunctfacility, located in 200 ft of water 10 milesfrom the Mississipi Delta, commenced pro­duction in 1992 and closed in 2000.

Sulfur produced from salt-dome structurescan be quite pure, but it often contains up to1 percent of bituminous residues, which

SULFUR AND SULFURIC ACID 1163

render it dark and can make it unacceptablefor sulfuric acid production. Some purifica­tion is obtained by filtering the dark sulfurthrough diatomaceous earth. Nevertheless,the most effective means of meeting maxi­mum commercial specifications for carbonis through blending dark sulfur with bright,recovered sulfur containing virtually no car­bon. That practice was widely used byLouisiana Frasch producers. The carbona­ceous material can be formed into larger, fil­terable particles (Car-Sul) by treating thesulfur with heat or sulfuric acid. Freeport'ssubmerged combustion distillation processwas used from 1966 until 1979 to purifyFrasch sulfur with up to 2 percent carbona­ceous material.

Recovered Sulfur

Hydrogen sulfide is recovered from naturalgas or refinery gases by absorption in asolvent or by regenerative chemical absorp­tion. 18

, 19 In either case a concentrated hydro­gen sulfide stream is produced that istreated further by the Claus process. A typi­cal Claus plant has a feed stream of at least45 percent H2S, but with modifications canhandle streams containing as little as 5percent H2S. For gas streams with low con­centrations of hydrogen sulfide, direct con­version of the hydrogen sulfide to sulfur isaccomplished in the solvent system, forexample, the Stretford process or CrystaSulfprocess.

The Claus process is based on the reactionof H2S with sulfur dioxide according to thehighly exothermic reaction:

2H2S + S02 ------.. 3S + 2H20

In practice, sulfur dioxide is produced in situby partial oxidation of the hydrogen sulfidewith air or oxygen in a furnace. In the split­flow arrangement, one-third of the H2Sstream is burned and then recombined withthe remaining two-thirds before entering theClaus reactor. In the straight-through version,the entire H2S stream is sent through theburner and the extent of H2S combustion iscontrolled by the air feed rate.

1164 KENTANDRIEGEL'S HANDBOOK OF INDUSTRIAL CHEMISTRY AND BIOTECHNOLOGY

STACK

WASTEHEAT

BOILER

AIR BLOWER

NATURALGAS

SOUR GAS

SULPHURCONDENSER

LOWPRESSURE

STEAM

SULPHUR

#'

BOILER FEED WATER

#3CLAUSREACTOR

#4 PASS

#3 PASS

#2 PASS

#1 PASS

WASTEHEATRECLAIMER

LOWPRESSURE

STEAM

AIR BLOWER

FILTER

BYPASS

Fig. 25.3. Claus process flow diagram.

A flow diagram for a typical Claus processis shown in Fig. 25.3. The hydrogen sulfide isburned in a fuel-fired furnace (950-1250°C)with air to produce sulfur and a gas streamcontaining H2S and S02' Process controlsmaintain the H2S: S02 ratio near 2, in accor­dance with the stoichiometry. Heat is removedfrom the gas stream in a waste heat boiler tocontrol the process gas temperature. Theprocess gas is passed through one or morecatalyst beds to convert the H2S and S02 tosulfur, which is removed in condensersbetween each bed. A high temperature shiftsthe equilibria toward the reactants, whereas alow temperature causes sulfur condensationon the catalyst bed, leading to decreasedcatalyst activity, requiring bed switching/regeneration to achieve modem recoverystandards. In practice the temperature isclosely controlled for each Claus reactor, withhigher temperatures at the first reactor wherecompounds such as COS and CS2 are con­verted to S. Much research and development

into H2S conversion has occurred in the pastdecade. Three-stage Claus units are now capa­ble of achieving recoveries of 98+ percent,a significant improvement over the 85­90 percent range of the late 1980s.20

Figure 25.4 shows a typical sulfur recoveryplant based on the Claus process. The tail gasfrom the Claus reactors may be furtherprocessed to remove any remaining sulfurcompounds. Combined H2S removal efficien­cies of 99.5-99.99 percent are achievable."This may be done by low-temperature Claus­type solid-bed processes (e.g., the Sulfreenprocess), wet-Claus absorption/oxidationprocesses (e.g., the Clauspol 1500 process),or hydrogenation of the off-gas to form H2Sfor recycle (e.g., the SCOT process). Residualsulfur compounds in the tail gas are thenincinerated to S02' The residual S02 in theoxidized tail gas may be scrubbed by any ofseveral processes (e.g., the Wellman-Lordprocess) before being vented to the environ­ment. It is feasible to bring the H2S content of

SULFUR AND SULFURIC ACID 1165

Fig. 25.4. Republic Refining Co. 440 metric ton per day Claus sulfur recovery unit (left) and SCOTtailgascleanup unit (right). (Courtsey Ortloff Engineers, Ltd.)

the treated tail gas to as little as 150-10 ppmvolume, depending on the solvent used."

Production and Consumption of Sulfur

World production of sulfur in all forms is esti­mated to have reached 63 million tons in2004,21 compared with 58.1 million tons in1990.22 This similarity in volumes concealsenormous structural changes in the industry'ssources of supply. Table 25.1 summarizes thedramatic shifts in the sources of sulfur duringthis period. It shows, for example, the virtualdemise of Frasch and native sources of supply,and a 54 percent reduction in sulfur valuesderived from sulfuric acid pyrites roasting off­set by huge increases in recovered sulfur(59%) plus added sulfuric acid and S02 recov­eries from metallurgical smelting, (59%).

From 1990 to 2003, voluntarily producedsulfur (Frasch, native mining, and on-purposepyrites roasting) declined 18 million tons. Atthe end of the decade it accounted for only 10percent of the world's sulfur output. On theother hand, by-product sulfur recovered from

hydrocarbon and metal smelting climbed from54 to 83 percent of the total supply by 2003.

The shift devastated commercial enterprisesin the voluntary extraction industry. Sulfurprices embarked on a decline which culmi­nated with spot sulfur prices falling to the$10s per ton in Tampa, FL, and Vancouver,Canada, by mid-200 1. In contrast, prices dur­ing 1990 were $140/ton and $108/ton, respec­tively. The primary difference was that, in2001, the newly developed disparity betweenvoluntary and involuntary volumes of outputprecluded a curtailment in sulfur output in theface of a dramatic, yet temporary, decline indemand (see Figs 25.5-25.7). The increase indemand following 2001 took prices in Tampaabove $65 per long ton during 2003.

Following the dramatic decline of northAmerican Frasch mining during the 1990s,global leadership in elemental sulfur outputhas shifted from the Americas (northAmerica) to the rest of the world (RoW; Table25.2). Substantial shifts occurred within eachhemisphere as the Frasch industries inMexico, the United States, Poland, Eastern

1166 KENTAND RIEGEL'S HANDBOOK OF INDUSTRIAL CHEMISTRY AND BIOTECHNOLOGY

TABLE 25.1 World Production - All Forms of Sulfur22 (Million Metric Tons)

2003 (e) Tons % 1990 Tons % Difference Tons %

Base processesFrasch 0.8 I 1.0 19 (10.2) (93)Native 0.6 I 3.1 5 (2.5) (81)Pyrites 4.6 ~ 10.0 11 (5.4) (54)

Subtotal 6.0 10 24.1 41 (18.1) (75)

ByproductRecovered 38.1 62 24.0 41 14.1 59Metallurgy 13.2 22 7.7 13 5.5 71Unspecified 4.6 1 2.4 ---.1 2.2 92

Subtotal 55.9 21 34.1 59 21.8 64

Total 61.9 100 58.2 100 3.4 6

Note: The 2003 USGS Mineral Industry Survey of Sulfur contains detailed country information(http://minerals.usgs.gov/minerals).

