Elution strategies for recovery of nickel and cobalt from laterite tails through scavenging resin-in-pulp
Littlejohn, P., Vaughan, J. and Nikoloski, A. (2012) Elution strategies for recovery of nickel and cobalt from laterite tails through scavenging resin-in-pulp. In: 2012 ALTA Nickel/Cobalt/Copper Conference, 26 May - 2 June, Perth, Western Australia.
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Ion exchange resin has been used to recover value metal in the uranium and gold industry through resin-in-pulp/leach and similar carbon-in-leach/pulp processes for decades. More recently, resin-in-pulp processes have gained attention as a potential method to improve the efficiency of nickel operations. While every nickel laterite operation is unique, many involve an acid leach, neutralization and oxidative precipitation of impurities followed by counter current decantation to separate valuable liquor from the unwanted metal residue and precipitate. Counter current decantation (CCD) of this material is challenging at best, with large CCD tanks having a large plant footprint and requiring high capital investment. Depending on the settling characteristics of the precipitate, 5% or more of the leached nickel and cobalt can be lost to the slurry underflow through solution entrainment, co-precipitation, and sorption processes on the high surface area solids. For a site producing 40,000 tonnes per annum nickel and 2,500 tonnes per annum cobalt this represents yearly losses of approximately $40 million USD, given current LME spot prices (as of March 2012).
The tremendous waste of value that these high losses of nickel and cobalt represent are the primary driving force behind the development of resin-in-pulp (RIP) scavenging from laterite tailings. RIP scavenging involves contacting ion exchange resin with nickel laterite tailings at conditions where the valuable metals load onto the resin. As the resin beads are larger than the slurry particles, they can be separated from the slurry using vibrating sieving. Following this, the resin is washed to remove residual slurry and solution, and then eluted to recover metal value. While exact values vary, typical caron process tails contain roughly 300 mg/L nickel and 50 mg/L cobalt in slurry. High pressure acid leach tailings may contain 200 mg/L nickel and 35 mg/L cobalt in slurry. With efficient resin-in-pulp contact, upwards of 90% of this otherwise lost metal value can be recovered.
Although the chelating ion exchange resins proposed for use in nickel laterite RIP are selective for nickel and cobalt over other unwanted metals, laterite tailings solutions contain a relatively small amount of these metals of interest. Depending on the composition of the original ore and the method of leaching, the neutralized slurry can contain large amounts of solution phase magnesium and manganese (in the case of acid leaching) and vast amounts of ferric iron, silica, aluminium, and chromium in the solid phase. The presence of other cations that compete with nickel and cobalt for resin loading sites complicates resin-slurry equilibria. In general, there is a trade off between recovery of nickel and cobalt and purity of loaded resin. To recover a high amount of the nickel and cobalt value, one must accept the presence of impurity metals on the resin. When resin is eluted, these impurity metals can follow value metals into the eluate.
To date, the majority of resin elution work has focused on metal recovery via acid contact (usually H2SO4). When one has produced a resin loaded with a high fraction of value metals, quantitative elution in this fashion is attractive. Using strong acid, metal is recovered in a small volume of eluent with rapid kinetics. However, as more impurities are loaded onto a resin, strong acid elution becomes less attractive as quantitative elution of a low purity resin produces a low purity eluate. In such a case, a method of selectively recovering value metal from resin is desirable.
One selective elution method involves two stages – dilute acid to remove weakly bound impurity metals, followed by strong acid to recover the remaining metals. For the iminodiacetic acid resins most commonly investigated for nickel RIP, this method achieves selectivity of nickel and cobalt over magnesium, calcium, and manganese, but does not separate nickel and cobalt from ferric iron or chromium. Another option is the use of ammoniacal elution. Nickel and cobalt readily form stable amine complexes and have high solubility in strong ammonia solutions, unlike the majority of other metals present in nickel laterite processing. By taking advantage of this chemistry, nickel and cobalt can be effectively eluted separate from impurity metals.
The value of selective elution depends on the purity of the loaded resin. With less impurities, the additional expense of a second stage of elution or the higher cost of ammonia reagents relative to sulphuric acid make selective elution less attractive. In order to better determine the performance of different elution methods, a resin produced through scavenging RIP of plant tails from Queensland Nickel was treated using three elution strategies – single stage strong acid, two stage weak acidstrong acid, and single stage ammoniacal elution.
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|Murdoch Affiliation:||School of Chemical and Mathematical Science|
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