Métallurgie Magnola Inc. (MMI) is a company based in Danville - Québec, Canada - specializing in the production of magnesium from mine tailings located close to the plant. In 2003 the company was forced to suspend operations because of the intense competition coming from China.
During MMI's production process, a residual mainly composed of silica and iron was produced. This residual was returned to a leakproof tank (secure pond), specially designed for this purpose. Furthermore, during this operation, metals impurities and chlorides were brought to the tank (see Table), creating a brackish solution.
In April 2003, with the suspension of the operations, the brackish water contained inside the pound was not used in the process, it was thus necessary to find a measure to manage the water surpluses brought into the pound by precipitations (rain and snow) to avoid any accidental overflow.
To stabilize and then reduce the level at the MMI tailings site, H2O Innovation recommended the installation of two membrane process lines: i.e., two OMEGA systems each comprising 36 reverse osmosis membranes, fed by 50 m3/hr of solution from the tailings dam.
The MMI tailings-site project was carried out in two phases:
- Phase 1: Winter 2004 - installation of pre-treatment facilities and the first OMEGA system, to stabilize the level at the secure site.
- Phase 2: Fall 2004 - installation of the second OMEGA system, to reduce the level of the brackish water in the secure pound.
As a pre-treatment for the reverse osmosis, H2O Innovation adds chlorine, followed by a contact tank to oxidize most of the metals in the residual solution, which otherwise could easily precipitate on the membrane surfaces and clog them. The precipitated metals are then separated from the effluent by passing them over a filtrating media. Excess chlorine is then neutralized, and a dosage of an anti-scaling solution is applied to prevent the formation of lime deposits or other inorganic materials on the surface of the membranes. The effluent is passed through 5-micron pre-filters before being finally filtered by the reverse osmosis membranes.
Both systems also include a regular washing tank for cleaning the membranes and thereby increasing their useful lives. In addition, the treatment line is fully controlled by an automatic, programmable central system, and employs a comprehensive telemetry system enabling H2O Innovation to provide appropriate remote monitoring of the data, and if necessary to make adjustments.
- An overall flow of 100 m3 per hour is treated
- The systems' recovery rate is 55%, even for cold water, thereby enabling a production of 27.5 m3/h of permeate per treatment unit
- The osmosis permeate that is discharged directly into the natural environment is of very good quality (the concentrations are well below the strictest standards for discharges) and the concentrate itself is returned to the tailings dam.
- The reverse osmosis filtration system functions well, enabling the tailings on the site to be stabilized