关键词: 电化学, 钛滤膜, 钙硬度, 分离, 结晶

Abstract:

The formation of scale in the industrial circulating cooling water system has caused a series of problems such as decreased heat transfer efficiency and pipeline corrosion. In this study, an electrochemical-microfiltration coupling process was developed for water softening. In this reaction system, Ti/SnO₂-Sb-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 indicated that 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 2μm. The calcium hardness removal efficiency increased from 28% to 86% with increasing current density from 1mA/cm² to 5mA/cm², and then it dropped to 78% with further increasing current density to 10mA/cm². 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 5mL/min to 20mL/min with the energy consumption decreasing from 3.06kWh/kgCaCO₃ to 1.38kWh/kgCaCO₃, which was much less than that in the conventional electrochemical waster softening system. It was found that the fouling was mostly due to the filter cake formation and internal pore blocking. And 78% membrane flux could be recovered after backwash with polarity reversal. The XRD and SEM analysis results showed that the crystal structure of CaCO₃ on titanium filter membrane surface was mainly calcite form. The electrochemical-microfiltration and backwash processes were driven by electron and thus avoided the consumption of detergents, which provided new strategy for water softening in the circulating cooling water.

Key words: electrochemistry, titanium filter membrane, calcium hardness, separation, crystallization