Production of Copper Arsenite In 1937 the Tennessee \alle? iuthorit: nndertool\ to deielop a n arsenical lariicide t h a t moould be cheaper and more effectite t h a n Paris green in airplane dusting for malaria control. After t h e selection of copper arsenite a4 t h e most promising of a series of metallic arsenicals studied i n t h e laborator:, a process for it. production from scrap copper arid w hite arsenic (arsenic trioxide) was de\eloped. The two main steps of t h e process are t h e dissolution of copper in a n ammonium chloride solution and t h e addition of arsenic trioxide to precipitate copper arsenite. The product is separated by filtration, washed, and spra? -dried, and t h e animonium chloride solution is r e q r l e d . The process w a s iniestigated in a pilot plant with a capacit? of 600 pounds of copper arsenite per da:. P r o b l c n i ~incident to continuous *tea& -state operation
not encountered i n the laboratory iniestigation i t ere met and solied. Information or1 equipment, operation, and reco\eries mas obtained from i+hichit w a s estimated t h a t copper arsenite could he produced a t a substantially lower cost t h a n Paris green. Pilot plant product I+ as used in airplane dusting tests, which indicated t h a t copper arsenite w a s superior to Paris green a5 a lariicide for malaria mosquito control. The present paper describes the stud? primaril? as a pilot plant case histor:, pro\iding inforniation on such points as historical bachground; pilot plant design and construction; organization a n d t: pe of personnel; methods of obtaining, reporting, and utilizing operating and anal>tical data; and the t > p e of prohlenls encountered. The reasons for carr?inp out the pilot plant work and t h e results and henefits of t h e vorL are discussed.
A
.-tudic,s aiid t o consideration.: of space', icost, and operating convenience : the basis for selecting cquipnient : t h e type, nuniticr, and dulirs of the o p i a t i n g pc~rsonncl: the collection and analof saniplcs and t h r use of thca anal>-tical re.sults in control he opcration; and t h r colltvtion of opcratiiig data arid thcir tabulation, c-orrelation, aiid u,se in operation. From i~ h a d w vien-point an attempt i c made t o convey somen.hat the chronological pattcxrn of a pilot plant irivc~stigatiori: the type of difficultios encountc.red : the leads folloivetl or rejectcd; tliv part playcd by noritrchnical or est ous considerations; mil the , of trial and accidcrit. that interplay of guessing and ana ont i i i t o c~vc~i~y i n w t ~iga t ion.
T THE request of the Health arid Safety Department of the
Tennessee S-allry .iuthorit!-, n.hich is responsible for the Authority’s program of malaria control, the Chemical Engineering 1)cXpart n i ~ i i tcarried out a s>-steniaticstud)- on a laboratory scale of t h e preparation and testing of a series of insoluble arsenitrs of the coninion metals. This led t o thc selection of cupric marwiiite (Cu.b20,) as tlie most promising compound and to the development of a pr0ce.s for its production from white ai.-enic (ar3cnic trioside) and scrap copper ( 3 ) . Cupric m-arst.niie, for conveiiicxnce called copper arsenite in this n-ork, ha.? l x ~ ren ported in the literature (21 but is not produced coiiinicrcially. I t has a throretical content of 22.0‘; Cu arid 71.4(-; compared Tritli 25.1c; Cu, 58,6c; (arctic a r i d ) for Paris green, (CuO coppc~i~ i- a coiisidtmbly IIIOIT t‘spr~ wiiir, and ~ i i i r earcltic acid is a n expc~nsireingredient of Paris g r w n that i. riot preacnt in copper ai,wnitc>,it appeawd that the lattrr \r-ould be chmper than Pari; grt’t’n if a fairly siniplc, and efficiciit m:~thod for its nianufacturc could be developed. A pilot plant for such a process Tvas designed arid constructed, arid \vas o1wrated long enough t o pcrfcct and prove the process, to produce sufficic,nt material for airplane dusting te.;ts of its Pffcctivcnes, and t o provide iiifoi~nationfor a rcliahle estimate of ttic cost of production. The follon-ing dcacription of thcx pilot plant productiori of copper arhenite is plrsented not primarily for the sake of its valuc~t o thi, larriridr. firld but rather n-ith the intention of providing dctailrd iiiforiiiation on the methods used in designing and operating a typical pilot plant, in the hope that such informat ioii will hc. usvful t o students and teachers of chemical engineering aiid t o engineers c.ngagetl in pilot plant work. In accordariccl n i t h this ohjrctivc~this paper is conccrned mainly n-ith such topirs as the wij- in which the design of the pilot plant 11-as related t o t h e bmic information proridcd by thr initial laboratory
~31’s
1)ECISION TO BUILI) I’ILO’I P L A l T
Early in the spring of 1939 thc lalmratory invcstigatioii of wqrnical Inn-icides had prog d far i~iiouglito indicatc: that king of tlie conipounds studied copper arsenite ir-as the most I and to demonstrate the technical feasibility of a fairly siniple method of preparing i t ; this mc’tliod was subsequently patented age the status of the project ir-as revien.etl, and irahle to carry thc n.ork to a pilot plant scale in ordt,r t o tcst t h e fcssihilitj- of tlie proccss in engineering cquipnicnt, tloterininc~suitatilc~cquipnic.nt an(l niaterials of (Ionstruction for a largr sriilc plant, ohtain inftrrniation for estimating the production cost on a large scal(~,anil provide enough prod. .\lthough the n c ~ for d the pilot plant 1r-I)rli tr-aLjclrar, the information at that time ai-ailablc froin the lahotudics was quite limited, and it n-oultl liavc b w n drsirable . t h c v studies further licfoix’ tlcsigning rlic pilot plant. However, the timing of thr pilot plant, and indcvd the' cntire investigation, n a b strongly influcncwl by t h e fact t plane dusting season for malaria control in the Tenn falls grnerally from May through September, 50 that tlic op-
