Mine water treatment tests results for a continuous flow reaction system

"An experimental study has been carried out to assess sulphate precipitation via ettringite process from acid mine water (AMD) originating from closed non-ferrous Romanian mining areas. Such a treatment is required, because by conventional treatment of the AMD with calcium hydroxide, the conformity for sulphate will not
be achieved, because the concentration correlated to gypsum solubility exceeds the limit value by roughly three times. Although sulphate is considered benign or with low toxicity, some countries have set limits for the wastewater discharged to natural receivers, from 500 to 1000 mg/L or 600 mg/L in Romania. The process of sulphate precipitation as ettringite using solid aluminium sources is known to be slow. Tests for sulphate precipitation, using monocalcium aluminate cement as source of reactive aluminium for the ettringite formation were done using continuous flow mixed flow reactors in series (or continuous stirred reactors, CSTR).
This reaction system allows the direct determination of the apparent reaction rate versus sulphate concentration which can be used further for various design tasks. The reaction rate for sulphate concentration below 600 mg/L may be less than 5 mg/L/min, depending on working conditions. The mixed flow reactors series has a net advantage over single CSTR, the best choice for a full scale treatment plant is a mixed flow
reactors series also, as the results indicate that a system with a single mixed flow reactor is to be much larger."