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: ie 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' ie - 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

Flocculation

Flocculation, in the field of chemistry, is a process wherein colloids come out of suspension in the form of floc or flakes by the addition of a clarifying agent. The action differs from precipitation in that, prior to flocculation, colloids are merely suspended in a liquid and not actually...

, coagulation

Coagulation

Coagulation is a complex process by which blood forms clots. It is an important part of hemostasis, the cessation of blood loss from a damaged vessel, wherein a damaged blood vessel wall is covered by a platelet and fibrin-containing clot to stop bleeding and begin repair of the damaged vessel...

 and suction. The most notable advantages of the gravitational methods are their cost effectiveness and in some cases excellent reduction.

Gravity Table

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 inclination 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
ie: metal from plastic, rubber from plastic, different grades of plastic

Preferential flotation

Heavy liquids such as hhh tetrabromoethane

Tetrabromoethane

Tetrabromoethane is a halogenated hydrocarbon, chemical formula C2H2Br4. Although three bromine atoms may bind to one of the carbon atoms creating 1,1,1,2-tetrabromoethane this is not thermodynamically favorable, so in practice tetrabromoethane is equal to 1,1,2,2-tetrabromoethane, where each...

 can be used to separate ores from supporting rocks by preferential flotation. The rocks are crushed, and while sand, limestone

Limestone

Limestone is a sedimentary rock composed largely of the minerals calcite and aragonite, which are different crystal forms of calcium carbonate . Many limestones are composed from skeletal fragments of marine organisms such as coral or foraminifera....

, dolomite

Dolomite

Dolomite is a carbonate mineral composed of calcium magnesium carbonate CaMg2. The term is also used to describe the sedimentary carbonate rock dolostone....

, and other types of rock material will float on TBE, ores such as sphalerite

Sphalerite

Sphalerite is a mineral that is the chief ore of zinc. It consists largely of zinc sulfide in crystalline form but almost always contains variable iron. When iron content is high it is an opaque black variety, marmatite. It is usually found in association with galena, pyrite, and other sulfides...

, galena

Galena

Galena is the natural mineral form of lead sulfide. It is the most important lead ore mineral.Galena is one of the most abundant and widely distributed sulfide minerals. It crystallizes in the cubic crystal system often showing octahedral forms...

 and pyrite

Pyrite

The mineral pyrite, or iron pyrite, is an iron sulfide with the formula FeS2. This mineral's metallic luster and pale-to-normal, brass-yellow hue have earned it the nickname fool's gold because of its resemblance to gold...

will sink.

Clarification/Thickening

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.

Sinking chamber

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.

Sources

• Prosessitekniikan Yksikköprosessit (Finnish (Unit processes of process technics)) by Juhani Pihkala
  • (Book's website http://www.edu.fi/oppimateriaalit/prosessitekniikka/)