Southern African Pyrometallurgy 2011, Edited by R.T. Jones & P. den Hoed, 

Southern African Institute of Mining and Metallurgy, Johannesburg, 6–9 March 2011 

359 

Chromite—A cost-effective refractory raw 

material for refractories in various 

metallurgical applications

N. McEwan, T. Courtney, R.A. Parry, and P. Knupfer 

Vereeniging Refractories (Pty) Ltd 

Keywords: refractories, chromite, Marico 

Abstract – This paper examines the role of refractory-grade chromite in 

refractories. The chromite deposit at Verref’s Marico Chrome mine is described. 

The refractory properties of chromite and their role in basic refractories are 

described. The properties and metallurgical applications of chrome-containing 

refractories are described and finally environmental issues associated with chromecontaining refractories are discussed. 

INTRODUCTION 

Chrome-containing refractories have been around since 1879 and are critical for 

various metallurgical applications. Refractory-grade chromite is important as a 

source of chromite in these chrome containing-refractories. 

The specific desirable properties of chromite that give chrome-containing 

refractories their specific properties are discussed in this paper. The properties 

and metallurgical applications of chrome-containing refractories are examined, 

and finally environmental issues associated with chrome containing refractories 

are discussed. 

MARICO CHROME CORPORATION 

Introduction to Marico Chrome Corporation 

Marico Chrome Corporation is a 50:50 joint venture between Vereeniging 

Refractories (Verref) and Samancor Chrome SA. 

Vereeniging Refractories was established in 1882 and is the oldest and largest 

refractory company on the African continent. In 1946 the company was 

registered and listed on the Johannesburg and London stock exchanges. It was 

listed as a subsidiary of The Vereeniging Estates Limited, later known as 

Anglocoal, a division of Anglo American. The name of the company was 

changed in 1967 to Vereeniging Refractories Limited, to reflect its main line of 

business. In 1989 the business was split into Amcoal Colliery and Industrial 
360 

Operations Limited, under the name Verref. The last change of ownership was 

in 2001, when Verref became an independent and privately owned company.1

Verref employs about 700 people in its mining and refractory operations. It has 

the capacity to produce about 150,000 tonnes per annum of shaped and 

unshaped products in a wide range of qualities, including magnesia, 

magchrome, chromag, doloma, alumino-silicate, high-alumina, zircon and acid 

resistant materials. Basic and alumino-silicate castables, mortars, gunning and 

ramming materials are also produced by Verref.2

Vereeniging Refractories serves the industries of southern Africa and also 

exports to central Africa and to overseas markets. 

Samancor was formed by the amalgamation of SA Manganese Ltd and African 

Metals Corporation Ltd (Amcor) in 1975. It was known as SA Manganese 

Amcor Ltd until 1985, when the name was changed to Samancor. In 1983, 

Gencor became the single biggest shareholder in Samancor, with about 40% of 

the shares, giving it effective control.3

By 1986, Samancor had a number of interests in the production of 

ferromanganese, chromium-alloys, ferrosilicon, graphite electrodes, and other 

carbon products, phosphate fertilizers, phosphoric acid and sodium 

tripolyphosphate, and interests in mineral deposits like chromium ores, 

dolomite, limestone, vanadium, serpentine and, of course, the huge manganese 

ore deposits.3

In 1998, after Gencor’s unbundling exercise, Billiton established a joint venture 

with Anglo American to purchase and de-list Samancor Ltd. Billiton owned 

60% of the shares. Billiton subsequently merged with BHP to form BHP Billiton 

(in 2001). The Samancor Chrome and Samancor Manganese divisions were 

split, with the Kermas group acquiring 100% of Samancor Chrome. The 

effective date of sale was 1 June 2005.3

Samancor Chrome currently operates two sets of mines and three alloy 

producing plants in the North West and Mpumalanga provinces of South 

Africa.3

Marico Chrome Mine is a relatively small-scale producer located 60 km north of 

Zeerust and close to the Botswana border. The mine produces about 40,000 

tonnes per annum of metallurgical and refractory-grade chromite for domestic 

and export markets. Mining as Marico Chrome Mine commenced in 1978 and 

the mine has an estimated 9 million tonnes of ROM reserves with a 40–50 year 

life of mine.2

The Marico Chromite Deposit 

The Marico deposit is part of the far western limb of the Bushveld igneous 

complex (see Figure 1). The Bushveld complex is approximately 460 km long 

and 245 km wide and is one of the most remarkable geological formations in the 
361 

