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 magnesia-chrome 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 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 alumina-chrome 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

• 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.
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 chronium-containing 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 chrome-magnesite 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.

   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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