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Contact Sulfuric Acid Plant
T
HE pmoJpectcrrs wlio years ago fullowed the t'~r1umbia River on tlie (lanariian side of tlie international looimdnry ntid discovered mineral-laden v&s ill ahat is kiiown as the I3,ussland district, British Columbia, l i t t l e tiiought that tiitly were layitig the foundation of 11great industrial enterprise. I3etwren then atid now tho Consolidated Mining & Smelting Coiiipaiiy of C:anada, I,td., came into being; copper and lead smelters irnd a zinr refitiery sprang into operat.ion at Trail, just nort.li of tlie l-nited Stnte,s hiundary line; smoke laden witli sulfur 'lh~xiiie over tho boundary line into the State of Wasliingtor Iriteruational Joint Commission met to cunsider the effect,s(if smelter smoke on vegetation on the United States side C J ~ the bottndary ; technologists developed and adopted incans for recovering sulfur dioxide from smoke; a sulfuric acid j h i t was estabiislied to utilize the sulfur dioxide; plios~horicacid arid anhydrous nmmonia supplicz wore made available by appropriate processes; ntid finally, because olierc mcrc iio industrial centers near enough to utilize these cliemicals, they were converted into anitrioriiuni sulfat,e, moiioatiiniriniuIri pl~osphate,and "triple soperpliospliate," st,amIard fertilizers which are marketed a i d used in adjoining ngriciiltural areas.
dioxide in calcining iqierat.iuris. lit t,lir I r w I smelter, 1)niglitISoyd sint.crittg niaeliiiies are uicd to reduce tlic sulfur content of tiie ore containing 14 to 15 1 m ceiit siilfiir to a sulfur content of 1.4 t o 1.5 per cent in the calcine in twu sititmirig operations. Prior to tlie iitilization uf sulfur dioxide for the prodrrction of
sull'nric acid, tlie zinc-conc:cntrate roasting plant housed twenty-five Wedge mecliartical furnaces, each 25 feet in diameter. Each furnace liad seven Iieart.iis, air-cooled rabble arms, atid coal-dust firiitg equipment. The capacity of ewti roasting funlace was 36 t,o 38 dry totla uf coneettt~retcspcr day. The roaster gas passcd throiigti a plate-type Cottrell treater to remove dust and arsenic aiid was delivered to the atmosphere by a 400-foot concrete stack. Tbe continued operation o f tlie copper, lead, arid zinc nictellurgical plants finally brought the smelter smoke issue into the forcgmuod. The daily capacity of these units is 425 tons of lrad, 400 tons of zinc, and 00 toris of copper. The sulfur, originally in eonihinatiuti with these metals in the form of sulfides, is reinoseci for the most part as sulfur dioxide and delivered to the atrnospliere during calcinirrg and other mt%alliirgieal operations. T h e cnlargcment of these plants from timt? to time arid the ctiristriiction of larger arid higher smoke stacks contributed to a wider distribution of smoke i i v ~ r it larger area mil itlsii to a Iiiglier concentrat,ion of the sulfur dioxide in the atruosplrere OS the same area. Irr the natural coime of evcrit,stliere amsc tire p r o l h n of the ,/ [lie siilfriric acid i k n t as ii, titiit of tlic coiirrlinated operaof srrielt,er sirioke on vcgctat,iori. There werc variables t i m s of tlie conipariy wa,s iiitrodoced in tlie pmductiiin laeal sit,itatiorr that created imirsiial procedures in program in a logical seqiieric everrt.s. In tiie carly nineties ilcaling with tlir wnnke prublcm. The smelter is locnted it copper strielt,er vas ciinstr !d at Trail to ti'& tiit, solfirle n h o t I2 m i l c s h t r i the iiiterriat~iwial lwutafary betwecii ores from the Rosslaiid di et only a few miles ilistaot. (Innaiin and tlii: I'iritcd States. ('lainis were made that tlierc The usitat operat.iinis of cooceiitrating, roasting, smelting, was iiijiiry to veget,ntior, cxtcrirling for somi: milcs into tlic converting, and electrolytic refining of the copper c~nstitiitt?d State of Wasliingtcm. The i>xtnlt,mil cIiars,ct,t?rof tlir injury tlie unit processes of the copper smeltcr. With tlir roasting, wen: inwxtig:%tedby t l w cntific dtaffs of the C a n a i h i and sineltiiig, and eoiivert.ing operations cairit' the initial strps in I;nit,txl States I)epartmr:nts of Agriculture, arid by t,lic creating later a smelter snioke pniiileor. Consdidatcd Mining & Smelting Company. Tlie results of In the subsequent development of the tniiiiiig regions 01 thwc: investigutioos were roportt.il to the International Joiiit British Columbia, silverhearing lead ores, zinc ores, :in4 1:onitiiission in Dccerrher, 1!Y20. 