CA1186867A - Phosphoanhydrite process - Google Patents

Abstract

ABSTRACT
A wet process for producing phosphoric acid and phospho-anhydrite by acidulation of phosphate rock with a phosphoric acid/sulfuric acid mixture containing a very high recycle content of small sized anhydrite seed crystals. A strong phosphoric acid is obtained having a concentration of at least about 35% P2O5. Further, the phosphoanhydrite may be readily converted to an industrially usable gypsum product.

Description

I cl ~ n(l (1 r t ~ vc~ i. f) r I;ield o~ th~ :tnve Ilt, ion _~
'['he present :invcnt:ioil rclates to the production o~
phospllor:ic acid and more partic~llarJy rercers -to an improved wet process for convertin~ phosphate roc~ into a concentrated phosphoric acid with eoncornitant produetion of all improved ealcium sulfate.
In conventlonal wet methods of producinc- phosphorie acicl, finely divided phosphate rock is diqested with a mixture of phosphoric aeid, sulfuric acid and wa-ter causing a reae-tion between the calcium phospha-te in the rock and the sulEuric acid to produc~ phosphoric acid and a precipitate of calcium sulfate (either the anhydr~te form - CaSO4 with no water of hydration;
the hemihydrate form - CaSO~. 1/2 ll2O; or the dih~drate form -CaSO4.2H2O). Over the years numerous improvemen-ts and refine-ments in these methods have been proposed, many o~ which are described in Phosphor:ic Acid edited by A. V. Slacl~ ~published by Mareel Dekker, Inc., New York, 1968). Many oE these refine-ments are diree-ted to produeing the ealeium sulfate eo-produet in various forms.
Mos-t wet proeesses are direetecl to produeiny the cal-CiU~I sulfate product in the dihydrate form, or phospho~yps-~m~
In most commereial operations, the plants are run at abou-t 75-80 C and 30% P2O5 in the leach zone. Ilere the reaction of sulfuric aeid and apatite (phosphate rock) is over in about 15 minutes. ~lo~.~ever, a residence time of about a ;~ours is needed for g~psum crystal ~row-th to sufficiently larqe erystals for filtration. Generally, -the practical ma~imum P2O5 eoneen--trat:ion of the pllosphoric acid cor a dihyclrate process is around 2~-30~ (40-~1~ H3~O~I), whieh must subsequently be eon-eentrated to hi~h levels, wllile the operational temperaLure is about 75-80C. ~bove these te~ eratures alld P~O5 coneen-tration limits, the phosphoqypsum beeomes unstable durin~

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)r~ ic)r~ o ~ t~ f~d all~o~lr)t-; Or tll(~ rn~ s'c~h:l~ h~rni-hy{lrate are Eormed, w:i~tll attendant harde~ ancl settirl~l up in mixincl Ve5Se LS and ~rea-kly increclsed react:ion t:imes. Pur-ther concentration of the phosphoric ac:id -to more concentratec1 pro-ducts requires enercJy :intensive and expensive evaporators. A
phosphoric aeid of about ~2~ P2O5 (60% ll3PO~) is needed beeause fertilizer manufacturers use this concentration to make di-ammonium phospha-te.
~ less commercially at-tractive variant on the wet proeess is the hemihydrate process. The hemihydrate proeesses may be useful in obtaining a calciumsulfa-tefor buildin~ ma~-erials manufaeture. However, the operator must be careful that the filters do not cool sinee the hemihydrate ean set to qypsum~
Then jaekhammers are needed to remove i-t. Furthe , -the hemi-hydrate produc-t is objectionable because impurities such as radium in the ore and excess phosphorie acid are carried over in-to -the hemihydrate. Here again, the phosphoric aeid product, about 32% ~25~ must be eoneen-trated for fertilizer produetion ~eeording to data published in -the Slack text volume 1 par-t 1, in an ar-ticle on hemihydrate and anhydrite proeesses in Europe by P. M. R. Verstee~h and J. T. Boontje, phospho-anhydrite can possibly be produced in a phosphorie aeid attaek system provided the tempera-ture and eoneentration of the-aeids are sufEiciently hi~h, about 135C and about 30'6 H3PO4. For example, it is there proposed -that anhydrite eould be made at 95-100 C in a mixture of 42~ P2O5 phosphorie aeid and 3-3.5 sulfuric aeid; or at 85-90C in the presenee oE 48~52~ P2O5 pllosphoric aeicl. In -the Eirst case, a 75-32~; sul uric aeid would be required; and in the second case, a 78% sulfuric aeid ~,loulcl be required for the phosphate I-oek in the leaeh zone. The reerence appears devoid oE any men-tion of usin~
anhydri-te seed erystal in the proposecl processes.

