Gold or Silver Mining and Metal Finishing industries often complain about the high concentration of cyanide in their effluent. Metal finishing industry facilities employ a broad range of processes in order to form and shape the raw materials, to alter the appearance and properties of the material surface, and to construct the products. Metals and alloys in the form of raw stock are used as starting materials to product a wide range of metal components in the industry.
During the unit operations, plating baths, cleaning baths, rinse baths and other baths are periodically replenished until they are exhausted or spent. Although, low amount wastewaters are generated from these dumps, they are main wastewater sources of the metal finishing industry facilities and create the greatest part of pollution load. Hence, they play a determining role on the selection of wastewater treatment system units and their design parameters. The work pieces are rinsed off to remove the fluid or solid films deposited on their surfaces during the unit operations. A large amount of wastewater with low pollutant concentrations occurs from the rinsing steps. Owing to high volume, pollution loads originating from rinses may become important. The total volume of rinsing wastewaters generally determines the wastewater treatment system capacity of the facilities.
Due to a wide range of process activities and the great variety of chemicals and materials used in the unit operations, there is a long list of pollutants existing in the metal finishing industry wastewaters. The list covers metals, cyanide, organic wastes, total suspended solids, and other inorganic pollutants such as sulphate, chloride, phosphates, fluoride and ammonia. Similar type of wastewater characteristics are found in Gold & Silver Mining wherein cyanide bearing tailings (acidic in nature) have to be treated and neutralized before discharge.
Cyanide is a well-known hazardous compound but is commonly used as a complexing agent for cadmium and precious metals plating and for other solutions such as copper and zinc baths. In the form of strongly complexed metal such as nickel cyanide complex species, cyanide creates several treatment problems. As cyanide concentrations may reach several thousand mg/L in stream segregated flows of the metal finishing industry, this pollutant is regulated by EC and US EPA.
Organic wastes resulting from the metal finishing industry operations vary in a wide spectrum and are principally classified into two main groups: toxic organics, and oil and grease. The origins of toxic organics are complicated. Hence, for ease of evaluation, they can be sorted out into three groups: solvents; complexing agents; and surfactants. Wastewaters with high cyanide concentrations commonly originate in electroplating and heat treatment operations and also mine tailings from Gold and Silver Mines. Many other unit operation, contribute to
cyanide wastes. Cyanide concentrations may reach up to 500 mg/L. The flows containing cyanide are subjected to stream segregation for treatment before being combined with other wastewaters. Recently, ozonation is becoming more and more common application for the oxidation of cyanide, particularly in European countries. Free cyanide is effectively oxidised by ozonation. This method has also proven to be a successful oxidation process in the treatment of some metal cyanide complexes such as zinc, cadmium, silver, copper and nickel cyanides, but ferri and cobalt cyanide complexes are resistant to ozonation. Ozone oxidises cyanide to cyanate under alkaline condition.
Optimum ozone dose has been reported as 1.8 g to 2.2 g ozone per gram of cyanide for the partial oxidation of cyanide to cyanate by US EPA (1983) and Barriga-Ordonez et al. (2006), and 4.6 g to 5.0 g per gram of cyanide for complete oxidation based on excess ozone application by US EPA (1983). Under the optimised operation conditions of ozonation application, cyanide can be oxidized to below detection limit (0.2 μg/L) in electroplating wastewaters and mine tailings.