Optimisation of a complex stoping schedule using Mixed Integer Programming
BHP Olympic Dam mines a massive and complex orebody of great lateral and vertical extent for copper, uranium, gold and silver. The mining method is sublevel open stoping and the scale of the deposit results in an inventory of thousands of individual stopes. It is not practicable to schedule this number by conventional manual means never mind attempt to optimise the stope mining sequence. As there was no suitable stope sequence optimisation software tool available on the market, BHP decided to develop its own application.
BHP has a software package called BlasorFlow® that is used for open pit optimisation. It applies a mixed integer programming formulation (MIP) to generate an optimised pushback sequence. Economic scenarios for prices, costs and discount rates are specified. Alternative mining, stockpile and ore processing configurations can then be modelled and scheduled.
The latest version of BlasorFlow includes an option for stope schedule optimisation according to practical stope mining constraints. These constraints include a specified exclusion zone around each stope within which no other stope can start mining until that stope has been mined-out and filled and a maximum number of stopes that can be mined concurrently within each ventilation district. A new pre-optimisation tool takes stope data and processes it into a form suitable for import to BlasorFlow.
Testing of ‘BlasorSSO’, as the new tool is termed, shows that the MIP functionality produces a schedule with a more sustained high-grade profile over the first several years and these results form a guideline for higher-value detailed stope scheduling. In addition, the output allows the best mine expansion strategy and optimum stope design cut-off grade to be investigated.