Estimation of gold inventory in large carbon in leach tanks

The preparation of the month end gold production report is required by the mine owners to regularly assess the performance of the plant against budgeted values. However the preparation of a gold production report is often complicated by the large inventory of gold in the carbon in pulp (CIP) or carbon in leach (CIL) tanks used to adsorb leached gold from the solution. Indeed a 600 m3 CIP/CIL loaded with 15 g/L of activated coal charged with 8000 g of gold/ton of coal may retain up to 72 000 g (2200 troy oz) of gold. The estimation of the inventory implies the determination of the concentration of carbon in a tank and of the gold content of the species contained in the tanks. This paper assesses the errors associated with the sampling of CIL tanks to measure these concentrations and describes the impact of these errors on the estimation of the gold inventory in the CIL tanks of the Agnico-Eagle Mines Limited Lapa concentrator.

Research highlights
► The estimation of the gold inventory in carbon in leach tanks of a gold ore processing plant requires several measurements that are stained with random and possibly systematic (bias) errors.
► The measurement of the carbon concentration in a tank may introduce a systematic error since only a small portion of the tank content is sampled and the tank may not be perfectly mixed.
► A systematic error on the carbon concentration strongly impacts the estimation of the gold inventory in a tank.
► The use of an error propagation method shows that the error on the estimated gold content in a carbon in leach tank is of the order of 10%.
► The effect of a systematic error on the variation of the inventory between the beginning and the end of a month is within the error on the estimated change of inventory.