THIOTEQ contactor

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Contactor: chemical stage

The chemical stage in Figure 2 consists of a gas-liquid contactor. The sulfide is transported to the contactor with the help of a gas recycle with a carrier gas (a mixture of CO2 and N2) and metal-loaded water (e.g. acid mine drainage) is fed to the contactor. Metals such as copper precipitate as a sulfide according to the following reaction:

Cu2+ + S2- → CuS (4)

The properties of the sulfide gas from the bioreactor and the contactor design result in metal sulfides with good settleability and filterability. The contactor off-gas, containing primarily N2, CO2 and water vapour is recycled to the bioreactor. There is no significant gaseous discharge during normal operation. However, a typical plant is equipped with a caustic gas scrubber to capture any gas bleed, which might occur on an infrequent basis, with the resulting NaHS added to the gas-liquid contactor.

Copper can be precipitated as a sulfide usually without pH adjustment and without any significant amount of precipitation of other heavy metals present in the water, to produce a product with a high copper value, usually greater than 50%. Other metals can be recovered as separate high-grade sulfide products, although pH control using an alkali source might be required to maintain optimum precipitation conditions. Toxic metals, such as arsenic, antimony, cadmium, and lead, that typically occur in smaller amounts in mine drainage and other industrial effluents are also removed and report to one of the sulfide products. The precipitated metal concentrates are recovered in a clarifier and then dewatered using a filter press to meet smelter feed moisture requirements. Metals such as copper, zinc and nickel can be precipitated separately into high-grade concentrates when the number of precipitation stages is increased. These metal sulfides can be shipped to smelters as high quality concentrates.

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