Open Cut » Environment
This report summarises the results of ACARP Project C3008/AMIRA Project Number P401, focussing on the generic aspects of the project. The generic aims of the ACARP component of the project and the mine site specific aims of the AMIRA component of the project were broadly achieved. The project involved co-disposal trials conducted by the University of Queensland. (UQ) and Warman International Ltd from 11 to 28 August 1993 at Goonyella – Riverside Mine from 4 to 19 October 1993 at Hunter Valley No. 1 Mine, and associated laboratory testing. The co-disposal trials conducted by Warmen International Ltd from 24 October to 18 November 1994 at Warkworth Mine, and associated geotechnical testing carried out by D J Douglas & Partners Pty Ltd and UQ, are also described.
A total of twenty co-disposal trials was conducted at the Goonyella – Riverside, Hunter Valley No. 1 and Warkworth mines. The results of laboratory testing of Gregory Mine washery wastes, are reported. Also described are the results of field studies, and associated laboratory testing, carried out by UQ on the full scale co-disposal delta at Jeebropilly Colliery.
All of the co-disposal deltas investigated drained readily and could support personnel almost immediately after deposition. However, moisture equilibrium with the atmosphere was not achieved for some months at Goonyella – Riverside Mine, with almost no rainfall during the monitoring period, or at Hunter Valley No.1 Mine, which experienced significant rainfall.
The in-situ densities of the co-disposal washery wastes were comparable to that achieved by the mechanical compaction of coarse reject alone. The in-situ porosites were generally comparable to those of gravel-bedding natural streams, although the Hunter Valley No. 1 porosites were significantly higher due to the platey shapes of the coarse reject particles.
The profiles of the co-disposal deltas conformed to exponential profiles, in agreement with existing river (sediment) transport theory. They can be represented in a non-dimensional form which enables the profiles of deltas of any length to predicted. The range of curvature of the profiles obtained was relatively small. The curvature (concavity) may be accurately predicted, but the initial and overall slope predictions are indicative only.
Both the particle size and particle buoyant density varied exponentially with distance down each co-disposal delta. Hydraulic sorting varied relatively little from delta to delta, but relatively little sorting took place. Sorting conformed to existing river transport theory, but generally showed minor reverse sorting (increasing particle size but decreasing density down the delta).
Much of the tailings segregated from the trial deltas, due to the relatively high discharge velocity and relatively low solids concentration, but also because of the gap-grading of the combined coarse reject and tailings. However, eliminating gap-grading by limited crushing of the coarse reject would have only a small effect on fines segregation, because of the suspension of fine particles in the flow.
Predictive equations for the fraction by mass of the total input washery wastes (coarse reject and tailings) retained on the co-disposal deltas have been derived for washery wastes which breakdown readily, and for those which do not. The predictions are likely to be accurate.
Except at the surface, co-disposed Goonyella - Riverside and Hunter Valley No. 1 washery wastes showed little evidence of weathering over 12 months. Experience at Jeebropilly Colliery has been that co-disposed coarse reject and tailings are not prone to spontaneous combustion, while loose-dumped coarse reject is. Any potential for acid mine drainage would also be reduced by co-disposal.
Guidelines have been developed highlighting the potential problems, and economic and environmental advantages, of co-disposing of coal washery wastes by combined pumping.
Conclusion
Twenty co-disposal trials were conducted at Goonyella - Riverside, Hunter Valley No 1 and Warkworth mines. The Goonyella - Riverside and Hunter Valley trials involved The University of Queensland (UQ) both on site and in laboratory testing and subsequent analysis of the data. For the Warkworth trials, UQ involvement was limited to laboratory testing and data analysis.
In the Goonyella - Riverside trials, either Goonyella coarse reject and tailings (7 trials) or Riverside coarse reject and tailings (1 trial) were used. The ratios or coarse reject to tailings and the solids concentrations tested ranged from 1.4:1 to 3.1:1 (or possibly more) and from 42 to 60% respectively. In the Hunter Valley No. 1 and Warkworth trials, in which a variety of coarse reject and tailings were used, the ratios of coarse reject to tailings and the solids concentrations tested ranged from 2.8:1 to 3.6:1 (or possibly more) and from 31 to 56% respectively.
The full scale co-disposal delta at Jeebropilly Colliery was also investigated by UQ. There, the input ratio of coarse reject to tailings was 5.25:1 and the solids concentration was 30%.
Field monitoring and laboratory testing of samples recovered from the co-disposal deltas, and analysis of the results obtained, has enabled the characterisation of the pumped co-disposal deltas.
The delta profiles, hydraulic sorting by particle size and buoyant density, deposition and fines segregation, weathering over time, and the potential for acid generation have been characterised. Relationships have been developed to predict co-disposal delta profiles, hydraulic sorting and deposition and fines segregation. Pumped co-disposal guidelines have been developed.
The results of the project have shown the pumped co-disposal of combined coarse reject and tailings to be an economical alternative to the conventional separate disposal of the two coal washery wastes, offering environmental advantages including the more efficient use of the available waste storage, potentially increased water recovery, and facilitated rehabilitation to a high level of future land use.
Pumped disposal has been adopted at several coal mines worldwide and is being considered at numerous other mines. It will find application at existing and new mines where its potential economic benefits are recognised and can be realised. Although pumped co-disposal presents a number of problems, including pipe wear and the segregation and operation of pumped co-disposal will minimise their impacts.