Fosterville Gold Mine improves gold recovery with advanced technologies

Challenge

• Reduce gold losses
• Reduce capital cost
• Implement integrated water balance process

Solution

• Metso Outotec BIOX®
• Metso Outotec HiTeCC™
• Metso Outotec ASTER™ Process

Benefit

• Improves gold recovery
• Low CAPEX and operating costs
• Water recirculated in plant

The Fosterville operations of Agnico Eagle Mines Limited is a high-grade, low-cost gold mine located 20 km east of the city of Bendigo in the state of Victoria, Australia. Fosterville’s ore is processed at the mine’s ~830,000 tonnes per year mill.

Commissioned in 2005, the operation then comprised of a crushing and grinding circuit followed by flotation, Bacterial Oxidation (BIOX®) and CIL areas. In 2008, Fosterville Gold Mine pioneered the development of a high temperature preg-robbing mitigation process. This highly successful technology, HiTeCC (high-temperature caustic conditioning), commercialized in 2009 at the site has since been demonstrated to increase overall plant recoveries by up to 12%.

As a non-discharge site, Fosterville Gold Mine have maintained their commitment to strengthening the sustainability of the site’s water balance through the application of innovative technologies. Among them is the Metso Outotec ASTER™ process. As a frontrunner in sustainable technologies, Metso Outotec was in a unique position to understand the requirements and partner with Fosterville to deliver an effective solution for the reuse of benign solutions.

Optimizing overall gold recovery with Metso Outotec BIOX® process

The primary ore at Fosterville is highly refractory, with less than 10% gold dissolution achievable on the flotation concentrate without any pre-oxidation. Due to the nature of this ore body, an oxidative pre-treatment step is necessary to destroy the sulphide mineral lattices. As part of Fosterville’s project development and due diligence, various oxidative technologies were investigated. Following the results (which ranked certain parameters such as safety, technical risk, operability, maintainability, economic risk etc.) Metso Outotec’s BIOX solution was selected as the preferred oxidative pre-treatment technology. The biooxidation of refractory sulphide ore concentrates results in the liberation of the occluded gold for recovery via cyanidation.

The Fosterville BIOX plant has been operating since April 2005 and has proven to be reliable, achieving recoveries through the BIOX circuit consistently over 99%. The first gold bar was produced in May 2005 and has been in successful operation ever since.

Monitoring metrics

The plant consists of a feed surge tank, six BIOX reactors with live operating volumes of 900 m3, three 9 m diameter counter current decantation thickeners and a diluted BIOX acidic ferric-arsenate liquor (neutralised in the six neutralisation reactors).

In order to maintain consistent recovery, the health of the BIOX circuit is monitored via four key metrics: Dissolved oxygen levels, ferrous levels, temperature and pH. If any of these metrics start to drift out of operational limits, then consequently, action is taken. Contaminants that may impact the circuit include step changes in minerals in the ore, such as stibnite, gangue, hydrocarbons and cyanide species. Sharp changes in process operating parameters, such as rapid increases in feed flow rate and reductions in the air supply, may also impact the circuit’s performance.

Mitigating an aggressive preg-robbing ore with Metso Outotec HiTeCC™ Process

In 2008, Fosterville spearheaded the creation of a high temperature preg-robbing mitigation process, today known as the HiTeCC™ (high-temperature caustic conditioning) technology.

A pilot plant consisting of two trains (six vessels per train) was utilised to validate early batch test work which back then highlighted the potential to recover preg-robbed gold and also to abstract out a process design basis. Each train consisted of different reactor volumes to allow comparisons in residence time as well as different test conditions (such as temperature, carbon concentrations, reagent additions) to be studied simultaneously. Heating of the pulp was achieved by circulating hot water through heating jackets around the reactors, and the reactors were fitted with baskets to retain carbon and allow for easy carbon transfer.

This process evolved into a robust and cost-efficient alternative for improving gold recovery when treating preg-robbing and double-refractory ores and concentrates. The preg-robbed gold is efficiently desorbed from the carbonaceous matter and then recovered onto fresh activated carbon.