16 r-------- - - - ------ - --- - - - - ------ - --,

ooo

oo

ooo 0

o 0oo

14

'"~ 10 1----1U--+-------------------------~

u;:Iii 8 ~----------------_e~:_::_-__=_--.--__1~ • Production * Imports

~ O~ O~_~ 6 f-- .- - - ------ ---- - - --- --l... Exports1;;,....;..- ---'

4 1--- 7""''''''"- - - -- - - - - - - - - - - - - - - - - - - - - - -----1

2 h=--:--'tIt--..,r('----'-'""::5<il:;;:;:ir~'-----------.:.:.---------_i

1980 1981 1982 1983 1984 1985 1986 1987 1988 1989 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000

Fig. 25.5. Trends in elemental sulfur supply and consumption in the United States."

Russia, and Iraq collapsed and recovered sul­fur volumes increased (Canada, the UnitedStates, Central Asia, the Middle East, andJapan, primarily). In 2003, the United Statesand Canada, at 9 and almost 8 million tons,respectively, are the first and second largestproducers of elemental sulfur in the world.Canadian output, after rising to 8.8 milliontons late in the last decade, fell back to 8.1

million tons in 2003 due to declining gasoutput. Production from oilsand processingfacilities should bring estimated output backto the former peaks in 2005.23 U.S. outputdeclined due to the closing of the remainingFrasch mines, and Mexico's output collapsedconcurrent with mine shutdowns. Europeanvolumes fell 45 percent: increased refineryand gas extraction was insufficient to offset

SULFUR AND SULFURIC ACID 1167

t:. Byproductocid

* Toeal

• Frasch"

<>~

2 r -- - - - - - - - - - - - - - - - - -...::.:::::...__- -

14

12

..........*10

II>Z~u 8

§:::I'Z0 6

~::;:

4

1980 1981 1982 1983 198-4 1985 1986 1987 1988 1989 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000

Fig. 25.6. Trends in the production of all forms of sulfur in the United States."

200 ,....----------------------------------.,

<> llas<den constan t 2000dollars• hc llU1prices

~ ISO I----~I"_-___:~~,,,o_---.-::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::~---luii2!;j::<

""w::; 100 I--I-:...--~--___",L::.=---=..____----~,------------------l

"~8uju~ so 1-- - - - - - ----- - - - - - - - - 1in.-- - --n-- - - - - - - - --l

o L-'-----'-_ ..I.---'-_ --'----'_ -'--_ '-____'___--'-_-'----''--...J..._~--'-_ _'___'_.....L...______'_....J

1980 198\ 1982 1983 1984 1985 1986 1987 198 1989 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000

Fig. 25.7. Trends in sulfur pricinq."

the effect of Polish Frasch curtailments andreduced volumes in France's Lacq gas field.Increases in Central Asia and the Middle Eastare both primarily linked to gas processing.The perhaps tempered growth in Middle Eastoutput must be viewed in the context ofrebuilding many oil and gas processing cen-

ters lost in the Iraq-Iran and Iraq-Kuwaitconflicts. A significant number ofprojects areplanned for this region. In Asia, Japan andKorea became increasingly important crudeprocessing centers, with attendant increasesin sulfur output. India's production is now halfa million tons versus none a decade ago.

1168 KENTAND RIEGEL'S HANDBOOK OF INDUSTRIAL CHEMISTRY AND BIOTECHNOLOGY

TABLE 25.2 World Production of Elemental Sulfur" (Million Metric Tons)

Countries-Regions 2003(e) tons % 1990 tons % Difference tons %

Canada 8.1 19 5.9 16 2.2 37United States 8.9 21 10.3 27 (1.4) (14)Mexico .LQ ---.2 -.l.J. ~ f.Lll (52)

Subtotal 18.0 42 18.3 48 (0.3) (2)

Europe 4.7 11 8.5 22 (3.8) (45)Central Asia 7.3 17 4.5 12 2.8 62Middle East 6.3 15 4.3 II 2.0 47Asia .-1.4 -.8. .Ls ~ 1.6 89

Subtotal 21.7 51 19.1 50 2.6 14

Other 2.9 1 -.M --.2. 2.3 283

Total 42.6 100 38.0 100 4.6 12

Note: The 2003 USGS Mineral industry Survey of Sulfur contains detailed country information(http://minerals.usgs.gov/minerals).

The United States remains the largest pro­ducer and consumer of sulfur in all forms. U.S.production and consumption data are providedin Table 25.3. Fertilizers and agro-chemicalsrepresent the largest use for sulfur, 69 percent.Phosphatic fertilizers consume 7.1 millionmetric tons, 57 percent, in the form of sulfuricacid. Agro-chemicals account for 1.3 millionmetric tons, 10 percent, all in the form of ele­mental sulfur. Sulfur, surprisingly, representsthe largest fungicide and pesticide productapplied in the United States. Refining use, foralkylation, is the second largest segment ofconsumption. It is also one of the most diffi­cult segments to track accurately. This arisesfrom refineries not reporting a portion of theirproduction and using that material directly forinternal use. Accordingly, a portion of actualsulfur output does not get reported within thescope of "production of sulfur," causing totaluses to exceed production by 1.1 millionmetric tons.

U.S. production of elemental sulfur has fluc­tuated dramatically in recent years due to thecessation of Frasch mining operations. Outputwas 10.0 million tons in 1999,9.3 million tonsin 2000, 8.5 million tons in 2001, and is esti­mated to have rebounded to 9.2 million tons in200423 due to a 700,000 ton increase fromrefining. From this point forward, U.S. output

should rise although re-injection of acid gassesmight lead to occasional declines. Such a pro­ject is about to start at an ExxonMobil gas pro­cessing installation in Wyoming.

SULFURIC ACID

Sulfuric acid is the largest-volume chemicalmanufactured in the world and its consump­tion is often cited as an indicator of thegeneral state of a nation's econorny." About41 million tons of sulfuric acid were pro­duced in the United States in 2003, of whichapproximately 70 percent was used in fertil­izer production. Its use extends to nearlyevery major chemical sector. This versatileacid is truly the "workhorse" of the chemicalindustry.

Pure sulfuric acid is an oily, water-white,slightly viscous liquid with a melting point of10AoC and a boiling point of 279.6°C. It isinfinitely miscible with water, forming sulfu­ric acid solutions characterized by their weightpercent H2S04, Oleum may be formed by dis­solving S03 in sulfuric acid to attain fumingsulfuric acid, with concentrations nominallygreater than 100 percent H2S0 4, Historically,sulfuric acid concentrations were determinedby measuring the solution density usinghydrometers calibrated in degrees Baume

SULFUR AND SULFURIC ACID 1169

TABLE 25.3 U.S. Production and Consumption of Sulfur in all Forms in 2003(Million MetricTons, SulfurEquivalent)

Production UsesFrasch 0.0 Fertilizers and agro-chemicalsRecovered 8.9 Phosphatic 6.8 H2SO4

Other 0.7 Nitrogenous 0.2 H2SO4

Total 9.6 Other Ag Chems ti SSub total 8.5

SupplyElemental S

Produced 8.9 Petroleumrefining 3.7 SInventory ---.!U H2SO4

Other 0.7 Sub total 3.8Imports

Elemental 2.8 Copper oresAcid 0.3 (leaching) 0.4 H2SO4

ExportsElemental (0.8)Acid (0.1) Inorganicchemicals 0.5 S/H2S04

Total 11.8 Other ---.!LQ S/H2S04

Consumption Total 13.8ElementalS 10.9Acid/other -.J1.2 Imbalance 2.0

Total 11.8

Source: Ober,1.,Sulfur-2003, Mineral Industry Surveys, U.S. Geological Survey, U.S. Dept. of Interior, adjusted byCon-SuI, Inc.24

(Be). This practice is waning, although somespecifications and tables of properties stillinclude this measurement.

