Effect Of Impurities On Gypsum Conversion To Ammonium Sulfate With Ammonium Carbonate

The deposit of gypsum, a by-product from wet phosphoric acid process, are drawing more and more attention over world. In China, the gypsum deposit has been a severe environmental problem accompanying with the production of phosphate or phosphorus-based complex fertilizer. Using ammonium carbonate aqueous solution to convert gypsum to CaCO3 and (NH4)2SO4 is an effective way to reuse gypsum which can significantly reduce the gypsum deposit. But impurities in gypsum by product greatly influence the crystallization of CaCO3 and the properties of conversion products. A systematically study on the effect of impurities is necessary for the industry of the conversion process.In this paper, the mechanism, the instrinctive kinetics of the reaction between gypsum and ammonium carbonate and the influence of the main impurities in gypsum are investigated.The instrinctive kinetics fits the first order reaction model. Its activation energy is 68.92KJ/mol. The rate constant k is 5.99x1010exp(-68920/RT).The impurities to some extent influence the conversion degree of S, filtration behavior, properties of product, the crystalline forms and crystalline habits.In the presence of water-soluble P3O5, the conversion degree is lowered and the filtration becomes difficult. The results of XRD and SEM indicate that water-soluble P2O5 reduce the rate of transformation from vaterite to calcite. And water-soluble P2O5 make crystalline grains smaller and irregular.In the presence of either F" or SiF62-, larger size product crystal can be obtained.SiF62- lower the conversion degree of S, however the conversion degree of S increase with F- ion concentration up to 1.0%, but the filtration process became easier. The results of XRD and SEM indicate that the conversion product CaCO3 is calcite.The existence of Fe3+, Mg2+ results in the transform of CaCO3 crystal from vaterite to calcite and the conversion degree is lowered and the filtration becomes difficult.. The surface of CaCO3 crystal became coarse.A13+ in gypsum lowered the S conversion, but the filtration goes successfully. A13+ benefit to regular spherical vaterite and smooth CaCO3 surface.When P2O5 and F- co-exist in reaction system, F- ion plays the main role in crystallization transform of crystalline forms. Water-soluble P2O5 also change the habits of CaCO3 crystal.