The mechanism for CO disintegration is well established. In the presence of iron compounds, carbon monoxide decomposes by the Boudouard reaction[Ref. 1]

2CO -> C + CO2.

Carbon is deposited on the surface of the iron compound producing a volume expansion, which in turn produces cracking of the refractory. High permeability of the refractory will contribute to the rate of carbon deposition. Thus, as cracking occurs, the rate of carbon deposition increases and these cracks propagate under the increasing splitting force of the carbon deposit.

Carbon monoxide disintegration is controlled by the catalytic activity of the iron compounds and disintegration occurs between 572 and 1,292°F (300 to 700°C) and is most pronounced at temperatures of 842 to 1,022°F (450 to 550°C). CO disintegration will not occur when the free iron content is less than 0.1 weight percent (wt.%) or when the iron oxide content is less than 0.2 wt.%. So, like polio, CO disintegration of furnace lining materials should be a thing of the past. However, high quality fireclays containing low iron contents are becoming scarce. Thus, it may be advantageous for the heat treat engineer to review how refractories are made and the mechanism of CO disintegration so that informed decisions can be made with regard to pricing a refractory lining.

Many furnace refractory linings are produced from fireclays, which includes all refractory clays that are not "white burning." White burning clays are reserved for making dinnerware and are low in impurities, thus, are more uniform in color when fired. Fireclay refractory will contain by weight 25-45% Al2O3, 70-50% SiO2, 0-1% MgO, 0-1% Fe2O3, 0-1% CaO, and 1-2% Ti2O with an apparent porosity between 10 and 25%. These fireclays are often designated with respect to their alumina (Al2O3) content as either pyrophyllite (Al2O3©4SiO2© H2O) or kaolin (2SiO2©Al2O3©2H2O). For many heat treat furnace applications, an insulating brick with higher porosity is desired. The clay is then mixed with a fugitive organic such as sawdust, which is subsequently burned out during the firing to leave a porous refractory.

Strength and refractoriness is controlled by the alumina (Al2O3) content. A more refractory ceramic is obtained using higher alumina clay. During the firing stage the aluminosilicate clay converts to mullite, which grows as columnar grains. Impurities and excess silica form a glassy phase that is left between the mullite grains. High alumina clays will form interlocking mullite grains and this provides higher temperature strength. Firing temperature will control the degree of conversion to mullite. A Seger cone number often designates refractoriness and a chart of the aluminosilicates is shown in Fig. 1. A high Seger number indicates more refractoriness.

CO disintegration is possible when Fe2O3 (hematite) is present after the firing. An excellent article by Chein and Ko[Ref. 2] shows that the size and distribution of hematite is critical in regard to CO disintegration. In general, the disintegration is more severe as the size of the Fe2O3 particle increases and as the concentration increases. In a reducing environment, the hematite grains are converted to magnetite (Fe3O4) and magnetite is a stronger catalyst for the Boudouard reaction. Also, the conversion of hematite to magnetite produces a 16 to 25% volume expansion and this contributes greatly to the cracking process. The amount of expansion depends upon oxygen partial pressure and temperature. It's important to note that the hematite conversion will also occur in other reducing environments such as hydrogen and water mixtures with out the deposition of carbon. Thus, control of the fireclay iron content and distribution is critical. IH

[1] O. Boudouard, Ann. Chim. Phys., vol. 24, no. 5, p. 85 (1901).
[2] Y.T. Chein and Y-C Ko, Amer. Cer. Soc. Bull., vol. 62, no. 7, p. 779 (1983).