Continuous-Mixing Process for Manufacture of Concentrated Superphosphate G. L. BRIDGER, R. A. WILSON', A S D R. B. BURT Tennessee I h 1le.y '4 11 t hori ty , IVY lsori D u m , .4 I n ,
I
A coritiriiroua-nii\inf unit for the manufacture of concentrated superphosphate w as deteloped arid is being operated successfully at a production rate of 30 to 35 toils of superphoq~hateper hour. Finely ground rock phosphate and phosphoric acid are fed continuously by a constantweight feeder and a rotamatic controller, respectitel?, into M funnel-shaped mixer i n M hich mixing is accomplished by turbulence of the reactants, and the mixture is diqrharged, while it is still fluid, onto a con*eyer belt. The
mixer is self-cleaning and contains no moving parts. The mixture sets quickly on the belt to a porous, relatively dry superphosphate which is disintegrated and conveyed to curing piles. Advantages of the process oyer the previously used batch-mixing process are loww investment, lower operating labor requirement, lower power requirement, lower maintenance requirement, better mixing, less down time for maintenance and cleanup, and more flesible and virtually automatic operation.
T
~ d and r ~ crumbly and is easily broken up. The durations of the Three stages depend on the source, grade (PZOj content), and particle size of the rock phosphate; the concentration of the phosphoric acid; the proportion of acid to rock phosphate; the temperature of the reactants; and the intensity of mixing, \Thich is particularly important because of the thixotropic nature of the reacting misture. The fluid stage can be prolonged greatly by intensive mixing. These properties of the reacting mixt,ure led t o the concept ( b , 6 ) of a process in Tvhich [a)mixing would be accomplished continu,)usly in the fluid stage during which little or no poiver would be required, ( h j the fluid mixture mould be discharged into a slow$peed mixer-disintegrator or preferably onto a b,dt, conveyer ou \rhich the reaction would continue through t,he plastic stage, and ,c) when a belt conveyer is used, disintegration of the material r o u l d be accomplished during the last stage by a suitable mill and by transferring it from one conveyer to another vhile it was being wnvej-ed to the curirig pile. MIXERSTRIEDUSGUCCES,FCLLY. Much of the early work iras devoted to development of a mixer having hish-speed blade3 I J Y patltlles for accomplishing fluid-stage mixing. One of the early experimental units ( 5 ) consisted of a miser containing high-speed paddles followed by a slow-speed mixer similar to b pugmill. The high-speed miser consisted of two horizontal shafts operating at 180 revolutions per minute and provided n-ith "des to propel the reactants through a horizontal vessel. The slon-qeed niirrr consisted of a single horizontal shaft operating at 40 r.p.m. and provided n i t h blades to propel the mixture through a somewhat larger vessel. This unit was successfully used for mising phosphoric acid and liniestonc continuously, but, n-hen pliosphoric acid and rock phosphate m r e used, the high-speed mixer caked completely after a few minutes of operation. I t was found possible, however, to mix rock phosphate arid phosphoric acid continuously in the sloiv-speed miser: consequently, a small plant unit' having a capacit?. of 3 tons of superphosphate per hour was built. This miser consisted of two horizontal shifts in a vessel 32 inches nide and 120 inches long, operated by a 3Ghorsepower motor. One shaft had three rows of blades and operated a t 25 r.p.m., and the other shaft had two r o m of blades and operated a t 38 r.p.m.; the blades had a pitch of 45". This unit achieved fairly successful operation, but its poiver consumption was considerably greater than that for batch mixers, since both the fluid and plastic stages of mixing were carried out in i t ; consequently. no further development on this type of mixer was c$rried out.
EIE hatch-mixing process for manufacture of concentrateti supprphosphate (t,riple superphosphate) that was used for many years in t,he Fertilizer Works of theTennesseeValleyAuthority has been described previously (1,b). This process consisted in dumping weighed batches of finely ground rock phosphate and strong phosphoric acid (usually 787, HaPOa) a t 150' to 200" F. Into a sigma-blade mixer driven by a 30-horsepon-er motor and mixing until a fairly dry and crumbly product was obtained; batch?$ of about 3/4 ton of superphosphate were made, and the time per cycle was about 21/? minutes, of which 1 minute rras for actual mixing. The product was stored in large piles, in which Final reaction and drying took place, and after a period of about 3 months it was dug out, ground, and bagged for shipment,. I t \vas not considered economical in the batch process to use lower reactant temperatures and lower acid concentrations, which haw been shown to give higher conversions of P,05 in the rock to a soluble or "available" form ( I ) , because longer niisirig cycle> would have been required to produce a dry product, and thus mixing capacity n-ould be reduced. .lnothrr disadvantage of tiit hatch process was that the thoroughness of mising obtaincd in the plant, batch mixers n'as much less than that shown in thi. lalioratory to be necess?ry for high conversion ( I ) . Laboratory studies of the physical changes that occur during :he reaction between rock phosphate and strong phosphoric acid 3, i ,.i indicated ) a process in Tvhich mi);ing ~ o u l dbe completc~l hefcw the slurry began to solidify; such a process nnuld have R nun1lx.r of potential advan!agi.s over the prccesq in which mixing IS continucd until a solid product is ohtained. The pi.ocess allpeared to be better adapted to continuous operation than to batchwise operation. The present paper describes laboratory experiments and tievelPqmvnt ~ o r that k led to a successful continuous mixing process vmbodying the advantages mentioned; data are presented on the operation of a plant-scale continuous-mixing nnit, which has wnersedcd the batch-mixing units EARLY WORK ON COSTINUOUS-IIIXING PROCESS
IVhen rock phosphate and phosphoric acid are mixed, the milrure undergoes three distinct stages of physical characteristics In the first stage the mixture is quite fluid and is easily mixed; in the second stage the mixture is plastic and is mixed only with ,lifficulty; and in $he last stage the mixture becomes relatively Present address, J l o n s s n t o Chemical Company, Anniston, I l a
