| Industrial by-product gypsum is a by-product produced in the process of industrial production or waste gas treatment,including phosphogypsum,titanium gypsum and desulfurization gypsum etc..The main component of industrial by-product gypsum is calcium sulfate dihydrate?CaSO4·2H2O?.Its massive stockpiling not only wastes space,pollutes the environment,but also causes serious waste of calcium and sulfur resources.With the construction of a resource-efficient and environment-friendly society and the increase of citizens'awareness of environmental protection,there is an increasing demand of resource recycling,environmental protection and sustainable development.As an underestimate resource,industrial by-product gypsum needs to be further researched and reused,especially phosphogypsum and desulfurization gypsum,which take the largest proportion of industrial by-product gypsum.In this paper,calcium sulfate dihydrate and insoluble impurities in gypsum were separated by salt leaching and acid leaching,so as to improve its purity and utilization.Ammonium acetate solution method was used to determine the optimum dissolution conditions by studying the concentration of ammonium acetate solution,dissolution temperature,solid-liquid ratio and dissolution time.Under the optimum dissolution conditions,crystals crystallized by saturation principle were characterized by XRD to determine the phase composition.Calcium sulfate dihydrate was separated from insoluble impurities in gypsum by acid leaching method.Calcium sulfate dihydrate was purified from filtrate through supersaturated crystallization stage.The optimum technological conditions and yield of calcium sulfate dihydrate whisker were determined by studying acid concentration,dissolution temperature and dissolution time.Hydrochloric acid was used as the acid leaching agent to prepare calcium sulfate dehydrate whiskers in the first 4 cycles of acid leaching solution,and the supersaturated crystalline acid leaching samples in the next 4cycles of acid leaching solution were used as raw materials to prepare calcium sulfate hemihydrate whiskers and anhydrous calcium sulfate whiskers by the hydrothermal method.The effects of acid concentration,temperature and time on the phase and microstructure of acid leaching products were studied.The optimum process conditions for the preparation of calcium sulfate hemihydrate whiskers and anhydrous calcium sulfate whiskers were obtained.The prepared calcium sulfate whiskers were characterized by XRD,SEM,thermal analyzer and whiteness analyzer in order to determine their application areas and provide an experimental basis for industrial production and utilization.The results show that ammonium acetate,as a weak acid and weak base strong electrolyte solution,has a remarkable effect on dissolving calcium sulfate dihydrate in phosphogypsum.The optimum technological conditions are as follows:when the temperature is 90?,the concentration of ammonium acetate solution is 8.0 mol/L,the ratio of solid to liquid is 10.0 g?100 mL,the dissolution time is 20 min,the maximum dissolution amount of phosphogypsum is 8.86 g/100 mL,The dissolution rate is 88.60%.The optimum dissolution conditions in sulphuric acid system are as follows:when the reaction temperature is 90?and the concentration of sulphuric acid is 1.0 mol/L,the dissolution amount of phosphogypsum and desulphurization gypsum are 1.68 g/100mL and1.57 g/100mL,the crystallization amount is 0.58 g/100mL and 0.55 g/100mL,and the extraction rate is 34.64%and 34.78%,respectively.The solubility of phosphogypsum and desulfurization gypsum is 8.3 and 7.8 times higher than that of dihydrate calcium sulfate in water.The optimum dissolution conditions of desulphurized gypsum in the nitric acid system are as follows:when the temperature is 90?,the concentration of nitric acid is 4.0 mol/L,and the dissolution time is 20 min,the dissolution amount of desulphurized gypsum is 7.08g/100 mL.The amount of dissolved calcium sulfate dihydrate was 35.1 times higher than that in water,the crystallization amount was 3.63 g/100 mL,and the extraction rate was51.20%.The optimum dissolution conditions of phosphorous gypsum in the nitric acid system are as follows:when the reaction temperature is 90?,the concentration of nitric acid is 4.0 mol/L,the dissolution time is 10 min,the amount of desulphurization gypsum is 6.87g/100 mL.Compared with the solution of dihydrate calcium sulfate in water,the dissolution of calcium sulfate was increased by 34.0 times,the crystallization amount was 3.52 