Open Cut » Coal Extraction
This study was a two year project undertaken by the Julius Kruttschnitt Mineral Research Centre in conjunction with the open cut coal mining industry.
The project was designed to provide a coordinated investigation of coal reconciliation procedures, coal damage, loss and dilution mechanisms, and economic analysis of mining practices designed to limit coal loss and dilution.
Coal loss is costing the open cut coal mining industry an estimated $480M per year in saleable revenue. The extent of coal loss varies between 5% to about 20% of in situ coal. These figures are conservative due to the compensating effect of dilution material that is currently being counted as coal in many operations.
An increase in coal recovery of 1% would increase revenue between $1M and $3M per year for most mining operations. Much of the coal currently being lost could be recovered at marginal cost and therefore attract greater than average profit.
A review was conducted incorporating seventy percent of the industry by number of mines and ninety-five percent by ROM production. Data were collected on the state of coal reconciliation, production reporting and planning practices in the industry, on the mechanisms of coal loss and dilution of concern to operating mines, and on mining practices being used by the industry to limit coal damage, loss and dilution.
All data that the project team were authorised to publish are tabulated in this report. The review has found that coal reconciliation practices are driven more by accounting and statutory requirements than by engineering or operational needs.
While the majority of the industry are conducting some sort of tonnage reconciliation, quality reconciliations are the exception rather than the rule.
The extent of dilution in ROM coal is considered to be a valuable measure of mining performance. This can be derived from a reconciliation of ROM and in situ coal quality.
Few mines monitor coal mining performance closely enough to provide useful feedback to design engineers, production supervisors, and operators on the performance of mining and suitability of current working methods.
In some cases mines are trialing alternative mining methods aimed at improving coal recovery without an adequate coal reconciliation procedure in place. It is believed that this approach is counter productive as good ideas are being lost purely as a result of poor trial procedures and lack of measurement.
A number of trials were conducted within this project and supported by careful measurement. An overburden buffer was left on the seam roof from a 45m³ shovel and selectively removed by a 12m³ back-hoe. This resulted in an increase in coal recovery of at least 10%. A rubber tyred front end loader used to remove an overburden buffer left on top of coal was found to improve coal recovery by 10%.
Coal loss was found to increase by about 6% and dilution increased by 5% in wet mining conditions. A similar increase in loss and dilution could be expected mining at night if adequate lighting is not provided. These trials demonstrated that careful measurement can demonstrate improvements which may not be perceptible to the eye.
Guidelines for routine reconciliation practices are presented in this report. Guidelines are also presented on investigatory reconciliation practices aimed at assessing the results obtained from mining trials or to confirm whether or not planning assumptions (i.e., budget loss and dilution) are being routinely met.
The principal mechanisms of coal damage, loss and dilution are identified and discussed. The mechanisms causing the majority of coal damage and loss result from overburden blasting and excavation, and from final seam roof clean-up operations.
There is an urgent need to develop a routine technique that will enable the accurate detection of the seam roof. Uncertainty with regard to the location of the seam roof leads to blast damage, which results in excessive uncovery and clean-up losses.
The industry is not good at following recommended operating procedures and guidelines for mining practices. Field work conducted within the project demonstrated that lack of adherence to recommended operating procedure was causing up to 10% to 25% additional coal loss. Significant improvement to mining recovery can be achieved if the 'artistic license' of mining is replaced by careful adherence to the plan.
The Cost of Care
There is a perception in the industry that care costs rather than pays. Part of the reason for this is that common reconciliation reporting systems cannot identify the revenue foregone as a result of the levels of coal loss and dilution currently being tolerated by the industry. This project has demonstrated that care pays.
Guidelines for mining practices have been presented to control coal damage, loss and dilution resulting from drill and blast, overburden removal and coal mining operations. Field work conducted within the project enabled some of the mining practice guidelines to be implemented and demonstrated in a physical mining environment.
The economics of open cut mining depend on the efficient use of large scale mining equipment designed for moving large volumes of overburden and not for the selective separation of overburden from coal. In some cases, coal is sacrificed in order to optimise the performance of the overburden removal operation (ie cast blasting).
A balance is required between the benefits currently derived from the scale of mining operations and the economic imperatives of maximising coal recovery. The results from this project suggest that current industry practice places this balance in favour of scale at the expense of coal recovery.
Simple cost models provide a good mechanism for determining the economic balance point just discussed. The computer based workbook developed within the project contains many simple cost models that may be tailored to particular mine site conditions for conducting such economic analyses.
The workbook is USER interactive in nature and allows first pass analyses which can be used to rank the economic viability of different approaches being considered. Clearly, there is no substitute for more thorough analysis than can be conducted within the workbook. The workbook encourages the USER to think more holistically about the mining operation.
Impact on Coal Preparation
In addition, the impact of mining practices on the coal preparation plant must also be included in any economic assessment. Many operations acknowledge that preparation plants are adversely affected by unforeseen variation in ROM coal quality and that washing performance could be improved by greater coordination of mining and washing programs.
This project has by no means provided the definitive solution to the management of coal recovery in open cut coal mines. It has, however, led:
- to a greater awareness of these issues within the industry;
- to a data base of industry experience, and
- provided some guidelines and tools for the improved management of coal recovery.
Further work is required on the general monitoring of coal production, particularly in terms of quality and the impact of alternative mining practices on preparation plant performance and the subsequent economics of the mining operation as a whole.
The future of coal reconciliation monitoring lies in the area of real-time feedback to the mining operation that can be used in a pro-active way to manage coal loss and dilution.