Importance of Ore Moisture
It has been established that an error in the moisture sample may indicate more or less gold ore than actually received at the processing plant and can affect gold ore accountability or settlement between mine and processing plant or metallurgical accountability in the plant. This difference has an impact on the geometallugy of the gold mining operation. Basically, errors on the high moisture side with some ore types may results from a high percentage of fines in the sample taken for moisture determination, especially when taken from the as-received mine run ore. Some gold operations have errors on the low moisture content and this problem is produced if moisture sampling is delayed until after the ore is crushed, since some moisture is lost by evaporation during the process. Some tests on homogeneous coarser material in which the coarse lumps are porous similar to fines have indicated that differences in moisture content are of minor importance in the fraction coarser than 25 mm.
In some gold operations, a common practice for moisture determination is to weigh each truckload of ore on platform type sales and take at least 5 kg hand sample of the truck content s for moisture determination. In other circumstances, a sampling cutter or special belt samples taken of the crushed material may be employed to determine moisture. Ore weight may be determined by a beltscale installed on the conveyor belt. Without any doubt, handling of the moisture sample after it is obtained is an important part of the operation. Normally, the sample must be kept in a close, moisture-proof container and for maximum accuracy must be weighed and dried entirely.
When the sample preparation is performed quickly, crushing, mixing and cutting to obtain a more manageable sample of 1.0-1.5 kg does not significantly alter the result. Drying the material to a constant weight at a maximum temperature of 110 oC is normal. It is important to mention that excessive heat can produce a partial oxidation of the sample and the results could be no realistic. Then, the drying temperature must be correlated with that employed in the analytical laboratory in order to consider the same basis for gold content and moisture determination. In the processing plant, the material is reweighed and resampled on the conveyor belt in order to get a realistic measurement of the plant feed and help to evaluate the losses of material from stockpiles to the processing plant.