Research On Preparation Process Of Potassium Sulfate From Sulfate Solid Waste By Conversion Method

The main by-product desulfurization gypsum?CaSO₄·2H₂O?from the thermal power plant or chemical plant flue gas desulfurization and the by-product Glauber's salt?Na₂SO₄· 10H₂O?from the PVC enterprise denitrification have become the main solid waste of sulfate type in China,not only occupied the land,but also polluted environment.However,China's potassium,sulfur resources is extremely lacking,especially potassium sulfate,long-term dependence on Russia,Germany and other countries import.It is undoubtedly a new way to complish the high utilization of the inorganic salt resources in the northwest and the utilization of the sulfuric acid solid waste by using the Northwest Salt Lake low-quality potassium chloride and industrial sulfate-type solid waste desulfurization gypsum and Glauber's salt conversion to produce high value-added potassium sulfate.In this paper,the ammonium sulphate mother liquor of calcium carbonate whisker of the desulphurized gypsum obtained form a thermal power plant in Ningxia and the mirabilite residue of a PVC enterprise was prepared as a source of sulfate radical,Qinghai Salt Lake low quality potassium chloride as potassium source,potassium sulphate was prepared by the conversion method.Firstly,the phase equilibrium data of the related system are studied,the solid-liquid phase equilibrium data and physicochemical parameters of the mutual quaternary system K⁺,NH₄⁺//Cl^-,SO₄^2--H₂O and K⁺,NH₄⁺//Cl^-,SO₄^2-?CH₂OH?₂-H₂O system at 298.15K and 333.15K were measured respectively,the solid-liquid phase equilibrium data and physicochemical parameters of the mutual quaternary system Na+,K⁺//Cl^-,SO₄^2--?CH₂OH?₂-H₂O and its sub-system Na+,K⁺//SO₄^2--?CH₂OH?-2-H₂O system at 328.15K were measured respectively.The phase diagram is constructed and the crystallization zone is able to guide and optimize the conversion process to produce potassium sulfate.Secondly,the theoretical calculation and technological analysis of the process of producing potassium sulfate by Glauber's salt conversion method were carried out by using the obtained phase diagram.The results show that the recovery of the mother liquor by indirect conversion can improve the yield of potassium sulfate.At the same time,the process of producing potassium sulfate by ammonium sulfate conversion method was also calculated.In addition,based on the phase diagram analysis,the process conditions for the production of potassium sulfate from potassium chloride and mirabilite residue were investigated.The optimum reaction conditions were as follows:reaction temperature 25?,reaction time 100 min,mixture ratio?mass ratio of Glauber's salt and potassium chloride?was 1.26:1,the stirring strength was 200 rpm.Under the optimum condition,the production of potassium sulfate from waste residue conversion method of two step conversion yield of K₂O is 89%,K₂O content in the product is 50.42%,1.8%content of chlorine ion,the quality of agricultural potassium sulfate reached the national first-class standard.In addition,the optimum conditions for the preparation of potassium sulfate from ammonium sulphate mother liquor prepared by desulfurization gypsum were as follows:the initial concentration of ammonium sulfate was 34w%,mixture ratio?n?NH4?2SO4:nKCl?was 0.5,the stirring rate was 200rpm,reaction time 120min,reaction temperature 40 ?.The best level combination experiment obtained the yield of K2O>64%,at this time the product K₂O%>50%,w?Cl^-?<0.15%,w?NH₄⁺?<0.3%,and the product potassium sulfate accorded with the standard of agricultural superior product.Because of the existence of potassium-ammonium-based solid solution in the salt-water system containing K⁺ and NH₄⁺,and the crystallization area of the solid solution is about 80%,so the separation of potassium-ammonium-based solid solution is theoretically analyzed in this paper.A mathematical model for the separation of potassium-ammonium-based solid solution by multi-stage countercurrent crystallization was proposed.The different evaporation strategies were used to simulate the variables of the specific process.The separation of potassium-ammonium-based solid solution can not only improve the yield and quality of potassium sulfate,but also provide reference for the efficient and controlled separation of valuable elements in solid solution of salt-water system.The research results of this paper have found an effective treatment method for the high utilization of ammonium sulfate mother liquor of calcium carbonate whiskers was prepared by desulfurization gypsum,desulphurized gypsum and mirabilite,with have good economic and social benefits and application prospects.