Mineral Processing and Extractive Metallurgy: Transactions of the Institutions of Mining and Metallurgy

The Beaconsfield gold mine processing plant is a complex operation and utilises crushing, grinding, gravity gold concentration, sulphide flotation, bacterial oxidation, cyanidation and zinc cementation to recover gold from the partially refractory ore. This paper discusses the challenges, developments and advancements throughout the processing plant since start-up which have culminated in a 25% increase in the mill throughput over the nameplate design and a concomitant gold recovery increase from 75 to 92%. The improvements have been attained via advancements in mechanical integrity, metallurgical review, process control and operator training. The current operation has produced over 900 000 oz of gold from September 1999 to closure at the end of June 2012.

Gold deportment and ore characterisation studies are key tools in gaining an appropriate understanding of complex gold ores for process development and optimisation. These studies have historically involved imprecise chemical methods with slow and costly optical microscopy for gold identification. The traditional approach requires highly skilled petrographers, is prone to human error, and is not scalable, reducing the usefulness in practical analysis. The proposed methodology utilises a decision tree and knowledge base in a staged approach to gold ore characterisation building-up progressively more detailed information about the sample mineralogy, gold deportment and key mineral properties, as the process is followed. While the automated QEMSCAN system is used for mineral and gold identification, interactive and focused data analyses are used to give more efficient and accurate results. This is complemented by smart sample preparation and chemical methods to enhance and validate the data, making the staged approach methodology reliable, scalable and therefore, practical and economical.

Centrifugal concentrators such as Knelson concentrators (KCs) are commonly used in most gold processing plants to recover fine liberated gold particles. The decision to employ a gravity recovery circuit is primarily based on the amount of gravity recoverable gold (GRG) in the ore among other considerations. However, the conditions required to optimise the gold recovery in such devices used in plants cannot be determined by GRG test results as the separation of particles is based on the particle bed dynamics within the given unit which are different to those encountered in a three inch KC used for the GRG test. The significant operating variables that affect the performance of a KC are the size distributions of the gold and gangue, rotational speed of the bowl, fluidising water flowrate and the cycle time. Of these variables, the rotational speed is generally fixed, and the size distributions of the gold and gangue are determined by the comminution circuit employed. Thus, the fluidising water is the key variable that can be used to control the dilation of the particle bed to maximise the recovery of gold. This paper examines the validity of the GRG test for performance evaluation and discusses an alternative method based on performance curve of the device that can be used to evaluate the optimum fluidising water flowrate of an operating machine. The principles of determining the performance curve of the machine and its use are also discussed and illustrated with laboratory and plant data.

A statistical ‘randomised block design’ full plant trial of magnetic conditioning of flotation feed was undertaken at Rio Tinto's Northparkes Mine in New South Wales. Magnetic conditioning of flotation feed has been shown to improve the flotation of <38 μm paramagnetic minerals consistent with selectively aggregating these paramagnetic minerals. The process was evaluated using shift composite samples that were analysed for copper and gold. Because the process targets <38 μm minerals, all samples were sized and the size-by-size recoveries were calculated for each sample. The size-by-size analysis targeted the effect on fine mineral, reduced the plant noise and shortened the test period required to get statistically significant results. An increase in gold and copper recovery was found in the <20 μm fraction to a high level of statistical significance. For the >20 and <38 μm size range, the copper recovery increased to a high level of significance, whereas the increase in gold recovery in this fraction was to a lower statistical significance. There was no statistical improvement in the >38 μm fraction for either metal. Approximately half the copper and gold losses at Northparkes are in the <20 μm size fraction. While the sulphide copper minerals chalcopyrite and bornite are known to be paramagnetic; gold is not paramagnetic. Gold's response to magnetic conditioning, however, is consistent with a number of literature references where, depending on the gold's mineralogical disposition, gold has been shown to respond to magnetic treatment.

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.