world, comprising a suite of rock types from basal sedimentary layers, through 

intrusive and extrusive igneous formations, to associated thermal 

metamorphism. It was formed between 2040 and 2060 million years ago. 

Figure 1: Geological setting of the Groot-Marico Chromite Deposit4

Throughout the main Bushveld complex one can recognise some general trends. 

1. The chromium content of the layers decreases upwards: 

· LG6.........................46–47% 

· MG layers..............44–46% 

· UG2........................43% 

2. An associated upward decrease in Cr:Fe ratio: 

· LG 6........................1.56 to 1.60 :1 

· MG layers..............1.35 to 1.5 :1 

· UG2........................1.26 to 1.4 :1 

3. The alumina content decreases upwards through the geological succession 

4. Chromite grain size varies in size from 2 mm to 50 μm from lower to upper 

 layers 

There have been many theories as to the relationship between the Marico body 

and the western Bushveld complex. These vary from the suggestion that the 

Marico basin is entirely separate to its being regarded as an eruptive feeder to 

the main western area.4

The lenses, or kidney-shaped, deposit (figure 2) is composed of mafic rocks 

forming an elongated body along a north- south central synclinal axis. The body 

is shallow at the rim and steepens towards the centre. 
362 

Exploration and mining activities have revealed 5 chromite layers, which, while 

different in nature, have been linked stratigraphically with the LG layers of the 

main western Bushveld complex.4

Figure 2: The Groot-Marico Chromite Deposit4

Although part of the Bushveld Complex, the Marico Deposit does show some 

significant differences from the chromites of the main Bushveld Complex, and it 

is some of these differences in the nature of the chromite that give Marico 

Chrome its superior properties with respect to refractory production. 

Typically the Marico chromites are— 

1. Higher in Cr2O3 content 

· Up to 49% for the refractory-grade product 

2. Lower in SiO2

· Typically below 1% 

3. Have higher Cr-Fe ratios 

· Typically 2:1 compared with 1.6:1 in LG1 

4. Lower magnetite content 

· This results in more refractory MgO·Cr2O3 in the spinel phases and 

an associated higher refractoriness 

The combination of these characteristics makes the Marico chromites suitable 

for refractory applications: the low levels of impurities (SiO2 and Fe2O3) and the 

relatively higher proportions of chromium to iron reduce the potential for iron 
363 

oxide bursting and volume expansion at high temperatures and result in higher 

refractoriness. 

CHROMITE AS A REFRACTORY MATERIAL 

Refractory materials 

Refractories are a branch of ceramics, which is mankind’s oldest art. The word 

refractory is defined as resistant to change, and in the metallurgical industry 

refractories are materials that are resistant to change at elevated temperatures. 

Refractories are used in any application where a supporting furnace structure 

must be protected from the temperature required for the metallurgical process, 

or where heat loss must be limited.5

Refractory materials were probably inadvertently first used during the 

transition from the Stone Age to the Bronze Age during the chalcolithic 

(copper) period, with the earliest evidence in the Timna Valley between the 

southern tip of the Dead Sea and the Gulf of Aqabah in 6000 BC. These early 

smelting installations were simple bowl-shaped hearths in small pits, with 

above-ground stone enclosures. With the advent of the Iron Age at about 

2000 BC higher temperatures were required and the use of refractories would 

probably have become a more conscious pursuit.6

Modern refractories technology began in the late 18th century with the growth 

of the iron industry and later the steel industry during the Industrial 

Revolution. The greater demands placed on refractories required materials 

other than alumino-silicates and silica; magnesite and chrome brick were all 

introduced in the late 1880s. It was only in 1931 that the superior hot strength of 

blends of chromite and magnesite was recognized and chrome-magnesite bricks 

were introduced with tonnage usage in open hearth and electric steel-making 

furnaces. 

