Effect Of Wet-process Phosphoric Acid Technology On Phosphogypsum Quality

Phosphogypsum (PG) is the byproduct of wet-process phosphoric acid(WPA),and its annual discharge has came to 50 million ton. PG could not be used in large quantities because of its unstable quality, so it was deposited mainly as rejected materials at present. This not occupy land aera, but spoil the environment, and cause the adverse influence on sustainable development of society. Because of the effect of crystal structure and impurities, PG is hardly to be utilized directly, and it not meet the remend before pretreatment. These raises the processing cost, also the present treatment method isn't able to be used to deal with PG in bulk. Consequently,it is imperative to reduce the difficulty using PG through improving PG quality.The main reason PG has a lot of impurities and the bad crystal structure is that the current WPA blindly chase high targets of extraction rate but neglect PG quality. Reducing the content of impurities in PG(particularly the residual phosphorus content)and optimizing PG crystal morphology by readjusting WPA is an effective way in which could improve PG quality.Through simulating WPA, respectively investigating the effects of rock phosphate powder, extraction process and washing process on PG quality. The leading factors obviously affect PG performance are water soluble P,P₂O₅ in crystal and crystal morphology based on which to estimate PG quality in this paper.Chemical composition of three types of rock phosphates were analysed and these three ores were served as raw materials to manufacture PG under the same condition, then test the residual phosphorus content and found that: PG produced by rock phosphates with more Fe has higher content of P₂O₅ in crystal, and that with more P has higher content of water soluble P. The effect of rock fineness on the content of P₂O₅ in crystal was greater than that on water soluble P, but the effect on crystal morphology was not distinct.The influence of extraction condition on PG quality was great: The supersaturation fell after the concentration of liquid-phase SO₃ was properly raised, crystallization condition reformed, crystal morphology fine, particle size regular,the residual phosphorus content lower; When the stirring rate rose, the content of P₂O₅ in crystal was reduced evidently, at the same time,it was not good for crystallization and reduction of water soluble P; If the reaction time could keep at 2.5～3.5 hours, the residual phosphorus content was lower and crystal morphology steady; With the concentration of liquid-phase P₂O₅ rose, reaction rate dropped, he residual phosphorus content increased, crystal was refined, especially while the concentration was above 20 percentages, PG quality went off by a large margin; The higher the reaction temperatures, the bigger the crystal size, however,its shape change little; Low grade rock phosphate should use higher slurry ratio of liquid to solid, PG crystal morphology wsa better and the residual phosphorus content lower under the ratio 4:1.After changing washing temperature, washing ratio of liquid to solid, washing times and PAM dosage, we found that: washing process mostly influence the content of water soluble P in PG, and the moderate flocculating agent could evidently reform washing effect.This study show that possible process readjustment could improve PG quality, it found a method to solve the question PG is hardly to be used.