The ore that is obtained from the mine needs to be processed to extract the valuable components. Ores are usually made up of different minerals pressed together, often of small sizes. In order to be able to extract the valuable components it is necessary to reduce the size of the ore down to the size of the component minerals. This size reduction requires a lot of energy, and a balance has to be found between the energy used for creating smaller particles, and the efficiency of liberating the valuable components from the rest. Hence size reduction and liberation are studied together.
The size reduction process is performed in several stages, and often combined with separation processes with recirculation of oversize material. It is also important to avoid ending up with particles that are too fine. This is both because it involves unnecessary energy, but also it can make the subsequent downstream processing more difficult because of the slower sedimentation velocity of fine particles.
Size reduction can be broadly described as crushing and grinding. Crushing is the first part with the ore reduced to a size of about 20 mm. This usually involves squeezing the ore until it breaks and is usually a dry process, without water being added. The crushed ore is then further reduced in size by grinding, which can also be made in multiple stages, with separation and recirculation of oversize particles between stages. There are also many different types of grinding equipment, including tumbling mills or stirred media mills, and whether there are balls or rods added to help with the grinding. The grinding can be performed either as a dry or a wet process. The traditional grinding processes are performed wet, with the addition of water to help move the ore particles. Dry grinding is used in some mineral processing operations, such as where the ore is sensitive to contact with water, or where the availability of water is limited. Dry grinding is used currently in the production of cement from limestone, but it is anticipated that dry grinding may become more widely used in the future.
Image: Grinding equipment at the Khibny PhosAgro mineral processing plant, during a visit by SEESIMA project participants.
The SEESIMA project worked with different aspects of the size reduction/liberation processes:
- A simple description of size reduction operations, by analogy to chewing food, was provided as part of a profiling of the SEESIMA project on the Kolarctic web page.
- KSC studied the optimisation of the layout of the grinding and classification/separation stages and implemented the results in industrial mineral processing plants. Read more here.
- LTU performed a PhD thesis study on improvements to the dry grinding process, including:
- Using Grinding Aid additives to reduce the energy consumption. Read more here.
- The effect of additives on the liberation, separation and downstream processes. Read more here.
- The performance of alternative, more environmentally friendly additives compared with the current usage. Read more here.
- The potential for the dry grinding aid additives to also have positive effect on the performance of wet grinding.
- SINTEF summarised and updated an earlier study made on electropulse disaggregation, which has been promoted as an alternative means of achieving more energy efficiency size reduction and liberation. Read more here.