Metal Finishing Guide Book


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which the salt slurry can settle and be discharged. The supernatant liquor can be returned to the feed circuit where it will mix with the incoming feed for reprocessing through the evaporator. Reverse Osmosis After evaporation, reverse osmosis (RO) has the longest operating history. Most commercial recovery installations have been on nickel plating operations. On the positive side RO is a relatively mature technology and uses considerably less energy than evaporation for the same rinsewater feed rate. A typical recovery scheme is given in Figure 4. On the negative side, the degree of concentration of the separated bath by RO is limited. If maintaining appropriate permeate quality [10-100 ppm total dissolved solids (TDS)], the practical maximum concentration of the reject (or concentrate) is 10,000 ppm (1.4 oz/gal) TDS. If permeate quality is not an issue, then 50,000 to 80,000 ppm (6.7-10.7 oz/gal) TDS reject concentration can be achieved. In many cases, if the recovered solution is returned directly to the plating bath, there may not be sufficient natural water evaporation from the bath to accommodate the volume of recovered RO concentrate. Similar to evaporation, RO returns essentially all of the undesirable impurities. RO has gained favor in recent years as a pretreatment for incoming process water, which has high TDS, and in some cases, for clean up of contaminated process water for recycle to the process. RO is a pressure-driven membrane process. The driving force of this process, the hydrostatic pressure gradient, is the difference in hydrostatic pressure between two liquid phases separated by a membrane. In reverse osmosis, particulates, macromolecules, and low molecular mass compounds, such as salts and sugars, are separated from a solvent, usually water. This is accomplished by applying a hydrostatic pressure greater than the osmotic pressure of the feed solution. The osmotic pressure of a particular feed solution varies directly with the concentration of the solution. In typical applications feed solution have a significant osmotic pressure, which must be overcome by the hydrostatic pressure applied as the driving force. This pressure requirement limits the practical application of this technology. The transmembrane flux (permeate flow) is a function of hydrodynamic permeability and the net pressure difference—the hydrostatic pressure difference between feed and filtrate solutions minus the difference in osmotic pressure between these solutions. The osmotic pressure of a solution containing low molecular mass solutes can be rather high, even at relatively low solution concentrations. In practice, it is practical to use RO to separate water (solvent) from all other substances of a solution in order to concentrate the solution and/or to generate or recover clean water for process reuse. The applied pressure is generally between 200 and 700 psig. In some cases, such as advanced reverse osmosis and high-pressure applications, the pressure may be as high as 1,000 to 2,000 psig. Depending on both the characteristics of the dissolved constituents and on the practical operation of the equipment, the dissolved constituents are rejected differently. This phenomenon is called the membrane rejection rate. The fraction of nonrejected substances is called leakage. The leakage of the various salts is dependent on the following parameters: size of dissolved molecules, ion radius electrical load of the ions, and interacting forces between ions and solvents. The rejection of organic substances is mainly dependent on the molecular weight and size of the molecules. 606

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