Research On The Basis And Crystallization Technology Of Separating Salt From High Salt Wastewater Of Coal Chemical Industry

The high water consumption and high pollution of coal chemical industry has always been the bottleneck problem in the development of coal chemical industry,with the increasing attention to environmental protection and waste water recovery in the global scope,the treatment of high salt waste water of coal chemical industry has attracted more and more attention,and has become a research hotspot.In this paper,the high salt wastewater from a coal chemical plant in ningxia was studied.Firstly,the crystallization process of sodium sulfate and sodium chloride was studied on the basis of the determination of basic thermodynamic data,then carry out experiments and design small tests.However,chemical oxygen demand(COD)can not only affect the thermodynamic data,but also affect the quality of crystalline salt and reduce the product purity and particle size.Three organic compounds,potassium acid phthalate,phenol and alcohols were selected in this paper to explore the effect of COD on the thermodynamic data and crystallization of sodium sulfate and sodium chloride.Research shows that the solubility of Na2SO4 and NaCl are affected in the solution containing potassium acid phthalate,when COD value increased from 528.26 mg·L-1 to 5267.6 mg·L-1,the solubility of Na2SO4 mass fraction increased from 32.70%to 34.74%,the solubility of NaCl mass fraction slightly down from 26.36%to 26.18%,the density and viscosity value of saturated solution are gradually reduce,density values of Na2SO4 saturated solution from 1.3286 g·cm-3 to 1.3261 g·cm-3,the viscosity value decreased from 2.7600 mPa·s to 2.6601 mPa-s and the pH value gradually decreased with the increase of COD value.When COD value was 528.26mg·L-1,the pH value of NaCl saturated solution was 3.274,Na2SO4 saturated solution was 4.125,and when COD value was 5267.6mg·L-1,the pH value of NaCl saturated solution decreased to 2.974,and the pH value of Na2SO4 saturated solution decreased to 3.606.Sodium sulfate,sodium chloride-water system phase equilibrium data are determinated in solution containing phenol or alcohols,COD values were 2628.5 mg·L-1 and 2625 mg·L-1,comparision with COD value of 2678.8 mg·L-1 containing potassium acid phthalate in the solution,found the solubility curve almost overlap,the density and viscosity curves of the saturated solution have the same trend.Secondly,the effect and mechanism of organic compounds on crystallization of Na2SO4 and NaCl were investigated.By analyzing the crystal shape and particle size,found that organic potassiumacid phthalate stimulates the growth of the crystal,the crystal size is large and uniform,average crystal size of Na2SO4 and NaCl are 1240?m and 660?m,respectively,when there is no organic matter in the solution,average crystal size of Na2SO4 and NaCl are 654?mand 449 ?m,respectively,Na2SO4 and NaCl crystal grain size increase were 89.60%and 46.99%,respectively.Organic phenol and alcohols will inhibit the growth of crystals.In the solution containing phenol,the average particle size of Na2SO4 and NaCl were 524 ?m and 447 ?m,respectively.In the solution containing alcohols,the average particle size of Na2SO4 and NaCl were 495 ?m and 287 ?m,respectively,both smaller than that in water.Na2SO4 and NaCl crystal system are triclinic crystal system and orthogonal crystal system,respectively.Finally,the phase diagram analysis was carried out to explore the process of crystallized Na2SO4 and NaCl,and FBRM technology was used to guide the optimization of the process and design the pilot process.The technological process is as follows:firstly,evaporation and concentration of high-salt wastewater,second,isothermal evaporation crystallization of sodium sulfate at 343.15 K,third,mother liquor is cooled to 273.15 K to obtain Na2SO4·1OH2O,finally,the remaining mother liquor is isothermal evaporation again at 343.15 K to obtain sodium chloride products,only a small amount of slag salt is produced.The theoretical yield of sodium sulfate sand odium chloride reached 96%and 58%respectively,and the discharge of slag salt was reduced by 84%.However,due to the loss in the experiment,the actual yield was lower than the theoretical yield.Sodium sulfate products comply with GB/T 6009-2014 in II class grade,purity of 98%or higher,sodium chloride products comply with first class standard for refined industrial salt,the purity of 98.5%or higher.The purity of Na2SO4 extracted from wastewater was greater than 80%,and the average particle size was 68.5 ?m.