~r~3 r:lt/ -- 2 -i'7 ~JeC;cripti.( ~>f. the 1':r:i.o1: Art . . ~
l~... p1tcr1l ~ 53:L 977 d:i;clo-~;esa di.hyc1r1t(? wc~t iJro-cess for pro(lucin~ phosphor:i.c acid i`1ith the su{1-1est:ior) that the phosphocJypsum obtained be thereafter treated with 1.5-33%
;u:1.fuxic aci.d at a tem1?erature of from 60C to the boili.n~J
point of the sulfuric acid solution Eor periods of more than 30 minu-tes (~enerally 2-3 hours) to obtain a calcium sulfate anhydrite. Nothing is known about the purity or practical usabili-ty of this anhydrite.
Summary o-f the Invention There is a need at the present time to provide im-proved processes :Eor reducinc.1 the ener~y :requiremen-ts and equipment capitalization expenses in phosphoric acid production;
further -to provide eeonomical means of obtai.nincJ hi~her P2O5 content phosphoric aeid produc-t; and further to provi.de pro-eesses whieh eonver-t the previous waste ealcium sulfate eo-product in-to material. acceptabl.e Eor commereial utilization in ~ypsum board and other indus-trial and construction materials.
One objec-t oE -the presen-t invention is to provide a process Eor economically and efficiently producing increased yields of stron~ phosphoric aeid. Strong phosphoric acid as defined herein is phosphorie acid of at least about 35%
P2O5 (48~ H3PO~), and preferably about 42~ P2O5 t60% 1~3PO4) content. ~nother objec-t oE this invention is the production of a phosphoanhydrite which may be co!lverted to qyl~sum products useful in the construetion and buildinc~ materials industry.
It has been diseovered that tne-^e is a very narro~
ra1l~e o:Etemperature and phosphor.ic acid a1lcl sulfllrie acid concentration conditio1ls ~herein a very stable insoluble an-hydri-ke is produeed; and Eurther, -that in -the presence oE

lar~e amounts oE recycle anhydrite seecl crystals thC? rat~.? of anhydrite cr~sta:Llization i very rapid. In addition it. has al c;o been fo~11ld thlL i.n a separ.lte reactor ullclel- certaill rl.t/ /~ ~ 3 t~ C1~ :jrI~ L(I~:! I>:r-OP~):rt:;Or1 C).C
t~ YC11^ate 5(?(-~ cr~stal;~, -the ptlc)spl~lo~rihydrite rn.ly he rapidly cx)nvert~d-to a flypsum product suitab:Le for use in c:orlstructior and industrial materials.
Theprese~tinvention comprises diclestin~ phosphate rock with a mix-ture oE phosphoric acid, sulfuric ac;cl, water and recycle anhydri-te seed crystals. Finely divided phosphate rock, sulfuric acid, phosphoric acid, small sized anhydrite seed crystals and wa-ter are slurried in a Eirs-t mixinc~ zone between about 60-110C, preferably about 75-95C. The quan-tity of sulfuric and phosphoric acids charcre to the mixiny zone is such as -to provide in the slurry about 62-73% total phosphoric acid and sulfuric acid content, with about 1~4 weight % being sulfuric acid. The slurry will have about 20-50% total solids comprisinc~ a weicht proportion ran~Jinc~
frGm abou-t 10:1 to abou-t 100:1 of anhydrite seed crys-tals to finely divided phosphate rock. ThereaEter, the slurry is separated into a raffinate of abou-t 35-45~O P2O5 phosphorie aeid and a reeyele slurry for the attaek -tank. Optionally, a portion of the phosphoanhydrite is separated by series separation from the recycle slurry, and mi~ed wi-th additional sul~urie acid and a sulfate hydration accelerator plus a larqe proportion by wei~ht of dihydrate seed crystals to conver-t the phosphoanhydrite to a purified ~ypsum produc-t of low radio-activity.
Brie~ Description of the Drawinqs Yigure 1 is a plot of phosphoric acid concentrations, with approximately 1.5~ sulEuric acid present versus temper--ature showirlcJ the states of hydration of -the calcium sulfate and the area of interest in the present invention ~e~ion Il) in heavie~r lines. Fi~ure 2 is a labelled f:low sheet diacrram matieally illustratincr-the various vessels, all of whicll are eonventioncll, employed in the process, ancl ~ ures 3 & 4 arc sca~ in(J olect30n mic-rol~lloto~l-clplls oE valious calcium r~ j rlt/, , ~

