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Metals refining
Feb 13, 2020

Improve your operations with our gold-processing solutions

It has now been almost four years since the integration of the BIOX®, ASTER™, and HiTeCC technologies into Metso Outotec (Outotec at the time). The BIOX process, which is part of the hydrometallurgy business line, was a natural fit for our gold portfolio.
Gold Processing Solutions

Outotec can provide clients with process solutions for a variety of gold ore types, ranging from free-milling to refractory gold ores and covering almost the entire process flow sheet, as shown in Figure 1 below. Outotec’s process solutions include comminution, flotation, pressure oxidation, BIOX, roasting, leaching, and HiTeCC, as well as gold recovery facilitated by analyzers and automation systems. Tailings and effluent treatment are an integral part of our solution offering, in the form of the ASTER technology for the destruction of thiocyanate.

Outotec’s capability to combine process and automation design is vitally important. The provision of process design, plant engineering, equipment, automation, and project management eliminates the need for expensive and counterproductive interfaces during implementation.

Outotec’s gold-processing solution
Figure 1. An overview of Outotec’s gold-processing solution offering

In general, we can provide clients with the following services spanning the full project life cycle: 

  • Innovative and proven technologies
  • Laboratory test programs and pilot test campaigns
  • Scoping and feasibility studies
  • Basic and detailed engineering
  • Proprietary key equipment and technology supply
  • Process automation and instrumentation
  • Training, commissioning, and start-up services
  • Operation and maintenance services
  • Spare parts
  • Plant audits, retrofits, and equipment upgrades.

OKTOP® BIOX Agitator Development

The BIOX Generation 4 design focused on achieving a significant reduction in CAPEX and OPEX for BIOX plants. Development of the design focused on two main operating cost items: the power required for oxygen mass transfer in BIOX reactors and reducing cyanide consumption during cyanide leaching post BIOX. A separate article in this newsletter discusses the progress made with the development of the MesoTherm process.

I would like to highlight the success achieved in the development of the OKTOP 3105 agitator for the BIOX application. Soon after the integration of BIOX into Outotec, an R&D program was launched at our Research Center in Pori, Finland to develop an agitation system that can deliver superior oxygen mass transfer with reduced power input.

The development program was based on the same stringent toll-gate procedure used previously by Biomin:

  • The first phase was to develop and test multiple potential configurations in the laboratory using a 500 l vessel to identify the best configurations to deliver the required process results.
  • The most promising configurations were then scaled up for testing in Outotec’s 21 m3 reactor at the Fairview BIOX plant in South Africa.

The Fairview program consisted of two phases:

  • Phase 1 – water testing the different configurations to confirm the small-scale results and identify the best one;
  • Phase 2 – slurry testing the selected configuration under typical BIOX primary reactor conditions to measure kLa and sulfide oxidation performance under real conditions.
OKTOP® 3105 impeller configuration
Figure 2. The OKTOP® 3105 impeller configuration
 Toll-gated development program for the OKTOP 3105
Figure 3. Toll-gated development program for the OKTOP 3105 agitator configuration
Performance of the OKTOP 3105
Figure 4. Performance of the OKTOP 3105 impeller compared to standard configurations

Cam & Motor BIOX Plant

It is also our great pleasure to introduce the latest member of the BIOX family. RioZim Ltd is a Harare-based gold and diamond producer. The company’s gold portfolio is anchored by the Cam & Motor gold mine located near the town of Kadoma, approximately one hour’s drive south-west of Harare, Zimbabwe.

The Cam & Motor mine is one of the oldest mines in Zimbabwe. The site is now a modern mining and processing facility with a design milling capacity of 2,700 tpd of ore. The plant is currently treating oxide ore from the Cam & Motor pit and some surrounding satellite deposits through a standard mill-CIL circuit. In anticipation of the treatment of refractory ore, the mine constructed a modern flotation plant in 2017–18 to produce a sulfide concentrate.

RioZim selected BIOX as the preferred technology for the treatment of the refractory gold concentrate. The current BIOX plant was designed for an ultimate capacity of 200 tpd of concentrate, but the project will be implemented in two phases: phase 1 will have a capacity of 100 tpd, which will be expanded to 200 tpd in phase 2.

The preliminary lay-out for the Cam&Motor BIOX plant is as follows:

  • Phase 1
    • Three BIOX reactors: 2P-1S (yellow)
    • Two CCD thickeners (green)
    • Three neutralization tanks (blue)
    • Services and reagents
  • Phase 2 (magenta)
    • Three BIOX reactors: 3P-3S
    • One CCD thickener
    • Three neutralization tanks
    • Additional blower and cooling capacity

Construction is already well under way with orders for the long-lead items secured and civil construction in progress. Commissioning is expected to take place during Q2–Q3 of 2020.

Location of the Cam & Motor mine
Figure 5. Location of the Cam & Motor mine (source: RioZim website)
CIL plant and tailings thickener
Figure 6. The CIL plant (background) and tailings thickener (foreground) at Cam & Motor
layout of the RioZim BIOX plant
Figure 7. Preliminary layout of the RioZim BIOX plant showing both phase 1 and phase 2 equipment

Obuasi BIOX Recommissioning

The commissioning of the Obuasi BIOX plant in 1994 was a major milestone for the BIOX process, confirming both its scalability and its suitability for operating in remote locations. The Obuasi Mine was placed on limited operations towards the end of 2014, and on care and maintenance from 2016. 

In June 2018 the Parliament of Ghana ratified the regulatory and fiscal agreements that cover the redevelopment of the mine and the Environmental Protection Agency issued environmental permits. With the key permitting and regulatory processes finalized, the mining services contract could be signed as another key milestone in delivering a modern, mechanized underground mining operation with 5.8M oz of ore reserves and 34M oz in mineral resources, and a mine lifetime extending over 20 years.

Following receipt of all the requisite approvals from the Ghanaian government, including parliamentary ratification and environmental approvals in June 2018, redevelopment of the Obuasi high-grade ore body has started in earnest. The first blast took place in February 2019 with the first gold production scheduled for the end of 2019. The initial production rate will be 2,000 tpd (ROM)ore for the first 12 months; production is expected to reach 4,000 tpd by the end of 2020 and ramp up to an ultimate capacity of 5,000 tpd. Annual production is expected to average 350,000–450,000 oz at an average head grade of 8.1 g/t for the first 10 years.

Fosterville ASTER Plant

The Fosterville mine is a high-grade, low-cost underground gold mine located 20 km from the city of Bendigo in the State of Victoria, Australia. The current operation was commissioned in 2005 and includes a BIOX plant. In 2010 Fosterville commissioned the first HiTeCC plant, developed by the mine technical team, for the mitigation of the preg-robbing associated with the treatment of the Fosterville ores.

Fosterville has been evaluating the implementation of the Outotec ASTER process for a number of years to improve the mine water balance by allowing treatment and then recycling of tailings dam water back into the process. In 2019 Fosterville decided to continue with the implementation of the ASTER plant, which will be the fourth application of ASTER technology.

The plant will have a design capacity of 792 m3/d of solution with the predicted thiocyanate and cyanide concentrations at 5,000 ppm and < 10 ppm respectively. The plant will consist of four ASTER tanks, each with a design volume of 225 m3, followed by a static settler to produce a clear product solution to feed to the plant process water and a thickened sludge underflow that can be recycled to the primary ASTER reactors.

Construction of the plant is well under way and commissioning is expected to take place in early 2020.

Metals refining