Chromite
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Chromite

A mineral deposit is defined as a mineral occurrence of sufficient size and grade that might, under the most favorable circumstances, be considered to have economic potential. Deposits sharing a relatively wide variety and large number of attributes are characterized as a “type,” and a model representing that type can be developed.

The mineral Chromite ((Mg, Fe)Cr2O4), is a member of the spinel group of minerals and is ubiquitous, constituting about 1 percent, within the ultramafic part of ophiolite sequences. When chromite is concentrated, it can occur in two forms, stratiform and podiform chromite deposits. Stratiform chromite is associated with large, mafic to ultramafic layered intrusions, such as the Bushveld in South Africa or the Stillwater in Montana, found in continental crust. This paper is about podiform chromite deposits, which are usually in the shape of pods, ranging from pea-size nodules to large bodies hundreds of meters in extent that may take the form of tabular, cylindrical, or highly irregular bodies. 

 

 

What is Chromite?

Chromium is the 24th element on the periodic table and is what makes steel stainless. It gives steel a high resistance to corrosion and makes it appear shiny. Chromium is found within the ore mineral chromite (FeCr2O4) which is iron-black to brownish-black in color, has a metallic luster, and a hardness value of 5.5 (scale is from 1 to 10). Chromite is also one of the first crystallizing minerals to form from cooling mafic magma (rich in dark, ferromagnesian minerals) or ultramafic magma (containing mainly mafic minerals). The chromite deposits found in the Ring of Fire are stratified and fall into the Chromium/Nickel/Copper/Platinum Group Elements category.

Economic Classification

Chromite ores have been classified into metallurgical, refractory, and chemical ores (table 1) for their end-use applications by the metallurgical, refractory, and chemical industries, respectively. Metallurgical ore, which has a high Cr2O3 content and a high chromium-to-iron ratio, is desirable for making ferrochromium or ferro-silicochromium used in chrome-hardened and corrosion-resistant (stainless) steels. Refractory ore, which has high alumina and low silica content and a low chromium-to-iron ratio, is used in manufacturing refractory bricks. These bricks have excellent mechanical strength and resistance to spalling at elevated temperatures in the furnace linings of steel mills, although changing technology has reduced the demand for refractory-grade chromite. Refractory ore, with high iron content, is also used for foundry molding sands. Chemical ore, in which the Cr2O3 content can be lower than in metallurgical ore, is converted to sodium chromate, sodium dichromate, lead chromate, and other chromate compounds necessary for electroplating, paint, textile, tanning, wood treatment, water treatment, and other chemical applications.

 

Use Type

Cr2O3 (%)

Cr:Fe ratio

Al2O3 (%)

Impurities (maximum)

Comments

Metallurgical grade A

>64

>2.5

low

4–9.5% C, 3.0% Si, 0.06% S, 0.03% P

Lump ore, nonfriable

Metallurgical grade B

56-64

1.8-2.5

low

4–6% C, 8–14% Si, 0.04% S, 0.03% P

Lump ore, nonfriable

Metallurgical grade C

46-55

<1.8

low

6–8% C, 6% Si, 0.04% S, 0.03% P

Lump ore, nonfriable

Refractory

30-40

2.0-2.5

22-34

12% Fe, 5.5% CaO, 1.0% MgO

>57% Cr2O3+Al2O3, hard, dense, nonfriable lump ore

Chemical

40-46

1.5-2.0

low

5% SiO2, Low Mg

Fine-grained, friable ore preferred

 Table 1. Different types of Chromite Ore

Chromite Occurrence

Chromite in economic concentrations confined to ultramafic rocks, however, it is concentrated only in dunite layers within layered basic intrusions and dunitic chromite pods within the mantle section of ophiolite bodies. Otherwise chromite occurs as disseminate grains in concentrations up to about 1 per cent. It is commonly concentrated as crystal aggregates in stringers and lenses or in massive pods.

The major deposits are of two types: alpine-type (or the basal portion of ophiolite. complexes) and stratiform types (layered basic intrusions). Examples of these include the Tiebaghi ultramafic massif in the New Caledonia ophiolite complex and the Merensky Reef and Steelpoort seam in the Bushveld Complex of South Africa respectively. Typical Alpine-type, podiform deposits consist of massive pods or lenses containing several thousand tonnes of chromite; stratiform complexes may contain cumulate strata of several million tonnes. In alpine-type complexes, economic deposits usually consist of a cluster of chromitite pods in which individual pods vary in size from a few tonnes to several million tonnes although pods over one million tonnes are rare. For example, in the Acoje district of the Philippines, there are 15 pods that contained an aggregate total of 1.7 million tonnes (1978 reserves) of chromite. The main orebody in the Coto district measured 550 by 290 by 55 meters and contained 6.3 million tonnes of chromite (Leblanc and Violette, 1983). Chromite also occurs in the dunitic parts of Alaskan type mafic-ultramafic intrusions. However, significant concentrations of chromite in these intrusions are rare.

 

The actual process of concentration of chromite in podiform bodies is not clearly understood. However, a variety of processes, generally involving crystal fractionation, crystal accumulation in magma chambers or conduits in residual mantle harzburgite tectonite, have been proposed. In the case of stratiform bodies, crystal fractionation and settling in a basaltic melt account for observed features such as graded layers, load casts, and slumping (Cameron, 1%3, 1980). These features arc more prevalent in the relatively undisturbed stratiform bodies than the altered and tectonically deformed alpine-type complexes. In relation to the number of ultramafic complexes known worldwide, there are few economic chromite deposits. It should be noted that metallurgical grade chromite is restricted to podiform chromite occurrences and that podiform occurrences may have variable grades.

Chromite Usages

Ninety percent of the chromite mined across the world is converted into ferrochrome for use in stainless steel production. Ferrochrome is an alloy composed of iron and chromium, created in an electric arc furnace when processing chromite ore. The importance of ferrochrome lies in the fact that there is no substitute for it. Mined chromite is also used to manufacture refractory bricks, furnace linings, and foundry sand. It is used in these objects because it has a very high melting point and can withstand very high temperatures.

 

Chromite is used in a multitude of applications, both in the ferrous and nonferrous metals industries as well as the industrial minerals sector. Its major application is in

the manufacture of specialty metals, including stainless steel, chrome-moly steel and nichel-chrome. It is used as an alloy to provide increased hardness, wearability and resistance to chemical attack. It is also used for hardfacing metals and to provide a tine, smooth finish to precision parts such as hydraulic piston rods and automotive engine valves.

Chromite is used extensively as a refractory material. It is used in the manufacture of refractory brick, basic brick, foundry ‘sand’, castables, mortar, and ramming and gunning compounds. Chromite has a high melting temperature range of 1545 to 173O⁰C, depending on its composition, making it a desirable component in refractory brick in ferrous and nonferrous furnaces. Chromite does not react in high-temperature basic environments where silicates would. Thus, chromite and chromite-compound brick and tile are used in ferrous industry furnaces, glass furnaces and as a lining in high pH, hot spent-liquor tanks in the pulp and paper industry.

Chromite is also used widely in the chemical industry. Most commonly it is processed into sodium dichromate which is then used to produce other chromium compounds used as antifouling agents, pigments, mordants and dyes, and as a leather tanning agent. Chrome compounds are also used for chrome electroplating, etching and anodizing, and in oxidants and catalysts.

Economic forecasts predict a small but steady increase in the demand for chromite and ferrochrome. Industrialized countries have a steady demand for both products but, in most cases, little increase or even a decline in consumption is projected. However, developing countries that are beginning to industrialize are potential new markets for chromite and ferrochrome.

 

Examples include China and India. With regard to increased demand for chromite ore, countries where electric power is cheap may be future producers of ferrochrome; examples are Brazil, Canada and India.

World Chromite Supply

The global resources of chromite are recognized to be adequate for the needs of the world for many more years, with world reserves being estimated at 7,600 million tonnes. South Africa, the dominant chromite ore supplier, is expected to hold its 35% share in the supply, with some marginal increases expected in Turkey, Russia, and Zimbabwe. There is some worry about supply shortages of ferrochrome. Figure 1 shows a pie chart illustrating the location of world chromite reserves (South Africa having by far the largest), and Figure 2 shows a pie chart illustrating world chromite production for 2011 (South Africa again dominating).

Iran Chromite producer

 Figure 1. World chromite reserves graph

Chromite exporter

  Figure 2. World chromite production graph

 

Country

Production capacity

(1000s of tonnes)

Percentage of Total

South Africa

15,340

44.9%

Kazakhstan

6,300

18.5%

India

3,250

9.5%

Madagascar

2,280

6.7%

Turkey

1,850

5.4%

Finland

1,300

3.8%

Russia

1,300

3.8%

Brazil

900

2.6%

Philippines

440

1.3%

Iran

400

1.2%

Albania

240

0.7%

China

230

0.7%

Zimbabwe

170

0.5%

UAE

100

0.3%

Oman

30

0.1%

Total

34,130

100%

Table 2. World chromite production capacity of major producers

Table 1 above further demonstrates the world chromite production capacity of the top producing nations. China currently consumes the largest amount of ferrochrome, accounting for some 50% of the total supply. The price of ferrochrome has risen as a result of increased world demand, mainly stemming from China and India. Due to stainless steel production increasing about 5.7% per year, continuous growth will most likely cause increased ferrochrome consumption, potentially reaching 10.4 million tonnes in 2015. The consumption of stainless steel by Asian countries is estimated to increase 7 to 10%, while consumption in Europe is hoped to rise by 10%. The economic recovery of the ferrochrome and stainless steel sectors has resulted in the current higher demand for chromite. This strong demand has helped the weak performance of a number of South African mines that have had logistical and infrastructural problems, in part due to heightened energy costs. At present, there is tight supply and rising prices (Industrial Minerals 2011). This has promising connotations for Cliffs’ entrance into the chromite market and would potentially position Canada as an important global chromite supplier. According to the ferrochrome production rate that Cliffs is forecasting (1,250 to 1,750 tonnes per day), Canada is likely to surpass South Africa in ferrochrome production (Golder Associates 2011).

 

 

 

 

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