Experimental Study On Removing Hard And Silicon From High-salt Wastewater

In the process of zero-discharge treatment of high-salt wastewater,calcium,magnesium ions and silica in wastewater would cause scaling problems in membrane equipment and pipelines.In order to ensure the long-term stable operation of the high-salt wastewater zero-discharge process,the hardness and silica must be effectively removed in the high-salt wastewater pretreatment part.In addition,the current zero-discharge pretreatment process for high-salt wastewater has problems such as long process flow,poor system stability,large floor space,and high operating costs.In view of the above problems,this paper takes the high-salt wastewater of a typical coal chemical plant as the experimental object,explored the optimal treatment conditions for the removal of hardening and silicon from the high-salt wastewater by the chemical dosing coagulation method,and improved and optimized the traditional pretreatment process.process.Besides,an integrated hardening and silicon removal process of dosing coagulation and tubular microfiltration was designed and processed.There are three forms of silica in high-salt wastewater,namely Si_a（monosilicic acid and dipolysilicic acid）,Si_b（oligomeric silicic acid）,and Si_c（high polysilicic acid and inactive silicon）.Combined with previous research results,a method for the determination of silica content and existing forms in specific water quality of high-salt wastewater was established,and the principle and results of qualitative and quantitative experiments were summarized.In this paper,the single factor experiment and orthogonal test of coagulation and de-siliconization were carried out,and the effect of the dosage of chemicals on the removal of calcium and magnesium ions was studied.Magnesium（MgO,MgCl₂）and aluminum（NaAlO₂,PAC）coagulants were used in coagulation and desiliconization experiments,respectively,to study the effects of different coagulants,dosage and reaction pH on the removal of different forms of silica.The main mechanism of silicon removal of different coagulants was then analyzed.Combined with the experiments of de-hardening and desiliconization,the experiments of adding drugs and coagulation at the same time of de-hardening and de-silicon were carried out.The synchronous dosing method and the step-by-step dosing method were used to explore the removal sequence and dosing method of hardness and silica.The experimental results showed that the optimal experimental conditions for the removal of hardness were that the addition amount of NaOH is 450 mg/L,the addition amount of Na₂CO₃ was 1100 mg/L,and the removal rates of calcium and magnesium ions were 91.5%and 86%,respectively.The optimal experimental conditions for silicon removal were that at pH 8,the addition of NaAlO₂ is 600 mg/L,and the removal rates of total silicon,Si_a and Si_c were 90%,92.1%,and 80%,respectively.Simultaneous dosing method with the addition of desiliconizing agents at the same time,the removal rates of calcium and magnesium ions were consistent with the results of the desiliconization experiments,while the removal rates of all silicon,Si_a,and Si_c were quite different from those of the desiliconization experiments,which is only 39.8%,37.26%and40.29%,respectively.In the step-by-step dosing method,silicon dioxide was added first to remove silicon dioxide,and then a hardening agent was added to remove hardness.The removal rates of all silicon,Si_a and Si_c were basically the same as the experimental results of silicon removal in wastewater,and the removal rates were 90.1%,95.2%,84.3%.The removal rates of calcium and magnesium ions were 91.5%and 88.3%,respectively,and the removal rates of magnesium ions were slightly higher than those in the hard removal experiment.Based on the high-salt wastewater dosing and coagulation experiments,the integrated hardening and silicon removal process of chemical dosing coagulation and tubular microfiltration was designed,and small-scale tests and field pilot tests were carried out.In the small test,the removal rates of calcium,magnesium ions and total silicon were all close to 90%,the turbidity of the effluent is less than 1 NTU,all suspended solids（SS）were removed,and the recovery rate of wastewater was close to 100%.The pilot-scale test equipment continued for 24 hours,the wastewater treatment results were basically consistent with those of the small-scale test,and the equipment was operating normally.