Ettringite Precipitation vs. Nano-Filtration for Efficient Sulphate Removal from Mine Water

Sulphate is the most common anion in mine waters due to the oxidation of sulphide minerals. Even though sulphate is not a toxic substance it can attack concrete structures, and it can affect water usage downstream of mining areas. Despite the European Water Framework Directive, EU Member States have very different approaches to dealing with sulphate containing mine waters. For example, in Germany the site-specific sulphate pollution situation and ecological parameters of surface waters, based on an environmnetal impact analysis, are used to determine the sulphate effluent limits. In Romania and some other countries, a relatively low fixed legal limit (600 mg/L) has to be complied with. In this context the consulting firms WISUTEC GmbH, EcoInd Bucharest and UIT Unwelt-und Ingenieurtechnik GmbH have developed two effective technologies to treat highly polluted mine water including sulphate removal. The first technology utilises heavy metal precipitation followed by the precipitation of ettringite for sulphate removal; whilst teh second utilises nano-filtration (NF), after a chemical "pre-treatment" stage, to produce a high quality permeate. Reagents tested for NF pre-treatment included hydrated lime, sodium hydroxide and magnesium oxide. Similarly, a range of reactive aluminium sources (calcium aluminate, aluminium rich cements, aluminium hydroxide and others) were tested to optimise the precipitation of ettringite. Both flow shhets developed by laboratory test work are well-suited to treat acid rock drainage (ARD) at the Rosia Montana mine site in Romania.