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Gravity separation is an industrial method of separating two components, either a suspension, or dry granular mixture where separating the components with gravity is sufficiently practical: i.e. the components of the mixture have different specific weight. All of the gravitational methods are common in the sense that they all use gravity as the dominant force. Gravity separation is used in a wide variety of industries, and can be most simply differentiated by the characteristics of the mixture to be separated - principally that of 'wet' i.e. - a suspension versus 'dry' -a mixture of granular product. Often other methods are applied to make the separation faster and more efficient, such as flocculation, coagulation and suction. The most notable advantages of the gravitational methods are their cost effectiveness and in some cases excellent reduction. Gravity separation is an attractive unit operation as it generally has low capital and operating costs, uses few of any chemicals that might cause environmental concerns and the recent development of new equipment enhances the range of separations possible.
Gravity separating tables are used to separate dry granular products by specific weight and are most common in the agricultural and recycling industries. They work on the principle of fluidization and have an inclined reciprocating grading deck.
From an inlet with an adjustable feed rate, the material is fed onto the inclined reciprocating grading deck which normally has a special woven wire mesh surface. High volume, low pressure air is passed through the mesh from a fan, the amount of air available is adjusted to suit the product being processed. This adjustment is to ensure that the lighter product is lifted to the top of the material and the heavier product remains on the surface of the grading deck, resulting in stratification of product.
Due to the reciprocating motions of the inclined deck the heavy product will walk uphill and climb to the highest side, whilst the lighter product will drift down towards the low side. The angle of inclinaeoeod of the deck and speed of reciprocation interact with air pressure to control this movement.
Examples of application
Agriculture: Gravity Separation tables are used for the removal of impurities, admixture, insect damage and immature kernels from the following examples: Wheat, Barley, Oilseed Rape, Peas, Beans, Cocoa Beans, Linseed
Recycling: Gravity Separators are used to remove viable or valuable components from the recycling mixture i.e.: metal from plastic, rubber from plastic, different grades of plastic
Heavy liquids such as hhh tetrabromoethane can be used to separate ores from supporting rocks by preferential flotation. The rocks are crushed, and while sand, limestone, dolomite, and other types of rock material will float on TBE, ores such as sphalerite, galena and pyrite will sink.
Clarification is a name for the method of separating fluid from solid particles. Often clarification is used along with flocculation to make the solid particles sink faster to the bottom of the clarification pool while fluid is obtained from the surface which is free of solid particles.
Thickening is the same as clarification except reverse. Solids that sink to the bottom are obtained and fluid is rejected from the surface.
The difference of these methods could be demonstrated with the methods used in waste water processing: in the clarification phase, sludge sinks to the bottom of the pool and clear water flows over the clear water grooves and continues its journey. The obtained sludge is then pumped into the thickeners, where sludge thickens farther and is then obtained to be pumped into digestion to be prepared into fertilizer.
When clearing gases, an often used and mostly working method for clearing large particles is to blow it into a large chamber where the gas's velocity reduces and the solid particles start sinking to the bottom. This method is used mostly because of its cheap cost.
- Prosessitekniikan Yksikköprosessit (Finnish (Unit processes of process technics)) by Juhani Pihkala
- (Book's website http://www.edu.fi/oppimateriaalit/prosessitekniikka/)