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Vol. 39, No. 11
INDUSTRIAL AND ENGINEERING CHEMISTRY
1522
ARSENIC TRIOXIDE
COPPER
T
WATER
0 TANK
P \
4
r
z
z
A
M
.- _ _ _ - -
c
FILTER
AMMONIA SCRUBBER
PREClPl TATOR OFF-GAS TO STACK
MULT ICLONE
LIQUOR STORAGE
WATER STORAGE
41-
ORIFICE METER
-W-
a
CHECKVALVE
I$-
S A F E T Y VALVE
THERMOMETER
3 P R E S S U R E GAGE Figure 1.
Flow Diagram of Copper Arsenite Pilot Plant
portunity for field testing of larvicides i$ limited t o this part of t l i c t year. The choice then had to be made of proceeding iiiiiiicdiately to dwipn : ~ n dconstruct a pilot plniit without tlie Iic’iic.fit of more thornugh chcliiiical studies and of preliminary n.oi,l; i n rngineering cquipincrit, or of putting off the possibility of ;I fivltl twt \YXS of t h r proi!iic.t for a full year. On I1Inrc.h 15 the, tl(~ci~ioii made t o go alic.:itl. The goal \\-:IS wt of having t lie pilor ! i h i i t ready for opri,ation liy June 15, in order t o produw 3 tijiis of experimental larvicide for airplane tes-ing. The pilot p h i i t process and dcsign were based on the follon-iiig rcsearcli r(l.iiIt q , available on hpril 1, 1939. I \ F o R u i n o v FROM LABORATORY R E S E A R ( : I I
The process for making copper arscnitcx that rctsnlt i 3 d f i o i i i t small scale studies of the Chemical Research Diviqiori coii,-istrd essentially of dissolving metallic copper in amiiioiiiuiri cliloritlt~ solution and reacting the resulting solution with arsenic, triosick, to precipitatc coppcr arsenite. T h r w studies had l i c c t n cari~ic~il r e nit11 a noriryclic procedurc: thcl Iaboratoi~y inued aftcr April 1 and gave additional r c ~ n l t s that later proved useful in tlie operation of thc pilot plant, liut these iwre not available in designing the plant. The laboratoi,y work provided the following reconiiiic,ndatiriti~anti iiiformatioii for design of tlie pilot plant:
DISSOLT-ISG (‘OPPER. (’irculate a batch of lor: smnioiiiuni chloride solution :tt boiling temperature for 1 h o u r over c o p p ~ r wire with coiitiiiiious aeration. L-se 500 milliliters of solution per kilogram of cJpper n-ire. The copper \vi11 tli.>wlve at a rate of 18 grams per kiluyram per hour. PRECIPITATISG Cor SESITE. ;\dd 100 gr:i,nis of arsi~iiic trioxide per liter :tiid preferably in :t scparnt GEKER.ILI Y F O R L ~ T I ~ S .
very satisfactory: stainless steel (AkncricanIron arid Ptecl Institute Types 304 and 316) s h o n d some corrosion but probably
cwultl lie used: lead cnrr(ided sonii.\vh:tt more than stainless Of a number of coating materials tested, Torilesit and Pliolitc shon.ec1 promise. .kcidproof hrick, stoneware, and x o o d a l ~ ow r c suggested.
strel.
1)ESIGN ANI) COh STRUCTION ’
Thtl ,size of the pilot plant, like the decision to build it, n-as inflwnced by the time factor. T h e n c d for supplying 5 tons of product by midsummer (although not all of it n-odd be needed at one t h e ) hrlpcd t o 9c.t a lower limit for the plant capacitv. Thc ohjcctive CJf ohtaining reliable information for estimating the vast of producing copprr arecnite on a large scale v a s considered i n wicrting both thc &,sired capacity and the type of equipment to he useil. A n uppcsr limit for plant size was set hy the budget,. \\-eigIiiiip these factors arid othi,rs. a production ratr of 25 pounds p i ~ rh o u r or 600 pounds per thy, wliirh would call for about 3 K ( Y I ~ S of good opc,ration for the production of 5 tons, was scllccted a s :i fipurcl tliat left 1 onable room for meeting the goal after e problems that would be encountered. m:iliing allon-ance fo Ttougli wtiniatw of equipment sizes confirriled the suitability of this c.:ipacxity figure. T V A pilot plants arc desigiied by the Process 1)evclopment Jliviqiori, except for the largest and most complcs om’s: 011 these hrlp is rcwivcd from the Chemical Plant Design Diviuion, xhich c~an-iesout the design of the laige scale chemi ch:ingc,.L sustained production rate a ging more than 500 pounds pet' day. The product \vas uniformly good, and stead tion substailtially free from the effects of undesirable precipitatioli was adequately demonstrated. The part,icle size of the product, was found to be substantially independent of the conditions nitiirit,ained in the precipitation or spray-drying opcrations. The previous cost estimates were confirmed. Laboratory and field test,s gave further evideiico of tho larvicidal valur of copper arsenit 1,. DESCRIPTION OF PILOT PLAZTT Follon.ing is a description of the pilot plant t i s init,ially installed, along xvith a few of the reasons guiding the selection anti design of equipment. Reference is made t,o subsequent changes in some of t,he equipment : 0thi.r changes n-ill be noted in discussing the operation. A diagrammatic flow sheet of the plant, is shown in Figure 1. Figure 3 shows a grneral photographic view of the plant, \vhich was locattd against, one ivall of a large, heated 1)uilding of tile ronstniction, dvvotcd ciitirc,l\- to pilot, plant installations. COPPERU I S S O L ~ E RThe . coppcr tlissolvcr consist8dof a steel tower l i n d with Durn acidproof brick, 0 feet high and 18 incht,s in inside diameter. The dissolver {vas made t,all and narrow rather t>haiishort arid wide (t,hat is, a t o w r r a t h r r than a tank) t o providr adequatc liquor depth for effertivcx avration. T3ric.klining was used because it, romhinrd corrosion n
