Mineral processing is the process of separating commercially valuable minerals from their ores in the field of extractive metallurgy. Depending on the processes used in each instance, it is often referred to as ore dressing or ore milling. Beneficiation is any process that improves (benefits) the economic value of the ore by removing the gangue minerals, which results in a higher grade product (ore concentrate) and a waste stream (tailings). There are many different types of beneficiation, with each step furthering the concentration of the original ore. Key is the concept of recovery, the mass (or equivalently molar) fraction of the valuable mineral (or metal) extracted from the ore and carried across to the concentrate. Processing of rare-earth minerals, often found in mineral sands, is often done by a mineral separation plant.
History
Before the advent of heavy machinery, raw ore was broken up using hammers wielded by hand, a process called "spalling". Eventually, mechanical means were found to achieve this. For instance, stamp mills were being used in central Asia in the vicinity of Samarkand as early as 973. There is evidence the process was in use in Persia in the early medieval period. By the 11th century, stamp mills were in widespread use throughout the medieval Islamic world, from Islamic Spain and North Africa in the west to Central Asia in the east. A later example was the Cornish stamps, consisting of a series of iron hammers mounted in a vertical frame, raised by cams on the shaft of a waterwheel and falling onto the ore under gravity. Iron beneficiation has been evident since as early as 800 BC in China with the use of bloomery. A bloomery is the original form of smelting and allowed people to make fires hot enough to melt oxides into a liquid that separates from the iron. Although the bloomery was promptly phased out by the invention of the blast furnace, it was still heavily relied on in Africa and Europe until the early part of the second millennium. The blast furnace was the next step in smelting iron which produced pig iron. The first blast furnaces in Europe appeared in the early 1200s around Sweden and Belgium, and not until the late 1400s in England. The pig iron poured from a blast furnace is high in carbon making it hard and brittle, making it hard to work with. In 1856 the Bessemer process was invented that turns the brittle pig iron into steel, a more malleable metal. Since then, many different technologies have been invented to replace the Bessemer process such as the electric arc furnace, basic oxygen steelmaking, and direct reduced iron (DRI). For sulfide ores, a different process is taken for beneficiation. The ore needs to have the sulfur removed before smelting can begin. Roasting is the primary method of separating, where wood was placed on heaps of ore and set on fire to help with oxidation.
2 Cu2S + 3 O2 → 2 Cu2O + 2 SO2 The earliest practices of roasting were done outside, allowing large clouds of sulfur dioxide to blow over the land causing serious harm to surrounding ecosystems, both aquatic and terrestrial. The clouds of sulfur dioxide combined with local deforestation for wood needed for roasting compounded damages to the environment, as seen in Sudbury, Ontario and the Inco Superstack. The simplest method of separating ore from the gangue consists of picking out the individual crystals of each. This is a very tedious process, particularly when the individual particles are small. Another comparatively simple method relies on the various minerals having different densities, causing them to collect in different places: metallic minerals (being heavier) will drop out of suspension more quickly than lighter ones, which will be carried further by a stream of water. The process of panning and sifting for gold uses both of these methods. Various devices known as 'bundles' were used to take advantage of this property. Later, more advanced machines were used such as the Frue vanner, invented in 1874. Other equipment used historically includes the hutch, a trough used with some ore-dressing machines and the keeve or kieve, a large tub used for differential settlement.
Types of separation
Disaggregation Beneficiation can begin within the mine itself. Most mines will have a crusher within the mine itself where separation of ore and gangue minerals occurs and as a side effect becomes easier to transport. After the crusher the ore will go through a grinder or a mill to get the ore into fine particles. Dense media separation (DMS) is used to further separate the desired ore from rocks and gangue minerals. This will stratify the crushed aggregate by density making separation easier. Where the DMS occurs in the process can be important, the grinders or mills will process much less waste rock if the DMS occurs beforehand. This will lower wear on the equipment as well as operating costs since there is a lower volume being put through.
Physical separation After the milling stage the ore can be further separated from the rock. One way this can be achieved is by using the physical properties of the ore to separate it from the rest of the rock. Prior to any physical separation process, sizing of ore particles is important for effective separation. This is done by using either Industrial Screens or Classifiers. These processes are gravity separation, flotation, and magnetic separation. Gravity separation uses centrifugal forces and specific gravity of ores and gangue to separate them. Magnetic separation is used to separate magnetic gangue from the desired ore, or conversely to remove a magnetic target ore from nonmagnetic gangue. DMS is also considered a physical separation.
Chemical separation Some ore physical properties can not be relied on for separation, therefore chemical processes are used to separate the ores from the rock. Froth flotation, leaching, and electrowinning are the most common types of chemical separation. Froth flotation uses hydrophobic and hydrophilic properties to separate the ore from the gangue. Hydrophobic particles will rise to the top of the solution to be skimmed off. Changes to pH in the solution can influence what particles will be hydrophilic. Leaching works by dissolving the desired ore into solution from the rock. Electrowinning is not a primary method of separation, but is required to get the ore out of solution after leaching.
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