Hyperfiltration-Induced Precipitation of Sodium Chloride
In a previous WRRI-funded project (Whitworth and DeRosa 1997), heavy metals including copper, cobalt, and lead, were successfully precipitated from initially undersaturated chloride solutions forced through clay membranes (hyperfiltration). Those metal chlorides are quite soluble, but not as soluble as sodium chloride, a common constituent of almost all waters. However, theoretical calculations suggested that it is not only possible to precipitate sodium chloride from an initially undersaturated solution passing through a clay membrane, but that it can be done at fluid pressures of less than 400 psi-pressures significantly less than those used in most reverse osmosis systems. Therefore, we tested the concept that clay membranes can concentrate and precipitate NaCl in a series of hyperfiltration experiments. Some of the experiments resulted in formation of sodium chloride crystals on the surface of the membrane when undersaturated NaCl solutions were forced through the membranes. We used both bentonite and kaolinite clay membranes in these experiments. We discovered in the early experiments, that ethanol, which is commonly used to displace water, forms an azeotrope when mixed with water and this results in NaCl precipitation when the NaCl concentrations are high enough. To avoid this problem, we displaced the water in the experimental cells and membranes with cooking oil before examining the clay membranes for the presence of NaCl crystals. Consequently, we did observe NaCl precipitation when undersaturated solutions were passed through both kaolinite and bentonite membranes. Thus, the initial concept appears to be valid. Further work will be required to develop this into a commercial application.