818 1.3 SAG Mill Design During the design of a SABC circuit, SAG and ball mill dimensions and installed power are selected to ensure the circuit can accommodate the expected ore competency while achieving the target grind size and nameplate capacity. Ore competency is characterized by two key attributes: (a) resistance to impact breakage—or competency in AG/SAG milling—as measured by the JKDWT or SMC Test®, and (b) grindability to achieve the final product size in a ball milling circuit—or competency in ball milling—as measured by the Bond Ball Mill Work Index (BMWi). A comprehensive ore characterisation campaign is therefore essential to capture the variability of ore competency for both SAG and ball milling across the deposit. For mill design purposes, representative ore characteristics are commonly taken as the most competent 80th percentile values of both Axb and Bond BMWi (David, 2013). Figure 61 presents the Comminution Footprint, showing Axb values and corresponding Bond BMWi for 261 ore samples from a deposit spanning the life of mine. While a broad trend exists between the two parameters, they are not expected to be strongly correlated, as they represent different breakage attributes of the same ore measured on different size ranges using distinct tests. The 80th percentile ore competency for this deposit, indicated by the intersection of the solid red lines, corresponds to an Axb of 34.4 and a Bond BMWi of 14.55 kWh/t. A reference ‘shifted properties’ ore is also shown in green, maintaining the Axb of 34.4 but with a Bond BMWi of 16 kWh/t. Figure 61 – Comminution Footprint for 261 Ore Samples Across a Deposit SAG and ball mill dimensions and installed power are selected to handle the 80th percentile of Axb and Bond BMWi, which dictate both the total comminution power required and its allocation between the SAG and ball milling sections (e.g., 52% is required in SAG and 48% is required in ball mill). If a circuit with these mill dimensions processes an ore with different ore properties within the comminution footprint—such as the reference “shifted properties” ore—the overall power requirements and the split between SAG and ball milling will change accordingly. The comminution circuit must then utilize its full operational flexibility to meet the new requirements. If these are not met, as is often the case, the circuit will respond with corresponding changes in throughput, SAG mill power draw, and final grind size. 1.4 Dealing with Variability AG/SAG milling technology has become widely adopted in the minerals processing industry due to its 0 20 40 60 80 100 120 140 160 180 5 7 9 11 13 15 17 19 21 23 Harder ß Axb àSofter Softer ß Bond BMWI, kWh/t à Harder 261 Ore Samples 80th Percentile Shifted Properties
RkJQdWJsaXNoZXIy MTM0Mzk2