The practice of minerals processing is as old as human civilization. Minerals and products derived from minerals have formed our development cultures from the flints of the Stone Age man to the uranium ores of Atomic Age.
The goal in mineral processing is to produce maximum value from a given raw material. This goal can be a crushed product with certain size and shape or maximum recovery of metals out of a complex ore. The technologies to achieve these goals are classical, complementary and well defined.
All operations have different process environments due to mineral hardness and size range. It is important to know in which “range” we are operating as this will affect many process parameters, (wear rate, uptime, operation costs etc.).
The operating stages in minerals processing have remained the same for thousands of years. Of course, we have come far in development of equipment and processes since then, but the hard, abrasive and inhomogeneous mineral crystals have to be treated in special ways in order to extract maximum value out of each size fraction.
|Starting point of mineral processing
|Size reduction & control:
|Processes to produce requested size distributions from feed material
|Processes to improve value of minerals by washing and/or separation
|Processes to produce requested end products from value and waste minerals.
|Operations for moving the processes forward with a minimum of flow disturbances
|Measures to protect the process environment above from wear and emissions of dust and sound
Operation – Dry or wet?
- When no water is needed for processing
- When no water is allowed for processing
In all other cases due to:
- Better efficiency
- More compact installation
- No dusting
Crushing of rock and minerals
By tonnage this is by far the largest process operation in minerals processing. The goal is to produce rock or (more seldom) mineral fractions to be used as rock fill or ballast material for concrete and asphalt production. Quality parameters are normally strength, size and shape. The size fractions, see below, are priced according to defined size intervals and can be reached by crushing only.
Crushing and grinding of ore and minerals Size reduction of ores is normally done in order to liberate the value minerals from the host rock. This means that we must reach the liberation size, normally in the interval 100 – 10 microns, see value curve 1 above. If the raw material is a single mineral (Calcite, Feldspar a.o.) the value normally lays in the production of very fine powder (filler. In order to maximize the value in size reduction of rock and minerals, see below, we need both crushing and grinding in various combinations.
Neither crushers nor grinding mills are very precise when it comes to the correct sizing of the end products. The reason is to find partly in the variation of the mineral crystals’ compounds (hard-soft, abrasive – nonabrasive), partly in the design and performance of the equipment. Size control is the tool for improvement of the size fractions in the process stages and in the final products. For the coarser part of the process, screens are used (in practice above 1-2 mm). In the finer part we have to use classification with spiral classifiers,
Enrichment – Washing
Washing is the simplest method of enrichment used to improve the value of rock and mineral fractions from sand size and upwards. Removing of surface impurities like clay, dust, organics or salts is often a must for a saleable product. Different techniques are used depending on how hard these impurities are attached to the rock or mineral surface
Enrichment – Separation
Most value minerals (both metallic and industrial) are priced by their purity. After liberation by size reduction and size control all minerals are free to be separated from each other. Depending on the properties of the individual minerals they can be recovered by different methods of separation.
After the enrichment operation we end up with a value product (concentrate) and a non-value product (tailings). These products are probably not sellable nor disposable due to the content of process water, particle size, or chemical composition. By upgrading we mean the methods of increasing the value of these products by sedimentation, mechanical dewatering, drying, calcining or sintering and recovering the process water from the tailings, making them disposable.
Without a proper set up for materials handling no processing system will perform. Different process stages may be in various locations, may have various feed conditions, are on different shift cycles etc. Materials handling of dry material is based on the operations of loading, unloading, transportation, storing and feeding. Materials handling of wet material, called slurry handling is also based on the operations of transportation (by slurry pumps and hoses), feeding (by slurry pumps) and storage (by slurry agitation).