111the siiinm(%-of 1930 the complex ores containing both rrietals, lrecanir: auailalile at coiumissiirrr ~ ~ ~ ~ ~ ~ to~ the ~ ~Governments ~ e i t i ~o fe the i l t:nit,i:il n r l Canada that tlir damages for past injury to ttitl Kimberley, 250 rriiies from Trsil. Ilifftwntial flotritioii carnc! into the getieral plan of metallurgical operatioris of the private o w n m of agriciiltitral lands in tlic State of Wadihgcompany, making available both lead and zittc ciincrntrates. ton lie paid, arid t,lrat the Consolidated Mining & Smelting contimir ilie cfforts slrendy under way t o eliminate Coincident with tlie development of concrntrat,ion hy flota- ('oio~~aoy tion, it was decided to erect at Trail a lead srrielter arid later ati the possibility of thi: continued occurrence of damage from electrolyt.ic zitic plant. Both of these plants prodii smelter smoke in the area investigated. 717
718
I N D U S T H I A 1. A N D E N ( i I ?I E E H I & G C H E31 1 S ' 1 " 1
V d 24, No. 7
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Joly, 1932
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U S T R I A L A N I f E Pi G I N E E 1%I N G C N E M I S T R Y
The fiow slieet or ground plan of the plant shows the follnwing unit operations: i. Production of sulfur dioride in calcining dopnr.tment of
electrolytic zinc plant. 11. Purification of roaster gas. d. Before division to the four acid plant unit8 1. Hot nlate Cottrell precipitator for dust and nrsenie reimdvat. 2. Weak acid scrubber. K. After division of gas to the sepamte uiiiLs 1. Water scrubher. 2. Cottrd precipitator. n. Removal of sulfuric acid mist. 3. Drying. a. Weak acid. 80 p e r cent. D. Strong acid, 93 Der cent. C. Entrained acid removal by coke filter. 111. Conversion of sulfur dioxide 50 sulfur trioride. A. Heat exchangers. B. Converters. C. Cooling station. IV. Absorption of snlfur trioxide in 98.5 per cent sulfuric acid. V. Acid cooler. VI. Storage of 93 per cent acid. \TI. Deliver? to fertilizer plant by pipe line instaliritioii ~~
In ~iiakiiigarranytmentx to utilize tlie sulfur dioxide US tlie zinc roa8ting plant, modifications in operating nititliods and in plant design were necessary in order to increase the suliiir dioxide content of the roaster gases. Calcining in Wedge hearth furnaces was discarded in favor of what is frcqucntly termed "flash roasting," a procednrc which is suggested by the fact that the zinc plant handles flotation concentrates in a very fine state of division-2W mesh and finer. The COLIcentrates are brought into the proper physical condition for calcining by a two-stage drying process and by disintegrating lumps in a ball mill. The dry finely divided concentrates are then calcined in a hot chamber in a manner similar to the combustion of powdered coal in metallurgical furnaces. The sir hlown into the furnace with the concentrates is supplemented by sccondary air admitted in proper amounts for the purpose of controlling the combustion reactions. In this method of roasting, the combustion rate is high and therefore no auxiliary heating or f&g is necessary a, is the case whcn zinc concentrates are roasted in a hearth furnace. The flash method of calcining produces a large amount of flue dust of such a degree of fineness that it cannot be successfully recovered in dust chambers of the usual construction. The cyclone collector was therefore substituted for dust chambers. The positive oxidizing conditions and the high temperature of the flash method produce a flue dust that is completely roasted and therefore can be sent directly to the zinc leaching plant; in contrast, the flue dust from a hearth calcining fnrnace must he returned to the furnace for a second treatment. The roaster gases from the furnace outlet contain 9 per cent of sulfur dioxide which is satisfactory for sulfnrie arid prodnc-
