Development of an Aspen model for the treatment of acid mine drainage

Abstract

Although numerous methods exist to treat mine water that is rich in CaS04 they all have inherent disadvantages. A means of treating acid mine drainage is by forming a precipitate known as ettringite. Ettringite is a low solubility calcium hydrosulphoaluminate that is stable between pH values of 11.4 and 124. Ettringite is made up of calcium, sulphate, aluminium and a large amount of water. The formation of this precipitate is a result of calcium sulphate which is brought into contact with an aluminium containing agent. Decomposition of ettringite takes place by reducing the pH to a near neutral value. A 5 stage process is proposed to treat acid mine drainage of which the formation of ettringite forms the cornerstone of this process. The process incorporates the formation of more than one precipitate, namely; metal hydroxides, gypsum, CaS04 and CaCOJ. To facilitate the formation of ettringite, gibbsite is recycled as a result of ettringite being decomposed. The results obtained in this paper are as a result of modeling this process on an Aspen Plus simulator. The simulation package is useful for investigating how this process behaves under non-ideal conditions and under various sensitivities. The process and its behavioral patterns are also analyzed in order to ascertain its economic viability.