1265
,
1266
I N D U S T R I A L A N D E N G I N E E R I N G CHE.M I S T R Y
Vol. 39, No. 10
the relatively large capacity of a niixer of this type, it Jvas not feasiblt, to develop it on a laboratory or pilotplant scale; consequently, the plant..scale oscillating mixer unit was coriverted to this type of unit for the initial development. The miser ooii+ted of a funnel-shaped bowl iiii ivhich acid !vas introduced tangciitially at, several points around tht, upper p u t and pulverized rock waintroduced into the side of the bon.1 itt a velocity high enough to t,liroa- i i I O the crnter of the hoivl. The fluiti lirciduct c!ischarged onto a conv twit, U s i n g occurred principitlly iii the discharge extension of the howl. :although some additional mixing oilcurred on the receiving belt b ~ c a u s r of the turbulence of the niisturc. bc.iore it. set up. This miser was operated for about a year (1, 2) and m-a? found to give a product as satinfactory as that from the plant batrh niisers Honever, the niising \vas still riot so thorough as desired, tho mychanical operation of the miser was riot entirely satisfactory, and too much atit,iitioii t)y the operators \vas required. The principal diffirultier w r e caused by ( a ) fluctuations in the rock feed rate andocrasioual Hooding or jamming of the rock-feeding system, which consisted of :I hopper a n d a screiv-type feeder; (b) fluctuations in the a d Feeil i,ate, irhich was controlled by a constant-level bos rrith an orificc. ilischarge, although this was riot so serious as the rock-fecsding diffiiwltien; and ( e ) improper design of the miser bowl due t,o in.ufficient fundament,al information on the setting charac:t.cxrid icp ~ l the f reacting mixture. Further studies were uiidertakeri, therefore, to perfect this type' ( i f miser. -1s a first step, lahoratory esperiments m r e made to Iicttermine the setting characteristics of the reacting acid-rock tiiisturr under conditions 5imilar to those obtained in this t y p of ( 1
Othcr types of high-speed misers t,hat \vert. tried iucluded it centrifugal pump t,ype, a miser consisting of a bonl haviiig t w I vertical shafts with intermeshing paddles arid a centrallv located discharge opening in its bottom, and a miser consisting of a yyliiidrical bolvl having four vertical blades attached to ita bottom anti four vertical blades attached t o a traveling head at t h r t,op of tht, mixer (8). In the latter the moving blades traveled in s u v l ~:I fashion t h a t they scraped all part,s of thrmsc4vee against the st,ationary blades and t8hesides of the miser, and the miser was thereforts essentially self-cleaning. The miser discharged through H centrally located hole in its bottom. Seitlier the rentriftgal punip nor the double-vertical-shaft misers were successful becausc. of caking and plugging and poor mixing. The traveling-hi:ail miser was the most satisfactory blade-t,ype miser developed, and a plant-scale unit having a mixing bowl 12 inches in diameter !vas built. This unit ivas operated successfully a t a rate of 16 tons 01' superphosphate per hour and could be operated for about 8 hours brfore a shutdown for periodic cleaning out, xvhich required only H fevi minutes. Hoa-ever, it, was subject to considerable mechanical strain and breakage if it, was allowed to cake exciwivc~lyor if hard lumps of material got into it, and occasionally the bo\vl became completely caked; t,his necessitatrd longer shut downs frit, cleaning. A number of mechanical mixers were t,ried in studies to product. concentrated superphosphate ( 5 ) by a spray-towrr process, iii which rock and acid while still fluid would be mixed continuously and sprayed countercurrent to heated air rising in a ton-er. 8omc of these were modifications of the high-speed blade type and ceritrifugal pump types mentioned. Others consisted of two fluid mixing nozzles in which both mixing and atomizing were acroniplished, and a device in which mixing occurred on the inside surface of a rotating bell, and the mixture was thrown from the bell in the form of droplets by centrifugal force. Xone of these mixers could be operated under practical conditions without frequent plugging. FIRSTSUCCESSFUL MIXER. Because of the difficulties encountered with mixers having moving parts, a program !vas undertaken t o develop a mixer in which mixing in the fluid stage ~ o u l d bc accomplished entirely by the turbulence of the acid and rock streams, and which would be self-cleaning by t8heaction of highvrlocity streams of acid and fluid reacting mixture. Because of
L.%BORATORY \lEASUREMENT OF FLUID TIAXE
It was desired to measure primarily the fluid time of the r e a c ~ iiig mass under conditions similar to those that would exist in the r'unncl-type miser-namr,ly, a period of intensive mising of only a onds (corresponding to the turbulence in the mixer bowl iiiid on the belt immrdiately aitcr discharge) followed by a period in which there would be LIU mechanical agitation (corresponding t i l the period on the conveyer belt after the turbulent mixture btsi'onies quiescent). The pLevious studies on setting time were not ;tpplicable for the funnel-type miser, because in these studies inrtBnsive mechanical mixing was continued throughout the fluid stage, so that this stage extended longer than if less mixing had twen used. For example, from previous measurements of the .catting time determined by nicasurement of power required by a .ipma-blade batch miser (31, fluid times of 11/2 to 2 minutes \\-auld have been predicted for the continuous mixer. Similarly, 111 previous measurements of the fluid time by a torque met,hod ( A ! fluid times of about, 30 seconds would have been predicted. How. t'ver, observation of the operation of the plant funnel-type miser indicated that the reacting mass set, to a porous, plastic inass in twnsiderably less t,han 30 seconds. Laboratory apparatus I hi3refoi-e devised in which rock and acid were mised intensively for a fell- seconds, and the mixture was discharged while still fluid into a n inclined trough vhere relatively little additional mising took place, but the end of the fluid stage could be observed by wssation of floi~. The total time from charging the acid and rock into rhr miser until flow i w i w d in the trough was takrn as t )
