Selective sulphidation of a nickeliferous lateritic ore

Abstract

The processing of nickeliferous laterites to produce nickel metal is both complex and energy intensive. Since most laterites are found in less developed regions, the capital costs for the infrastructure can exceed those for the process itself. The low temperature sulphidation of lateritic ores to produce an intermediate nickel concentrate, for further processing, offers a number of potential advantages, such as lower energy consumption and a relatively simple flowsheet. In this research, the sulphidation of a nickeliferous lateritic ore was investigated between the temperatures of 450–1000 °C and sulphur additions of between 25 and 900 kg of sulphur per tonne of ore. The experiments showed that the nickel oxide can be selectively sulphidized to a nickel–iron sulphide. It was found that both the grade and the sulphidation degree largely depended upon the temperature and the sulphur additions, with temperatures above 550 °C exhibiting the highest recoveries and grades. A DTA/TGA with mass spectrometer was used to further elucidate the nature of the phase transformations that occur upon heating of the ore in the presence of sulphur.

Introduction

Nickel consumption is expected to rise steadily in the future due to the increased stainless steel demand, particularly in China. This continuing rise in nickel demand will no longer be met through the treatment of nickel sulphide deposits, which currently account for over 60% of nickel production. Instead, future nickel production will have to increasingly rely on nickel laterites, an oxidic ore which is estimated to account for over 70% of known nickel resources (Dalvi et al., 2004).

Nickeliferous lateritic ores are located in tropical and sub-tropical regions and are formed through the weathering of olivine-rich rocks known as peridotites (Boldt, 1967). Whereas the host rock may contain between 0.2 and 0.3 wt.% nickel, the limonitic zone of an ore body may contain between 0.5 and 1.7 wt.% nickel and between 40 and 60 wt.% iron (Dalvi et al., 2004). Limonitic ores are treated using either one of two industrial processes. In High Pressure Acid Leaching (HPAL), the ore is leached with sulphuric acid in an autoclave followed by purification and either electrowinning or hydrogen reduction of the solution to produce a high purity nickel product. In another industrial process, known as the Caron process, the nickel in the ore is reduced at temperatures between 500 and 800 °C followed by an ammoniacal leach which extracts the nickel and cobalt into solution prior to further hydrometallurgical refining (Dalvi et al., 2004).

Both of these extraction processes require substantial capital expenditures in both infrastructure and equipment. To further complicate the issue, nickel laterite deposits are often located in remote areas where access to power and transportation may be difficult. Due to the high capital costs and complex operations there is a significant interest in alternative nickel extraction methods, which are less energy intensive, and can produce an intermediate product on-site, which is suitable for refining. One major avenue for research has been the selective reduction of nickel from lateritic ores, followed by physical upgrading via either magnetic separation or flotation.

The current research investigates the selective sulphidation of nickel from a limonitic ore at temperatures below the melting point of the oxides. The objective is to form nickel–iron sulphide particles which are large enough for physical separation from the gangue material.