g/100mL,and the extraction rate was 51.22%.The optimum dissolution conditions of desulfurized gypsum in the hydrochloric acid system are as follows:when the temperature is 90?,the concentration of hydrochloric acid is 3.5 mol/L,and the dissolution time is 20 min,the amount of desulfurization gypsum is6.18 g/100 mL.The amount of dissolved calcium sulfate dihydrate was increased by 30.6times than that in water,the crystallization amount was 3.43 g/100 mL,and the extraction rate was 55.39%.The optimum dissolution conditions of phosphorous gypsum in hydrochloric acid system are as follows:the reaction temperature is 90?,the concentration of hydrochloric acid is 3.5 mol/L,the dissolution time is 10 min,the amount of desulphurized gypsum is 6.22 g/100mL,compared with the solution of dihydrate calcium sulfate in water,the amount of crystallization was 3.12 g/100 mL,and the extraction rate was50.11%.In order to reuse the acid,only the industrial by-product gypsum can be separated from the impurities,the acid can be reused several times,and good results still can be obtained in 8 cycles.The optimum experimental conditions for preparing calcium sulfate dihydrate whiskers from phosphogypsum in hydrochloric acid system are hydrochloric acid concentration of 3.5mol/L,reaction temperature of 90?,reaction time of 10 min,four times before circulation,static cooling at room temperature of 12 h,filtration and drying of 12 h,maximum aspect ratio of crystal whiskers from phosphogypsum acid leaching solution of 77,86,92 and 85,respectively.The optimum experimental conditions for preparing calcium sulfate dihydrate whiskers from gypsum are hydrochloric acid concentration of 3.5 mol/L,action temperature of 90?,action time of 20 min,four times before circulation,static cooling at room temperature for 12 h,filtration and drying for 12 h,maximum aspect ratio of crystallization products from acid leaching solution of desulfurization gypsum is 95,135,114 and 73,respectively.At this time,purity is about 100%,white.Whiteness is 96.5%and 95.4%,respectively.In hydrochloric acid system,the optimum experimental conditions for preparing calcium sulfate hemihydrate whiskers by acid leaching of phosphogypsum sample PYJ are hydrochloric acid concentration 6.0 mol/L,reaction temperature 80?and reaction time 0.5h.At this time,the whiskers are uniform in size,smooth in surface,clear and distinct in edges and angles,the longest diameter ratio is 136,and the purity is about 100%,whiteness can be reached.98.6%.The optimum experimental conditions for preparing calcium sulfate hemihydrate whiskers by acid leaching TYJ from desulfurized gypsum sample are hydrochloric acid concentration 5.5 mol/L,reaction temperature 80?and reaction time 1.0h.At this time,the whole whisker is fibrous,and the surface of whisker is slightly fragmented,the longest diameter ratio is 87,and the length range is 55420.The width is310?m,and the purity is about 100%and the whiteness is 98.1%.The optimum conditions for preparing anhydrous calcium sulfate whiskers from acid leaching samples PYJ and TYJ in the hydrochloric acid system are hydrochloric acid concentration 6.5 mol/L,reaction temperature 80?and reaction time 2.0 h.At this time,the whiskers are uniform in size,smooth in surface,clear and distinct edges.The length ranges of acid leaching products of acid leaching samples PYJ and TYJ are 2.57.9?m and 1.57.3?m microns with widths of 0.20.4?m and 0.090.4?m,respectively.The purity of anhydrous calcium sulfate whiskers prepared at this time is about 100%,whiteness is 99.2%and 98.7%,respectively.Ammonium acetate leaching and acid leaching process of gypsum by-product separate calcium sulfate from insoluble impurities and solve the problem that impurities commonly exist at present affect the performance of gypsum resource utilization products.At the same time,ammonium sulfate can be obtained from ammonium acetate solution.Calcium sulfate dihydrate can be obtained in sulphuric acid,nitric acid,and hydrochloric acid solutions.In the hydrochloric acid system,calcium sulfate hemihydrate whisker and anhydrous calcium sulfate whisker with higher added value can also be obtained,which greatly improves the added value of resource utilization and has strong process feasibility.The acid leaching solution can be recycled,which effectively reduces the processing load on the environment,and achieves the goal of ecological utilization of industrial by-product gypsum,with high economic and environmental benefits. |
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