Improved Recovery in Highwall Mining Using Backfill

This report presents the studies undertaken within ACARP project C3052, entitled 'Improved Recovery in Highwall Mining Using Backfill'.  The aims of the project are to examine current fill practice as it applies to highwall mining, to determine, at a conceptual study level, whether fill could be economically applied, to trial key aspects of fill behaviour and to outline a feasibility study for any worthwhile ongoing development.  he objectives are to review the issues, to map out the possible filling methods and to cost and rank them, to undertake laboratory testing of fill materials and to outline field testing and feasibility of fill strategies.

The methodology for the review involved:

• setting criteria for the fill materials and for the filling methods,
• examining the different mining strategies that may apply in thin and thick seam mining,
• reviewing relevant current fill practice in both coal and metalliferous mining worldwide,
• examining advances in applicable technology in the cement and concrete industries,
• selecting a range of relevant filling methods and costing them at a conceptual level (at the expected productivity on the basis of complete filling),
• examining other issues that may affect costs,
• setting the required parameters of the fill material for laboratory testing and testing of the selected fill materials, and
• resolving the appropriate areas for modelling, field and feasibility studies.

In Australia, filling has not been adopted in coal mines, often due to a view that it would be uneconomic and of little benefit at prevailing export prices of coal. This may be true in underground situations with current mining reserves where high recoveries can be achieved without subsidence becoming a major cost issue.

In highwall mining however, recoveries are not high and both subsidence of the highwall and the maintenance of good stable and safe working conditions during extraction are of prime importance.

Metalliferous mining practice has been to develop fills tailored to specific applications when conventional fills would not apply. For highwall mining, the emphasis for filling needs to be on the lateral distribution of fill, on tight filling and on using the available materials. In many cases there is no local cementitous material, the immediate tailings supply (such as fine washery reject) is less than the overall fill demand and the cost of using reclaimed tailings is high if not pre-planned.

Therefore the fill materials will predominantly be aggregates from either coarse washery reject or spoil pile material.

In this study, information from the literature survey was used to detail the criteria for fill selection and current fill practice and experience. The discussion on criteria for fill selection included discussion on geotechnical criteria, environmental criteria, economic and production criteria, and safety and regulatory requirements.

The discussion on current fill practice and experience looked at metalliferous mining practice (placement techniques, material properties, benefits and costs), coal mining practice (overseas experience in Germany, Poland, India, South Africa, United Kingdom, United States and Australia) and alternative technology (integrated materials, co-disposal, cellular foam concrete, water absorbing / water reducing materials and accelerators and retarders).

The discussion of mining requirements considered single pass and two pass operation on thin seams and single pass, full height and single pass per lift, two lifts for thick seams.

Relevant data for filling options and costs were obtained from site visits to both Oaky Creek and Moura Mines.

Seven different methods were defined and were cost based on a fill production of 15,000m³/wk on two 10 hour shifts per day, seven days a week. The methods costed were:

• placing a fluid aggregate fill via boreholes;
• conveying an aggregate fill into the holes and ejecting it on retreat;
• trucking an aggregate fill into the holes and slinging it into place;
• trucking an aggregate fill into the holes and ramming it into place;
• placing a hydraulic fill from boreholes or fill lines in the holes;
• placing pneumatic fill on retreat from a line pushed into the hole; and
• using the mining unit to carry fill lines or to convey the fill and place fill on retreat.

The most cost effective methods of placement are to use the miner on retreat either by using the coal transporting equipment in reverse (method 7) or by laying a pneumatic system on the miner for use as it retreats (method 6). In each case a slinger or stower would eject the material over the miner to the face for the retreat or would move to a forward ejection position for the retreat. Separate conveyors or pipelines for pneumatics or hydraulic placement are other useful options.

Further Work

The conceptual studies for this project should be supported by field testing which should concentrate on confirming methods for localised placement of fills and for assessing the performance of hydraulic fill placed in inclined excavations.

Geotechnical modelling can be used to check the effectiveness of the various fill alternatives and the effectiveness of different filling strategies, even if it is based on preliminary information, because there is a need to minimise the number of alternative strategies and equipment options that could be developed.

The field study and geotechnical modelling should be followed by a feasibility study to confirm their operational practicability in particular site situations.

The first task would be to do a detailed costing study specific to the site targeting for the feasibility study.

Site Specific Costing Study

The site specific costing study is aimed at completing similar conceptual studies to those completed in this report but targeted specifically to the costs for a site and to the mining strategies likely to be adopted by the site. The site chosen should be the site for the feasibility study to determine the likely benefits to that site and to set a framework for the review of the feasibility study. The likely cost of the study is $25,000 t $30,000.

Field Testing

It is proposed that field trials be conducted to trial borehole and hydraulic placement techniques in formats and of a scale that are applicable to highwall mining. The primary issue is obtaining commitment and choosing the test site.

In addition, it is suggested that a re-entry vehicle be built for simple hole inspections to gain experience on equipment re-entering 'holes'. In the event that filling is useful, remote hole access will be needed to check on ground conditions, methane build up and excess groundwater in old holes. This will be done well prior to equipment entry, even as a check prior to commitment to using fill.

No budget is given for the field testing as it is likely to be site dependent. It was originally intended to undertake field testing at a site which would have borne the cost of the trials.

Geotechnical Modelling

The aim is to evaluate the geotechnical benefits of filling and so to estimate the additional recovery that could occur.

This is not seen as a major geotechnical problem, except in terms of predicting the behaviour of thin pillars and the effect of adjacent fill on their performance. Experts in this field have been approached for their contribution to the prediction of the performance of thin pillars both with and without fill.

Because of the considerable number of alternative mining situations that could be examined the work will be restricted to three or four basic situations that have been selected to illustrate what is involved in filling and the geotechnical benefits of fill. A preliminary budget estimate for the geotechnical modelling is $15K.

Feasibility Study Requirements

It is proposed that a feasibility study be undertaken to define the project requirements that are necessary to prove up a method or methods for filling of highwall mining operations to the end of the feasibility stage. At the end of this study, the technical feasibility should be resolved, the operating and capital costs known to 15% accuracy and a schedule developed for the development and implementation of the relevant approach. Trials of approaches and equipment development will need to be included in the costings. The study program has been drawn together as a preliminary guide to further development as there are quite a number of options depending on the particular highwall situation.

The provisional budget estimate for this work is $3,000,000.