Reverse Osmosis Recycle of Electroplating Chemicals at pH Extremes

Long-term reverse osmosis tests were conducted with electroplating wastes on a new membrane referred to as NS-100. This membrane consists of a polyurea layer, formed by the reaction of tolylene diisocyanate with polyethylenimine, deposited on a porous polysulfone support film. The membranes were tested as liners within 0.5411. diameter fiber glass tubes. A total of 2360 hr of continuous reverse osmosis operation was achieved, 1220 hr onpH 1.2 acid copper rinse water and 1 140 hr on pH 12.8 alkaline zinc cyanide rinse water. The membranes exhibited remarkable chemical stability during exposure to these two p H extremes. Excellent rejections of copper, zinc,and cyanide ions were demonstrated throughout the length of the study, as well as good to excellent organics rejections. OLLUTION ABATEMENT has been of acute concern to the metal P finishing industry in recent times. As more stringent regulations are enforced in this country, more effective methods of treatment must be employed in order to limit the discharge of pollutants such as copper, zinc and cyanide to their prescribed levels. Problems of pollution control for the metal finishing industry are compounded by the necessity of having to treat voluminous amounts of rather dilute plating solution rinse waters. If discharged into the environment without treatment, these rinse waters can pollute our natural resources, inhibit or destroy biological activities in the natural environment and in biological sewage treatment processes, and adversely affect materials of construction. Specific examples of detrimental elfects include the toxicity of heavy metals and cyanides to various forms of aquatic life’, the deleterious effect of copper and chromium on biological sewage treatment processes (because of their toxicity to the microflora)*, and the corrosive effects of acids and bases on sewer lines and metal and concrete ~tructures~’~. Several methods presently exist whereby waste waters containing cyanide and metal ions can be treated for clean-up. Many of these techniques are aimed toward the destruction and/or removal of the contaminating species from the water. This task is often accomplished by the addition of chemicals to t h c effluent stream to convert the undesirable constituents to elrher a less harmful state or a state whereby they can be effectively removed. Although these techniques are effective in improving the quality of the water effluent, they can introduce solid waste disposal problems. Such is the case in the Precipitation of potentially harmful and difficult-to-handle metal hydroxide sludges. Reverse osmosis is an attractive alternative to existing ‘echniques in the treatment of metal finishing wastewaters. It is Particularly appealing to reclaim valuable chemicals from the Process stream as well as purifying water for recycling purposes. In theory, all waste discharge would be entirely eliminated. The *North Star Division. Midwest Research Institute, Minneapolis, Minn. JUNE, 1976 savings realized in reduced water consumption and recovered chemicals can be credited against capital and operating costs for the treatment systems. Reverse osmosis can be used by itself or it can supplement other water purification methods. It can be used, for example, to treat water from a continuous cyanide destruction process for recycling back to the plant operations, or it can increase the metal ion concentration prior to an ion exchange treatment process. Several researchers5-’’ have examined the technical and economic feasibility of treating various wastewater streams from metal finishing operations by reverse osmosis. Computations, based on laboratory test results, have shown this process to be economically viable for treatment of nickel plating streams6.8,i0. Obviously, the degree to which reverse osmosis can be adapted to a recycling process in a plating operation must be determined individually, on a case-by-case basis. The objective of this research effort was to determine the practicality of the NS-100 membrane for treating metal finishing wastewaters with reverse osmosis. The focus of this program was two-fold: I ) to fabricate high performance tubular NS-100 membranes in a consistent and reproducible manner