Underground » Environment - Subsidence and Mine Water
The overall objective of this scoping study is the identification of an affordable process which will treat the typical saline water produced by the coal industry. The process may produce water of quality suitable for use or discharge and provide a disposal solution for the contained salts. The major activities considered were the development of an overall generic understanding of the significance of the issue and gathering of information from a number of mines to form the basis for case studies of various remediation options. A technical review of relevant technologies and a consideration of potential strategies including a simple economic analysis allowing comparison. This was then followed by development of conclusions and recommendations for advancement of the subject.
The following conclusions regarding disposal of saline mine water were drawn from the material presented in this report:
- Remediation of saline water is an issue for about 50% of coal mines, and for one third it is considered to be significant.
- The saline mine water has between 2,000 and 10,000 ppm total dissolved solids with an average of about 5,000 ppm.
- The water contains a mixture of cations and anions. Sodium is the predominant cation. Chloride, sulphate and bicarbonate are the dominant anions.
- The mixture of salts, particularly sodium sulphate, results in the solids concentrate having very little current market value. Storage of mine water on site is not a long-term option.
- Use by other local mines or industries would provide the easiest and least costly solution.
- Concentration of the saline water followed by a suitable disposal of the solid or concentrated residue is thought to be the best approach.
- Membrane techniques are capable of converting the majority (>85%) of the volume of minewater to saleable water and a concentrated brine. Of the membrane techniques, electrodialysis reversal (EDR) appears to be the most prospective due to recent technology improvements.
- Evaporation in solar ponds or via enhanced spray drying appears to have prospects.
- Two different solutions are required, one in which sufficient land is available for ponds (e.g. most Bowen Basin mines) and others where there are tight restrictions on available land (e.g. Hunter Valley mines).
- Disposal of the concentrate by entombment may be possible on some mine sites.
- Transport of the concentrate to the ocean and disposal by an environmentally acceptable system is an approach that should be considered further.
- None of the simple cost analyses met the target cost of $500 per ML, however spray drying followed by sea disposal or entombment was close at about $750 per ML, and electrodialysis reversal (EDR) followed by sea disposal could provide a solution for mines with limited land being available at a higher cost of about $1200 per ML. The use of EDR also offers the possibility of returns from the sale of water for drinking or agricultural purposes.
- The concept of spray drying needs to be demonstrated on a reasonable scale to confirm that the approach can be employed in a cost effective manner. This will require the design, construction and installation of a spray tower at a suitable minesite, the construction of a small evaporation dam and its operation/monitoring for a sustained period to confirm its performance.
- The evaluation of an EDR system at a suitable minesite for a sufficiently long period of time to confirm its capability of producing a sufficiently concentrated brine for disposal, the quality of the water it produces and its ability to maintain performance with limited attention and scale formation.
- A study of the biological impact of disposal of the concentrated brine in an ocean environment needs to be undertaken by a suitably qualified laboratory to a suitable standard for discussion with appropriate government officials.