The Improvement Process Of Continuous Production Of Sodium Fluoride

Na2SiF6 is one of the kinds of chemical products in many category,it has attracted much attention,especially in industry,agriculture and construction industry.As raw materials or additives,it can be used as preservatives of leather and wood,building hygroscopic agent and coagulator and agricultural pesticides etc.Sodium fluorosilicate mainly utilizes wet-process phosphoric acid by-product fluosilicic acid as raw materials to produce it.It has been using the batch production process,however,this process has been low production efficiency and high labor intensity.It's only suitable for small-scale and artisanal production.Recently,there have been some sodium fluorosilicate continuous production process and devices,however,this continuous process still using single stage crystallizer(synthetic groove)to produce.It couldn't avoid generating a large number of small crystals and the crystals can't easily to grow up and the products have been settling and throwing dry difficult,have low purity,easy caking on the way of transport,exhaust gas and wastewater treatment difficult,resulting in increasing the production costs.In this paper,aiming at the defects and problems of domestic existing in the large and continuous production of sodium fluorosilicate production process devices.Basic experiments for this thesis:Firstly,the crystallization kinetics of sodium sulfate and sodium silicate were studied in a simple crystal device.When the residence time constant,by changing the concentration of fluosilicic acid and study the concentration of fluorosilicic acid in the crystal into nucleation rate B0 and growth rate G,the influence of crystal slurry suspension density MT,the kinetic equation for B0=3.29 × 10-4G0.63MT0.95.When the concentration of fluosilicic acid constant,by changing the residence time and to study the effect of residence time on the above three parameters,under the condition of the kinetic equation for B0=1.95 × 10-3G1.08MT1.47.Secondly,in a batch crystallizer by orthogonal design and single factor alternate method combination,to study the effect of sodium fluosilicate production process conditions such as the fluosilicic acid and sodium sulfate concentration,excess salt ratio,crystallization reaction temperature,time,stirring speed,whether adding seed crystal,crystal structure and feed location and the influence on the crystallization of the product.The results show that with 11.7%fluorosilicic acid and 24%of sodium sulfate as raw material and in sodium sulfate addition amount is 120%of the stoichiometric amount,300 r/min,the stirring speed,41 degrees of reaction temperature and 60 min of reaction time,raw materials add location for fluorosilicic acid in draft tube bottom,sodium sulfate in the upper part of the guide barrel,circulating mother liquor position between the draft tube and the mold walls in the middle position,obtained under the conditions of sodium fluorosilicate crystal not only purity meet the requirements and large grain size,uniform particle size distribution.In addition,the results also show that the products obtained in DTB crystallizer in average particle diameter and particle size distribution has more advantages and with sodium sulfate instead of sodium chloride to prepare sodium fluorosilicate as raw material,we can obtain the better,for large particle size of sodium fluorosilicate crystal shape,so that it's feasible to use sodium sulfate as raw material for the production of sodium fluorosilicate crystal.Then,based on the above research,in a view of the problems existing in the existed continuous production process.The experimental on the basis of single stage crystallizer increased multiple grain growing tank to simulate the sodium fluosilicate multi-stage continuous production process,explores the optimum conditions,and under these conditions respectively with the sodium sulfate and sodium chloride as raw materials for production of sodium fluorosilicate,purity and particle size of the products obtained were compared.Results show that when the third order reaction crystallization tank stirring speed is set to 6:3:1,and the crystallizer and grain growing tank of insulation based on,keep other process parameters and batch reactive crystallization process parameters are the same,obtained by the experimental design of multistage continuous production process of sodium fluorosilicate crystals in all aspects are good in the optimum conditions for single crystal obtained products and in a continuous production process,with sodium sulfate instead of sodium chloride as the raw material for preparing sodium fluorosilicate is feasible.Finally,in continuous test based on the proposed multistage reaction crystallization of sodium fluorosilicate continuous production process of amplification scheme,the theory can solve the fundamental problem of the existing processes for continuous production of sodium fluorosilicate,is widely used in domestic large,single stage reaction crystallizer,continuous technical renovation and application of sodium fluorosilicate production device to provide guidance and reference.