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INDUSTRIAL A N D ENGINEERING CHEMISTRY
gas from the Simpson and the Ellenburger formations of the Ordovician System.is low, the estimated gas reserves are high; any gasoline plant processing such gas in large quantities will make available a considerable tonnage of hydrogen sulfide. For example, a plant which processes 1OO,O0O,OOO cubic feet perday of Ellenburger gas with an average of 0.2 mole %of hydrogen sulfide will produce the equivalent of about 8 long tons of sulfur per day. Oil and gas reserves, and hence hydrogen sulfide reserves, available for future recovery are closely related to the discovery dates of the fields concerned. Though the oil production in the Permian Basin dates back to about 1923,the data in Table I11 show that new discoverieshave moved forward steadily since that time. The total number of field discoveries in Table 111 does not necessarily agree with the data of Figure 2, for some fields may have more than one major producing horizon. Even though most discoveries prior to 1940 were in the normally high-sulfur Permian Series, over one hundred fields of that age have been discovered since that date. Because fields may vary in proved production area from 40 to as much as 75,000 acres, a mere tabulation of the number of fields is of little value. However, it is of vital interest to note that of the 961,000acres of oil fields in the Permian Basin 896,000
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acres have production from horizons of Permian age. Of the gasoline plants proposed, under construction, or in operation, twenty-five have source production of Permian age only, seven of Permian and non-Permian age combined, and ten of exclusive non-Permian age. Although the newer plants are being built to process gaa from the older horizons, i t is evident that the operators of the plants do not anticipate any serious depletion of the higher-sulfur-bearing gases of Permian age in the near future. Hydrogen sulfide is now available in sizable quantities in the Permian Basin of West Texas and New Mexico; but the area is large, and hence the availability in any one region is limited by the amount of gas that is processed in that vicinity. Although reserves are known to be large, they will vary so widely from one area to the other that they can be estimated only aa from as low as 10 to as high aa 25 or more years a t c u m n t rates of production.
ACKNOWLEDGMENT Among the primary sources of information on which this paper is based were V. E. Cottingham, chairman, North Basin Pools En ' eering Committee. W. J. Murray, chairman, Texas RailroayCommission; R. k,. Purvin and associates, Dallas; and D. H. Tucker, El Paso Natural Gas Company. REOBIVED ~ p r i li a , 1960.
Economic Utilization of Sulfur Dioxide from Metallurgical Gases R. A. KING The Consolidated Mining and Smelting Company of Canada, Limited, Trail, B. C. T h e peculiar conditions regarding the dispersion of smelter smoke a t Trail are outlined, and the development of the various sulfur-recovery processes employed is traced, with mention of the technical and economic considerations involved. The Trail smelter i s located in a mountainous area where topographical and meteorological conditions are unfavorable for adequate dispersion of the sulfur dioxide contained in metallurgical waste gases. Expansion of the smelter output in the middle 1920's resulted in some atmospheric pollution, and proximity to the United States border lent International complications. Many of the developments were based on integration of new plants with existing ones and utilimtion of by-product materials from established operations. The availability of ammonia made possible i t s employment as an absorbant for sulfur dioxide. The acidification process for releasing sulfur dioxide from the absorbing solution was practical because a supply of sulfuric acid was at hand and the company was already producing and marketing ammonium sulfate. By-product oxygen made possible operation of the sulfur dioxide reduction process, and later the availability of by-product oxygen and substantially pure sulfur dioxide made possible the development of the cyclic process of sulfuric acid manufacture. The IS years prior to 1943 witneesod the development of processes and installation of fertilizer plants a t Trail, with the reault that sulfur dioxide has changed from a waste material to a valuable raw material in ehort supply. To meet the demand for acid for fertilizer production, roasting of iron ooncentratu for add reaovery h a m e necessary In 1943. At present all master and dntering
plant gas is treated for sulfur recovery, and iron concentrate is roasted intermittently, depending on the demand for acid and the availability of customs zinc concentrates. Total current loss to the atmosphere from Trail operations is less than 9% of the sulfur charged. The sulfur released to the atmosphere annually in 1947 and 1948 was less than in any other year since 1904.
T
RAIL'S problem of dilution of the sulfur dioxide emitted from the smelter originated in 1894,when August Heinse's copper plant commenced operation on Rossland copper-gold ore, In 1897 the Canadian Pacific Railway bought out Heinse to obtain his railway rights and thus became the owner of the Trail smelter. In 1901 a lead plant was added to treat the rich ores from the Slocan district, and in 1906 the present company, The Consolidated Mining and Smelting Company of Canada, Limited, was formed, still controlled by the Canadian Pacific. The Sullivan mine in East Kootenay was acquired in 1910, and all subsequent developments were based on this property. Operation of the zinc plant commenced in 1916,but the Sullivan mill was not completed until 1923,following successful solution of the problem of separating mixed lead, zinc, and irgn sulfide ores high in iron. It was not until 1926 that a claim for smoke damage was filed from the Northport, Wash., area. This date coincided both with plant expansion doubling the metal output and consequently the emission of sulfur dioxide and with the erection of two stacks 409 feet high. An International Joint Commission waa appointed in 1927 to
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INDUSTRIAL AND ENGINEERING CHEMISTRY
investigate the alleged damage to vegetation in the state of Washington by fumes from the Trail smelter. After lengthy hearings, the total damages up to 1932 were assessed by this body a t $350,000.
Vol. 42, No. 11
Columbia River about 11 miles by river channel north of t,he international boundary. From the Arrow Lakes, about 20 miles above Trail, to Marcus, Wash., approximately 35 miles down stream from the boundary, the Columbia flows through a valley
Aerial View of Trail Metallurgical Plants with Fertilizer Plants in Background
Following further claims, an Arbitral Tribunal was set up to award any damages occurring during and after 1932 and to determine what measures might be adopted to abate the nuisance. Damages totaling $78,000 were awarded for the period 1932 to 1937, although claims for nearly $2,000,000 were filed by the United States. For 1938 to 1940 the United States claimed $35,OOO damages, but the tribunal ruled that no damages had occurred. The work of the tribunal included extensive meteorological investigations in the Columbia Valley near Trail, conducted by E. W. Hewson and G. C. Gill (a). In addition, an extensive investigation organized under the National Research Council of Canada covered measurement of sulfur dioxide content of the atmosphere at various points in the Columbia River Valley, surveys of conditions of field and forest crops and the sulfur content of vegetation, and experimental work on the effect of sulfur dioxide on conifers and crop plants under controlled conditions
(4).
