Mineral processing

Mineral processing, otherwise known as mineral dressing, is the practice of beneficiating valuable minerals from their ores. Industrial mineral treatment processes usually combine a number of unit operations in order to liberate and separate minerals by exploiting the differences in physical properties of the different minerals that make up an ore.

Many plants also incorporate hydrometallurgical or pyrometallurgical processes as part of an extractive metallurgical operation. Geometallurgy is a branch of extractive metallurgy that combines mineral processing with the geologic sciences.

Mineral processing involves four general types of operations: comminution or particle size reduction, sizing or separation of particle sizes by screening or classification, concentration by taking advantage of physical and surface chemical properties, and dewatering or solid/liquid separation.

A number of auxiliary materials handling operations are also considered a branch of mineral processing such as storage (as in bin design), conveying, sampling, weighing, slurry transport, and pneumatic transport.

Comminution

Comminution is particle size reduction of materials. Comminution may be carried out on either dry materials or slurries. Crushing and grinding are the two primary comminution processes. Crushing is normally carried out on "run-of-mine" ore, while grinding (normally carried out after crushing) may be conducted on dry or slurried material.

Sizing

Sizing is the general term for separation of particles according to size.

The simplest of sizing processes is screening, or passing the particles to be sized through a screen or number of screens. Screening equipment can include grizzlies, bar screens, and wire mesh screens. Screens can be static (typically the case for very coarse material), or they can incorporate mechanisms to shake or vibrate the screen.

Classification refers to sizing operations that exploits the differences in settling velocities exhibited by particles of different size. Classification equipment may include ore sorters, gas cyclones, hydrocyclones, rotating trommels, rake classifiers or fluidized classifiers. When the feed material contains particles of different densities as well as particles of different size, a degree of concentration takes place during classification because settling velocities are also dependent on particle density.

An important factor in both communition and sizing operations, is the determination of the particle size distribution of the materials being processed, commonly referred to as particle size analysis. Many techniques for analyzing particle size are used, and the techniques include both off-line analyses which require that a sample of the material be taken for analysis and on-line techniques that allow for analysis of the material as it flows through the process.

Froth flotation

Separation by froth flotation relies on the differing surface potentials of the particles. Hydrophobic particles are recovered to the froth, whereas hydrophilic particles are discharged with the tailings stream. Some mineral particles are naturally hydrophobic, whereas others require specific reagent additions to change their surface potentials. Oxide ores, such as spodumene and tantalite can be treated using oxalic acid based collectors. Sulfide ores can be recovered using xanthate or dithiophosphate type collectors.

Gravity concentration

Particles can be classified based on their specific gravity. Gravity concentration processes include:

• Heavy media or dense media separation
• Shaking tables, such as the wilfely table
• Spiral separators
• Centrifugal bowl concentrators
• Jig concentrators are continuous processing gravity concentration devices using a pulsating fluidized bed.
• Multi gravity separators

Electrostatic separation

Non-conducting particles maintain an electrostatic charge induced electrically, and so remain pinned to a charged drum. Conducting particles do not maintain the electrostatic charge and so fall off the drum, thus minerals such as ilmenite and rutile can be separated.

Magnetic separation

Minerals such as magnetite and pyrrhotite are naturally magnetic, and so can be separated from non-magnetic particles using strong magnets.

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