Mineral beneficiation-ore liberation

Sinonine deeply know that liberation is the first and the most important step in Mineral Beneficiation. The second step, separation, is impracticable if the first step, liberation, is not accomplished successfully.

Liberation can be defined as the freeing or detachment of dissimilar mineral grains. The operation employed to liberate the dissimilar mineral grains is Size reduction or Comminution. If the particles of ore consist of a single mineral, they are termed as Free particles.

If the particles of ore consist of two or more minerals, they are termed as locked particles. If the locked particles contain valuable minerals at considerable quantity, they are termed as middling particles.

Grain size: It is the size of a mineral as it occurs in the ore.

Particle size: It is the size of any particle whether free or locked particle.

Grain and Grain size pertain to uncrushed ore and Particle and Particle size pertain to crushed or ground ore.

Liberation size: It is the size of a mineral particle at which that mineral is completely liberated. It is the size of a free particle of required (valuable) mineral.

Various mineral grains, present in the ore, exist in physical combination with each other. To detach the valuable mineral grains from all other gangue mineral grains, it is essential to reduce the size of the ore particles.

If one mineral species in an ore is to be separated physically from all other species in the ore, all grains of the desired species must be physically detached from all remaining species in the ore. In an ore containing mineral A, B and C (Figure 1) if all grains of mineral A (considered as valuable mineral) are to be separated from the

ore, all grains of A must be detached from minerals B and C (gangue minerals). When such detachment is complete, mineral A is said to be liberated. However liberation of mineral A does not require liberation of minerals B and C in the ore.

If the physical properties of the adjacent minerals are sufficiently dissimilar, or if the bond between them is notably weaker than either of them, fracture may take place (when it is comminuted) preferentially at the boundary. Comminution results in true freeing or detachment of minerals. Then it is known as Liberation by detachment or Intergranular liberation. When the run-of-mine ore is reduced in size to grain size of valuable mineral, all the valuable mineral particles exist as free particles, no matter whether remaining particles are free or locked particles of gangue minerals.

Figure 1 A particle of an ore containing A, B and C minerals.

Accordingly, the reduced ore consists of:

Free particles of valuable minerals

Free and/or locked particles of gangue minerals

At this condition, particle size = liberation size = grain size

This condition normally does not occur in practice. If the physical properties of the adjacent minerals are not so dissimilar, comminution to grain size does not result in rupturing the bond between adjacent dissimilar minerals. Fracture occurs across the grains producing locked particles. Hence to detach valuable mineral, the ore is further reduced in size. Thus the reduced ore consists of:

Free particles of valuable mineral

Very few locked particles of valuable and gangue minerals

Free and/or locked particles of gangue minerals

This condition is practically considered as equivalent to freeing. This type of liberation is known as Liberation by size reduction or Transgranular liberation. Liberation of most of the ores comes under this category.

At this condition, particle size = liberation size < grain size

Let us consider a particle in two dimensions (Figure 2). Let ABCD be 8 × 8 cm size particle consists of valuable mineral particles (shaded portion) and gangue mineral particles (white portion) of 2 × 2 cm size each in equal proportion. This particle is reduced by size reduction to a 2 × 2 cm size. If the fracture takes place along the boundary lines as shown in Figure 6.2(a), it yields 16 particles of 2 × 2 cm size each.

All 16 particles are free particles out of which 8 particles are of valuable mineral and 8 particles are of gangue mineral. Here all the valuable mineral particles are liberated. This type of liberation is called liberation by detachment.

In this case, Grain size, Particle size and Liberation size are equal.Suppose if the fracture takes place across the grains as shown in Figure 6.2(b), it yields 25 particles out of which 4 particles E, F, G, H are of 1 × 1 cm size, 12 particles are of 2 × 1 cm size and remaining 9 particles are of 2 × 2 cm size. Here:

E and G are free particles of valuable mineral.

F and H are free particles of gangue mineral.

All others are locked particles.

                    (a)                                                         (b)                                                 (c)

Figure 2 Liberation methods.

Hence valuable mineral has been liberated partially to a lesser extent. If the resulting particles are again crushed and fracture takes place along the boundary lines as shown in Figure 2 (c), it yields 64 particles, all are free particles of 1 × 1 cm size out of which 32 are valuable mineral particles and 32 are gangue mineral particles. Here all valuable mineral particles are liberated. This happens when the particles are reduced to 1 × 1 cm size which is less than grain size. 1 × 1 cm is the particle size and also the liberation size.

If the second fracture also does not take place along the boundary lines, it yields still locked particles which need further reduction in size. This reduction is continued till all valuable mineral particles occur as free particles. Then the particle size, hence liberation size, is less than 1 × 1 cm. This happens because the fracture takes place across the mineral grain. This type of liberation is called liberation by size reduction.

In this case, Liberation size (also Particle size) is less than the Grain size. It is important to note that the particles are reduced by size reduction in both the cases but it matters whether the fracture takes place along the boundaries or across the grains. Locked particles will be produced when the fracture takes place across the grains.

Figure 3 shows an example of a typical comminution product wherein black refers valuable mineral and white refers gangue mineral. Only 2 are liberated free valuable mineral particles, 8 are free gangue mineral particles, 6 are locked particles containing valuable and gangue minerals. These locked particles are known as middling particles.

Figure 3 Typical comminution product.

Beneficiation is carried out at this size to separate 98% valuable minerals so that 2% of the valuable minerals are lost. To separate 100% valuable minerals, the ore is to be further reduced in size to get 100% liberation which consumes additional power. If the cost of this additional power is more than the cost of 2% valuable minerals, more than 98% liberation is not desirable.

The beneficiation method to be used depends on liberation size of the ore which inturn depends on type of the ore. Ore types can be conveniently classified as follows:

Massive ores In these ores, reasonable amount of crushing makes the valuables liberated. Example: Coal, bedded iron ores.

Intergrown ores In these ores, valuables can be freed only partially by crushing and require certain amount of grinding to complete the liberation.Example: Chrome ore.

Disseminated ores In these ores, valuables are sparely distributed through a waste rock matrix and require fine grinding to liberate the valuables. Example: Gold ore.

To liberate the valuables, the ore particles are to be reduced in size by the application of the forces. When the forces are applied on the ore particle, fracture takes place depending upon the method of application of the forces.

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Sinonine Technology Team