| The brine used in the soda ash industry contains considerable amounts of sulfate ions,which must be removed before using it to make soda.This process is called denitrification in the industry.In Guangdong Nanfang Soda Ash Industrial Co.Ltd.the sulfate concentration of brine is lower than before for the source of brine had changed,and traditional cryogenic denitrification process is costly.To this end,the company is developping the calcium method denitrification process aiming to use distilled ammonia waste to remove sulfate from brine.This method not only does not need purchase extra denitrification agent meanwhile can reduce waste emissions,but also the process is simple,in addition,it need less investment in equipment and low energy consumption.So undoubtedly it is good for energy saving and emission reduction.However,there were some problems existing in the development of the process,that is,calcium sulfate suspended in the system and it is difficult to settle,the process is too long which seriously affecting the production and the by-product salt gypsum is difficult to directly utilization hence it would produce much solid waste.Based on these problems,in this paper,we used polyacrylamide as settlement assistant and optimize the process parameters to ensure the removal of sulfate ions and make the calcium sulfate particles quickly settled.In addition,we studied the preparation process of α-calcium sulfate hemidrate(α-HH)from calcium sulfate dehydrate(DH)which provide the basis for comprehensive utilization of by-product salt gypsum.The main contents and conclusions are as follows:(1)Used polyacrylamide as flocculant and evaluate the effect of sulfate removal and the effect of settlement on the product,and use the small test results to carried out industrial application.The results showed that after used polyacrylamide in calcium method denitrification process the concentration of sulfate in the treated brine was below 3 g/L which meet the requirements of industry production,and the best sedimentation effect was obtained when using anionic polyacrylamide with molecular weight of 12 million,and the flocculant dosage was 20 mg/L,the temperature was 50~60℃,pH was at 4~8,stirring time was 30 min.It was proved the product was calcium sulfate dihydrate,and it has thick and plate-like structure.The addition of polyacrylamide flocculant is helpful to improve the crystal morphology and increase the crystallinity of the product.The above results of laboratory research had been applied to the actual calcium denitrification process in company.The designed brine treatment scale is 30 m3/h,and the continuous operation for several months has achieved good results which can meet the actual production requirements.(2)The α-HH was prepared by normal pressure salt solution method.The reaction conditions and the effects of additives on the reaction process and products were investigated.The results showed that both CaCl2 and NaCl could promote the transformation of DH into α-HH.The higher the concentration of CaCl2 or NaCl,the faster the dehydration rate;when used the mixed solution of 10% CaCl2 and 5% NaCl,the conversion rate of DH was 95.52% after 5 h and the average diameter of the product α-HH was 149.22 μm,the average diameter was 9.07 μm,and the aspect ratio was about 16.45.pH had no obvious effect on crystal morphology.The addition of crystal modification agent can affect the reaction progress or change the morphology of the product in different level.Alum does not change the overall morphology of the crystal,but it can decrease the crystal size.The addition of succinic acid and EDTA could make the morphology of α-HH changed from fibrous form to short column form.Citrate acid and PAM could inhibite the conversion of DH to α-HH seriously when their addition amount reached a certain level.These reaults were attributed to these organic acids and PAM could be adsorbed on the surface of crystals,thus changed the superposition rate of atoms and growth rate of crystal,and finally affected the reaction process and changed the product morphology.(3)α-HH was prepared by microwave-ultrasonic method.Due to the cavitation of microwave vibration and the high energy of microwave,DH can be converted into α-HH at lower boiling point and the dehydration reaction rate is faster compared with conventional heating and stirring.When the reaction system were be heated to 105℃ in 10 minutes and keep the temperature constant then react for 3 h,the conversion rate of DH was 97.06%.(4)Use the by-product salt gypsum as raw materials and distilled ammonia waste as reaction medium to prepare α-HH,the results show that salt gypsum could transformed into α-HH by normal pressure salt solution method just like natural gypsum.When the slurry concentration was 10% or 15%,the salt gypsum could be nearly completely converted into α-HH,the ratio of α-HH in the product was more than 98% after 5 h.The product is mainly α-HH crystal with coarse hexagonal prismatic structure,smooth surface and high crystallinity.And the α-HH crystal has the largest size and the better crystallinity when the slurry concentration was 15% reaction for 5 h.Hence in order to obtain high product quality and production efficiency,it is recommended to use 15% slurry concentration and 5 h reaction time.These results provide the basis for comprehensive utilization of by-product salt gypsum. |
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