Study On Crystallization Preparation Of Phosphogypsum-based High Strength Gypsum And Influence Of Impurities

Phosphogypsum is a solid waste produced by phosphorus chemical industry.With the rapid development of phosphate fertilizer industry in China,phosphogypsum has become the second largest industrial waste after fly ash.Phosphogypsum is mainly composed of calcium sulfate dihydrate,which is an important potential resource of gypsum.From the point of view of resource utilization and environmental protection,it is urgent to improve its comprehensive utilization.αhigh-strength gypsum has been widely concerned and applied for its advantages of high mechanical strength,thermal stability and good biocompatibility.It is an effective way to improve the utilization rate of phosphogypsum resources to prepare αhigh-strength gypsum with high added value from phosphogypsum.In this paper,α high-strength gypsum was prepared by semi-liquid phase dynamic autoclaving method.The effects of preparation technology,organic acids and inorganic salts（acids）crystallizers,and different impurities on the crystal morphology,water consumption of standard consistency,setting time and mechanical properties ofα-type high-strength gypsum were studied,and the crystallizer with the highest strength for preparing α high-strength gypsum was selected,and α high-strength gypsum crystals with regular hexagonal short columns were successfully prepared.The phase structure and morphology of the crystal were analyzed to explore the effect of crystallizer and the influence behavior of impurities.The main work carried out is as follows:（1）Preparation of phosphogypsum-based high-strength gypsum by semi-liquid phase dynamic autoclaving method: The process conditions of phosphogypsum transformation to prepare α high-strength gypsum were optimized by the orthogonal experiment optimization method,and the effects of reaction temperature,constant temperature time,and slurry moisture content on the mechanical properties and crystal morphology of the products were investigated.The preparation process conditions were optimized and determined as follows: the dosage of 1# crystallizing agent is 0.3%（mass fraction）,the dosage of 2# crystallizing agent is 1.27%（mass fraction）,and the reaction constant temperature time is 20 min,the obtained αhigh-strength gypsum can reache α40 level.（2）The effect of crystallizing agent on the preparation of α high-strength gypsum by phosphogypsum transformation was studied: inorganic salts/acids（aluminum sulfate,oxalic acid）,organic acids（succinic acid,maleic acid,ethylene diamine tetraacetic acid,malonic acid）and self-made compound crystallizers（1#,2#）on the water consumption,setting time,mechanical strength,crystal micromorphology and structure of the standard consistency of α high-strength gypsum were mainly studied.The results showed that the self-made compound crystallizing agent had the greatest effect on α high-strength gypsum,the crystallizing effect was the best,the crystal structure integrity was high,and the mechanical strength was high.（3）The effects of different impurities in phosphogypsum on α high-strength gypsum were studied,including phosphorus-containing impurities（phosphoric acid,calcium hydrogen phosphate,calcium dihydrogen phosphate,calcium phosphate）,soluble fluorine and organic matter on the setting time,mechanical properties,crystal morphology and phase structure of α high-strength gypsum.The results showed that the addition of phosphoric acid,calcium dihydrogen phosphate and calcium hydrogen phosphate in phosphorus-containing impurities had obvious retardation effect on αhigh-strength gypsum,while calcium phosphate had no obvious effect on the setting time;Phosphoric acid had a great influence on the mechanical properties of αhigh-strength gypsum,and the strength of α high-strength gypsum would increase first and then decrease with the increase of the content of calcium dihydrogen phosphate and calcium hydrogen phosphate,respectively.Soluble fluorine had obvious coagulation effect on α high-strength gypsum and would reduce its mechanical strength.Organic matter had little effect on setting time and mechanical properties.（4）The influence of calcium dihydrogen phosphate and calcium hydrogen phosphate on the crystallinity of α high-strength gypsum was studied by using control variable method under the condition of adding impurities.It was found that calcium hydrogen phosphate has a greater effect on the crystallinity of α high-strength gypsum,while calcium dihydrogen phosphate has a smaller effect on the crystallinity of αhigh-strength gypsum.