Factors in a Gold Process Selection

The factors affecting process selection can be grouped into six areas.

Geological: The grade and reserve of economic minerals in an orebody determine the type and scale of process technology that can be applied. Low grade ores and tailing materials (0.5-1.5 g/t) usually require low cost treatment, such as heap and dump leaching. Higher grade ores, typically more than 1.5 g/t, may be treated by higher cost processes, such as grinding, leaching and Carbon in Pulp, for which the additional cost are more than offset by higher gold sales revenue. Complex sulphidic and carbonaceous refractory ores require yet higher grades to justify the additional expense of oxidative pretreatment.

The geometry of an orebody not only affects the mining method, but may also dictate the sequence of mining different regions, and possibly different ore types, within the orebody which can be important processing consequences. Variations in ore properties, such as ore hardness, mineral composition, alteration, degree of fracturing and clay content, invariably reduce process efficiency and can significantly affect process selection. For example, orebodies containing pockets of mercury or copper may need special processing techniques, and ores containing highly altered or fractured regions may have particular materials handling requirements. Such variations can be smoothed out to a large extent by blending, where the variability of the ore determines the amount of blending required. However, the process selected must be able to cope with ore type variations that are inevitable, even after blending.

Mineralogical: The mineralogical properties of an ore determine its response to the various process options and indicate the potential environmental impact of its treatment. The mineralogical characteristics are determined from the ore composition and textural properties. Such date is used in conjunction with metallurgical testwork results and information from other similar orebodies for process selection and flowsheet development.

The quality of mineralogical information required for effective process selection depends on the t ype and variability of the orebody. Ores with simple mineralogy, or with similar geological and mineralogical properties to those of other well understood deposits, require less rigorous analysis than those with complex or unknown mineralogy, although even subtle changes in mineralogy can greatly affect process selection and overall process economics. For example, the mineralogy of ores from the Witwatersrand region in South Africa is well established and relatively consistent, while more complex refractory ores of the Pacific Rim and parts of North America are more variable and generally require more detailed investigation.

Metallurgical: The metallurgical response of an ore to a proposed treatment scheme directly determines the economics of the process, or combination of processes used. The factors to be considered in the evaluation are:

• Recovery of gold and other valuable minerals.
• Quality of product, and the need for further processing.
• Treatment rate.
• Capital cost.
• Operating cost.
• Environmental impact.
• Technical risk.

The factor 1-3 affect the revenues generated by the project, items 2-6 affects process costs, and 7 is the level of uncertainly associated with a process. This lat factor depends on the track record and complexity of the technology applied, and the ability of the project to absorb unexpected costs associated with the application of higher risk technology. The optimum flowsheet selection yield the greatest economic benefit, while meeting the other critical project requirements such as compliance with environmental policy and maintaining acceptable levels of risk.

During metallurgical evaluation it is important hat all the sources of available information are utilized. The result of testwork performed on representative ore samples from the project under development usually provides the most accurate date; however, mineralogical information, process design data and operating experience from other similar operations and orebodies should be considered.

Environmental: Environmental considerations have played an increasingly important role in the development and exploitation of all mineral resources in the last years. Legislation has been passed to restrict the use of environmental unacceptable processes and to control others. This legislation has been developed at different rates and to varying degrees around the world and can have major effect on process selection and operation.

Process selection must be considered the environmental impact that each unit process has on the following: water quality, air quality, land degradation, visual impact, noise, flora and fauna, rare and endangered species, and cultural resources. These are affected by the following aspects of chemical extraction processes: the types and amount of wasted produces (solids, liquids, and gases), the short and long term stability of waste products, alteration of minerals and metal by the process, the process water balance and the need for discharge, and the method of waste disposal and treatment.

Any proposed flowsheet must be capable of conforming with regulatory requirements and any environmental impact, whether regulated or not, should minimized by good process design, effective waste management and reclamation procedures, to the extent necessary, by detoxification or treatment waste streams.

Geographical: The location of the orebody and the proposed treatment facility may have an important effect on process selection. The main factors include: climate (rainfall, temperature ranges); water supply; topography; infrastructure; availability of equipment, reagents and supplies; communications, political environmental; availability of skilled and unskilled labour, and sites of archaeological or religious importance.

Economic and Political: Economic and political factors which may affect process selection are many and varied. The most important of these are the price of gold (and other metals value, e.g. silver, uranium, platinum group metals), tax rates and structures and the prevailing economic and political climate, both locally and worldwide.