Almost all the metals and other inorganic materials that have been exploited occur as the compounds that constitute the minerals that make up the earth’s crust. The forces and processes that have shaped the earth’s surface have concentrated these minerals in widely different amounts. When this concentration is sufficiently great so that the mineral can be economically exploited and recovered, the deposit is referred to as an ore or orebody. However, even then the minerals are not usually available in a form with the purity necessary for immediate processing to the desired end product. In his sixteenth century work on mineral processing Agricola (1950) wrote: “Nature usually creates metals in an impure state, mixed with earth, stones, and solidified juices, it is necessary to separate most of these impurities from the ores as far as can be, before they are smelted.”
Valuable minerals must first be separated from those of no commercial value, which are called gangue. Ore processing refers to this initial treatment of mined material to produce a mineral concentrate of a sufficiently high grade to be satisfactorily processed further to the pure metal or other end product. The differing characteristics of the minerals making up the ore are exploited to separate them from each other by a variety of physical methods that generally leave the chemical composition of the mineral unchanged. (The processing of coal is specifically discussed in the article “Coal preparation”)
Crushing and Grinding
The particle size of the material arriving at the processing plant will depend on the mining operation employed and on the ore type, but it will be relatively large. Comminution, the progressive reduction in the particle size of lumpy ore, is carried out for two reasons: to reduce the material to a more convenient size and to liberate the valuable component from the waste material as a first step towards its effective separation and recovery. In practice, comminution usually consists of the crushing of larger-sized material, followed by the breaking of the material to finer sizes by tumbling it in rotating steel mills.
It is not possible to progress from very large lumps to fine material in a single operation or using one machine. Crushing thus is usually a dry operation that typically takes place in stages which are designated as primary, secondary and tertiary.
Primary crushers reduce the ore from anything as large as 1.5 m down to 100 to 200 mm. Machines such as jaw and gyratory crushers apply a fracture force to the large particles, breaking the ore by compression.
In a jaw crusher, ore falls into a wedge-shaped space between a fixed and a moving crushing plate. Material is nipped and squeezed until it breaks and released and nipped again further down as the jaws open and close, until it finally escapes through the gap set at the bottom.
In the gyratory crusher, a long spindle carries a heavy, hard steel conical grinding element that is moved eccentrically by a lower bearing sleeve within the crushing chamber or shell. The relative motion of the crushing faces is produced by the gyration of the eccentrically mounted cone against the outer chamber. Typically this machine is used where a high throughput capacity is required.
Secondary crushing reduces the particle size down to 5 to 20 mm. Cone crushers, rolls and hammer mills are examples of the equipment used. The cone crusher is a modified gyratory crusher with a shorter spindle that is not suspended, but supported in a bearing below the head. A roll crusher consists of two horizontal cylinders rotating towards each other, the rolls drawing the ore into the gap between them and after a single nip discharging the product. The hammer mill is a typical impact crusher mill. Comminution is by the impact of sharp blows applied at high speed by hammers attached to a rotor within the work-space.
Grinding, the last stage in comminution, is performed in rotating cylindrical steel vessels known as tumbling mills. Here the mineral particles are reduced to between 10 and 300 μm. A grinding medium, such as steel balls, rods or pebbles (pre-sized lumps of ore much larger than the bulk feed of material), is added to the mill so that the ore is broken down to the desired size. The use of pebbles is termed autogenous grinding. Where the ore type is suitable, run-of-mine (ROM) milling may be used. In this form of autogenous milling the entire ore stream from the mine is fed directly to the mill without pre-crushing, the large lumps of ore acting as the grinding medium.
The mill is generally loaded with crushed ore and grinding medium to just under half full. Studies have shown that the breaking produced by milling is a combination of both impact and abrasion. Mill liners are used to protect the mill shell from wear and, by their design, to reduce slip of the grinding media and improve the lifting and impact portion of milling.
There is an optimal size to which ore must be ground for effective separation and recovery of the valuable component. Undergrinding results in incomplete liberation and poor recovery. Overgrinding increases the difficulty of separation, besides using an excess of expensive energy.
After crushing and milling, the products are usually separated simply according to their size. The primary purpose is to produce appropriately sized feed material for further treatment. Oversize material is recycled for further reduction.
Screening is generally applied to fairly coarse material. It may also be used to produce a reasonably uniform feed size for a subsequent operation where this is required. The grizzly is a series of heavy parallel bars set in a frame that screens out very coarse material. The trommel is an inclined rotating cylindrical screen. By use of a number of sections of different sized screens, several sized products can be simultaneously produced. A variety of other screens and screen combinations may be employed.
