Home > Minerals processing: The nickel laterite challenge

Minerals processing: The nickel laterite challenge

Editorial
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While nickel laterites are attractive from a mining perspective, they are mineralogically complex to efficiently extract. 

Emerging economies and the demand for stainless steels and other alloys are driving demand for nickel. With continued challenges and delays in increasing output from existing operations and bringing new operations on-line, supply will struggle to match this increase in demand and thus a simplistic view of the outlook for nickel can be summarised as 'robust'.  

Nickel occurs in essentially two ore types; magmatic sulphides and ('wet' or 'dry') laterites.

The sulphides typically are found hundreds of metres below ground with principle mineralisation including pentlandite (Ni, Fe)9S8 .

Major deposits such as those in Canada and Russia have historically provided the bulk of nickel production.

Nickel laterites on the other hand are found nearer or at the surface and are generally classified as limonitic (the upper layer, based on nickeliferous limonite; (Fe, Ni)O(OH)) or saprolitic (the lower layer, based on hydrous nickel silicate; (Ni, Mg)3Si2O5O5(OH)).

Laterites are found in two general regions and are referred to as wet or tropical laterites (from Cuba, Indonesia, New Caledonia etc) or dry laterites as found in Australia. 

As a consequence of many years of exploitation of sulphide ores these ore bodies are being depleted and/or more expensive to mine (usually due to depth).

Companies are increasingly looking to derive nickel production from nickel laterites.

Whilst these near surface ores are relatively attractive from a mining perspective they are generally mineralogically complex and lower in nickel (and cobalt) grade presenting issues associated with developing economically attractive processing options.

In the recent past the nickel laterite processing industry has had a chequered history, with challenges associated with continuing profitable production on existing operations (such as Cawse, Murrin Murrin, Bulong) and managing runaway project costs and time delays on new projects (such as Ravensthorpe and Goro).

A number of hydrometallurgical technology options have been and continue to be employed as means of alleviating the challenges presented by the ore body and the individual process limitations with recent 'favourite' flow sheets being built around ammoniacal based processes (Caron) or sulphuric acid based processes such as high pressure acid leaching (HPAL), Heap Leaching (HL), atmospheric leaching (AL) and combinations or variants of these latter options.

Choice of the flow sheet end point also varies site to site with production of saleable 'intermediate' products such as mixed sulphide or mixed hydroxide usually being favoured over processing through typically solvent extraction to a final metal product. 
 
Taking a strategic approach 
In Australia more than half of our nickel reserves occur as nickel laterites and although the nickel (and cobalt) grade is low and the mineralogy complex these are increasing going to be targets for processing.

With this in mind, Researchers within the CSIRO Minerals Down under National Research Flagship (MDU) have identified nickel laterites as a strategic target and they are engaged with a variety of research programs aimed at reducing the cut-off nickel grade of Australian laterites that would be considered economically viable to process. Success in this area will be measured in the billions of dollars value to the Australian economy. 

CSIRO's Minerals Down Under Flagship is also a leader in innovations in the exploration, mining and processing areas and is working across Australia to better understand and quantify Australian opportunities to exploit nickel laterites.

Innovations around surface and airborne sensors, more efficient drilling, automated characterisation, intelligent mining machines, down-hole analysers hyperspectral core logging and rock characterisation, just to mention a few, continue to assist industry.  
 
Process challenges 
One of the first process related challenges with nickel laterites is the low and variable grade. Most attempts to upgrade feed material into a hydro­metallurgical plant have largely failed to achieve meaningful increases in nickel concentration without loss of a significant proportion of the nickel.

With the range of sophisticated mineralogical characterisation tools available to CSIRO we are able to rapidly determine the potential (and metallurgical cost) for ore upgrading on individual ores.

Analytical and geometallurgical tech­niques that identify the distribution of the value metals through varied mineralogical matrices and the gangue mineralogy are critical to understanding, modelling and then optimising leach conditions for best nickel (and cobalt) recovery and minimal excess acid additions (i.e. operating cost).

With collaborators, CSIRO has also explored a number of standard and novel beneficiation options with the degrees of success again being reliant on the mineralogical matrix and the value distribution therein. 

Building on this understanding models have been developed to optimise the performance of both the mild HL and extreme HPAL options for leaching nickel and cobalt into solution with reduced reagent consumption and subsequent neutralisation costs. Subsequent processing then depends very much on understanding the solution chemistry and opportunities and methods for impurity rejection. CSIRO researchers have been able to recently complete and publish research in each of these areas.

In parallel to assisting industry optimise 'traditional' leaching options we have been working with industry partners to develop alternative acid leaching options and in at least one of these cases we are currently building a one-tonne-a-day demonstration plant which will demonstrate very efficient reagent recycle processes and potentially offer to industry a radical new option with very different process economics.

The issue of recovery of the value metals, nickel and cobalt from the solution remains. This includes post major impurity (mainly iron) removal as an economic improvement to producing intermediate product. 

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