Bulk ore sorting improves profitability and sustainability
Pre-concentration is more than an industry buzzword. Removing barren material early enough can bring significant savings, especially in today’s low-grade mining operations. It also reduces the environmental footprint of the mine due to lower energy consumption, greenhouse gas emissions and water losses per ton of product.
High-grade ore deposits are depleting fast. Consequently, the mining industry has to take on lower-grade deposits, which are more difficult to extract. These deposits require the mining, movement and processing of larger volumes of material per ton of valuable mineral. The transportation, comminution and processing of these large volumes is expensive and energy intensive.
These low-grade ore bodies generally contain a large proportion of liberated barren gangue, or, in other words, material of no worth. What if there was a way to eliminate this barren material from the process early on? This is exactly what pre-concentration is about.
Pre-concentration is the rejection of gangue from coarse feed. It increases the grade of the ore proceeding to the next stage of processing and avoids feeding the plant with material that will cost more to process than the respective value of the contained valuable mineral. Less tons of ore are treated per ton of product, thus reducing the costs, energy and water consumption per ton of product. This effectively creates more value with less impact, improving the resource efficiency and sustainability of the operation.
Because gangue tends to be high in silicates and typically harder and more competent than the valuable minerals, removal of this hard and barren material prior to comminution stages also has the potential to significantly reduce energy consumption and processing costs, and may also reduce ore transport requirements.
What is bulk ore sorting?
Bulk ore sorting is a pre-concentration technology in which large volumes of barren gangue are separated from a fully loaded conveyor belt based on the grade as measured or inferred from a sensor measurement. With bulk ore sorting, ore that previously didn’t qualify for processing may be upgraded, making it economic to treat and improving the resource utilization. More valuable metal may be extracted from the resource while the processing plant treats less tons at higher feed grade.
Bulk ore sorting could also be used to separate ore types to treat via different process routes, or to reduce dilution and ore loss in mining operations by improving grade control. It is an efficient way to deal with uncertainties of grade, particularly where the complexity of mine geology makes the estimation of grade difficult. This helps the mining operation to achieve the planned cut-off grade and optimize extraction of the resource.
Current sorting technologies can’t handle large volumes
Ore sorting relies on measuring a property that is different in the valuable and waste components using some form of sensor. A variety of sensors are available, and those commonly used in industrial applications include photometric, electromagnetic, radiometric and x-ray. Sensor-based ore sorting is not new, and it has been shown to be technically feasible. However, in the minerals industry, it is currently only used in some niche applications, such as industrial minerals (e.g. calcite, rock salt or talc), diamonds and other gemstones.
Current sorters separate individual particles. They require careful feed preparation so that individual particles can be detected and measured, and ejection is usually achieved by blasts of compressed air. Therefore, current sorters have very low capacity (up to 300 tph for larger particles and much less for smaller particles), making them unviable for high tonnage pre-concentration.
To make sorting viable for pre-concentration, it should be applied to bulk quantities of ore, such as on a loaded truck tray or a fully loaded conveyor belt.
New developments underway
The Australian Commonwealth Scientific and Industrial Research Organization (CSIRO) is developing a sensor using magnetic resonance (MR) that has the ability to rapidly measure batches of ore on large primary production conveyors. The MR sensor is well suited to a bulk ore sorting application, as it is penetrative and can measure large throughputs on fully loaded conveyor belts. In addition, the measurement response time is rapid, thus allowing diversion of different grade streams in an ore sorting application. However, the MR sensor measures an individual mineral (not element) and may have limitations measuring ores with complex mineralogy. The sensor is currently developed for chalcopyrite, a dominant copper mineral, and with further development could potentially be applied to other minerals.
Using several types of sensors together may be a way to overcome the limitations of the different sensor types. A control system is also required to interpret the data from the sensor or sensors and make an “accept” or “reject” decision, and a diversion system, such as a diverter gate, is required to separate the valuable batches of ore from waste. Metso has developed conceptual designs for implementation of bulk ore sorting on plant feed or in-pit conveyors treating up to 3,600 tph and with belt speeds of up to 5 m/s.
