Chromite

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. In combination with magnetite in varying proportion, it is used to manufacture refractory bricks. In chemical industries, it is used for the manufacture of sodium chromate and dichromate. Other chromium chemicals are derived from these compounds, which find a number of uses in the industry.

1. Metallurgical Use:

In the manufacturer of high carbon and low carbon ferrochormite, by and large, high-grade chromite is used. However, lower grade chrome is required for the manufacture of charge chrome. There is no BIS specification for metallurgical grade chromite. The user industries prefer the following grades:

(a)

Ferro Chrome:

i

Cr2O3  

46% (min.)

ii

SiO2     

2 - 6%

iii

A12O

3 - 10%

iv

Cr : Fe

2.6 : 1 (min.)

v

S

0.5% (max.)

vi

P

0.2% (max.)

vii

Physical Characteristics

Lump 25 mm to 150 mm

viii

Fines up to 10 mm are also used.

(b)

Charge Chrome:

i

Cr2O3  

40% (min.)

Ii

Cr : Fe

1 : 6 (min.)

2. Refractory Use:

There is no BIS specification for refractory grade chromite. The sub-committee on refractory raw material of the DGTD in their report dated August 1985, has examined the specification of chromite for refractory industries, and has specified the following 4 grades:

Special grade           

Grade I

Grade II

Grade III

Cr2O3   52 54%

52 54%

46 48 %

40 42%

SiO2      2% (max.)

5% (max.)

6 - 9 %

9 12%

CaO     1% (max.)

1% (max.)

1% (max.)

1% (max.)

FeO     20% (max.)

20% (max.)

20% (max.)

20% (max.)

The ore should be lumpy in nature and on crushing through a hammer type of impact mill, not more than 30% of less than one-millimeter size should be generated.

On Dry Basis:

 i

Cr2O3  

44% (min.)

 ii

FeO

20% (max.)

 iii

A12O

3% (max.)

 Iv

SiO2     

7% (max.)

 V

CaO

3% (max.)

 Vi

MgO

14% (max.)

3. Chromite as used in foundry:

The BIS (IS: 6788 1973) has prescribed the following specification on dry mass basis. The loss on ignition shall be 1% (max.).

Constituent 

Requirement

I

Cr2O3

44% (min.)

Ii

FeO

20% (max.)

Iii

SiO2

7% (max.)

Iv

CaO

3% (max.)

Future Trends:

It is seen that the specifications for the metallurgical use are becoming less rigid. For the manufacturer a ferro chrome in the requirement Cr2O3 has reduced from 48% to 46% (min.) and Cr: Fe ration from 2.8:1 to 2. 6:1.The specification of ores for charge chrome industries is far more rigid. This has resulted in broadening the base of supplies of metallurgical grade ore.

Similar relaxation in the specification is also noticed for the refractory industry. R & D efforts are a foot to utilize chromite fines in the manufacture of refractory. The Mineral Development Board has taken initiative to initiate research to utilize fines for the manufacture of mag-chrome sinters. These trends indicate that chromite fines may be used in the manufacture of refractory and in that event the shortages experienced in the supply of chromite lumps for refractory industry will not be there.

The situation with regard to end-use classification of chromite reserves appears to be satisfactory. The Geological survey of India, the directorate of Geology & Mining. Orissa, The Orissa Mining Corporation and the Mineral Exploration Corporation Ltd., Have conducted exploration in the most important chromite hearing areas of Orissa and have categorized reserve as given below:

  • Metallurgical Cr2O3 48% (min.); Cr: Fe = 2.6: 1 (min.)
  • Charge chrome Cr2O3 42% (min.); Cr: Fe = 1.6: 1 (min.)
  • Inferior grade Cr2O3 less than 42%

The reserves of refractory grade chromite have not been estimated separately by the exploration agency. The estimates contained in the inventory are based on the dispatches from the deposit to the refractory industry. If a substantial part of the production of a deposit is dispatched to a refractory industry then the entire reserves of that deposit has been classification as of refractory grade.

After examining in details, the changing specification of chromite for various industries and the future trends the following end-use grade for the purpose of reserve estimation.

1. Metallurgical:

(a) Ferro Chrome:

Cr2O3
Cr Fe


46% (min.)
2.6 : 1 (min.)

(b) Charge Chrome:

Cr2O3
Cr Fe


40% (min.)
1.6 : 1 (min.)

Physical lumpy

Cr2O3  

+ 40%

Cr2O3 + A12O3

60% (min.)

CaO

1% (max.)

FeO

20% (max.)

SiO2

Less than 10%

Physical lumpy

3. Chemical:

Cr2O3  

42% (min.)

CaO

3% (max.)

FeO

20% (max.)

MgO

14% (max.)

4. Beneficiable Ore:

Cr2O3  

15% (min.)

The BSI (IS: 4737 1982) has prescribed the specification for chromite for chemical industries:

S. No.

Characteristics (on dry basis)

Requirement

I

Chromic oxide (as Cr2O3), percent by mass     

44.00

II

Total iron (as    FeO), percent by mass

20.00

III

Alumina (as A12O3), percent by mass

14.00

IV

Silica (as SiO2), percent by mass

7.00

V

Lime (as CaO), percent by mass

3.00

VI

Magnesia (as MgO), percent by mass

14.00

The IS: 10819 1984 has prescribed the specification for chromite for Refractory industry.

S. No.

Characteristics
(on dry basis)

Grade-I
Percent by mass

Grade-II
Percent by mass

I

Loss on ignition

1.5 (max.)

1.5 (max.)

II

Cr2O3

50 (min.)

40 (min.)

III

Total iron (as FeO)

18 (max.)

18 (max.)

IV

A12O3 

14 (max.)

14 (max.)

V

SiO2

4 (max.)

4 (max.)

VI

CaO

0.8 (max.)

0.8 (max.)

VII

MgO

15 (max.)

15 (max.)

Physical requirement of both the refractory grads chromite shall be bard, massive, fine grained, serpentine free and lumpy ore and the size range shall be of 150 mm + 50 mm.

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