A mine ventilation system is related to the magnitude and direction of air movement through the various working places in the mine. The supply of air is referred to as air distribution, and it is accomplished by adopting a ventilation circuit suitable for the particular mining method used for extraction. In sublevel stoping, primary development openings such as shafts and ramps are used for main airways for ventilation, while the individual levels can be used as intakes and outlets using unidirectional air distribution.
Sublevel stoping mines are likely to have extensive workings on each level, as well as between levels, and therefore require ventilation from combined vertical and horizontal circuits. The stopes are designed to allow flowthrough ventilation between the sublevels connected by the stopes. The overall objective is to supply fresh air to each level from a downcast pressure source, radiating outward and upward through the working places to exhaust airways leading to upcast shafts.
In general, the airflow should be in an opposite direction to the stope retreat direction, so that dust and fumes are kept away from the operators. Consequently, the ventilation design for a stoping block will consist of access to fresh air, either from fresh air raises or a decline, as well as a return air exhaust system. The preferred approach is to ventilate each stope with a separate split of air, with the air introduced to the working places from the lowest level. Separate exhaust openings may be required to prevent contaminated air from entering other stopes in a stoping block. Ventilation shafts and airways must be located and maintained in ground which will not be caved and lost during the lifetime of the operation. In addition, short circuitry and dust hazard created by air leakage up or own partially filled orepasses must be prevented.
Global Economic Assessment
A number of global design considerations must be analyzed and economically evaluated to arrive at the optimum design for a stoping block. The outline of the orebody is determined by cutoff grade evaluations that account for the cost of block development, mining cost, haulage, surface cartage, mineral processing, and general overhead costs. A financial model is used to determine the viability by comparing the unit cost of all the steps involved in mining and processing with the estimated revenue. This could be an iterative process as, once the cost of development is included, some stoping blocks may prove not to be economical. However, they may become economic if development is carried out through those blocks to access other more economic areas.
Thomas and Earl (1999) have described a computerized stope optimization tool that can be applied in the strategic planning of underground stopes. The technique can be used to generate an extraction sequence in conjunction with an optimum stope configuration that maximizes the net present valueof an operation. The tool is used to generate inventories for a series of cutoff grades, and the results are scheduled to produce net present value (NPV) versus tonnage relationships.