Carbon-in-leach Research Articles

Insights from simulation of CIL/CIP circuit Part 2 – Simulation studies

In Part 1 of this paper, the development of a semiempirical model of a carbon in leach (CIL)/carbon in pulp (CIP) plant for recovery of gold and silver was detailed. With appropriate practical parameters, the model was able to reproduce typical tank profiles for gold and silver contained in the solids, solution and active carbon. The model was used in a series of simulations to explore the influence on plant performance of: (1) circuit design encompassing tank size and number, and tankage configuration; (2) using some of the initial tanks for leaching only (CIP); (3) the impact of a thickener recycling a percentage of tailings solution to the head of the circuit; (4) leaching and adsorption parameters; (5) operational parameters such as ore feed rate and percent solids in the slurry; and (6) rate of countercurrent movement of active carbon. The results of these simulations highlight factors which need to be considered in the design and optimisation of a CIL/CIP circuit and provide an insight into identifying deficiencies in performance and the possible ways to counteract them by manipulation of the factors above.

Assessment of options for economic processing of preg-robbing gold ores

One of the challenges facing the gold industry in the twenty-first century is the continuing need to identify new reserves of economically treatable ores. Discoveries of large, metallurgically simple orebodies are becoming increasingly rarer, forcing companies to investigate options for the processing of ores which may present recovery, economic and/or environmental issues, requiring innovative approaches to their treatment. Ores which contain naturally occurring carbonaceous material (‘preg-robbing ores’) is an example of one such ore type. Newmont’s Jundee operation contains zones of carbonaceous ore within the oxidised ore body. A program consisting of detailed laboratory work and extensive plant trials has been undertaken to assess the options of the processing of this material. Laboratory tests demonstrated the advantage of carbon-in-leach (CIL) over direct cyanide leaching and carbon-in-pulp (CIP) for treatment of these ores. The laboratory program identified several factors which would enhance overall gold recoveries when preg-robbing ore was being processed, including: keeping preg-robbing ore separate from non-graphitic ore, maximising gravity recovery and ensuring the plant operates in ‘pure’ CIL mode i.e. no leaching before first adsorption tank. Plant trials were undertaken to assess the economic sustainability of treating moderate preg-robbing ores through a modified gravity/leach/adsorption circuit. Results indicated that where good operational control of the circuit was maintained to ensure high gravity recovery, minimal leaching prior to carbon contact and maintenance of good, active carbon inventory, then acceptable and sustainable overall gold recoveries could be obtained. However, plant trials did not produce gold recovery as high as laboratory tests. Using these relatively simple plant modifications potentially allows a plant to treat ores with preg-robbing index (PRI) values up to 1. When PRI values rise to 1·3–1·6 leach recoveries can drop from >85 to <40%, indicating a more intensive approach may be required, including kerosene addition and higher carbon inventories and activities.

Factors influencing the rate of gold cyanide leaching and adsorption on activated carbon, and their impact on the design of CIL and CIP circuits

The carbon in pulp (CIP) and carbon in leach (CIL) processes became firmly established in the gold mining industry in the 1980s, initially in South Africa and Australia, from where they spread rapidly to all the gold producing regions of the world. The percentage of annual global gold production by activated carbon-based processes grew from zero in the 1970s to almost 70% by the turn of the century, which represented a phenomenal rate of acceptance of a new technology by a traditionally conservative industry. The main reason for this rapid acceptance of the new technology was the fact that the first few large industrial plants in South Africa convincingly demonstrated better gold recoveries than the traditional filtration/Merrill Crowe process, with lower capital and operating costs. And as the plants developed an operating track record over their first few years of life, they proved to be remarkably robust mechanically, and highly tolerant of plant upsets, changes in feed composition and solution phase contaminants that had caused great problems in Merrill Crowe plants. These stellar attributes of the carbon-based gold plants have led to complacency and laziness in the industry, both at the new plant design stage, and with on-going optimization of existing plants. In many cases, basic “rules of thumb” that were developed as design criteria for the early CIP plants are still used today, with no appreciation of the factors that may cause one plant to perform quite differently from another. There seems to be little incentive to improve performance when it is well known that most CIP and CIL plants operate quite well with minimal optimization and, in many cases, minimal understanding of the factors that influence performance. Consequently, almost all CIP and CIL plants are overdesigned at the construction stage and are then operated sub-optimally. This can lead to higher gold losses and/or higher capital and operating costs than necessary. This paper examines the factors that influence CIP and CIL plant design and performance, and demonstrates a very simple methodology that can be used to arrive at something close to an optimum plant design. It can also be used as an on-going tool by plant metallurgists to transform a fairly well run plant into an exceptionally well run plant.

