CRYSTALLIZATION OF GYPSUM IN WET PROCESS PHOSPHORIC ACID R I C H A R D L. G I L B E R T , J R . ChrmicoI Rcscmch and DtveIopment Laboratories, Agr~cUlNTolDinision, American Cyanamid Co., Princeton, N . J.
Impurities in wet process phosphoric acid, as well as PzOa and HzS04 concentrations, have major effects on crystal habit which influence the filterability of gypsum crystallized therefrom.
ET PROCESS PHOSPHORIC Acm is increasingly important as Wthe basis for most of the fertilizer PSOSused. Most of this acid is made by processes yielding gypsum as a by-product. Filterability, a function of crystal size and shape of the gypsum crystals, has major effects on process efficiency. Therefore, any process variables which affect crystal size and habit are important. For best filtration and washing, equant crystals of uniform size are most desirable. As the range of crystal sizes increases, the cake tends to become less permeable. Permeability of the cake is also affected by crystal shape: Platelike crystals give highly impervious cake, while prismatic crystals may produce a cake having excessive void space. Extremes of shape also tend to render the crystals more fragile, resulting in formation of fine fragments by attrition. One of the most important factors affecting crystal size is the number of crystals growing, which depends upon the number of nuclei formed in the crystallizer. Rate of nucleus formation is a function of supersaturation and, because the solubility of gypsum in phosphoric acid depends on PIOs and HaSol concentrations, the latter are among the important variables affecting crystal size. The habit of a crystalline material is defined as the common or characteristic form or combination of forms in which it crystallizes. The habit of any crystalline material depends upon the environment in which the crystals are formed and on the rate of crystal growth. The relative growth rates on various crystal faces are affected by impurities, which may block growth sites, as well as by supersaturation. Typical habits of gypsum are shown in Figures 1 and 2 (7). These habits are commonly seen in gypsum from wet process phosphoric acid, although crystals from phosphoric acid are often highly twinned and aggregated. A typical gypsum from a phosphoric acid plant (Figure 3) comprises rhomb-shaped crystals, twins, and clusters. The effect on gypsum habit of varying concentration of sulfuric acid in the phosphoric acid reaction system has been described (3):
While a considerable number of factors influence the size and shape of the gypsum crystals, the primary control is the sulfate concentration in the slurry. Typically, sulfate concentrations fall in the range from 1.0 to 4.0%. Stars, consisting of needlelike gypsum crystals radiatingf rom a central nucleus, usually indicate high sulfate concentration. At slightly lower sulfate concentrations rather large individual rhomb-shaped gypsum crystals with a length not exceeding two or three times the width are formed. At still slightly lower sulfate concentrations and within a rather narrow range, it is possible to grow agglomerates, or twins. These consist of rather large crystals, and up to about four are grown together on one or more faces, with the major axes of the crystals apparently having random orientation. Low sulfate concentrations will generally pmduce small crystals in the form of thin plates. 388
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1 a
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Figure 1. Typical habits of gypsum
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Figure 2. Gypsum twins
Figure 3. Typical gypsum from phosphoric acid plant (185x1 These effects probably are due primarily to changes in growth rate since sulfate concentration has a major effect on the solubility of CaSO,, and thus on the degree of supersaturation at any constant PZOSConcentration. Penetration twins have been shown (5) to be favored by increased supersaturation in crystallization of gypsum in aqueous systems. Foreign ions and organic materials in wet process acid would be expected to affect gypsum crystal habit; the plaster industry has for years used organic “retarders” and has added small amounts of foreign ions to affect the properties of plaster of pans. Ions of high “superficial electrical density” cause formation of tabular crystals of gypsum (4). Imn and aluminum are typical of such ions; presumably they are adsorbed on the 101 face, which typically is the fastest growing face. Japanese work (6, 7)has been reported on the effects of ferric and fluoride ions on gypsum habit. Ferric ions were shown to produce platy crystals. Fluoride was credited with the
secondary role of reaction with silica to free organic materials which then impeded the growth of gypsum crystals. Organic materials are well known crystal hahit modifiers; addition of a surfactant for this purpose in a wet phosphoric acid process has been patented (2). This work explored the effects of some impurities upon the habit of gypsum crystallized from phosphoric-sulfuric acid solutions. Exparimenlal
