The Cyanide Soluble Copper (CNsolCu) contained in Gold/Copper ores has often been the Achilles Heel in making these particular projects economic. It has been a headache for geologists and metallurgists alike. The copper has proven to be more than just a nuisance element, but a determinant in the projects’ economic feasibility. In addition to the economic implications, simply quantifying the impact has proven to be a distraction in the assessment of what should be a gold project. The measurement of total copper in the ore is not sufficient to understand the cost impact as it is the cyanide soluble copper that is relevant. Total copper may become important if it can justify the inclusion of a flotation circuit pre or post CIL to produce a profitable copper concentrate. However, this aspect should be considered separately as the true economics can be marginal.
GO FOR GOLD AND IGNORE CYANIDE SOLUBLE COPPER A NEW APPROACH TO GOLD/COPPER ORES.
By
Malcolm Roy Paterson
PT GreenGold Technology, Indonesia
Presenter and Corresponding Author
Malcolm Paterson malcolm@greengoldengineering.com
ABSTRACT
The Cyanide Soluble Copper (CNsolCu) contained in Gold/Copper ores has often been the Achilles Heel in making these particular projects economic. It has been a headache for geologists and metallurgists alike. The copper has proven to be more than just a nuisance element, but a determinant in the projects’ economic feasibility. In addition to the economic implications, simply quantifying the impact has proven to be a distraction in the assessment of what should be a gold project. The measurement of total copper in the ore is not sufficient to understand the cost impact as it is the cyanide soluble copper that is relevant. Total copper may become important if it can justify the inclusion of a flotation circuit pre or post CIL to produce a profitable copper concentrate. However, this aspect should be considered separately as the true economics can be marginal. The basis of this paper is to show that the application of a simple resin technology, the ReCYN Process, is a game changer in not only solving the cost impact of soluble copper but also in simplifying the assessment of copper/gold ores. The overall benefits are seen to extend beyond the processing costs, and to improved Reserve estimates. (1) The approach is to effectively ignore the cyanide soluble copper and focus on the gold. By incorporating the ReCYN process into the recovery flowsheet, the copper no longer becomes a cost. It can be shown that the additional cyanide consumed in dissolving copper is more than balanced by the recovery of the copper and its associated cyanide by the ReCYN process. The nett cyanide cost is then specifically related to gold recovery. As an example, the cyanide soluble copper can increase sodium cyanide consumption to over 15kg/t ore at certain times. Without the soluble copper, the cyanide consumption would be less than 1kg/t ore. This latter figure can now be used in the project economics without reference to CNsolCu, thus simplifying the Resource, Reserve and Process cost estimation. Cyanide soluble copper also causes downstream processing problems with adsorption onto carbon, impacting gold recovery and increasing detox costs. All these issues are able to be resolved since a higher cyanide level in the leach solution is now possible, due to the lower cost of cyanide recovered by the ReCYN process and the deletion of any detoxification costs.
There is expected to be an increase in the number of new gold projects that have ore associated with copper. Thus the challenge is expected to grow in importance.
This paper describes three diverse examples of the application of ReCYN Technology to problematic gold/copper ores.
Keywords: Cyanide, gold, copper, resin, detoxify, recovery, economic and environmental.
INTRODUCTION
Many gold projects in the world have a nuisance level of copper in the ore. The copper grade is too low to be considered an economic copper project but has sufficiently high cyanide soluble copper (CNsolCu) levels to be problematic as a gold project. The measurement of CNsolCu becomes a complex task as there is often no simple relationship between total copper and CNsolCu in different parts of an ore body. Estimating CNsolCu involves a leaching process and is sometimes carried out on each 1m interval of the core, a long and expensive addition to the exploration program.
The Resource estimation process imposes a cost on ore containing CNsolCu and the subsequent impact on Ore Reserves can be considerable (1).
Grade control in mining becomes more complex as CNsolCu has to be considered as well as gold grades. Even close space drilling does not ensure ore selectability due to minimum mining volumes.
Process plant operation becomes more difficult as control of free cyanide levels in the leach requires greater attention to the plant ore feed composition. Sudden increases in CNsolCu can reduce free cyanide levels, impact gold recovery and cause excessive copper loading onto the carbon.