'7 ulEate products producecl by the present :invention.
Detailed Descript:Lon oE the Preferred E'mbodiments It is theorized that all phosphoric acid processes oE the wet process type are carriecl out under conditions where-in an insoluble anhydrite is the thermodynamically stable form of calcium sulfate. The crystal s-ta-tes actually precipi tated, however, in mos-t co~nerial wet processes are the met-astable varieties, the hemihydrate and dihydrate. Conversion of these into the stable anhydrite modification under the conditions prevailing in the dihydrate and hemihydrate wet processes is extremely slow. This is because the activation ener~y required to cross the enerqy barrier for conversion is very high.
Referring to Figure 1, under practical process conditions in wet processes for producinq phosphoric acid, the rate of calcium sulfate crystal growth is proportional to the supersaturation level of a high concentration of reacting calcium and sulfate ions. The solids deposited per unit of time is proportional to the available crystal surface area (or for a given crystal seed,its specific surface), a low water content (i.e., a hi~h P2O5 phosphoric acid concentra-tion) and high temperature. In Region I, the dissolution of finely divided phosphate rock takes places through the dis-solving action of phosphoric acid, and -to some degree sulfuric acid. The calcium ion that is brought into solution in this re~ion combines with sulfate ion to precipita-te the unstable hemihydrate. This hemihydra-te in turn dissolves and re-crystallizes as the dihydrate, particularly if dihydrate seed crystals are available forsurface precipitation. This re~ion is where most commercial processes operate. However, above the practical 28-32~ P2O5 concentrations and 75-80C for the dihydrate process, the calcium sulfate dihydrate becomes un-stable and increasin~ amounts of the metastable hemihydrate are formed, especially if large amounts of hemihydrate seed , , rlt/\ ; - 5 -crystals are pre~ent. ~gain, conversion oE the hemihydrate to a stable insoluble anhydrite is slow. In reqion l:[ aloncJ
the border to region III of Figure 1, the conditions are sufEicient to precipitate anhydrite on an insoluble anhydrite seed crystal, and any hemihydrate will readily convert to anhydrite since the hemihydrate phase is unstable. The rate of precipitation is dependent upon the tempera-ture, solu-tion agitation, surface area of seed crystals, solids conten-t of the mixture, sulfuric acid concentra-tion and the time allowed for phospha-te rock acidulationand anhydrite recrystallization.
The process of the present invention should be operated in the leach zone at a temperature between about 60 C
and about 110C, pre-ferably about 75-95C. Substantially below about 75C, increased coolin~ capacity would be needed and sufficient hemihydrate would be formed to interfere with the process. Subst.antially abo~e 95C is undesirable for additional heat would be neededto maintain the reaction.
Sufficient solution agitation is accomplished in conventional mixing vessels for phosphate ore or rock digestion.
rrhe process may be carried out ineither multitank digesters or a single tank systemwith multiple zones, compartments or cells.
The preferred anhydrite seed crystals have an optimum mean particle size of about 1-10 micrometers, more pre~erably 1-4 micrometers. The seed crys-tals may be derived from any origin, but preferably are recycle product For continuous operation.
The anhydrite seed crystal recycle rate is preferably about 20:1 to 100:1, and more preferably abou-t 60:1-80:1, by weight, of anhydrite to phosphate rock ore. At substan-tially less than about 20:1 in region II of Figure 1, hemihydrate seed crystals preclpitate. Above about lOO:l recycle, the time for reaction becomes excessive. Maintaining this ratio rl-t/~ - 6 -is readily accomplished by controlllncl the me~ering of ore in-to the m:ixing zone. If the ore is metered too fast then some hemihydrate Eormation occurs, and there is insufficient balancing of the reaction conditions to ~ercome -the energy barrier shift Erom the metastable hemihydrate to the anhy-drite.
The "free", excess above stoichiometric, sulfuric acid concentration in the present process rancles Erom about 1 to 4%, with about 1.5-2% being optimum. If substantially less than 1~ free sulfuric acid is present, the acidulation rate of the ore is decreased and the phosphate ore partieles tend to beeome encapsulated with relatively insoluble dicaleium orthophosphate. Also, inereasing amounts of silicate gels are formed, which interfere with calcium sulfate erystal qrowth.
Above about 4~ free sulfurie acid, the acidulation rate of the ore is also deereased and the ore particles tend to beeome eneapsulated with ealeium sulfate eoatings.
In the digesting zone, as shown in E'igure 2, generally Erom 3-10 mixing eells are utilized in the practiee of the present proeess. Upon mixing the phosphate rock, recycle and any makeup feed materials, the mixtureconstitutes a slurry of about 20-50% solids. At subtantially less than 20% solids, more phosphate ion is co-precipitated with calcium sulfate thus causing phosphate loss in -the process; while above 50%
solids, the slurry is difficult to mix. The slurry has about 62-73% total phosphoric acid and sulfurie acid content in the proportions indicated hereinabove. Below about 43% P2O5 (58% H3PO4) and 4% sulfuric acid, free water becomes available for calcium sulfate dihydrate formation and -tends to move the operation too far toward Region I of Figure 1. Above a eon-eentration of about 50% P2O5 (71.5~ H3PO4) phosphorie aeid and 1.5~ sulfuric acid, viscosity of the slurry becomes high enough that increased amounts o phosphate ion are co-precipitated rlt/~