Uses of Sulfuric Acid

The primary industrial uses of sulfuric acid arein phosphate fertilizer manufacture, petroleumrefining, copper ore leaching, synthetic rubberand plastics, and pulp and paper mills." It isused as a solvent, a dehydrating agent, areagent in chemical reactions or processes, anacid, a catalyst, and an absorbent, and in manyother applications. In spite of its wide usage,sulfuric acid rarely is contained in the finalproduct. Sulfuric acid ends up as gypsum inphosphate fertilizer manufacture, for exam­ple. In many other processes the sulfuric acidis converted to a waste product that requiresdisposal or reuse. Because disposal of wastesulfuric acid is becoming increasingly unac­ceptable environmentally, the recycle of sulfurvalues from waste sulfuric acid has becomemore widespread.

Nearly all sulfuric acid is manufactured bythe contact process in which sulfur trioxide isabsorbed into 93-98 percent H2S04, The acidmay be sold at various strengths, usuallydepending on the requirements of the con­sumer. It generally is marketed on a 100 per­cent basis, but normally is shipped as93 percent H2S04 (66°Be), as 98 percent acid,or as 20-22 percent fuming oleum. Table 25.4shows common acid strengths and end uses.Concentrated acid may be stored in mild steeltanks, but dilute acid must be contained inlead-lined or plastic tanks. Bulk shipments ofconcentrated acid are made in steel tanks onships, tank barges, or railcars. Reagent gradeacid is commonly sold in 5-L glass bottles.

Development of theSulfuric Acid Industry

Sulfuric acid is formed in nature by theoxidation and chemical decomposition of nat­urally occurring sulfur and sulfur-containing

1170 KENTANDRIEGEL'S HANDBOOK OF INDUSTRIAL CHEMISTRY AND BIOTECHNOLOGY

TABLE 25.4 Acid Strengths and End Uses

Percent Percent Oleum SpecificH2SO4

oRe (% Free SO]) Gravity Uses

35.67 30.8 1.27 Storage batteries, electric utilities.62.18~9.65 50-55 Normal superphosphate and other fertilizers.77.67 60.0 1.7059 Normal superphosphate and other fertilizers;

isoproyl and sec-butyl alcohols.80.00 61.3 1.7323 Copper leaching.93.19 66.0 1.8354 Phosphoric acid, titanium dioxide, steel pickling,

regenerating ion exchange resins.98-99 Chlorine drying, alkylation, boric acid.104.50 20 1.9056 Surfactants, nitrations.106.75 30 1.9412 Hydrofluoric acid.109.00 40 1.9737 Explosives.111.24 50 1.9900 Reagent manufacture, organic.113.50 60 1.9919 Sulfonations, blending with.114.63 65 1.9842 Weaker acids.

Source: Chemical Economics Handbook, SRI International, Dec. 1990.

compounds. It is made by the action ofbacteria (Thiobacillus ferrooxidans) on coalbrasses or iron disulfide discarded on refusedumps at coal and copper mines; it is pro­duced in the atmosphere by the oxidation ofsulfur dioxide emitted from the combustion ofcoal, oil, and other substances; and it also isformed by chemical decomposition resultingfrom geological changes.

Although there were vague references to"spirits" expelled from alum by Arabianalchemists in the tenth century and byRoman alchemists in the thirteenth century,the first distinct mention of sulfuric acid hasbeen credited to Basil Valentine in the late1400S.44 He burned sulfur with saltpeter inglass retorts or bell jars with a little water,and he also calcined copperas (ferrous sul­fate heptahydrate) with silica, with bothprocesses yielding sulfuric acid, although hetook them to be different substances. Theseprocesses for making sulfuric acid continueduntil 1746 when Dr Roebuck constructed alead chamber in England for sulfuric acidmanufacture. This marked the beginning ofthe "chamber process" for sulfuric acid,which was to continue in use for the next twocenturies.f

The first lead chamber was 1.8 m square,and 8: I mixtures of sulfur and saltpeter iniron carts were rolled into it and burned with

intermittent admission of air." As in the glassretorts, the sulfur trioxide that was formedcombined with water to produce sulfuric acid,which condensed on the walls and collected inpans. Steam was introduced into the chambersin 1774, and continuous addition of air wasbegun in 1793. It then was recognized that thesulfurous acid from the burning of sulfur wasoxidized by air and needed saltpeter only as acatalyst. In 1827 Gay-Lussac invented a towerfor recovering the nitrogen oxides escapingfrom the chamber. The nitrogen oxides werecondensed in sulfuric acid but could not beeconomically recovered from the acid untilthe invention of the Glover denitrating towerin 1859. The introduction of these two towerscompleted the chamber process except forvarious refinements to reduce costs. Theprocess could produce acid with up to 77 per­cent H2S04 but generally yielded strengths ina 65-68 percent range. Higher-strength acidwas produced by boiling chamber acid toremove water. A platinum still for producingconcentrated sulfuric acid from chamber acidwas first built for the Harrison Works inPhiladelphia in 1814.

The developing markets for sulfuric acid inthe late eighteenth century increased thedemand for Sicilian sulfur. By 1832, sulfurprices had risen to $80/ton, and stocks rose;then in 1833 the market broke, with the price

at $15/ton. Sicilian government attempts tostabilize sulfur at $70/ton failed, in partbecause of an 1833 discovery by a Frenchchemist that sulfur dioxide could be obtainedby roasting pyrites in a furnace. Processimprovements in pyrite roasting by 1870made pyrites competitive with sulfur as araw material. By 1880, with the singleexception of the United States, the sulfuricacid industries had gone to a pyrites basis.By 1909 virtually all U.S. sulfuric acid camefrom pyrites and as by-product acid fromzinc and copper smelters. In the late 1890s,development of the Frasch process for sulfurmining lowered the price of sulfur. Thischange, coupled with new developments insulfur burners, led sulfuric acid manufactur­ers back to sulfur as a raw material source.During World War I gypsum was burnedwith coal to produce S02 for sulfuric acidproduction. This process continued inEurope until the 1950s, with the last plant (inGreat Britain) shut down in 1975. Today sul­furic acid production from pyrite roastingremains important in Spain, the FormerSoviet Union, China, Japan, South Africa,Turkey, and some eastern Europeannations.f The Bosveld No.2 plant in SouthAfrica operated on gypsum until the early1980s when it was shut down.

The contact process for sulfuric acid datesfrom 1831, when a Briton patented a methodfor converting sulfur dioxide to sulfur triox­ide by passing the gas through a heated tubefilled with finely divided platinum. The sul­fur trioxide was adsorbed in chamber acid toproduce concentrated and fuming sulfuricacids. Commercialization of the newprocess was delayed by lack of markets forthe concentrated acid and poor understand­ing of the process parameters. About 1870,demand for fuming sulfuric acid spurredGerman development of the contact process.In 1901, BASF reported that the governingprinciples for the successful manufacture ofcontact acid were well understood. Theseincluded cleaning of the sulfur dioxide gasstream, use of excess oxygen, and tempera­ture control of the catalyst bed. However,the reliance of the contact process on feed

SULFUR AND SULFURIC ACID 1171

acid produced by the chamber process lim­ited its development until about 1930. Itbecame known that the acid in the final S03absorption tower had to be kept in the range98.5-99.0 percent H2S0 4 to be effective.Accurate dilution techniques to allow recy­cle of the absorber acid were developed inthe late 1920s. The contact process thencould continuously produce diluted acid,and no longer required chamber acid. Thedevelopment of poison-resistant vanadiumcatalysts for S02 conversion allowed the useof smelter gases in the contact process. Withits lower capital and operating costs, thecontact process then supplanted the cham­ber process for sulfuric acid production. By1940 the United States had equal numbersof chamber and contact processes. Today nocommercial chamber plants are operating inthe United States.

