Electrochemical-Microfiltration Coupling Process For The Efficient Crystal Separation Of Calcium Hardness In Circulating Cooling Water

The circulating cooling water system was widely used in various industries,such as petroleum and chemical industry.As the concentration of circulating water increases after flowing through the heat exchanger surface,the concentration of scale ions formed in the system also increases.The deposition of scale on the surface of the heat exchanger would cause problems such as reduced heat exchange efficiency and pipeline corrosion.Therefore,effectively controlling the hardness ion content in the water in the circulating water system was a key measure to ensure the safe and stable operation of the system.As an“environmentally friendly descaling technology",electrochemical method can not only effectively reduce the hardness concentration of water,but also prevent pipeline corrosion.In this study,an electrochemical-microfiltration coupling process was developed for water softening.In this reaction system,Ti/SnO2-Sb2O5-RuO2-IrO2 titanium filter membrane was used as the cathode for OH" production by H2O electrolysis and therefore created a basic environment at its surface,where CaCO3 would become highly supersaturated.Meanwhile,this titanium filter membrane served as a microfiltration unit to effectively separate CaCO3 particles.Polarity reversal strategy was applied in the membrane backwash phase.At this time the titanium filter membrane was used as the anode for in-situ generation of H+by H2O electrolysis,which could dissolve the scale attached to the membrane surface and in the membrane pores.The experimental results were as followed:the smaller the membrane pore size was,the more calcium hardness was removed.Specifically,the calcium hardness removal efficiency could reach 79%when membrane pore diameter was 2 μm.The presence of the PTFE microfiltration membrane in the electrochemical device can effectively slow down the diffusion rate of H+generated in the anode to the cathode chamber.The calcium hardness removal efficiency increased from 28%to 86%with increasing current density from 1 mA·cm-2 to 5 mA·cm-2,then it dropped to 78%with further increasing current density to 10 mA cm-2.In addition,increasing the alkalinity facilitated the removal of calcium hardness.The calcium hardness removal efficiency rose from 53%to 83%when elevating the[HCO3-]/[Ca2+]molar ratio from 0.7:1 to 1.4:1.In contrast,the calcium hardness removal efficiency reduced from 84%to 46%as the flow rate increased from 5 mL·min-1 to 20 mL·min-1 with the energy consumption decreasing from 3.06 kWh/kgCaCO3 to 1.38 kWh/kgCaCO3.The XRD and SEM analysis results indicate that crystal structure of CaCO3 on titanium filter membrane surface was mainly calcite form.It was found that the fouling is mostly due to the filter cake formation and internal pore blocking.And the membrane flux could be recovered after backwash with polarity reversal.The presence of various components in the complex water body had a certain influence on the descaling performance of the electrochemical-microfiltration coupled process.The removal efficiency of calcium hardness hardly changed with the continuous increased of conductivity.The presence of Mg2+would significantly inhibit the removal of hardness.When the concentration of Mg2+rose from 100 mg·L-1 to 400 mg·L-1,the total hardness removal efficiency decreased from 73%to 63%,and the calcium hardness removal efficiency dropped from 57%to 48%.The presence of SiO32-in the solution had almost no effect on the removal of calcium hardness.Increasing the content of scale inhibitor would increase the efficiency of scale inhibition,but at the same time it would reduce the removal efficiency of calcium hardness.When there was an EDTMPA scale inhibitor with a phosphorus concentration of 5 mg·L-1 in the solution,the scale inhibition efficiency and the calcium hardness removal efficiency were 43%and 46%,respectively.The electrochemical reaction system can remove about 15%of Mg2+,but it had little effect on the removal of SiO32-and phosphorus.The electrochemical-microfiltration was driven by electron and it can achieve a high calcium hardness removal rate with low energy consumption.It was a green and efficient water treatment technology.At the same time,the titanium filter membrane can be used as the anode,and the anode backwash technology was used to replace the traditional membrane cleaning agent to clean the scale on the membrane surface.This technology provides a new idea for the removal of hardness ions in the circulating water system.