The metallurgical reduction plants at Trail are located on a terrace 180 feet above the river, and the roaster gases are discharged from stacks 409 feet high. This is not sufficient height to ensure satisfactory diffusion at all times. When the wind velocity and turbulence are high the dispersion of smoke is satisfactory, but during periods of calm the gases accumulate in the valley and diffuse very slowly. Topographical features of the Trail area have an important bearing on the sulfur problem. The smelter is situated on the
ranging in width from about 0.5 to 3 miles, which acts as a natural trough for the smoke. The valley is bounded by mountain ranges rising 1500 to 3000 feet above the river level Kith occasional higher peaks, and is cut by several tributary streams. These features, together with the winding course of the river and valley, foster very complex air motions, making wind prediction most difficult. Both large and small eddies are produced by this irregular topography. These eddies can be explained when the wind velority is high, but they are almost impossible to account for when the winds are very light. During periods of light winds, the smoke can often be observed taking different directions from the various stacks-for example, the smoke from the lead sintering plant stack may be moving up river, that from the zinc stack down river, and the smoke from the blast furnace stack across the valley. Different meteorological conditions in summer and winter also have a bearing on sulfur dispersion. Temperature gradient in the valley has a marked effect on turbulence and consequently on diffusion of the smoke. In summer there is a more rapid decrease of temperature with height and more turbulent air motions result. In winter this decrease is not so noticeable, isothermal and even inversion temperature conditions being experienced at times, and the air in the valley is usually very stable. During these calm periods, which may last &s long as a week, the smoke from the smelter may accumulate under favorable atmospheric. conditions a t a stable isothermal atmospheric stratum or inversion layer, high above the valley. Diffusion from this type of
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I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
accumulation is relatively slow. Occasionally a change in atmospheric conditions will cause these accumulations to drop, bringing about circumstances which may result in sulfur dioxide fumigation of vegetation. The control measures specified by the tribunal are based on contemporary weather observations. The operating regime laid down regulates the amount of sulfur dioxide released to the atmosphere from the lead sintering plant based on the time of day or night and meteorological observations including readings of wind direction, velocity, and turbulence, and sulfur dioxide content of the atmosphere a t Columbia Gardens, near the boundary. These readings are recorded in a central control office and the operation of the sintering plant is directed from this office. The maximum concentrations of sulfur dioxide allowed to persist a t Columbia Gardens are 0.3 p.p.m. for 40 minutes in the summer and 0.5 p.p.m. for 60 minutes in the winter. A similar plan of smoke control had been instituted by the company in 1934, based on visual observation of the extent of diffusion of smoke in the valley and sulfur dioxide readings a t recorder stations. The regime imposed by the tribunal was considerably more severe, causing interference with operating efficiency. Obviously, the only practical solution waa treatment of all the gases for recovery of sulfur dioxide. This has since been accomplished. At the present time smoke control is under the direction of a special department operating from a control office strategically located in a penthouse on top of the general office building. The sulfur dioxide concentration in the atmosphere i s recorded continuously on instruments a t Columbia Gardens, B. C., Northport, Wash., and Waneta, B. C., on the border. Readings from the first two locations are relayed to automatic recorders in the control office, which also collects the required meteorological data. The Smoke Control Department exercises control over the operating plants with respect to sulfur dioxide emissions. In addition, control standards have been established for all stack emissions. Trained observers are on duty day and night. For several years losses of sulfur dioxide to the atmosphere have been maintained well within the limits imposed by the regime. As a result of constant vigilance by smoke control and operating personnel, sulfur dioxide emissions have decreased steadily since the sulfur-recovery plants were installed, and continuing improvement is foreseen.
HISTORY OF SULFUR DIOXIDE RECOVERY AT TRAIL The first recovery of sulfur dioxide a t Trail was effected in R small chamber plant operating on zinc roaster gas. The first unit was built in 1916, and duplicated in 1917, bringing the total capacity to 30 tons per day of chamber acid. These units were installed to make the refineries independent of outside sources of acid, because the supply was both unreliable and expensive. It has always been obvious that the sulfur as well as the metal content of the concentrates should be recovered and marketed. The threat of damage claims and pending international complications merely accelerated thia development. Preliminary studies indicated that the production of fertilizers offered the logical outlet for the large supply of sulfuric acid potentially available, particularly as the Canadian prairies would ultimately require large quantities of phosphate. This market waa undeveloped. however, and it waa apparent that a more flexible operation adaptable to a varying market demand was desirable. Accordingly, the decision waa made to manufacture ammonium sulfate and ammonium phosphate fertilizers, in spite of the poor outlook for nitrogen fertilizers a t that time (late 1920’s). The contact process for sulfuric acid manufacture waa selected because it was generally recognized as the most economical and because a small local market existed for oleum and 93% acid. Two eources of sulfur dioxide existed a t that time: gas from the Dwight and Lloyd sintering machines (lead plant) containing 0.5
,2243
