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Chromium in Refractories

Prepared by Dr. Mariano Velez

Ceramic Engineering Dept.,

University of Missouri-Rolla 

USA (Sept. 2000)

• Introduction

 Refractory materials are chemical compounds that are used as structural materials forming insulation

linings and/or as containment vessel in high temperature and corrosive environments in many industrial

processes. The use of chromium in refractories is second in importance to its metallurgical applications.

The mineral chromite is the only ore of chromium 1. About 15% of the total world chromite consumption

is from the refractories industry 2. A typical analysis of a chromite suitable for refractory purpose is 38 to

48 percent Cr2O3, 12 to 24 percent Al2O3, 14 to 24 percent Fe2O3, 14 to 18 percent MgO, and less

than 10 percent SiO2.

 The usefulness of chromite as a refractory is based on its high melting point of 2,180 °C (3,960 °F),

moderate thermal expansion, neutral chemical behavior, and relatively high corrosion resistance.

Chromite enhances thermal shock and slag resistance, volume stability and mechanical strengh. In

contact with iron oxide, it forms a solid solution (a homogeneous crystalline phase composed of different

minerals dissolved in one another) with iron oxide and expands considerably, causing the refractory to

crumble (bursting). Adding magnesia can prevent this phenomenon 3.

 Chrome-based refractories 4,5 are typically used in cement kilns, secondary steel refinig furnaces,

foundry sands, glass melting furnaces, and incinerators. In some cases alternative materials -- such as

magnesium-aluminium spinels, spinel-bonded magnesite and high alumina refractories -- have replaced

chrome-containing refractories. However, these materials do not always meet performance or cost

requirements.

• Current Applications

 The iron and steel industry consumes about 70% of the total tonnage of refractories produced globally.

The cement and lime industry consumes 7%, the ceramics industry 6%, the glass industry 3 to 4%, and

the oil industry about 4% 6.

 Chrome refractory bricks of 100 percent chromium ore have been largely replaced by bricks composed

of mixtures of chromite and added oxides (i.e., magnesia) for greater refractoriness, volume stability,

and resistance to spalling ( cracking/rupturing of a refractory shape). A large quantity of Cr2O3 raw

material is also being used in the production of refractories, either in the form of synthetic grain, such as

MgCr2O4 and Cr2O3 or as additive 7. Magnesia-chrome brick can be severely affected by hydration during

storage. The MgO (periclase) grains become superficially hydrated by the formation of a Mg(OH)2

(brucite) layer. Upon subsequent heating , the brucite layer decomposes, producing a loss of bond

between the periclase grain and the matrix, severely decreasing of the bricks 8. 

• Copper Metallurgy

 Nearly all copper producing furnaces have adopted refractory practices based on the use of magnesiachrome refractories 9. In converter furnaces, the lining of the furnace bottom and the tuyere zone (zone

of greatest wear) is usually fused cast magnesia-chrome or chrome-magnesite brick. The prospective

replacement of magnesia-chrome by spinel magnesia-alumina spinel brick for copper smelting, converting

and refining is left undetermined at the time 10.

• Aluminium Metallurgy

 The aluminium industry uses alumina-chrome refractories with bricks having a chromic oxide content

ranging from 9-10 to 90%. The refractories are fired to develop a solid-solution bond, having a minimum

of silicate bond phase. On the other hand, high chromic oxide content refractories are being fused to
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obtain extremely dense refractories to be used in areas of high wear. The commercial products are highly

resistant to the corrosive action of the variety of fluxes, slags, and glasses derived in the molten

aluminum related processes.

• Steelmaking

 Iron and steel planst are the major consumers of refractories, although new technological

improvements have led to lower consumption 11. Magnesia-chrome refractories are commonly used in

secondary steelmaking plants because of their high resistance to a wide variety of slags and their

stability at high temperatures. For instance, the reaction vessels in the argon-oxygen decarburization

(AOD) process are lined with burned dolomite, burned magnesia or fused magnesia-chrome refractories.

• Cement kilns

 Several types of bricks are used in cement rotary kilns, and must have good mechanical behavior and

high chemical stability. Magnesia-spinel (MgO-MgAl2O4) refractories are mainly used for cement rotary

kins. In Japan, ultra high temperature fired magnesia-chrome briks are mostly applied in the burning

zones of the kilns 12.

• Glass Melting

 Glass contact refractories are limited to compositions including Al2O3, Al2O3-SiO2, ZrO2, with or

without Cr2O3 additions, and dense Cr2O3 13,14. High alkali borosilicate fiberglass (which is highly

corrosive) is melted using fused chrome-AZS (alumina-zirconia-silica refractories) or fused aluminachrome refractories. Low-alkali borosilicate glass is melted using dense sintered chromic oxide. Chrome

refractories (10 and 16% Cr2O3) offer high corrosion resistance to soda-lime glasses and are used as

paving, sidewall and back-up linings and increasingly in the foreheart components. Sintered chrome,

chrome-magnesite, or magnesite-chrome is used in the bottom of the checkers, as well as on the

structure surrounding the checkers and walls. During operation, the checkers develop minor amounts of

soluble Cr+6 on the surface 2. 

• Special Refractories

 Chromium-borides, Cr-carbides, and Cr-silicides are relatively new high temperature materials intended

for special applications 15 (see Table I). Chromium and boron compounds, because of their high melting

point, have potential for structural applications, which require high temperature strength, chemical

stability, and hardness and strength 19.

