At several gold operations, blending is a part of the gold ore control at the mine site during mining as a strategy of compensating for gold grades variability in the gold deposit. Sampling and assays play an important role in these gold operations and in forming stockpiles of different gold grades. It is important to adopt an extensive program of ore control and analyses and the formation of blended ore stockpiles at the mine site in order to improve the mill performance. It is common to observe truckloads of gold ore leaving the mine and sampled. When assays results are received the material goes to the ore storage area and the coordination for disposition of the load to one stockpile is performed. These stockpiles can be blending piles, sub-marginal piles or waste piles.
Essentially, the blending pile is built with a succession of graded layers. For example, materials of less than 0.8 g/t gold are used in the bottom layer of blending pile to reduce losses into the underlying soil. After this low grade ore is leveled, a second layer of 0.8 – 1.2 g/t gold is piled and leveled. High grade ore designates by limits of 1.2 to 4.2 g/t gold are piled in the third layer. In some occasions, an extra high grade ore exceeding 4.2 g/t may be considered. Once the material is piled, the ore blending pipe contains a gold content influenced by the grade and proportion of the material. The proportion of each material is based on the type of recovery process employed and the optimum gold grade. In the previous sample, the average gold content of the pile is projected 2 g/t gold.
In this way, control of gold ore grade is important. In other cases, stockpiles are built at the mill site in several ranges of gold content and the blending is performed by taking material from each stockpile in an established ratio in order to get a uniform feed grade. Other alternative is to build stockpiles in various gold grade ranges near the mine site and new piles of high or average grade are formed as the material is transported to the process. Then, gold ore lots from these piles may be crushed, sampled and placed in individual fine ore bins with final blending to uniform feed performed by mixing feed from the various fine ore bins.
Consequently, blending of the gold ore comprises several steps at mine site. It is important to prepare a map showing gold grade and disposition of the material in the place based on assays from different drill holes, which can be performed to control blasting and loading operations. In this way, based on gold grade content, the material is moved to a mine receiving bin to surge stockpiles, to low grade stockpiles or to waste dumps. Blending of truck loads at the respective bin may be the initial step in controlling the ore grade.