Crushing and grinding are both stages of ore comminution prior to beneficiation. However, crushing refers to the fragmentation of particles larger than 5mm, primarily through compression. The efficiency of crushing and grinding stages differs significantly due to the different particle size ranges and forces involved. Crushing, which deals with large ore blocks, has a higher probability of fragmentation, with probabilities ranging from 50% to 100% in various crushers, resulting in higher efficiency. In contrast, grinding processes smaller particles and involves random impacts, where steel balls may or may not make contact with particles, and even if they do, fragmentation is not guaranteed due to insufficient force. Research indicates that the fragmentation probability in ball mills is below 10%, leading to low grinding efficiency.
Given the higher efficiency of crushing and the lower efficiency of grinding, it is beneficial to increase the crushing task and reduce the grinding task in ore comminution. This approach, known as multi-stage crushing and minimal grinding, focuses on maximizing the efficient crushing stage while minimizing the less efficient grinding stage. Additionally, the energy consumption analysis of comminution shows that the energy consumption of coarse crushing is logarithmically proportional to the reduction ratio, while the energy consumption of fine grinding is directly proportional to the reduction ratio minus one, with a difference of an order of magnitude. Therefore, the theoretical basis supports the idea of increasing the crushing task and reducing the grinding task. Multi-stage crushing and minimal grinding have been widely recognized and applied in ore processing plants worldwide.
To implement the multi-stage crushing and minimal grinding approach, several methods have been adopted in the mineral processing industry: (1) transitioning from open-circuit crushing to closed-circuit crushing to achieve finer final particle size; (2) increasing the number of crushing stages, such as going from two stages to three stages or from three stages to four stages; (3) replacing fine crushing machines with rod mills (grinding to 3-5mm); (4) using more efficient ultra-fine grinding machines to achieve lower fine particle sizes.
To further realize the multi-stage crushing and minimal grinding approach, researchers have studied the optimal point at which the crushed material should be handed over to the grinding stage. Different starting points and research methods have led to varying conclusions. Some researchers, considering the low energy consumption of both crushing and grinding, graphically analyzed the combined energy consumption using Bond's formula and concluded that handing over the material at a size of 12.7mm to the grinding stage results in lower energy consumption. Soviet researchers, aiming for the lowest cost, calculated that large-scale beneficiation plants should have a final crushing size of 4-8mm, while small-scale plants should aim for 10-15mm. Professor Li Qiheng suggested balancing both crushing and grinding while considering productivity to determine the optimal crushing size. From the perspective of minimizing the combined energy consumption, I used mathematical methods to calculate that handing over the material at a size of 3-4mm to ball mills results in lower energy consumption. Although the research conclusions vary, they indicate that a final crushing size of 15-12mm may not be optimal. Reducing the crushing size to below 10mm, with particles above 5mm, would greatly benefit grinding productivity and the overall comminution process.