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tion by tile coiltact process. The percentage is reduced by leakages in the purification system. The economic significance of using waste sulfur dioxide for sulfuric acid production becomes apparent when it is noted that the plant under maximum production of 375 tons per day requires 129 tons of elementary sulfur, equivalent to 258 tons of sulfur dioxide on tile basis of 95 per cent conversion. The fact that this amount of sulfur dioxide would otherwise he wasted is a very important factor in the consideration of the enterprise from . treatment of the tlie standpoint of Ixoduction ~ ( J s ~ s The r o a s t e r g a s e s after leaving the zinc plant is under the control of the acid plant organization. I'naI%~xCKPlos
PROCESS There are a nuiiiber of steps in tlie purificatioii process in the coursc of the gas to the c o n v e r t e r s . Dust and arsenic removal is effected in a hot Cottrell t r e a t e r which removes 96 wr c e n t of t h e d u s t c a r r i e d by tlie gas after leaving the cyrlone separators. The dust removed a t t,his point goes t o the z i n c leaching plant. The Cottrcll treater is of the d a t e and win? type. Tlien: ib a total ~,feiylit. sections; each section contains sixteen 10 X 12 foot corrugated iron plates, 8 inclies apart, with the high tension wires placed midway between tlie plates. Unidirectional current at 50,OW volts producd by synchronous motor-driven 25 kv.-amp. rectifiers serves this plant. The entire treater liandles 40,000 cubic feet of gas per minute, which enters at a temperature of 720" P. and leaves at 450" F. The gas from the hot Cottrell treater passes next to the weak acid scrubbers. There are four of these: three are circular. with a diameter of 20 and a heieht of 28 feet: one is squafe, 12 x 12 feet in crna section, and 2 5 feet'iiigli. Each scrubber is constructed of lead, lined with acid-proof brick of English manufacture, and filled with brick checkrrwork of the general type of construction found in Glover towers nsed in chamber process plants. The gas passes in counterflow to the acid stream which has a strength of 70 to 80 per cent. The circulation of the acid is accomplished in the following manner: From a large storage tank, supported several feet above the scrubber, the acid flows by gravity to a distribution tank on the top of the tower. The acid is distributed over the brick checkerwork of the tower througli thirteen lead pipe nozzles attached to pipe lines leading from the distribution tank. Four feet below the outlet of each nozzle there is a suspended concave disk on wliich the falling acid strikes. This has the effect of spraying the acid over the checkerwork in a fashion similar to the distrihution of sawage on a trickling filter. From the bottom of the tower tlie acid passes to three water coolers, each containing four banks of pipes. Sediment is removed by settling, and then the acid is pumped back to the storage tanks above the scrubbing tower by a Lewis hard-lead acid pump. When it is necessa?y to supply fresh acid to the I
J u l ) , 1932
I S D UST R I A L A S D E N G I N E E R I N G C H E XI I ST R Y
top of the heat exchangers a t a temperature of 25" C. (77" F.) and under a pressure of 28 ounces. It then flows downward outside the pipes which are heated by the hot sulfur trioxide from the converter. Leaving the heat exchanger a t a temperature of 310" C. (590" F.), the sulfur dioxide can be admitted to the converter a t two points, C and D. The main stream of gas enters the converter a t C above the catalyst layer, and is further preheated by contact with the outside of the hot catalyst tubes as it passes horizontally from one side of the converter to the other, guided by baffles and upward through a perforated baffle to the space above the catalyst tubes. I n this manner some of the heat of the conversion reaction is absorbed by the entering gas, prior to the second stage of conversion. Adjustments in the final temperature of the reacting gas stream ran be made by admitting gas a t 310" C. direct from the heat exchanger through the by-pass, E , to a point, D, in the converter. The conversion reaction then takes place as the gas stream passes downward over the catalyst in the tubes. Intermediate cooling occurs through heat absorption by the entering gas and in the space above the 18-inch catalyst layer in the bottom of the converter, where the second stage of conversion takes place.
