Comminution — crushing and grinding — liberates the valuable mineral from gangue. It is usually the largest energy consumer and one of the most capital-intensive areas of a processing plant, so its design has a direct impact on project economics.
Determining energy requirements
The standard method for estimating ball-mill energy is Bond's equation:
W = 10 × Wi × (1/√P80 − 1/√F80)
where W is the specific energy (kWh/t), Wi the Bond work index, and F80 and P80 the 80% passing sizes of feed and product in microns. The work index is measured by the standard Bond ball mill laboratory test.
For autogenous and semi-autogenous mills, specialised tests such as the JK Drop Weight test and the SMC test are used to obtain breakage parameters (A×b).
Common configurations
- Three-stage crushing and ball milling: suited to hard ores and medium throughputs.
- SAB circuit: semi-autogenous (SAG) mill followed by a ball mill; common in large copper and gold plants.
- SABC circuit: adds a pebble crusher to break critical-size material from the SAG mill and increase capacity.
- High-pressure grinding rolls (HPGR): a lower-energy alternative for some hard ores.
Mill operating parameters
- SAG mill: ball charge typically 8–15% by volume; speed around 70–80% of critical.
- Ball mill: closed circuit with hydrocyclones; circulating load typically 250–350%.
- Ball size, liner design and pulp density directly affect grinding efficiency and wear.
Simulation and optimisation
Once laboratory parameters are available, the circuit is modelled with process simulation software such as MODSIM and USIM PAC to size mills, motors and cyclones and to assess feed variability. In operating plants, optimisation includes adjusting ball size, tuning cyclones and managing recirculating loads.
Company experience
In the project to upgrade the autogenous mill return load at Central Iron Ore, Bafq, the presence of steel balls and the high moisture of the return load led to a dedicated crusher segment design and the replacement of flat screens with a trommel — an example of how thorough feed characterisation shapes grinding-circuit design.