Cost-effective ASTER™ thiocyanate detoxification, Runruno Gold Mine, Philippines

The ASTER™ process was developed to deliver an improved and integrated water balance for re-use in BIOX applications. This simple technology has proven to be an effective process for degradation of thiocyanate and cyanide in tailings dam water at thiocyanate concentrations up to 4 g/L and free cyanide levels as high as 50 mg/L.

ASTER is an acronym for Activated Sludge Tailings Effluent Remediation, which is a biological thiocyanate and cyanide destruction process, rendering a non-toxic solution that can be recycled to upstream metallurgical circuits or safely discarded to the environment.

About the Customer

FCF Minerals Corporation (FCF), a wholly-owned subsidiary of Metals Exploration Plc (Metals Exploration), is a Philippine-based resources company and the owner of the Runruno gold project.

Metals Exploration is a natural resource exploration and development company operating in the Pacific Rim, focusing on the Philippines. The Company’s principal asset is the Runruno Gold-Molybdenum Project on the island of Luzon in the northern Philippines which has attracted a series of awards and recognition both internationally and nationally.

INTERNATIONAL AWARDS

• Mining Journal Outstanding Achievement Award
• Best Community Development Initiative Award (Asia Mining Congress – Singapore)

NATIONAL AWARDS

• Presidential Mineral Industry Environmental Award (PMIEA)
• Safest Mines Award - Exploration Category
• Mining Forest Award
• Platinum Award
• The Kabalikat Award

About the Project

The Runruno Gold Project is located on the island of Luzon, approximately 200 miles north of Manilla in the Philippines. The mine is a surface mine operation and uses the proven BIOX® and carbon in leach processes to recover gold, and the ASTER process for detoxification of the CIL residue prior to discharge to the Residue Storage Impoundment (RSI). Work to date has defined a mineral resource of 1.39 million ounces of gold, and the operation will produce 96,700 ounces of gold per annum over a mine life of 10.4 years. The ASTER circuit is designed to treat 5 000 m3 of slurry at a solids concentration of 4-6 % solids and 500 ppm thiocyanate.

Processing at Runruno

The ROM ore is crushed in an MMD sizer prior to milling in a single-stage SAG mill. A gravity circuit on the mill discharge is used to recover gravity recoverable gold. The Outotec BIOX process is used to treat the refractory gold sulphide concentrate ahead of conventional CIL treatment. The CIL circuit residue is washed in a cyclone circuit to produce a thickened slurry and an overflow solution containing most of the cyanide and thiocyanate, as well as 4 – 6 % solids. The overflow solution is treated using a detox process to reduce the free cyanide concentration to below 50 ppm prior to ASTER treatment to reduce the total cyanide and thiocyanate concentrations to below 1 ppm. The combined flotation tailings, neutralized effluent and detoxified CIL residue is pumped to the RSI. Process water is recovered from the RSI and recycled to the processing plant as required.

Background

Metals Exploration commenced drilling at the Runruno project in 2005 followed by the announcement of an inferred resource of 2.0 million ounces of gold in 2006. Development of the project continued with the completion of a scoping study in 2008. BIOX was selected as the preferred technology based on the lower capital and operating cost of the process combined with the ASTER process for treatment of the CIL residue prior to discharge to the TSF. BIOX and ASTER commissioning was completed in 2016 with the first BIOX product pumped to the CIL section.

The ASTER Process

ASTER is an acronym for Activated Sludge Tailings Effluent Remediation, which is a biological thiocyanate and cyanide destruction process, rendering a non-toxic solution that can be recycled to upstream metallurgical circuits or safely discarded to the environment. The process was specifically designed to treat cyanidation tailings effluent produced during the biological processing of refractory gold ores using the BIOX process, where thiocyanate is the primary contaminant. The Outotec ASTER process delivers an improved and integrated water mass balance for subsequent upstream re-usage of the treated solution in BIOX applications.

Commercialized in 2010, the ASTER Process relies on a range of aerobic microorganisms to oxidize thiocyanate and cyanide in effluent streams, enabling the recycling of the treated water to BIOX plants. ASTER can also be used to improve water balance at hydrometallurgical operations and enhance the safe management of gold leach residue solutions.

