Selecting the preferred process for the treatment of a refractory gold ore or concentrate involves a number of aspects including the metallurgical and mineralogical characteristics of the ore, any impurities, the size and location of the project, local conditions and regulations, and utility requirements. The selected process route must also conform to stringent environmental standards.
Metso Outotec can evaluate the pretreatment technologies and develop a project together with the client all the way from the conceptual study phase to commercial operation. Typically, all three pretreatment technologies are technically feasible for most sulfide ores and can be tailored to meet the requirements of the project in question; however, robust project development including test work and engineering will help to identify the optimal choice. Metso Outotec has participated in several refractory gold projects from conceptual study through to commercial operation and we have thorough experience and understanding of the specific project drivers affecting technology selection, including CAPEX and OPEX drivers.
Technology selection is typically done in the early project development phase, for example in the pre-feasibility study phase, based on CAPEX and OPEX estimates. When Metso Outotec is involved early on in the project we can help to identify the best technological fit for the client’s project and deliver the optimum solution.
The roasting process
Roasting of sulfide ores is an established technology for gold production. Roasting can be carried out in one or two stages depending on the ore type. Single-stage roasting consists of dead roasting in an oxidizing atmosphere; in two-stage roasting the first stage is operated under reducing conditions to remove impurities such as arsenic and antimony while the second stage uses an oxidizing atmosphere to complete the oxidation process. For refractory ores, oxygen enrichment is an effective way to reduce process gas volume, which has a significant effect on plant size and consequentially CAPEX.
The major objectives of the roasting process are to eliminate the sulfur and carbon contained in the concentrate, remove impurities such as arsenic, and to provide a calcine product and SO2-containing gas. A roasting temperature of 500–800 °C is generally suitable depending on the process and the mineralogical requirements.
The products of the roasting process are calcine and roasting gas. The calcine consists of Fe2O3, Fe3O4, FeSO4, and gangue material, while the roasting gas contains SO2, SO3, O2, N2, and H2O as steam. Depending on the chloride content in the roaster feed, HCl may also exist in the roasting gas in corresponding concentrations; this is removed from the process together with the effluent stream in the wet gas cleaning section.
Feeds with a high arsenic concentration require a special treatment that uses a two-stage roasting process. Arsenic typically occurs as arsenopyrite (FeAsS). Under normal roasting conditions, with a slight excess of oxygen in relation to the stoichiometric requirement for converting the iron content of the pyrite to hematite, the majority of the arsenic is oxidized to As(V) and will react with the hematite to form stable iron arsenate (FeAsO4). In this way, about 70–80% of the arsenic content in the feed becomes bound to the cinder. This conversion can be optimized in a CFB reactor using special process conditions. The remaining arsenic, which is oxidized to As(III), is volatilized and entrained by the roaster gas and must be separated in the wet gas cleaning system.
Metso Outotec has built several gold roasting plants where each process concept has been optimized according to the chemical and mineralogical composition of the feed material. The different flowsheets for the roasting process are influenced by operating temperature for maximum gold recovery, oxygen concentration in the roaster off gas, impurity control in calcine product and off gas, requirements for heat recovery, and finally the CAPEX aspect, with the goal being to develop the most economical flowsheet possible. Notable references include:
- Syama, Mali: slurry feeding, low CAPEX
- Minahasa, Indonesia: low sulfur content in feed, partial heat recovery
- Newmont, USA: optimized heat recovery, ore preheating, high capacity
- Tongguan, China: two-stage roasting, arsenic removal
- Cengiz, Turkey: conventional dead roasting process, maximum heat recovery
The pressure oxidation process
A typical pressure oxidization (POX) circuit for refractory ores consists of the following main process steps: comminution and flotation followed by carbonate removal, chloride removal, pressure oxidation, vent-gas scrubbing, conditioning, solid-liquid separation, and neutralization. Pressure oxidation technology is applied for both ores and concentrates.
The purpose of the pressure oxidation step is to oxidize sulfide minerals such as arsenopyrite/pyrite into sulfates and release the gold that is contained in the sulfide crystal structure. The gold is not dissolved but remains in the leach residue. Oxidation is typically carried out at 190–230 °C and at an oxygen partial pressure of 3–7 bar by feeding gaseous oxygen with a purity above 98% into the autoclave. Reactions are highly exothermic, and the temperature in the autoclave is controlled by injecting cooling water into each autoclave compartment separately.
