The importance of the ability of the primary grinding is characterized by the presence of gravity recoverable gold in the circulating load. It means that part of the gold can be recovered in this part of the process plant. There is an “x” amount of gravity recoverable gold in the ball mill discharge, which includes both gravity recoverable gold freshly liberated, “f” and gravity recoverable gold already liberated that reported the hydrocyclone underflow and was not ground into non-gravity recoverable gold in the ball mill. Then gravity recovery (“d”) is equal to R*x and the gravity recoverable directed to the hydrocyclone is equal to (1-R)*x, and a proportion, “c” is classified to the hydrocyclone underflow, c*(1-R)*x. This gravity recoverable gold is then ground in the ball mill and a fraction “b” survives. In this way, at the ball mill discharge, the amount of gravity recoverable gold is equal to b*c*(1-R)*x as gravity recoverable gold that has survived grinding, plus an amount “f” that has been liberated, for a total of b*c*(1-R)*x + f, which is also equal to “x”.
For example, values of 0.80 for “f”, 0.95 for “b”, 0.99 for “c” and 0.1 for R. Basically, the value of R can be obtained by taking the product of how much the circulating load is treated, how much is recovered in the primary recovery unit and how of the gravity recoverable gold in the primary concentrate is recovered in the cleaning section (gold room). The value of R could be obtained if 25% of the circulating load is treated, with a primary recovery of 50% and a gold room recovery of 80%.
The total gold recovery using the formula is 52%. It is important to indicate that this data is realistic, although “b” is low for industrial circuits because gravity recoverable gold must be ground down many size classes before becoming non-gravity recoverable gold. Also, a population balance model would estimate the circulating load “x” and “d” as vectors using matrix values of “b”, “c” and R, and a size-by-size description of the gravity recoverable gold content “f”. If the values of “f”, “b”, “c”, and R are lower, there is an impact on the gold recovery and usually, the impact of classification and grinding is the greatest. A drop in “c” produced by a coarser grind is the most likely. At a constant grind size, “b” will be affected most by the choice of grinding circuit, single stage SAG milling yielding the lowest “b” values and SAG milling with a fine transfer size followed by ball milling yielding the highest “b” values. Surprisingly the effect of the gravity recovery is the lowest.
Primary grind circuit with gold recovery unit