tliernial insulat,ion and also ticctiusc there was uncertainty regarding thv r(4stance of stainlvss stccl and othclr metals to the ammonium chloridr mot her liquor. h bed of copper wire 3 . 3 to 4 fret t l o i ~ prcsstcd on a perforatcd stainless steel plate undernc>at,liwhicah :til. \vas introtlucid. .lnimonia could lw introduced froni :I port:ible rylincli~rinto this air line. The dissolver was roil made of 3/4-inch A.I.S.I. Type ~vall\\-aspenetrated at, points n i w t m pipe nipples, which, through the use ed simultaneously as outlets for a liquidI w c l gaga, for thc introduct,ion of thermometers, and for t,he reinov:$l of liquid samples. Recirculated liquor was introduced at thi, t o p and distributed over the copper by a stainless steel cone. Thc copper charge consisted of 1000 pounds of scrap KO. 12 cablc chopped into 1.5-inch lengths. PRECIPITATOR. The precipitator was a brick-lined tower similar in construction and dimensions to the dissolver. Recirculation \vas riot, used in the precipitatpr, but agitation was providid by a l/,-horsepover Lightnin miser introduced at a 45' angle through the side wall. Stainless steel (Type 316) stirrers a-ere used both in t,he precipitator and storage tank and proved fairly satisfactory, but failed one by one and were replaced with Diirimet, xhich gave much betttxr servicc. An emergency bronze stirrer installed in the precipitator dissolved coinplctely in a day or t x o . Arrenir triosidr (technical grade .IsLh) was introduced ttit,ougli the top of t h r tower b y mcans of a doubledoored hopper. \\hirh \\-as closed on t o p by a hinard flangc sc&d with a rubber gati1cc.r and undcrncath t)y a 6-inch quick-opening gate valve. This hoppcr i II in Figure 4,which is a vivw of the u p p r o1)cratiiig pltitform. Th(1 hot gaws leaving the preGASCOOLEE.INU SCRURBER. cipitator and dis~olvi~r, containing water vapor and ammonia vapor, passed through a common manifold into a shrll-and-tubc ~ w r coolctl v in ort1i.r t o k w p the temperaliquor lo\\- cnough for good ammonia recovery. Vnlvc. arr:ingemrnt s wcrc sach th:tt air flow could bo
November 1947
INDUSTRIAL AND ENGINEERING CHEMISTRY
n r ~ r s i i g i i~i il sihrics iristtxtd 01' iii ~)arallelthrough tliix tlissolvor : ~ n d precipitator. T h e cooled gases passed into h scrubber consisting of a n open cypress tank 4 feet in diameter and 4 feet high, filled wit,h filtrate mother liquor, through which the gas was bubbled by means of a distributor made of 1/4- and l/s-inch pipe. TASKS,PUMPS, ETC. T h e slurry obtained from the precipitator was filtered by gravity through filter cloth stretched across the top of a wooden-stave tank. This tank, like the other tanks s h o m i n Figure 3, was ideritical with the scrubber tank. Kooden tanks were used because of ready availability, cheapness, and the uncertainty regarding the suitabilitl- of stainless steel and other metals. They proved quite sAtisfactory as long as they were operat,ed a t a fairly constant liquid level but leaked badly when the level was raised to a place that had been left dry long enough t o shrink the wood. On the basis of rapid scttling obtained in preliminary laboratory tests, it \vas originally planned t o separate the product from the mother liquor by decantation However, the method proved unsatisfactory in initial pilot plant trials k c a u w settling n-as too rapid; the product settled into a d m s e cake that could not be effeet,ively reslurried for washing even hy vigorous agitation. For the same reawn it could not he removed fi,om the settling taiik by pump, as planned, but had t o be dug out laboriously. T h e method of separation was then changed t o gravity filtration b y stretching a cloth across the same tank used for settliiig, arid this was a decided improvement x i t h regard t o product ri.coT effective xashing, and reduction in labor. -1further inipi,ovenient ivas later made by converting thc-gravity filter t o :t surtiou filter operating essentially like a Biiehner funiicl. -1wooden tank (Figure 3, S o . 5 , was provided for the stouigi' of mother liquor to be used in dissolving roppcr. The tank wah (%quippedn i t h a removable coil, n-hich could he connccttd to circulate either steam or cooling Tvater, and x i t h a portable Lightnin miser with a stainless stecl Ftirrcr. Two 0thc.r tank., 4 and 6, can be seen in Figure 3 but arc not e h o ~ in n thc flow diagram. Tauk-I-wasused alternattxly with tank 3 as a f i l t c a r . Twli G n-as used for the storage of ~i-asli\rater that wa~aIO IJY i~ec~ycli~cl. 1lul)bcI hose was used for the lines handling pro tiotli for its corrosion resistance and to pt~rmituse oi lines : clc9citicd UIJ[III di>,.pitcx its i i , l i i t ivc,ly liigli c w t and d t e r such alti~rnati 3 a 1ie:ittd Iiulvwizci, ( l i i l i r mill I l i : ~ ( lh e n sonsidere~tlI~cc.:iusc~i t c.oml)incd t l r c s f u i i i , t i o i i I J ~pulvi,t,izc:i. and dryer, and partiidLr1~-1x~c.au.i~it r:it.i.ic~tIt lit, iiiatc*i~i:dft~oni a \i--atrr suspcmsioii t o i~ parkagtd Iirotluct in a siiiglix iil)ei~atioiiiii a scaled unit, iiivolving 110 handling :ind a niininiuiu ( i f esposurc of personnel t o the iri,itatiiig arid toxic cffccts of tile poivdered product. I t s cost was boriic' oclunlly by the' coppc'i' ars:ciiite pilot other 1)rojcc.t iti ~vIii(,lii t X:IS l:iti>rusctl. 11)r the, clryiiig ~ ) ~ ~ e i ~ : i t i I\i Jil' ii c o l ~ t : i i n c ~ ( from l the combustioii of Iiy-product cartioii iiionoxitlc gas 1'1,ciiii I!iiosphorus rotarJ- kiln, plant electric. furnace operation iii an esp~t~inic~iit:il d i i c h was a pcmianent instci1l~ti1~11 fc~rg~,iit>r:tluse and was
tanks. ISSTRUVEXTATIOS. Air flon-Y and steam flows were measured b y I I I ( ~ Pof orifice meters. The air orifiws \vc'ria calibrated against a standard orifire equipped with straightening vanes, and the steam orifices were calibrated b y weighing the condensate obtained dui,ing a measured period. Stemm traps were located ahead of the oi~ificc~s. Pressures at the inlet and esit of both towers were measured. Steam p t ~ sures were measured by 100-pound Bourdon gages, and air pressures were measured by mercury manom-
eters.
~
Figure 4.