Plant throughput can be limited in periods of high copper input due to detox circuit limitations.
From the above, it is clear that the nuisance value of CNsolCu can be high and often a material distraction when developing a gold project. The inclusion of the ReCYN process into the flowsheet allows the CNsolCu problem to be effectively ignored from a cost perspective and the focus returned to the gold. The reason for this is the recovery of the copper, and its associated cyanide in the ReCYN process balances the cost of the additional cyanide required to dissolve the copper in the ore. Cyanide levels within the leach circuit still require careful control to cater to the variable input, but this can be achieved with an automated system that measures Free and WAD cyanide.
THE LONG ROAD
Many excellent papers have been published detailing attempts to overcome the CNsolCu problem (2,3,4), with each concluding that there is no single answer to this problem, or proposing a possible but non-commercialised technology. The reviews detail various processes for removing the copper up-front, reducing copper solubility, and methods to recover the dissolved copper and associated cyanide. The general conclusion has been that none of the existing processes can be universally applied to solving the CNsolCu dilemma. The Red Dome Gold/Copper project (5) that operated between 1986 to 1997 in Queensland is a classic example of the process complexity that is encountered with CNsolCu and the innovations applied to cope with the problems. The success of the project was not solely due to its size and Au/Cu grades, but very much due to the ingenuity of the operations team. With a more limited resource base, the challenge can be insurmountable. An example is the Ravensthorpe Gold Copper Project in WA where for over 50 years, numerous mining companies have attempted to commercialise the deposits at Kundip. There were many reasons the project did not move ahead, including low metal prices and resource size, but always underlain by the CNsolCu issue. Often the copper problem becomes all-consuming, with resources disproportionately directed to first understanding the extent of the problem, quantifying its impact during processing, and then trying to find a technology to overcome the influence. There is a long history of research (2) directed at finding an alternative solvent to cyanide and adsorbants that are more selective. It is evident that no alternative process has so far provided a satisfactory answer. The answer may now be provided following a recent Alta published paper that promoted the benefit of retaining cyanide as the preferred solvent for gold leaching while addressing the negative aspect of cyanide in tailings dams (6). This 2017 paper promoted the use of a proprietary resin adsorbant
technology known as the GreenGold ReCYN Process. Initially promoted as a cyanide recovery and detox process, it has the added ability to provide an answer to the problem of cyanide soluble copper.
The basis of the technology was first described in 1956 in South Africa by Eric Goldblatt (7). The road since that time has certainly been long, including the involvement of the author and others in developing the technology for the last 30 years.
THE CHALLENGE
A major challenge with gold/copper ore types has been to first quantify the problem. Simply assaying for total copper is only the first step; the real challenge is quantifying the soluble copper throughout an ore body. Often the distribution of soluble copper is not consistent, with variations occurring not just with oxidation level, but often with more complex geology and mineralisation confusing the spatial distribution. Sometimes a relationship between copper head grade and CNsolCu is described by a formula, but with varying success. A common approach is to apply a cyanide leach test (BLEG or Leachwell) to each drill core interval of ore and apply that result as a modifying factor in the Resource estimate, influencing the cut-off- grade. The results are then related to a mining schedule that theoretically allows the resultant impact to be predicted and costed. The complexity of gold/copper ore bodies makes this assessment process risky and is why so much effort has been dedicated to understanding and trying to quantify the impact of soluble copper in a gold project.
THE ANSWER
Elegance is born of simplicity.
The ReCYN process has been promoted for use in gold/silver cyanidation projects to support cyanide’s continued use as the preferred solvent. It reduces operating costs, improves project economics and has significant environmental benefits.
There is now an added benefit; it provides a universal answer to the problem of cyanide soluble copper.
Cyanide soluble copper can now be effectively ignored from a cost standpoint; it is no longer a distraction or project killer. The focus is directed to the main revenue stream, gold.
Insoluble copper is considered separately and economically justified in its own right, independent of the gold project. If it is shown to be economically viable and justifies the capital and operating cost of the additional flotation and concentrate handling circuits, these circuits can then be added to the flowsheet. It cannot be assumed that the non-soluble copper is worth recovering, especially if a proportion of the copper has been removed in cyanidation.
