To the background of a tightening economic environment for the Australian coal industry, this investigation was undertaken through 1987 and 1988 for the purposes of assessing the likely impact of emerging dragline/crusher/conveyor technology on two sub-economic coal deposits.
Because of the excessive amount of work and time required to assess both deposits to a level of worthwhile details, one deposit was selected on which to concentrate. The geology, hydrology, topography, coal quality and geotechnical characteristics of that deposit, together with the limitations of proposed machinery, were evaluated to select a block of about 40 million tonnes of the reserves best suited for this study. Depths to top of coal varied between 70m and 150m.
Interaction between overburden removal equipment and coal extraction methods was identified early in the investigation to be a major issue. Alternatives were investigated and a workable though innovative solution proposed involving steep hiwall access ramps and conveyor transportation of the coal. All coal conveying activity was segregated to one side of the mine layout, with the overburden system totally on the opposite side.
Overburden stripping was planned using two draglines working in tandem. The lower one would sidecast conventionally, having a geometry to best suit this application. The upper machine with custom geometry would be in a prestrip mode, dumping unshot tertiary clay or blasted competent sediments into a hopper/crusher feeding a round pit conveyor. This system would backfill across the top of the inpit dragline spoil heaps by means of a spreader/stacker.
The Surpac package on a microvax computer was adapted to dragline pit design and assisted with this complex mine design. Where possible, the parameters applied were drawn from actual experience in Central Queensland operations.
Equipment scheduling to supply 2mtpa coal production was undertaken using VP-Planner spreadsheet. A development schedule was established, from which capital expenditure, workforce numbers and operating costs were derived.
These enabled a financial evaluation, recognising the time value of money, to proceed. The price selected in this evaluation which yielded zero nett present value with a 19% discount rate applying, became the nominated base price. Sensitivity studies were then undertaken, varying the projection parameters used in the evaluation to establish the effect on that nominated price. The nominated base price was $29.36/t. At this price, for the quality of coal in the deposit, the prospect would remain uneconomic for quite some time to come.
Key advantages of the scheme when compared with conventional mining methods were identified as being
- environmental improvements,
- reduced workforce numbers, and
- minimised unit operating costs.
These contributed to a significantly lower required selling price than for all previous studies.
Disadvantages were that
- the development was capital intensive,
- production was totally lost during major conveyor relocations and breakdowns,
- the cost of holding insurance spares would be high, and
- rigidity of the total system would impose operational constraints on mine design and scheduling.
A scheme which replaced conveyors with large rear dump trucks for hauling the upper dragline material to spoil was then investigated. This alternative only became apparent at the end of 1988 with the advent of a new generation of larger trucks. Complete new capital expenditure, workforce and operation cost schedules were derived. Financial evaluation and sensitivity studies were also completed.
The nominated base price for this modified mining method was $29.81/t.
Benefits of the dragline/hopper/truck system compared with the dragline/crusher/conveyor option were
- elimination of the crusher, conveyor and spreader/stacker,
- reduced development capital,
- high truck efficiency due to the dragline supplying all the vertical movement of overburden, and
- a high degree of operational flexibility.
- more infrastructure facilities,
- higher life of mine capital cost,
- greater workforce numbers, and - higher unit operating costs.
Both of the systems studied at length are technically practical. There are economic advantages to be gained from such innovative marriages of equipment for deep stripping.
The dragline/hopper/truck combination is particularly attractive for adapt ion into the Australian coal industry and is worthy of further investigation and development.