It is routine in a processing plant to sample the feed, tailings and product streams in order to make a metallurgical balance – to account for the metal processed and where it ended up. For gold plants, measuring and estimating the amount of “goldin- circuit” (GIC) or gold inventory is also important. It represents a valuable asset and has to be evaluated regularly to ensure that all of the gold is accounted for.
Graphically represented right, the GIC can change over time depending on how much of the metal in the feed is removed as product or in tailings. If the GIC inventory measured cannot account for all of the metal, then it needs to be investigated.
The main components of the GIC are shown in the figure above.
Significant imbalances in the GIC inventory suggest problems with the metallurgical balance method. Perhaps the main feed weightometer is not calibrated properly or the measurement/ sampling methodology of inventory components is not correct or gold is being lost by theft.
Sometimes, the amount of gold accounted for in the GIC inventory is less than what is calculated from the starting GIC plus the estimated metal in the feed: how can this happen?
In the figure above, a typical monthly GIC report shows that this plant had 99kg of gold in inventory – this represents about $4 million in lockedup value. Of this total, most of it was in the leach tanks; however, 11% or $460,000 in gold was associated with “miscellaneous”: caught up behind mill liners, in spillage sumps and various items of equipment. Careful steps are taken to minimise this miscellaneous gold lock-up, which is often considered a high risk area for theft. In some cases, where practical, these high risk areas will result in some form of modification to equipment or security systems.
At the end of the accounting period, the plant operations team will make a concerted effort to minimise the inventory by converting as much of the GIC into doré in the gold room so that it can be reported as final product.