Table I 16-18. Summary of relevant properties of special chromium compounds

General term Compounds identified Main properties

Cr-borides Cr4B, Cr2B, Cr5B3, CrB,

Cr3B, CrB2

High melting point

High electrical conductivity

Resistant to chemical attack

Cr-carbides Cr23C6, Cr7C3, Cr3C2 High melting point

Very hard

Cr-silicides Cr3Si, Cr2Si, Cr5Si3, Cr3Si2,

CrSi, CrSi2

High melting point

Hard and brittle

Resistance to attack by acids

• Hexavalent Chromium Compounds in Refractories

 When chrome-based refractory materials are exposed to severe environments (high temperature, high

pressure contact with different chemical species and phases), a possibility exists that toxic by-products

or waste materials will form. Certain chromium compounds are classified as carcinogenic substances

20,21. 

 Hexavalent chromium compounds have been found at levels, which exceed the EPA (United states

Environment Protection Agency) limits when chrome-bearing materials are mixed with calcium aluminate

cements (castable) at even relatively low temperatures 22,23. 
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Environmentally hazardous CrO3 forms in refractories along grain boundaries (narrow zone in ceramics or

metals corresponding to the transition from one crystallographic orientation to another). As chromite

comes into contact with alkali and alkaline earth oxides, the transition of Cr3+ into Cr6+ in air is

accelerated and occurs at notable rates 24. Hence, the content of alkali and calcium oxide in chroniumcontaining refractories or materials coming into contact with then should be minimized.

 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 pct CaO) 25. The use of fused grains

decreases the formation of Cr6+. In the case of magnesite-chrome refractories, temperature, basicity

(CaO/SiO2), and chromite phase size play important roles in Cr6+ formation. The formation of Cr6+ can

be minimized by carefully controlling the amount of calcium oxide in the refractory and by avoiding the

use of a fine chromite phase during brickmaking.

 Today, there is a trend to try to subtitute high-chrome brick refractories with other materials: copper

industry 9,10, steel industry 26,27 and cement industry 28. In the glass industry there has been the

subtitution of chromite refractories with other suitable materials (i.e., fused-cast AZS, dense zircon).

Some companies are taking back used chrome refractories from its customers and reprocessing them into

new products 29. Another approach to reduce chromium in refractories has been the use of suitables

coatings to improve the slag resistant behavior of magnesia-chrome bricks 30. Still another practice is

the use of fused-chrome magnesite, which is more expensive than its counterpart, sintered chromemagnesite brick used in the steel industry. In this case, there has been an increase in refractory life, as

well as a lower consumption of refractory per ton of steel produced. 

• Refractory Waste Management

 Most refractories are disposed as landfill 31. Recycling and waste management of refractories is

extremely difficult since associated cost are difficult to quantify. For example, it is expensive to transport

used refractories from users to recyclers. Also, since many recyclable, spent refractories are low value

items, it is almost always cheaper to buy new raw materials than to use recycled materials. Only about

10% of total annal refractory production is recycled, even though successful recycling programs have

existed for some time 32,33. Europe recycles a higher tonnage of refractories than the US., especially

magnesia-chrome from steel plants 34, while in Japan the used chrome bricks from cement kilns or glass

furnaces are returned to the manufacturer 29. 

 A preliminary research programm 35,36 has outlined a refractory recycling financial model that includes

the major factors that impact the cost of management strategy. It quantifies the financial impact of

tangible (i.e., loss of production due to slow processses) an intangible (i.e., public relations, risk of major

ecological liabilities) costs and benefits. A strong commitment by company management toward reducing

waste refractory is required for successful recycling 37. Start up costs is the main reason companies are

slow to recycle 38. A successful refractory recycling program includes several steps, of which sorting of

the materials by type, is the most critical. Upon sorting, there might be several sizing steps like

communication, and final separation. In general, refractory waste minimization techniques currently

include (1) improved refractory materials that yield substantially longer vessel campaigns, (2) zoning of

refractory linings to yield consistent wear, and (3) use of water cooled panels in place of refractory

materials 39.

 Refractory recycling from glass and steel furnaces has been presented as an economically and

environmentally sound alternative to landfilling. For instance, a method has been developed to recycle

chromium-bearing refractories 40. The role of Cr2O3 in the corrosion resistance of Al2O3-Cr2O3 castables

for waste melting furnaces has also been discussed 41.

• Conclusion

 Chrome-based refractories are being used in Portland cement rotary kilns, secondary steel refining

furnaces and transfert ladles, specialty steel melters, foundry sands, glass manufacture (textile and

fiberglass, throat areas of glass tanks), steel and glass reheat regenerators, copper, nickel, and lead

smelters, utility boilers, coal gasification furnaces, and waste incinerators. Although alternative materials

have been proposed to replace chrome-containing refractories, these substitutes do not always meet

performance or cost requirements. 
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 What is the actual impact of Cr-based refractories on the environment ? It is well agreed that soluble

Cr6+ is unwanted, however Cr-refractories continue to be produced by many countries throughout the

world (Japan, China, for instance) and the US. The consensus is that if the refractories have been in

contact at some moment with alkali materials then there is a strong posibility of Cr6+ being formed. To

sustain the already wide usage of chromium refractories in the steel, non-ferrous, and glass industry

further research (i.e., decision modeling 35, logistical and technological approaches 9,10,39) needs to be

conducted to either improve refractory compositions and/or treat the refractory wastes.

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