As high-grade deposits are depleted, mines are faced with lower feed grades. This requires the mining and processing of much larger volumes of material per tonne of product.

The aim of preconcentration is to remove barren material as early in the process, and at as coarse a particle size as possible, to reduce the amount of material requiring downstream comminution and processing.

Preconcentration can significantly improve the economics of a project by upgrading below cut-off grade material and/or increasing production rates. Furthermore, gangue is usually high in silicates, harder and more competent than valuable minerals, so removal of this barren material prior to further comminution can significantly reduce energy consumption and processing costs. Ore transport requirements such as hoisting and truck or conveyor haulage may also be reduced, and there is less water consumption and a smaller volume of fine, wet tailings to dispose of per tonne of product.

There are several technologies that may be applicable for preconcentration including: gravity concentration, screening, sensor based ore sorting (SBOS) and magnetic separation. The suitability in each case depends on the ore properties as well as site and economic conditions.

Selecting a method

Preconcentration by gravity processes requires a distinct density difference between the ore and gangue minerals at fairly coarse sizes. Finely disseminated ores are generally not suitable, as the density difference only becomes apparent when the particles are finely ground.

Jigging or dense medium separation (DMS) is an efficient gravity concentration option that is suitable for a wide variety of ores at a relatively low capital and operating cost. For example, Banks Island Gold in British Columbia experienced higher dilution than expected due to old workings (8g/t gold versus 16g/t gold), but preconcentration with DMS rejected up to 50% of the mined mass and the tailings were disposed of underground.

In some cases, valuable minerals can be softer or more friable than gangue minerals and are concentrated in fines after initial breakage; therefore, pre-concentration can be achieved by screening and discarding the coarse barren material. Despite being simple, reasonably high capacity, and having low costs and technical risk, there are few applications of preconcentration by screening. This may be partially due to the fact that while the relationship between grade and particle size in plant feed is commonly understood, it difficult to measure representatively across the orebody due to limited access, coarse particle size and large tonnages. This makes it difficult to demonstrate the business case and limiting the application of preconcentration by screening. One of the most well-known examples was the Bougainville copper pre-screening plant in PNG which increased production and resource utilisation by a combination of lowering the cut-off grade and increasing concentrator throughput.

Preconcentration may also be achieved using SBOS, which measures a property that is different in the valuable and waste components using a sensor or combination of sensors. The current commercially available technology is based on the measurement and separation of individual particles, and thus is throughput limited.

This technology has proven to be successful in smaller, niche applications. However, to be practical in preconcentration applications for large-scale mining operations, SBOS needs to be applied to bulk quantities of ore such as at the shovel, a loaded truck tray or batches of ore on a fully loaded conveyor belt.

Bulk ore sorting on a fully loaded conveyor belt would require:

• A fast and penetrative sensor or combination of sensors;
• A control system to interpret the data and make an ‘accept’ or ‘reject’ decision, and
• A diversion system such as a diverter gate to separate the valuable ‘batches’ of ore from waste.

A complete solution for bulk ore sorting is not yet commercially available. There are online grade measurement systems, but many of these need to be adapted to increase speed and linked with a control and diversion system to allow accurate separation and discarding of waste. Most existing sensors are not currently suitable for bulk ore sorting as they are either not sufficiently penetrating or too slow for effective separation. However, it may be possible to adapt existing sensors (at increased costs) and/or use a combination to overcome limitations.

Applications and benefits

Preconcentration effectively upgrades the plant feed; fewer tonnes of ore are treated in the processing plant per tonne of product, thus reducing the costs, energy and water consumption per tonne of product. If conducted as close as possible to the mining face it can also significantly reduce ore transport requirements.

There are many different ways in which preconcentration can be applied, and the potential benefits and overall impact on project economics depends on the ore deposit and characteristics, mining and processing methods, site conditions, local economic climate, etc. Some examples include:

• Increasing production and reducing costs for an existing operation with limited processing plant capacity: Preconcentration upgrades the plant feed thus increasing the amount of product per tonne of ore treated. In a process-constrained operation this increases the production rate and revenue while the amount of ore treated (and associated costs, energy and fine wet tailings) are reduced per tonne of product. A techno-economic assessment was conducted to evaluate the viability of bulk ore sorting for the PanAust Phu Kham copper deposit in Laos for the current processing plant. The evaluation demonstrated that bulk ore sorting has the potential to increase the amount of metal in product and/or reduce processing plant throughput (and costs) to improve the annual inferred cash flow for the conditions at the time. Even greater benefits would be realised later in the mine life when harder ore makes the operation SAG mill limited (Valery et al., 2016). 

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• Improve resource utilisation by lowering cut-off grade: Due to upgrading of the plant feed by preconcentration, the cut-off grade can be lowered and mining reserves increased. In greenfield projects, the size of downstream processing equipment may be reduced (reducing the capital and operating costs) or the production rate may be increased.
• Improve grade control and reduce mining and transportation costs: Preconcentration of ore underground or at remote satellite sites can reduce transportation costs. Measurement with a bulk ore sorter can also decrease misclassification of ore and waste, reducing dilution and ore losses. It can provide selectivity and accurate implementation of cut-off grade thus requiring less selective (costly) mining processes.
• Recover marginal material: Low-grade or waste stockpiles can be upgraded by preconcentration making them economic to treat and maximising resource yield. An assessment of particle ore sorting to preconcentrate low-grade gold ore stockpiles at Central Norseman Gold indicated high gold recoveries at high mass rejection rates. The financial evaluation suggested it would be highly viable with strong cash flow generation at stockpile grades as low as 0.7g/t gold (Parry and van Wyk, 2016).
• Optimise process route: Different ore types can be separated and the process route can be optimised for each. For example, higher-grade material may be recovered by a higher cost or higher recovery process such as flotation, while lower-grade material is processed by lower cost or lower recovery processes such as heap leaching.
• Reduce environmental footprint: The environmental footprint of the mine can be reduced due to lower energy consumption, greenhouse gas emissions and water losses per tonne of product. In addition, less fine wet tailings are produced, and in the case of ore sorting, dry coarse waste may be useful as aggregate or other fill purposes.

Walter Valery is global director – consulting and technology, and Kristy-Ann Duffy is process consultant / mining and mineral processing, both at Hatch

References

• Parry, A N, and van Wyk, G, 2016. Upgrading Low-grade Gold Ore Stockpiles by Preconcentration Using Ore Sorting – an Assessment of the Economic Impact and Viability, in Proceedings of the 13th AusIMM Mill Operators’ Conference, (The Australasian Institute of Mining and Metallurgy: Melbourne), Perth, WA, 10 – 12 October, 2016, pp 179 - 188.
• Valery, W, Duffy, K, Holtham, P, Reple, A, Walker, P, Rosario, P, 2016. Techno-Economic Evaluation of Bulk Ore Sorting for Copper Ore at the PanAust Phu Kham Operation, IMPC 2016 - XXVIII International Mineral Processing Congress, September 11-15, Québec City Convention Center.