An Acid Recovery and Mineral Carbonation Process for the Nickel Industry

The Murrin Murrin nickel laterite process in Western Australia emits about 14 tonnes and 8 tonnes of CO2 during power generation and neutralisation respectively, while consuming over 30 tonnes of neutralising agent [1] and about 40 tonnes of sulphuric acid per tonne of nickel metal produced. Under Australia’s former carbon tax this represents a cost of nearly $14 million AUD per annum, and while this tax has been removed a carbon pricing scheme could be reinstated in Australia and this could threaten the economic sustainability of this operation.

Nickel laterites are a form of naturally weathered ultrafamic rock containing saprolites (e.g. 1.8-3 wt% Ni). This presents the opportunity to integrate a mineral carbonation process within the nickel refining process, and as a medium scale emitter of CO2 (<2.5 Mtpa) this may be a good approach to reducing emissions in this industry [2].

Our group at the University of Queensland is developing a process for nickel laterites to regenerate acid and base while capturing CO2 emissions as mineral carbonates. In the process sulphuric acid is used to extract nickel, which also causes magnesium to dissolve. Normally this magnesium is neutralised to magnesium sulphate and represents about 4 tonnes of tailings per tonne of nickel metal. In the proposed process triethylamine is used as a base to raise the pH of this solution before bubbling CO2-rich flue gas though the solution to precipitate magnesium carbonate. This can then be used for neutralisation while sequestering about 1.5 tonnes of CO2 per tonne of nickel metal. The aqueous product stream containing acid-protonated amine is then distilled to recover free amine and acid which are both recycled within the process. A major benefit of this process for CO2 sequestration is that it does not rely solely on a carbon tax to be economically attractive because there may not be a need to purchase acid or base.

This presentation will focus on the regeneration of the acid and base via fractional distillation. Solutions of sulphuric acid with excess triethylamine at various concentrations were distilled using various heating protocols to study the effect on the energy requirements, percentage recovery of the amine and water co-evaporation. This information will be used to determine the optimal ratio of acid and base and the associated energy requirement. For the regeneration process to be successful enough amine must be regenerated to sufficiently raise the pH for carbonation with minimal co-evaporation of water to minimise energy requirements and ensure the overall process is carbon negative.

References:

1. Environmental Protection Authority (1996). Nickel/Cobalt ore mining and processing operations, Murrin Murrin, 60km east of Leonora Retrieved from Perth, Western Australia: http://www.epa.wa.gov.au/sites/default/files/EPA_Report/B816.pdf
2. Sanna, A., Uibu, M., Caramanna, G., Kuusik, R., & Maroto-Valer, M. M. (2014). A review of mineral carbonation technologies to sequester CO 2. Chem. Soc. Rev., 43(23), 8049-8080. doi:10.1039/c4cs00035h