The simplicity of this process ensures low CAPEX and operating costs, consistent performance, and safe, easy operation. Fosterville was the world’s first commercial installation with their plant commissioned on-site in 2009, and this successful technology has been demonstrated to increase overall plant recoveries by up to 12%.

The pilot plant trial confirmed that the HiTeCC treatment of the CIL plant residue was the best method for increasing the overall gold recovery from the ore, with a recovery increase of over 10%. The pilot plant trialling was also extended to evaluate the ability of the process to recover gold from the historic leach tails and demonstrated that gold in the old tailings residue could also be recovered. Thus, it was decided to design the HiTeCC treatment plant with additional capacity to allow reclaimed old tails to be processed concurrently with fresh CIL residue slurry.

The plant is configured as a cascade of six reactors with the counter-current flow of CIL tailings slurry and freshly added activated carbon. The first four reactors are fitted with lids to retain heat and reduce steam entering the operator working zone on the top of the reactors. The loaded carbon from the HiTeCC circuit is recovered to a carbon column, regenerated, and then returned to the CIL circuit.

As of 2021, the HiTeCC plant has been operating for almost 12 years and has proven to be an effective technology for the recovery of preg-robbed gold.

Supporting an integrated water balance with the ASTER™ process

Metso Outotec’s ASTER™ process (Activated Sludge Tailings Effluent Remediation) was developed to deliver an improved and integrated water balance for re-use in BIOX applications. It has been successfully implemented in three metallurgical facilities around the world, with the Fosterville facility (commissioned in Q1/2021) as the fourth commercial application of the ASTER technology. A fifth ASTER plant is currently in the construction phase in Zimbabwe.

The process configuration is modular, with a number of primary oxidation reactors in parallel, feeding a set of secondary reactors in series. This simple technology can treat leach tailing solutions with concentrations of up to 5,000 mg/L of thiocyanate and degrade them down to concentrations as low as 0.1 mg/L.

Designing a cost-effective metallurgical operation

The metallurgical team at Fosterville set up a pilot plant facility and commenced an ASTER test work program in partnership with the Metso Outotec team. The test work campaign focussed on adapting locally sourced microbes to the Fosterville tailings solution, followed by optimizing the process parameters as well as the performance of the culture through further adaptation to progressively higher duties. The test work was successful and further confirmed the suitability of the ASTER process. The results also generated the required destruction rates and other process design criteria allowing Fosterville and Metso Outotec to develop a specific ASTER process design for the Fosterville ASTER plant.

This particular ASTER plant has been designed to treat a 792 m3/d, 5,000 mg/L SCN feed. The processing will be solution based and thus the reactors have no installed agitators with solution homogenisation brought about by aeration only.

The Fosterville plant consists of six tanks each with an operating volume of 180 m3, followed by a static settler. The static settler enables recycling of thickened biomass to the ASTER primary tanks, thereby increasing the biomass concentration in the primary ASTER reactors and increasing the thiocyanate degradation rate. The feed to the ASTER reactors is heated to maintain the optimum temperature for the ASTER culture. Construction of the plant commenced in 2019, with commissioning in Q1/2021.

Strength in partnerships

The collaboration and partnership approach between Metso Outotec and Agnico Eagle’s Fosterville Gold Mine has allowed the mine to further enhance their operations and output. Since its implementation, BIOX has proven to be reliable by achieving recoveries through the BIOX circuit consistently over 99%. The HiTeCC treatment of the CIL plant residue has increased overall gold recovery by over 10%, and the ASTER process has resulted in the degradation of SCN levels as high as 4,000 ppm in leach tailings solutions to below 0.1 ppm.

Close collaboration, a dynamic relationship and the implementation of these innovative technologies from Metso Outotec have assisted in establishing Fosterville Gold Mine as one of the largest and lowest cost gold-only producers in Australia. There’s strength in partnerships and that’s why, together, we are the partner for positive change.