Traditional approach to Grinding Circuit Selection

For AG/SAG mill circuit selection, piloting is still regarded as being the best option for estimating what the performance of the full-scale circuit will be. Tests are normally conducted under a range of conditions, the choice of circuit then being made on the basis of a number of criteria, which normally include factors such as minimum specific energy and/or maximum power utilisation efficiency. The specific energy is easy to determine as it is unambiguously defined as the power draw divided by the throughput, with different operating conditions (e.g., ball load and speed) and circuit configurations (e.g., open circuit, closed circuit, with or without pebble crusher, etc.) resulting in different specific energies. However, product grind size from each also varies, leaving the designer with the problem of determining which is the most energy efficient. Historically, this has often been done by applying Bond’s equation to determine the operating work index, which is considered by some to be indicative of the efficiency of the circuit. This has been recently challenged on the basis that the Bond equation is fundamentally flawed and hence any conclusions regarding energy efficiency based on its use are likely to be erroneous (Morrell 2004a). A further problem is that, whereas the piloting may provide sufficient information to select the best circuit, it may only apply to the ore that was tested during the programme. Many deposits have highly variable comminution characteristics leaving unanswered the very important question: “Will the chosen circuit work as well on other ore types?” When pilot testing is not carried out at all, this problem is exacerbated as the circuit design has to rely entirely on laboratory-scale ore characterisation data.