,
TENN I 32.7 % 9 0 5
TENN
, 33.6 *A P 2 0 5
TENN ,33.4 % Pro5
m
9
FLA.. 3 4 0 % P 2 0 5 FLA.. 3 3 . 2
OA
P205
1
I
TENN., 30.1 % PzOs TENN.,
O 0 0.85
r
1267
INDUSTRIAL AND ENGINEERING CHEMISTRY
October 1947
l
=,
29.3 */e P z O s ~
,
~
~
1
0.90 0.95 1.00 1.05 1.10 1.15 ACIDULATION, PRODUCT P 2 0 5 / C a 0 MOLE RATIO
Figure 2. Effect of Acidulation on Fluid Time of Superphosphate Prepared from Yarious Rocks
Tennessee rock; this is probably due to both chemical and physical differences between Florida and Tennessee rocks. The rocks in Table I w r e listed in order of increasing fluid time in a n attempt to correlate chemical composition with fluid time. HOKever, because of the many variable components, the effect, of any specific component could not be evaluated. The effect of particle size of Tennessee rock on fluid time is shoivn in Figure 3. I n these experiments separate portions of the trick n-ere ground for various periods of time in B ball mill and the ilegrre of grinding indicated as percentages through 200- and 325inesh srrerns. It was found that the minus 325-mesh fraction was most significant in determining setting time, and the results \rere therefore correlated rrith the prrcentage of 325mesh fraction. Rock usually used in the Fertilizer \To phate manufacture is 75 to 80% minus 200 me minus 325 mesh. The results in Figure 3 indicate that any significant increase in the percentage of minus 325-mesh material in the plant material n-ould decrease the fluid time very sharply, which nould increase the difficulty of achieving thorough mixing i n the fluid stagr. Other experiments indicateil that increasing rhr par,ticlr qize of the plant material to as large as 50% minus 200 mesh and 35CGminus 325 mesh increased thit fluid t>imeahout 5OC$ for both Tennewee and Florida rocks. Thc effect of acid temperat,ureon fluid time of superphosphates niade from Tennessee and Florida rocks is shown in Figure 4. The fluid time wa9 increased greatly as the temperature was reduced from that employed in the batch-mising process (153' to 200" F.J. Thiq indicatc~that, in addition to the improvenlent' in P?Oj convrrsion if cooler acid w r e used in a continuous mixing unit, less difficult operaticNn of the miser I and probable better mixing would be achieved, whereas with the batch mixer:, the use of cooler ac id n-ould have decreased th(Jir capacity. The effect of acid concentration on I fluid time for Tenneesee rock of t,wo PRODUCT P205/Co0 MOLE grades is shonm in 0 F i g u r e 5, N i n i 70 60 mum fluid times ocPER CENT OF ROCK THROUGH 325 MESH cwred a t 75 and Figure 3. Effect of Particle Size of Rock on Fluid Time of Superphos- 79% HaPOcfor the tn.o rocks. phate
measure of the fluid time. The length of flow on the trough vas also measured as an additional indication of the fluidity of this reacting mixture. e laboratory apparatuh is > h o ~ ini i Figure 1. Tlir niixin 1 was Sl/? inches long (over-all) and 1 inches in dianietei thr opening a t the bottom was 13:, inches in diameter. The agitator was designed t o conform t o the shape of the conicd dection of the vessel, and the agitator blades, irhich were driven a t 1045 r.p.m., irere curved to impart a downward thrust to the reacting mixture so that quick discharging would be obtained. .Irubber stopper, which was attached to a pivoitd ami for quick opening. fittcd into the bottom of the vesvl. Rock was fed t o the niixei, from a pivoted trough n-hich could be tilted quickly. Acid was fed from a glass receptacle fitted with a rubber stoppclr that collld he opened quickly; the discharge timr for the acid IWR less than 2 wconds in most of the experiments. The trough into which the mixture discharged was made of steel with tidw 3 iiichcs \yidix a t a 120" angle and was placed at a!i angle of 10" with the, horizontal; for some of the experiments it wac: neces?ary- to ust' a l o n g c ~trough than thil one shon-n in Figure 1. I n making a test, the apparatus \vas cleanrd thoroughly sncl hrought to a temperature of 70' * 2' F. One hundred fifty gr:iiii.s of rock and the desired quantity of acid (180 to 235 grains) IVC~L' discharged simultaneously into the niixcr Jrhile tht, agitator was turning. Five seconds after the beginning of thy introduction of the acid and rock the mixer \vas dischargctl intcl this flow trough. The time at'n-hich the niixtui n.hic.11 \viis usually quite diginct, was noted, and thc total length of flow on the trough !vas measured. For cach coiiditioii fivc tests irere made and the average used. The 'rcproducihilit!. \\-as usually n-ithin 2qc for setting times greater than 10 st.conds: the reproducibility of the flow distance. measurenimta \vas noi this nood. TlTe cliemical composition and particli. size. of the ph~.pIitit ,, rorks that were used in t,he laboratofy experiments are shown in Table I. Phosphoric acid from the TV.1 Fertilizer Korks was used; this acid containrd 7 7 . 8 7 HaP04,0.02% CaO, and 0.015 F,, __-...-._c w c p t in the experimrnts on effrct oi ticid concentration. The results of experiments on the effect o f acidulation of Florida and Tennessee rocks of various grades on fluid time are shown in Figure 2. It was found that acidulation has a relatively minor effect on fluid time, whereas changes in the grade of Tennessee rock of about 4uc P20j increased the setting time about tourfold. The fluid time of Florida rock of slightly higher P?Os content than any of the Tennessee rocks tested was considerably less than the higheqt grade
-
Con,gositiorl, ?-------------. Dry basis
Type u f !doisRock" tiire', PzOj C'aO siOz' FezOd 11?Os F 5 0 :3 20 12.8 -1.0 Tennessee 0.0 40 3 29.3 3 . 20 2 . 8 3 9 Tennessee 4 1 . 0 1 5 . 4 0 0 30.1 3 36 2.3 3 0 Tennessee 0.9 13.6 9.9 31.7 3 . 8 4 3 6 Florida 0 . 8 8 2 0.0 47.8 33.2 1 8 3.63 6 5 0.6 Florida 1.1 48.4 34.0 3 3 3 . 4 4 2 . 6 Tennessee -14.1 8 0 33.0 0.: 2 3 2 8 3 39 Tennessee 32.2 Y 3 13.7 0. I 3.43 1.6 2 9 Tennessee 5,s 15 5 33.4 0.7 1 8 3.55 5,1 3.4 26 3 Tennessee 0.7 33.6 2 5 2 8 5,3 Tennessee 2.1 46.5 33.2 3:;O 2.1 3.1 7.4 Tennessee 0.6 45,Q 32.i 3.29 .4 2 2.5 Tennessee 9.0 43 3 1.8 31.6 8 3 3.41 2.5 6.2 Tennessee 0.5 45.4 33.1 a Listed iri, urder of increaeing fluid time of resultant superphosphates. Wet basis. C Determined a s perchloric acid-insoluble material.
