GHD | Oceana Gold New Zealand Ltd. | 12552081 | Waihi North Project 10 2.2 WTP Description The WTP has been in operation since 1988 and has been subject to upgrades in 1999 and 2011. A reverse osmosis (RO) plant was built and commissioned in 2008 to provide an additional treatment option for trace elements removal. The WTP has performed consistently well with only infrequent and minor recorded non compliances with consent conditions. The WTP currently incorporates four parallel streams with three of these dedicated to trace elements removal only. The fourth stream has two phases of treatment; oxidation of cyanide to destroy the cyanide complexes followed by trace elements precipitation and removal. It is proposed that the WNP will see the WTP incorporate six parallel streams dedicated to trace elements removal and two parallel streams for the destruction of cyanide complexes followed by trace elements precipitation and removal. Cyanide oxidation is achieved using a combination of hydrogen peroxide, copper sulphate and lime. A series of tanks are used for reagent mixing followed by retention to provide time for chemical reaction. Hydrogen peroxide in the presence of copper destroys 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. Lime and ferric chloride are added to all four water streams to facilitate trace elements precipitation and removal. Trace elements tend to occur in a soluble form when the pH of water is low. Raising the pH with lime in the presence of ferric chloride results in insoluble hydroxides and carbonates forming. Following mixing and retention, a polyelectrolyte (flocculant) is added along with more lime to form flocs that can be settled out. Clarifiers at the end of the treatment process allow the suspended solids and precipitated trace elements to be removed from the water. The suspended solids and precipitated trace elements fall to the bottom of the clarifiers forming a slurry. The slurry is pumped to the tailings pond via a thickener. Carbon dioxide is added to the clean water overflow from the clarifier to reduce the pH of the water to meet the compliance limits. The RO plant can be utilised (if required) in order to further reduce concentrations of trace elements to very low levels utilising a partially permeable membrane. The proposed upgraded water management system is illustrated in Figure 7 and the proposed upgrade WTP including the various waste streams is presented in Figure 8.
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