Nickeliferous pyrrhotite – “Waste or resource?”

Abstract

Currently nickel producers are keeping a close eye on both the economic and production developments taking place at the Talvivaara open pit mines and their bio-heap-leach operations located in Finland. The concept of open pit mining combined with heap leaching is a popular concept in the copper industry and practiced on oxidised copper ore bodies with less than 1% Cu. In general, this process consumes large quantities of sulphuric acid when based on oxide mineralogy. Talvivaara is processing very low-grade, but complex Ni, Zn, Cu sulphide ore (0.27% Ni, 0.57% Zn and 0.14% Cu). Its full contained metal value at 70% base metal recovery is estimated at 40–50 USD/MT ore using long-term metal prices (all elements). Low-grade complex disseminated nickel sulphide ore bodies are fairly abundant worldwide, but in general not yet economically treatable through the conventional mine-mill-smelter route. In the Sudbury basin of Ontario there is a vast resource of ∼0.8% Ni-bearing pyrrhotite tailings, which has been disposed of separately in shallow lakes since the early 1990s in compliance with known environmental and sustainable development standards. The estimated full contained metal value at 70% nickel recovery from pyrrhotite is estimated at 90–100 USD/MT using the same long-term metal prices as for Talvivaara. Supply of sulphuric acid is abundant from nickel smelter operations in the basin. There are, in addition, many other low-grade ore bodies where nickel-bearing pyrrhotite, with or without pentlandite exsolution flames, is readily available. This commercial context raises the question whether Ni-bearing pyrrhotite can be considered a sustainable resource. In this paper some historical context behind pyrrhotite processing in the Sudbury area is provided. Flow sheet design considerations based on both technical and economic factors are reviewed with the objective of providing the nickel industry with tools for informed decision-making on the possibilities for exploitation of this low-grade nickel sulphide raw material.