Since the 1940s most developments in thecontact process have focused on energyrecovery and pollution abatement. The pro­duction of S02, its conversion to S03, and thedilution of H2S04 are exothermic processesthat are exploited to reduce energy costs atsulfuric acid plants. Sulfur emissions havebeen sharply reduced by using two S03absorbers, although one absorber is the eco­nomic choice. Tail gas scrubbing processeshave been developed to further reduce sulfuremissions from sulfuric acid plants. No singleabsorber plants have been built in the UnitedStates since the 1970s, although there werestill some in operation as recently as 2000.As of the early 1970s, U.S. EPA regulationslimited new sulfur-burning sulfuric acid plantemissions to 2 kg S02 and 0.075 kg acid mistper metric ton of H2S04 produced, andmetallurgical plants to 650 ppm volumeS02' During the 1990s some local require­ments exceeded the EPA regulations andseveral plants were constructed with S02 lim­ited to 100 ppm S02 (about 0.5 kg S02 permetric ton H2S04 produced). One doubleabsorption plant followed by an ammoniabased DynaWave scrubber, built underMonsanto Enviro-Chem license in Ulsan,Korea, started operation in 1999 with S02at 30 ppm.

1172 KENT ANDRIEGEL'S HANDBOOK OF INDUSTRIAL CHEMISTRY AND BIOTECHNOLOGY

Fig. 25.8. Modern double absorption sulfuric acid plant with view of sulfur furnace in foreground.(Courtesy Monsanto Enviro-Chem.)

Manufacture of Sulfuric Acidby the Contact Process

The basic steps in the contact process are:(l) production of sulfur dioxide; (2) coolingand, for smelters, cleaning of the process gas;(3) conversion of the sulfur dioxide to sulfurtrioxide; (4) cooling of the sulfur trioxide gas;and (5) absorption of the sulfur trioxide insulfuric acid." Figure 25.8 is a photograph ofa contact process plant. A simplified diagramof a double absorption contact sulfuric acidprocess is shown in Fig. 25.9. Because sulfurdioxide is produced by several processes, it isconvenient to separate the discussion of sulfurdioxide production from its conversion tosulfuric acid.

Sulfur Dioxide Production

Sulfur is converted to sulfur dioxide by burn­ing molten sulfur with dried air in a sulfurburner to yield a 1000-1 200°C gas streamcontaining 10-12 percent S02. The burner ismounted at one end of a sulfur furnace, andthe gas passed through a waste heat boiler atthe other end. The gas temperature is reduced

to 420-440°c on leaving the boiler, whichproduces 40-60 bar steam.

In the simple pressure-nozzle burner, theliquid sulfur is atomized by pumping it at8-15 bar through the nozzle. In a two­component burner, the sulfur is atomizedprimarily by the combustion air stream. Itoperates at lower pressure and has a widersulfur throughput range, 5-170 tons/day, thanthe simple burner. Lurgi's rotary burner canbum up to 400 tons/day of sulfur. Largersulfuric acid plants, such as the Monsantodesigned Anaconda plant in WesternAustralia, use multiple sulfur guns in a singlefurnace to bum 1400 metric tons per day ofsulfur. A process gas with 18 percent S02 canbe achieved in Lurgi's two-stage sulfur com­bustion process. However, S02 gas producedfrom burning sulfur in air at 18 percentcannot be used directly in a contact sulfuricacid plant without the addition of dilution airas there is not enough oxygen in the gasstream to react with the S02 to achieve a nor­mal conversion to SO}. In the two-stageprocess excess sulfur is burned in the firststage, consuming all of the oxygen. The low

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1174 KENT ANDRIEGEL'S HANDBOOK OF INDUSTRIAL CHEMISTRY AND BIOTECHNOLOGY

residual oxygen level limits the formation ofnitrogen oxides, which otherwise would bevery high at the combustion temperature ofl750°C. The combustion gas then is cooledto 620-650°C, and additional dried air isinjected to burn the residual sulfur in an after­burner. A second waste heat boiler cools theprocess gas to 420-440°C before sending it tothe converter.

Pyrites and other iron sulfides are roasted toproduce an iron oxide cinder and an off-gascontaining 7-14 percent S02 which is con­taminated with varying amounts of arsenic,lead, zinc, and other metal oxides. The off-gasmust be cleaned before it is sent to theconverter. Various types of pyrite roastingequipment have been used in the past, includ­ing shaft furnaces, multiple hearth roasters,rotary kilns, and dust roasters. Fluid-bedroasters now are widely used for theirsuperior process technology, throughput rates,and economics. The roasting process must becontrolled between 850 and 940°C. At lowertemperatures the reaction is incomplete,whereas at higher temperatures the ironoxides and sulfides form a eutectic melt thatinhibits the reaction rate. Fluid-bed roasterssurpass other types in temperature control andtemperature uniformity throughout the bed.Fluid-bed technology for S02 production wasintroduced in the early 1950s by Dorr Oliverin the United States and by BASF in theFederal Republic of Germany.

Copper, lead, zinc, and other sulfide oresmay be processed by roasting or smelting.Roasting or sintering of sulfide ore is essen­tially identical with pyrite roasting. Sulfurmelting generally occurs at higher tempera­tures. Older reverberatory furnace smeltersproduce off-gas with only 1-2 percent S02'too low for its economical recovery as acid.By using an oxygen-enriched air feed, the off­gas can be raised to 6-8 percent S02' Bathsmelters (Mitsubishi, Noranda), where the oreconcentrates are heated and reacted in theslag/matte melt, produce an off-gas with10-20 percent S02.25 Flash smelters (lnco,Outokumpu), which involve suspension andreaction of the concentrates in an oxidizinggas stream, operate at l200-1300°C and pro-

duce a waste gas with 10-15 percent S02'Oxygen enrichment of the feed air can raisethe S02 level in the off-gas to 30-80 percent.The Kivcet process smelts with pure oxygenand produces off-gas with 80-85 percentS02.26 Normally strong gases are diluted withair to 14 percent S02 in order to limit outlettemperatures in the converter first pass to lessthan MO°C and to provide sufficient oxygen toconvert the S02 to S03' In 1996 the originalOlympic Dam sulfuric acid plant in SouthAustralia was modified to operate with 18 per­cent equivalent S02 gas strength. This plantused a Monsanto preconverter and a cesium­promoted catalyst.27 Figure 25.10 is a photo ofa metallurgical gas sulfuric acid plant.