In the mid-1960s low-silica, magnesia-chrome and reconstituted fused-grain 

magnesia-chrome refractories were introduced worldwide, with Verref 

commissioning a fusion plant in 1967. Since then fused magnesia-chrome 

chrome-mag products have been increasingly used in extractive metallurgical 

applications ranging from ferroalloys and steel, to base metals (lead, copper, 

cobalt and nickel), to PGMs. Two industries where their use has been 

discontinued because off the high level of hexavalent chrome formed are glass 

and cement. 

The principal refractory consuming manufacturing process that we can easily 

identify with would be— 

1. Iron and steelmaking 

2. Non-ferrous metal production 

3. Cement production 

4. Glass production 

5. Petrochemical production 
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If a refractory material is defined as having a melting point of greater than 

1500°C, then there are sixty compounds identified in the Handbook of 

Chemistry and Physics that meet this criteria.5

Table I: Compounds with melting point > 1500 °C

Compound Number 

Borides 13 

Carbides 19 

Nitrides 10 

Silicides 7 

Oxides 11 

Total 60 

If in addition to a high melting point, the limits of reasonable abundance and 

reasonable price are introduced, the list reduces to the six oxides on which, 

together with carbon, silicon carbide and silicon nitride, the vast range of 

refractory materials are based. 

Table II: Refractory oxides 

Oxide 

Melting 

point (°C) Classification 

Silica SiO2 1728 Acid refractory 

Alumina Al2O3 2010 

Chrome Cr2O3 2265 

Zirconia ZrO2 2670 

Neutral refractory 

Lime CaO 2614 

Magnesia MgO 2800 

Basic refractory 

Of these oxides, chrome in the form of chromite is the most cost effective. 

Depending on its chemical composition each type of refractory material will be 

more compatible with certain chemical and physical environments than with 

others 

Refractories are also classified according to their form: 

1. Shaped refractories—essentially bricks 

2. Unshaped refractories—commonly called monolithics 

Both of these categories can be sub-divided into further categories. 

Suitability of chromite as a refractory raw material 

The usefulness of chromite as a refractory is based on four factors: 

1. It has a high melting point 
365 

Because of their unique properties the Marico chromites, with a higher 

proportion of Mg than Fe in the spinel phase, are postulated to have an even 

higher liquidus temperature than other chromites (see Table III). 

Table III: Theoretical liquidus temperature of Marico chromite4

Spinel Molecular

mass (%)

Melting 

point (°C)

Calculated melting

 point (°C)

MgO·Al2O3 30.58 2105 643.7 

MgO·Cr2O3 25.89 2400 621.0 

FeO·Cr2O3 36.26 2160 783.0 

FeO·Fe2O3 7.27 1600 116.3 

Estimated liquidus temperature 

of Marico chromite 

2164 

2. Moderate thermal expansion 

Refractory materials will expand when heated up and shrink when cooled 

down. If no permanent changes occur in the original dimensions this effect 

is known as reversible thermal expansion. Chromite has a linear expansion 

of about 1.3% at 1400°C, which is almost 50% of that of magnesia (MgO). As 

a consequence, when added to magnesia refractories, chromite will improve 

the thermal shock resistance of the refractory (see Table IV). 

Table IV: Thermal expansion at 1000°C 

Brick quality 

Thermal 

expansion 

Magnesia brick 1.4% 

Magnesite-chrome brick 1.1% 

Chrome-magnesite brick 1.0% 

3. Neutral chemical behaviour 

In addition to refractoriness, a lining material must be compatible with the 

process slag chemistry. Figure 3 shows the range of slag lime-to-silica ratios 

with which magnesia, mag-chrome and high-alumina refractories are 

compatible. Chromite-containing materials can tolerate slags ranging from 

slightly acid to basic, and can be used in place of tabular-alumina brick or 

magnesia brick in most applications. 

