Study On Purification Of Byproduct By Phosphogypsum By Sulfuric Acid Treatment

Phosphogypsum is a by-product solid waste of phosphate fertilizer industry, has the high concentration of CaSO4·2H2O, can be mainly applied to the production of the low-end building materials and low-value industrial sulfate, and is fewer high-value utilization ways. Due to impurity effects of phosphogypsum, presently the utilizing rate of phosphogypsum accounted for only 20% of the emissions. Study of impurity removal on the phosphogypsum and high added value whisker prepared with the phosphogypsum have become one of the research focus.Acid leaching method, as a way of purifying phosphogypsum, has the characteristics of low energy consumption and slight secondary pollution, etc. This paper aims to leach the phosphogypsum with the sulfuric acid and by-produce the calcium sulfate whisker during purification. The paper mainly did a research upon the process disciplines, the phase behavior changes and recycling treatment leaching the phosphogypsum with the sulfuric acid, discussed the variants of acid leaching solution, phophogypsum as well as whisker after the circulation processing, and made a relative exploration on preparing the calcium sulfate whisker and powder with the phosphogypsum.The research shows that:(1) The solubility of pure gypsum and phosphogypsum in the dilute sulfuric acid solution increases as the temperature rises, and increases at first then decrease with increasing sulfuric acid concentration. In the experimental range (temperature:0～80℃, sulfuric acid concentration:0～30%), the biggest solubility in pure gypsum occurs at 10% sulfuric acid concentration,80℃, with a value of 1.1336 g/100 g of sulfuric acid, at this point, the maximum crystallization rate in the filtrate is 0.5552 g/100 g; the maximum solubility of phosphogypsum is 1.1226 g/100 g of sulfuric acid, corresponding maximum crystallization rate is 0.5094 g/100 g of sulfuric acid.The dissolve process of pure gypsum and phosphogypsum in 10% and 30% sulfuric acid solution is different. Only having CaSO4·2H2O dissolution and crystallization in 10%sulfuric acid, the dissolution-crystallization balance of CaSO4·2H2O is until achieved; while existing the dissolution of CaSO4·2H2O and dissolution and crystallization of CaSO4 in 30% sulfuric acid, the solubility of gypsum is influenced by CaSO4 dissolve-crystallization equilibrium. After 30 minutes dissolved at 80℃ and 30% sulfuric acid solution, the conversion rate of pure gypsum dissolved into CaSO4 is 15.59%,67.32%for that of phosphogypsum.(2) The phosphogypsum is treated with sulfuric acid of 30% concentration in the liquid circulation experiment. The phosphogypsum impurity effect of the acid leaching solution after repeated treatment has not diminished, removing P rate is about 0.435% every time, but affected by the impurity accumulation and deposition, the crystallization rate decreases at first then increases.After the solid recycled phosphogypsum is processed intermittently 8 times with 30% sulfuric acid, P removal rate is 99.6%, the whiteness degree increased from 33.1 to 69.5, and the main phase composition changed from dihydrate to anhydrous.After the phosphogypsum is processed 8 times with 10%sulfuric acid in an advanced solid circulation experiment featured with the existing stirring, no washing and no drying, its F impurity can be removed, the P content reduced from 0.9631%to 0.4507%, cumulative whisker production rate reached 2.1319 g/100 g 10% sulphuric acid solution; after 8 times treatment of 30% sulfuric acid, phosphogypsum developed into two layers; the upper one reduced the P, F content to 0, SiO2 content from 14.2958% to 2.2025%, while the removal rates of impurities P, F in the lower one are respectively 88.4%,39.9%, content of SiO2 increased from 14.2958%to 82.3749%, and cumulative whisker production rate reached 0.7828 g/100g 30% sulfuric acid solution.After the phosphogypsum is processed 8 times with 10% sulfuric acid in an advanced solid circulation experiment featured with no stirring, no washing and no drying, its F impurity can be removed, the P content reduced from 0.9631% to 0.4988%; after 8 times treatment of 30% sulfuric acid, its F impurity can be removed, the P content reduced from 0.9631% to 0.3004%.(3) Phosphogypsum can completely dissolved after boiling and stirring 2 minutes in 10%sulfuric acid solution, the solubility of boiling condition greater than the solubility of other reaction temperature, its average value is 1.7512 g/100g of sulfuric acid, and corresponding maximum crystallization rate reached 0.6932 g/100 g 10% sulfuric acid solution.Concentrating the filtrate of phosphogypsum processing with 10% sulfuric acid, the morphology and composition of solid product obtained related to the concentration: the filtrate concentration reaches 20%, solid product is long flake form, and the filtrate concentration exceeds 30%, solid product is CaSO4 powdered crystal.After processed with 40%sulfuric acid at 80℃ for 30 minutes, the phosphogypsum is developed into two layers:the top layer is of 76.4-whiteness anhydrite powder, containing 87.97% CaSO4 and 3.88% SiO2, while the down layer mainly contains 45.26% CaSO4 and 49.82%SiO2.