Classification is the separation of particles according to their settling rate in a fluid. Differences in density, size and shape are effectively utilized. Classifiers are used to separate coarse and fine material, thereby fractionizing a large size distribution. A typical application is to control a closed-circuit grinding operation. While size separation is the primary objective, some separation by mineral type usually occurs due to density differences.
In a spiral classifier, a rake mechanism lifts the coarser sands from a slurry pool to produce a clean de-slimed product.
The hydrocyclone uses centrifugal force to accelerate settling rates and produce efficient separations of fine-sized particles. A slurry suspension is introduced at high velocity tangentially into a conical shaped vessel. Due to the swirling motion, the faster settling, larger and heavier particles move towards the outer wall, where the velocity is lowest, and settle downwards, while the lighter and smaller particles move towards the zone of low pressure along the axis, where they are carried upward.
Concentration separation requires particles to be distinguished as being either those of the valuable mineral or as gangue particles and their effective separation into a concentrate and a tailing product. The objective is to achieve maximum recovery of the valuable mineral at a grade that is acceptable for further processing or sale.
The oldest and simplest method of concentration is the selection of particles visually and their removal by hand. Hand sorting has its modern equivalents in a number of electronic methods. In photometric methods, particle recognition is based on the difference in reflectivity of different minerals. A blast of compressed air is then activated to remove them from a moving belt of material. The differing conductivity of different minerals may be utilized in a similar manner.
Heavy medium separation
Heavy medium or dense medium separation is a process that depends only on the density difference between minerals. It involves introducing the mixture into a liquid with a density lying between that of the two minerals to be separated, the lighter mineral then floats and the heavier sinks. In some processes it is used for the preconcentration of minerals prior to a final grind and is frequently employed as a cleaning step in coal preparation.
Heavy organic fluids such as tetrabromoethane, which has a relative density of 2.96, are used in certain applications, but on a commercial scale suspensions of finely ground solids that behave as simple Newtonian fluids are generally employed. Examples of the material used are magnetite and ferrosilicon. These form low-viscosity, inert and stable “fluids” and are easily removed from suspension magnetically.
Natural separating processes such as river systems have produced placer deposits where heavier larger particles have been separated from lighter smaller ones. Gravity techniques mimic these natural processes. Separation is brought about by the movement of the particle in response to the force of gravity and the resistance exerted by the fluid in which separation takes place.
Over the years, many types of gravity separators have been developed, and their continued use testifies to the cost-effectiveness of this type of separation.
In a jig a bed of mineral particles is brought into suspension (“fluidized”) by a pulsating current of water. As the water drains back between each cycle, the denser particles fall below the less dense and during a period of draining small particles, and particularly smaller denser particles, penetrate between the spaces between the larger particles and settle lower in the bed. As the cycle is repeated, the degree of separation increases.
Shaking tables treat finer material than jigs. The table consists of a flat surface that is inclined slightly from front to back and from one end to the other. Wooden riffles divide the table longitudinally at right angles. Feed enters along the top edge, and the particles are carried downwards by the flow of water. At the same time they are subject to asymmetrical vibrations along the longitudinal or horizontal axis. Denser particles which tend to be trapped behind the riffle are shuffled across the table by the vibrations.
All materials are influenced by magnetic fields, although for most the effect is too slight to be detected. However, if one of the mineral components of a mixture has a reasonably strong magnetic susceptibility, this can be used to separate it from the others. Magnetic separators are classified into low- and high-intensity machines, and further into dry- and wet-feed separators.
A drum-type separator consists of a rotating non-magnetic drum containing within its shell stationary magnets of alternating polarity. Magnetic particles are attracted by the magnets, pinned to the drum and conveyed out of the magnetic field. A wet high- intensity separator (WHIMS) of the carousel type consists of a concentric rotating matrix of iron balls that passes through a strong electromagnet. Slurried residues are poured into the matrix where the electromagnet operates, and magnetic particles are attracted to the magnetized matrix while the bulk of the slurry passes through and exits via a base grid. Just past the electromagnet, the field is reversed and a stream of water is used to remove the magnetic fraction.
Electrostatic separation, once commonly used, was displaced to a considerable extent by the advent of flotation. However, it is successfully applied to a small number of minerals, such as rutile, for which other methods prove difficult and where the conductivity of the mineral makes electrostatic separation possible.
The method exploits differences in the electrical conductivity of the different minerals. Dry feed is carried into the field of an ionizing electrode where the particles are charged by ion bombardment. Conducting particles rapidly lose this charge to a grounded rotor and are thrown from the rotor by centrifugal force. Non-conductors lose their charge more slowly, remain clinging to the earth conductor by electrostatic forces, and are carried around to a collection point.
Flotation is a process of separation that exploits differences in the physico-chemical surface properties of different minerals.
