Process Engineering for Pollution Control and Waste Minimization_8

Một bản tóm tắt các cơ hội cho các công nghệ màng trong điều trị của các dòng quá trình khai thác mỏ và khoáng sản đã được trình bày bởi Awadalla và Kumar (4). Nghiên cứu này chỉ ra một loạt các ứng dụng bao gồm cả mỏ thoát nước axit (AMD), xử lý nước nổi,
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Process Engineering for Pollution Control and Waste Minimization_8 cells in a rectangular reaction tank. A high-voltage electrode coated with poly- ethylene is wound on the perforated baffle plate separating the extraction and stripping cell. When a high-voltage electrostatic field is applied to the reaction tank, the aqueous drops in the organic continuous phase disintegrate into numer- ous smaller droplets under the action of the electrostatic field. This provides a great deal of surface area for separation. The extractant dissolved in the continu- ous organic phase acts as a shuttle to transport metal ions from the extraction cell to the stripping cell. A summary of opportunities for membrane technologies in the treatment of mining and mineral process streams was presented by Awadalla and Kumar (4). This study indicated a variety of applications including acid mine drainage (AMD), treatment of flotation water, copper smelting and refining wastewater, mill wastewater, removal of ammonium and nitrate ions, membranes in the aluminum industry, treatment of groundwater, treatment of uranium wastewater, treatment of dilute gold cyanide solutions, recovery of zinc from pond water, rare earth (RE) concentration, and separation of selenium from barren solution. AMD contains pollutants such as iron, manganese, calcium, magnesium, and sulfate ions. Although lime neutralization is considered the “best available technology economically achievable,” it is no longer considered environmentally acceptable because of the low-level contamination of heavy metals which cannot be removed. Alternatively, almost complete removal of dissolved solids can be achieved by the use of ion exchange, distillation, and reverse osmosis (RO) to produce high-quality water which can be used by municipalities or industry. The use of RO is best implemented as a supplement to neutralization processes. The RO concentrate stream is neutralized and clarified prior to discharge or recycled. Coupled RO/ion exchange can be used when high concentration of calcium sulfate and/or iron fouling is a problem. For the case of water reuse in which completely demineralized water is not essential, a charged ultrafiltration process using negatively charged noncellulosic membranes was utilized. For the case of AMD for coal conversion processes, high-ultrafiltration recovery with high removal of calcium sulfate and iron and good flux are required. Recovery of up to 97% is achievable by introducing an interstage settling step. Commer- cially available charged ultrafiltration membranes by PSAL (millipore type of noncellulosic skin on cellulosic backing) were used in this study. Cost for treatment using UF with interstage settling are $1.33/1000 gal of AMD, including membrane replacement cost, pumping cost, and lime cost. In order to avoid problems with recycling wastewater from flotation mills which contain the breakdown products of collector-frother reagents, the water must be purified before recycling to the mining operation. The traditional method for treatment of flotation water involves lime precipitation, ozonation, adsorption on activated carbon, and biological treatment (4). Biological treatment requires excessive holdup and is dependent on the climate, the presence of toxic heavyCopyright 2002 by Marcel Dekker, Inc. All Rights Reserved. metals, and sensitive control of the microorganisms. Reverse osmosis has been used for the recovery of flotation reagents. Commercial RO membranes have been used to remove 95% of organic carbon, calcium, and magnesium from the flotation feed stream. Scrubber blowdown from a primary copper smelting plant and acid process- ing water from a selenium-tellurium plant have been treated using negatively charged noncellulosic ultrafiltration membranes (4) . Removal of over 85% of As and Se from the acid processing water was made possible when the pH was adjusted to 10 and the solids were settled prior to ultrafiltration. Scrubber blowdown was effectively treated without pH adjustment to a pH of ...