4. Relatively high corrosion resistance 

Chromite has exceptionally good resistance to pyrometallurgical slags. Slags 

that are acidic and contain high levels of iron—in other words, are silica-rich 

fayalite (2FeO·SiO2)—rapidly attack and deeply penetrate alumino-silicate 

refractories. The resistance of chromite against fayalitic slags—which are 

common in many non-ferrous metallic smelting processes—is exceptional. 

Figure 4 is a photomicrograph of the working face of a magnesia-chrome 

brick and demonstrates the resistance of chromite to silicate-slag attack. The 
366 

chromite grain in the centre of the field stands proud of the surrounding, 

altered magnesia grains. 

Figure 3: Refractory use as a function of slag lime-to-silica ratio

Figure 4: Resistance of chromite to silica-rich fayalitic slag 

Field width, 1000 mm 

CHROMITE-CONTAINING REFRACTORIES 

The early chrome refractories consisted of moulded and fired chromite. These 

refractories had several problems because of their bursting and crumbling as a 

result of alternative exposure to oxidising and reducing atmospheres. They also 

shrank and softened at high temperatures.8 The addition of magnesia “solved” 

many of these problems, and this led to the development of the magnesiachromite, chrome-magnesite series of refractories during the 1930s.8

The effect of a silicate melt on brick properties was established in the 1950s and 

’60s, and in the steel industry in particular the demand increased for lower-SiO2

bricks for OH and electric-arc furnaces. Magnesia-chrome brick become the 
367 

preferred quality because of its superior slag resistance and stability at high 

temperatures.8

In the firing process FeO in the chromite oxidises to Fe2O3 and diffuses at high 

temperature into the MgO. Magnesio-ferrite (MgO·Fe2O3)—a refractory 

spinel—is formed. The chromite is “stabilised”, thereby reducing the risk of 

undergoing subsequent redox reactions. There is also development of direct 

bonding between MgO crystals and MgO and chromite grains. 

A combination of low silica and good bonding gives bricks a high hot strength 

and good spalling resistance. High Cr2O3 contents give rise to low wettability 

by fayalitic slags and a high chromite-spinel content gives rise to low slag 

solubilities.7

By fusing (melting together) the chromite and magnesia the chromite spinel is 

completely stabilised and completely dispersed as small spinel crystals 

throughout the magnesia. Optimum performance is then obtained. 

Chromite-containing refractories are divided into three main groups (according 

to the chrome oxide content (see Table V). 

Table V: Chrome-containing refractories9

Brick quality Cr2O3 (%) 

Magnesite-chrome brick <30 

Chrome-magnesite brick >30 

Picrochromite >75 

Five types of these brick are manufactured.9

1. Silicate bonded 

The magnesia crystallites and the chromite grains are bonded together by 

silicates. These bricks have limited refractoriness, but can have good thermal 

shock resistance and high pressure flexibility. 

2. Direct bonded 

By lowering the impurity content and high-temperature firing, one can 

produce a direct-bonded brick in which the chromite reacts with the MgO to 

form a highly refractory spinel, MgO·(Al, Cr, Fe)2O3

3. Chemically bonded—generally with magnesium salts and unburned 

4. Co-burned 

Magnesia clinker and chromite grains being sintered before brick-making 

5. Fusion cast 

Magnesia clinker and chromite grains being fused before brick-making 

Applications for chrome-bearing refractories include— 

1. All pyrometallurgical extraction processes for Cu, Ni and Pt. They are the 

preferred refractory these applications 

2. The steel industry, in which quantities of fused-grain brick are still used in 

vacuum degassers 

3. CLU converters in the ferroalloy industry 
368 

4. Foundry electric-arc furnaces 

ENVIRONMENTAL ISSUES ASSOCIATED WITH 

CHROME-CONTAINING REFRACTORIES 

Chromium exists in a number of different oxidation states which give it the 

ability to modify other chemical compounds, or to act as a catalyst in promoting 

chemical reactions. Hence its wide use in the chemical industry. The most 

important oxidation states are—10

1. Cr(III)—trivalent chromium 

Trivalent chromium is the most stable oxidation state of chromium. Trivalent 

chromium compounds are stable, generally have low solubility in water, and do 

not present a significant environmental hazard. The most common example of 

trivalent chromium is green chrome oxide (Cr2O3), which is widely used as a 

pigment in paints and as a component in alumino-silicate refractories. 