ASTER Advantages

The ASTER Process has a simple flowsheet, which makes it easy to operate while the Outotec OKTOP® ASTER Reactor design ensures high process performance and robustness. The process is supplied with a full range of services offering and knowledge transfer.

The ASTER process utilizes non-pathogenic naturally occurring bacteria with no harmful chemicals used in the process. The microorganisms used in the ASTER process can adapt, within limits, to changes in flow rates and substrate concentrations in the short term, providing robustness to the process. In addition, over the longer term, natural adaptation can increase the tolerance of the bacteria to specific contaminants significantly.

The major benefit for ASTER is that it can treat high levels of thiocyanate in the feed and reduces it to levels that allow the product solution to be recycled back to the process plant, thereby enabling a closed water balance or, subject to local environmental legislation, to be safely discarded to the environment.

Process Development for the Runruno Application

Extensive continuous ASTER pilot-plant test work was conducted in South Africa to validate and confirm the ability of the process to meet the project-specific requirements. The test work results demonstrated the adaptability of the ASTER bacteria culture to the Runruno feed slurry and confirmed that the selection of ASTER as the thiocyanate (SCN) degradation technology was feasible.

The results proved the stability of the process and, based on the results, the Runruno ASTER design specification was confirmed. The Runruno ASTER plant was configured as three primary reactors operating in parallel followed by one secondary reactor resulting in near-complete degradation of all thiocyanate in the ASTER feed and ensuring that the residue slurry is safe to discharge to the RSI and the water recycled to the process water tank.

The Runruno ASTER is both the largest ASTER circuit to date with a capacity of 5000 m3/day, and the first ASTER™ application treating a diluted feed slurry containing ~ 4 to 6 % solids. 

Close Customer Collaboration

We worked in close collaboration with our customer, providing test work, consulting and process engineering services through the development of the project, from first ASTER scoping tests through to bankable feasibility study. The test work confirmed that the ASTER bacteria adapted easily to the Runruno CIL solution as well as the dilute slurry, resulting in complete removal of all thiocyanate.

Process Design and Scope

We managed the development of the process during the test work phases and the generation of the process design criteria for the design of the full-scale ASTER plant. This included extensive continuous ASTER test work to confirm the technical viability of the application of the process for this duty. We were also responsible for the development of the process design specifications for the ASTER plant including the ASTER feed section, ASTER reactors, and associated reagent addition systems and services.

Support Services Throughout

As part of the standard scope delivered to every ASTER project, we supply comprehensive consulting and advisory services during the detailed design of the plant. This service is extended through construction and includes construction verification and commissioning planning. As part of the scope, we also supply the bacterial inoculum for the process. Our team of Outotec process engineers work closely with the site personnel and assist during the commissioning of the commercial plant until commercial production.

Results

The Runruno ASTER plant was the first application of the process for the treatment of a dilute slurry, and the successful commissioning and operation of the Runruno ASTER plant, therefore, represents the next generation in the development of this technology.

The ASTER plant was commissioned in 2016 first treating residue for the direct leach of the oxide ore followed by treatment of the BIOX CIL residue.
To date the performance of the ASTER plant has exceeded expectations, consistently delivering a detoxified solution containing less than 1 ppm cyanide and thiocyanate. The robustness and adaptability of the process ensured that the performance could be maintained during variations in the feed rate and feed chemical composition.

The successful operation of the ASTER plant at Runruno allows the mine to effectively manage their water balance, maximizing the return of process water from the RSI and limits the use of freshwater.

Conclusion

The ASTER process for the biological destruction of SCN and CN in the CIL residue prior to discharge to the RSI was successfully implemented. The process offered technical, economic and environmental advantages over alternative processes. We provided our expertise and experience in the testing, design, and commissioning of ASTER plants and worked in close collaboration with our customer during project development from first batch ASTER amenability tests through to commercial production. A bespoke test work program was developed and executed for our customer, generating the required level of confidence for bankable feasibility while taking into consideration the project-specific technical and environmental requirements.

Runruno’s modern ASTER plant represents the next generation of in the development of the ASTER process. The stability of the process and consistent results achieved confirmed the technical robustness and efficiency of the ASTER process.

*This case study has been written prior to the merger of Metso and Outotec.