Exploiting the heterogeneity
Most mining deposits are naturally heterogeneous and lend themselves well to bulk ore sorting, but it should be implemented as early in the process as possible before excessive mixing occurs. Material presented to the sorter needs to have sufficient grade variability occurring in large enough batches of material for effective separation. But every time the ore is rehandled, transferred, crushed or blended, the degree of mixing increases; reducing the variability and thus the potential for effective separation of batches of barren gangue from ore. Therefore, either in-pit or plant feed conveyors provide the best opportunities for bulk ore sorting; early in the process the sorter can exploit the natural heterogeneity of the deposit.
However, current mining practices are generally designed to blend out the variation and provide a consistent, stable feed to the processing plant. The mining industry typically takes a ‘one size fits all’ approach and endeavors to treat all the ore from a deposit through one extraction process. The process is designed for the average or typical ore. This often means that the process includes a blending strategy to ensure a stable feed grade to the plant. Maximizing the value of bulk ore sorting requires a shift in mining practices. The goal should be to exploit the natural variability in the deposit rather than blend it out.
A perfect complement to in-pit crushing and conveying
Because the natural heterogeneity of the ore makes sorting more effective, a bulk ore sorter should be placed as early as possible in the process. This also avoids extra costs and the lost capacity associated with treating non-economic material. In fact, the benefits of discarding barren material early on are carried through all the downstream process steps.
In almost all cases, bulk ore sorting would need to be implemented after primary crushing to present material at a size that can be handled by the sorter. Each additional crushing or material transfer stage incorporated prior to sorting increases mixing, costs and unnecessary energy consumption, so these should be avoided if possible.
Conducting bulk ore sorting at the mining face, either in-pit or underground, allows ore and waste to be directed to the appropriate destination (waste dump or process) immediately. The feasibility of this approach will depend on having suitable space available for the sorter as well as on the mining method employed and potential impact on mining productivity.
Underground pre-concentration, prior to haulage or hoisting, produces solid waste underground, which can be combined with tailings and cement from the surface as required and disposed of as fill. In open-pit mines, bulk ore sorting could potentially fit well with in-pit crushing and conveying systems. The sensor(s) would be located on a conveyor leaving the pit, and would use a flop gate to separate waste and ore onto their respective conveyors. Furthermore, a system equipped with multiple flop gates could be used to separate different ore types onto separate stockpiles, which could be either blended according to the downstream process requirements or sent to different process routes.
Multiple benefits in both Greenfield and Brownfield operations
By removing coarse barren material, pre-concentration has the potential to significantly reduce the amount of material that requires downstream processing. If conducted close to the mining face, it can potentially reduce ore transport requirements by rejecting barren gangue and transporting less ore to the processing plant. Pre-concentration effectively upgrades the plant feed; less tons of ore are treated in the processing plant per ton of product, thus reducing the costs, energy and water consumption per ton of product.
In existing operations with fixed plant capacity, the production rate can be increased after sorting due to the increase in feed grade. In Greenfield operations, the size of downstream processing equipment can be reduced – reducing the capital and operating costs – or the production rate can be increased.
Additionally, bulk ore sorting can reduce dilution and ore loss in mining operations by improving grade control. In some cases, mining costs may be reduced, with the bulk ore sorter providing selectivity thus allowing less selective mining processes.
Better recovery, less tailings
Pre-concentration can upgrade previously uneconomic material to valuable ore. It may enable the recovery of valuable components from waste dumps, low-grade stockpiles and marginal reserves that would otherwise be uneconomic to treat.
The environmental footprint of the mine is also reduced, due to lower energy consumption, greenhouse gas emissions and water losses per ton of product. Less fine wet tailings are produced, requiring a smaller tailings storage facility and minimizing the surface impact. Even though the waste dump area may, in some cases, increase, the dry coarse waste from the sorter could be useful as aggregate or for other fill purposes.
Despite the apparent benefits, the uptake of pre-concentration has been slow. This is possibly due to perceptions of unacceptable metal losses, insufficient understanding of ore characteristics, and a lack of understanding of the systemic impact. However, metal losses in pre-concentration are offset, if not completely compensated for, by the increased recovery in downstream processes due to the higher feed grade, and the overall economics of the project benefit from reduced costs and/or increased production.
Limitations in sensor capabilities and, most importantly, the low throughput of the existing individual particle sorters is also a significant deterrent to application. A bulk ore sorting system, on the other hand, is much simpler, less expensive and has a much smaller footprint than the current individual particle sorting technologies available. The additional costs associated with bulk ore sorting are likely to be outweighed by the reduction in either downstream processing costs or by the increase in production.