Study On The Process Of Acidifying Ammonium Sulfate To Decompose Phosphate Rock

Large quantities of sulfur are imported yearly to produce sulfuric acid which used for the production of phosphate and compound fertilizer.The production of phosphate and compound fertilizer not only consume a large amount of sulfuric acid,but also produce large amounts of phosphogypsum,the current utilization rate of phosphogypsum is less than 20%yet.Sulfur contained in phosphogypsum mainly used to produce ammonium sulfate or potassium sulfate,however,ammonium sulfate is not available to decompose phosphate rock,but can make phosphorus and calcium in phosphate rock separate relatively simple,such as performing acidification for ammonium sulfate first,and then using it to decompose phosphate rock for preparation of phosphate fertilizer,the utilization problem of sulfur produced in wet recycling process can be solved.Therefore,this issue intends to acidify ammonium sulfate,acidified product of which is used to decompose phosphate rock,choice of acidifiers for ammonium sulfate,process conditions and dynamics of decompositon of phosphate rock with acidified ammonium sulfate are studied,which provide basic data for ammonium sulfate to improve recycling utilization of sulfur in phosphorus chemical enterprises.First,chemical reactions carried out in the research are analyzed theoretically;acidifiers are screened by thermodynamic calculations.In the range of 20-90℃,AG value of reactions which used ammonium sulfate acidified by nitric acid,hydrochloric acid,sulfuric acid and phosphoric acid to decompose phosphate rock is negative,therefore,the reaction is spontaneous;but when acidified with acetic acid and citric acid,the △G is positive,so the reaction does not occur spontaneously.When using ammonium sulfate acidified by nitric acid,hydrochloric acid,sulfuric acid and phosphoric acid to decompose phosphate rock,the products may be CaSO₄·2H₂O,CaSO₄·0.5H₂O,NH₄H₂PO₄;with nitric acid as acidifier,products may contain H₃PO₄,when using sulfuric acid as acidifying agent,there may be presence of（NH₄）2HPO₄.Secondly,the process of acidified ammonium sulfate to decompose phosphate rock is investigated.Optimum conditions of acidified ammonium sulfate to decompose phosphate:nitric or hydrochloric acid as acidifiers,reaction time 80 min,reaction temperature 80 °,H+ concentration 5.4 mol·L-1.Under these conditions,the phosphate decomposition rate reached more than 99%.The phosphate decomposition rate is mainly affected by crystallization of CaSO₄·2H₂O through analyzing phase and morphology of the residue from ammonium sulfate acidification process.When using nitric acid and hydrochloric acid as acidifying agents,CaSO₄·2H₂O crystalline precipitate only a small amount,which does not affect the decomposition of phosphate rock,so conversion rate is high;when sulfuric acid and phosphoric acid used as acidifying agents,wrapped and hidden phenomenon may exist in liquid,which lead to conversion rate decrease.Then,kinetics of ammonium sulfate acidified with nitric acid to decompose phosphate rock shows that,liquid-solid reaction process is in line with unreacted shrinking core model which belongs to the single particle reaction model,its dynamic model can be described by the Drozdov equation of phosphate decomposition kinetics model.According to the Arrhenius equation,the apparent activation energy Ea is 4.531kJ·mol-1,this shows the decomposition process of phosphate with acidified ammonium sulfate is diffusion controlled process in solid product layer.Dynamics model of nitrate acidification ammonium decomposition phosphate is:In 1/1-η-βη=0.1404 exp（-4531/RT）tFinally,the subsidiary production calcium sulfate whisker from filtrate is discussed.The filtrate is dealed with four different ways-direct cooling aging treatment,adding ammonium sulfate solution,adding seed and condensing filtrate,products of which are calcium sulfate dihydrate.In addition to the enrichment process is not conducive to calcium sulfate crystals by-product,the other three may by-product crystals.