Experimental Study On Separation Of Sodium Chloride And Sodium Sulfate From Concentrated Brine Of Coal Chemical Industry

It is difficult to dispose the concentrated brine produced by coal chemical enterprises,as it needs a high processing cost.Disposing concentrated brine in coal chemical industry usually takes the following steps.Firstly,the pretreatment,membrane separation and thermal evaporation were combined.These combined processes can realize the concentration and evaporation crystallization of concentrated brine.Secondly,the water resources in the concentrated brine can be recovered,and miscellaneous salt can be obtained by the ultra concentrated water treated by evaporation crystallization.Finally,the zero discharge of wastewater can also be realized.After the concentrated brine is evaporated and crystallized,the organics and high salt content components are concentrated and solidified.The main components of crystalline solid are sodium chloride,sodium sulfate,sodium nitrate and some organic matters.At present,there is no effective way to treat the crystal salt.Besides,the post-treatment cost of the crystal salt is very high,which can result in the secondary pollution for long term accumulation and easy leaching.At the same time,a large number of sodium chloride and sodium sulfate in the crystal salt cannot be separated as industrial grade salt to realize resource utilization.Only if the sodium chloride and sodium sulfate can be separated in concentrated brine,the resource utilization and solid waste reduction can become a realization.The separated substance can be made into industrial salt,which can completely solve the environmental risk of crystal salt and truly realize zero discharge of concentrated brine in coal chemical industry.The removal of COD and SiO₂ by ultrafiltration membrane and the separation of NaCl and Na₂SO₄ by nanofiltration membrane are mainly studied in this paper.The results are as follows:The operating pressure of the ultrafiltration membrane should be controlled at0.6MPa based on the condition of reasonable operating resistance range and operating time of the ultrafiltration membrane.When the concentration ratio is 20-40times,the membrane flux decreases significantly while the concentrated brine passing through the ultrafiltration membrane.The concentration multiple of ultrafiltration should be below 40 times.The content of COD in ultrafiltration water is less than500mg/L,and the average total removal rate of COD in the solution is greater or equal to 48.37%.The content of SiO₂ produced by ultrafiltration water is less than30mg/L,and the average removal rate is greater or equal 79.90%.To separate sodium sulfate and sodium chloride from concentrated brine by nanofiltration membrane,the best operation condition is at 0.8MPa operation pressure and 6 hours operation time.The content of Na₂SO₄ in the concentrated water of nanofiltration increases gradually with the increasing of time.NaCl content in nanofiltration wate increases with the increasing of time at first,then keeps stable,and lastly begins to decline in the later period.In order to ensure the purity of NaCl and Na₂SO₄,the concentration multiple of nanofiltration should not exceed 6 times.NaCl accounts for more than96%of the total TDS,and Na₂SO₄ accounts for more than 95%of the total TDS after separation.These experimental results show that the separation of concentrated brine from coal chemical industry by ultrafiltration and nanofiltration.The test indices of evaluation are as follows:sodium chloride can reach the superior standard of industrial wet salt in GBT 5462-2015;the purity of sodium sulfate can meet the class III first-class standard of GBT 6009-2014 industrial anhydrous sodium sulfate.The total recovery of sodium chloride?80%,and the combined process can effectively separate sodium chloride and sodium sulfate.