Chemical Analysis and Heat Treatment of Chromite

INTRODUCTION

Chromite is an oxide of chromium and iron. The chemical composition of chromite is FeO.Cr2O3 or FeCr2O4. Its molecular weight is 223.84 gm. Chromite in its purest form has 46.46% Cr, 24.95% Fe, 28.59% O or it may be referred as 67.90% Cr2O3, 32.10% FeO with Cr : Fe ratio of about 1.8 : 1.Chromite is the only economic source of chromium and has a wide range of uses in metallurgical, chemical, refractory industries. It is the chief element for the production of stainless steel, refractory, ceramics, chemicals, electrodes, foundry etc. The properties of chromium that makes it most versatile and indispensable are its resistance to corrosion, oxidation, wear and enhancement of hardenability. The important countries where considerable reserves of chromite ore are known and mined for production of ferro-chrome and charge-chrome etc. are Brazil, Republic of South Africa, Kazakhstan, India, Russia and Finland. World chromite reserves are estimated to total around 7.6 billion tons, with the majority of this located in south Africa(77%).

STRUCTURE OF CHROMITE

To facilitate the production of chromium, the first requirement is to understand the structure of the mineral chromite. Chromite (FeCr2O4) belongs to the spinel group of minerals, having a general formula of AB2O4. A is a divalent element, generally Fe or Mg and B is a trivalent element such as Cr or Al.

[pic 1]^[1]

Spinel has a face-centred cubic crystal system, of which oxygen occupies the lattice sites and the metals occupy the vacant sites, i.e. voids.

Total no of lattice sites= 1/8 * 8 + ½ * 6= 4

Total no of tetrahedral holes= 8

Total no of octahedral holes= 1 +   ¼ * 12 = 4

O2- ions are located at the lattice sites whereas the A2+ ions are located at the tetrahedral holes and B3+ ions at the octahedral holes. Only 1/8 of the total tetrahedral holes and 1/2 of the total octahedral holes are occupied in the crystal.

Hence, total no of O2- ions per unit cell = 4

totalno of Fe2+ ions per unit cell= 1/8 * 8= 1

Total no of Cr3+ ions per unit cell= ½ * 4= 2

Therefore the stoichiometry of the crystal is maintained.

The lattice parameter of chromite spinel is 8.344 Å. The radius of oxygen ion is 1.26 Å, fe2+ ion is 0.92/0.75 Å and that of cr3+ ion is 0.755 Å.

MINEROLOGY OF ORE

Mineralogy of the ore refers to the identification of different phases (compounds) associated with the principal mineral, its composition and characterization which is essential to carry out necessary operations on the ore for further applications. The different phases associated with chromite are hematite (ferric oxide), goethite, iron silicates, calcium aluminate, calcium silicates, calcium oxide, silica, quartz, alumina, gibbsite, kaolinite, etc. The chromium to iron ratio is important in the processing of ferrochrome. All the gangue minerals are to be removed by suitable operations, hence, the mineralogical study of the ores is needed.