What is classification?
Classification is a method of separating mixtures of minerals into two or more products on the basis of the velocity with which the grains fall through a fluid medium. In mineral processing, the fluid medium is usually water, and wet classification is generally applied to mineral particles which are considered too fine to be sorted efficiently by screening.
How classification works?
When a solid particle falls freely in a vacuum, it is subject to constant acceleration and its velocity increases indefinitely, being independent of size and density. Thus a lump of lead and a feather fall at exactly the same rate. In a viscous medium, such as air or water, there is resistance to this movement and the value increases with velocity. When equilibrium is attained between the gravitational and fluid resistances forces, the body reaches its terminal velocity and thereafter falls at a uniform rate.
The nature of the resistance depends on the velocity of the descent. At low velocities motion is smooth because the layer of fluid in contact with the body moves with it, while the fluid a short distance away is motionless. Between these two positions is a zone of intense shear in the fluid all around the descending particle. Effectively all resistance to motion is due to the shear forces or viscosity of the fluid and is hence called viscous resistance. At high velocities the main resistance is due to the displacement of fluid by the body, and viscous resistance is relatively small; this is known as turbulent resistance. Whether viscous or turbulent resistance predominates, the acceleration of particles in a fluid rapidly decreases and the terminal velocity is quickly reached.
Classifiers consist essentially of a sorting column in which a fluid is rising at a uniform rate. Particles introduced into the sorting column either sink or rise according to whether their terminal velocities are greater or lesser than the upward velocity of the fluid. The sorting column therefore separates the feed into two products an overflow consisting of particles with terminal velocities lesser than the velocity of the fluid and an underflow or spigot product of particles with terminal velocities greater than the rising velocity.
What are the types of classifiers?
There are many different types of classifiers which can be broadly grouped into two classes depending on the direction of flow of the carrying current.
Horizontal current classifiers such as mechanical classifiers are essentially of the free-settling type and accentuate the sizing function.
Vertical current or hydraulic classifiers are usually hindered-settling types and so increase the effect of density on the separation.
What is free settling?
Free settling refers to the sinking of particles in a volume of fluid which is large with respect to the total volume of particles, hence particle crowding is negligible. For well-dispersed ore pulps, free settling predominates when the percentage of solids by weight is less than about 15%.
The free-settling ratio is larger for coarse particles than for fine particles. This means that the density difference between the particles has a more pronounced effect on classification at coarser size ranges. This is important where gravity concentration is being utilised.
What is hindered settling?
As the proportion of solids in the pulp increases, the effect of particle crowding becomes more apparent and the falling rate of the particles begins to decrease. The system begins to behave as a heavy liquid whose density is that of the pulp rather than that of the carrier liquid. This condition is called hindered-settling. Because of the high density and viscosity of the slurry through which a particle must fall in a separation by hindered settling, the resistance to fall is mainly due to the turbulence created. Hindered-settling reduces the effect of size, while increasing the effect of density on classification.
What are Hydraulic classifiers and how they work?
These are vertical current classifiers and are usually hindered settling type. These classifiers use water additional to that of the feed pulp. Water is introduced so that its direction of flow opposes that of the settling particles. They normally consist of a series of sorting columns through each of which a vertical current of water is rising and particles are settling out. The rising currents are graded from a relatively high velocity in the first sorting column, to a relatively low velocity in the last, so that a series of spigot products can be obtained, with the coarser, denser particles in the first spigot and the fines in the latter spigots. Very fine slimes overflow the final sorting column of the classifier. The size of each successive vessel is increased, partly because the amount of liquid to be handled includes all the water used for classifying in the previous vessels and partly because it is desired to reduce, in stages, the surface velocity of the fluid flowing from one vessel to the next. The greatest use for hydraulic classifiers in the mineral industry is for sorting the feed to certain gravity concentration processes so that the size effect can be suppressed and the density effect is enhanced.
What are horizontal current classifiers and how they work?
These are free settling type classifiers and there are different varieties. Some are discussed below: –
Settling cones – These are the simplest form of classifier, in which there is little attempt to do more than separate the solids from the liquid, i.e. they are sometimes used as dewatering units in small-scale operations. They are often used in the aggregate industry to de-slime coarse sands products.
Mechanical classifiers – Several forms of classifier exist in which the material of lower settling velocity is carried away in a liquid overflow, and the material of higher settling velocity is deposited on the bottom of the equipment and is dragged upwards against the flow of liquid by some mechanical means. The pulp feed is introduced into the inclined trough and forms a settling pool in which particles of high falling velocity quickly fall to the bottom of the trough. The settled sands are conveyed up the inclined trough by some mechanical means.
The rake classifier – It is a mechanical classifier utilising rakes. The rakes are actuated by an eccentric motion to dip into the settled material and move it up the incline for a short distance. The rakes are then withdrawn, and return to the starting-point, where the cycle is repeated; the settled material is thus slowly moved up the incline to the discharge. In the duplex type one set of rakes is moving up, while the other set returns.
Spiral classifiers – This classifier uses a continuously revolving spiral to move the sands up the slope. They can be operated at steeper slopes than the rake classifier, in which the sands tend to slip back when the rakes are removed. Steeper slopes aid the drainage of sands, giving a cleaner, drier product. Agitation in the pool is less than in the rake classifier which is important in separations of very fine material.