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cast-iron tray on top of the tower. From the center conipartment the acid is distributed over the entire tray, the function of which is to distribute the acid uniformly over the quartz packing. The tray is 3 inches deep and contains a series of holes inch in diameter, centered 5 inches apart. I n each inch above the hole there is a glass tube which projects bottom of the tray, through which the acid flows. Acid of 99.5 per cent strength is drawn off on opposite sides of the bottom of the absorption tower. To the acid from one of these outlets sulfuric acid is added to bring the concentration to 98 per cent. This acid is cooled and pumped by means of a Lewis pump to the absorption tower. The 99.5 per cent acid drawn off from the opposite side of the absorption tower is diluted with water to 93 per cent. This represents the acid production of the plant, which passes through a water cooler and then to storage in one of two 2100-ton steel tanks. Through a 4-inch pipe line the sulfuric acid is pumped to the fertilizer plant storage tanks, a distance of 6000 feet. The difference in level between the two storage systems is 400 feet, and it is interesting to note that each of two four-stage Allis-Chalmers centrifugal pumps deliver the acid at the rate of 1 ton per minute. The contact plant, together with its delivery pipe line, constitutes the connecting link between the metallurgical works (the source of the waste sulfur dioxide) and the new chemical plant designed for fertilizer production.
FERTILIZER PLAKT
FIGURE 1. FLOW SHEETOF H E ~ T ExCH\\GER i \ ~CO>\ERSIOVU ~ I T
Temperature measurements are taken periodically a t points in the converters by the uce of Republic electric pyrometers. Four of these measurements are taken within the catalyst mass. Summarizing a set of temperature readings, the gas delivered t o the converter has a temperature of 310" C. (590" F.). The gas is preheated so that within the catalyst mass in the pipes the readings are 570", 603", 535(', and 552" C. (l058", 1117", 995", and 1026" F.). The average of two readings above the catalyst layer is 440" C. (824" F.) and the sulfur trioxide gas leaves the converter a t a temperature of 460' C. (860" F.). Under these operating conditions the average conversion is 94 per cent. I n fact, the conversion reaction is carried out under conditions quite within the theoretical limits which govern an equilibrium reaction of this character. In addition to the temperature measurements of the conversion process by electric pyrometers, an Inglehart automatic recorder is used to indicate the sulfur dioxide rontent of the entering gas. The accuracy of the recorder is periodically checked by chemical analysis. The sulfur trioxide gas passes to the heat exchanger and then to an air cooler en route to the absorption tower. From the air coolers the sulfur trioxide gas passes to a n absorption tower which in its essential parts is similar to the drying and scrubbing towers previously descrihed. This tower, however, is constructed of steel, lined with brick, and filled with quartz. The tower is 1.5 feet in diameter and 26 feet high. The sulfur trioxide enters the base of the tower and flows countercurrent to the 98 per cent acid used for absorption. Acid of this strength has sufficiently low vapor tension to prevent the forniation of sulfuric acid droplets. The absorption acid is puniped into the center compartment of a
Erected on a n allotted area of 60 acres of ground, for the purpose of utilizing the contact sulfuric acid, the fertilizer plant, representing an expenditure of $10,000,000, employing three hundred men, utilizing electrical energy a t the rate of 34,000 h. p., and producing 300 to 400 tons of fertilizer products per day, is the most impressive part of the new development program of the Consolidated Nining 8: Smelting Company. An abundance of cheap power, developed by three companyowned electric power plants on the Kootenay River, and sulfuric acid from waste smelter fumes are important considerations in viewing the economic factors of the enterprise. Indicating more directly the role of sulfuric acid, it is utilized in two divisions of the fertilizer plant. I n one, ammonium sulfate is produced by the interaction of synthetic ammonia and the contact acid. In the other division, phosphoric acid is manufactured from phosphate rock shipped from Montana and Idaho. The phosphoric acid, in turn, is used to produce monoammonium phosphate and "triple superphosphate." For the production of synthetic ammonia by Fauser catalyst columns, both hydrogen and nitrogen are required. The hydrogen is produced by electrolysis, using an installation of Knowles, Fauser, and Stuart (bell type cells), and also the Pechkranz filter-press type of cell. Two Claude complete units produce the necessary nitrogen from liquid air. Located far from industrial and manufacturing centers, the sulfuric acid plant of the Consolidated Mining 8: Smelting Company has more than usual significance. It has the effect of increasing the interlocking features of the entire production program of the company and sets up a degree of coordination somewhat unusual in metallurgical and chemical operations. R E C E IE ~D . 4 p d 18, 1932
" N o s I ~ F L A ~ ~ ~AIRPLANE ~ . ~ B L EFCEL. " Development of a "noninflammable" motor fuel for use in airplanes is being studied by the French Government in conjunction with private French organizations, and recent tests are said to have heen highly satisfactory according to L'Air. Elimination of the fire risk is said to have been accomplished in the new fuel, which does not leave the residue of carbon accumulation.