1525
Upper Operating Platform of Copper Arsenite Pilot Plant
1.526
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 39, No. 11
copper, ruiiiiing thcw aii:dysea was part of his job. The shift engineer had a B.S.degree in chemical engineering and had one or two years' experience in pilot plant operation and similar work. The field engineer had a similar background with somewhat more experience. The project leader, in addition to such experience, had for the past few years been in charge of development projects of scope similar to the copper arsenite work. The aide was a chciriicd c:ngiiieering student ill his junior year working for a 3-month period as part of a cooperative training curriculum; such joiis are also held by permanent employees whose qualifications are based on pertinent college training or on high school education plus practical experience. The plant \vas operated by a crew (nonproi'cssionalr of t\vo men per shift, whose qualiticalions consisted of grammar school or high school education and whose previous technical espci,icnce was generally confined to sevo~,aI years of service in their cui,rent jobs. The hcad operator's responsibilities included the operation of the plant, periodic recording of data. and securing of samples, plus performancc of Figure 5. Spray Dryer Used in Copper Arsenite Pilot Plant some of the operating duties. His helper performed most of the operating chores, such as charging the tower r i t h copper, operating thc f i l t c ' i , t'tc., as ell as the general task of keeping the area clean. located adjacent t o the dryer. The slurry to be dried \vas stored .A sccond Iirtlper served on the day shift only to operate the in a lead-lined steel tank and \ms circulated by a 1-inch Pinneer spray drycr and to take the necessary samples and (ht:i, metal centrifugal pump. The tank W:E provided with tivo portThe operators w r e under the administrat,ion of the Service Serable Lightnin misers ( l / d horsepowr) to keep the copper srsciiitc tion but ITere fully subject to the supervision of the shift engineer. in suspension. The rate of f e d was controlled by recirculating Most of the maintenance n-ork and equipment changes \vew the slurry continuously, a sniall portion being bled into the desiccator through a valve consisting of a 6-inch length of 3 / 4 - i n ~ h carried out on the day shift, when craftsmen xere available on a full-time basis for pilot plant work by application to the Service rubber tubing that could be const,rictedwith a screiv clamp. PERSONNEL
1 PROJECT LEADER I Figure 6 is a n organizational chart of the personnel, profesSUPERVISION PLANNING sional and nonprofessional, cmployed full time in the copper arINTERPRETATION OF DATA REPORTS senite pilot plant investigation, n-ith a brief indication of their principal dut,ies. Because of the relatively small size of this pilot plant, the number of personnel was probably smaller and the organization simpler than in many cases. The chart represents the period when the pilot plant was being operated continuously on a five-day, three-shift basis. The invwtigation !vas SHIFT E N G I N E E R * the direct, responsibility of a project leader. The fipld engineer SUPERVISION NOTEBOOK RECORDS acted as general assist,ant to the project engineer, having been CONTROL ANALYSES SHIFT MAINTENANCE assigned to t,he project, from the start. Xs previously mentioned, SUGGESTIONS he had assisted in the design, prepared the drairings, and f~llo~v-cd DATA TABULATION the construction. During the regular operation he took care of SUBMITTING SAMPLES the maintenance jobs and the equipment changes other than those SUPPLIES handled during the night shifts. H e prepared draviings for such changes and carried out calculations for the interpretation of OPERATOR * OPERATION results and preparation of reports. SUPERVISION A technical aide (nonprofessional), under t,he direction of the DATA SHEETS SAMPLES field engineer, carried out the tabulation and routine calculation of the pilot plant data on a daily basis; once a day he transmitted the samples from the pilot plant to the analytical laboratory, and he secured the various supplies, routine and special, requested by the shift, engineer. The shift engineer, Rho was assigned to the I project only during pilot plant operation, was in charge of the I H E L P E R (DRIER1 OPERATION OPERATION operation for a n 8-hour shift. I n addition to his main job of HOUSEKEEPING RECORDS running the plant successfully, he entered a log of the shift history in a permanent record not,ebook,saw that the proper records THREE-SHIFT; OTHERS DAY-SHIFT ONLY and samples were being taken, and made suggest,ions as to the improvement of the operation of the process. After the developFigure 6. Organization Chart of Full-'Time Personnel for Pilot Plant Investigation ment of rapid methods of analysis of the liquor for arsenic and
*
November 1947
INDUSTRIAL AND ENGINEERING CHEMISTRY
Section. For emergency jobs during the night shifts, maintenance men could be had on call from the adjacent full scale plant operations. A note should be added coriceriiing the supervisory levels above the top block of Figure 6. T h e project leader held informal discussions of the work progress with his supervisor, the chief of the Process Development Division, at irregular intervals, perhaps once or tIvice a xeek. Once a niontli written progress reports were discussed a t a meeting of the project leaders and the division chief, and the program was revien-ed in the light, of current results and revised if advisable. A fornial report to serve a s a permanent, record of the ivorli was prepared a t the conclusion of the 1039 campaign, and another in 1940; these reports included conclusions d r a w i from the n-orli and recommendat,ions for further work or for utilization of the results. It may be superfluous to mention t h a t these conclusions and recommendations had generally been transmitted by niemoranduni or otliern-ise, and acted upon, before the reports theniselves had been issued in final form. PROCEDURE
The operating procedure is illustrated bl- the follon-ing set of actual instruc:ione, which n.ere in effect during the final period ~ t ' pilot plant operation. (The spray dryer instructions are not included.)
Operafzng Insfructiom Copper Arsenite Pilot Plant, Run P2, June 1940 M o s t of the following instructions apply to each cycle:
1. At end of reaction period, shut off air, ammonia, and steam to precipitator. S o t e level. P u m p slurry onto filter, having previously started suction. Solution remains in precipitat,or 60 minutes after first arsenic addition. (Initial precipitate is generally olive-green and gradually turns light blue upon aeration. If sample is still green after 60 minutes, continue aeration and consult supervisor for special instruction.) 2. Transfer solution from dissolver to precipitator. Start air a t A&' = 5 inches mercury, and steani as indicated on board. 3. Transfer 52 gallons (6l/2inches) from tank 7 (scrubber) t o dissolver, controlling air fioiv to precipitator t o avoid suction a t top of towers. Start air at ilP = 2 inches ivater, and steani as indicated, and recirculate. 4. 1Ieann-hile slurry is being filtered. Take 10 gallons of cake, adding st,rongwash m t e r from storage tank, heat, and 1%-ash wash to filtrate. Transfer filtrate to t a d ; 7 , removing sample E vhile pumping. Add 1200 nil. concentrated hydrochloric acid to t,anli 7 . (aand E were used t o designate, respectively, samples of Fquor froin the precipitator and the filtrate a. After solution has been in precipitator to A&' = 0.3 in+ mercury. Take sample h 20 pounds arsenic. Start ammonia, adding minute period ( AP = 3.8 inches oil). G . Analyze sample A for copper and arsenic. Calculate total arsenic to be used, using curve for -1 = 30 on chart (Figure 7 ) . Add required amount. 7 . After collecting three cakes on cloth, wash Jvit,h 30 gallons of cold water, and transfer filtrate t o n-ash-water storage tank. \\-ash with 30 gallons additional water a n d discard filtrate. Rcmove cake to storage box. SAMPLES.Take liquid samples A and E from precipitator and filtrate, rt:spoctivelj-, once every batch, as indicated iii procedure. Take duplicate samples of mother liquor from tank 7 , once every four batches, just before transferring solution to dissolvert h a t is, before batches 4, 8, 12, etc. (one sample for ammonia and chloride, and one for copper and arsenic), Determine copper and arsenic in all samples. I~EADISGS. Record both tower levels just before emptying and just after filliiig. S o t e level and time for taiilr 7 aflcr adding or removing solution. Record temperature in both ton-ers tirice per batch. Record time and amount of arsenic additions. Record time of taking s: S o t e number oi spray d r SOTC. I3e carci'ul iti :Lddiiig solid arsenic t o avoid dusting. Keep h a m 1 on sc:tlc an(l weigh 1~;- tliffererice; remove arsenic n i t h scoop from barrel tiircc:ly t o hopper. Keep m o d e n plug in tank 7 drain when not pumping out.
