Study On The Production Of Calcium Sulfate Whiskers From Chlor-alkali Salt Mud

Salt mud is a kind of waste residue produced when raw salt is refined in the process of ionic membrane caustic soda production.With the increase of chlor-alkali production capacity year by year,and the production of salt mud also has increased.The composition of salt mud is complex,and the comprehensive utilization is difficult.The problems of environmental pollution and waste of resources are becoming more and more prominent.Calcium sulfate whisker has the advantages of high strength,high toughness,high temperature resistance,acid and alkali resistance,etc.It is a cheap,green inorganic functional additive.Calcium sulfate whiske was prepared from calcium resources in salt mud,which opens up a new field for the application of salt mud in high added value and middle and high grade materials.In this paper,the salt sludge of Tianye chlor-alkali plant in Xinjiang was used as raw material to study its main components.After acid leaching of salt mud,purification and reaction with sodium sulfate,the calcium sulfate whisker was preparaed of by ultrasonic method,and the magnesium salt was recovered from filtrate after preparation of whisker.The technological scheme and parameters of producing calcium sulfate whisker and magnesium ammonium phosphate slow-release fertilizer from salt mud were preliminarily determined.At first,XRF,XRD analysis and moisture content determination of salt mud were carried out.The results showed that the composition of salt mud was CaCO₃ about 21.5%,Mg?OH?₂about 9.18%,SiO₂ about 10.22%and the water content was about 41.62%,NaCl about 8.38%and a little Fe₂O₃,Al₂O₃,etc.Using hydrochloric acid as leaching agent,the optimum technological parameters of calcium and magnesium leaching were obtained,which were as follows:solid/liquid mass ratio 100g/400mL,dissolution temperature about 60?,hydrochloric acid concentration2.0mol/L,stirring rate 400r/min,calcium and magnesium leaching rate 98.23m.By analyzing the impurity ions affecting the growth of calcium sulfate whisker,and by using hydrolysis method,the removal rate of impurities such as Fe³⁺?Al ³⁺and Ni²⁺can reach90.3%,under the condition of which Mg²⁺was retained,the agitation rate was controlled,the reaction temperature was 90?,the stirring rate was 400 r/min,and the solution pH=6H⁷and reaction time 3h.The effect of Ca/Mg ratio on the morphology of calcium sulfate whisker was investigated in the CaCl2 purification solution.It was concluded that the purified solution could be used as raw material for preparation of calcium sulfate whisker directly,without the need to adjust the concentration of magnesium ion.The effects of 95%ethanol and different surfactants on the growth of calcium sulfate whisker were studied.The optimum process of producing calcium sulfate whisker is that the content of ethanol solution is 0.5,the concentration of CaCl₂ is 0.2mol/L,and the concentration of sodium sulfate is equal concentration,ultrasonic,and so on.The optimum technology is:1:95%ethanol/V solution 1:4%CTAB=0.5,the concentration of CaCl₂ is 0.2 mol/L,and purified solution is 0.2 mol/L.The CaSO₄.0.5H₂O whiskers with a long diameter of about 100²20 um,and a smooth surface of about 5¹0 um,were prepared by aging for 1 hour each.The yield of CaSO₄.0.5H₂O whiskers can reach 90.1%.Using calcium sulfate whisker filtrate and ammonium dihydrogen phosphate as raw materials,on the condition of Mg²⁺:NH₄⁺:PO₄^3-1.1:1:1?molar ratio?at room temperature of pH=9,controlling reaction time 1.0 hours,aging 0.5 h,filtering natural air drying,the white powder product,with production rate of 87.6%is yield,in fact,that is magnesium ammonium phosphate sediment,according XRD analysising.