The materials used were typical strong acids from American Cyanamid’s Dorr-Oliver type phosphoric acid plant at Bradley, Fla. made from uncalcined Florida rock. Concentrations were adjusted with reagent grade materials: HaP04, H ~ P O I , ALOa, Fez03, HISiFs, HF. Use of only reagent grade materials would have permitted better control of the kind and amount of impurities. Originally, it had been planned only to precipitate gypsum from synthetic solutions of various PIOS and HsSO4 concentrations, and to correlate crystal habit with concentrations. However, the first phase precipitated in reagent grade systems was always calcium sulfate hemihydrate which transformed to gypsum on holding. The presence of hemihydrate limited the extent of supersaturation which could be attained for gypsum growth, and the hahit of gypsum which recrystallized from these solutions was the same, regardless of PIOSand H ~ S O concentraI tions. Gypsum could be crystallized from these solutions only when gypsum seed was present. When plant acids were used, however, the initial phase precipitated was always gypsum, in the range of PzOi and H,SO, Concentrations studied. In crystallizing gypsum, as constant an environment as possible for crystallization was maintained. The following procedure was used to maintain constant concentrations of P 2 0 s and H2SO4. In a Dolvethvlene Erlenmever flask were daced 100 grams of acid of’& dedired composiiion. The fl&k was placed in a hot water bath and brought to 70° C. I n a second flask were placed 100 grams of the same solution to which were added 3.5 grams of concentrated reagent grade H~SOI. In a third flask, 100gramsof “calciumcharge” wasmade up. This was of the same PIOs concentration as the above solutions and contained 3.7 grams of reagent Car(POn)l, equivalent to the excess HBSOIadded to the second flask. The second and third solutions were heated to 70” C. and poured simultaneously into the reaction flask. The flask was then stoppered and shaken in the water bath for 3 to 4 hours. Samples were taken at the first appearance of crystals, and at intervals thereafter, by inserting a filter stick and applying vacuum. The crystals on the filter stick were immediately washed with acetone and air-dried. The dried samples were examined microscopically. I n a series of samples which showed sufficient habit modification to he of interest, a photomicrograph was made, usually of the last sample in the time sequence. The mounting medium for these samples was mineral oil, n = about 1.48. Results
The effect of organic matter present in phosphoric acid is shown in Figures 4, 5, and 6. Figure 4, gypsum precipitated in the laboratory from a plant acid, shows typical rhombshaped crystals and swallow-tail twins. Figure 5 shows the elongation of the crystals precipitated in acid from which organic materials had been removed by treatment with active carbon. Figure 6 shows the same effect achieved by calcination at 1500’ F. of the phosphate rock from which the acid was prepared.
Figure 6. Gypsum precipitated from acid made from calcined r o c k ( 1 8 5 X ) VOL 5
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OCTOBER 1966
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Effect of foreign ions in plont acid on gypsum
Although CaS04.'/% Hx0 was the first material precipitated Table 1. Effect of Impurities on Cas04 Crystallization in from reagent grade solutions of phosphoric-sulfuric acids, it was HaPOr-H2SOrH20 possible to force the precipitation of gypsum by addition of Impurity Phme Precipitated impurities to the acid solutions. Table I lists the effects of addition of various combinations of impurities to a solution HF Hemihydrate FenOs Hemihydrateand gypsum containing 28% PZOSand 1% HsSOI. A1201 Hemihydrateand gypsum Aluminum and silicofluoride ion iin combination are thus H2SiF8 Hemihydrate A1,O1 Hemihydrate shown to have major effects on calciui,, " s..,L.&~ ~ r yl^ll._.l:._ ~ s ~ a n~~ ~ d na u n . ~ Fen02 ~ Hemihydrate FeaOI HsSiFe To show the effect of foreign ions in plant acid on gypsum Gypsum 0 habit a factorial experiment was mn, using three levels of P20s concentration and two levels each of AlrOs, FezOs,and F. (HtSiFe was used to add fluorine.: The lower level in each case wa analysis. For the higher level, doubled by addition of reapent -~ grade Ahus.. kelUa. ~, and Tab..=. =... I, .,,v= ,,Y.,C HzSiFe. The lower level analyses of the acid samples used Factorial Study are shown in Table 11. F PzOa H~SOI AlnOa Fez08 A photomicrograph was made of the final sample from each nmn.r~*inn-: +L"..,:A" --..:w.-:.._I.:.. -L0 . 7 0 0.94 27.7 2.20 y.-y'ly..y.. LII L ' I U Y L U ' L U " ' W'"' Y L C S"I"LI"II 29.8 1.65 0.81 1.03 after 4 hours at 70' C. 32.3 1.88 0.83 1.10 For the sake of brevity, photomicrographs of only one series,
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