QUANTIFYING THE BENEFIT
The ReCYN process treats the CIL tails to recover Free Cyanide and WAD cyanide-metal complexes. The Free cyanide is recovered and recycled to the leach plant. The WAD cyanide consists of the complexed metal cyanides, mainly copper. The cyanide associated with the complexed metal cyanides is recovered and recycled to the leach plant, and the associated metals recovered for sale. The cyanide consumption specific to copper is approximately 3kg NaCN for 1kg CNsolCu, i.e., a ratio of 3:1, matching the theoretical complex of Cu(CN) 4 3- . This ratio approximates to an equal dollar value for copper and cyanide, i.e. copper consumes its’ own value in cyanide. The minimum cost impact due to cyanide consumption is, therefore, $6/kg of CNsolCu.
For a 2Mtpa gold project and an average CNsolCu of 0.1%, the additional operating cost is $12M/a. The full impact is more than doubled when the loss of copper and detox costs are considered.
By contrast, the ReCYN process is cost-neutral.
By being at least cost neutral, the cost impact of cyanide soluble copper is eliminated, and the project focus can be on the gold. Depending on reagent costs the net benefit for copper recovery could be positive. Besides the benefit in reduced operating cost, there is potential for an increase in gold/silver recovery, higher throughputs and an upstream positive impact on Reserves (1). These benefits must be assessed on a project by project basis.
THE COST
The capital cost for a ReCYN plant is not very different from that of a conventional detox plant using INCO or Caro's acid. The difference is the ReCYN plant has a nett operating cost-benefit, not a nett cost. Cyanide is recycled at 50% of the cost of new cyanide, and there is a further cost benefit from WAD cyanide metals. The benefit of zero detox cost means that even for brownfield projects the plant often has a payback of fewer than 12 months.
PROJECT EXAMPLES
GreenGold is presently involved in three projects that are incorporating ReCYN technology to solve the cyanide copper problem. Each project is very different in size and application, which demonstrates the wider appeal of the technology.
The Martabe Gold Project, Sumatra, Indonesia.
Agincourts’ Martabe Gold Project has been operating for six years and is a large, long life gold/silver project, producing 300koz/a Au. It has a nuisance copper problem, which will increase its impact with deeper ores. Care is exercised in separating high copper material on the ROM stockpile to prevent process upsets in the plant. The problem ore is bled into the plant to prevent any dramatic impact on cyanide consumption and to avoid overloading the detox circuit. In 2018, Martabe undertook a detailed assessment of the ReCYN process to replace their Detox circuit which had become a bottleneck and sensitive to CNsolCu levels. What became evident was the magnitude of the benefit that is realised by incorporating the ReCYN Process. Whittle Consulting undertook a detailed Reserve optimisation study in 2018 (1) and demonstrated the extent of the economic impact ReCYN Technology would have on the project. The following graph illustrates the cumulative benefit from each area of influence.
Note the diverse areas that are impacted by the process, ranging from detox savings through to the Reserve estimate.
The ReCYN plant at Martabe is presently under development and due for commissioning in October this year. It is understood the project risk is reduced for Martabe as they have an existing Detox plant using SMBS/Peroxide, which can be reverted to at any time.
Mt. Morgan Gold/Copper Project, Queensland, Australia.
