Mining activities in Peru are subject to strict environmental requirements in terms of Maximum Permissible Limits (LMP). Currently, the principal technology used by the mining industry in Peru for effluent treatment involves the use of lime to cause precipitation of metal hydroxides by modification of pH. However, this method has deficiencies that include the necessity to have access to a large supply of lime, substantial generation of sludge that must be disposed, of, low process thermodynamic efficiency, and inability to achieve the levels of LMP required by Peruvian law.
ARCADIS has developed new technologies for abatement of heavy metals concentrations (Fe2+, Fe3+, Pb2+, Cu2+, Zn2+, MoO42-, AsO33-, AsO43- and others), as well as well as removal of ultrafine solids and soluble compounds (SO42-, Cl-, Ca2+, and others) that may be present in mining effluent. Included in these technologies are two of interest: Recirculation and Source Reduc-
tion through Process Optimization, and Precipitation with Sulphides followed by Dissolved Air Flotation (DAF). ARCADIS believes these technologies—already proven at operations in Chile—would be successful at Peruvian operations as well.
The first technology is designed to optimize the water management process throughout the circuit in order to decrease the volume of effluent to be treated, which helps to reduce the size of the treatment plant and improves effluent quality control. This can be accomplished by preparation of a detailed model of the entire process, describing mass and ionic flow behavior using a tool developed by ARCADIS in Matlab-Simulink, a software environment for multidomain simulation and model-based design for dynamic and embedded systems. This tool can model, with proven precision, all system units in steady state; test changes and modifications such as variations in water quality, flow separation or union, as well as changes in dosage and replacement of reagents; simulate the effect of recirculation or reuse in water quality; and can even be used to estimate total opex and capex of the alternative process.
This approach has been successfully applied at several mining projects. One project called for study of water management at a copper mine to abate dissolved sulphate in the effluent from the concentrate pipeline. The technology was demonstrated to be effective for sulphate reduction (from 2,000 to 300 mg/l) by incorporating pulp washing to improve water quality and redistribute water flow without a need to treat the effluent. This allowed a reduction in capex for the treatment plant, and improved water quality by generating an effluent with low concentrations of sulphate and dissolved solids, less conductivity and reduced hardness.
In the DAF approach, effluent is initially treated to achieve precipitation-coagulation of metallic sulphides. This is possible due to the high reactivity of used sulphides (reagents) and the low solubility of the formed solids (clots) in a wide pH range. Once the clots are formed, a second stage (Dissolved Air Flotation) of solid-liquid separation uses flotation by micro bubbles (smaller than those used in flotation of mineral concentrates) generated by a process involving compression-decompression of water. The main advantages of this technology in comparison with precipitation with lime are that it can provide lower contaminant values in the final effluent and less generation of sludge—but that sludge also can be sold because of its high metal content. This method also requires less residence time in the reactor, a smaller area required for installation and fewer elements to be controlled. Sludge generated by conventional lime addition produces highly soluble hydroxides, creating an additional problem for its disposal.
DAF technology has been successfully tested in effluent treatment plants at Chilean mines including Collahuasi, Los Pelambres and Esperanza, and at Codelco’s Ventanas refinery. In each case, the technology was able to achieve precipitation of almost all divalent metallic ions, producing concentration levels lower than that stipulated by local regulations.
Information for this article was provided by ARCADIS, an international company providing consultancy, design, engineering and management services in the fields of infrastructure, water, environment and buildings. Contributors to the article include: Carlos Morillo, project manager, Water Management Area, Arcadis Chile; Omar Gaete, project manager, Water Management Area, Arcadis Chile; Kathia Tabra, effluent project leader, Arcadis Peru; and Giulliana Tiravanti, effluent project leader, Arcadis Peru.