MINERAL DRESSING

The first process most ores undergo after they leave the mine is mineral dressing (processing), also called ore preparation, milling, and ore dressing or ore beneficiation. Ore dressing is a process of mechanically separating the grains of ore minerals from the gangue minerals, to produce a concentrate (enriched portion) containing most of the ore minerals and a tailing (discard) containing the bulk of the gangue minerals.

Since most ore minerals are usually finely disseminated and intimately associated with gangue minerals, the various minerals must be broken apart (freed) or “liberated” before they can be collected in separate products. Therefore, the first part in any ore dressing process will involve the crushing and grinding (which is also known by a common name called “comminution”) of the ore to a point where each mineral grain is practically free.

1. Comminution

Crushing and grinding are usually carried out in a sequence of operations by which the lump size is reduced step by step. Normally there are 3 stages of crushing and 2 stages of grinding.

Three stages of crushing

  • i. Primary Crushing (coarse crushing): In primary crushing, ore or run-of-mine ore (up to 1 m in size) is crushed down to about 10 cm and it is done in a jaw or gyratory crusher.
  • ii. Secondary Crushing (intermediate crushing): In this case, ore is crushed from 10 cm to less than 1 – 2 cm size; for this purpose jaw, cone or roll crushers are used. These secondary crushers consume more power than primary crushers.
  • iii. Tertiary Crushing (fine crushing): By tertiary crushers ore is crushed from 1 – 2 cm to less than 0.5 cm. Short head cone crushers, roll crushers, hammer mills can be used for this purpose.

The two stages of grinding

  • i. Coarse Grinding: Rod mills are generally used as coarse grinding machines. They are capable of taking feed as large as 50 mm and making a product as fine as 300 microns.
  • ii. Fine Grinding: Fine grinding, which is the final stage of comminution, is performed in ball mills using steel balls as the grinding medium. The ball mill, after feeding 0.5 mm material may give a product that is less than 100 microns. Grinding is usually done wet.

The principle purposes of grinding

  • i. To obtain the correct degree of liberation in mineral processing.
  • ii. To increase the specific surface area of the valuable minerals for hydrometallurgical treatment; i.e. leaching.

Mineral processing combines a series of distinct unit operations. The flowsheet shows diagrammatically the sequence of unit operations in the plant.

Mineral dressing flowsheet
Mineral dressing flowsheet

Comminution and concentration are two primary operations in mineral processing as it can be seen from the above flowsheet, but many other important steps are involved.

  • Sizing – by screens and classifiers.
  • Dewatering – by thickeners, filters and driers, etc.
  • Auxiliary operations – conveying, sampling, etc.

2. Sizing Methods

There are two methods of industrial sizing.

  • i. Screening
  • ii. Classification

Screening

Screening is generally carried out on relatively coarse material, as the efficiency decreases rapidly with fineness, and it is generally limited to materials above about 250 microns in size, finer sizing normally being undertaken by classification.

Industrial sizing is used in closed circuit with a crusher or a ball mill. For the large lump sizes coarse grizzlies made of rails or trommel (revolving) screens made of punched plate may be used. For finer material, screens are usually made of woven metal wire.

The material that passes through the openings (apertures) of a particular screen is known as the undersize and material that remains on the screen is the oversize.

Laboratory screening: The other use of screening is as a measuring technique with the purpose of determining the relative amounts of various particle sizes in a given material. The particle size distribution of crushed or ground ore is determined by means of a screen analysis. For this purpose standardized screening scales are developed. Most common is the American Tyler Screen Scale (Tyler Standard Series) where the screen number(mesh number) is given as the number of meshes (openings) or wires per linear inch (1 inch=2.54 cm): the diagonal of each screen opening is equal to the edge of the previous screen. So, the linear dimension of each opening differs by a constant factor of √2 = 1.414.

Tyler screen scale starts with 1.05 inch (26.67 mm), for smaller particle sizes the dimensions are usually given in microns (1 micron = 10-3 mm). Thus, 200 mesh (#) is equal to 74 microns in the Tyler Screen Series.

The result of a screen analysis is given as the fraction of the sample which passes through one screen but which is stopped by the subsequent screen so we can say that a certain percentage is + 26.67 mm (coarser than the coarsest screen), another percentage is – 20 mesh + 28 mesh (dimensions between 0.833 mm and 0.589 mm) and finally that a certain percentage is – 200 mesh (finer than 0.074 mm or 74 microns). The mesh number does not directly indicate the size of the aperture, and the aperture can be calculated from the mesh number if the wire diameter is known.

1 inch (25.4 mm) = Number of wires * Wire diameter + Number of apertures * Aperture size

Tyler standard screen
Tyler standard screen

Classification

Classification is defined as a method of separating mixtures of mineral particles into two or more products according to their settling velocities in water, in air or in other fluids as given in below figure. Industrial classification may be carried out in different types of classifiers and these classifiers are; hydraulic classifiers, mechanical classifiers and cyclones. Basically they all work according to the principle that the particles are suspended in water which has a slight upward movement relative to the particles. Particles below a certain size and density are carried away with the water-flow, whereas the coarser and heavier particles will settle.

In classification mostly we use wet cyclones (hydrocyclones) in which rapid spinning of the pulp centrifuges the solid particles.

Classification by hydrocyclones
Classification by hydrocyclones

3. Concentration

The second fundamental (main) operation in mineral processing, after the release, or liberation, of the valuable minerals from the gangue minerals, is the separation of these values from the gangue, i.e. concentration.

Concentration is usually accomplished by utilizing some specific difference in physical (or chemical) properties of the metal and gangue compound in the ore.

In concentration the following terms are used:

  • Head is the feed to a concentrating system.
  • Concentrate is defined as the valuable mineral(s) separated from ore undergoing a specific treatment.
  • Tailing is the fraction of ore rejected in a separating process. It is usually the valueless portion, i.e. discard or waste.
  • Middlings are the particles of locked valuable mineral and gangue, i.e. liberation has not been attained. Further liberation can be achieved by further comminution.
  • Recovery is the percentage of the total metal, contained in the ore that is recovered in the concentrate.

Physical Concentration Methods

  1. Separation dependent on optical and radioactive properties of minerals, i.e. hand pickling, optical sorting, radioactive sorting, etc.
  2. Separation dependent on specific gravity (density) difference of minerals, i.e. heavy-media separation, gravity concentration by use of tables, jigs, cones, etc.
  3. Separation utilizing the different surface properties (i.e. surface chemistry) of the minerals, i.e. froth flotation, etc.
  4. Separation dependent on magnetic properties of the minerals, i.e. low and high, dry and wet magnetic separation, etc.
  5. Separation dependent on electrical conductivity properties of the minerals, i.e. electrostatic separation, etc.

So mineral processing is concerned mainly with the physical methods of separation of minerals. Pyrometallurgy and hydrometallurgy may also deal with raw materials but those processes change the character of some or all of the constituents of the raw materials.