The Fosterville Gold mine is located approximately 20 km to the East of Bendigo in central Victoria. The gold at Fosterville occurs as solid solution within disseminated arsenopyrite and pyrite. The Fosterville ore bodies contain various amounts of native carbon in the form of bituminous coal. This carbon (non-carbonate carbon or NCC) has been the predominant mechanism for gold loss from the processing facility through ‘preg-robbing’. Processing of the Fosterville ore is achieved initially through crushing, grinding and flotation to extract the sulphides. The sulphide concentrate is oxidised using bacterial oxidation, before being leached in a conventional CIL circuit. A high portion of the native carbon (NCC) in the mine ore is naturally hydrophobic in nature, and subsequently reports to the flotation concentrate stream, and ultimately onto the CIL circuit. This NCC has a notable preg-robbing ability. Treatment of black shale ores, which have elevated NCC levels, has historically resulted in CIL recoveries as low as 35%, with around 60–80% of the gold loss from the leach circuit attributed to preg-robbing. Standard technologies for the dealing with a carbonaceous leach feed have been trialled with limited success. The discovery of the significance of heat on leach recoveries, triggered extensive testwork on a range of leach feed and tails samples. Pilot plant testing demonstrated that ‘heated leaching’ of the CIL tails was the best process with an average recovery increase of 7·5% being achieved. Following a successful pilot study, an engineering feasibility study was completed by Minerva Engineering, and the project economics determined. With strong recovery gains evident and a predicted project payback of 1–1·5 years, approval for the installation of the full scale plant was granted. Detailed design commenced in September 2008 with installation commencing in January 2009. The circuit was successfully commissioned in April 2009. Following commissioning, the heated leach circuit has achieved recovery gains of 4–14%, and has proven itself to be a significant contributor to the overall plant performance.

Cyanide detoxification by chemical means (sulphite, hydrogen peroxide or Caro's acid) has typically been adopted by the gold industry to meet the International Cyanide Management Institute (ICMI) code and/or regulatory compliance for discharge into tailings storage facilities (TSFs), treatment of TSF return water or the discharge of excess water from the mining operation. This paper presents and discusses results from a number of different evaluations that have been conducted to reduce the cyanide concentration in cyanidation tails and process streams by chemical means. Preoxidation before cyanidation and the ore mineralogy were both found to potentially have a significant impact on reagent requirements in the treatment of cyanidation tails. The solution speciation provided important insights into the reagent consumption, which becomes very high in targeting residual weak acid dissociable (WAD) cyanide concentrations below 1 mg L⁻¹. Preliminary evaluations of polishing and alternative processes for reducing the WAD cyanide showed potential to achieve these low concentrations and reduce the treatment costs.

A semi-empirical model was developed to represent a carbon in leach/carbon in pulp (CIL/CIP) circuit for recovery of gold and silver. The model uses a simple two rate representation of the leaching kinetics. Parameters for the leaching kinetics may be obtained by sampling the feed to a circuit and the tailings from each tank and assaying the solids for both gold and silver to obtain the assay profiles of the solids down the tanks. The parameters in such a leaching model would be expected to be a function of the ore type, the grind, cyanide concentration, whether oxygen was used and the general chemical environment. The leaching parameters are primary inputs to the model for any simulation. Standard power law relationships are used express the practical equilibrium between the gold and silver cyanides adsorbed on the active carbon and the tenor of the solution. There is an underlying assumption that the residence of time of carbon in the tanks is sufficient for the carbon loading and solution concentration to be close to equilibrium. The parameters for this relationship are also derived from plant data. The model was constructed by writing a mass balance for gold and silver in and out of each tank in the train and for the circuit overall. Other inputs to any simulation are the feedrate of ore, per cent solids of the slurry in the tanks, gold and silver loadings of the input ‘barren’ carbon and the rate of movement of carbon through the circuit counter current to the slurry. With appropriate practical parameters, the simulation is able to reproduce typical tank profiles.

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.

Using iron and sulphur oxidising bacteria to catalyse the breakdown of sulphides that host the gold is an important biological method for the pretreatment of refractory gold ores. Following this biological treatment, a combination of chemical and physical methods is used for leaching (such as the cyanide process) and concentration (such as carbon in pulp or electrowinning) of the gold. Although these methods are well accepted by industry, they harbour limitations in the processing of low grade refractory ores and regulatory agency/public acceptance of cyanide use. Thus, it is beneficial for industry to develop environmentally friendly, as well as cost efficient, leaching and concentration techniques that are based on micro-organisms. This may soon be possible by adapting the results of recent multidisciplinary research, which has shown that micro-organisms are capable of driving a biogeochemical cycle of gold dispersion, transport and re-concentration in the supergene environment. The indigenous microbiota in biologically active soil microcosms from a number of Australian sites are capable of solubilising up to 80 wt-% of the gold contained in soils and deeper regolith materials. Studies using molecular microbial techniques have shown that a metallophilic bacterium, Cupriavidus metallidurans, is present in biofilms on gold grains from a number of Australian sites. Cupriavidus metallidurans is capable of actively accumulating gold from solution and therefore contributes to the formation of secondary gold grains and nuggets. Identifying the biochemical and physiological pathways that lead to the dispersion and accumulation of gold in regolith and quantifying the reaction kinetics of these processes may thus lead to the development of industrial bio-processing capacities for gold containing ores.