Initial test work and flowsheet development was focused on low temperature atmospheric leaching of both cobalt and nickel at high acid concentrations which gave high metal extractions (+90% Co and +80% Ni) but at uneconomic capital and operating costs at prevailing metal prices. This work was carried out by Independent Metallurgical Operations (IMO) and is summarized below.
During the test work it was noted in diagnostic analysis of feed and tails samples that 85-90% of the cobalt was contained in the manganese mineral asbolane, which was less than 1% of the feed. Subsequent test work by RMDSTEM was then focused on selective leaching of this fraction of the ore using sulphur dioxide gas in an atmospheric, low temperature (<50oC), low time (<8hrs) agitated leach. Larger scale laboratory tests confirmed the validity of this approach. The results of this work are summarised below.
In July 2009 the MTJV engaged Independent Metallurgical Operations (IMO) to perform a process development study which mainly involved metallurgical testwork, interpretation and flowsheet development. At the completion of the process development study in February 2010 a single flowsheet (Figure 7) had been identified and accepted for advanced process development. Testwork indicated this flowsheet was capable of extracting over 90% of the contained cobalt and around 80% of the contained nickel from the resource (Figure 5). IMO’s testwork was carried out on various bulk samples comprising drill chips, diamond core and material dug from shallow pits.
IMO Process Description and Flowsheet
A novel method for leaching the ore at atmospheric conditions was developed by IMO specifically for the Mt Thirsty Cobalt Deposit which has some unusual mineralogy compared to typical oxide deposits. Conditions are controlled within the leaching process to reduce manganese minerals contained within the ore while allowing leached impurities such as iron to precipitate as jarosite during the leaching stage. Jarosite precipitation allows removal of a major impurity from solution in a form which is easy to settle and regenerates acid.
Downstream processing consists of separating the leach residue solids from the pregnant leach solution in a conventional counter current decantation circuit. The very good solid‐liquid separation properties of the leach residues produced allow for low flocculent consumption and high underflow densities.
Cobalt and nickel are recovered from the pregnant leach solution via precipitation with sodium sulphide. This results in the formation of a high grade and high purity mixed sulphide product containing approximately 10% cobalt and 44% nickel. Manganese can be recovered from solution after a secondary neutralization process by precipitation with soda ash to yield a high purity carbonate product.
A cobalt‐nickel mixed sulphide product and a manganese carbonate product is produced for sale to third parties for refining (Figure 6). This moves the more complex refining operations to a low cost environment with greater proximity to end markets.
Sulphuric acid requirements would be provided by a sulphur feed acid plant. In addition to producing acid the plant would also produce steam for power generation and sulphur dioxide gas which is used in the leaching process. The steam would be directed to onsite turbines which would generate enough electrical energy to meet the capacity of the processing plant and utilities.
Limestone is used as a low cost neutralising agent in this process. This can be supplied from an existing lime sand quarry located near Esperance and delivered to the plant site by road train.
A simple metallurgical diagram of the IMO process flowsheet is shown below (Figure 7):
RMDSTEM Testwork and Conceptual Flowsheets
Several phases of metallurgical test work were conducted by RMDSTEM on split samples taken from a 200kg bulk sample composited from air core samples drilled in June 2012.
This test work demonstrated that approximately 80% of the cobalt and over 20% of the nickel (associated with manganese enriched ore) can be extracted in 4 to 5 hours from Mt Thirsty oxide ore using low temperature (40oC) agitated leaching in closed tanks with very low acid consumption and low iron release. The low acid consumptions achieved of 25-50kg/tonne of ore compares favourably against previous studies targeting high nickel recoveries (as opposed to targeting cobalt), and this represented a major breakthrough for the Mt Thirsty Cobalt Project as acid consumption is a major operating cost item.
Based on their test work results RMDSTEM proposed two simple conceptual flowsheets (refer Figures 9 and 10) representing a completely different, low cost chemical system for processing the oxide ore compared to previous flowsheets that were capital intensive and aimed at maximising both nickel and cobalt recoveries. The new flowsheets deliver a 500-micron pulp to the leach tanks which is then sparged with SO2. The SO2 is delivered by burning liquid sulphur and there is an SO2 re-absorption system that recycles excess SO2. Recovery of cobalt and nickel is by precipitation with MgO to form a hydroxide concentrate.
(a) Thickener Flowsheet
Since there are no nickel bonds to break, a leach time of 2 hours was indicated by leaching tests to extract the cobalt. The whole leach pulp and solution is transferred to either a single large or several smaller paste thickeners, mixed with a flocculent and a paste formed.
The overflow pregnant leach solution (PLS) will contain 86% of the leached metals. A wash with clean process water and thickening in a second paste thickener will recover a further 11% of the soluble metals with a total recovery of 97% of cobalt and nickel in solution. Cobalt and nickel can be precipitated as a hydroxide.
(b) Resin in Pulp Flowsheet
In this scheme, the leached slurry is contacted with a broad spectrum Ion Exchange resin comprised of large beads. These can be recovered by the use of screens, and are handled similarly to carbon in a carbon in pulp gold plant. The initial resins studied appear to be able to recover + 99% of the combined cobalt and nickel in solution. This loaded resin is forwarded to an Ion Exchange Strip plant which produces a strong solution of cobalt and nickel sulphate. This is then precipitated as a combined Co-Ni hydroxide and shipped to the appropriate refineries. The capital cost of the equipment is marginally lower, but the large capital cost of a resin charge makes the two flowsheets almost equal cost wise.