Optimizing Lime Treatment that Uses Decommissioned Open Pit

Water management at this nickel mine in Northern Québec involves the operation of a Low Density Sludge (LDS) lime treatment system that uses a decommissioned open pit as a reactor and sludge settling basin. The source of mine impacted water being treated is surface run-off and water collected in several active and old open pits and underground workings. Contained in the mine water is elevated levels of dissolved nickel ranging from 2 to 40 mg/L that must be reduced to < 0.25 mg/L before it can be discharged into the pristine local receiving environment. Due to the extreme cold climate at the site, the LDS treatment operates seasonally between July and September providing a very limited window of no more than 80 days for water treatment and discharge.

Historically, the treatment process operated in a batch mode that involved three sequential steps. In the first step, mine water was pumped into the pit to the maximum possible operating level while lime was being added to raise the pH of the entire pit to 11.5 to precipitate nickel-hydroxide solids. This involved recirculating the water around the pit for a period of several weeks to ensure the same conditions existed throughout. In step two, water pumping and lime addition stopped and the sludge solids were allowed to settle. Finally, in the third step, the pit supernatant was pumped to an acid addition reactor located on shore to lower the pH from 11.5 to the target range of pH 7 to 8 suitable for discharge into the environment. Two full batches were completed during each treatment season.

System Limitations
The main concern with this system was the limited treatment capacity. The volume of water treated and discharged during the short season was much lower than the volume of contaminated water generated each year. This resulted in a gradual accumulation of untreated water at site and increased risks associated with the long-term storage of untreated water at the mine.

Our Solution
In 2016, BQE Water was asked to debottleneck and optimize the existing LDS system to permit treating a larger volume of water within the same short season, preferably without the need for any major capital expenditure. The mine essentially wanted to increase the number of batches that could be completed within a season from two to three. In response, we formulated a strategy that eliminated batch treatment by turning the system into a continuous one. Crucially, the strategy involved completing a technical assessment, followed by carefully planned in-situ testing and close monitoring during the early part of the treatment season such that the new continuous system could be either rapidly implemented or quickly reversed back to batch operation if necessary. This was done to minimize the risk of lowering the treatment capacity during the 2016 season if the new proposed approach did not work while allowing for the benefit to be maximized if the approached worked.

What We Did
Using the pit geography and bathymetry, we established that by placing the point of high pH water being pumped into the pit and the point of supernatant being withdrawn to the acid addition reactor in strategic locations within the basin, it should be possible to achieve target effluent water quality without the need to recirculate the pit water. This premise was tested at the start of the season when a pH profile and water quality parameters important for environmental compliance were monitored across the pit immediately following the start of lime addition. We found that with the exception of pH, all other parameters met discharge regulations at the supernatant withdrawal point. The continuous treatment scheme with simultaneous lime and acid addition were then implemented using a rigorous Management of Change protocol to update all operating procedures and to train operators on the new procedures.

The Result
By implementing a continuous operating system, the treatment capacity was increased by more than 40% during the first season and more than 50% the following season, resulting in a significant reduction in the overall inventory of contaminated water stored at site. More importantly, the system change required zero capital.

Below is a comparative diagram of the original batch treatment and the revised continuous treatment.Despite the success achieved, the operation revealed an opportunity to improve both the robustness of the continuous operation and overall treatment capacity even further. The continuous treatment system is limited by its exposure to climatic elements such as strong winds, movement of melting icebergs, rain, surface run-off and spring pit turnover. All these factors disrupt the settling of the amorphous lime sludge and causes elevated concentrations of total suspended solids and total metals to exceed the applicable discharge limits at the supernatant withdrawal point. Because of this, close monitoring of the pit water quality and a temporary pause in operation until climate conditions change is required.

Future Steps
With the volume of contaminated water expected to increase as a result of further mine expansion, the client is interested in ongoing improvements and increases in the treatment capacity of the system. Based on operating results since 2016, we are now assessing different options to minimize the negative impact of climate elements on system performance. In addition, we are expanding the assessment to include possible changes that would minimize the volume of waste sludge generated and deposited in the pit as this is an important component of life cycle considerations.

Parting Thoughts
What allowed us to be successfully in this project was: 1) our intimate knowledge of the existing operation, its constraints, management logistics and overall performance from having been contracted to operate the system for several years prior to optimization; and 2) our ability to assess the problems holistically and step outside the box. Rather than focus on enlarging the equipment to permit the treatment of a third batch, we rethought the problem and offered a new control narrative that significantly increased the plant throughput without any capital investment.

This project represents an example of an outsourced water treatment operation where the service provider is retained not only to provide labour but also system expertise and added value.

Written by
Alain Consigny, PEng