AG/SAG Mill Circuit Grinding

As little as 10 years ago, a “conventional” comminution circuit in the minds of many metallurgists would have conjured up pictures of crushing–ball mill or rod mill–ball mill circuits. Today, it is not common to find such circuits in operation, let alone being built. AG and SAG mills now dominate circuit design in gold and base metals applications and can rightfully lay claim to being conventional, leaving technologies such as high-pressure grinding rolls (HPRGs) the title of “new.” Regardless of how one categorises these technologies, today a much wider spectrum of proven equipment is available to the circuit designer than, say, 25 to 30 years ago. Although such choice may be seen as an improvement, it provides a particular challenge in terms of assessing which circuit is the most energy efficient and how in the first place the ore type should be meaningfully described in terms of its breakage characteristics. For ball milling, Bond’s ball work indices and equations have become the standard for describing grindability and efficiency. However, to date, there is no universal equivalent for autogenous/semiautogenous milling. The Julius Kruttschnitt Mineral Research Centre (JKMRC) drop-weight test parameters, A and b, have become popular when characterising rock for autogenous/semiautogenous milling, but they are specifically for use in autogenous/semiautogenous modeling and cannot be used for power-based calculations. In addition, the JK (JKTech) parameters are obtained from breaking relatively large quantities of material and hence cannot be obtained from small samples such as those provided by drill core. The recently developed Drop-Weight Index (DW i) may provide the solution.