Estimation of gold inventory in large carbon in leach tanks

The preparation of the month end gold production report is required by the mine owners to regularly assess the performance of the plant against budgeted values. However the preparation of a gold production report is often complicated by the large inventory of gold in the carbon in pulp (CIP) or carbon in leach (CIL) tanks used to adsorb leached gold from the solution. Indeed a 600 m 3 CIP/CIL loaded with 15 g/L of activated coal charged with 8000 g of gold/ton of coal may retain up to 72 000 g (2200 troy oz) of gold. The estimation of the inventory implies the determination of the concentration of carbon in a tank and of the gold content of the species contained in the tanks. This paper assesses the errors associated with the sampling of CIL tanks to measure these concentrations and describes the impact of these errors on the estimation of the gold inventory in the CIL tanks of the Agnico-Eagle Mines Limited Lapa concentrator.

Cyanide recovery by tailings washing and pond stripping

Modern gold producers are experiencing the operational dichotomy of minimizing wildlife exposure to toxic levels of cyanide in tailings and process waters, while decreasing operating costs for exploitation of lower-grade ores. The widely accepted tailings weak-acid dissociable (WAD) cyanide level of <50 mg/L is typically achieved by oxidation of cyanide using reagents such as SO 2/air, H 2SO 5 or H 2O 2. Consumption rates and costs of these processes can be substantial; moreover, destruction of the reagent that is often the single largest operating cost centre for the process plant represents an additional economic downside. Technical and economic aspects of an alternative process for achieving these eco-efficiency objectives with minimal additional hazardous chemicals are considered and compared against other process alternatives for a selected scenario. The WPS (washing – pond stripping) process comprises tailings slurry washing using counter-current high-rate thickeners and pond or tank stripping of cyanide from the thickener overflow. Water balance issues are avoided by use of cyanide-stripped water along with any make-up water requirements in the washing stage. The cyanide-rich caustic scrub solution is recycled to the leach. Metals and other species that may build up in plant process waters even in the absence of tailings thickening can be treated by a variety of conventional measures. The WPS process is based on proven unit operations and hence utilizes existing plant infrastructure and other resources wherever possible. Single tailings thickeners are used in some gold plants for the recycle of water, along with a portion of the cyanide and soluble gold losses; this equipment can be better utilized by washing with recycle water to raise recoveries to high levels. Moreover, removal of cyanide from the overflow water improves management of the water balance and reduces cyanide consumption in the mill. The existing process water pond or tank is augmented with a covering commonly used in the water treatment industry for biogas production and odour removal. This low-pressure system provides a long retention time for the HCN stripping process, and the positive effect of pond solar heating on the Henry’s Law constant can also improve the thermodynamic driving force for stripping. The covered pond eliminates wildlife deaths in this area of the site; by also venting the carbon-in-leach (CIL) tanks in-circuit cyanide losses are further minimized, resulting in lower lime consumption by operating at lower pH conditions. Scrubbing systems are increasingly being designed in to new and existing gold circuits for the removal of cyanide, ammonia, volatile organic compounds (VOCs) and mercury from carbon regeneration kiln and gold room off-gas streams. Moreover, the occupational health and safety issues regarding elevated HCN levels above leach tanks employing hypersaline waters may require venting and scrubbing systems to be introduced. The additional soluble gold that is recovered in the WPS process can be a significant economic driver along with the advantages of cyanide recovery over destruction. The environmental risk of ongoing cyanide tailings management is minimized, while providing economic benefits from the recovery of cyanide, water and gold.