Chromium in chromite is also in the trivalent form. 

2. Cr(IV) 

Cr(IV) oxide (CrO2) is a black, conducting, ferromagnetic compound used in the 

production of audio and video tapes. 

3. Cr(VI)—hexavalent chromium 

The most common examples of hexavalent chromium compounds are chromic 

acid and the dichromates of sodium and potassium, which are used in the 

chemical industry and to surface-treat steels to improve corrosion resistance. 

Hexavalent chromium compounds are soluble, toxic, and are known to increase 

the risk of respiratory cancer. 

When chrome-based refractory materials are exposed to high temperatures and 

pressures combined with certain chemical phases, a possibility exists that toxic 

by-products can form. In particular the transition in the oxidation state of the 

chrome from Cr3+ to Cr6+ is of particular concern, as hexavalent chromium 

compounds are classified as carcinogenic and harmful to health. 

As chromite comes into contact with alkali and alkaline earth oxides the 

transition from Cr3+ to Cr6+ is accelerated. In particular it is clear that exposing 

chromium-containing materials to alkali or calcium oxide-rich environments 

will most likely result in the accelerated formation of Cr6+. The reaction in 

chromium-containing refractories begins along the grain boundaries and can 

thus spread throughout the structure of the refractory at a fairly rapid rate 

where circumstances and the environment favour it. 

The Cr6+ content, following the CaO-Cr2O3 phase diagram, increases with 

exposure to temperatures below 1022°C and with an increase in CaO (from 0 to 

42% CaO). In the case of magnesia-chrome refractories temperature, basicity 

(the CaO-to-SiO2 ratio) and the chromite grain size all play a role in Cr6+

formation. Thus the formation of Cr6+ can be minimized by carefully controlling 
369 

the levels of CaO in the refractory and by avoiding the use of fine chromite 

during brick making. The use of fused magnesia-chrome or chrome-magnesia 

grains will also help minimise the potential to form Cr6+ within the refractory 

structure.8–11

Because of the likely formation of Cr6+ when exposed to alkali or CaO 

environments there has been a move away from chromium-containing 

refractories in those applications where these chemical and certain physical 

conditions exist—a prime example of this being the cement and glass industries. 

This move has also taken place in other industries and applications where the 

formation of Cr6+ is not at all likely, but a view has been taken that chromiumbased refractories are environmentally damaging and could be harmful to 

health. 

The move to replace chrome-containing refractories has seen development of 

several other possibilities. These include magnesia-alumina spinels, spinelbonded magnesia, very high alumina materials, zirconia-containing materials, 

and various fused-cast products.9

Exposure limits to Cr6+

Occupational exposure limits to hexavalent chromium range from 1.0 to 

0.01 mg/m3 on an 8 hour TWA. Values vary from one country to the next. The 

exposure limit soon to be adopted by the European Union will probably be 

0.01 mg/m3. The limit in South Africa is 0.05 mg/m3.

12

If the maximum nuisance dust level of 10 mg/m3 is assumed, of a material at a 

hexavalent-chromium level of 450 ppm (unused chromium-bearing refractories 

vary between 20 and 200 ppm) exposure to hexavalent chromium would be 

0.005 mg/m3, which is well below the TWA maximum.10

Limits for landfill disposal are typically— 

1. 0.5 mg/L Cr(VI) in the leachate—Germany 

2. 1.5 mg/L Cr(VI) in the leachate—Japan 

3. 30g/ha/m Cr(VI)—South Africa. The leachable Cr6+ is determined 

according to the USEPA TCLP test or the acid-rain test.12

Exposure limits and analytical techniques 

One of the problems with comparing exposure limits is that environmental 

limits and the limits of occupational organizations are based on different 

extraction and analytical-test methods, which all yield different results. In 

addition, the results from different laboratories do not always agree. 

The Ceramic Research Association in Britain has carried out extensive work on 

the development of a reliable and significant test method for the determination 

of hexavalent chrome.13
370 

CONCLUSIONS 

Chromium plays an essential role in a wide range of industrial processes. In 

refractories, chromite is a cost-effective material that has properties ideal for a 

number of metallurgical applications ranging from ferroalloys and steel, to base 

metals (lead, copper, cobalt and nickel), to PGMs. The specific properties of 

Marico chromite further improve the properties of refractories required for 

these industries. 

Under certain operating conditions, however, toxic and hazardous hexavalent 

chromium is formed. The major industries in which this occurred have moved 

to alternative lining materials and the indications are that in other user 

industries hexavalent chromium will not present an occupational or 

environmental disposal hazard. 

In areas where hexavalent chromium may be identified as a problem, a joint 

approach by the refractory producer and the refractory user, through product 

development or recycling of used lining materials, will be the most costeffective solution. 

ACKNOWLEDGMENTS 

This paper is published with the permission of Vereeniging Refractories and 

Samancor Chrome. The contributions of colleagues are gratefully 

acknowledged. 

REFERENCES 

1. W. Ebersohn (Editor), Verref, a company rich in history, A Succeed/ESSENTIAL Special 

Publication, 2007. 

2. T. Courtney, Presentation, “Trusting in Experience and Innovation”, Vereeniging 

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of Mining and Metallurgy, Johannesburg, 5–8 March 2006, pp. 286–288. 

4. L.A. Collins and D.R. Human, The Groot-Marico Chromite Deposit, Western Transvaal,

Mineral Deposits of Southern Africa, Vol I & II, 1986, pp.1299–1235. 

5. R.A. Parry, What are refractories?, VTS Module 02 Rev 04, Vereeniging Refractories (Pty) 

Ltd, 2005. 

6. K. Sugita, “Historical Overview of Refractory Technology in the Steel Industry”, Nippon 

Steel Technical Report 98, July 2008. 

7. P. Knufer, Presentation, VTS Module 05 Rev 00 Basic BBR 716 Basic Rm & Ref., 

Vereeniging Refractories (Pty) Ltd, 2005. 

8. R Engel, Chrome Bearing Refractories: Is There a Future?, The Refractory Engineer, May 

2010, pp 12–14. 

9. P. Hloben, Refractory Materials—Major Industrial Applications, Rexxon Corporation, 

Bryanston, South Africa, 2000. 

10. R.A. Parry, Alternatives to chrome-containing refractories, presented at the Institute of 

Refractory Engineers, Vereeniging Refractories (Pty) Ltd, 22 June 2000. 

11. T. Courtney, personal communication, Vereeniging Refractories (Pty) Ltd, 5 Sep. 2010. 

12. International Chromium Development Association: Health Safety and Environment 

Guidelines for Chromium (2007 edition), pp.29–31. 

13. P. Hodsin, Hexavalent Chromium in Refractories in the UK,. Report of CERAM Research 

Working Group, October 1998. 
371 

Niell McEwan

Technical Manager, Vereeniging Refractories 

Niell completed his schooling at Carletonville High School in 1987, 

and studied Metallurgical Engineering at the University of 

Potchefstroom (now North West University). 

He started his working career at Samancor in the Chrome division in 1993 and 

fulfilled the positions of Metallurgist, Production Engineer, and Production 

Superintendent at Middelburg Ferrochrome and Ferrometals. Achievements 

included commissioning and operating the first charge chrome furnace with 

Outokumpu pelletising and preheating technology in South Africa. 

In 2002 Niell joined Vereeniging Refractories and fulfilled the positions of 

Operations Manager, Business Development; Project Manager; and Technical 

Manager. 

Niell has also completed an MBA from Herriot Watt University in the UK 

(2002) and a Masters of Engineering in Project Management (MPM) from the 

University of Pretoria (2010). 
372