to 1.0% sulfur dioxide, measured after the Cottrell treaters, and gas from the Wedge roasters (zinc plant) containing about 2.5% sulfur dioxide, also after the treaters. The contact process re-. quires considerably stronger gas than this for economical operation, 80 the first problem was to reduce the volume and raise the sulfur dioxide content of the waste gas. This was accomplished by the development of the suspension roasting process for treating zinc concentrates. Eight Wedge-type roasters were converted to the suspension system. The capacity of these converted units was equivalent to 25 of the original machines, and the gas strength a t the outlet of the treaters was over 6.5%. Operation of the first contact unit, a 35-ton-per-day modified Grillo process plant with platinum catalyst, commenced in 1929. This was in reality apilot unit to check the feasibility of operating a contact plant on zinc roaster gas and to investigate the market. Facilities for the production of oleum were also installed. This unit was followed by three National Process Company units which were put into operation in 1931, rated a t 112 tons per day each, employing vanadium masses. Following the installation of the National Process units, the sulfur dioxide absorption and reduction processes were developed. This development was dictated by two considerations. As attempts a t raising the concentration of the sintering plant gases using existing equipment had been unsuccessful, alternative processes had to be devised. Secondly, the depressed economic conditions of the early 1930’s indicated that it would be several years before the fertilizer market could absorb all the sulfuric acid potentially available from the waste gases. Shipment of liquid sulfur dioxide from a remote location such as Trail was not economic, but a market for sulfur was potentially available a t the Pacific coast sulfite pulp mills, so research work on the reduction of sulfur dioxide was undertaken. Preliminary studies on the recovery of 100% sulfur dioxide from roaster gas and reduction of this to elemental sulfur started in 1932. A number of absorbants were investigated and ammonia, which was available locally from the company’s fertilizer operations, was finally selected as the most feasible and econoniical. A 3-ton-per-day sulfur pilot plant was in operation in 1934. The experimental work was conducted on lead sintering plant gas, as this provided the most difficult concentration problem that would be encountered a t Trail. This research work led to the first large scale units, a semicommercial absorption plant operating on zinc roaster gas and a 40-ton-per-day reduction unit, which went into production in the summer of 1936. These plants were followed closely by a larger absorption plant operating on zinc roaster gas, a plant to treat the lead sintering plant gas, and two additional reduction units, bringing the total rated capacity to 150 tons of sulfur per day. In 1938, two Monsanto Chemical Company sulfuric acid units, rated at 125 tons per day each, were completed and put into operation. Vanadium catalyst was employed again in these plants. This raised the rated capacity of the installed units to 600 tons of 100% acid per day. At that time the original Grillo plant was being used only periodically in the production of oleum. I n 1943 greatly increased supplies of fertilizer were considered essential to the war effort, and a number of changes were made to, meet the fertilizer requirement of the Combined Food Board The sulfur dioxide reduction plant was closed down in July of that year and the 100% sulfur dioxide was used to enrich the gas to the contmt plants and increase their output. The Grill0 process plant, which had been idle for some years, was redesigned to operate on a cyclic process involving the use of pure sulfur dioxide and pure oxygen. After all the mechanical difficulties had been solved, a daily production of over 200 tons of 100% acid was maintained from this unit. The original platinum catalyst has been replaced with vanadium. I n that year also, the demand for sulfuric acid exceeded the supply from lead and zinc operations for the first time. Sullivcm
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Lead Sintering Plant Gas. SMELTERABSORPTION PLAXT. Gas from the sintering machines passes through a humidifying
iron concentrates were roaated to augment acid production. Suspension roasters were used and the gas was added to the zinc roaster gas. At the present time acid is produced from iron concentrat- intermittently, depending on the demand for acid and the availability of customs zinc concentrates. Two other processes have been put into operation as means of increasing the over-all recovery of sulfuric acid. Retreatment of sulfuric acid plant tail gas to recover unconverted sulfur dioxide commenced in 1945. Treatment of zinc plant stripped electrolyte to recover zinc and sulfuric acid commenced ixi 1947. The present capacity of the sulfuric acid plants a t Trail, inrluding the cyclic plant and the five conventional contact units, is 1300 tons of 100% acid per day. TAIL GAS TO ATMOSPHERE
LEAD
flue and a treater for dust removal before entering the absorption plant, where the sulfur dioxide content is absorbed in aqua ammonia, forming a solution that is essentially ammonium bisulfite.
The is treated in two parallel systems, each co,nprising a lead coolingtower,wood-sup orted and wood-pack&, 25 x 26 x 48 feet high, and three leaaabsorption towers, also wood-sup ported and packed, 25 feet square and 32 feet high. Flow of minute1 is Water in the cooling tower, UP to 1600 gallons countercurrent t o the gaa flow. The three absorption towers are having two partitions with the required constructed aa a openings for gas dividing the structure into three towers. The
AOU A AMMONIA
TAIL GAS TO ATMOSPHERE
'
ZINC ROASTING
ZINC PLANT
+
DRYING TOWER-DRYING
+
TOWERS
li
ACID PLANT TAIL GAS RETREATMENT
S t s ~ . m - t E L I M I N A T O R C I O O o / ~ SO2
AMMONIA
v
CAKE TO STOCK OR Z I N C P L A N T
Figure 1.
AMMONIUM SULPHATE SOLUTION STORAGE
TAIL GAS TO ATMOSPHERE
SULPHURIC ACID STORAGE
Flowsheet of Present Sulfur Dioxide Recovery Operations
A paper by Lepsoe and Kirkpatrick (8) provides a general description of sulfur dioxide recovery a t Trail. A broad, over-all picture is given in a more recent paper by Kirkpatrick (6), which describes the economic utilization of waste gases by the integration of base metal and chemical fertilizer operations. A Symposium on Atmospheric Contamination and Purification was presented a t the San Francisco meeting of the AMERICANCHEMICAL in March 1949 (1.2); reference was made to the Trail SOCIETY problem by Katz ( 5 ) and Swain (11).