with the anhydrite, again causing phosphoric acid losses in the process. ~esidence time in this ~.one through the ~ifFerent cells is abou-t 1-4 hours, depending upon temperature, recycle ra-te oE anhydrite seed crystals, volume of the particular cells and the like.
After digestinq the rock, as shown in Figure 2, -the slurry passes to a filter -to separate the desirable phosphoric acid produc-t from a recycle slurry of anhydrite filter cake and residual phosphoric acid. A portion o:E the recycle slurry may be converted-to~_alcium sulfate suitable for use inindustrial products by filtering, washing, passing, the anhydrite to a hydrating section~ preferably 1-3 mixing cells, and therea~ter, a series separation by conventional means produces a coarsedihydrate product sui-table for use in gypsum materials, small sized anhydrite relics containing im-purities, and clarified -Eiltrate for recycle.
EXAMPLE
Phosphoanhydrite (CaSO4) and stron~ phosphoric acid were produced by the process of the invention by the acidula-tion of phosphate rock ore with a phosphoric acid - sulfuric acid mix containing a very high recycle content of anhydrite seed crystals.
The reaction mixture fed to the digesters comprised 44~ P2O5 phosphoric acid (60.7% H3PO4) and 1.5% H2SO4- At these acid concentrations, essentially all water is tied up through hydrogen bonding to the acids so that little or no water is available to form calcium sulfate hydrates. The acid mix-ture contained 5~ anhydrite solids as seed crystals.
The temperature of -the reaction mixture was 85C, which put the reaction conditions well into Region II in Figure where anhydrite is stable.
The ground phosphate rock and ~0~ sul~uric acid were me-tered in slowly enough so that acidulation could take "
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place readily, ancl a ratio oE at least 2() to 1 byweic3hto~
anhydrite seed crystals -to phosphate rock was present. Two hours residence time was allowed after the last phosphate addition for comple-tecrystallization of anilydrite. The mix-ture was fil-tered while hot and washed with hot water.
After drying, it analyzed 38.05% CaO, 52.24% S~3~ 0.04%

