Water treatment

The water treatment plant has the capacity to treat up to 15,000 cubic metres per day of non-cyanided water and 5,000 cubic metres per day of cyanided water.

Treating water containing cyanide

Water decanted from the tailings pond (1), and collected in the underdrainage system (4) contains cyanide and passes through a pre-treatment cyanide destruction stage. Hydrogen peroxide, copper sulphate and lime are added to the cyanided water (2) in a rapid mix tank. Cyanide oxidation begins.

The solution moves through a number of retention tanks (3) to ensure that there is sufficient time to allow cyanide oxidation reactions to take place during times of peak flow. Hydrogen peroxide in the presence of copper destroys all free cyanide through chemical oxidation. Weak acid dissociable (WAD) cyanide is also oxidised during the process. On oxidation, cyanide yields simple carbon and nitrogen compounds.

Treating non-CN water

Mine water (5), collection pond water (6) and some underdrainage water (4) do not require treatment for cyanide removal. This water is treated to remove metals, trace ions and suspended solids.

Similarly, cyanided water goes through this same process once the cyanide has been destroyed. The water passes into tanks (7) to which lime and ferric chloride are added (8). The lime raises the pH of the water to 9.5-10. This is necessary to reduce the concentration of copper, and other metals, to the low levels required to allow the water to be discharged. In the presence of ferric chloride and lime, insoluble hydroxides and carbonates form. To separate the solids from the water, polyelectrolyte (flocculant) is added (9), along with more lime. Polyelectrolyte is used in potable (drinking) water treatment. It achieves a very low concentration of residual solids carry over. The result is a very high clarity water discharge.

All treated water then passes through clarifiers to allow the suspended solids and metals to be removed from the water. The suspended solids and metals fall to the bottom of the clarifiers (10), forming a slurry. The slurry is pumped to the tailings pond (11) via a thickener.

Carbon dioxide is added (12) to the clean water overflow from the clarifier to reduce the pH of the water to meet the compliance limits. The water then passes to polishing ponds (13). The polishing ponds hold the treated water for a sufficient time to allow laboratory testing, and the results to be received and interpreted prior to the water discharging to the Ohinemuri River. Water that meets the discharge criteria is discharged (14) to the Ohinemuri River. If the water does not meet the discharge criteria, it is recycled (15) back through the plant, used in processing, or piped to the tailings storage facility.