Off-gas from roasting and smelting opera­tions may contain dust, S03' halogens, NOx,

arsenic and other toxic metal fumes, and mer­cury.28,29 These components must be removedfrom the gas stream before it is sent to theconverter. Although S03 is produced in theconverter, its presence in the cooled gasesupstream of the converter will cause exces­sive corrosion by forming sulfuric acid mists.The cleaning plant steps are: (1) hot-gasdedusting; (2) wet scrubbing; (3) gas cooling;(4) mist removal; and, if necessary,(5) mercury removal. The hot gases generallyare passed first through a waste heat boilerto reduce the temperature to 250-400°C.Cyclones followed by hot-gas electrostaticprecipitators (50-90 kV) remove nearly all(99+%) of the dust. The gas then is contactedwith weak (5-30%) H2S04 in an open spraytower which removes metal vapors and addi­tional solids, cools the gas to 50-80°C, andconverts S03 to acid mist for later removal. Ifhigher levels of particulate removal arerequired, a venturi, Swemco or DynaWavescrubbers may be used. The saturated gas isnext cooled in a packed tower or shell andtube heat exchanger to condense excess water.Shell and tube heat exchangers of graphite oralloy construction are generally used only insmaller size sulfuric plants or where the gastemperature has a tight approach to availablecooling water. Silica packing is used in thetowers, or sodium silicate is added to the weakacid circuits to remove fluorides as fluosilicic

SULFUR AND SULFURIC ACID 1175

Fig. 25.10. Modern metallurgical sulfuric acid plant with view of preheating furnace in foreground.(Courtesy of Kennecott, Monsanto Enviro-Chem, and Manly Prim Photography.)

acid. In cases where the fluoride levels arevery high, additional liquid-gas contactingstages are provided to reduce the gas phaseconcentration of fluorides. Acid mist isremoved in wet electrostatic precipitators fit­ted with lead tubes and star wires or, morerecently, with FRP or PVC tubes speciallytreated to maintain a conductive liquid filmsurface (Lurgi)" and composite wires fabri­cated with barbs to promote corona dis­charge.i" If present, mercury is removed in anadditional tower by scrubbing with mercurychloride solution (Boliden)," hydrochloricacid solution, or 70-85 percent sulfuric acid.

Waste sulfuric acid sludges from petroleumrefineries are disposed of by conversion toS02 for production of fresh sulfuric acid. Theheavy organic components of the sludges canbe decomposed thermally at 800-1300°C(Lurgi, Monsanto Enviro-Chem) or reduc­tively at 20Q-600°C with coal in a rotary kiln(Chemico). Thermal decomposition is accom-

plished in a fuel-fired vertical or horizontalfurnace. The acid sludge also can be injectedinto fluid-bed pyrite roasters as a means ofdis­posing of the acid and reducing fossil fuelconsumption. Dilute acid sludges must beconcentrated to 60-75 percent H2S0 4 for eco­nomical conversion. This is generally done byusing waste heat from the decompositionprocess. The sulfur dioxide gas stream fromthese processes requires cleaning, as describedabove for roasting and smelting plants.

Calcium sulfate may be decomposed tocement clinker and sulfur dioxide gas in a coke­fired rotary kiln at 900-1400°C (Miiller­Kiihne)." However, the unfavorable economicsof this process relegate it to countries that donot have other sources of sulfur. Phospho­gypsum (gypsum produced by the acidulationof phosphate rock) may be decomposed in thisway as a means of recycling the sulfur values inthe large waste phosphogypsum piles at fertil­izer plants (OSW-Krupp and FIPRlDavy

1176 KENTANDRIEGEL'S HANDBOOK OF INDUSTRIAL CHEMISTRY AND BIOTECHNOLOGY

McKee). This process is hampered by the moreextensive gas cleaning requirements for decom­posing phosphogypsum as compared with nat­ural gypsum. During the early 1980s there wassome interest in recycling phosphogypsum. By2000, because of the high cost of theseprocesses, there was little commercial interest.Environmental forces also are behind the recy­cling of ferrous sulfate from metals industrypickling liquors. This "green salt" is decom­posed to sulfur dioxide and iron dioxide inpyrite roasters. Elemental sulfur, coal, or fueloil may be used as supplementary fuels.

Refinery waste gases may be burned toeliminate hydrogen sulfide and other sulfur­containing contaminants. Streams containingsmall amounts of H2 S or constituents unsuit­able for Claus plants may require combustionto S02 as the means of disposing of the toxicgas. The resulting effluent gas usually is lowin S02 and contains water vapor and carbondioxide. Flue gases from fossil fuel powerplants also fall into this category. Recovery ofthe sulfur values from these dilute gases usu­ally is driven by environmental considerationsrather than economics. In the United States,power plant flue gas often is scrubbed withlime to convert the S02 to gypsum sludge forlandfill disposal. Alternatively, the Bergbau­Forschung process recovers sulfur dioxide bydry adsorption on activated coke at BO°C.The S02 is released by heating the coke to600-650°C. Sulfur dioxide can be absorbed ina sodium sulfite solution (Wellman-Lord) toproduce sodium bisulfite. Pure moist sulfurdioxide can be recoveredby heating the sodiumbisulfite.

Single vs. Double Contact Process

The single absorption contact process for sul­furic acid is characterized by four main processsteps: gas drying, catalytic conversion of S02to S03, absorption of S03, and acid cooling.The maximum S02 conversion for a singleabsorption plant is about 97.5-98 percent. Byadding a second S03 absorber with one or twocatalyst beds between absorbers, the S02 con­version can be increased to 99.5-99.8 percentor even as high as 99.9 percent with a cesium­promoted catalyst, resulting in lower S02 emis-

sions. The so-called double absorption processis now the industry standard.

If water vapor is present in the gas stream orthe gas temperature or metal surface tempera­tures drop below the dew point, liquid acid isformed by condensation of H2S04 vaporrather than by absorption of S03' Therefore,the S02 laden process gas sent to the convertermust be dry to protect the downstream processequipment against corrosion. The drying gen­erally is done in a packed tower with recircu­lating concentrated (93-98%) sulfuric acidkept at 50-60°C by indirect cooling. The toweracid stream is heated by condensation of thewater and by dilution of the acid. The towertemperature is used to control the moisturelevel of the gas sent to the converter. Acidfrom the drying tower is cross flowed to theabsorber or is sent to storage tanks for ship­ment. When sulfur is burned, the combustionair to the sulfur burner is dried because thecombustion of sulfur does not produce water.Off-gases from pyrite roasters and metallurgi­cal smelters are dried as part of the gas clean­ing process. For roaster gases with low S02concentrations, a predryer may be addedupstream of the main dryer.

Oxidation of S02

Oxidation of S02 to S03 is accomplished inmulti-stage, fixed-bed catalytic convertersequipped with interstage boilers or heatexchangers to remove the heat of reaction.Typically, four stages are compartmentedwithin a single vertical converter, which maybe brick-lined, steel, or cast iron. Newer con­verters are stainless steel, and some have fivestages for higher conversion. Isothermal tubu­lar converters are no longer suited to modemhigh-capacity plants. The extruded cylindricalcatalyst pellets are usually 4-9 percent V205

with alkali metal sulfate promoters on a silicacarrier (diatomaceous earth, silica gel, or zeo­lites). The reaction temperature for vanadiumcatalyst is generally 410--440°C. In the late1980s a cesium-promoted catalyst becamecommercially available from Topsoe andMonsanto. These low-strike catalysts operateat 360--400°C.Higher temperatures (~600°C)

reduce the SOz conversion and lead to struc­tural damage of the catalyst. High-pressuredrops across catalyst beds from catalyst dustformed during processing require periodiccatalyst removal and screening to removedust. Ring-shaped catalysts developed byTopsoe and others have lower dust pressuredrops and are now in wide use. Other catalystshapes used are ribbed rings and cylinders.The usual catalyst loading per one toniday sulfuric acid capacity is 150-200 L in adouble absorption plant and 200-260 L ina single absorption plant. Bayer developedand operates fluid-bed converters that utilizespecial 0.3-1 mm abrasion-resistant catalysts.