Typically, oxidized autoclave slurry is washed in a counter-current decantation circuit prior to cyanide leaching. Besides acid, detrimental elements for cyanidation such as iron and copper are washed out of the solution. The acidic solution from the autoclave and the acid pretreatment step are neutralized with lime; depending on local availability, limestone or quicklime may be used. The purpose of the neutralization step is to neutralize the acidic solutions and precipitate components such as iron, arsenic, zinc, and lead so that the neutralized slurry complies with local environmental regulations and can be stored in the tailings pond.
Recent Metso Outotec gold POX deliveries include:
- Petropavlovsk Pokrovskiy POX Hub, Russie
- Mazedaki, Turkey
- Mansurah Mansurah, Saudi Arabia (under construction)
The BIOX process
The BIOX® process for the treatment of refractory gold concentrates has been in commercial operation for 35 years following the commissioning of the first plant at the Fairview Gold Mine in South Africa in 1986. To date, 12 successful BIOX plants have been constructed in 10 countries, with the 13th nearing completion in Zimbabwe. The BIOX process has been proven to operate successfully over a wide range of concentrate feed grades and mineralogies and in varying climatic conditions and site altitudes.
During the BIOX process sulfide minerals such as pyrite, arsenopyrite, and pyrrhotite are oxidized in a microbially assisted chemical leaching process. These oxidation reactions are highly exothermic, generating on average 30 MJ per kg of sulfide oxidized depending on the mineralogy of the concentrate. The BIOX bacterial microbial culture needs to be maintained at a temperature of around 40 °C, and the reactors are cooled by circulating water through internal cooling coils. The amount of cooling required per reactor is dependent on the extent of the sulfide oxidation and the magnitude of heat losses from the reactor, mainly in the form of convection and radiation.
Recent Metso Outotec BIOX deliveries for gold concentrate treatment include:
- Fosterville BIOX Plant, Australia
- Suzdal BIOX Plant, Kazakhstan
- Kokpatas BIOX Plant, Uzbekistan
- Runruno BIOX Plant, Philippines
- Cam&Motor BIOX Plant, Zimbabwe
The Suzdal plant in Kazakhstan was the first BIOX plant to operate at sub-zero temperatures. The original design capacity of the plant was 192 t/day of concentrate at 12% sulfide sulfur. Detailed heat balance calculations performed for the Suzdal BIOX process indicated that all the reactors except the last secondary reactor would require cooling, even at the minimum design temperature of –50 °C. Convection heat losses from this reactor could be reduced by installing insulation material with a low thermal conductivity on the outside of the reactor.
Metso Outotec’s MesoTHERM technology uses a combination of microbial consortia as a hybrid, two-stage biooxidation process where mesophiles are used to realize the primary-stage sulfide oxidation and thermophiles to bring about the latter stage, near complete sulfide oxidation. MesoTHERM technology has been shown to reduce cyanide consumption by as much as 50% compared to conventional leach consumptions from mesophile BIOX processing alone, from 20 kg NaCN/ton to around 8–10 kg NaCN/ton.
Technology selection criteria
The selection of the preferred or optimum process for a specific project will depend on many factors including technical, financial, operational, and regulatory aspects, and the selection of an appropriate technology supplier is also important. The table below lists several aspects that need to be considered under each category; other aspects can be added based on the project-specific drivers.
Technology selection is usually done during the pre-feasibility stage of a project and is based on limited or batch test work and PFS-level CAPEX and OPEX estimates. It is therefore critical to have in-depth knowledge of all the processes being evaluated in order to be able to perform a fair evaluation of the processes at such an early stage. Outotec has extensive experience in the accurate interpretation of test-work results and in the extrapolation of results from batch tests to continuous performance, in order to determine the process design criteria for the plant. Outotec also has an extensive database that can be used to benchmark the test-work results against similar projects and to predict the performance of the working commercial plant.
Once the technology is selected the process is typically evaluated in more detail as part of a definitive or bankable feasibility study. This study should include a detailed review of the decision framework used to select the technology.