Can electric pulses improve tungsten recovery?
Europe has a long mining history, and suffers from a case of ‘low hanging fruit’ – the easiest to mine and most profitable ore deposits were mined first, a long time ago, leaving behind many technically complex or challenging orebodies to extract metals from, largely due to being either too small and scattered, of low metal content, or mineralogically complex, making economic metal recovery difficult with current state-of-the-art technology and processes.
The FAME project was part of the first wave of Horizon 2020 projects designed to secure a European supply of critical raw materials where production was currently dominated by non-EU countries.
The FAME project sought to address the technical, economic and environmental issues that arise from working these complex deposits through a holistic approach and seek to use flexible and possibly mobile technologies that can ensure economically viable extraction of these resources and minimise the social and environmental impacts that arise. Part of this work investigated the use of a non-traditional crushing method: Electric pulse fragmentation (EPF).
EPF was selected as it has been demonstrated to have the potential to selectively break the more metal bearing particles in the material feed[1] or weaken them[2]. Concentration of the metals at the crushing stage can save costs in transportation of material by reducing the amount of waste rock (gangue) transported to a mill, and by reducing the energy spent in comminution by ensuring less gangue arrives at the mill. Weakening of the material can improve energy efficiency by reducing the energy required to mill a particle, allowing overall reduced energy usage, resulting in energy (and thus ecological and economical) savings.
This article published as part of the FAME project, compares EPF and traditional crushing of a tungsten (W) ore – In this case a scheelite bearing skarn ore. Scheelite is difficult to process as it is extremely brittle leading to overgrinding and losses of tungsten in later stages of mineral processing.
Method
Ores were EPF treated in a Selfrag Lab system[3] to a fixed energy input to the sample determined by machine settings. The product size distribution was then measured, and the W content of each size fraction determined. A similar amount of material crushed mechanically to roughly the same size distribution for a comparative test. Both products were then crushed to 1mm and weakening tests conducted.
Figure 1 From Bru et al., 2020.
Results
Images of the EPF treated material show that fractures are generated near metal bearing ore minerals, confirming that EPF generates fractures within a rock and that these fractures generally occur at mineral grain boundaries.
During milling, EPF treated material required less energy to reach the target size than mechanically treated material, although additional energy was used as part of the EPF treatment making it overall more energy intensive than mechanical crushing in this test. This shows that these fractures can have a significant effect on comminution energy, and this must be further investigated to optimise the EPF treatment.
Product size analysis (Figure 7) shows that EPF produces fewer fines (<1mm) after ball milling. This is an advantage where overgrinding can be an issue. Conventional crushing also concentrated W into the finest fractions (<20µm), which could causes overall loss of W in the process as it is too fine to recover. EPF showed increased concentration of W in the 20 - 125 µm size fractions (Figure 15), indicating that EPF keeps W relatively coarse, making it easier to recover.
Summary
+ Weakens rock, reducing energy needed to mill.
+ Concentrates metals into finer fractions, without overgrinding.
- EPF & Conventional crushing to a target size used more energy overall than conventional crushing alone.
- New disruptive technology which needs plant redesign.
There are positives and negatives to EPF treatment of ores. The main negative is relating to the increased energy usage when combined with mechanical crushing, with the positives relating to potential less losses to fines. This then becomes an economic argument – if the coarser liberation of metal ores achieved by EPF can reduce the amount of W lost to tailings, this justifies the increased energy consumption and can be an attractive option. It is also worth mentioning that these tests were not optimised in terms of EPF treatment and crushing, and the net benefits may be understated. Studies have shown that the fractures caused by EPF are consumed by the next crushing stage[5], meaning any benefit gained from the treatment would have largely been lost after material was crushed to 1mm for the ball mill test. Thus, the effects are probably under-reported. In a mine environment where the ore would go directly from EPF treatment, possible as a tertiary crusher, to a mill, we may see the full benefits of EPF treatment.
[1] Shi, F., Zuo, W., Manlapig, E., 2015. Pre-concentration of copper ores by high voltage pulses. Part 2: Opportunities and challenges. Miner. Eng. 79, 315–323. https://doi. org/10.1016/j.mineng.2015.01.014.
[2] Shi, F., Zuo, W., Manlapig, E., 2013. Characterisation of pre-weakening effect on ores by high voltage electrical pulses based on single-particle tests. Miner. Eng. 50–51, 69–76. https://doi.org/10.1016/j.mineng.2013.06.017.
[3] http://www.selfrag.com/products/selfrag-lab/
[4] Tschugg, J., Öfner, W., Flachberger, H., 2017. Comparative laboratory studies of conventional and electrodynamic fragmentation of an industrial mineral. BHM Berg- Huettenmaenn. Monatsh. 162, 319–325. https://doi.org/10.1007/s00501-017- 0638-z