1527
The readings were recorded on standard nunibereti d a t a sheets, each covering 1 day's operation. A log of the sliift'h operation, n i t h notes on special occurrences, repairs and mainlcii:ince n.ork done or needed, special samples, and suggcjted iniprol-cliieiits xvas entered in a bound notebooli Ivitli duplicate tc.itr-out, 1i:Lgep. Information and instructions also were entered iii tliia IiCJte~Jll(Jk by the project leader and the field engineer. J1:ita PIII Iiotebooli pages ivere numbered, respectivclj-, \\ ith iujnrcpxting serial numbers and w x e filed perni:~~iciitlyaii cr final reports covering the n-ork had been completed. r l - 1 ~h t : i sheets and tear-out notebook pages were piclied up daily, :ind tlie results were tabulated together n-ith tlie ana1ytie:il results, which were received on standard forms from the hbolxtoiy. Itoutiiic calculations of averages, totals, material Iialancc~s,ctc., ivcie niade ILYSI~.
In the 1939 citiiipiiign tlie lollon iiig : During the pi,eliniiiiary ,,peration
o n the day shift only, a complete set, of liquid janiples v a s taken, n-hich included samples from the dissolver and precipit,ator a t t,he end of each hourly batch and samples of the filtrate, scrubbing solution, strong wash, and mother liquor a t the end of each filtration cycle. During continuous 24-hour operation, in order to keep the analytical load from beconiing excessive because of the increased rate of operation, liquid sanipling v a s reduecd to samples of *every sixth batch from t,he dissolver, togethcr ~ ~ - i t l i samples of the filtrate, scrubber solution, aiid mother liquor talien at t h e end of alt,ernate filtration cycles. All solutions were analyzed for copper and arsenic; in addition, the tank solutions were analyzed for ammonia and occasionally for chloride. All analyses for the 1939 pilot plant operation were carried out by the general analytical laboratory, which is set u p to handle the analytical Lvorlr for all the research and developincnt projects, and results generally \\-ere available 24 hours after sampling. The studies carried out in the bench scale plant in the spring of 1940 resulted in a considerable change in the sampling and analysis procedure for the 19-10 pilot yltuit opcration. The liquor samples were immediately analyzed by tlie shift engineer for copper and arsenic by rapid, approximate methods. Copper determination was by a colorimetric method employing tlic cupric ammonium complex; arsenic deterininat,ion vas by titration n-it,h sodium bromate. B y dividing t h e arsenic addition t o the precipitator into two steps, the time required foi, analysis \!-as not lost from the precipitation period. The rapid analytical niethods also were used for det,erniining the copper aiid arscnic contents of the filtrate from each batch. The only solution samples submitted t o the analytical laboratory were motlier liquor saniplc taken after every fourth batch and analyzed for copper, arsenic, ammonia, and chloride. Samples were taken of each product of a givcii spnij- dryer operation, Tvhich amounted t o 200 t o 300 pounds. Siricc riming or quartering v-as impractical because i ~ i ' tlic toxic-ity of the material, samples were obtained by tlippiiiy into tlie batch at a number of points. T h e samples mere :~nalyzcdfor coppe~',arsenic trioside, arsenic pelitoxide, free arsc,nic trioside, ammoilia, chloride, and nioisturc. Screen aiialyscs, u k i g scr to 325-mesh size, also were made, and these w r e sugple1i1(!i!ted by microscopic examination for rough estiriiatts of ultiniste particle size. Portions of tliese saiiiyles were submitled t o tlic Malaria Control Sect,ionfor pan tests, and the product itscli w n s subniittid for fiold tests. I n pan tests the larvicidal efIicicitcy of tlie inaterial n-as compared with t h a t of Paris green uiitlvr 1ai1or:~tor~coritlitions. Equal weights of the tn-o niaterittls, niiscd 11-ith9 parts of soapstone in a niortar and pestle, yvere dusted onto the surface of water contained in shalloir pans, each c.oiit::iiiiny substantially thc smic numtwr of .4nophe/cs 1 : i t ~ v ~: L~i ci i ~l i i i a i i i t : a ~ ~ J i l i t : t l ltenipelature. t T l i c 131vx i ~ ~ i i i i t i i i i i iali y 2, 3, and 4 hours were counted. The rat,io of tlie percentage
INDUSTRIAL AND E N G I N E E R I N G CHEMISTRY
1528
I
i 200
20
40
60
80
IO0
C, I N I T I A L COPPER CONCENTRATION, G M PER LITER
Figure 7.
Calculation of Solid Precipitator
As203
Addition to
killed by the material being tested to that killed by the Paris green in a 3-hour period was taken as its Paris green coefficient. Field tests were made under the actual conditions in which larvicide was applied in routine mosquito control operations. Equal amounts of both Paris green and copper arsenite, diluted in the customary manner with 3.5 parts of soapstone, r e r e dusted by airplane over two comparable areas of the reservoir. The number of live Anopheles larvae in each area was determined before and after dusting by dipping out samples in a standardized manner a t a number of definite locations marked by stakes. The results of the pan and field tests of copper arsenite have been reported by Hinman, Cron-ell, and Hurlbut (I). NOTES 08 OPERATION
I n the initial operation of the pilot plant, which started on June 15,1939, batchwise tests meremade intermittentlyon the day shift during the first 2 months. I n the course of this time changes were made gradually in both the equipment and procedure, largely by a trial-and-error process. Both pan tests and field tests were promptly made of the first products and served as a guide in improving the process. Continuous operation started on August 15 and was continued for a month. Xoted beloTr are some of the more pertinent observations and changes, both for the equipment and process, that \rere made during these periods. It became apparent early in the operaEQUIPXENT SOTES. tion that the formation of precipitate in various parts of the system mould be a source of operating difficulties. These difficulties were overcome gradually by minor changes in the equipment, and they were virtually eliminated in the 1940 operation through better control of the mother liquor composition as discussed in the next section. The l/&xli openings through which air originally was introduced into the dissolver and precipitator clogged almost immediately but were replaced successfully with l/n-inch openingq. The 1.5-inch annular space betlreen the steam coils and the wall of both t o w m filled solidly with precipitate, creating a dead space, which cut down heat transfer and made i t difficult to remove the coils for cleaning. This difficulty m s eliminated by using smaller coils, which increased the clearance from 1.5 to 3 inches. The precipitator coils became coated with a hard layer of precipitatc, which apparently protected them from corrosion but a159 cut down the heat transfer capacity to a serious extent. Occasional Jrashing with acid helped some-ivhat, but the problem rms not solved in the pilot plant work. Plugging of the tranifcr liiic.; at the trcs and ells was frequent. Prior to the 194.0 operation, perrriurient cleanout rods working through packing glands n-ere inrtalled a t the critical points; this madr it easy to keep the lines open.
