Ard treatment - batch vs. Continuous flow reaction systems for sulphate precipitation, a technical note

Sulphate is the specific anion present in the acid rock drainage (ARD). Although sulphate is considered benign or with low toxicity, some countries have set limits for the wastewater discharged to natural receivers, often 500 mg/L or 600 mg/L. By conventional treatment of the ARD with calcium hydroxide, some gypsum can
precipitate, depending on the ARD initial sulphate content, but the conformity for sulphate will not be achieved, because the concentration correlated to gypsum solubility exceeds the limit value by roughly three times.
An experimental study has been carried out to assess sulphate precipitation via ettringite process from acid rock drainage originating from closed non-ferrous mine site in the North Romanian mining area.
Tests for sulphate precipitation, using monocalcium aluminate cement as source of reactive aluminium for the ettringite formation, showed important differences regarding sulphate precipitation yield between batch and continuous flow reaction systems, due to reaction system itself. For continuous stirred tank reactor system (CSTR) a total space-time of 2.6 h is required when operating with cement dose equivalent to 265 mg Al for 1 g sulphate in order to safe meet the sulphate discharge limit of 600 mg/L for mine water with initial c(SO42-) = 2650 mg/L and at a temperature of 15 oC.