Anion Permeable Membrane
Figure 6. Schematic diagram for electrodialysis.
Posttreatment requirements—To avoid fouling tendencies, all manufacturers recommend periodic reversal of the applied voltage while simultaneously rerouting the feed and concentrate.
Several vendors are currently manufacturing electrodialysis systems for treatment of wastes from gold, chromium, silver, and zinc cyanide plating operations and from nickel plating operations. Other successful electrodialysis applications include recovery of metals from tin and trivalent chromium baths and the recovery of chromic acid and sulfuric acid from spent brass etchants. The latter electrodialysis system was developed at the Bureau of Mines and Is now available from Scientific Control of Chicago, Illinois.17
The Bureau of Mines is conducting research to develop methods for recovery of metal and acid values from spent pickling solutions used in the manufacture of stainless steel, titanium, and other specialty metals. Specific metals in the spent solutions may include iron, chromium, nickel, titanium, and zirconium. Experimentation has been performed in the area of chemical kinetics and electrodialysis. Woven polymeric membranes are being evaluated for this purpose. This work is being collaboratively conducted with the stainless steel industry. Industrial testing has achieved success in minimizing acid (HF/HNQj) consumption, reducing waste generation, and increasing productivity through improved control techniques.
More than 100 electrodialysis systems are now used commercially for the recovery of metals from electroplating wastewaters. At least four manufacturers (Baker Brothers/Systems, Stoughton, Massachusetts; Scientific Control, Inc., Chicago, Illinois; Innova Technology, Clearwater, Florida; and Ionics Inc., Watertown, Massachusetts) are currently manufacturing electrodialysis equipment.
The environmental impacts are limited to those resulting from pretreat-ment and posttreatment. Pretreatment operations generate wastes such as spent filter elements, oil and grease, and sludge from scaling components. Post-treatment requirements are minimal, but they could involve treatment of process streams that accumulate contaminants in a near zero discharge system.
Typical capital costs of electrodialysis systems for treatment of plating rinsewaters range from 130,000 to $45,000. Capital costs for the "Chrome Mapper" system available from Innova Technology range from $9,900 to $30,000, including installation and power supply. This system has been used successfully for the recovery of chromic acid from electroplating wastewaters. Systems are sized according to bath temperature, drag-out concentra tions, number of rinse tanks, concentration of the bath, and the volume of spent solution to be treated per unit time.
Scientific Control sells electrodlalysls units to recover chromic/sul-furic acid brass etchants. Sizes of available units range from units capable of removing 0,05 lb of copper per hour to those that remove 0,5 Ib/h. Capital equipment costs (1986 dollars) for these units range from $24,000 to 180,000. These costs do not include Installation, plumbing, and a ventilation/exhaust system. Additional costs for the exhaust system could range from $5,000 to $15,000, depending on the size required. Membranes will need to be replaced approximately every 9 months, at a replacement cost of 10 to 15 percent of the original equipment costs.
HIGH-TEMPERATURE METALS RECOVERY (HTWR)
Several types of high-temperature metals recovery (HTMR) processes are currently available or under development for the recovery of metals from sludges generated either directly by industrial processes or from the treatment of industrial wastewaters. These HTMR processes may involve plasma-based or high-temperature fluid-wall reactor systems (which use electricity as the energy source} or coal/natural gas-based technologies. The HTHR processes have several potential advantages: 1) maximum volume reduction, which reduces the ultimate disposal requirements of any residual materials; 2) potential for destruction of other toxic organic constituents in the wastes; and 3) the potential for energy recovery through the combustion of waste products. Disadvantages include 1) high capital and operating costs, Z) high maintenance requirements because of high-temperature operations, 3) need for highly skilled and experienced operators, and 4) the potential for adverse environmental impacts, primarily from atmospheric discharges. Because of differences in the design and configuration of HTHR processes, no unique process description is applicable to all HTMR processes. Pretreatment and posttreatment requirements also vary by the type of process.
Figure 7 shows an example of an HTMR process. The process shown, which is being operated by INMETCO in Ellwood City, Pennsylvania, Is capable of recovering metals from wastes containing nickel, chromium, and iron. This process was specifically developed to make use of byproduct wastes produced by stainless steel manufacture (i.e., swarf, flue dust, and mill scale). Other wastes are blended into it at 0 to 20 percent with feed restrictions. The INMETCO process involves pelletiiing, reduction in a rotary-hearth furnace, and smelting in an electric furnace. The IW4ETC0 plant can treat approximately 47,000 tons of wastes per year to produce 25,000 tons of a stainless steel pig containing iron, nickel, and chromium. In the INMETCO process, the wastes received at the plant (slices, slurries, grindlngs, swarfs, dusty metallics) are first pretreated to insure a material of uniform size. The wastes are then blended with coke or coal fines and water, and the mixture is conveyed to a 14-ft disk pelletlzer that produces 3/8- to 1/2 - In.-diameter pellets, strong enough to resist disintegration in subsequent operations .
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