Vol. 39, No. 11
Several other equipment changes were made prior to the 1940 operation that contributed to both the ease and effectiveness of operation. Convcrsion of the gravity filter t o a suction filter has been mentioned. The mother liquor storage tank \vas eliminated; instead, all the mother liquor was stored in the scrubber tank. Through this simplified operation it was feasible to operate with two pumps instead of three, and ammonia recovery in the scrubber Jras improved also. The addition of hydrochloric acid (instead of ammonium chloride) to the mother liquor to replace chloride losses also improved ammonia recovery. A shortcoming of the pilot' plant %-asthe crowded arrangement, particularly the inadequate head room over the filt,er and storage tanks. The ammonia fumes given off by the hot solut,ion aggravated this difficulty. The conditions above the filter n-ere improved by cutt'ing 1.5 feet off the top of the wooden tank. It was evident that for prolonged operation a system of hoods for fume removal vould be desirable. Another change that was indicated as desirable was the use of oversize pumps and lines to minimize the proportion of time spent uiiproductively in transferring solutions, and thus increase the over-all production rate of the plant. The operator controiled the operation of the spray dryer by constant adjustment of the feed rate t o keep the temperature of the exit gases in the desired range of 210" to 250" F. I n the initial operation slurry was fed by gravity from an overhead tank, thc feed rate being adjusted by means of a valve in the feed line. This was unsuccessful because the rapid settling of the copper arsenite particles caused clogging of the valve and xide fluctuztion in rate. The problem finally m s solved by a feed arrangement in which slurry !vas const'antly recirculated through an overhead linc from a vigorously agitated feed tank. Part of the slurry rras diverted from this line in an upnard direction through a control valve that consisted of an adjustable pinch clnmp on a 6-inch length of 1-inch rubber tubing; from this valve the line turned dorvnn-ard to the dryer. Even with this arrangement it was necessary to clean the valve every three or four minutes by a brief injection of tap n-ater through a branch line. PROCESS SorEs. Observations and changes in the process n-ere as follorrs: Addition of A i r and Ammorzia. The proper rate of aeration during the dissolving and precipitating steps was determined by trial. Theoretically, s o m w h a t less than 1.0 cubic foot per minute rras needed in the dissolving step, for the oxidation or" metallic copper to the cuprous state, but rates up to 10 c.f.m. were tried. At rates above 2 c.f.m. foaming occurred and hecame iiicreasingly viorse wit,h increasing rate. %, rate of 1.0 c.f.ni. tras adop4;ed and prove satisfactory. Analyses of the offgas, Lvliich were made occasionally, agreed roughly with the calculated oxygen consumption. I n the precipitation step aeration was needed for oxidation of the cuprous ion t'o the cupric state, and it Lras found best to complete the oxidation early in the precipitation period. On the other hand, it was difficult t o attain temperatures above 200" F. a t high rates of air flow, and it rms found that high temperature v a s essential in the latt,er part of the period to conrert the precipitate from the green color in which it formed initially to the light blue that was characteristic of a good product. These requirements were reconciled by aerating at a rate of 10 c.f.m. for the first 15 minut,es, then reducing the rate to 3 c.f.m. for the 60 minutes following the initial arsenic addition. hithough various conflicting theories as to the function of ammonia addition were held a t different' times, it was finally concluded that its primary function m s the obvious one of making up for losses of ammonia from the system. KOsystematic ammonia addition JVW provided for initially, but almost a t once the need hec:ime apparent, and the addition of 3 / 4 pound per batch to the dissolver >?-asfound by trial to balance the losses fairly well in the
November 1947
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
1939 operatioil. During the bench scale studies, i t appeared aciditioii oi animoiiia tu tlie precipitation step favored this operation; chnsequently, in the 1940 pilot plant operation aniinonin \vas added t o t,he precipitator instead of the dissolver. Although this proved satisfactory, there was n o conclusive evidence that tlie change was significant. Both i l l the bench scale and pilot plant work it was found that tlie ratio of amnionia t o chloride in the mother liquor had 110 ~ n a r l t eeffect. ~l Hoivever, this ratio \vas kept much more uniform iu 1940 than in 1939 by adding 1 pound of hydrochloric acid ( 1 0 0 5 basis) t,o the mother liquor with each batch of filtrate. TT'uter Balance. One of the problems to be solved in the pilot plailt operation x a s adjusting the ivater balance in the cyclic part of the process. K a t e r ]vas itdded t o the process chiefly in the torm ot' wash water. It was removed, aside from mechanical losses, as vapor in the gases leaving the scrubber and as moisture iii the washed product going t o the dryer. I n general, input and outgo had to be balanced in order to avoid a cumulative change in the mother liquor concentration. It was obviously desirable t o use as niuch wash x a t e r as possible to keep mother liquor loss in the product t o a minimum. Practically speaking, the permissibl(1amount of Lvash ivater could be increased only by increasing tlie scrubber temperature to increase the water content of the saturated gases leaving the scrubber. Hon-ever, such a n increase in temperat,ure increased the loss of ammonia vapor in these gases. -1fairly satisfactory balance was struck a t a scrubber temperature oi about 95" F.; however, operation vias not POlonged enough to permit a n accurate determination of the optimum condition. An obvious method of improving the a-ater balance is to separate the condensate from t h e gases leaving the cooler before they pass into t,he scrubber, and t,o use this as wish water. This was tried; the separ:at,ion v a s accomplished by use of a collecter with a liquid seal. 1Io\vever, the high ammonia content of this conderisate made its use as Lyash viater impractical because of ammonia losses aiicl iritolerable working conditions around tlie filter. Washing efficiency mas improved by using the available wash water trTice iri a, countercurrent progression and adding the ryash solution t,o t h e filtrate after its second use. The cake Jyas washed finall>- \\-it11 additional water which was discarded : this, of course, did riot improve the recovery of mother liquor, hut its use proved desirable for