Dynamic modelling of a carbon-in-leach process with the regression network

The regression network provides a connectionist framework in which both parametric and non-parametric modelling can be implemented. It is shown how mechanistic knowledge can be built directly within the connectionist structure that results in a semi-empirical network model. In doing so the inherent freedom of a specific model is restricted so that the generalisation performance of such a model improves accordingly. It is described how a semi-empirical regression network kinetic model is developed for the dynamic modelling of the carbon-in-leach (CIL) process for gold recovery. By providing for mechanistic knowledge in the connectionist structure and catering for poorly understood aspects of the process by use of non-parametric regions within the structure of the semi-empirical regression network, the regression network kinetic model displayed significant superiority in generalisation properties over other non-parametric regression models if evaluated during dynamic simulation runs.

The mechanism of enhanced gold extraction from ores in the presence of activated carbon

Carbon-in-leach (CIL) is a common process alternative to carbon-in-pulp (CIP) when an ore is preg-robbing. The activated carbon will compete with the preg-robbing ore constituents in the leach tanks, with gold cyanide more selectively adsorbing onto the carbon due to its stronger adsorptive capacity. However, other mechanisms may also play a role in enhancing the extraction of gold from an ore in the presence of activated carbon. There may be an equilibrium effect in solution, where the gold concentration is decreased due to adsorption onto activated carbon, resulting in a greater driving force for further dissolution of gold, according to Le Chatelier's Principle. In this paper the use of activated carbon to improve gold dissolution from a number of ores of different mineralogy was investigated. Plant work, batch experimentation and diagnostic leaching were used to research this effect. An appreciable improvement in gold extraction was found when activated carbon was used. Both of the suggested mechanisms were found to play a role in enhancing gold extraction, with the controlling mechanism most strongly dependent on the ore's mineralogy. The preg-robbing characteristics of the ore and the cyanide concentration used for gold leaching were also important factors in the enhancement of gold extraction.

Efecto del oxígeno disuelto en la adsorción de oro en carbón activado

The effect of the oxygen dissolved on the adsorption of gold in a activated carbon such as these used for carbon in pulp (CIP) and carbon in leach (CIL) processes were studied. The research was oriented to dilucidate the effect of the oxygen dissolved in the gold solution on the kinetics and distribution of the gold adsorbed in the carbon under differents conditions of ionic strength, pH and gold concentration. It was found that the level of the oxygen dissolved influences directly the amount of gold adsorbed on the activated carbon, being this effect more relevant for low ionic strength solutions. The pH and initial gold concentration has no effect on this behavior.

Counter-current vs co-current flow in carbon-in-pulp adsorption circuits

Carbon-in-pulp (CIP) and carbon-in-leach (CIL) technology is applied successfully to recover gold from cyanided pulps. These plants are operated on a counter-current basis allowing for small concentrations of gold to be recovered at the expense of inefficiencies such as back-mixing, additional carbon breakage by interstage pumps and difficulties in carbon inventory management. In this study, linear regression techniques were utilised in order to study the adsorptive behaviour of carbon in gold bearing solutions with the aim of determining whether a partial co-current mode of operation will be feasible in CIP operations. It has been found that these empirical techniques are able to predict the adsorption rate of gold onto activated carbon and that the point of discontinuity of the constant rate adsorption period can accurately be determined. Thus, the behaviour of a single activated carbon particle can be predicted making use of standard linear regression techniques and using inputs such as, slurry density, rate of agitation, carbon loading and carbon and gold concentration. This information can then be utilised to maximise the use of the constant rate adsorption period by using a partial co-current mode of operation and reducing inefficiencies such as back-mixing and gold loss due to the abrasive action of interstage pumping on the adsorbent.

