Study On Synthesis Of Magnesium Oxysulfate Whiskers From Bittern And Salting Out Process Of Mother Liquor

Domestic bittern resources are mainly used to produce potassium chloride,potassium sulfate,bromine and magnesium chloride,but there are no high value-added magnesium salt products.Due to the low recovery rate of bittern resources in the process of bittern processing,the comprehensive utilization rate of bittern is less than 20%,and the unused bittern is discharged into the sea and pollutes the environment.Therefore,it is urgent to develop new processes to improve the utilization of bittern resources.In this paper,based on the comprehensive utilization technology of bitter brine using magnesium resources,the synthesis of high value-added magnesium oxysulfate whiskers and separation process of subsequent mother liquor were studied.Magnesium oxysulfate whiskers[5Mg(OH)₂·Mg SO₄·3H₂O,MOS]were synthesized by hydrothermal reaction of bittern as raw material and Na OH as precipitant.The effects of raw material ratio,reaction temperature,concentration of liquid alkali and reaction time on liquid phase composition and product morphology were investigated.The products were characterized by XRD and SEM.The optimum conditions were determined as follows:the raw material molar ratio of Mg²⁺to OH^-was 1:1.5,the concentration of liquid alkali was 4 mol/L,the reaction temperature was 200?and the reaction time was 7 h.Under this condition,the length of MOS whisker is about 60?80?m,the aspect ratio is about 200,and the magnesium ion conversion rate was 81.57%.The crystallization kinetics equation of MOS whiskers was established by experimental data simulation.The results showed that the hydrothermal synthesis of MOS whiskers was controlled by crystal growth.The demagnesium mother liquor was concentrated by evaporation.The phase diagram of the five-dimensional metastable system of Na⁺,K⁺,Mg²⁺//Cl^-,SO₄^2--H₂O at 25?was used to predict the evaporation process of mother liquor.The results showed that the actual evaporation process was basically consistent with the predicted salt precipitation path.When the mother liquor density was less than 1.2607 g/cm³,it was the main precipitation stage of Na Cl.When the density was higher than this value,picromerite occured.According to the salting-out laws of mother liquor,a new process for preparing sodium chloride,picromerite and by-product bischofite were designed.In this case,the comprehensive utilization and development of bitter resources can be realized.The remaining magnesium ions in the mother liquor can be converted to basic magnesium carbonate or magnesium hydroxide products to simplify the subsequent separation of the mother liquor.Therefore,the conditions for preparing basic magnesium carbonate by reaction of mother liquor with Na₂CO₃,and the conditions for preparing magnesium hydroxide by reaction of mother liquor with Na OH were investigated.The experimental results showed that the basic magnesium carbonate product can be obtained by reacting with Na₂CO₃,but the magnesium ions in the mother liquor can not be completely converted into the product,HCO₃^-and CO₃^2-were introduced.The Mg²⁺in the mother liquid was completely converted into magnesium hydroxide crystals with the theoretical molar ratio of Mg²⁺to OH^-of 1:2.The magnesium ion yield was 100%.The composition of the mother liquor was changed from pental system of Na⁺,K⁺,Mg²⁺//Cl^-,SO₄^2--H₂O to quaternary system of Na⁺,K⁺//Cl^-,SO₄^2--H₂O after making Mg(OH)₂.According to the phase diagrams of Na⁺,K⁺//Cl^-,SO₄^2--H₂O system at 0?and 25?,Na Cl-KCl-H₂O ternary system at 25?and 100?,Na Cl,Na₂SO₄?10H₂O and KCl were separated from the remaining mother liquor by evaporation concentration crystallization and freezing crystallization.