the sake of eliminating chlorides ironi the product as thoroughly as possible in order t o minimize corrosion of the spray dryer, which othern-ise was appreciable. Control of Precipitation Step. I n the 1939 pilot plant operation tlic production capacity rapidly decreased xvith continued operation because of t h e accumulation of insoluble arsenic-copper compounds iri the bed of copper. I n addition, the product, while satisfactory, \TXS quite variable in coniposition and' oftcri contained considerable amounts of free arsenic. Tcmporarg- improvements were obtained b y such methods as removing thc copper and cleaning it mechanically, or cleaning i t by circulating either fresh ammonium chloride solutioii or hydrochloric acid through the bed. H o w v e r , these v e r e makeshift arid uneconomical procedures. It was apparent t h a t the cause for this behavior n-as the cizcessive content of dissolved arsenic in t,he filtrate from prccipiM o r batches and hence in the mother liquor, which led t o precipitation of t h e troublesome compounds when this mother liquor !?-as recycled to the dissolver. This behavior did not become apparent during the first 2 months of operation, which was on a day-shift basis, because when the mother liquor n-as allowed to cool overnight in the storage tank, considerable precipitation occurred and lowered the concentration of dissolved arsenic in the solution charged to the dissolver. Horever, with three-shift operation the mother liquor remained hot, little precipitation occurred, and the trouble developed. Cooling the mother liquor was an impractical procedure for steady operation because the
i h t
1529
precipit at(^ i'~JI'lllec1i i i t lie storagc t aiik ~vereof variable composition and generally had poor larvicidal action, sa t h a t they represented an excessive \vast(%. The copper concciitration in the solution before precipitation ranged from 20 to 60 grams per liter; after precipitation the solution contained from 5 to 20 grams of copper per liter and irom 50 to 90 grams of arsenic trioxide per liter. At these high concentrations of arsenic, precipitation in the dissolver and other parts of the system could not be avoided. However, there xere indications that, if the concentration of arsenic trioxide in the solut,ion leaving the precipitator could be kept' belon- 30 grams per liter, unn.anted precipitation could be fairly well eliminated. In t,he bench scale studies made the following spring, rapid methods of analysis for copper and arsenic were developed, so t h a t the amount of arsenic added could be based on the actual coniposition of m c h batch. T h e follov,-ing conditions were then varied in an esploratory nianner over the range indicated, in search for a method of controlling the process more effectively: R a n g e uf \-ariation__ EGsolver Precipitator Condition 100 123 100-100 Air i n p u t , LX of ntirniiil 0-100a 0-200 NH3 i n p u t , 9 of nornifil 5-20 5-20 NHaCl concentration, C; 0.89-1.22 0.67-1.22 S H s i C l mole ratio 45-150 45-150 Time, minutes 212 140-212 Temperature, F (1-39 38-184 .iszOs concentration. g I C u concentration, g./1 0-3 1 6-160 0.59-1.96 AszOaiCu mole ratio before 1 ) I ) l I l . ... 1-2 Repeated pptri a Based on normal addition to precipitator; no ammonia was normally a d d e d t o dissolrer in benrh v a l e te-ts.
Obviousll-, oiily a liiiiitcd :tiiiouiit of \vork could lie douc OII each factor, even though 200 separate precipitations r e r e carried out. The effect of most of these factors v a s found to be either negligible or too miall to account for the pilot plant behavior. Review of the accumulated results of a large number of experiments suggested t h a t the factors most strongly affecting the residual arsenic concentration after precipitation \yere the initial concentrations of arsenic and copper. EIorvever, an attempt t o correlate the data for about forty tests made under reasonably uniform conditions on the basis of these two concentrations was unsuccessful. It -xas then found unexpectedly that, within the range of practical operation, the rate of precipitation was governed solely by the initial copper concentration, and this led t o the following simple relation:
1= a
+ DB + cC'
(1)
where A = residual -4~203concentration (after prvcipitation), g.A M = t,otal initial A l ~ 2 present,, 0 a e. ('I. C = initial copper concentration, g.11. a, b, c = constants The specific values of a,b, and c depend on operating conditions, such as time and temperature. For the experimental conditioris (-&minute reaction period after arsenic addition: temperature. 200" to 212" F.; SH?,/Clmole ratio nominally 1.0; S l 1 4 U concentration, 7.5%;:arid vigorous agitntiori) Equatioii 1 ticcomcs:
A = 2.0
+ 0.77B - 1.3C
(2)
This equation was used for control purposccs as fOllO\VS: A , the arsenic concentration after precipitation, was wt itt a value shown by experience to give satisfactory steady-state operation with only minor formation of precipitate in the inothrr liquor storage t a n k and copper dissolver; 30 grams per h e r proved an effective upper limit. Immediate analysis of the copper-rich solution after transfer to the precipitator by the colorimetric method gave a value of C, and B was then calculated from Equation 2. Knowing by analysis the concentration of arsenic trioxide already in solution and the volume of solution, the amount
1530
INDUSTRIAL AND ENGINEERING CHEMISTRY
of solid arsenic trioxide to be atldcd t,u I)riiig t,lie t o ~ dv o i m ~ i i tration u p to B r a s determined. Use of this cqu:tti~iriu t c facilitated for routine operation iiy preparation of tlici cliwt shown in Figure 7, in which a series of lines are given for diffc~writ values taken for A . The use of this chart proved very effective. Iri the 1939 operation several rule-of-thumb methods for determining t,he amount of arsenic addition had been tried, such as niakirig thc ratio of arsenic to copper the same as in copper arsenite, or addiiig a fked amount of arsenic. They were consistently uiisuccessful, and residual Asz03coricentrations as high as 70 or 80 grama per liter were frequent. However, with the help of tlie c h r t , thc residual As203 conccntrat,ion averaged 15 grains per litcr for the 1940 operation; individual values ranged from 5 t o 42 grains per liter, but few exceeded 20. Xoreover, the averagr rate of copper dissolution was as high on the last day of the M a y run as on tlie first. I n this operation, -4 was set at 30, but a factor oi safety \vas provided by extending the reaction period from 45 t o 60 minutes. Even with this comparatively stcdy-at:itc opcratioii the) coniposition of successive batches leaving tlic tliswlver varied n-idcly in a manner iiihereiit in the operation. It \vas t,liis variation t h a t made a control