Absorption of S03

Sulfur trioxide from the converter isabsorbed in 98 percent HzS0 4 recirculatedcountercurrently through a packed towermaintained at 60-80°C by indirect cooling.The optimum concentrationofthe absorber acidis near the Hzo-HzS04 azeotrope, 98.3 percentHzS04, where the S03, HzS0 4, and HzO vaporpressures are at their lowest values. Absorptionefficiencies in excess of 99.9 percent gener­ally are obtained. On leaving the converter, theprocess gas is cooled first with feed gas in agas-gas heat exchanger and then with boilerwater in an economizer to 180-220°C beforeit enters the absorber. An impingement sepa­rator, or Teflon or glass fiber mist eliminator,is placed in the top of the absorber to removeacid mists. If oleum is produced at the plant, itis made in a separate oleum tower upstream ofthe absorber. A portion of the S03 stream tothe absorber is diverted to the oleum towerwhere it is absorbed in a recirculating streamof oleum.

In double absorption plants an intermediateabsorber is placed between the second and third(or between the third and fourth) converterbeds. By removing S03 from the gas stream atthis intermediate point, higher SOz conver­sions are attained in the downstream converterbeds, and the overall SOz conversion isincreased. The cooled gas from the intermedi­ate absorber is reheated by hot converter gas ingas-gas heat exchangers before returning to

SULFUR AND SULFURIC ACID 1177

the converter. An oleum tower may be placedbefore the intermediate absorber.

Acid Cooling

Absorption of S03 in concentrated sulfuricacid and the formation of HzS04 from S03and HzO produce heat in the absorber, as doesacid dilution from the addition of makeupwater. Process control requires that the acidbe cooled before it is recirculated to the dryeror absorber towers or sent to storage. Earlieracid coolers of parallel banks of stacked, irri­gated, cast iron sections have been largelyreplaced by stainless steel shell and tube orplate exchangers, with or without anodic pro­tection. Hastelloy, Sandvik SX, ZeCor, andSaramet alloys and Teflon linings are alsoused in acid piping and coolers.32

-35

Tail gas emissions are controlled by improv­ing the SOz conversion efficiency and byscrubbing the tail gas. In a double absorptionprocess plant, a five-bed converter has0.3 percent unconverted SOz, as comparedwith 0.5 percent for a four-bed converter.A Lurgi Peracidox scrubber may be used toremove up to 90 percent of the residual SOz inthe tail gas from a double absorption plant.Hydrogen peroxide or electrolytically pro­duced peroxymonosulfuric acid is used to con­vert the SOz to H2S04 in the Lurgi scrubber.

Other Modifications tothe Sulfuric Process'"

Tail gas from single absorption plants maybe absorbed on activated carbon (Sulfacid)or scrubbed with ammonia (MonsantoAMMSOX) or sodium sulfite (Wellman­Lord). Metallurgical acid plants differ from sul­fur-burning plants in that the cleaned SOzprocess gas must be heated before it is sent tothe converter. Many of these plants have weakSOz streams that require large gas-gas heatexchangers for temperature control. Four plantsin the Former SovietUnion processing 2-4 per­cent SOz use an unsteady-state oxidationprocess in which the cold (40-70°C) SOzgas isreacted on hot catalyst beds without interveningheat exchangers.F'" As the temperature front

1178 KENTAND RIEGEL'S HANDBOOK OF INDUSTRIAL CHEMISTRY AND BIOTECHNOLOGY

moves through the bed to the exit side, theflow is reversed. Cycle times are 30-120min, and single-bed conversions of 80-90percent are reported as compared with 55-60percent for conventional processes at higherexit temperatures.

The thermal capacity of a 1000 ton/daysulfuric acid plant is about 63 MW This heatliberation must be controlled in a manner thatmaintains optimum gas temperatures in theconverter system and optimum acid tempera­tures in the dryer and absorber circuits. Tailgas emissions also are affected by the energybalance. Figure 25.11 shows an energy flowdiagram for a contemporary sulfur-burningsulfuric acid plant. About 97 percent of thetotal energy input derives from burning sulfur,and 3 percent comes from the electricity con­sumed to drive the gases through the plant.Most plants can recover 55-60 percent of theenergy as high-pressure steam (40-60 bar,400--480oq , but about 40 percent is lost aswaste heat dissipated to the environment inthe form of hot water from acid coolers.

During the late 1970s acid plants wereoptimized to generate more steam. Steam canbe produced at pressures up to 80 bar from

III

high-temperature sulfur burners such as theLurgi two-stage combustion system. Thehigh-pressure steam is let down to low pres­sure steam through a turbogenerator thatco-generates electricity. In double absorptionplants, economizers were installed upstreamor downstream of the heat exchangerservicing the intermediate absorber. Boilerfeed water is preheated to 90-95°C in thiseconomizer to increase steam production.The energy production from acid plantswas increased to 70 percent by installinglow-gas-temperature economizers, low­pressure-drop catalysts, and suction dryingtowers, by increasing the S02 feed gas con­centration, and by preheating the boiler feedwater with hot acid. Further energy recoveryrequires higher operating temperatures for theabsorbers and acid coolers. Venturi concur­rent absorbers operating at acid temperaturesof 130-140°C are installed in several plants.To recover acid heat directly as steam, itwas necessary to increase the acid tempera­tures from 11O-120°Cto about 200°C.

As of 2005 Monsanto Enviro-Chem hadbuilt 21 Heat Recovery System (HRS) unitssince demonstrating the first HRS at Namhae

TAil GAS2.511

'---lXI~" ~~~CT ACID

28% 7lIL--.1~--~. WASTE HEAT

FROM ACID----~~-----~~ ;:;"",. ___' COOUNG 37%

Fig. 25.11. Sankey energy flow diagram for a 1000 ton/day sulfur-burning double absorption sulfuricacid plant (feed gas 10% S02)' A: Blower; B: Sulphur furnace; C: Waste heat boiler; D: Catalyst bed 1;E: Steam superheater; F: Catalyst bed 2; G: Boiler; H: Catalyst bed 3; J: Intermediate heat exchangers;K: Intermediate absorber; L: Converter bed 4; M: Economizer; N: Final absorber; 0: Air dryer; P: Acidcoolers. (Courtsey Lurgi GmbH, Frankfurt, Germany.)

Chemical in South Korea in 1987.38,39 Theprocess is based on 310 stainless steel, whichresists corrosion in 98.5 percent H2S04 at tem­peratures up to 220°C. The intermediateabsorber at Namhae takes 194°C gas fromthe converter third stage economizer andabsorbs the S03 in 199°C, 99 percent acid.Recirculated acid from the absorber is cooledfrom 220°C in a 10-bar HRS boiler. The addedenergy recovery for this process is reportedto increase the total recovery to 90 percent.Monsanto's proposed Monarch process com­bines HRS technology with the wet catalyticconverter process (Lurgi) to increase heatrecovery and shift it to high-pressure steamproduction for electric power generation.36

Other Sources of Sulfuric Acid

Spent sulfuric acid usually is diluted in theprocess in which it is used: titanium dioxidepigment processing, plastics manufacture, andso on. The dilute acid may be used in processesrequiring dilute acid or may be concentratedfor reuse by a number of vacuum evaporationprocesses (Simonson-Mantius, Chemetics)40

or by thin-film evaporation (DuPont, Bofors).In the submerged combustion distillationprocess, water is evaporated from the diluteacid by forcing hot flue gases from a fuel-firedburner below the acid surface (Chemico). Theconcentration of 75 percent acid to 95-98 per­cent H2S04 by the Pauling-Plinke process isdone by feeding the 75 percent acid to a strip­ping column fitted with a stirred cast iron potmounted in a furnace. The acid concentrationin the pot must be kept above 80 percent tominimize corrosion.