Per Cent
I'20s/Ct.O
Through _ _ 200 325
mole ratio
mesh
mesh
90 86 83 96 83 88 90 89 88 85 87 93 87
72 68 74
0 0 0 0 0 0 0 0 0 0 0 0
287 290 287 274 2i8 294 291 290 287 282 282 288 0 28R
56 50 64
71 66 64 71 68 73
55
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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
1268 8C
6C
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0 In W'
2 4c -I
D 3 _1
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2c
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80 I20 A C I D TEMPERATURE, "E
Figure 4.
160
21
Effect of Acid Temperature on Fluid Time of Superphosphate
I t was found that the length of flow of the mixture in tlic trough ranged from 6 inches (in the experiments with tlie most finely ground rock used) t o 180 inches (in the experiments with the moat dilute acid used). Plots of length of flow against tlic scver:il w r i ables gave curves of almost exactly the same type as those in n-liicli fluid time was plotted, with the exception of the plot of lcxnglh of f l o ~against' acid concentration. I n this instance no niiriiniuni point was observed, but the length of flow continued to decreast. v i t h acid concentrations greater than 75 and 79% H 3 P 0 4 becausc, , of the viscous nature of the mixtures made Kith the stronger acids h feiy experiments were made Jvith C.P. phosphoric a , d t o d t ~ termine whether the small amounts of impurit,ies in thc txli,ctricfurnace fertilizer grade of acid influenced the setting characteristics. Pract,ically i d h i c a l results were obtained with both acids, and i t was concluded t h a t the impurities in the fwtilizi>r-grade acid do not influence setting characteristits.
Vol. 39, No, IO
Ln the actuating mechanism oi thc eontru+r, arid thus impair its vnsitivity. A recording thermocouple is installed for measurement of the acid temperature. K h e n using acid fairly free from suspended solids, the rotainetei performance is good. The indicated quantity of acid fed b y th? rotameter is usually within 2y0 of the amount calculated from volumetric measurements of the acid storage tanks. T h e instan::meous fluctuations of the recorder pen are within 1% of the sc' 1.alue. ROCK-I'EEDISG S Y s n x . Ruck phospliatr grourid t o 7.5 to 80"; minus 200 mesh is conveyed from storage silos to a iced tiin Iw Iaated above a llerrick FeedoIveight constant-wight feeder (ea. pacity, 6 to i S tons per hour) nhich regulates the flon-of pulverized ioclc to tlic inisor. Rock from the bin is delivered t o the fced1~7 rhrough a 9-inch variable-speed screw conveyer d i i g n e d to rut, iull at all times. The speed of this conve?-cr is rcguhted by t h t'cwlcr nieclinrii>m. The rock discharges from the feeder iiito a ti-iiicli screiv conveyer inclined at, 30" with the horizontal (Figure I ~ running I st. sufficient spi:ctl to deliver the rock from the f c d e r :o a vertical spout ovcr the center of tlic niising bowl. Tlic. in~.liricd~ C I ' C K feetlcr li:~? been -found liencfirial in smoothing O U I iiiiiior pulsations inherent in the feeder operation. The amouii! , !'ed by thta ftw1t:r i i usiirtlly within 2?, of the indicated w I u ( ~ and rviLli unusually g.oo11 wlil)ra,tion it, is posGkile to reduce tlic w r o r 111 witliin lCo.
