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Chromite

Chromite, a brownish black cubic mineral belonging to the spinel group, is the only ore mineral from which metallic chromium and chromium compounds are obtained. Chromite is iron magnesium chromium oxide: (Fe, Mg)Cr2O4. It is an oxide mineral. It is the only ore of chromium metal. It is a high temperature mineral often found in the lower parts of magma bodies from the fractional crystallization process. Chromium is an important metal and has a wide range of industrial uses. Chromite forms in deep ultra-mafic magmas and is one of the first minerals to crystallize. It is because of this fact that chromite is found in some concentrated ore bodies. While the magma is slowly cooling inside the Earth's crust, chromite crsytals are forming and because of their density, fall to the bottom and are concentrated there. It is a mineral found in ultra basic rocks such as peridotite. It is also found in serpentines and other metamorphic rocks derived from the alteration of ultra basic rocks. Although its primary origin is ultra-mafic rocks such as peridotites, chromite is also found in metamorphic rocks such as serpentites. Chromite, as is indicated by its early crystallization is resistant to the altering affects of high temperatures and pressures. Thus it is capable of going through the metamorphic processes unscathed, while other minerals around it are being altered to serpentine, biotite and garnets. This characteristic also explains chromites use as a refractory component in the bricks and linings of blast furnaces.

PHYSICAL PROPERTIES:

i. COLOR: brownish black to a deep dark black.

ii. STREAK: Dark Brown.

iii. LUSTER: Metallic to Greasy.

iv. TRANSPERANCY: Crystals are opaque.

v. CLEAVAGE: None

vi. HARDNESS: 5.5

vii. SPECIFIC GRAVITY: 4.1 – 5.1

viii. SPECIFIC GRAVITY: 4.1 – 5.1

ix. CRYSTAL SYSTEM: Isometric

x. CHEMICAL CLASSIFICATION: Oxide

x. CHEMICALCOMPOSITION: Iron Chromium Oxide (Fe Cr2 O4)

CHEMICALPROPERTIES:

Theoretically chromite is FeO Cr2O3 containing 68% Cr2O3 and 32% FeO. But chromite conforming to the above composition is rarely found in nature. Iron is replaced by magnesium and similarly chromium by ferric iron and aluminum. It is this replacement that improves Cr: Fe ratio in chromite. Chromite is consumed in metallurgical, refractory and chemical industries. By far, the metallurgical industries are the main consumers of chromite followed by refractories and chemicals. It is used in the manufacture of high carbon and low carbon ferro-chrome as also charge-chrome.

i. METALLURGICAL USE:

1)

Cr2O3

46% min

2)

SiO2

2-6 %

3)

Al2O3

3-10%

4)

FeO

20% (max)

5)

CaO

1% (max)

6)

S

0.6% (max)



ii. REFRACTORY USE:

 

Grade 1

Grade 2

Grade 3

Cr2O3

52 - 54%

46 – 48%

40 – 42%

SiO2

5% (max)

6 – 9% (max)

9 – 12% (max)

CaO

1% (max)

1% (max)

1% (max)

FeO

20% (max)

20% (max)

20% (max)

Loss on Ignition

1.5%

1.5%

1.5%

Al2O3

14%

14%

14%

MgO

15%

15%

15%



iii. USED IN FOUNDARY:

i)

Cr2O3

44% (min)

ii)

FeO

20% (max)

iii)

SiO2

7% (max)

iv)

CaO

3% (max)



USES:

• used to harden steel, to manufacture stainless steel, and to form alloys.

• used in plating to produce a hard, beautiful surface and to prevent corrosion.

• used to give glass an emerald green color. It is responsible for the green color of emeralds and the red color of rubies

• wide use as a catalyst

• Di-chromates such as K2Cr2O7 are oxidizing agents and are used in quantitative analysis and also in tanning leather

• compounds are used in the textile industry as mordants.

• compounds are used in the textile industry as mordants.

• used by the aircraft and other industries for anodizing aluminum

• the refractory industry uses chromite for forming bricks and shapes, as it has a high melting point, moderate thermal expansion, and stable crystalline structure

• used in tanning leather.