What is the heap leach pad inventory?
It is a question that every heap leach operation seeks to answer and to quantify. Heap leach operations yield ranges in recovery relative to placed ounces and recoverable gold. Detailed metallurgical accounting and heap leach models can provide estimates on remaining inventory, similar to the graph below:
So, what does each line represent?
Total Gold Placed – Fire Assay Grade on the pad
Total Extractable Gold – Fire Assay Grade multiplied by ultimate expected recovery from metallurgical test work
Extracted Gold – Gold complexed from the ore into solution
Recovered Gold – Gold out the bottom of the pad and recovered through the process plant
Remaining Extractable Gold – The difference in Total Extractable Gold and Extracted Gold
Remaining Recoverable Gold – Total gold remaining for potential recovery, the difference in Total Extractable Gold Placed and Recovered Gold
Gold in Solution – Gold that has been complexed but not recovered, the difference in Extracted Gold and Recovered Gold
But more importantly, how do we get the remaining inventory?
The inventory grows over time as operations progress. Large jumps in the inventory typically occur when issues in operations arise. Short-cycling, double-stacking, high-clay areas leading to poor percolation, poor pH, lime addition issues, and CN issues to name a few. The Remaining Extractable, Remaining Recoverable, and Gold in Solution can be very large numbers depending on the operation. Realizing these ounces is typically very profitable, but can be difficult, and perhaps cost prohibitive.
Forte has assisted numerous operations with evaluating the opportunity to recover the inventory from the pad and aided with analysis as to the potential benefit. Methods for recovering this inventory include material re-handling, on/off rest and leaching cycles, and solution wells. Material re-handling can change flow paths and allow for changes in solution to rock contact which can increase recovery, however this is typically cost prohibitive. Rest cycles allow for extraction to continue and can reduce overall solution in the system and is less expensive to implement, however this does not necessarily assist with solution to rock contact at depth for dry areas or areas beneath compacted zones. Solution wells can allow for solution to be introduced in previously dry areas or under-irrigated areas at depth, however depending on the number of wells required and the types of wells, these can be cost prohibitive as well.
Forte has found success with utilizing these options for recovering the pad inventory and we've assisted our clients with reducing inventory within the pad while entering into closure operations. Forte has
utilized various modeling techniques, coupled with on-site testing programs, to evaluate the opportunities for each site to realize value as pad operations come to an end.
We can work with your site team to analyze the opportunity to reduce inventory and realize recovery for heap leach pads as they progress towards closure. Analysis can be conducted to understand residual recovery related to required pumping operations to manage solution on-site. This can assist with profit post mining and stacking operations.