Technology

	BIOX® ASTER fact sheet [PDF - 1MB]
	Aster™ brochure [PDF - 1MB]
	BIOX® brochure [PDF - 4MB]

INTRODUCTION

Gold Fields owns the BIOX^® and ASTER™ biological processes. The BIOX^® process was developed for the pre-treatment of refractory ores and concentrates ahead of a conventional cyanide leach for gold recovery, while the ASTER™ process, an acronym for Activated Sludge Tailing Effluent Remediation, is a biological thiocyanate and cyanide destruction process to deliver an improved and integrated water balance.

THE BIOX^® PROCESS

It has been more 25 years since the BIOX^® technology was first commercialised, and this is now being used successfully around the world for the pre-treatment of refractory sulphide ores. What started off as a small research project in the 1970s has, over the last two decades, culminated in a mature, commercially viable process with 11 commercial BIOX^® plants and one demonstration plant having been commissioned: there are currently 10 plants in operation.

The BIOX^® process was developed for the pre-treatment of refractory ores and concentrates ahead of a conventional cyanide leach for gold recovery. The gold in these ores is encapsulated in sulphide minerals such as pyrite, arsenopyrite and pyrrhotite thus preventing the gold from being leached by cyanide.

The BIOX^® process destroys the sulphide minerals and exposes the gold for subsequent cyanidation thereby increasing the overall gold recovery that can be achieved. The BIOX^® process has many real advantages over alternative refractory processes such as roasting or pressure oxidation. These include:

• Improved rates of gold recovery
• Significantly lower capital cost
• Low operating cost
• Robust technology that is suited to remote locations
• Low levels of skills required for operation
• Environmentally friendly
• Ongoing process development and improvement
• Backed by over 25 years of operating knowledge by an experienced team that provides valuable technical support

The process utilises a mixed population of bacteria to break down the sulphide mineral matrix thereby liberating the occluded gold for subsequent cyanidation. The bacterial cultures in the BIOX® reactors are, however, not controlled but rather allowed to adapt to the concentrate and operating conditions. It is therefore important to control the pH and temperature within narrow ranges to maintain the right balance of bacterial species to optimise the rate of oxidation.

The BIOX^® process involves the continuous feeding of the flotation concentrate slurry to a series of stirred reactors. The bacteria require nutrients to sustain growth and nitrogen, phosphorous and potassium are added to the primary reactors as a solution of ammonium, potassium and phosphate salts. The bacteria also require sufficient carbon dioxide to promote cellular growth. Carbon dioxide is obtained from the carbonate minerals in the ore and from the air added to the process.

The BIOX^® culture operates best at a temperature of 40°C. However, it is possible to run at 45°C in the primary stage and even at 50°C in the final secondary reactors. The oxidation reactions of sulphide minerals are exothermic. Therefore, it is necessary to cool the process to maintain the slurry temperature within the optimum range.

The typical operating pH range in the BIOX^® process is 1.2 to 1.6. Lime, limestone and/or sulphuric acid are used to control the pH in the reactors. The oxygen consumption during direct sulphide oxidation is high and large volumes of air have to be injected and dispersed in the slurry. This is one of the main engineering challenges in the design of a full scale bio-reactor.  

THE ASTER™ PROCESS

The ASTER™ technology was developed in the late 1990s and relies on a range of aerobic microorganisms to oxidise thiocyanate and cyanide to levels below 0.5 mg/L.

The ASTER™ process was developed to deliver an improved and integrated water balance for re-use in BIOX® applications. Previously, the recycling of tailings process water containing concentrations of thiocyanate and cyanide in excess of 10 mg/L was impractical owing to the almost zero tolerance of the microorganisms used in the bioleaching of sulphide minerals to these species.

Moreover, additional motivation was born out of the environmental legislation associated with the disposal of cyanidation tailings and water discharge becoming increasingly stringent worldwide. Various processes exist today for treating thiocyanate and cyanide species and are generally categorised as a recovery or a destruction process. The latter processes typically involve the breaking of the carbon nitrogen bonds thereby destroying the cyanide species producing less toxic species. Low levels of cyanide and high levels of thiocyanate in tailings solutions make the ASTER™ process fit for purpose as a destruction process.

The ASTER™ process offers competitive advantages to existing destruction Processes, including:

• Efficient thiocyanate and cyanide removals to ≤ 0.2 mg/L
• Optimal microorganism performance to tailings feeds containing 4 000 mg/L thiocyanate and 100mg/L cyanide
• Simple metallurgical process with low capital cost
• Robust technology that is suited to remote locations
• Low levels of skill required for monitoring
• Low operating costs
• Backed by a significantly experienced team

The year 2010 was a significant milestone for the ASTER™ process with the first commercial plant being installed at the Barberton Mines, Consort plant, in South Africa. This was the culmination of a 15-year research and development programme toll-gated through the various laboratory, pilot and demonstration stages to realise a final process development translated into a commercial plant through a simple process engineering design.

The process is currently being tested on batch and pilot plant scale for projects in Kazakhstan, Philippines, Australia and Peru. The operating window of the process is also continuously expanded through a focused research and development programme.