Mt. Morgan was the largest gold and copper producer in Australia at the turn of the last century and funded the BP company. The legacy are the usual holes in the ground containing very low pH waters which are also contaminated with copper and other metals. These waters require continuous detoxification before river discharge, at a significant ongoing cost to the Queensland Government. In 2018 Carbine Resources Pty Ltd proposed a re-treatment program for the tailings that would solve the environmental problem and be economically positive. The project failed to be sufficiently commercially attractive to attract funding, partly due to excessive Royalty obligations. The final version of the flowsheet did incorporate the ReCYN process, but the rest of the flowsheet was not optimised to take full advantage of the ReCYN Plant. Consequently, the capital and operating costs were high and when combined with the excess Royalty and project purchase cost, the project was not bankable. GreenGold has re-assessed the process flowsheet and proposed a simplified version, making better use of the ReCYN process. The capital and operating costs are substantially lower, and a different Royalty arrangement has made the project look much more attractive. Several fundamental flowsheet changes are made possible by the ReCYN process, including the placing of flotation after CIL. Cyanide soluble copper has been a significant impediment in developing this project for many years. Since the last commercial scale process plant ceased operations in the district in 1971, several companies have looked at developing the project. ACH Resources Pty. Ltd. (ACH) acquired the mining leases in 2016 and have carried out a programme of infill drilling and test work to support a development case. A significant amount of attention has been focussed on the cyanide soluble copper problem as it follows much of the gold. A flotation plant was included in the initial proposed flowsheet with the CIL treatment of the float tailings and sale of the concentrate for gold and copper credits. Typically, the gold and copper are not efficiently recovered by flotation where CNsolCu is present in the ore, and substantial CNsolCu reports to the float tail, affecting cyanide consumption and detox cost. The copper concentrate grade is also not always at a saleable level. The copper is not of a high enough grade to provide any substantial revenue benefit and therefore becomes a nuisance component. The copper represents less than 10% of the revenue under the flotation scenario and even less of the net value but consumes a large portion of the time and effort. ACH is now considering a revised flowsheet that excludes the flotation step. The ore is simply cyanide leached for gold and copper dissolution, gold adsorbed onto carbon and the tailings treated through the ReCYN process for cyanide and copper recovery. The high copper levels produced in the leach are kept off the carbon by using high Free cyanide levels. This is made possible because of the cyanide recovery process and ensures maximum gold and copper dissolution. The ReCYN plant tailings can be treated through a flotation plant if it is economically justified. Following the leach, where a substantial quantity of copper is removed, flotation may not be economically feasible. Gold recovery in the leach is very high (+95%), and there is little gold value in the concentrate. The revised flowsheet has a much-reduced capital and operating cost and focusses on the gold, where the majority of the value is. The above is not an exhaustive review of the flowsheet changes but demonstrates the basic improvement allowed by the ReCYN Process. The project is presently at the DFS stage. Ravensthorpe Gold Copper Project, Western Australia.
CONCLUSIONS
New technology is not easily accepted by the mining industry for the valid reason that the high capital investment normally required for a gold project imposes a significant financial risk.
The ReCYN Process is in the category of new technology as it has a limited commercial track record. Six years of successful continuous operation at a mine in Indonesia it is not sufficient to be considered a proven technology. A second ReCYN operation was commissioned last year, and a third large scale plant is currently under construction (6).
These applications were accepted mainly by providing attractive economic detox solutions, and where the production risk is limited in a Brownfield's operation.
It is now realised there are wider benefits to the technology with the latest extension to the specific task of solving the cyanide soluble copper dilemma.
The concept that mining companies can look at an ore body with a simplified view, that is not blurred by the cyanide soluble copper problem may help bypass some of the usual conservative views associated with new technology and allow for its early adoption.
It is still necessary to measure CNsolCu in the ore body for plant design purposes, but it is no longer a significant determinant in the valuation.
ACKNOWLEDGMENTS
The author would like to acknowledge the consultants and mining companies referenced in the Paper for permission to use their information. The views of the author do not necessarily represent those of the referenced Companies.
REFERENCES
1. Whittle G, Pan J, Whittle Consulting, ReCYN™ Case Study – Martabe Operations, Application of ENTERPRISE OPTIMISATION considering Green Gold Technologies Pte Ltd.’s ReCYN™ process Revision v1.1, 1st June 2018. 2. Xianwen Dai a.*. Andrew Simons a.b. Paul Breuer a , A review of copper cyanide recovery technologies for the cyanidation of copper containing gold ores. Alta 2011 Gold Conference. 3. José R. Parga1, Jesús L. Valenzuela2, Héctor Moreno3, Jaime E. Pérez1 Copper and Cyanide Recovery in Cyanidation Effluents, Advances in Chemical Engineering and Science, 2011, 1, 191-197
4. Brown C A, The Red Dome Experience. Proceedings of the Oretest Colloquium ’99, Scarborough, WA, 10 th November 1999.
5. Paterson M R, The ReCYN Process – Changing the negative perception of Cyanide.ALTA 2017
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