Research On Concentrating Fluosilicic Acid Solution By Vacuum Falling Film Evaporation

The concentration of fluosilicic acid from phosphate fertilizer plant as a by-product of phosphate fertilizer is only 10-20%,so the transportation cost was very high and low concentration of fluosilicic acid can't meet requirement of high concentration for producing anhydrous hydrogen fluoride.Traditional method of concentrating fluosilicic acid is to decompose fluosilicic acid by concentrated sulfuric acid under the condition of high pressure and high temperature and then absorb the decomposed gas by water to form fluosilicic acid of high concentration.However,the cost of this method is very high.In the paper,the fluosilicic acid was concentrated by vacuum falling film evaporation.And this method has advantages of simple process,low cost of investment as well as high efficiency.In a word,the research is innovative to some extent.Firstly,surface tension and viscosity of industrial fluosilicic acid were determined in the paper.The results showed that surface tension of fluosilicic acid increasing with the increase of the concentration of fluosilicic acid and the trend was quickly before it is slow.Also,viscosity of fluosilicic acid had a linear increase with the increase of the concentration of fluosilicic acid.Secondly,fluosilicic acid with concentration of 17% was used as the raw material to investigate the influence of flow,temperature,vacuum degree/absolute pressure and concentrating time on condense efficiency.The optimal flux of 1300 ml/min and the optimum temperature range of about 53 ? were determined.Also,we found that the order of the factors which influence fluosilicic acid condense was concentrating time > temperature > pressure within high vacuum(100-150 hpa pabs)range.Within low vacuum(150-200 hpa pabs)range,the order of the factors which influence fluosilicic acid condense was that concentrating time ? pressure > temperature.Within the lower vacuum,it can't meet requirements of condensing fluosilicic acid.Finally,the optimum process conditions of concentrating fluosilicic acid of 17% by vacuum falling film evaporator were researched: one was 1300 ml/min-100 hpa-54 ?,two was 1300 ml/min-125 hpa-54 ? and three was 1300 ml/min-150 hpa-53 ?.Besides,condense efficiency of fluosilicic acid which have different concentrations was investigated under the optimum process conditions.Results showed that condense efficiency of process two was better when fluosilicic acid of 20% as raw material was concentrated to fluosilicic acid of 30%,and condense efficiency of process one was better when fluosilicic acid of 10% as raw material is concentrated to fluosilicic acid of 25%.At the same time,the process time of concentrating fluosilicic acid which had a volume of 1000 ml and concentration of 17%,20.19% and 10.31% respectively was determined by the experiment.By comprehensive comparison,process two of 1300 ml/min-125 hpa-54 ? is most appropriate.