method esseiitial. Thus the copper concentration before precipit,ation ranged from 10 to GO grams per liter, and the required additions of solid arsenic ranged from 20 to 57 pounds. The success of the method is indicated by the fact O3 to Cu before precipitationthat, while the mole ratio of t h a t is, of C t o B on a molal basis-varied from 0.67 to 1.9, thcl ratio in the corresponding products varied only from 0.94 to 0.99. Product. The 19-10 product' \vas considerably more uiiiioim than t h a t made in 1939, was substantially free from uncombined Asz03, and had a smaller particle size. Cnder the microscope it appeared to consist of ultimate particles 2 to 6 microns in diameter, some of which were combined t o form loose agglomerates 10 to 30 microns in size. By comparison, commercial Paris green had a particle-size range of 10 to 50 microns. T h e final test run gave a n over-all recovery of about Gc;,of tlie arsenic and copper as useful product. I t appeared t h of the remaining 15y0consisted of very firie particles that from the spray dryer Multiclone. P a r t of this firie material was scrubbed out by the spray water in the exit stack but was nor utilized. It would probably be feasible to recover m o s t of this fine material in a bag filter, and this would be particularly desirable, since dusting tests indicated t h a t the larvicidal action increases with decreased particle size. Assuming the recovery of this fine material, material balance figures indicated t h a t , in a steady production of copper arsenite on a moderate scale, it was reasonable t o expect 95yorecovery of copper and arsenic and a consumption of ammonia and hydrochloric w i d not esceeding 2.5 to 3.0 pounds each per 100 pounds of product. VALUE O F PILOT PLAST WORK
T h e pilot plant investigation confirmed, in this case on a semicommercial scale, tlie technical soundriess of the process \vorketi out in the laboratory for producing copper arsenite. Further than that, it disclosed that steady-state cyclic operatioil, such as was necessary for economical commercial productioii, presented problems not apparent in the laboratory rioricyclic experiments, and i t then pointed out a practical solution of these problems. T h e work led t o the development or selection of types of equipment and materials of construction suitable, although not necessarily optimum, for commercial operation, and provided data for the design of a commercial plant. Procedures and techniques for the operation and control of such a plant were developed. Sufficient information was obtained for niakirig reliable investment and product,ion cost estimates. -4potential benefit, not utilized in the present case, was the training of supervisory and operating personnel who could have placed a commercial unit
Vol. 39, No. 11
iritu iiiitial opei~niioli etfioiently. ,111 t:stiiiiate of t l i t i health hazards )vas obtained, and safe practice \vas Product was made available in sufficient q u field tcstirig. This is aln ays an important furiction of pilot work. In t h r present case the comparative field tcssts disclosed t h a t a iiiucii smaller proportion of the copper arsenite than of Paris giwri r t w h t d the designated area of the r a t e r surfact,. This differenrc., which w a d not foreseen from laboratory larvicidal tmtn, \vas caused by the smaller particle size of the copper arsciiite. I n spite of tho lower efficiency of application of the copper arsenite, its over-all effectiveness was greater than t h a t of P u i s green. These results led to a more general study of methods of improving the distribution of larvicide in the field and of the relation of larvicide particle size t o effectivencw, which resulted in brncfits to the malaria control program. .i final brief coiiinieiit on the cours(1 of the investigation may be iiiatle i i i tho light of hindsight. T h e performance of the pilot plant agreed \vel1 with t.hat predicted from th(, laboratory work iii most r c q e c t s , such as react,ion rates, chcmicai behavior, arid product quality. Siiveral comparisons have been indicated iii the prccediiig pages. As a further example, it \vas found that the average ratt. for dissolving copper in the final pilot plant run was 13 grains per kilogram per hour, compared with the anticipatcd figure of 18 grains per kilogram per hour; this difference might reasonably be expected because of channeling arid coating of the 1)cd of copper in the pilot plant dissolver. Tlic. rhief problem encountered in the pilot plant work not :ttliquat(~lyforecast by the small scale research was the cumulative effcci. of undesirable prccipitatcs formed during steadystate cyclic operation. I t might well be argued that, if the pilot plant n.orli had been deferred until the bench scale study had bccii carried out, t,his problem nould have been encountered and wvorkcd out niore cheaply on the smaller scale. Successful d>--st>LtiL operation then might ~vcllhave been achieved in the initial campaign, u-hich under this scheme would not, have bcvn uiidertalten until 1940, and most of the cost of the 1939 t plait operation might have been saved. On the other hand, n though steady-state operation n-as not achieved during the 19313 campaign, it did fulfill its major objective of providing copper arsenite larvicide for field tests. Thus it appears that the preseiit investigation was carried to the pilot plant scale too soon from the standpoint of the optimum over-all cost of developing the process, but that this \vas justified by a special circumstance. It, probably is true, however, that such special circumstances are more t,he rule than the exception iii pilot piitlit invcstigations and are one more re:tsoii n h y there is little danger t h a t process development work will ever he reducc,d io a dry routine. ACKNOWLEDG\IENT
The x o r k ivas carried out a t the request of the Tennessee 1-alley Autliority Health and Safety Department, and appreciation is expressed to E. L. Bishop aid members of his staff for tiitsir advice and assistance. Thr: ituthor is indebted t,o E. It. Itusliton, who carried out the initial laboratory iiivevtigations :tiid devised tlie process, for advice and assistance throughout t l i v n-ork. :kkno~vledgmerit is Inade to .J. €1. LVxlthall for helpful criticism and advice, and to \I-.H. Cook, D. 0. LIyatt, ,J. C. Barber, A. W.Beinlich, Jr., and other engineers mho participated in t,he pilot plant work. LITERATURE CITED
Hinrnan, E. €I., Crowell. It. L., and Hurlbut, H. S., Am. J . T r o p . M e d . . 22, 271-81 (1942). ( 2 ) Mellor. J. X-. "Comurehensive Treatise on Inoreanic and Theoletical ( ' h e n i i ~ t i"~ Val. I X , London, Longmans, Gieen and Co , Ltd , 1929. (3) Rushton, E. R.,U. 9 . Patent 2,263,594 (Nov. 25, 1941). (lj
I
RECEIVEDApril 14, 1947
