ALTA_2020 - ReCYN III The New Generation of Gold Plants

INTRODUCTION Free and complexed metal cyanides in leach slurries can be present at high levels of several hundred ppm. Combining the high solution concentrations with favourable resin equilibrium values results in high resin loadings and low solution values in a co-current adsorption system. A typical loaded resin value for the CuCN 3 2- complex is >50kg/t resin, achieved with a residual solution level of <1.0 ppm CuCN 3 2- . Unfortunately, the same equilibrium relationship does not apply to gold, where typical leach solution levels of 2ppm Au must be reduced to <0.01ppm Au, whilst maintaining practical gold loadings of several kg/t resin. A counter-current adsorption system would be more appropriate for gold recovery, but this would result in a separate adsorption circuit and include the complexity of interstage screens and reverse pumping as with carbon. A simple answer has been to divide the adsorption circuit into two co-current streams, whereby complexed metals, including gold, are adsorbed in the first section, and Free cyanide adsorbed in the second half of the train along with any scavenged metals. A further advantage of the split adsorption is the copper is recovered with a high CN:Cu ratio, a direct benefit for copper elution. The second challenge in introducing the gold recovery section is in the handling of the copper recovered during gold elution. Here, an advantage is taken of the differential elution of copper and gold, whereby the copper is substantially eluted before gold starts eluting. The eluted copper is added to the copper precipitation circuit and recovered along with the primary copper production. ReCYN III is accepted as a compromise of three differing chemistries and objectives. However, the differences are reconciled in a combined process with economic benefits. The following sections provide an introduction of the ReCYN III process development and its application to the Mt. Morgan Tailings project. RECYN PROCESS DEVELOPMENT There is a long history of attempts to use a strong base macroporous anionic resin for cyanide and metal recovery. Eric Goldblatt first proposed a method more than sixty years ago when trying to find a use for spent resin from a uranium plant (2) . A more concerted effort to commercialise the concept was made in the 1980s by Talon Metals, a Canadian company, whose interest developed from unrelated areas. Starting in the medical field, then in the preservation of wine and fruit juices and then in the cleaning of metal-contaminated sites, the technology followed a tortuous route to the gold industry. Talon combined forces with Signet Engineering in 1988 to commercialise a process using a proprietary resin known as Vitrokele. The development met with mixed success, mainly due to the initial focus being on gold recovery and not recognising the impact of high cyanide soluble copper levels. In 1996 the focus changed to cyanide recovery and detoxification. Signet constructed a commercial plant in 1998 for the Mirah project in Indonesia, incorporating a continuous elution circuit for cyanide recovery and batch elution circuits for copper and gold recovery. The gold recovery was a scavenging circuit as the primary precious metal recovery incorporated a carbon process. The plant commissioning was delayed due to the Asian Financial Crisis and remained under care and maintenance for fourteen years. During this time, a combination of circumstances resulted in the cancellation of the Talon/Signet Vitrokele programme. Since 1998, several similar resin technologies have been proposed and tested to the point of piloting, but none commercialised. The Mirah cyanide recovery plant was commissioned in 2014 and operated successfully for five years. The copper and gold recovery circuits were commissioned but not used for any extended time due to the low metal levels. The project achieved tailings discharge compliance solely with Free cyanide recovery. The Mirah circuit ran with high cyanide levels of up to 1200ppm NaCN due to the high silver content in the ore. It is noted that the inclusion of cyanide recovery for the Mirah Project made the project economically feasible. In 2016 the owners of the Mirah Project purchased the Mount Muro Project in Central Kalimantan, and part of a significant upgrade included the conversion of the plant from Merrill-Crowe to CIL and the addition of a cyanide recovery circuit. The process plant was re-commissioned in 2017, and the cyanide recovery circuit is successfully recovering two tonnes of sodium cyanide per day with no detox requirement. ALTA 2020 Gold-PM Proceedings 2

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