The identification of constituents in waste waters from gold mining using ICP-MS

ABSTRACT Gold mining has been undertaken in the Northern Territory since the 1870's in the mining district around Pine Creek, 180 km south of Darwin. This is a tropical region experiencing a monsoonal wet season each year from November to April. During the last 10 years, the use of the carbon-in pulp (CIP)/carbon-in-leach (CIL) process coupled with larger machinery to move material has permitted the mining of relatively low grades of ore and has created larger waste rock dumps than in the past. Oxidation of any sulfidic material incorporated in waste rock occurs during the dry season. With the onset of the wet season, rainwater percolates through the waste rock, flushing out acidic solutions containing a range of trace elements which may be indicative of the origin of the waste material itself. ICP-MS (inductively coupled plasma-mass spectrometry) has proved to be a particularly useful analytical tool for generating trace element patterns of waste rock materials and associated waste water. Certain elements and their compounds such as arsenic are readily absorbed by biota and are not transported via fluvial systems. However, species present in anionic forms are not readily adsorbed by particulate matter and appear to be more readily transported. Trace elements showing an indication of origin of waste rock are rare earths, groups of transition metals, gold itself and tellurium (co-association with sulfur). The predictive value of certain trace elements is demonstrated through a comparison of mine waste waters at different sites with uncontaminated surface and groundwaters. A comparison is also made with alluvial gold mining tailings where element are primarily in particulate forms.

The dynamic modelling of ill-defined processing operations using connectionist networks

This paper porposes a new methodology to model ill-defined processing operations using neural nets (NNs). A process with many variables (large dimensionality) cannot be modelled adequately if limited process data are available. This problem of multidimensionality is addressed and an approach suggested to reduce the dimensionality using NNs. An NN is trained on process data for the global variable space, whereafter the first-order partial derivatives of the process are estimated with the NN and used to perform perturbation analyses. As a result, the variable space can be subdivided into subspaces with reduced dimensionality. The final product of this modelling methodology is a combined model of phenomenological expressions and NNs. The model can be incorporated successfully in a dynamic simulator of the process. A new approach to conduct modelling on the basis of continuous data collected directly from an industrial processing unit is also proposed. The modelling methodology is applied to a typical processing operation, i.e. the carbon-in-leach (CIL) process.

Styles of centrifugal pumps used in pumping carbon to minimize carbon attrition

Attrition of activated carbon in either the carbon-in-pulp (CIP) or carbon-in-leach (CIL) method of gold extraction can lead to, at the least, loss of expensive carbon that has to be made up, and at the worst, loss of both the carbon and any gold that was attached to it. Carbon attrition therefore represents a significant factor in both the efficiency and cost of the extraction process, and efforts to reduce attrition through process and equipment selection pay dividends. In evaluating carbon attrition potential, one would note that an individual carbon particle is in jeopardy virtually any time it is in motion. Since the motivators of this motion are primary centrifugal pumps, the purpose of this paper is to review the current styles of these pumps now in use and to examine their performance in minimizing carbon attrition.

Modern trends in gold processing — overview

This paper reviews the state of the art in processing and extraction of gold. The ore bodies which were considered uneconomical at one time are becoming economical due to new and advanced methods of extraction. The paper discusses the gold treatment methods on free milling ores with conventional cyanidation and refractory ores with direct and pretreatment techniques for the recovery of high gold values. In the extraction aspect, the paper discloses two different extraction schemes on treating refractory ores, namely pretreatment followed by gold leaching and direct leaching. Pretreatment process involving roasting chemical oxidation and bio-oxidation have been discussed. Direct leaching of gold ores such as heap leaching, carbon in pulp (CIP), carbon in leach (CIL) and resin in pulp (RIP) are summarized. This paper also dicloses in a detailed manner the research approach on the development of alternative leach reagents which could improve environmental concerns as compared to the use of cyanide. Special emphasis of the review is focussed on the technical and economic guidelines for developing a small gold mine on the basis of capital and operating cost analysis.

Developments in Gold Leaching: A Literature Survey

The gold industry has seen intense activity in the areas of exploration and metallurgy'in recent years. Many developments have taken piace through which ore bodies that were once considered uneconomical to exploit are becoming economical grade. Even the traditional cyanidation process has been modified during the course of time to make it more economical and efficient. Developments such as heap leaching, in-situ leaching, carbon-in-pulp (CIP), carbon-in-leach (CIL) resin-in-pulp (RIP), and pretreatment of refractory ores have already made an impact on gold industries. Research and development work is also being carried out on less toxic, more efficient lixiviants which may eventually replace the toxic cyanide, solving the disposal problem currently facing the industry. This paper aims at summarizing the developments in the extraction of gold using conventional, as well as unconventional, lixiviants. Some developments in the area of gold recovery from pregnant solutions have also been briefly discussed.