9r
I
TENNESSEE ROCK 0 33.4% b o 5 A 3 2 . 7 % P205
DESCRIPTION OF PLANT-SCALE U N I I
The funnel-type continuous miser x a s further developed OII a plant scale t o overcome the operating difficulties previously mentioned. T h e laboratory data on fluid time nere of considerable aid in arriving at the final operating conditions. h description and discussion of the operation of the continuous mixer p i t as finally developed follow. A diagrammatic flow sheet of the process is shown in Figure 6, and a general view of t,he equipment i? shown in Figure 7. ACID-FEEDISGSI'sTEif. Phosphoric acid from )-urd storage tanks is pumped to adjustment tanks where it is diluted from approximately 82y0 H3PO4t o the desired concentration (usually iri the range 74 t.o 78% H8POI). Good agitation, which is acconiplished b y liberal use of air, has been found necessary for accuratr, adjustment of acid strengt,h. From these adjustment tanks the acid is fed by gravity through a Fischer-Port,er rotamatic acid flow controller-recorder (capacity, 0 to 52,000 pounds per hout with a liquid specific gravity of 1.58) to acid-distributing nozzle+ in the mixer. All parts of this unit in contact with thc acid art* made of American Iron and Steel Institute Type 316 stainless steel. A removable strainer made of stainless steel installed in tht, acid line ahead of the rotamatic controller prevents large partirles, sometimes present in the acid, from reaching the controller. The strainer is 4 inches in diameter and 30 inches long and has slot openings 8 / 1 6 X a/a inch in size. As a further means of ensuring precise control of the acid flow, a n alumina gel drier is installed in the.compressed air line connected t o the controller; this operates a valve in the acid line, which in turn is actuated by the metering tube. It was found that, without the alumina gel drier, the moistiirc present in the compressed air would condense and acrnmula.tr
A
I O70
0.960
I
I 75
80
I
I
85
90
ACID CONCENTRATION, Yo H 3 P 0 4
Figure 5 .
Effect of Acid Coilcentration on.
Fluid Time of Superphosphate 3
i major problem encountered i n controlling the flow of finel? ground rock phosphate is that, under certain conditions, this material assumes fluidlike properties, and floods or flushes through the controlling equipment. To minimize this difficulty, careful attention must be given to the feed bin design t'o prevent arching 'Jf the material in thc bin which is followd by cave-ins that st,art the flooding. Several means for accomplishing this have bcer, published previously (9). The bin (capacity, 3l/: tons) uscd i r , the development of the continuous miser unit consisted of t,\vci adjacent vertical \yalls and two walls a t a 60' angle with thrhorizontal. With this bin it was 0bserve.d that,, during operatiof? the rock receded rapidly a t the intersection of the vertical walk and hung on the sloping walls. I n test operation with rock levels inaintairied a t 31/2, 6, or 8 feet from the top of the bin, flooding occurred almost every time fresh.rock was fed into the bin. During these t,ests feeding of rock to the bin w&sstarted when the rock had receded approximately 6 inches from the specified level, and fceding mas stopped when the level rsas approximately 6 inche!: tLbovc the specified level. The same experience was enrountered when tho rock vias allon-ed to recede from a full bin to tht: specified levels given. However, by operating with essentially a full hin at all times (r411ing of thc hin was started iyhen the livc.1 had
October 194?
INDUSTRIAL AND ENGINEERING CHEMISTRY
1269
tion from thc laboratory experiments on the SCI. ting characteristics of concentrated superphos._ phate, it was possible to calcuhte the approximat? I I __ diameter of the discharge extension to givc a ROTAMATIC slight holdup of fluid superphosphate in the bowl \ ROCC ACID-RATE I BIN CONTROLLER1 and not cause the material to set up in thc mixer. Under normal operating conditions at 3(1 AUTO MATIC tons of superphosphate per hour the retentior, time in the mixer is estimated to be about 2 seconds, with a 21/*-inch-diameter extension Several extensions with diameters from 1 3 / 4 to 3 inches were made for at,tachment to the m i w howl to allow a study of the effect of varioup rates of production on the thoroughnebs ut' m i x . ing. Extensions 6 and 9 inches in length were MIXER SQUIRREL-CAGE tried, but no improvement in mixing was obtained DISINTEGRAT3R Kith the longer extensions, and the shortrr (6-inch) extensions were easier to rod out Then CONCENTRATED r \ m - SUPERPHOSPHATE this became necessary. The est,ensions arc 0 CONVEYOR BELT TO CURING PILE fastened t,o the mixing bowl. by means of a 1 flange held on with wedges extending througl, i ; a slotted bolt. This arrangement enablcs Figure 6. Flow Sheet for Continuous Production of Concentrated quick and easy removal of the extension iri Superphosphate case the mixer becomes plugged with superphosphate. dropped 1 foot from the top a t the intersection of the vertical .A ventilation hood is provided Over the mixer to remove a walls), flooding is virtually eliminated. Control of the rock lev(,: Yligh! amount of dust and fumes arising from the miser. The in the feed bin is maintained automatically by means of a Icvg.! ldlust losses, however, are negligible. The major portion of the indicator operating on a capacitance principle, which actuates till, tiilorine-bearing fumes are liberated along the length of the con-. screiv conveyer feeding rock to the bin. belt twiicath the mixer. The belt is enclosed in a structurr M I X E R BOTL. The mixer consists of a n inverted truneatcc provided with doors for cleaning beneath the belt and with t x cone to xhich is a t h c h e d a cylindrical extension. Acid is introhaust lines for removal of the obnoxious fumcs. duced through four I-inch pipes spaced equidistant around til( Operation of the mixer is begun by starting the rock and then perimeter of the mixer. These' pipes rest flat against' the mixt,r t h c acid to the mixer. It is preferable to start the flow of rock wall and are adjustable, b u t normally they extend about 6 inchei just before that of the acid to prevent acid from spilling from the below the rim of the mixer. To each pipe is nttachcd a 45' ell irr conveyer belt. which is fitted a standard 3/4-inch pipe nipple which serves a b N With precise control of t,hc acid and rook rates to the mixer nozzle. Fishtail-type nozzles were tried, but, since no improveh i d , operation of this unit is trouble-free. 'rhc cntire mixing ment in mixing was obtained, their use ivas discontinucd T h iirlit is operated hv t,wo mrn. One mnn WY t i n t tlic yixcr a n d nozzles are turned flat against the mixer wall and all point in the same direction. They impart to the acid n sxirling motion. The head a t the nozzles is 30 feet of acid. Rock is introduced into the cent,er of t,he r n i s t s r bo&-1through a vertical spout extending to a levcti just below the acid nozzles. Details of the niixi:r are shown in Figures 8 and 9. The rock falls int(1 the vortex created by the tangential introduri i o r l of the acid to the mixer. The fluid level is niaiiitained slightly above the miser discharge extension, which is completely full a t all times. Tht, wid and rock are thoroughly mixed as they tiiicharge from the mixer, but additional mixirip i+ obtained by turbulence on the belt conveyer rpceiving the effluent. Early attempts t o fcwl the rock through one side of the mixer gave poor mixing and high dust loss, and resulted in frequent plugging of the mixer, because the rock penetrated the acid film and started a build-up of solid snpclrphosphate on the mixer wall. Rock distributor? vere also tried with the vertical spout, in a n effort to distribute t,he rock evenly around the periphery of the mixer. This also resulted in rock prnclration of the acid film causing a build-up of solid superphosphate in the mixer. Introduction of the rock into the vortex of acid xvas found to give the best opeiation and resulted in thorough mixing when a sufficient holdup of fluid superphosphate in Figure 7 . Plant Unit for Production of Concentrated Superphosphate tdhe mixer bon-1 was maint,ained. With informa-. hy Continimiic \ l i \ i n p ProrePHOSPHORIC ACID
ROCK PHOSPHATE
, L
~
-3;
-I
INDu
1270
Figure 8.
