Open Cut » Environment
This report presents the results of an ACARP funded study carried out by Coal Processing Consultants (an ACIRL and CSIRO Division of Coal and Energy Technology joint venture) on aspects of spoil pile self heating. The specific objectives of the project were;
- To trial fly-ash grouting as a method of fire control on part of a fully instrumented burning spoil pile.
- To evaluate the performance of the fire control method.
- To investigate the clays and overburden materials at selected mines to measure their suitability as barrier materials for covering spoil-piles to minimise self-heating.
- To produce an updated Operator's Manual.
The project was conducted in close cooperation with the following five companies; Bayswater Colliery Company (Bayswater); Drayton Coal Pty Limited (Drayton); Muswellbrook Coal (Muswellbrook); Pacific Power (Ravensworth Rehabilitation Site) and Peabody Resources (Ravensworth South).
The major findings of the project are as follows;
A flyash grouting trial was held on a section of hot spoil at Drayton. Both dry flyash and a water flyash slurry were used as the grout. The trial took place in a section of spoil which had been instrumented with 4 temperature and oxygen probes and contained an active fire. A total of 108 tonnes of flyash was injected over a period of three days. While the grouting had an immediate effect on the temperatures in the spoil pile it is too soon to conclude whether the grouting was successful in extinguishing the spoil pile fire. Further measurements will continue over the next 18 months as part of the ACARP funded project C6003 which began in early 1998.
The cost of the grouting trial was $18,000 for ~1200m3 of spoil. This corresponds to ~$15/m3 of spoil. It is considered that this cost could be reduced significantly for grouting carried out in a more commercial, rather than research, environment. Nevertheless the relative cost of the grouting compared to other options , such as excavation, water quenching or covering, will play a major role in determining whether grouting will find widespread application in surface mining for controlling spoil pile fires.
The properties affecting the performance of cover layers in inhibiting oxygen penetration into spoil were investigated. The diffusion of oxygen through selected inert overburden, including clay from Drayton, material from a pre-strip operation at Ravensworth and spent marine conglomerate from Muswellbrook showed that the diffusion coefficient for oxygen depended on the air filled voidage of the spoil. Calculations of the penetration of oxygen by diffusion through cover layers was found to require very thick layers of low voidage material to significantly reduce the flux of oxygen. Such low voidages are not normally achieved by tipping and compaction. Water is required in the cover layers to reduce the diffusive flux of oxygen, significantly. Consequently the water holding properties of potential cover materials are important. In this regard cover materials with a high proportion of fines (ie clays and silts) are required.
The desired characteristics of cover layers will depend on the proposed application. For fresh cold spoil, attention must be given to the availability of the cover layer material, its water holding capacity, its resistance to erosion as well as its response to geotechnical instability eg cracks in the spoil. For hot spoil undergoing spontaneous combustion the water in the cover layer must also be able to withstand the elevated temperature environment in which it will be placed. These features must be taken into account in determining the thickness of the cover layer and need to be determined on a site to site basis.
The ability to predict self heating in spoil piles was furthered during the study. This took the form of:
- comparison of the predictions from the CSIRO numerical model of self heating with measurements from two trial spoil piles and
- the use of the output from the dragline simulation program, 3D-Dig, as a materials distribution file for the CSIRO model of self heating