Methods to control excessive dilution

A common indicator for excessive dilution is the usage of planning “adjustment” factors (mass and/or grade multiplier). These could typically be block model factor (typically grade), mine design factor (typically tonnes and grade), mine call factor / schedule factor (typically tonnes and grade), process factor (typically contained metal), etc.

The key to controlling dilution is through process control management e.g. identifying and mapping the various mining processes that impact on dilution, measuring each process’s performance, quantifying what the dilution is and then analysing the cause(s) and potential cost-effective solutions. The solutions may sometimes require retraining, introducing new techniques, providing incentives, and will always require management support and commitment.

The following are common underground mining aspects for excessive dilution control:

  • Eliminate or minimize undercutting of the stope hangingwall by the stope access development. This may require defaulting to survey controlled development versus geological control, or refining the location of the development relative to the hangingwall positions defined by structural control or grade control or reducing round lengths.
  • Improve resource modelling (improved interpretation techniques to match style of mineralization, more infill drilling, more structural mapping, etc.).
  • Limit the stope wall or backfill effective exposures by mining smaller stopes. This could be from decreasing level intervals, reducing strike extent, or leaving pillars (permanent sill or rib or yielding island pillars).
  • Periodically review the mining method and method parameters to ensure they are appropriate to the geology. Methods are commonly pushed beyond their appropriate application when the rockmass conditions and/or orebody geometry changes (e.g. flattening of dip, changes in host rock conditions due to depth stress, etc.).
  • Apply ore-waste separation techniques such as resue mining, split-timed blasting, hand-sorting, belt sorter, heavy media separation, dense media separation, particle size separation, etc.
  • Limit the unsupported stope wall exposures by applying additional ground support between levels (e.g. cablebolts from strike drives in/or out of the ore, etc.).
  • Limit unsupported stope wall exposure times – backfill is generally integral to the mine method working successfully versus an un-necessary cost or waste disposal after-thought.
  • Conduct comprehensive stope void surveys to identify sources of dilution and location / profile (i.e. is dilution occurring between cablebolts indicating inadequate support coverage, etc.).
  • Conduct geotechnical assessments such as mapping the ore drive joint sets, stress modelling or review adjacent area performance records to indicate the potential for “unintended” dilutions, to enable refinements to the stope dimensions, shape or support designs prior to mining.
  • Ensure simple, clear and accurate survey set-outs based on asbuilts immediately prior to drilling versus after development (as floor and wall conditions can change during this period).
  • Drill production holes parallel to the key stope walls versus stab-holes – provided the holes are accurately placed. This can be an interesting situation as stab-holes can actually result in lower dilution as they may be less affected by drilling in-accuracies combined with good blast control.
  • Refine the production-hole stand-off to the final-design wall positions, refine the powder factor by adjusting the drill pattern – hole size and/or reduce the powder factor adjacent to critical walls. Drilling and blasting suppliers commonly provide technical support to operations to review issues and efficiencies – make use of them. Provide drill and blast designs and analyze results using contemporary software packages.
  • Sequence the mining extraction from poor-to-good rockmass and avoid closure pillar situations (but these are generally a trade-off with project economics via reduced development intensity).
  • For stress related dilution, consider conducting stope extraction in a de-stressed environment (i.e. stress shadowing) such as for parallel orebodies at depth or extended strike relief slot.
  • Increase grade control for ore that is visually difficult to differentiate from waste. This may involve more face sampling, different sampling techniques such as hand scanners, more infill drilling or even intermediate stockpiling until sample turn-around confirms if it is ore or waste.
  • Reduce the mining rate to enable management to focus on regaining control.
  • Review contractor tonnage-based or metre-based activities to ensure their work practices are aligned with the company’s dilution control practices.
  • Ensure company performance incentives are not counter to required dilution control.
  • Conduct marker trial test work to better understand the draw characteristics for the particular orebody (i.e. caving operations).

Peter Mokos

Principal Mining Engineer