sT R I A L
A N D E N,G I N
(:oncentrated Superphosphate Continuoii*
B E R I N G cH E M I sTR Y
Vol. 39, No. 10
mixer unit was 36 inches \vide.and 37 feet betn-ceri the center lines of the head and tail pulleys. The belt traveled at a speed of 25 ferlt per minute, which provided about 11/4-minute reteritiori time for the superphosphate on the belt. K i t h this belt a production capacity of 30 to 35 tons per hour of superphosphate can bc maintaincd. The retention time provided on the belt is an important factor. Although the superphosphate sets to a plastic state in less than a minute, i t is rather sticky a t this stage; therefore, it is necessary t o retain the material on the belt for a longer period t o allow it to harden so t h a t part of it will riot adhere t(J the belt,: adherence vould foul the return idlers and necessitate frequent cleanups. The retention t h e required on the belt varies xvith the strength and temperat,ure of the acid used. Table I1 gives the recoinmeiided minimum retcntion times for various teniperatures and strengths of acid based 011 extrapolation of the plant unit optrating results. A somewhat longer belt than that used in the present work nould have been dcsirable, since the belt Irtention time was the limiting factor in the capacit,y of the unit. ii .quirrel cage-type rotary disintegrator iDI~INTEGRATOR. niounted above the conveyer belt a t a point immediately ovcr thix head pulley to shred the superphosphate as it is dirchxrgctl froin the conveyer bclt. The disintegrator is 24 inches in tliameter and extends ovcr thv full width of the coiiveyc'r belt. The t ivelve disintegrator linives are niitde of stainle (.led edges and arc parallel t o the axis of the unit. The disintcgrator is driven a t 90 r.p.in. by a 5-horsepower motor. Cltwancc twt,\v-c:i,nthe edge of the knives and the surface of the, conveyer
\Iixer its auxiliaries are performing, and the other man attt.rids to yard duties, such as pumping acid from storage to theadjust,inenttanks. diluting the acid, and maintaining a supply of pulveriztd rock i i i storage silos. With convenient arrangemcnt of the equipiiieii t I W O men could take care of t x o or three units without difficulty BELTCOSVEYER. T h e discharge from the Iniscr is received by a rubber belt conveyer running horizontally under the mi. T h e center line of the mixer is approsimatelg 6 feet from thc ccnter line 0.f the tail pulley of the conveyer belt. T h e corivcyci runs on troughing idlers, and skirt boards about 4 i'c3ctt long (math. trom pieces of discarded conveyer belt) are plac,ed o n carh side o l the conveyer under t,he mixer. A straight,idler is installedappruximately 0 t o 10 feet from the center line of the niiser to flex t,lw belt and cause t,he superphosphate, which will be set up to a solid state by the time it reaches this point, t o break apart. In atldition, a series of knives to cut the superphosphate into narrow strips about 11/2 inches d e has been found benefirinl (Figure 71. T h e esact locations of the idler and knives are determincd by tests and, for a given belt, speed, are governed by the strength and lemperature of the acid used. T h e time observed for thc. aupc!t'phosph3te to set in t,he trsts was about 20 seconds. This conipares with 18 seconds as detc.rmincxd in the laboratory unt1c.r about the same conditions. The conveyer belt, usrtl in the development of the c.ontiiiuou>
Acid Concn.,
Temp.,
78 78 78
140
74 74
110
70 HaPOi
F.
110 40
LINE
Recomriieuded Retentiun Time RIin. 1.5 * 2.0 9.0 4.0 12.0
40 110 5.0 68b 80 6.0 68 b a T h e d a t a shown appl t o concentrated superphosphate produced fruiir Tennessee brown rock pxosphate. 6 slightly shorter retention time in each case would be required for Florida phosphate. b Superphosphate made n i t h 68,% acid cannot be cured satisfactorily by storage b u t would have t o be dried.
Figure 9.
Sketch of Continuous Mixer
October 1947
INDUSTRIAL AND ENGINEERING CHEMISTRY
belt, is adjustable, but a distanw of about 1 to l ' , ' ~ inches \\a,. found to give the best results. The disintegrator is housed in a box above the conveyer belt \vith built-in deflectors to send t,he shredded product in the proper direction. T h e disintegrator, which operates independently of the rest of the mixer, has given practically trouble-free operation and requires very little atwnt,ion. Disintegration of the superphosphate results in the formatior, of considerable fines, depending on the mixing conditions, and practically complete elimination of lumps over 2 inches in size. This facilitates handling of the product, permits the building of larger piles in t,he shed in which t h r superphosphate is stored for curing (because of a greater anglc of repose of the disintegratcd material), and greatly reduces the difficulties due t o large, hard lumps when the cured superphosphate is ultimatelv ground f o v ahipmwt.