Fe2O3~ 0.09~ A12O3, 0.08% F, 0.58Qo Pt 0.52~ E12O. 0~027 Mgand 5~75 SiO2. X-ray difractionanalysisshowed only insoluble an-hydrite and alpha quartz. Scanning electron micropho-tographs of the phosphoanhydrite are illustrated in Figures 3 & 4. The process yielded a strong phosphoric acid having a 44% PO5 concentration.
The co-produeed phosphoanhydrite may be disposed of or alternatively, converted into useful qypsum produc-ts.
It may be suitable as is, for example as an ingredient in eonstruetion materials suehasa Keene's eement. Howe~er, if original phosphate roek eontained eonsiderable radioaetive ma-tter it may not be suitable. In that ease it is preferred to remove sueh eontamination by various means. The phospho-anhydrite obtained in thls proeess is formed into a slurry with about 1-20~ aceelerator or aeeelerator mixture (such ~s 1.5 weight~ sulfurie aeid and 1.5 weight ~ sodium sulfate) and substantial proportions of coarse purified gypsum reeycle seed crystals (such as about 40 mierometers sized gypsum in 40:1 to 100:1 weigh-t proportions of seed gypsum to anhydrite).
The slurry isallowed to hydrate about 30% to 95~ of the an-hydrite, Figure 3 (leaving about 70-5~ anhydrite relic un-rehydrated) depending upon the level of radioactivity in the anhydrite, yielding gypsum crystals, illustrated in Figure 4 of about 30-100 micrometers (pre~erably clreater than about 50 micrometers). These are readily separated by conventional equipment to a substantially radium-free clypsum which is sui-t-able for conventional processing in thçmanu-~actlre of gypsum wallboard, gypsum plasters and other gypsum products.

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Claims (11)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A process for producing phosphoric acid and calcium sulfate from phosphate rock which comprises:
continuously feeding finely divided phosphate rock, sulfuric acid, phosphoric acid, water and anhydrite seed to a mixing zone and mixing them to form a slurry;
the quantities charged to the mixing zone being such as to provide a slurry comprising about 20-50 weight %
total solids, about 62-73 weight % total phosphoric acid and sulfuric acid content with about 1-4 weight % sulfuric acid, and about 10:1-100:1 by weight of anhydrite seed to phosphate rock, and mixing at a temperature between about 60° and 110°C;
continuously withdrawing a leach slurry from said mixing zone and passing it to a first filtering separation zone to separate calcium sulfate slurry and phosphoric acid;
recovering a strong phosphoric acid from the first separation zone; and recovering a slurry of calcium sulfate anhydrite in phosphoric acid from the first separation zone and returning it to the mixing zone.

2. The process of claim 1 wherein the slurry withdrawn from the first separation zone is passed to a second filtering separation zone to separate coarse calcium sulfate anhydrite filter cake and a slurry of fine anhydrite seed in phosphoric acid filtrate which is returned to the mixing zone.

3. The process of claim 2 wherein the anhydrite filter cake is passed to a crystallization zone to hydrate in the presence of a large proportion of dihydrate seed crystals, a substantial portion but not all of the anhydrite to coarse phosphogypsum; and separating a portion of the phosphogypsum that is low in radioactivity as purified product gypsum.

4. The process of Claim 3 wherein the anhydrite filter cake is continuously passed to a crystallization zone;
mixing in the crystallization zone at a temperature be-tween ambient and 40°C, the anhydrite with seed gypsum and soluble sulfate hydration accelerator to form a slurry, the quantities being charged in the crystallizing zone being such as to provide a slurry comprising by weight about 1-20% accelerator and about 1:10 to 10:1 proportions of coarse gypsum to anhydrite, said coarse gypsum being all at least about 30 microns;
continuously passing the slurry to a third separation zone to separate fine anhydrite relic and coarse gypsum;
recovering fine anhydrite relic contaminated with radio-active matter and passing it to disposal;
recovering about 30-90% by weight proportion of the coarse gypsum as product purified gypsum; and recovering about 70-10% by weight proportion of the coarse gypsum and recycling it to the crystallizing zone.

5. The process of Claim 1 wherein the mixing zone is maintained at 75-95°C.

6. The process of Claim 1 wherein the anhydrite charged to the mixing zone is about 5-15 micrometers particle size.

7. The process of Claim 1 wherein the slurry in the mixing zone has about 1.5-2% excess over stoichiometric sul-furic acid.

8. The process of Claim 1 wherein the slurry in the mixing zone has about at least 20:1 weight proportions of anhydrite seed to phosphat rock.

9. The process of Claim 8 in which the weight propor-tions are about 60:1 to 80:1 of anhydrite to phosphate rock.

10. The process of Claim 4 in which the coarse gypsum product has a particle size between 40 and 100 micrometers.

11. The process of Claim 4 in which about 30-95% of the anhydrite is hydrated to gypsum particles of about 30-100 micrometers; coarse gypsum of greater than about 50 micrometers is recovered as purified gypsum product and the particle size of anhydrite crystal remaining not rehydrated is about 5-25 micrometers.