DESCRIPTION OF PLANTS AND PROCESSES PLANTS OPERATING AT PRESENT
A t present all the gas from both lead and zinc roasting is treated for sulfur dioxide recovery. The following plants are operating: an absorption plant at the smelter operating on sintering plant gas, an absorption plant a t the zinc plant operating on zinc roaster gas, an acidification unit treating solution from both the absorption plants, five conventional sulfuric acid plants operating on enriched zinc roaster gas, and one recently developed cyclic process sulfuric acid plant operating on pure sulfur dioxide. I n addition, there are units treating sulfuric acid plant tail gases for recovery of sulfur dioxide and treating zinc plant stripped electrolyte for recovery of sulfuric acid. A simplified flowsheet depicting these operations is shown in Figure 1.
flow of gas is concurrent with the solution in the first tower, countercurrent in the second tower, and concurrent in the third tower. The circulating solution is pumped from the base of each absorption tower to a distributing spider at the top (flow being 1200 to 1500 gallons per minute in the first two towers and 600 to 800 gallons per minute in the third). A ua ammonia containing about 30% nitrogen is added t o the circaation. Temperature is controlled by passing the circulating solution through water coolers (aluminum tubes and steel shell) immediately after thc addition of ammonia, removing the heat of reaction. Temperature of the circulation is controlled at about 35" C. in the first tower and about 2 " lower in each. of the succeeding towers. Water from the tube coolers is used in the cooling towers before wasting to the sewer. Solution is bled forward from one tower base to the next, The plant make, containing about 240 grams per liter of sulfur as sulfur dioxide, is bled off the first tower to storage provided by two 60,000-gallon lead-lined wood stave t a n k . From storage the solution is pumped to the zinc absorption lant for acidification alon with the make solution from that rant. Gas is landled through the plant by two 8irocco double-inlet fans with Allan West speed controls, rated a t 150,000 cubic feet per minute at a static pressure of 10 inches of water. These are driven by 500-hp., 600 r.p.m. motors. Allis-Chalmers pumps are used for solution circulation. There are 12 pumps installed, rated at 800 gallons per minute and driven b 30-h motors. Plant control is based on analysis of the in?& a n a t a i l gas and the circulating solution from each absor tion tower. The familiar Reich test is used for gaa analyses. iolutions are assayed for sulfur dioldde by titration with 0.1N iodine solution and the pH is determined using special meters developed and built in the
.
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Cominco instrument shop. Aqua ammonia addition is manually mntrolled with rotameters for indicating the flow. This absorption system is handling currently about 300,000 cubic feet per minute of gas, 150,000 cubic feet per minute per unit, containing about 0.75% sulfur dioxide. The tail gas caryies about 0.10% sulfur dioxide. Absorption efficiency is of the order of 85%. Suspension Roaster Plant Gas. The gas from the suspension roasters passea through waste heat boilers and cyclones where the bulk of the dust is recovered. Most of the remainder of the dust is collected in a treater before the gas reaches the cooling towers. Four lead cooling towers, brick-lined and packed, known locally as Glover towers, are installed, one 10 X 10 X 24 feet high and three 20 feet in diameter by 28 feet high. In these towers the gas is cooled with circulatin water (250 gallons per minute per tower). After leaving the coo?in towers at about 40" C. the gas enters a header flue where the flow is split, part p i n g to the zinc absorption plant and part to the conventional acid plants. Sulfuric acid plant capacity is limited by gas volume, so that the conventional acid plants draw the volume of gas that they can handle and the absorption plant takes the remainder. In the absorption plant the gas is concentrated to practically 100% sulfur dioxide and delivered to the acid plants, the cyclic plant drawing what i t requires and the conventional plants receiving the remainder. ZINC ABNRPTIONPLANT.The volume of gas treated in this plant varies, depending on the number of roasters and acid plants operating. Normally the five conventional acid plants operate a t capacity, taking about 55,000 cubic feet per minute of gas, with the absorption plant taking the remainddr, which may amount to approximately 30,000 cubic feet per minute if all the suspension roasOers are operating. T h e gas is treated in four towers in series. Gas flow is countercurrent to the solution flow. The towers are 17 feet s uare and 35 feet high, built of lead, steel-supported, and w oda-packed. By-pass flues are provlded SO that any of the towers can be taken out of service for repacking without shutting down the plant. Aa in the smelter absorption plant, the solution is pum the base of each tower to a distributing spider at the top gallons per minute) and cooled with water in aluminum tube coolers after the addition of aqua ammonia containing 30% l$Ogl. Temperature of the circulation is controlled a t about in the first tower and about 2' C. lower in each of the succeeding towers, Solution is bled forward from one tower base to the next and the plant make is bled off the first tower to stora e, provided by two 60,OOO-gallon lead-lined wood stave tanks. !'he production from the smelter absorption plant is also stored in these tanks. Gas is handled through the zinc absorption plant by three Sirocco fans equip ed with hydraulic couplings. These fans are rated at 40,OOO cuiic feet per minute at a static pressure of 12 inches of water and are driven by 125-hp., 880 r.p.m. motors. Wilfley pumps are used on solution circulation (six installed), rated at 500 gallons per minute and equipped with 25-hp. motors Operation of this plant is facilitated by a continuous recorder giving the per cent sulfur dioxide a t the treater inlet and absorption plant inlet. Reich testa are run on the gas at the second tower outlet and on the tail gas, and the tower solutions are aaayed for sulfur dioxide content and pH. The plant is controlled using these figures and is operated to get 2% sulfur dioxide in the gas at the outlet of the second tower. Aqua ammonia addition is manuall controlled usin rotameters. Temperatures of the inlet androutlet gas are inicated and sulfur dioxide content of the divisional tail as (abeor tion plant plus acid plant retreatment tower tail gases? is recorfed on instruments in the control r m of this unit.
@
%$At present this plant is operating on an average of about 20,000 cubic feet per minute of gas containing approximately 5.5% sulfur dioxide. The tail gas analyzes slightly less than 0.2% sulfur dioxide. The absorption efficiency is almost 97%.
