(iii) Muffle furnace : This furnace is used when high temperature is required and the fuel and its products of combustion are not to be desired to come into contact with the material to be heated. The muffle is a chamber made of refractory material. The muffle is surrounded by hot flames and hot gases all around. In an electric muffle furnace, the closed chamber is surrounded by heating electric coils. Such a furnace is used for the extraction of zinc, for annealing and gold and silver assaying.

(iv) Electric furnaces : In these furnaces, electrical energy is converted into heat energy. Such furnaces are largely used where cheap power is available and very high temperatures are required and also for carrying electrolytic reduction. The electric furnaces are generally of three types :
(a) Induction furnace (b) Arc furnace and (c) Resistance furnace.

(a) Induction furnace : In this furnace the charge lying on the furnace bed or in a crucible constitute the secondary coil of an induction unit and the induced currents produced by making and breaking the primary circuit, heat up the material.

(b) Arc furnaces : Heat is generated by arcs and a temperature over 3000°C may be obtained. Carbon electrodes are used to carry the current and an arc is struck between them and the charge. Arc furnaces are of two types :

(i) Direct-heat (ii) Indirect-heat.

In the direct heat arc furnaces, arc is used to heat up a gas in which the arc is burning and in indirect heat arc furnace, the arc burns above the charge i.e., the arc radiates heat towards the charge.

(c) Resistance furnaces : Heat is generated by the resistance in the electric circuit. In some cases the material forming the charge may act as the resistance and in other cases, the body of furnace is made up of resistance material and this material cause heating. In some cases, rods of poorly conducting material are embedded into the charge which become intensely hot on passage of the current.

REFRACTORY MATERIALS

The materials which can withstand very high temperatures without melting or becoming soft are known as refractory materials. These are not affected by slags formed during the extraction of metals. These are used in the form of bricks for the internal linings of furnaces. Refractory materials used are of three types:

Silica (92% SiO₂, 2.7% Al₂O₃) and quartz can tolerate temperatures upto about 1750°C, bauxite upto 1800°C, alumina upto 2000°C and magnesite, chromite, etc., upto 2200°C. Some carbides such as silicon carbide is used as refractory for special purposes.

SINTERING

The conversion of small pieces of substance into large one. The substance is partially fused so that smaller pieces join together to form larger one.

COLLECTORS

These compounds adsorb themselves on polar groups to grains of ores and thus derive them on the surface to pass on into the froth eg. ethyl xanthate

ACTIVATORS AND DEPRESSANTS

These compounds activate or depress the floating property of one of the components of the ore and thus help in the sepration of different minerals present in the same ore. CuSO₄ is an example of activators, while sod and pot cyanides are the examples of depressants eg. in galena (PbS) is usually associated with sphalerite (ZnS) and FeS2₂. Floatation is carried out in presence of potassium ethyl xanthate (collector) and sod. Cyanide and alkali (depressants) the later compounds depress the floatation property of ZnS and FeS2₂ particals and hence only PbS particals go into the froth when air is blown in. After the removal of galena the procen is repeated by adding CuSO₄ (activators) which activates the floating character of ZnS particals and thus time ZnS comes with the froth. The acidification of remaining slurry leads to the floatation of FeS2₂.