1271
D l A M E T f R OF DISCHARGE EXTENSION, INCHES
PERFORMANCE OF UNIT
During the development of the Iniscr unit the acid C O I I C ~ I I tration was varied from 72 to 8Or: I&PO+ and the acid temperat u w was varied from 100" to 160" F. by means of a drip-typv cooler. Good mixing was obtained in this range of acid concentrations and ternpbratures and \vas probably better a t the loiver concentrations, although this ivas not confirmed quantitatively. However, with acid concentrations bclo\v 74% the belt conveyer CROSS-SECTIONAL AREA O F did not provide sufficient retention time, and the superphosphate D I S C H A R G E EXTENSION, SQ. IN. stuck to i t excessively. K i t h 74 to 80% H3PO1 acid, tc,mperaFigure 10. Effect of Discharge Extension Sizr tures as low as 110" F. could be used without excess~veaccumulaon Production Capacity of Continuous RIixer tion of superphosphate on the belt. -1longer belt would h a w permitted a greater range of operating conditions. Sufficient data ivere not obtained to check the effect of lower acid teniperaitics, analyses are made for available P&j, free acid, and inoistun tures on Pros conversion t h a t had previously been determined in the laboratory experiments ( 1 ) ; hon-ever, i t was observed that, H' a check on the operation of the automat,ic feeders and mixer. with the cooler acid, the liberation of fluorine-bearing gases waq greatly reduced. PROPERTIES OF COSTINUOUS-.MIXER SUPERPHOSPHATE The maximum production capacity of the mixing b o d wa> found to be directly proport,ional to the cross-sectional area of the The fresh superphosphate as it is discharged from the mixer belt bowl extension. This relation is shown in Figure 10. For a tias a characterist'ichoneycomb st'ructure (Figure 11). The matm()given diameter of extension, the nearer the production rate is to rial is relatively dry and is more easily disintegrated and conits maximum capacity the more thorough is the mixing. It is desirable to operate the mixer not, more than 15% beloll- its veyed to storage than is the batch-mixer product previously niadc. maximum capacity. Figure 10 indicates that the mixing hon-1 The cured product, after disintegration and screening through a used in this 11-ork would handle 50 tons of superphosphate per I-mesh screen, has a bulk density of 56 pounds per cubic foot'. hour with a 3-inch-diameter extension, but,, because of limited which is about the same as for batch-miser superphosphat,c. retention eime provided by the belt conveyer, a 21/2-inch-dianieter extension and a production rate of about 35 tons per hour ~ v e r ~ The chemical composition of cured superphosphates made from the highest used in the present n-ork. Good operation of the 'rcnnessee brotvn rock phosphate containing about, 32% P20jin miser was not obtained at, rates of 17 tons per hour and lower t hc continuous-mixer plant and in the batch-mixer plant arv coni(usifig a 13/4-inchextension), because the flow of rock and acid n-as not sufficient t o maintain the vortex action npedrd for good mixp n i w l i n thv following t,ahulatiori: ing, and the miser became plugged. CHEMIC4L COYTROL
The optimum proportion of rock and acid fed to the mixer is calculated, based on their coniposition and cost and the anticipated conversion for the mixing conditions used, so as to give the, lowest cost per unit of available P2Oj in the cured produet. Each silo of pulverized rock is analyzed. The acid concentration is adjusted according t o specific gravity measurements made with a hydrometer. dccuracy of the acid adjustment is checked by chemical analysis of daily composite samples of acid fed to thv mixer. The acid-rock proportion is obtained by the operatori from tables giving the desired settings for the automatic feeder& for the mixing rate t o be used. As a check on the accuracy of proportioning the rock and acid, analyses are made on shift coniposite samples of the fresh superphosphate obtained by a n automatic sampler, which takes increments a t 5-minute intervals a t the discharge of a n elevator in the handling system. Since the reaction between the rock and acid does not go to completion in the mixing unit, determinations of only total P20sand CaO contents are made, since other determinations are of little use in controlling the performance of the miser. The P 2 0 j / C a 0 ratio of the superphosphate is used to indicate the adequacy of rock and acid proportioning, but, if visual examins tion indicates abnormal-
Bat,ch 78 0.94 48.1 45.9 49 1 47.9 Continuous 78 0.98 48.5 Continunu3 75 0.98 47.5 Acid temperature, 130° t o 150° F. i, l l o l e ratio i n product. C 1IacIntire:Shaw-Hardin method ( 7 ) . d Loss in weight over concentrated HzSO4. li Percentage of rock P205 converted t o available
L1 :
1.;;
2.1)
3.0 5.4
.,.I
86 91
93
foriii.
