THE SUITABILITY AND IMPACT OF POWER STATION FLY ASH FOR WATER QUALITY CONTROL IN COAL OPENCAST MINE REHABILITATION

THE SUITABILITY AND IMPACT OF POWER STATION FLY ASH FOR WATER QUALITY CONTROL IN COAL OPENCAST MINE REHABILITATION
Report No. 745/1/01

EXECUTIVE SUMMARY

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:

To establish possible scenarios for the disposal of power station fly ash and its utilisation in rehabilitation practices in the coal-mining industry.
To predict the long-term chemical behaviour of such systems.
To estimate the long-term local and regional impact of such systems on the environment.

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:

The problem of spoil acidification is real and signs of pyrite oxidation are present at all three mines. Acidification has already become a problem at one mine.
A variety of factors contribute to the rate of acidification. A lack of sufficient natural base potential of the rock is one of the main reasons. Another key factor is the practice of coal discards disposal in opencast pits, without performing the necessary impact studies.
The base potential of fly ash varies from station to station. Values range between 50 - 140 kglt. Although this base potential is initially available as the fly ash leaves the power stations, it is fixated at the ash dams by carbon dioxide from the air. The rate at which the base potential is available is thus reduced by an order of magnitude.

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:

Application scenario Advantages Disadvantages Recommendation

In-pit, below the final decanting level
No long-term advantage

Heavy metal mobilisation from the ash after acidification of the mine water will present a problem
The sporadic application of the fly ash in ramps and voids will not have an overall beneficial impact on the mine water.

This should not be done unless the whole of the pit is to be covered with sufficient ash to eliminate oxidation of the pyrite in the mine.
The Matla fly ash disposal in and on top of Kriel Pit 3N is a good example of safe disposal because the whole pit will be covered with the fly ash, thus eliminating future pyrite oxidation.

In-pit, above the final decanting level
Fly ash could be applied as a cover to minimise rain-water and oxygen ingress into the spoil.
The fly ash could be treated by addition of cement or bentonite to reduce rain-water and oxygen penetration

The alkalinity to be released from an ash cover is insufficient to neutralise acid production in the spoil below.
The cost of ash application could be high because of the undulating spoil surface.
Vegetation and maintenance of the ash surface will be required.

Perform trial permeability and leaching tests in the field.
Design the ash cover to meet the objectives.
Implement as a cover in high risk areas, such as above coal discards.

Ash water introduction
This will introduce additional alkalinity into the spoil.
It will improve the mine water quality through gypsum and heavy metal precipitation.
Water is an easy medium to work with and can be introduced into the spoil through boreholes, where and when required.

Ash water will have to be tapped as soon as possible from fresh ash and injected through boreholes into the spoil to ensure maximum efficiency.
Ash water from the ash dams has insufficient alkalinity and will not be an effective neutralising medium

A trial investigation should be instigated by Middelburg South Colliery. They should inject lime water under controlled conditions int borehole WM12. Associated experimental design and measurements should be done by experienced individuals to ensure that definitive answers are obtained.

In terms of the original project objectives, the degree to which they have been met by the current investigation, is as follows:

Objective 1:

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.

Objective 2:

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.

Objective 3:

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.

Recommendations

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:

Delineation of problem areas such as coal discards and acid patches.
Consideration of the type of remedial strategy, i.e. ash cover versus ash or lime water injection.
Design of the ash cover to meet the objections, e.g. minimisation of water and oxygen penetration or sustained leaching of alkalinity from the ash.
In the case of alkaline water injection, design of injection and monitoring equipment and methods.
Implementation on a trial basis.

Application scenario	Advantages	Disadvantages	Recommendation
In-pit, below the final decanting level	No long-term advantage	Heavy metal mobilisation from the ash after acidification of the mine water will present a problem The sporadic application of the fly ash in ramps and voids will not have an overall beneficial impact on the mine water.	This should not be done unless the whole of the pit is to be covered with sufficient ash to eliminate oxidation of the pyrite in the mine. The Matla fly ash disposal in and on top of Kriel Pit 3N is a good example of safe disposal because the whole pit will be covered with the fly ash, thus eliminating future pyrite oxidation.
In-pit, above the final decanting level	Fly ash could be applied as a cover to minimise rain-water and oxygen ingress into the spoil. The fly ash could be treated by addition of cement or bentonite to reduce rain-water and oxygen penetration	The alkalinity to be released from an ash cover is insufficient to neutralise acid production in the spoil below. The cost of ash application could be high because of the undulating spoil surface. Vegetation and maintenance of the ash surface will be required.	Perform trial permeability and leaching tests in the field. Design the ash cover to meet the objectives. Implement as a cover in high risk areas, such as above coal discards.
Ash water introduction	This will introduce additional alkalinity into the spoil. It will improve the mine water quality through gypsum and heavy metal precipitation. Water is an easy medium to work with and can be introduced into the spoil through boreholes, where and when required.	Ash water will have to be tapped as soon as possible from fresh ash and injected through boreholes into the spoil to ensure maximum efficiency. Ash water from the ash dams has insufficient alkalinity and will not be an effective neutralising medium	A trial investigation should be instigated by Middelburg South Colliery. They should inject lime water under controlled conditions int borehole WM12. Associated experimental design and measurements should be done by experienced individuals to ensure that definitive answers are obtained.