ACID~FICATION UNIT. Mixed solution from the smelter and zino absorption plants, stored in the two bisulfite make storage
2245
tanks, is filtered through Shriver presses using vinyon cloth and filter aid (three prea3es, 40 plates, 42 X 42 inches) and the filtrate is stored in a 200,000-gallon lead-lined wood stave tank. The filter cake is returned to the smelter. The filtrate, containing about 240 grams er liter of sulfur as sulfur dioxide, is pumped from storage to t i e acidifier. Type AB 2.5 X 2 inch Wilfley pumps with 10-hp. motors are used in this serwce. The solution is heated first by heat exchange with the eliminator effluent, hot ammonium sulfate solution. It is then heated further with steam in B stainless steel tubular heater, and mixed with sulfuric acid in a Pachuca-type acidifier. Two acidifiers are installed, one operating, one standby. These are steel tanks, lined with Pyroflex and acid roof brick, 8 feet in diameter and 10 feet high. The evolved sulkr dioxide gas and solution overflow into the eliminator, where the remainder of the gas is boiled out of the ammonium sulfate solution with direct steam. Two eliminators are installed, one operating in series with each acidifier. These are constructed of steel lined with Pyroflex and acidproof brick, and packed with spiraf rings. One is 9 feet in diameter and 22 feet high and the other is 7 feet 6 iwhes in diameter and 34 feet high. From the eliininator the ammonium sulfate solution, containing about 15 grams per liter of free sulfuric acid and substantially free of sulfur dioxide, flows by gravity to one of two 6000-gallon pump tanks or to a 200,OWgaIlon ammonium sulfate stora e tank. These tanks are all wood stave construction and leaflined. Coils are installed in the pump tanks t o preheat the bisulfite feed to the acidifier. Aqua ammonia is added to the ammonium sulfate in the pump tank to neutralize the free acid and produce a slightly ammoniacal solution to minimize corrosion of equi ment. The ammonium sulfate solution, containing about 4 2 6 ammonium sulfate, is umped to the fertrlizer plant and evaporated in Oslo-ty e crystalk e r s to produce a crystalline product for market. %o AllisChalmers stainless steel two-stage centrifugal pumps, rated at 125 gallons per minute at 275 pounds per square inch, are used to deliver the ammonium sulfate solution through two rubber-lined pipe lines to the fertilizer plant, located at Warfield, about 400 eet above the Tadanac metallurgical plants. The flow of gas from the eliminator is split, the cyclic acid plant drawing what it requires through its drying and purifyin system and the balance going directly to the Conventional acitf plants, where it is mixed with zinc roaster gas a t the outlet of the Glover towers. An Elliott blower, rated a t 1500 cubic feet r minute at a static pressure of 18 inches of water, is provideEo supply 100% sulfur dioxide to the cyclic acid plant. The gas to the cyclic plant is cooled in a tower with a circulation of water. A small amount of elemental sulfur formed by decomposition of traces of thiosulfate and thionates in the bisulfite solution is removed with a coke filter, and the sulfur dioxide is dried with 98% sulfuric acid. Two cooling towers and coke filters are avrtllable, normally one operatin and one standby. The cooling towers are lead, steel-supported, %rick-lined, 7 feet 4 inches square, one 36 feet high and the other 48 feet high, both Packed with spiral rings. The coke filters are steel, 8 feet X 10 eet X 5 feet 9 inches high, lined with lead and acidproof brick. The drying tower is a steel vessel, brick-lined, 8 feet in diameter and 24 feet high, also packed with spiral rings. Water coolers are provided on both the cooling and drying tower circulating systems. Acid from the drying tower circulation is fed into the acidifier, as is the condensate from the cooling tower. The o erating rate of the acidification unit is established by settin tEe bisulfite feed to the acidifier at the desired flow (up to 150 g&ons per minute) with a rotameter. A p H recorder-controller is installed on the eliminator efRuent (ammonium sulfate solution). This controls the sulfuric acid feed to the acidifier, Addition of steam to the eliminator is automatically adjusted by a temperature recorder-controller on the eliminator effluent. In addhion to these instruments, the sulfur dioxide content of the enriched gas to the conventional acid plants is recorded on an instrument in the acidification control room. The operator assays the eliminator effluent and Warfield solution for sulfur dioxide, acidity, and ammonium sulfate (by specific gravity) hourly.
SULFURIC ACID PLANTS. The sulfuric acid plants rtre not described in detail in this paper, as the development and operation of the cyclic plant have recently been discussed by Snowball (IO) and the conventional plants are all standard units. Cyclic Acid Plant. Development and operation of the cyclic process sulfuria acid plant have been described in some detail by Snowball (IO). The original modified Grillo process plant, capacity 35 tons per day, was redesigned to operate on pure sulfur dioxide and oxygen and produces 200 tons per day of 100% acid.
INDUSTRIAL AND ENGINEERING CHEMISTRY
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Make-up gases, 500 cubic feet per minute of oxygen and 1000 cubic feet per minute of sulfur dioxide, are added to the circulating gases after they leave the absorption system, ahead of the coke filter. Both oxygen and sulfur dioxide are dried with sulfuric acid prior to their addition to the circulation. The circulating gases, about 6000 cubic feet per minute containing ap roxi mately 20% sulfur dioxide and 36% oxygen, are conveyed gy a i Elliott blower into a heat exchanger system. From the heat exchangers the hot gas passes to the converters (two in parallel), then back through the heat exchangers and through the sulfur trioxide coolers. I t then passes into the absorption train comprising two parallel banks of three absorption towers. The sulfur trioxide is absorbed in 97.8y0 sulfuric acid, which is used instead of 98.401, to avoid formation of oleum in the first absorption tower. From the absorption towers the unconverted gas passes through spray catchers and is recycled through the coke filter to the conversion equipment. Make-up is added to the unabsorbed gases following the spray catchers and ahead of the coke filter. About 1% nitrogen is present in both the sulfur dioxide and oxygen. A small purge of about 30 cubic feet per minute is vented to the tail gas retreatment tower from the last absorber to maintain the required gas composition. Air is blown through tubes in the converters to control the temperature. This air is then passed through a heat exchanger to preheat the circulating gases before they are heated by exchange with the converted gases. Catalyst temperatures range from about 700" C. in the top tray to 500" C. at the heat exchanger inlet.
Production Data, Cyclic Acid Plant KO.of units
Rated capacity Efficiency
1 200 tons per day Practically 1 0 0 ~ o
National Process Units. The Sational Process installation at Trail has been described by Cohleigh (I). Three units are installed and operating. Final purification of the gas to all five conventional acid units is effected in the purification system installed for the National Process units. Gas to the capacity of the conventional acid units is drawn by fans from the cooling (Glover) towers and blown into three water wash towers, one in each unit. These towers are lead, steel-supported, brick-lined, and brick-packed. From the wash towers the gas passes to mist treaters, tube and star wire type, for removal of sulfuric acid mist. After the mist treaters the gas enters a header and IS split between the National and Monsanto units. All the 100% sulfur dioxide gas that is not required for the cyclic acid plant is added to the gas to the conventional plants prior to the cooling tower fans. Oxygen may also be added at this point, although it is usually preferable to add the oxygen at the suspension roasters and get the benefit of an enriched air supply in both operations. In the National Process units the gas from the mist treaters is dried in two towers, first with about 65% sulfuric acid and finally with 95 to 96% acid. Following the dryers a coke-packed s ray catcher is provided to remove acid spray and the gas is byown by an Elliott blower into the heat exchanger-converter section. Three external exchangers and converters operating in parallel are provided for each unit. The inlet gas enters the external heat exchanger and is heated by hot converted gas. From the external heat exchanger the gas enters the heat exchanger section of the converter above the bottom tube plate and is further heated by extracting some of the heat of conversion generated in the tubes. The gas then passes through holes in the top tube plate and down through the catalyst-filled tubes where the first stage of conversion takes place. The partially converted gas then passes into a bed of catalyst located below the bottom tube plate, where the final stage of conversion takes place. The converted gas passes back through the external heat exchangers and an atmospheric sulfur trioxide cooler (one installed per unit) into the absorber where the sulfur trioxide is absorbed in 98.4% sulfuric acid. Plant make is bled off the absorber circulation, diluted to 93% with water and acid from the weak acid (65y0) dryer, and pumped to storage. Cross flows of acid among the weak dryer, strong dryer, and absorber circulations serve to maintain the circulating acids at their required strengths. Water is also added to the absorber circulation. The tail gas is scrubbed with aqua ammonia in the tail gas retreatment tower for recovery of unconverted sulfur dioxide. Catalyst temperature is manually controlled in the National Process units by adding cold gas after the external heat exchanger or after the internal heat exchanger (prior to the first stage of
Vol. 42, No. 11
catalysis). Catalyst temperatures run about 630" C. at the top of the tubes and about 500" C. at the converter outlet, depending on gas strength.
Production Data, National Process Units No. of units Rated capacity Production rate Volume Efficiency
3 112 tons per day per unit (5.5% SO2 gas) 220 tons per day per u+ (10% S0a gas) 11,000 cubic feet per minute per unit 6.5% SOz. 96% 10.0% SOZ, 91%
Monsanto Units. In the Monsanto units ga.7 from the mist treaters is dried in one stage with 93 to 96% sulfuric acid. Spray is removed with a coke-filled spray catcher before the gas is conveyed by a centrifugal blower through the heat exchangers to the converter section of the plant. In these units three heat exchangers operating in series are provided. Conversion is again effected in two stages. From the first converter the gas is cycled through the third heat exchanger for cooling to a more favorable temperature for conversion and is returned to the second converter. Gas from the second converter asses back through the first two heat exchangers, through the supfur trioxide cooler, and into the absorber where the sulfur trioxide is absorbed in 98.4% sulfuric acid. The dryer circulation is maintained a t the desired acid strength by bleeding the make back from the absorber circulation. The plant make is bled off the dryer circulation, diluted to 93% with water, and pumped to storage. The tail gas is retreated for recovery of unconverted sulfur dioxide in the retreatment tower. Catalyst temperatures are controlled in the Monsanto units by introducing cold gas between the second and third heat exchangers (manually operated) or directly to converter inlet (automatically operated). In addition, the third heat exchanger may be by-passed. Temperatures run from 410' to 430" C. at theinlet of the first converter, 590" C. at the outlet of the first converter, and 480" C. a t the outlet of the second converter, depending on the gas strength.
Production Data, Monsanto Units Number of units Rated capacity Production rate Volume Efficiency
2 125 tons per day per unit (6% SOa gas) 220 tons per day per unit (10% SOPgas) 11,000 cubic feet per minute per unlt 6.5% SOz. 97% 10.0% 502,91%
Zinc Plant Spent Electrolyte Recovery. Sulfuric acid is also recovered along with zinc from zinc department spent electrolyte. A large quantity of this electrolyte must be stripped and discarded daily for control of both the volume and purity of the electrolyte. This "stripping acid" is treated in the zinc absorption plant for recovery of the sulfuric acid and zinc. Stripping acid is pumped from storage tanks through a mixing chamber where aqua ammonia (30% nitrogen) is added. I t then passes through cooler pipes for temperature control, and into an agitator 10 feet in diameter and 14 feet high. At this point unfiltered ammonium bisulfite solution from the bisulfite make storage tank is added. The zinc is precipitated M an insoluble complex zinc ammonium sulfite and the sulfuric acid is converted to ammonium sulfate. The zinc preci itate is removed with a Bird continuous centrifuge and the soyution is pumped to the filtered bisulfite solution storage tank. Strip ing acid is fed to the agitator and the filtered solution is p m p e f from the centrifuge to the storage tank using 2.5-inch abour pumps with 20-hp. 1800 r.p.m. motors. Type AB 2.5 X 2 inch Wilfley pumps with 10-hp. motors are used on sulfite feed. The flow of stripping acid is set at the desired rate (up to 25 gallons per minute) by a manually controlled rotameter. A recording and integrating flowmeter is installed for measurement of the stripping acid. Aqua ammonia is added through a rotameter which is automatically controlled by a pH recorder sampling the solution after ammonia addition and cooling. The flow of water to the coolers is adjusted by a temperature controller on the solution after the coolers. The flow of bisulfite solution to the agitator is set at 25 gallons per minute by rotameter regardless of the acid flow rate. This supplies more than sufficient bisulfite at the maximum strip in6 acid rate. The centrifuge provides a convenient way of gtenng the excess bisulfite solution. At present this unit is recovering the equivalent of about 30 tons per day of 100% sulfuric acid. The zinc filter cake is returned to the roasters.
