Concentration Of Common Ores

Removal of the unwanted materials example:- sand, clay etc. from the ore is known as concentration, dressing or benefaction. It involves several steps and selection of these steps depend upon differences in physical properties of the compound of the metal present and that of gangue. The type of metal, the available facilities, and the environmental factors are also taken in consideration. Some of the important methods are described below.

Hydraulic Washing 

This is based on the differences in gravities of the ore and the gangue particles. It is therefore a type of gravity separation. In one such process an upward stream of running water is used to wash the powdered ore. The lighter gangue particles are washed away and the heavier ores are left behind.

Magnetic Separation

This is based on differences in magnetic properties of the ore components. If either the ore or the gangue is capable of being attracted by a magnetic field, then such separations are carried out (e.g. in case of iron ores). The ground ore is carried on a conveyer belt which passes over a magnetic roller.

Froth Floatation Method

This method has been in use for removing gangue from sulphide ores. In this process, a suspension of powdered ore is made with water. To it, collectors and froth stabilisers are added. Collectors (e.g., pine oils, acids, xanthates, etc.)enhance non-wettability of the mineral particles and froth stabilisers (e.g.,cresols, aniline) stabilise the froth.

The mineral particles become wet by oils while the gangue particles by water. A rotating paddle agitates mixture and draws air in it. As a result, froth is formed which carries the mineral particles. The froth is light and is skimmed off. It is then dried for recovery of the ore particles.

Sometimes, it is possible to separate two sulphide ores by adjusting proportion of oil to water or by using depressants . For example, in case of an ore containing ZnS and PbS, the depressants used is NaCN. It selectively prevents ZnS from coming to the froth but allows PbS to come with the froth.

Leaching

Leaching is often used if the ore is soluble in some suitable solvent. the following example illustrate the procedure:

Leaching of alumina from bauxite

The principle ore of aluminium bauxite, usually contains SiO2 iron oxide and titanium oxide(TiO2)as impurities. Concentration is carried out by digesting the powdered ore with a concentrated solution of NaOH at 473 - 523 K and 35 - 36 bar pressure. This way, aluminium oxide is leached out as sodium aluminate leaving the impurities behind.

The aluminate in solution is neutralised by passing CO2 gas and hydrated aluminium oxide is precipitated. At this stage the solution is seeded with freshly prepared samples of hydrated aluminium oxide which induces the precipitation.

The sodium silicate remains in the solution and hydrated alumina is filtered, dried and heated to give back pure aluminium oxide.

PHOTOELECTIC EFFECT

When light of sufficiently small wavelength is incident on a metal surface, electrons ejected from the metal. This phenomenon is known as photoelectric effect. The electrons ejected from the metal surface are called photoelectrons. Let us try to understand photoelectric effect on the basis of  the photon theory of light.

We know that there are large number of free electrons in a metal which wander throughout the body of the metal. However, these electrons are not free to leave the surface of the metal. as they try to come out of the metal attracts them back. A minimum energy, equal to work function (φ), must be given to an electron so as to bring out of the metal.

When light is incident on a metal surface, the photons collide with free electrons. In a particular collision, the photons may give all of it's energy to the free electron. If this energy is more than the work function (φ), it will come out. The electron after receiving the energy, may lose energy to the metal in course of collisions with the atoms of the metal. Only if an electron near the surface gets extra energy and heads towards the outside, it is able to come out. If it is given energy E which greater than φ, and it makes the most economical use of it, it will have the kinetic energy (E - φ) after coming out. If it makes some collisions before coming out, kinetic energy would be less than (E - φ). The actual kinetic energy of such an electron will depend on the total energy lost in collisions. It is possible that the electrons makes several collisions inside the metal and losses so much energy that it fails to come out. So, the kinetic energy of the photoelectron coming out may be may anything between zero and 

(E - φ), where E is the energy supplied to the individual electron. We can, therefore write,

K.E. max = E - φ

Metal	Work function	Metal	Work function
Cesium	1.9 e.v.	Calcium	3.2 e.v.
Potassium	2.2 e.v.	Copper	4.5 e.v.
Sodium	2.3 e.v.	Platinum	5.6 e.v.

Work functions of some photosensitive metals 

Let monochromatic light of wavelength (λ) be incident on the metal surface. In a particle picture photons of energy hc/λ fall on the surface. Suppose, a particular photon collides with a free electron and supplies all its energy to the electron. The electron gets an extra energy E=hc/λ and may come out of the metal. The maximum kinetic energy of this electron is therefore,

K max = hc/λ - φ

The above equation is known as Einstein's photoelectric equation. Einstein, after an average academic career, put forward this theory in 1905 while working as a grade Ⅲ technical officer in a patent office. He was awarded the Nobel prize in physics for 1921 for this work.

 

TWO MAJOR TYPES OF CELLS IN PHYSICS

There are two major types of cell namely:-

1. Galvanic cell
2. Electrolytic cell

GALVANIC CELL

 It's a device used to convert chemical energy into electrical energy. It consist of two electrodes in different compartments joined by a salt bridge. In this type of cells all the reduction and oxidation reactions occur spontaneously, means free energy decreases with operation of cell. Useful work is obtained by the cell. In galvanic cells anode works as negative and cathode as positive. Electrons released by oxidation process at anode go into external circuit and pass to cathode. To setup this cell, a salt bridge is used.Example of galvanic cell :- Daniel cell

Daniel cell

In this cell, Zn rod is dipped in zinc sulphate solution and Cu rod in copper sulphate solution. Both solution are connected through KCl salt bridge. when Zn to Cu occurs. Zinc atoms change to Zn2+ and electron reach at Cu electrode, where Cu2+ changes into Cu metal and this copper deposits on electode. In this redox reactions, the transfer of electrons between oxidizing and reducing agents ocurrs through the wire and thus chemical energy changes into electrical energy.

 

ELECTROLYTIC CELL

It is a device to convert electrical energy into chemical energy. Both electrodes are in the same solution. The reactions occurring in this type of cells are non-spontaneous, that is free energy increases with operation of cell. In this cell work is done on the system. In this type of cell anode is positive and cathode is negative. Electrons enter into cathode electrode from the external source and leave the cell at anode. No salt bridge is used in this cell. Example of this cell :- Dry cell

Dry cell  

It is a primary cell based on Leclanche cell invented by G. Laclanche in 1868. In a primary cell , the electrode reactions cannot be reversed by an external source of electrical energy. In this cell reaction takes place only once taht is the cell is not rechargeable.

It is generally used in torches, transisters, radios, calculators, tape recorders.etc. The potential of dry cell is apporximately 1.5 volts.