Chemical reagents called collectors are added to the pulp and react selectively with the surface of the valuable mineral particles. The reaction products formed makes the surface of the mineral hydrophobic or non-wettable, so that it readily attaches to an air bubble.
In each cell of a flotation circuit the pulp is agitated and introduced air is dispersed into the system. The hydrophobic mineral particles attach to the air bubbles and, with a suitable frothing agent present, these form a stable froth at the surface. This continuously overflows the sides of the flotation cell, carrying its mineral load with it.
A flotation plant consist of banks of interconnected cells. A first concentrate produced in rougher bank is cleaned of unwanted gangue components in a cleaner bank, and if necessary recleaned in a third bank of cells. Additional valuable mineral may be scavenged in a fourth bank and recycled to the cleaner banks before the tails are finally discarded.
Following most operations it is necessary to separate the water used in the separation processes from the concentrate produced or from the waste gangue material. In dry environments this is particularly important so that the water may be recycled for re-use.
A settling tank consists of a cylindrical vessel into which pulp is fed at the centre via a feed-well. This is placed below the surface to minimize disturbance of the settled solids. Clarified liquid overflows the sides of the tank into a launder. Radial arms with blades rake the settled solids towards the centre, where they are withdrawn. Flocculants may be added to the suspension to accelerate the settling rate of the solids.
Filtration is the removal of solid particles from the fluid to produce a cake of concentrate that can then be dried and transported. A common form is the continuous vacuum filter, typical of which is the drum filter. A horizontal cylindrical drum rotates in an open tank with the lower section immersed in pulp. The shell of the drum consists of a series of compartments covered by a filter medium. The inner double-walled shell is connected to a valve mechanism on the central shaft that permits either vacuum or pressure to be applied. Vacuum is applied to the section immersed in the pulp, drawing water through the filter and forming a cake of concentrate on the cloth. The vacuum dewaters the cake once out of the slurry. Just before the section re-enters the slurry, pressure is applied to blow off the cake. Disc filters operate on the same principle, but consist of a series of discs attached to the central shaft.
Only a small fraction of the mined ore consists of valuable mineral. The remainder is gangue that after processing forms the tailings that must be disposed of.
The two major considerations in tailings disposal are safety and economics. There are two aspects to safety: the physical considerations surrounding the dump or dam in which the tailings are placed; and pollution by the waste material that may affect human health and cause damage to the environment. Tailings must be disposed of in the most cost-effective manner possible commensurate with safety.
Most commonly the tailings are sized, and the coarse sand fraction is used to construct a dam at a selected site. The fine fraction or slime is then pumped into a pond behind the dam wall.
Where toxic chemicals such as cyanide are present in the waste waters, special preparation of the base of the dam (e.g., by the use of plastic sheeting) may be necessary to prevent the possible contamination of ground waters.
As far as possible, the water recovered from the dam is recycled for further use. This may be of great importance in dry regions and is increasingly becoming required by legislation aimed at preventing the pollution of ground and surface water by chemical pollutants.
Heap and in Situ Leaching
Much of the concentrate produced by ore processing is processed further by hydrometallurical methods. The metal values are leached or dissolved from the ore, and different metals are separated from each other. The solutions obtained are concentrated, and the metal then recovered by steps such as precipitation and electrolytic or chemical deposition.
Many ores are of too low a grade to justify the cost of pre-concentration. Waste material may also still contain a certain amount of metal value. In some instances, such material may be economically processed by a version of a hydrometallurgical process known as heap or dump leaching.
Heap leaching was established at Rio Tinto in Spain more than 300 years ago. Water percolating slowly through heaps of low-grade ore was coloured blue by dissolved copper salts arising from oxidation of the ore. The copper was recovered from solution by precipitation onto scrap iron.
This basic process is utilized for oxide and sulphide heap leaching of low grade and waste material around the world. Once a heap or dump of the material has been created, a suitable solubilizing agent (e.g., an acid solution) is applied by sprinkling or flooding the top of the heap and the solution that seeps to the bottom is recovered.
While heap leaching has long been successfully practised, it was only relatively recently that the important role of certain bacteria in the process was recognized. These bacteria have been identified as the iron-oxidizing species Thiobacillus ferrooxidans and the sulphur-oxidizing species Thiobacillus thiooxidans. The iron-oxidizing bacteria derive energy from the oxidation of ferrous ions to ferric ions and the sulphur-oxidizing species by the oxidation of sulphide to sulphate. These reactions effectively catalyze the accelerated oxidation of the metal sulphides to the soluble metal sulphates.
In situ leaching, sometimes called solution mining, is effectively a variation of heap leaching. It consists of the pumping of solution into abandoned mines, caved in workings, remote worked-out areas or even entire ore bodies where these are shown to be permeable to solution. The rock formations must lend themselves to contact with the leaching solution and to the necessary availability of oxygen.