| The basic steps of Odda process for the production of nitrophosphate include: decomposing phosphate rock with nitric acid, separating acid insoluble matters, freezing crystallization of acid hydrolyzate, separating and washing calcium nitrate tetrahydrate, neutralizing the mother liquor, concentrating slurry, granulating and drying, products packaging and so on. After decomposing phosphate rock by nitric acid, the calcium ions transfer to the form of Ca(NO3)2·4H2O that is completely soluble in acid solution, but can be only removed 75～80% as solid by liquid ammonia under -5℃, about a quarter of Ca(NO3)2·4H2O is remained in mother liquor. Due to the declining grade phosphate rock in recent years, manufactures will have to face the reality of using mid-low grade phosphate rock. It is predicted that high-grade ore resources containing P2O5≥30% in China can only be available by 2014. If mid-low grade phosphate rock is used, the system is bound to introduce too much calcium and other impurities. Because the redundant calcium, insoluble Ca3(PO4) will be generate during neutralizing the mother liquor process, as a result, the soluble phosphorus in nitrophosphate will degrade to affect the quality of product. Meanwhile, it is very unfavorable for the production of nitrophosphate, because the generated Ca3(PO4) scaled in the pipeline led the heat was not easy to transfer in the evaporation process. In order to ensure product quality of nitrophosphate and improve the conversion rate of phosphorus, the excess calcium must be removed in the system, that is adding a slight excess sulfuric acid (molar ratio of SO42-and Ca2+ is 1.1:1) in the mother liquor to achieve the purpose of removing redundant calcium ions by forming calcium sulfate.Calcium sulfate is a kind of slightly soluble and soft-viscous solid, in order to improve the filtering performance of calcium sulfate, it is important to research thick-long and easily-to-filter calcium sulfate particles in acid medium. There are several hydrate forms of calcium sulfate, mainly existing dihydrate and hemihydrate and anhydrate forms.In order to investigate the technical conditions of generate coarse and uniform crystals of calcium sulfate in nitric acid-phosphoric acid medium, the following two sets of experiments were designed in this paper. To research different effects on the relevant parameters of calcium sulfate, the first set of experimental systems changed the contents of phosphoric acid and nitric acid, respectively; and changed acid contents, when both of them coexisting. The second set of experimental system was added diffident contents of Mg2+, F- and acid-insoluble in the reacting solution which contained equivalent content of phosphoric acid and nitric acid with the acid system of Odda process. During the study, the morphology and particle size of calcium sulfate crystal was examined by microscope, and the crystal phase was characterized by XRD. In the implementation of the above two experiments, it was found that although the system has excess sulfuric acid, the removing rate of calcium in mother liquor can not achieve a satisfactory results. Thus, it was speculated that calcium sulfate in acidic medium has considerate solubility. Therefore, we designed another set of experiments to investigat the solubility of calcium sulfate in the room temperature (25℃) and reacting temperature (65℃) in different contents of phosphoric acid, nitric acid, and research the crystalline of the deposition solid-phase.In the first set of experiments, the micrographs showed that calcium sulfate crystal morphologies were rod-like, flat-like and needle-like in acid medium. Crystal morphology depended largely on the level of acid content. Under lower acid content, the obtained crystals were thick-long rod-like and more soft with a certain crystal gloss; while with the increase of acid content, the crystals gradually changed thin-tiny rod-like, even tiny needle-like. It is indicated that higher acid contents inhibited crystal growth in all directions. The XRD analysis showed that the hydrate form of calcium sulfate was changed from dehydrate to hemihydrate with the increasing of acid content, it was also affected intensively by acid. In high acid medium, calcium sulfate existed in hemihydrate form and appeared tiny needle-like to filer difficultly.There were not obvious varieties for calcium sulfate morphologies with tinier rod or needle-like in the second set of experiments. It was illuminated clearly that Mg2+, F-, and AI were not into crystal lattice, just changed the external characters of crystals. On the other hand, it was indicated that acid medium was the most important factor to change the morphology. XRD analyses showed that calcium sulfate existed in the form of dehydrate and hemihydrate mixture of form, rather than single phase and the proportion of them changed with different kinds and contents of impurities. In the range of researched impurity contents, the filtering time of solid-liquid separation was longer with impurity content increasing, and crystallinity was decreased, crystal condition was deteriorated.The solubility of calcium sulfate dehydrate in water was fallen with temperature rising. The solubility at 25℃was 0.68g/(100g solvent), at 65℃0.55g/(100g solvent). In the HNO3-H2O or H3PO4-H2O solvent systems, the solubility of CaSO4·2H2O increased with acid concentration increasing; and the solubility at 65℃was always higher than it under 25℃. In the mixed acid solvent system, the solubility still maintained higher at 65℃than it under 25℃. The general trend was the solubility of CaSO4·2H2O increasing with acid concentration increasing. The micrographs showed that calcium sulfate dihydrate crystals in acidic medium were larger than it in water system, and crystal shape was basically long rod-like or flat sheet. However, when in the mixed acid solvent system, that is, under a fixed 20% concentration of H3PO4 and increasing HNO3 concentration more than 15% the sediment crystallization appeared deterioration and crystal particles appeared small and tiny. XRD results showed that the depositions of calcium sulfate crystals in saturated solution were basically CaSO4·2H2O. However, when H3PO4 and HNO3 concentrations were both 20%, all the characteristic peaks of CaSO4·2H2O disappear, only appearing the characteristic peak of CaSO4·1/2H2O at 2θ= 25.41°, and the peak was not strong. In this situation, the crystal form occurred transformation and the crystals are in poor condition. The acid concentration and temperature should be the main factors on impacting crystal changes of calcium sulfate, and further study is still in progress.
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