Reactive Crystallization Of Calcium Carbonate Prepared By Phosphogypsum

Phosphogysum is the primary solid waste from phosphoric acid production. Oneton of phosphoric acid can bring about five tons byproduct of phosphogypsum. Alarge stockpile of phosphogypsum not only occupies vast areas of land, but alsocauses serious environmental pollution. One of the ways to recycle the wastephosphogypsum is to produce ammonium sulfate and calcium carbonate. Calciumsulfate is the main component of phosphogysum. Thus, this paper systematicallystudied the reactive crystallization process of calcium carbonate using calcium sulfateThe main contents include the following aspects:Calcium carbonate has three polymorphs: calcite, aragonite and vaterite. Thecrystal form has a significant effect on the crystal habit, particle size and particle sizedistribution of calcium carbonate. In this paper, the effects of seeding, additives, solidcontent of calcium sulfate, and other conditions on the crystal forms was investigated.The characterization of calcium carbonate crystals was obtained by PXRD and SEM.It shows that solid calcium sulfate can exclusively induce calcite without transformingfrom vaterite.The induction time of calcium carbonate under different conditions ofsupersaturation and temperatures was determined using FBRM and the data wasanalyzed and understood using both classical nucleation theory and clustercoagulation theory. Modeling result through classical nucleation model shows that:homogeneous nucleation happens at higher degree of supersaturation, whileheterogeneous nucleation happens at lower degree of supersaturation. The results ofthe cluster coagulation theory show that short induction time can be attributed to hightemperature, large cluster diffusion coefficient, high initial coagulation concentrationof clusters and small critical nuclei size.Preparing calcium carbonate is one of the ways to recycle phosphogypsum. Oneof the biggest problems of this process is the difficult filtration caused by the smallparticle size of calcium carbonate. In this paper, a new process of preparing calciumcarbonate under low temperature was developed. It’s found that temperature havedramatic effect on the particle size of the product. The effects of operative conditions such as solid liquid ratio, agitation, addition rate and excess of ammoniumbicarbonate, and reaction time on the crystallization of calcium carbonate werestudied and the process was optimized. The result shows that the main particle size ofthe product increased more than2times after optimization.