Study On The Factors Affecting Sediments In The Process Of Wet-process Phosphoric Acid Production

The phosphate rock is a main material in the wet-process phosphoric acid production. It contains many different elements, such as phosphorus,calcium,iron,aluminum,magnesium,fluorine,silicon,sodium and potassium. The domestic phosphate rock belongs to medium or lean ore, and it has a lot of impurities. The main problem of wet-process phosphoric acid production is how to prevent the impurities to precipitate itself from the filtrate during the vacuum filtration of slurry. The sediment composes of Na₂SiF₆,K₂SiF₆ etc., which is hard to be dissolved and cleaned. The period of cleaning the sediment in production is not more than a week. This makes a big problem to the production, and it drops the rate of production seriously. Many domestic and international scientists have made a lot of related researches, and obtained some achievements, but it did not solve the problem. Therefore, it is important to study the factors affecting the precipitation of impurities from the wet-process phosphoric acid production.When the wet-process phosphoric acid slurry is filtrated at 0.054～0.06Mpa, water in the filtrate evaporates, and then the temperature drops suddenly. Na₂SiF₆ and K₂SiF₆ deposit and adhere to the inside of filter from the supersaturated solution. So the method for solving the problem is that adding surface-active agent to increase the amount of Na₂SiF₆ and K₂SiF₆ dissolved in the filtrate, or in accordance with the theory of common ion effect, when H₂SiF₆, Na₃PO₄ and K₃PO₄ added into the wet-process phosphoric acid, it can reduce the amount of Na₂SiF₆ and K₂Si_F6 dissolved in the solution. So the sediment of impurities deposit from the solution before the slurry filtrates by vacuum filtration.Because of this, it was investigated that how many degrees of the filtrate temperature dropped and how much water evaporated in the filtrate under definite temperature and vacuum by simulating vacuum-filtration of wet-process phosphoric acid slurry firstly. The results indicated that the more vacuum is, the more degrees dropped of the filtrate temperature. The sediments are lower, the filtration capability is workable under 75～80 degree centigrade at 0.056～0.057Mpa. So the optimum temperature is 75～80℃in the wet-process phosphoric acid production, and the optimum vacuum degree of filtration is about 0.056～0.057 Mpa.Then, the thesis measured the solubility of potassium(sodium) fluorosilicate in phosphoric-sulfuric acid under deferent temperature. The results showed that the solubility of potassium(sodium) fluorosilicate in phosphoric acid solution increases as the temperature of solution increases, the solubility of sodium fluorosilicate in phosphoric-sulfuric acid solution was higher than that of potassium fluorosilicate at the same temperature. And it was also studied the additive effects on sediments in solution by adding H₂SiF₆ and Na₃PO₄ into the solution. The result indicated that the amount of additive was more; the solubility of Na₂SiF₆ or K₂SiF₆ was less. When the amount of Na₃PO₄ reached to 0.85 g/kg or 1.48 g/kg, Na₂SiF₆ or K₂SiF₆ in the solution was hardly dissolved. Also when the amount of H₂SiF₆ reached to 4%, Na₂SiF₆ or K₂SiF₆ in the solution was not dissolved. SO it can apparently reduce the quantities of sediments when we add H₂SiF₆(3～4%) and Na₃PO₄(0.085～0.128%) into the slurry.At last, the thesis studied the main factors existing in the solution itself which influence the crystallization of impurities, then a new method was put forward that how to prevent precipitation of sediments from the supersaturated solution when it is filtrated by vacuum filter. Based on the single factor experiment, the concentration of P₂O₅, SO₃ and additive were studied that can influence the solubility of Na(K)₂SiF₆. The results showed that the amounts of sediments have a inverse ratio to the concentration we chose. The optimum concentration of the additive is 3.0 percent, the concentration of phosphoric acid is as higher as possible and the concentration of SO₃ properly increases according to the actual situation, it can reduce the quantities of sediments.