November 1950
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I N D U S T R I A L AND ENGINEERING CHEMISTRY
Acid Plant Tdl Gas Retreatment. Tail gas from all six sulfuric acid plants is retreated in a single tower, 17 feet square and 42 feet high. This is a lead tower, supported on a steel frame lined with acid roof brick and packed with wood. All flue and fan washes, spiEs, and leakages as well as water from a scrubber treating absorption plant tail as are collected in this tower. Water is added as make-u if necessary A ua ammonia is added to the circulating s o h i o n . No ex’terna coolin is provided, BS the gas is dry and evaporation of water proviles what cooling is necessary. The make from this tower is fed to the fourth tower of the zinc absorption plant 8 stem. Lewis pumps are used for circulation. geich tests are run on the inlet and outlet gases and sulfur dioxide content and pH of the circulating solution are determined. A rotameter is used for aqua addition.
. --
UNACCOUNTED
lqooo 14,000
This tower treats almost 50,OOO cubic feet per minute of gas, running currently about 0.9% sulfur dioxide. The tail gas is averaging about 0.07%, giving an ahsorption efficiency of 92%. PLANTS NOT OPERATING AT PRESENT
These plants include the sulfur dioxide reduction plant, which was shut down when the demand for sulfuric acid for fertilizer production increased, and the exorption units (sulfur dioxide recovered from ammonium bisulfite solution by boiling), which were shut down when more ammonia became available for sulfur dioxide recovery. In addition, an autoalwe process for recovering sulfur and ammonium sulfate from ammonium bisulfite solution was developed through the pilot plant stage. Sulfur Dioxide Reduction Plant. Theoretical considerations involved in the reduction of sulfur dioxide have been discussed in two papers by Lepsoe. In the first paper Lepsoe uses thermodynamic values from the literature to calculate equilibrium ga< concentrations for various reducing agents, including carbon (6). In the second paper the kinetics of sulfur dioxide reduction with carbon are discussed (7). The reduction plant was shut down when the demand for sulfuric acid for fertilizer production exceeded the output of the acid plants. When last operated in 1943 the reduct,ion plant comprised three units, two rated at 40 tons of sulfur per day and thr third rated a t 70 tons of sulfur per day. The reduction process employs the well-known reaction between sulfur dioxide and incandescent coke. This reaction is slightly exothermic, but cannot support itself, 80 some oxygeii must also be blown into the furnace to make up heat lost by radiation. The reducing reaction does not go to completion in the furnace. Various side reactions take place, so the exit gases contain considerable quantities of carbonyl sulfide and carbon monoxide, and a t times small amounts of carbon bisulfide as well as elemental sulfur and carbon dioxide. Some sulfur dioxide is added to the producer exit gases for catalytic. conversion of this carbonyl sulfide to sulfur by reaction with sulfur dioxide. The reduction furnaces were especially designed by the Power Gas Corporation, Ltd. Ash is removed on a rotary grate and through a water seal at the bottom. Coke is received in gondola-type cars and dumped into bins (two installed), from which it is elevated to bunkers at the top of the plant using a skip hoist. Coke hoppers located above each as producer are fed by gravity from these bunkers. The ro8ucers are in turn fed by gravity from the hoppers through i i d e valves. Pure sulfur dioxide from the acidification unit is fed into the reduction system at two points. Part is blown along with oxyinto the bottom of the producer and part is added to the proucer exit gases. Gases from the top of the producer, mixed with the required amount of sulfur dioxide to convert the carbonyl sulfide, pass through a cyclone where coke dust is removed and then pass through a precatalyst column, a steel vemel lined and acked with fiebrick, where the greater part of the carbonyl surfide is converted. The $as then passes through a waste heat boiler for cooling to the optimum temperature for completion of the catalysis and then through the catalyst column, a vessel similar to the recatalyst column but packed with catalyst. From the catatyst column the gas passes through a second waste heat boiler where i t
y
Figure 2. Total Tone of Sulfur Charged and Dbtributed per Month at Trail Smelter is cooled so that all the sulfur is condensed as a liquid or a mist. The sulfur mist is removed from the tail gas in tube and wire-type treaters (two per unit) and the tail gas is scrubbed with water. Liquid sulfur from the two waste heat boilers and the treaters runs into settling pots and is pumped through a steam-jacketed Shriver press for removal of a very small uantity of extremely fme occluded carbon which gives a greenia tinge to the sulfur, From the filters, sulfur a t a purity exceeding 99.99% runs into heated tanks and is pumped to storage blocks.
Exorption Process. Another process that the company developed and operated for a while is known aa the “exorption” process. This waa used to maintain the output of pure sulfur dioxide for the reduction plant early in the recent war when the ammonia supply for the absorption plants was restricted. The exorption process is based on the reversible reaction: 2NHiHSOs = (NH4)tSOa
+ SO2 + H?O
Sulfur dioxide is recovered by heating solution of ammonium bisulfite, and the resulting ammonium sulfite is returned to the absorption cycle. Theoretically no ammonia is required when the exorption process ie operated in closed cycle with the absorp tion process, whereas 1 mole of ammonia is fixed for every mole of sulfur dioxide released in the acidification process. In practice, however, some sulfate is produced by autoxidation in the exorp tion cycle, and this has to be removed to prevent the system from salting up. The sulfate content can be controlled by crystallizing out excess ammonium sulfate or by bleeding off part of the exorp tion effluent to acidification. A variation of the latter expedient
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I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
was adopted in the commercial plant, although pilot plant work was also done with a crystallizer in the circuit. The commercial plant comprised six exorption units. Part of the solution from the absorption plants was diverted to exorp tion, while the remainder went to acidification. Bisulfite solution from the absorption cycle is heated by heat exchange with the exorption effluent solution and evolved gas, and finally in a tubular steam heater to 130" C. The hot solution is pumped into the side of a flash chamber, where release of pressure causes a rapid evolution of sulfur dioxide and steam. The effluent solution is drawn off the bottom of the flash chamber, cooled by heat exchange with the feed solution, and returned to the absorption circuit. The evolved gas is also cooled by heat exchange wit,h the exorption feed, and enters the acidification cooling and drying system prior to the cooling tower. The split of bisulfite solution between exorption and acidification is controlled so that the concentratmionof animonium sulfate in the absorption plant circuit is sufficiently low to avoid crystallization. Autoclave Process. A process for obtaining ammonium sulfate and sulfur by autoxidation of ammonium bisulfite has been developed through the pilot plant stage and patented (9).
