Abstract
Mining operations generate effluents containing pollutants such as ammonia, nitrite and nitrate as a result of blasting operations. Cyanide compounds such as free cyanide, cyanate and thiocyanate are also present when cyanide is used in the gold recovery process. In most cases, mine effluent stored in the ponds eventually needs to be discharged to the environment; however, the levels of contaminants often exceed the discharge limits hence cannot be discharged without treatment. Several treatment solutions exist for the removal of nitrogen compounds and cyanide. Reverse osmosis is often perceived as a good solution as it produces an effluent of high quality. However, reverse osmosis also produces a brine which is recycled to the ponds, gradually increasing the total dissolved solids (TDS) in the ponds over time. Biological treatment offers an alternative to reverse osmosis with the added benefit that nitrogen compounds are fully converted to innocuous nitrogen gas, which is released to the atmosphere, thereby offering a more sustainable treatment solution. Moving Bed Biofilm Reactors (MBBR) have been used successfully at several mines. In Quebec, a two stage MBBR was installed to remove OCN, SCN and NH4-N from the effluent prior to discharge. The MBBR plant has been in operation for 4 years; operating data will be presented to show that a fully compliant non-toxic effluent is discharged under a wide range of operating conditions. In Ghana, pilot trials were conducted at a gold mine to demonstrate complete removal of nitrogen compounds including CN, NH4-N, NO2-N and NO3-N using a four- stage MBBR system. Results from both systems are presented.
Highlights
Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.The McArthur-Forrest process in which cyanide is used as a leaching agent for gold is the most frequently used process in the gold mining industry
Foltested organisms die) was observed for either the Oncorhynchus mykiss or lowing commissioning of the Moving Bed Biofilm Reactors (MBBR), the ammonia, cyanate, thiocyanate and nitrite concentrations in the final effluent remain below values which can be considered lethal for both organisms, and the recorded mortality values are consistently below 50%
Following commissioning of the MBBR, the ammonia, cyanate, thiocyanate and nitrite concentrations in the final effluent remain below values which can be considered lethal for both organisms, and the recorded mortality values are consistently below 50%
Summary
Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.The McArthur-Forrest process in which cyanide is used as a leaching agent for gold is the most frequently used process in the gold mining industry. Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations. The chemical reaction which takes place follows the equation below: Licensee MDPI, Basel, Switzerland. Effluents from this process are laden with cyanide, which is highly toxic to wildlife [1]. The most common detoxification process used in the industry is the INCO Sulfur Dioxide/Air process. This process uses SO2 in the presence of air and of a copper catalyst to oxidize the cyanides to cyanates [2]: conditions of the Creative Commons
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