Upstream environmental desulphurisation and valorisation of waste rocks as a sustainable AMD management approach

Open-pit mining generates a large volume of waste rocks (WR) that are deposited as unsaturated piles on the surface. WR are mainly characterised by the high heterogeneity of their hydrogeological, mineralogical and physical proprieties. The main objectives of this study consist of: i) a detailed characterisation of WR from Canadian Malartic mine in Malartic city (Quebec, Canada), ii) the use of environmental desulphurisation of the three classified waste rocks main lithologies to reduce their acid generation potential and iii) the recovery of the residual precious metals (e.g. Au, Ag). The studied materials were sampled after blasting while ensuring their representativity. Each lithology was first screened based on the DPLS (diameter of physical locking of sulphides) to remove the inert fraction higher than 2.4 mm. Then, the fraction (< 2.4 mm) was separated into two fractions to optimize their integrated management 0-1 mm referenced as fine fraction and 1-2.4 mm referenced as coarse fraction. Chemical and mineralogical determinations showed that the three lithologies are more enriched in carbonates (mainly as calcite) than sulphides (mainly as pyrite). The acid base accounting (ABA) testing and the net acid generation (NAG) tests both showed that these lithologies (< 2.4 mm) and their corresponding fractions are non-acid generating except three fractions that are uncertain. In order to reduce the sulphur content, desulphurisation applied on the fraction < 2.4 mm using laboratory gravity separation (Knelson separator and Mozley table) was more efficient than flotation. All desulphurised materials from gravity and flotation methods were in the non-acid generating zone. The gravity method offered a sulphur recovery of about 84 wt%. The pyroanalysis of gold (Au) and silver (Ag) on the concentrate of desulphurisation with gravity methods display a gold content of 3,78 g/t and of 4,9 gA for silver. The precious metals recovery could offset partially the costs related to the waste handling and reprocessing.