| The wet process phosphoric acid was one of the important methods to produce phosphoric acid,and it was also the basic product of phosphorus chemical industry.It was widely used in phosphate industry and phosphate compound fertilizer industry.The production of wet process phosphoric acid was mainly to produce phosphoric acid by decomposing phosphate rock with sulfuric acid.Phosphoric acid was prepwered by traditional dihydrate wet phosphoric acid process,which is required a lot of manpower and material resources to digest the part of phosphogypsum.So a technical solution was proposed to produce high-purity white gypsum.The amount of waste residue produced by phosphoric acid decomposition of phosphate rock was very small,and the gypsum produced by the second step precipitation was of high whiteness and purity,which can solve the problem of phosphogypsum difficult.Phosphoric acid was used to decompose phosphate rock in two steps.In the first step,calcium dihydrogen phosphate liquid phase and reaction waste residue were obtained by reaction under appropriate process conditions(temperature,liquid-solid ratio,P2O5 concentration,reaction time,etc.),and solid-liquid separation was carried out under the appropriate type,dosage of additives and the selection of appropriate filter.In the second step,concentrated sulfuric acid was added to precipitate calcium to obtain high-purity gypsum,phosphoric acid.Partial phosphorus acid and washing liquor were returned to the system of the first step phosphoric acid decomposition.The effects of reaction temperature,liquid-solid ratio,phosphphate rock of medium and high grade were investigated by single factor experiment.The suitable technological conditions for the experiment were reaction temperature 75?,liquid-solid ratio 9:1,reaction time 3 h and phosphoric acid concentration 30%(calculated by P2O5).Under this condition,the decomposition rates of phosphate rock are 99.78%and 98.62%.Response surface methodology was used to further optimize the experiment,and the quadratic regression model equation of phosphate rock decomposition rate with reaction temperature A,concentration of B phosphoric acid and liquid-solid ratio of C was establwashed,which was,Y=91.92+5.60X_A+0.48X_B+11.60X_C-0.35X_AX_B-4.65X_AX_C+2.30X_BX_C+5.86X_A~2-14.74X_B~2-7.43X_C~2.The results showed that the optimum process conditions were reaction temperature 80?,liquid-solid ratio 10:1 and phosphoric acid concentration 27%.Under these conditions,the decomposition rate of phosphate rock was 96.6%,which was close to the predicted value of the model.The model could be used to optimize the process conditions.The equilibrium distribution of F and P in three phases of gas,liquid and solid in the process of acid hydrolysis of phosphate rock was studied.It provides basic theoretical guidance for the recovery and utilization of F in the process of phosphate rock decomposition by phosphoric acid.In order to obtain phosphoric acid product and return part of phosphoric acid to phosphate rock system,it was necessary to use sulfuric acid to precipitate calcium ions in decomposition solution to obtain phosphoric acid and gypsum.In this paper,the effects of excess coefficient of sulfuric acid,reaction temperature and time on the properties of gypsum after reaction were studied by single factor experiment.The optimum process parameters were obtained as follows:excess coefficient of sulfuric acid 1.1-1.2,reaction temperature of 65-75?,reaction time of 2 h.The content of free water was about 38.5%,and the content of water-soluble P2O5 was about 1.92%.The crystal form of gypsum was observed by polarizing microscope.The gypsum was sheet crystal with small size and contains some broken crystals.In the actual industrial production,due to a large number of crystal seeds,the crystal size of gypsum will be greatly improved,the free water and water-soluble phosphorus in gypsum would maybe reduced correspondingly.Under the experimental conditions,the whiteness of gypsum was 85-88. |
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