CLASSIFICATION | Mineral Processing

Classification


It is the process by which the particle of various size, shape and specific gravity separated in to uniform groups with the help of a fluid medium in which the particles are allowed to settle faster than the finer and lighter grains.

● The coarse and heavier grains settle faster than the finer and lighter grains.

● The condition for classification depends on the subsequent use of the classified product. In such a case classification is not strictly a sizing operation but a concentrating operation.

Factor affecting classification : Shape, Size, Specific gravity, Air bubbles and specific gravity of fluids.
● Classification is only suitable for finer grain sizes.
● It separate particle on the basic of the specific gravity.

Types of classifier :

Sorting classifier :

The settling conditions are hindered in sorting classifier. The Separation is achieved by sorting that is sizing modified by specific gravity and shape. It is usually applied to coarse product

● The usual type of sorting classifiers are
1. Simple lanunder classifier or Evan's classifier
2. Richard's Hinder settling classifier
3. Richard's pulsator classifier
4. Hydrotator classifier

Sizing Classifier :

Sizing classifier utilise free settling condition to effect sizing as much as possible unaffected by specific gravity and shape. This classifier do not required any additional water besides that is present in the suspension which is to be classified. Sizing classifier maybe subdivided in to settling cone having no moving parts and mechanical classifier having moving parts

Performance of the classifier 

Capacity : The capacity of a classifier is directly proportional to 

1. Cross section (A) of the sorting columns
2. The ring velocity of the fluid water or air in the sorting columns.
3. The solid contact in the classifier feed
4. Specific gravity of the solid (P)

Settling Pacticles

Settling Particles are two type

1. Free Settling 

2. Hindered Settling

Free Settling :

Free settling takes place when individual particle settle freely that is unhindered by other particle and others external forces. The rate of falling for settling under free settling condition depends on the following :

1. Specific gravity(p) of the particle between two particles of same size having different specific gravitys. The having higher specific gravity will settle faster

2. Size : between two particles of same specific gravity, the particle and larger size will settle faster
3. Shape : The settling speeds different very much on the shape, spherical particles settle faster than the long and narrow particles while the narrow particles settle faster than flat particles

4. Specific gravity : of fluids for the Fluids having different specific gravity particle settled in the lighter fluid

5. Coefficient of viscosity of the fluids : The settling depends on the viscosity of the fluid if the flow is laminar under turbulent condition the fall is dependent of (mio sign)

Hindered settling : 

● It takes place when particle of different size, shape and specific gravity is our crowd in single starting mass. When many particles are present, this is a mutual interference in the movement of the particle, due to this velocity of the particle is considerable less than that is computed under free settling condition. 

● Uder this condition the specific gravity of the fluid should be replaced by the specific gravity of the thick suspension fluid mineral pulp. The terminal velocity expresion for hindered settling is given Newton’s law of settling but with a single change that is Pf is replaced by (P suspension).

Moment of Solids in Fluids :

● This is deal with a particular important in case of gravity concentration, classification, heavy media separation, jigging and tabling.

● When an object immersed in a fluid the upward force acting on the object is equal to the weight of the fluid displaced. This is true in all case when both object and fluid are in static condition

● The particles is acted upon by the two opposite nature forces. The downward force acting upon it has a fixed value while the upwards force due to particles Movement in the funtion of its velocity.

● So the critical point is reached both the force balanced at that point the particle attends a maximum velocity which is known as critical or terminal velocity of the particle in fluid.

● Once this occurs particle moves down to the bottom of the column

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