Chemetics has developed a process fortreating spent alkylation sulfuric acid withnitric acid to produce a sulfuric acid that canbe used to acidulate phosphate rock, the majoruse for sulfuric acid. The organic contami­nants are converted to carbon particles that areremoved with the gypsum on filtration of thephosphoric acid. Special alloys are used in thefabrication of the acid reactor. Topsoe devel­oped and, by the year 2005 had built, morethan 45 Wet Sulfuric Acid (WSA) processunits. This process is especially suited for

SULFUR AND SULFURIC ACID 1179

low-strength, less than 4 percent, S02 gasstreams which would not be auto thermal ormeet water balance conditions in the conven­tional dry sulfuric acid contact process. In theWSA process wet S02 gases pass throughconverter beds where the S02 is oxidized toS03' The S03 reacts with water vapor to formH2S04 in the gas phase. The acid is condensedin proprietary WSA condensers. Sulfuric acidis produced at concentrations around 98 per­cent.41,48

Production and Consumptionof Sulfuric Acid

The world production ofsulfuric acid (1999 and1989) is given in Table25.5. There was virtuallyno change in the global use of sulfuric acid dur­ing the decade. But major regional shifts didtake place. Asia, driven by the Chinese pushtowards self-sufficiency in phosphate fertiliz­ers and increased manufacturing activity,nearly doubled its use during the decade. InAfrica, Morocco added fertilizer capacity, andH2S04 use was up 41 percent, although ele­mental sulfur use was unchanged during thestart and end of the decade. Significantchanges also affected the raw materials used tomanufacture sulfuric acid.

A large shift occurred from pyrite roastingto acid recovered for environmental reasons.Almost all nations reduced their use of pyritesas a result of environmental considerations.China's increase was only temporary: by thelate 1990s that nation was joining the rest ofthe world in shifting from pyrites to elementalsulfur. The production of sulfuric acid as aresult of pollution abatement regulations rep­resents the industry's only growing segment,with a 30 percent gain over the decade. Thattrend should continue. The closing of threecopper smelters in the Southwestern UnitedStates will, however, reduce SOF productionby around 1 million tons (Table 25.6).

Table 25.7 lists the sulfuric-acid-consumingindustries in the United States and showsthe trends in their acid consumption ratesthrough the 1980s and 1990s. Agriculturefurther increased its dominant use of sulfuricacid, accounting for 77 percent in 1999 vs.

1180 KENT ANDRIEGEL'S HANDBOOK OF INDUSTRIAL CHEMISTRY AND BIOTECHNOLOGY

TABLE 25.5 World Production of New Sulfuric Acid for 1999 and 1989(Million Metric Tons 100% H2S04) 42

1999 1989 Difference

Elemental Sulfur Total Elemental Sulfur Total Elemental Sulfur Total

Western Europe 8.5 17.5 12.3 24.9 -3.8 -7.4

France 1.4 2.2 3.5 4.1 -2.1 -1.9

Germany 1.2 3.0 1.6 3.7 -0.4 -0.7

Spain 0.3 2.8 0 3.7 0.3 -0.9

Central Europe and FSU 10.3 15.6 24.5 38.5 -14.2 -22.9

Poland 1.2 2 2.5 3.1 -1.3 -1.1

FSU 8.3 12 18.7 28.3 -10.4 -16.3

North America 37.6 46.2 37.8 44.6 -0.2 1.6Canada 1.6 4.8 1.2 3.5 0.4 1.3U.S. 33.2 37.1 33.3 37 -0.1 0.1Mexico 2.8 4.3 3.4 4.1 -0.6 0.2

Latin America 5.6 10.2 4.3 5.7 1.3 4.5Brazil 4.0 4.8 3.0 3.7 1.0 1.1

Africa 17.1 18.1 10.0 12.8 7.1 5.3Morocco 8.5 8.5 4.1 4.1 4.4 4.4Tunisia 4.8 4.8 3.2 3.2 1.6 1.6South Africa 2.5 3.1 1.1 3.1 1.4 0

Asia 16.5 42.0 12.7 30.7 3.8 11.9China 4.3 21.6 1.0 11.4 3.3 10.2Japan 2.2 6.9 1.2 6.9 1.0 0India 5.2 6.1 3.6 3.9 1.6 2.2

Oceania 1.1 2.5 1.2 2.0 -0.1 0.5

World Total 102.8 158.7 102.8 159.2 0 -0.5

Source: Con-SuI, Inc. and Freeport Sulfur Marketing Department estimates.

70 percent in 1989. Even so, lower phosphaticfertilizer production reduced sulfuric acid usein agriculture by almost 10 percent. The U.S.economic shift from manufacturing to serv­ices and higher valued products is highlighted

TABLE 25.6 Sulfuric Acid Productionfrom Pyrites and Other Forms (MillionTons 100% H2S04)

1999 1989 Difference

Pyrites SOF Pyrites SOF Pyrites SOF

World 16.0 40.0 25.4 31.1 -9.4 8.9Western Europe 2.1 6.9 6.9 5.7 -4.8 1.2Eastern Europe

FSU 1.0 4.3 5.9 8.1 -4.9 -3.8North America 8.6 6.3 2.3Asia 12.3 13.4 10.3 7.6 2.0 5.8

Source: U.S. Geological Survey, Con-Sul, Inc., Freeport SulfurMarketing Department.

by the 37 percent decline in sulfuric acid con­sumed by nonagriculture industries.

Nearly all the sulfuric acid consumed inagriculture was reacted with phosphate rock(principally Ca9(P04)6CaFz) to produce phos­phoric acid. Granular phosphate fertilizersare produced by ammoniating phosphoric acidto yield mono- and diammonium phosphates;ammonium phosphate is also produced as afertilizer. Petroleum refining is the largestnonfertilizer use for U.S. sulfuric acid. Theacid competes with hydrogen fluoride asa catalyst in petroleum alkylation reactions forgasoline production. Sulfuric acid acts as acatalyst in synthetic rubber and plastics manu­facture. Copper ore leaching is carried out bydistributing the acid over leach piles of the oreand collecting the copper-rich leachate forprocessing. Sulfuric acid from nearby smeltersis normally used in copper ore leaching.

SULFUR AND SULFURIC ACID 1181

TABLE 25.7 Sulfuric-Acid-Consuming Industries in the United States (Millions ofMetric Tons, 100% H2SO4)

Consuming Industries 1999 1989 1984 1979

Agriculture 24.5 26.9 26.8 24.1Phosphatic 23.7 26.4 26.4 23.2Nitrogenous 0.6 0.3 0.3 0.5Pesticides 0.1 0.1 0.1 0.2Other agricultural 0.1 0.1 0.1 0.2

Other industries 7.2 11.5 11.1 14.1Petroleum refining 1.6 2.1 2.1 2.4Synthetic rubber and plastics 1.2 I 0.6Rayon and cellulose 0.2 0.2 0.3Other chemicals 1.4 2.7 3.2 4.2Copper ore processing 2.2 1.8 1.0 2.1Uranium and other ore processing 0.3 0.4 0.4Iron and steel 0.2 0.3 0.9Other metals 0.2 0.2 0.2 0.1Pulp and paper 0.4 1.0 0.8 0.8Pigments and paint 0.5 0.4 0.3 0.6Other 0.9 1.6 1.8 1.6

Total 31.7 38.4 37.8 38.1

Source: U.S. Geological Survey and U.S. Bureau of Mines: Sulfur Annual Reports.