A solution of ammonium bisulfite cootaining lesser amounts of ammonium thiosulfate or ammoniunl polythionates is fed continuously into an autoclave containlng a saturated solution of ammonium sulfate with some free sulfuric acid (10 to 50 grams per liter). The autoclave is operated at a temperature above the melting point of sulfur (120 to 140" C.) and at a pressure of 30 to 40 pounds per square Inch. Sulfur and ammonium sulfate solutions are withdrawn continuously lrom the autoclave, the sulfur is separated, and the ammonium sulfate crystallizes out, the mother liquor being returned to the autoclave. The ammonium thiosulfate or polythionates may be present in the bisulfite solution, they may be formed in situ, or ammonium thiosulfate may be produced by adding aqua ammonia to ammonium bisulfite to adjust the bisulfite-sulfite ratio within a specified range and reacting the resulting solution with sulfur. This latter react,ion can be conducted advantageously in a ball mill a t ordinary temperatures using lump sulfur. A variation of the autoclave process was tried to remove noxious constituents from sulfur dioxide reduction plant tail gas. The tail gas was scrubbed with ammonium bisulfite solution from the absorption plant, ammonium thiosulfate being formed in situ by reaction with hydrogen sulfide in the tail gas. A relatively small amount of hydrogen sulfide is found in the reduction plant tail gas, arising from moisture in the system introduced with the coke. This process was abandoned, first, because serious corrosion of lead equipment was experienced, resulting in contamination of the sulfur with lead sulfate, and secondly, because scrubbing with ammonium bisulfite does not remove carbonyl sulfide, which hydrolyzes slowly in the atmosphprr to hydrogrn sulfide. After work on thiv process was discontinued, a water scrubber was installed on the tail gas. Subsequently a tail gas burner wm developed on a pilot plant scale that burned both hydrogen sulfide and carbonyl sulfide to sulfur dioxide without additional fuel. However, the reduction plant was shut down shortly after this development work was completed.
lrad, zinc, and iron sulfides .%bout 6500 short tons of ore per calendar day are mined and concentrated. Approximately 25y0 of this is rejected in a preliminary sink and float trratment, the remainder is ground, and the constituents arc separated by flotation. The ore mined contains about 1300 tons of sulfur per day, which is distributed in the products as follows: 30 tons rejected in the float, upward of 1000 tons left in the iron tailing, and 300 tons shipped to Tadanac in zinc and lead concentrates (200 tons in zinc and 100 in lead). TADANAC METALLURGICAL PLANTS
At Tadanac customs ores and concentrates are treated along with the Sullivan products and, if necessary to meet sulfuric acid requirements, Sullivan iron concentrate is roasted. Currently about 430 tons per day of new sulfur are treated in the Tadanac plants, approximately 120 tons i n the lead department and 310 in the zinc department. At present less than 9% of this new sulfur is releead to the atmosphere, over 80% is recovered as acid, 4yo is unaccounted for, and the remainder is fixed in plant wastes and products to market (not including acid). The distribution of sulfur charged to the various metallurgic~l plants at Trail for the years 1900 through 1948 is shown in Figure 2. The growth of the sulfur-recovery program from 1929 is clearly shown on this chart. The sulfur released to the atmosphere in 1947 and 1948 is less than in any other year since 1904. This improvement has arisen mainly from two operating improvements-retreatment of acid plant tail gas, and adjusting the operation of the sintering plant to the capacity of the smelter absorption plant. Retreatment of acid plant tail gas for recovery of unconverted sulfur dioxide has resulted in recovery of the equivalent of about 30 tons per day of sulfur formerly released to the atmosphere. Current production of sulfuric acid at Trail averages about 1100 tons per day, which is consumed roughly as follows: Metallurgical plants and sales .4mmoniurn sulfate fertilizer .4minonium phosphate fertilizers Total
65
370
1100 665
LITERATURE CITED (1) Cobleigh, W. M., IND.ENG.CHEM.,24,717-21 (1932). (2) Dean, R. S., and Swain, R . E., C. S. Bur. Mines, Bull. 453, 23-228 (1944). (3) Katz, Morris, IXD.ENG.CHEM.,41,2450-5 (1949). (4)Xata, Morris, et al., "Effect of Sulphur Dioxide on Vegetation," Ottawa, National Research Council of Canada, 1939. (5) Kirkpatrick, U'. S., Fourth Empire Mining and Metallurgical Congress, Great Britain, Paper H 1.9 (July 1949). (6) Lepsoe, Robert, IND. ENG.CHEM.,30, 92100 (1938). (7)Ibid., 32,910-18(1940). (8) Lepsoe, R o b e r t , and Kirkpatrick, W. 8 . .
SULXUR BALANCE SULLIVAN MINE AND CONCENTRATOR
The Sullivan mine is a massive sulfide replacement deposit containing chiefly
Vol. 42, No. 11
Absorption Towers, Zinc Absorption Plant
T r a n s . C a n . Znsi. M i n i n g M e t . , 40, 399-404 (1937). (9) Lepsoe, R o b e r t , and Mitchell, R. F., U.S. P a t e n t 2,359,319 (Oct. 3, 1944). (10) Snowball, A. F., Can. Chhem. Process Inds., 31, 1110-14 (1947). (11) Swain, R. E., IND. ENG. C H E M . , 41, 2384--8 (1949). (12)Symposium on Atmosp h e r i o Contrtmination and Purification, Ibid., 41, 2383-492 (1949). RECEIVED March 27, 1950.
