The comminution process is the most prominent feature of capital and energy utilization in modern mineral-processing plants. Selection of the mill decides the efficacy of the entire processing cycle, stability of operation, and economic feasibility of the mining project. Semi-autogenous Grinding (SAG) and Ball mills fulfill different and complementary functions in primary and secondary grinding systems. The operating choice of the type of the tumbler mill is dictated by several factors, such as dimensions of feed materials, desired fineness of the final product, ore amenability, and economics.
The circuit positioning and feed size make the main physical difference between semi-autogenous grinding (SAG) and ball milling processes. SAG mills take coarse crushed feed directly out of primary crushers and can accept particle feed sizes of up to 152 mm. These relatively short but large diameter machines contain a low-ball charge ranging from 8% to 21% and use big ore rocks as grinding media through high-energy impact grinding. Ball mills act as secondary grinding machines that grind ore feed previously crushed or feed from the SAG mill screens undersize. Containing a relatively high steel ball charge ranging from 30% to 45% of mill volume, these relatively long but narrow machines grind feed particles down to the micron-sized target products like P80 of 75 to 125 µm.
The ore breakage parameters and hardness factors indicate the required unit grinding energy. The traditional drop weight parameters (A × b), pinion energy for the SAG (kWh/t), and Ball Mill Work Indices (Wi) define the basic grinding energy requirements. Higher competency ores require higher energy to break large sizes, whereas softer and varying ores may cause significant instability if the circuit transfer size (T80) is not well-controlled. Ball mill offers better control over the particle size distribution and acts as an insurance against the operational instabilities which tend to hamper SAG mill performance.
Capital costs vs operational costs form the final determining factor on the flowsheet design. The use of the semi-autogenous grinding (SAG) mill reduces the overall capital cost and footprint of the plant layout because it replaces the secondary and tertiary crushing. In contrast, the ball mills will increase the overall steel media inventory and increase the need for crushing facilities; however, ball mills produce reliable fine grinding operation. Industry standards reveal that the use of the secondary crushing process, which involves HPGR along with ball mills, reduces overall circuit specific energy by up to 15-25% compared to conventional SABC. Nonetheless, SAG mill remains the industry standard option for capital limited projects and wet clay ores. The correct sizing of the SAG mill, based on the site-specific hard rock ore hardness, is crucial to avoid production bottlenecks.
SAG mills and ball mills operate as successive, complementary machines instead of mutually exclusive machines in modern plant layout. Coarse rock grinding is done through SAG mills since they give high impact breakage, and secondary fine grinding is done through ball mills to achieve mineral liberation. This process of successive operation in SAB or SABC circuitry combines high throughput primary grinding with fine grinding, hence improving energy efficiency and ensuring long-term profitability.
