Membrane-based Electrochemical Precipitation For Recirculating Cooling Water Softening

Recirculating cooling water is widely used in different industrial fields,such as steel manufacturing,metallurgy,power generation,chemical industry.In order to prevent scaling,it is necessary to carry out effective treatment for this kind of water.At present,available recirculating cooling water treatment methods include scale inhibitor addition,magnetization,chemical precipitation,reverse osmosis,electrodialysis,capacitive deionization and electrochemical precipitation.Among these treatment methods,electrochemical precipitation recently has attracted wide attention because of its environmental compatibility,high treatment efficiency,wide application range and convenient operation.However,electrochemical precipitation still has many problems,such as poor electrode performance,high device cost and low removal efficiency of Cl^-.This thesis systematically developed a new electrochemical precipitation technology with good stability,low cost and high efficiency for recirculating cooling water treatment.Major research works include three sections:Ti/IrO₂-RhO_x-Ta₂O₅electrode investigation,membrane-based electrochemical precipitation technology treatment performance,membrane-based electrochemical precipitation and desalination technology treatment performance.A novel electrode,Ti/IrO₂-RhO_x-Ta₂O₅,was successfully prepared using thermal decomposition.The electrode had long lifetime under the condition of polarity reversal.Physicochemical characterization showed that Rh doping could significantly optimize the microstructure and cyclic voltammetry characteristics of electrodes,and improve the uniformity of oxide coating.In addition,Rh³⁺and Rh⁴⁺could be transformed into each other during polarity reversal,which could avoid the oxide coating detachment and greatly increase the service lifetime of the electrode.The service lifetime of Ti/IrO₂-RhO_x-Ta₂O₅ was about 4 times of that of Ti/IrO₂-Ta₂O₅,and it could be over 11 years at a current density of 100 A/m² and a polarity reversal frequency of 12 h^-1.A novel process,membrane-based electrochemical precipitation（MEP）,was developed for water softening.Compared with traditional electrochemical precipitation,the MEP used cheap ion exchange membranes to replace most expensive electrodes,which not only improved softening performance,but also reduced device cost.Heterogeneous anion exchange membrane was the most suitable ion exchange membrane for the MEP due to its excellent softening ability and low price.Polarity reversal could detach the scale deposited on the surfaces of cathode and membranes effectively.After 10 min polarity reversal,the voltage decreased from 42.5 V to 27.3 V and plenty of the membrane surface was exposed.88.4%of the scale was detached successfully by polarity reversal.Long term operational experiment indicated that the MEP had good operational stability and could maintain good softening performance continuously and stably.The average precipitation rate was 64-85 g/h/m²,which was1.2-2.8 times higher than those of conventional electrochemical precipitation processes.The energy consumption was only 9-12 k Wh/kg Ca CO₃.The price of the used heterogeneous anion exchange membranes was 33$/m²,which was only 60.0%and1.1%of the price of the stainless steel cathode and the corresponding DSA anode,respectively.In order to overcome the problem that the MEP can not remove Cl^-and other impurities effectively,the membrane-based electrochemical precipitation and desalination（MEPD）was successfully proposed.The experimental results showed that the MEPD had good softening performance,desalination performance and operation stability.The average removal rates of Ca²⁺and Mg²⁺were 23.3%and 29.5%,respectively.23.8%of Ca²⁺and 22.4%of Mg²⁺were removed by electromigration,whereas 76.2%of Ca²⁺and 77.6%of Mg²⁺were removed by precipitation reaction induced by water splitting.The average removal rates of Na⁺and Cl^-were 12.0%and11.6%,respectively.In addition,polarity reversal and double anodes and cathodes switching both could effectively detach the scale accumulated on the membrane surfaces during the operation of the MEPD and restore the softening and desalination performances of the device,whereas the latter could prolong the service lifetime of the anodes.The service lifetime of used DSA in double anodes and cathodes switching was6 times as longer as that in polarity reversal.Long term operational experiment and accelerated lifetime tests of membranes indicated that the MEPD had good operational stability,and could maintain good softening and desalination performances continuously and stably.