THE SUITABILITY AND IMPACT OF POWER STATION FLY ASH FOR WATER QUALITY CONTROL IN COAL OPENCAST MINE REHABILITATION
Report No. 745/1/01
The acidification process of spoil in opencast collieries in South Africa has been studied for many years. This has provided sufficient information to suggest that acidification poses a potential threat. Many companies and individuals have been conducting research to find solutions to this problem. Over the past 10 years, it has become apparent that no unique solution will be developed to prevent, significantly reduce or treat acid water as the magnitude of the problem is too great. At best, a combination of actions could possibly reduce the impact of acidification in mines.
For many years, the option of applying power station fly ash to prevent, limit or ameliorate acid-mine drainage has been considered. The technique has been introduced at two underground mines, with limited success in terms of mine water quality management. The scope of these applications was aimed more at stabilising the overlying strata than neutralising mine acidity.
Fly ash has been applied in an opencast mine at Kriel Colliery. This mine has excess base potential and, for that reason, there is no danger that heavy metals will be leached from the fly ash after its introduction.
Most other collieries, as far as is known, have significantly lower base potentials. The potential for acidification, after fly ash introduction, is therefore real. The introduction of fly ash into a system that will become acid will have catastrophic consequences for mine water qual ity management.
With this description of the background situation, the objectives of this investigation were as follows:
Three opencast collieries and power stations were investigated. These were Rietspruit Colliery/Matla Power Station; Optimum Colliery/ Hendrina Power Station and Middelburg South Colliery/Duvha Power Station. Examination of the underground situation did not form part of this investigation. Conclusions from this investigation are as follows:
Three possible scenarios of fly ash application in opencast mines have been considered. These are: In-pit application below the water table; In-pit application above the water table; Introduction of ash water. The following is a summary of the advantages and disadvantages of these types of applications:
|In-pit, below the final decanting level||
|In-pit, above the final decanting level||
|Ash water introduction||
In terms of the original project objectives, the degree to which they have been met by the current investigation, is as follows:
To establish possible scenarios for the disposal of power station fly ash and its utilisation in rehabilitation practices in the coal-mining industry
This topic has been addressed extensively in this work. The preferred scenario is site specific. Generally, a cover of ash to minimise water and oxygen ingress into the spoil is preferred. The limiting condition is that the ash may not be placed below the final decanting levels of the pits. The introduction of ash water into the spoil is only an option at operating power stations, because of the higher alkalinity of water in contact with fresh ash.
To predict the long-term chemical behaviour of such systems
Extensive chemical and leaching tests have been performed on both spoil and fly ash. A complete data set is included in this document for future reference. The work has demonstrated that placing fly ash below water in pits introduces a high risk of metal leaching, so this practice is not recommended. Application of fly ash as a cover will improve the pit water quality and minimise water infiltration. The application of ash water will improve the pit water quality by raising the pH and precipitating sulphate in the pit.
To estimate the long-term local and regional impact of such systems on the environment
If fly ash were applied as a cover, the environmental benefits would be significant. Water and oxygen ingress into the spoil will be limited. Problem areas, such as coal discards in pits, should be identified and covered with fly ash. Cement or bentonite should be added to reduce water and oxygen ingress. If applied as cover material above the pit water table, no detrimental environmental impact will result.
The overall conclusion is that fly ash application in opencast mines can be done above the pit water table. Below the pit water table, this should only be done if detailed and site-specific investigations suggest no risk to the environment. The current document provides a sound directive in terms of decision-making and for planning additional experimentation in this regard.
This investigation has highlighted the possible outcome of three possible methods of fly ash related mitigation at opencast coal mines. The next step will be to implement the options under trial conditions by the mines concerned.
This should be initiated using the following process: