Companies are now judged by the extent of their concern for and contributions to the preservation and protection of the environment. With ISO 14001 certification and enforcement of the Law of Pollutant Release and Transfer Register, the handling of substances that could be a harmful or toxic will become a heavy burden for companies. Needless to say, magnesia-chrome bricks (hereafter referred to as MC brick) used in the burning zone and the tire region of rotary cement kilns, are the objects of significant concern. In particular, because used MC bricks may contain hexavalent chrome compounds, the most desirable option is to dispose of them in a managed-type disposal site, after being treated to render them harmless. Currently the recycling of used bricks is very difficult, and the places for disposal are decreasing each year, given the continually increasing quantity of waste, and as a result, the disposal cost is likely to increase rapidly, because of the handling and treatment steps (see Fig. 1). Asahi Glass Company Ltd. began to install chrome-free brick in the burning zone of rotary cement kilns in 1991, and to make trials with several customers, using magnesia-spinel-based brick (hereafter referred to as spinel brick) with the intent of providing more chrome-free brick for the kiln, which means improving the properties to include high adherence of the cement coating, good resistance to chemical reaction at high temperature and also good resistance to the penetration of cement, so that they can be substituted for MC brick. Though this chrome-free brick yielded reasonable results, which justified their use in place of MC brick at that time, unfortunately, it was not a popular choice, because in those days the use of MC brick was permitted without any restrictions. However, further improvement of the service life was achieved in a few kilns, and consequently the chrome-free brick has been continuously used in those kilns.
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