REFERENCES

I. British Sulphur Corp: Ltd, Sulphur, No. 193,33-36 (Nov/Dec. 1987).2. International Fertilizer Industry Association, Paris, France, 2004.3. (a) Sulfur Magazine, No. 259, p. 41, Nov.-Dec. 1998, Gerrie du Plessis. (b) Hyne, 1.B., "The Forming, Handling

and Transportation of Elemental Sulphur," Presented at Sulphur 1990 Conference, The British SulphurCorporation, London.

4. (a) Ibid, 45 (b). (b) Kemp, E., Hyne, 1. B., and Rennie, W. 1., "Reaction of Elemental Sulfur with Water UnderU.V Radiation," Internal. 1. Sulfur Chem., Part Al(1) 69-70 (1971).

5. (a) Personal communication, Pennzoil Sulphur Co. Operations at Antwerp terminal. (b) Dowling, N. 1.,Clark, P. D.,and Hyne, 1.B., "Understanding and Mitigating Corrosion During Handling and Transportation of ElementalSulfur," Sulphur 1996 Conference, The British Sulphur Corporation, London.

6. Sandvik Process Systems, lnc., 21 Campus Rd., Totowa, NJ, USA.7. Berndorf Belt Systems USA, 920 Estes Avenue, Schaumburg, IL, USA.8. Devco International, Inc., 6846 S. Cauton Ave., Suite 400, Tulsa, OK, USA.9. Enersul, Inc., 7210 Blackfoot Tr. SE, Calgary, Alberta, Canada.

10. Ibid. 51.II. Kaltenbach-Thuring SA, 9 Rue de l'Industrie, 6000 Beauvais, France.12. d' Aquin, G. E., Transporting Sulfur Pellets, U.S. Patent 6368029, Apr. 9, 2002.13. Hyne, 1. B., "Some Impurities in Elemental Sulphur-Origins and Elimination," Sulphur 1991 Conference,

The British Sulphur Corp, London.14. Thieler, E., Sulphur, Theodor Steinkopff, Leipzig, 1936.15. In 2001, Iraq and Poland were the only nations with operating Frasch mines.16. d'Aquin, G., "North American Sulphur Perspective," paper read at Sulphur 1998 Conference, British Sulphur

Corporation, London.17. d' Aquin, G., North American Quarterly Sulphur Review, Con-Sui, Inc., Tulsa, OK (Apr. 2002, Jan. 2005).18. British Sulphur Corp. Ltd, Sulphur, No. 193, 26-30 (Nov.lDec. 1987).19. British Sulphur Corp. Ltd, Sulphur, No. 192,30-39 (Sept./Oct. 1987).20. Rameshi, M., "State of the Art in Gas Treating," Sulphur 2000 Conference, The British Sulphur Corporation, London.21. Ober, 1., Sulfur-2000 Annual Report, Mineral Industry Surveys, U.S. Geological Survey, U.S. Dept of the

Interior, Reston, VA.

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22. Ober, 1.,Sulfur-1991 Annual Report, Mineral Industry Surveys, US. Bureau of Mines, US. Dept of the Interior;Ober 1.,Sulfur-2003 Annual Report, Mineral Industry Surveys, US. Gedogical Survey, US. Dept. of the Interior.

23. d' Aquin, G., North American Quarterly Sulfur Review, Con-Sui, Inc. Tulsa, OK (Jan. 2005).24. Ober, 1., Op. Cit.25. Sohn, H.Y.,George, D. B., and Zunkel, A. D., Advances in Sulfide Smelting, The Metallurgical Society ofAIME,

Warrendale, PA, 1983.26. British Sulphur Corp. Ltd, Sulphur, No. 190,33--40 (May/June 1987).27. Bhambri, N., Fell, R. C; Fries, R. M., and Ritschel, P. M., "Metallurgical Sulfuric Acid Plants for the New

Millennium," Sulphur 98 Conference, British Sulphur Corporation, London.28. Sander, U H. E, Fischer, H., Rothe, U, and Kola, R. Sulphur. Sulphur Dioxide. and Sulphuric Acid, Verlag

Chemie, Weinheim, 1984.29. Friedman, L., "The Sulfuric Acid Plant Recent Advances-Reviews and Analysis," paper read at the AIChE

Central Florida Meeting, Clearwater, FL, June 2, 1988.30. British Sulphur Corp. Ltd, Sulphur, No. 203, 24-32 (July/Aug. 1990).31. British Sulphur Corp. Ltd, Sulphur, No. 207, 37--41 (Mar./Apr. 1990).32. Davies, M., Hodgson, D. S., and Rodda, 1."Applications of SARAMET in H2S04 Plants," paper read at Sulphur 88,

Vienna, Austria, Nov. 6-9, 1988.33. Douren, L., "Sandvik SX-The Future Material for Concentrated Sulfuric Acid," paper read at Sulphur 87,

Houston, TX, Apr. 5-8, 1987.34. Holdschick, H., and Jurascheck, M. A., "Advances in the Development of Cast Materials for High Concentrated

Sulphuric Acid at Temperatures up to 280°C," paper read at Sulphur 88, Vienna, Austria, Nov. 6-9, 1988.35. Puricelli, S., Fell, R. C, and Randolph, D. R., "The State of Sulfuric Acid Technology," AlChE meeting,

Clearwater, FL, May, 2000.36. McAlister, D. R., Grendel, R. w., Schneider, D. R., Shafer, 1.R., and Tucker, 1.S., "A Sulfuric Acid Plant for the

I990s," paper read at Sulphur 1990, Cancun, Mexico, Apr. 1--4, 1990.37. Blicharz, M., "Different Methods of Sulphur Removal from Gas from Non-ferrous Metal Works," paper read at

Sulphur 88, Vienna, Austria, Nov. 6-9, 1988.38. Shafer, 1.R., "Namhae Heat Recovery System Update," paper read at Fertilizer Industry Round Table, Baltimore,

MD, Nov. 15, 1988.39. Smith, R. M., Sheputis, 1.,Kim, U B., and Chin, Y. B., "Sulfuric Acid Heat Recovery System (HRS) Operations

at Namhae Chemical Corporation, Korea," paper read at Sulphur 88, Vienna, Austria, Nov. 6-9, 1988.40. Lebel, G., "Pilot Plant Development of the Chemetics Ti02 Waste Acid Recovery Process," paper read at Sulphur

1990, Cancun, Mexico, Apr. 1--4, 1990.41. British Sulphur Corp. Ltd, Sulphur, No. 253, 67--69 (Nov.lDec. 1997).42. The British Sulphur Corp. Ltd, WorldSulphur and Sulphuric AcidAtlas, 5th ed., Purley Press Ltd, London, 1989.43. Duecker, W.w., and West, 1.R. (Eds.), The Manufacture ofSulfuric Acid, Van Nostrand Reinhold, New York, 1959.44. Fairlie, A. M., Sulfuric Acid Manufacture, Reinhold, New York, 1936.45. Heydorn, 8., Aguiar, D., and Ferguson, A., "Sulfuric Acid," in Chemical Economics Handbook, SRI

International, Menlo Park, CA, 1987.46. Matros, Y. S., and Bunimovich, G. A., "Reverse Process of S02 Oxidation in Sulfuric Acid Production," paper

read at Sulphur 1990, Cancun, Mexico, Apr. 1--4, 1990.47. Wyld, w., Sulphuric Acid and Sulphur Dioxide Raw Materials, D. Van Nostrand, New York, 1923.48. Polk, P. (Topsoe Houston, Tx) e-mail to Fell, R.C. Feb 3, 2005.