pH CONTROL IN BAUXITE RESIDUE NEUTRALISATION WITH SULPHURIC ACID AND SEAWATER

Smyth, C., Hunt, A.

Rio Tinto Yarwun alumina refinery operates with a seawater neutralisation circuit to precipitate residual caustic from bauxite residue before tailings storage. The refinery also utilises a sulphuric acid dosing system to assist with neutralisation due to an insufficient supply of seawater. A consequence of adding acid for neutralisation is that it can affect the aluminium concentration in the final effluent. To meet refinery effluent soluble aluminium targets the system was historically operated at lower pH targets than required by Yarwun’s environmental license. A review of previous lab work was conducted to determine the dissolved aluminium concentration in neutralised red mud supernatant as a function of the proportion of acid to seawater used in neutralisation. This information was used to develop a pH control strategy that respects both the dissolved aluminium and pH environmental targets whilst optimising acid consumption.