磁场对静态纳滤过程的膜通量及CaCO3结晶的影响

使用NF270型和NF型纳滤膜研究了经磁化预处理的钙溶液的静态纳滤过程,并与未经磁化预处理钙溶液的静态纳滤结果相比较.结果表明,当水样中Ca²⁺ 浓度为3.6 mmol/L,硬度等于碱度,初始水样体积为300 mL,体积浓缩倍数为2时,普通溶液的膜通量随滤出液体积呈现单调递减趋势;但处理磁化溶液时,膜通量随滤出液的体积先下降后上升,然后才下降,且平均膜通量比普通溶液的高,表明磁化预处理可以改善纳滤膜的部分操作性能.对膜的扫描电镜分析表明,处理磁化溶液的纳滤膜膜面上生成的晶体相对较多.综合纳滤膜通量和膜面晶体生成量的观察,认为磁场的引入可使膜表面结晶速率增加,致使膜面溶质浓度降低,通量上升,同时因生成的晶体全部留在膜面上,运行一段时间后有效膜面积减少,又导致通量下降.晶体生成量较多的膜,通量却相对较高,这与平常对膜污染的认识相矛盾.通过对结晶过程的理论分析,认为一定程度的结晶对膜操作性能有改善作用.

Influence of Magnetic Field on Membrane Flux and CaCO3 Crystallization in the Unstirred Dead-end NF Process

With reference to a control system operating under same conditions, unstirred dead end NF processes of magnetic pretreated solution containing Ca²⁺ were conducted with NF and NF270 nanofiltration membranes to study the influence of magnetic field on the membrane flux and the crystallization of CaCO₃ on membrane surface. Ca²⁺ in the solution was 3.6 mmol/L, and the hardness was equal to the alkalinity. When experimental solution with initial volume of 300 mL was concentrated 2 times, the flux of membranes treating general solution presents a monotonously falling trend, while the flux of membranes treating magnetic solution decreased firstly and then increased, and then falled again. The average flux in treating magnetic solution was higher than that of the general solution, which indicated that magnetic pretreatment could ameliorate parts of membrane performance. More crystals were formed on membranes treating magnetic solution according to the SEM observation and the chemical analysis. It was assumed that magnetization could speed crystallization on membrane surface, which resulted in concentration falling in the polarization layer and flux increasing accordingly. The formed crystals that may decrease effective membrane areas were all deposited on the membrane surface, so the flux decreased finally. Average flux was higher while more crystals were formed on membranes, which conflicted to the conventional concept of membrane fouling and could not be explained by the serial resistance model, implied the crystallization on membrane surface unnecessarily detered membrane performance.