ORE BENEFICIATION

•Improving the chemical or physical properties of an ore so that metal can be recovered at a profit. Also known as mineral dressing. •Concentrating the mineral content of an ore by ore-dressing, smelting and pelletizing. Beneficiation usually takes place close to the site of an ore body prior to its transportation to a manufacturing region.•The improvement in the physical or chemical properties of a material by the removal or modification of undesirable components or impurities which it contains.

• Mineral beneficiation is a process by which valuable constituents of an ore are concentrated by means of a physical separation process. As one of the initial steps of extractive metallurgy, the main purpose is to prepare the ore prior to downstream purification processes.

• The most common mineral beneficiation processes include: comminution, size classification, and concentration.

• Comminution – rock size reduction by crushing and grinding

• Size Classification - screening, de-sliming, cycloning Gravity

• Concentration – separation exploiting the differences in specific gravity of minerals via the use of centrifugal concentrators, shaking tables, and spirals

• Froth Flotation – bulk flotation, differential flotation, etc.

• Magnetic separation Optical Sorting – using advanced optical sorting technology

Comminution

• Comminution is particle size reduction of materials. Comminution may be carried out on either dry materials or slurries. Crushing and grinding are the two primary comminution processes. Crushing is normally carried out on "run-of-mine"ore, while grinding (normally carried out after crushing) may be conducted on dry or slurried material.

ball millThe ball mill is a key equipment to grind the crushed materials, and the ball mill is widely used in powder-making production line including cement, silicate, new-type building material, refractory material, fertilizer, ore dressing of ferrous metal and non-ferrous metal, glass ceramics, etc. The ball mill can grind various ores and other materials with dry type and wet type. There are many types of grinding media suitable for use in a ball mill, each material having its own specific properties and advantages. Key properties of grinding media are size, density, hardness, and composition.

A ball mill, a type of grinder, is a cylindrical device used in grinding (or mixing) materials like ores, chemicals, ceramic raw materials and paints. Ball mills rotate around a horizontal axis, partially filled with the material to be ground plus the grinding medium. Different materials are used as media, including flint pebbles and stainless steel balls. An internal cascading effect reduces the material to a fine powder. Industrial ball mills can operate continuously, fed at one end and discharged at the other end. Large to medium-sized ball mills are mechanically rotated on their axis, but small ones normally consist of a cylindrical capped container that sits on two drive shafts (pulleys and belts are used to transmit rotary motion).

Jaw crusher

• A jaw crusher consists of two metal plates that crush material as they close together. As a compression crusher, they generally produce the coarsest material; this is due to the preferential breakage of rocks along inherent lines of weakness. Jaw crushers are mainly used in primary crushing as a means of preparing rock for subsequent processing stages.

Cone crushers and gyratory crushers consist of a cone that crushes material as it rotates within a crushing chamber.

Diagram of impact jaw crusher. Diagram of horizontal shaft impactor

Impact crushers consist of a set of hammers that crush material as they spin within a crushing chamber.Impact crushers tend to be used where shape is a critical requirement and the feed material is not very abrasive.The crushing action of an impact crusher breaks a rock along natural cleavage planes giving rise to good product quality in terms of shape.

Hydraulic Washing (Hydraulic Classifier)

When the ore particles are poured over a vibrating sloped table with grooves and a jet of water is allowed to flow over it, the denser ore particles settle in the grooves while the lighter gangue particles are washed away.

Froth Floatation

• A mixture of water, pine oil, detergent and powdered ore is taken in a tank

• A blast of compressed air is blown through the pipe of a rotating agitator to produce froth

• Sulphide ore particles wetted and coated by pine oil rise up along with the froth (froth being lighter)

• Gangue particles wetted by water sink to the bottom of the tank (water being heavier)

• Froth containing the sulphide ore is transferred to another container, washed, and dried.

Note : Sulphide being more electronegative attracts the covalent oil molecules. The gangue being less electronegative is attracted by the water.

• Principle : This process depends on the preferential wettability of the ore with oil (pine oil) and the gangue particles by water.

• Method : The crushed ore is taken in a large tank containing oil and water and agitated with a current of compressed air. The ore is wetted by the oil and gets separated from the gangue in the form of froth (Fig. .7.5).

• Since the ore is lighter, it comes on the surface with the froth and impurities are left behind.

• Sulphide ores like zinc blende (ZnS) Galena (PbS) are lighter than their impurity. They are concentrated by froth floatation.

Magnetic Separation

• Powdered ore is dropped over a conveyor belt running over two rollers, one of which is magnetic.

• The magnetic ore particles get attracted to the magnetic roller and run along with the conveyor belt for a little longer than the non magnetic gangue.

• Gangue particles drop down first forming a heap.• Then, the magnetic ore particles drop down forming a

separate heap.• Thus, two separate heaps of ore and gangue particles

are formed.• Magnetic ores like pyrolusite (MnO2) and chromite

(FeO.Cr2O3)

Roasting and Calcination

Roasting is a process of heating the concentrated ore to a high temperature in the excess of air. If an ore is a carbonate or a hydrated oxide, it is heated in the absence of air to a temperature that is high but insufficient to melt the ore. The process is known as Calcination. Calcination removes volatile impurities and makes the ore porous.

Roasting Calcination

(i) The ore is heated in the excess of air.

(ii) Generally, sulphide ores are

roasted, so S02 is given off.

2ZnS+302 800°-900°c) ?Zn0+2S02

(iii)Volatile impurities are

removed as oxides (S02, P205,

AS203) and the ore becomes

porous and more reactive.

(i) The ore is heated in the absence of air.

(ii) Carbonate and hydrated ores

are calcined and so, C02 and

water vapours are given off.

ZnC03 -ZnO + C02

A1203.2H20 A1203 + 2H20

(iii) Moisture and organic impurities are removed.