Comparative study on different steel slags as neutralising agent in bioleaching

Abstract

A comparative study on bioleaching of a pyrite concentrate using five different steel slags as neutralising agent has been performed with reference to a commercial grade slaked lime. The acid produced during oxidation of pyrite was neutralised by regular additions of neutralising agent to maintain a pH of 1.5. Bioleaching was conducted as batch in 1-L reactors with a mixed mesophilic culture at a temperature of 35 °C. The different steel slags used were Argon Oxygen Decarbonisation (AOD) slag, Basic Oxygen Furnace (BOF) slag, Electric Arc Furnace (EAF) slag, Composition Adjustment by Sealed Argon Bubbling–Oxygen Blowing (CAS–OB) slag and Ladle slag, representing slags produced in both integrated steel plants and scrap based steel plants. The aim of the study was to investigate the possibility to replace normally used lime or limestone with steel slags, considering their neutralising capacity and eventual toxic effects on the bacterial activity. The bioleaching efficiency was found to be equally good or better, when steel slags were used for neutralisation instead of slaked lime and the extent of pyrite oxidation of pyrite was in the range 75–80%. Some of the slags used contained potentially toxic elements for the bacteria, like fluoride, chromium and vanadium, but no negative effect of these elements could be observed on the bacterial activity. However, slags originating from stainless steel production are less environmentally friendly due to the presence of chromium. The neutralising potential of the slags was high, as determined by the amount needed for neutralisation during bioleaching. The range of additions of neutralising agents required to control the pH at 1.5 for all the experiments ranged from 16–27 g, while 22 g was needed in the experiment with slaked lime. Hence, it was concluded that considerable savings in operational costs could be obtained by replacement of lime or limestone with steel slag, without negative impact on bioleaching efficiency. Recycling of steel slags would render an eco-friendly process and provide a means for sustainable use of natural resources.