Some Problems in the Crushing and Grinding Stage of Mineral Processing
2019-08-30 18:14:16xinhai
In order to select useful minerals from ores, it is necessary to crush the ores first so that the useful minerals in the ores can reach the state of monomer dissociation.
The preparatory work before separation usually consists of two stages: crushing-screening and grinding-classification. The crusher and screening equipment work together and the grinding and classification equipment form a closed-circuit cycle.
1.Technological principles of more crushing and less grinding
The concept of more crushing and less grinding is deeply rooted in people's minds, but many concentrators fail to really attach importance to it. Crushing mainly depends on the extrusion and impact of crushing equipment on materials, while grinding mainly depends on the impact and grinding on materials. Among them, grinding is a high energy consumption and low efficiency operation. The power consumption of crushing operation only accounts for 8%-12% of grinding operation. The energy utilization efficiency of crushing operation is much higher than that of grinding operation.
However,there are quite a few mineral processing plants with the disadvantage of incompatible crushing and grinding equipment and technology: guaranteeing the size of crushed products cannot guarantee the output, while guaranteeing the output may lead that the size of crushed products is too large. The crushing products with too large particle size enter the ball mill, which makes the grinding and classification process difficult to guarantee both the fineness and the output. The grinding particle size of most mines is larger than 10 mm, even up to 25-40 mm, which causes great energy consumption. The most economical and effective design or reform plan is to consider crushing and grinding as a whole, determine the reasonable crushing product size, give full play to the advantages of low crushing energy consumption, increase the processing capacity of crushing equipment, reduce the grinding particle size as much as possible,and implement more crushing and less grinding to achieve the best economic benefits. For example, a gold mine in Shandong Province transformed the two-stage and one-closed-circuit crushing process into two-stage and half-crushing process, solved the problem of reasonable matching between the production capacity and crushing ratio of the second-stage crushing equipment,and achieved obvious economic benefits in production.
2. Stage-grinding, stage separation technological process principle
The process that separating part of tailings and qualified concentrate by some suitable beneficiation methods after coarse grinding of raw ore, and then regrind and reprocess the middling is called Stage-grinding, stage separation technology. It is necessary to follow the principle of “Collect and discard as early as possible” in the grinding operation and even in the processing plant operation. This requires that the grinding operation should try to use the stage grinding to discard the gangue minerals in time, which not only reduces the burden of grinding operations, avoids over-grinding, reduces the loss of metal minerals, but also reduces the amount of subsequent separation operations and reduces costs.
3.Selective grinding technological process principle
Selective grinding refers to a kind of grinding phenomenon in which the ore fragmentation degrees is different in the grinding process due to the difference in hardness between minerals. The main purpose of the crushing and grinding operation is not to reduce the ore particle size, but to dissociate the useful minerals from the gangue minerals. Therefore, the ultimate development goal of the grinding operation is to obtain the highest mineral liberation degree with the minimum energy input. Selective grinding is widely used in mineral processing industry such as metal ore, non-metal ores and coal mines, especially in the molybdenum ore processing.
A reasonable grinding-classification process is mainly determined by the following factors:
1. Disseminated grain size and common characteristics of useful minerals and gangue minerals;
2. The hardness and density of useful ore and gangue minerals;
3. The argillization degree and oxidation of useful ore and gangue minerals;
4. The content and value of useful minerals in the ore;
5. The requirements of concentrate particle size in production practice.
The determination of the grinding sections is mainly based on the scale of the dressing plant, ore grindability, desired mineral crystal size, the feed size and the final product size.
When the upper limit of the particle sizeof the final product is greater than 0.15mm, one stage of grinding classification is adopted in the process.
When the upper limit of particle size of the final product is less than 0.15mm, the process adopts two-stage grinding classification.
Special conditions of the second-stage grinding classification process can be considered: large ore size (upper limit: 25mm or more), difficult grinding ore, large production scale, and final product size is coarse.
Another special condition of one-stage grinding classification process can also be adopted: soft ore, easy grinding ore, small production scale, and the upper limit of particle size of the final product is less than 0.15mm.
The whole mill classification process is composed of mill and classifier. And the grinding sections must be determined by technical and economic comparison according to specific conditions. In practice, most mineral processing plant adopt one or two grinding classification process, and less adopt three or more grinding classification process.