In-pit disposal of mineral residues

David Love, Guinea Flower (pty) ltd, Tshwane, South Africa

Introduction

The practice of in-pit disposal of mineral residues such as tailings, coal discard, and plant rejects in mines across the world has been studied more intensively in the past 30 years, especially in terms of its environmental implications. If an open-cast pit is managed to prevent post-closure decant, then the water level in the pit is necessarily kept at a specific elevation, dirty water is contained in the pit, and so residues can be too. Often, this is done by pumping dirty water out of the pit for treatment and release or reuse, meaning that local groundwater flow directions are towards the pit.

Technical Review

Disposing of mineral residue in a pit below the in-pit water table may sound environmentally counter-intuitive, but in fact has advantages. In addition to the self-evident availability of water, the principal environmentally-significant chemical property of a sub-aqueous environment is the limited availability of oxygen: the maximum dissolved oxygen availability is some thirty times less than under atmospheric conditions (1).

If the mineral residue is reactive, such as the sulfide-bearing tailings of many gold and base metal mines, or coal discard which is also sulfide-bearing, the sub-aqueous environment limits the exposure of potentially acid-generating sulfide-bearing residues to oxygen (1), and numerous case studies have shown the effectiveness of this in limiting acid rock drainage generation over the long-term (2) (3) (4). Although recent findings indicate a bacterial mechanism that can drive pyrite oxidation under anoxic conditions (5), this requires specific methanogenic bacteria, and extremely small grain sizes for the bacteria to attach to (6).

If the mineral residue is dissolving, such as gypsum- or epsomite-bearing residues, the load in the pit water and solubility constraints will dissolution (7).

Mineral residues may be disposed below the water table, with spoils or waste rock above them, or at the bottom and covered with water, like a pit lake on top of the residue (8) (4).

Disposing of mineral residue in a pit above the in-pit water table does have the geochemical advantages of sub-aqueous disposal, but still consolidates dirty water of the mineral residue, and of the pit, into one system. Duplexing the mineral residue on top of the pit causes any seepage from the mineral residue to go into the pit, and be managed by the same dirty water management system that the pit requires.

In-pit disposal reduces the amount of mined land and waste exposed at any point in time (9). This means that rainfall that falls onto the mineral residue does not also fall onto the pit (as it would if the two were separate) meaning that less rainfall in total is converted into dirty water. In catchments that already carry a heavy load from mining and industry, saving green land that can create clean runoff from rainfall is a benefit.

Furthermore, some mineral residues, such as clay-rich tailings, or coal fines, form low permeability layers when they dry (10), meaning that less rainfall is converted into infiltration than would be the case in a conventional pit without residue disposal.

Discussion

Disadvantages of in-pit disposal mainly relate to the inaccessibility of the waste after placement: whilst hydrological and geochemical conditions can be monitored by sampling from boreholes drilled into the waste, should adverse conditions develop, it is much more complex to introduce a treatment method to the waste at depth: only the water abstracted from the pit to prevent decant can be treated. Furthermore, if the land around the pit should landslip or otherwise fail, this could mean dirty water in the pit is no longer contained.

In-pit disposal is at times motivated by convenience, such as accessibility of the site, or the timeline to develop the dump (11), but disposal needs to be done within environmental constraints, and it is important that the mining and environmental milestones are integrated with the requirements of waste landform rehabilitation (12).

The applicable legislation also needs to be complied with, and should not be disregarded to motivate more convenient disposal strategies, without attempting to secure appropriate regulatory approval. Within southern Africa, some countries such as Mozambique (13) or Zimbabwe (14) specify threshold levels of contaminants in water which must not be exceeded, without specifying how to prevent the release of contaminants. South Africa’s landfill regulations (15) and DRC’s mining code (16) specify concept barrier designs that must be applied depending upon threshold levels of contaminants in leachate - in the latter case depending also upon geotechnical characteristics of the underlying ground. South Africa’s mineral residue regulations (17) require a pollution control measures to be suitable for the specific residue, on the basis of its characteristics and risk to the water resource. All countries in the region require a groundwater impact assessment, demonstrating how a mineral residue facility could potentially impact the groundwater, and what is to be done to prevent or mitigate this.

Conclusion

Notwithstanding differences in the details, all countries in the region impose a duty of care upon a mining operation to protect the water resource from contamination from mining activities, including residues, and a duty of care upon the state to ensure that this is done. Thus while environmental and operational benefits can be derived through in-pit disposal of mineral residue, the onus is on the mining operation to demonstrate that the mineral residue disposal method proposed at a specific site will satisfactorily protect downstream water resources.

This article may be cited as:

Love, D. (2024). In-pit disposal of mineral residues to minimise dirty water in stressed catchments. WISA Conference Summary of Oral Presentations (p. 31). eThekwini: Water Institute of Southern Africa.

References

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3. MEND. Review of In-Pit Disposal Practices for the Prevention of Acid Drainage - Case Studies. MEND Report : 2.36.1, 1995.

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13. Decree 18. Regulation on Environmental Quality and Effluents Emission Standards. Issued under : Environment Law 20/97, 2004.

14. S.I.274. Water (Waste and Effluent Disposal) Regulations. Issued Under : Water Act CAP.20:24, 2000.

15. GN R. 635. National Norms and Standards for the Assessment of Waste for Landfill Disposal. Issued under : National Environmental Management: Waste Act, 2013.

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17. GN R. 632. Regulations Regarding the Planning and Management of Residue Stockpiles and Residue Deposits from a Prospecting, Mining, Exploration or Production Operation. Issued under : National Environmental Management: Waste Act (as revised by GN R. 990 of 2018), 2015.