Pushing the Boundaries of Chemical Precipitation for Mine Water Treatment
By Lynda Smithard, PEng, McCue Engineering Contractors
To support the Environmental Assessment (EA) of a new gold mine project in British Columbia, McCue Engineering Contractors completed the preliminary design of a mine water treatment plant (WTP) using a water profile modeled for the future open pit sump water. The water profile was complex and the treatment plan specified by the consultant included heavy metals removal by chemical precipitation. Plant effluent quality was predicted largely based on published theoretical heavy metal solubility data.
Both the regulators reviewing the EA and other project stakeholders, including local First Nations, requested more, site-specific information to address the uncertainty as to what could be achieved with water treatment at the project site and what impact the treated water would have on the local environment.
Dealing with doubt
To address uncertainty with the treatment process and improve the inputs for the impact assessment model, McCue created a synthetic water sample from field leachate samples and laboratory-grade salts to match the modeled plant inlet water profile. The synthetic water sample was used to develop and validate a chemical water treatment process at a bench scale and provide effluent quality data for impact assessment modeling. The bench scale test program also provided valuable data to advance the design of the mine water treatment plant from preliminary to detailed.
McCue successfully utilized an unconventional chemical precipitation program to remove antimony, chromium and copper.
Validating the process
A few months after the bench test, McCue had the opportunity to validate the chemical precipitation process with a full-scale water treatment system at a metals contaminated site in Metro Vancouver, BC. The site required the temporary treatment of water contaminated with copper and zinc during the excavation phase of a major infrastructure project.
McCue successfully removed heavy metals from contaminated groundwater to meet both provincial and federal surface water quality standards for a marine discharge. Strict standards, particularly for copper and zinc, the presence of salts (from seawater intrusion), and construction activities in the excavation made the water treatment project technically challenging.
The chemical program selectively removed heavy metal ions while permitting sodium and chloride ions to pass through and be discharged to the marine environment, resulting in a significant cost saving for the property owner and a significant reduction in the carbon footprint by reducing the quantity of waste disposal required.
Moving forward
Results from McCue’s bench scale tests and the subsequent full-scale water treatment plant have contributed to the body of knowledge for heavy metals precipitation treatment performance for antimony, chromium and copper. Previously, literature indicated that no or negligible removal could be achieved for these parameters using chemical precipitation.
This work fills in many data gaps in the theoretical end points for water treatment by metals precipitation. It further establishes practical end points, which have been demonstrated to be lower than the theoretical ones. McCue’s work has provided valuable data on metals precipitation performance that could benefit EA work at other mine sites globally.
Filling in gaps in knowledge decreases uncertainty as to what level of water treatment and environmental protection can be achieved. Lowering uncertainty helps with gaining social and regulatory acceptance. Better data also helps advance engineering design work more efficiently. Combined, the work will lower both project risk and cost.
About this author
Lynda Smithard is an Environmental Engineer with 22 years experience, and is a registered professional engineer in British Columbia, Alberta and Yukon Territory. Lynda joined McCue in 2008 as an owner after 12 years in engineering consulting. Lynda leads the engineering team at McCue and is responsible for overseeing all engineering work. McCue designs, constructs, operates and maintains water treatment systems for mining, oil & gas and industry. Learn more at www.mccuecontracting.com.
The opinions expressed in this article are not necessarily those of Canadian Mining Magazine / Matrix Group Publishing Inc.
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