The batch-miser product represents the high& practical acidulation and the lowest practical acid concentration t h a t could bc, used consisteptly in t.he batch process because of limit,ations irl mixing cycle and handling equipment. On the other hand, with the continuous mixer it is practical to operate a t the higher acidulation and lower acid concentration s h o m . The conversion of t,he continuous-mixer product was several per cent higher than that of the batch-mixer product; this improvement in conversion was due to more thorough mixing as well as to higher acidulation and lower acid concentration. With 7570 acid and an acidulat,ion of 0.98, the cured continuous-miser superphosphate contained significantly more free acid and moisture, which increased handling, disintegrating, and screening difficulties somewhat. It is believed that these conditions represent the present practical limit,of acid concentration and
INDUSTRIAL AND ENGINEERING CHEMISTRY
1272
Vol. 39, No. 10
uieiit aiitl procedure described earlitbr i r i this paper were used. h set of mixing conditions typical of those used in normal superphosphate plants \vas chosen as the standard conditions; these were Florida rock phosphate (34.0% Pz06, 49.1r; CaO, 0.77, II,Oi pulverized to joy0minus 200 mesh, 56' 136. sulfuric acid (71 2 5 I1,SO1) a t 130' F., and an ncitl/rock weight ratio of 1.22 \\.ith these conditions a fluid, easily discharged mistuw was obtained. The fluid time was 113 seconds and the distance of flow OD the trough was 8 feet. However, the mixt,ure did not set Figure 11. Frech %perphosphate \lade in < : O ~ I I ~ I I ~ I OTIixer US up to a d q , crunibly state a8 when phosphoric acid wae widulation €or curing by storage. T h e conversions ui riiv CUI!. used, but iemained a5 a thick slurry for a t least 20 minutes (at t>inuous-niixerproduct correspond closely to the economic opt iI\ hich time it was removed from the trough) and was still plastir inum (about 92C0 a t present); any further improvement, thereliter 3 d a j 5 of storage a t room temperature in an open container fore, should be in the direction of decreasing the acid requircwcn! The standard conditions were then altered in attempts to find rather than increasing the conversion. This might be done by (aonditioristhat would give a practical fluid time for good mixing use of still lower acid concentration (about i'oyOHJ'O,) and lowcar and at the same time result in a ary product i n a feil- minutes after wid temperatures, and a different method of curing might also hi, mixing, so that it would be possible to convey the material diiicressary. rrctly to storage without the necessity of curing in a den. Acidulation was varied from 1.0 t o 1.6, acid temperature \vas varied EVALUATION irom 60" to 280' F., rock particle size was decreased to 99% ininus 200 mesh, and acid concentration ryas increased to 80% The continuous-mixer unit has displaced the bat,ch-t ype mixer;. IILSO4. S o n e of these alterations in conditions, either singly n r in the TVA Fertilizer Works. The advantages of the coiltin111 combination, resulted in the desired setting characteristics. uous-mixer process over the batch-mixer process are (UI IC. It is concludc~dthat the continuous miver could be used successoperating labor, power, and maintenance; (b) better mixing and iiullv for normal superphosphate manufacture, since the fluid timr therefore better P205conversion; (c) lower in m t m e r i t ; ((1) (.bdut ing the mixing under the standard conditions is even longer sentially automatic control; and ( e ) considerably more flcsibilit~ than for eonccntrated superphosphate. Proper regard would In operating conditions-such as acid concentration, acid ttmhive to be given to materials of construction of the mixer for use perature, and acid-rock proportion-which permits operat ion a: However, it does not apt J f iulfuric in place of phosphoric acid. miditions found to be optimum for PzOj conversion. Ttica fol[ J ~ W that the freshly mixed normal superphosphate could be conlowing tabulation gives some of the important operating iiidiu,s v c y d directly to storage without difficulty, becauscl 01 its plastir i'or the t v o processes, based on operating expericnw. I T l i c w 1 mdltlon (lata are for proportioning and mixing only and do r i o t incluri~ miveying of raw materials or product.! .tCKKOWLEDGMENT Ratch
:,,TI tilr l i o i i r
Procew
Number of mixer units
I'roces,5 1
Maintenance labor, man-hr./ton superph&phate 0.1: 0.02 Maintenance materials, cents/ton superphosphate Q., 0.9 I n addition, t h e percentage of scheduled time operated I S niuch greater for the continuous process t h a n for the batch process bec:tiise lcss niaint e n m c e a n d clean-up are required. b Includes foreman.
C'onsiderably more improvement in thcse indices can be expected with the continuous process than with the batch process, sincc these figures are based on lcss than the maximum capacity of the 1-ontinuous-mixer unit (50 tons per hour) and since the economic size unit for the continuous process is much larger than for the batch process. A superphosphate plant t)f sufficient size to require several continuous units could thertsfore achieve considerably lower requirements than are listed. APPLICATION T O hORIIlAL SUPERPHOSPHATE MANUFACTURE
Experiments were made in the laboratory on the setting characteristics of normal superphosphate to determine the possible application of the continuous inixer to its manufacture. The equip-
'The early work on the continuous mixing process was carried out under the direction of R. L. Copson and R. II. Xewton. The first plant-scale miser unit of the type finally adopted, which was improvised from one of the moving-blade units, was devised and toperated by S.A. Harvey, who also cooperated in the final development work. The laboratory experiments on setting characteristics were csrricd out with the assistance df W. W. Cerf. H. C. Claihornr assisted in the plant-scale development work. LITERATURE CITED
and Cerf, W.W.,IND. ENG.CHEM., 37, 829-41 ( 1 9 4 5 ) ~ ( 2 ) Cogson, R.L , ('hem. & M e t . Eng., 52, No. 5, 218-19 (1915). 13) Copson, R. L , Y e n t o n , R. H., and Lindsay, J. D., TND. E s n CHEM.,28, 923-7 (1936). 14) I b i d . , 29, 175-9 (1937). 1\5) Curtis, FI. A , , Chen. & M e t . Eng., 42, 488-91 (1935). (6) C u r t i s , H. A , . U. S.Patent 2,070,582 (Feb. 16, 1937). 17) AIacIntire, W.II., Shsw, W. M . , and Hardin, L. J., IND.EN(+ CHEX., AN.\L. ED.,10, 143-52 (1938). I$) Xewton, R. H., E. S. Patent 2,115,742 (May 3, 1938). !,9) Sandstrom, C. O., C h e m & Xelet. Eng., 47, 22-3 (1940). l j Bridger, G . L , B u r t , 11. B.,
PREEESTLD before t h e Division of Fertilizer Cheniistryat t h e 112th Meeting the \ M E R I C A N C H E V I C I L S O C I E T Y , S e w York, N. Y.
Of
