Gravity separation and Ore Beneficiation Oct12

Gravity separation and Ore Beneficiation...

Gravity separation and Ore Beneficiation Gravity separation is the oldest known ore beneficiation technique and is practiced extensively in ‘Nature’. Earliest recorded human use of gravity separation was recovery of gold by panning from the Upper Nile by ancient Egyptians, dating back to 1900 BCE. Gravity separation is a physical process which consists of the separation of different mineral types in the ore from one another based on differences in their specific gravities using the force of gravity, which can be influenced by one or more of other forces such as centrifugal force, resistance to motion by a fluid (e.g. air, water) etc. Hence, besides gravity, other factors, such as size, shape etc., also have an influence on the relative motion and thus in the separation. The effect of centrifugal action on the gravity force is given at Fig 1. It can be seen that as the gravity force increases, settling velocity of smaller particles becomes higher. Fig 1 Effect of centrifugal force on gravity force Separation of the ore particle by gravity is dependent on two factors namely (i) settling rate of the particles, and (ii) difference in specific gravity when compared against the medium in which they are being separated, this gives differential settling rate and has been termed the ‘concentration criteria’.Settling rate of a particle is dictated by ‘Stoke law’ and is equal to kd2g(Ds-Df), where k is a constant, d is particle diameter, g is force of gravity, and Ds is the specific gravity of solid and Df is the specific gravity of the fluid medium. The ‘concentration criteria’ (CC) gives an idea of the amenability of separation of two ore particles and can be expressed by (Dh-Df)/(Dg-Df) where Dh is the specific  gravity of heavier component of the ore,...

Classification of Materials and Types of Classifiers...

Classification of Materials and Types of Classifiers Size control of particles finer than 1 mm, are out of the practical range of conventional screens. Separation of such particles is carried out by classification. Classification implies the sorting of particulate material into different size ranges. It is a method of separation of fines from coarse particles and also lighter particles from heavier particles. The two product streams resulting from any classifiers are (i) a partially drained fraction containing the coarse particles, and (ii) a fine fraction of particles. Usually the principle of the classification is based upon the various densities, specific gravity, terminal falling velocities of particles in liquid and in air. 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. Classification is performed on the basis of the velocity with which the material particles fall through a fluid medium generally water or air. In view of the fact, that the velocity of particles in a fluid medium is dependent not only on the size, but also on the specific gravity and shape of the particles. In classifiers, use is made of the different rates of movement of particles of different sizes and densities suspended in a fluid and differentially affected by imposed forces such as gravity and centrifugal fields, by making suitable arrangements to collect the different fractions as they move to different regions. Based on their separation principles, classifiers are classified into two major types. They are (i) wet classifiers, and (ii) dry classifiers. Wet classification with hydro-cyclones using separation by centrifugal force typically covers the size range of 10 micrometers to 100 micrometers while wet classification with spiral classifiers using separation...