Modern Gasworks Practice

200 MODERN GASWORKS PRACTICE oven wall containing silica is taken past 575° there is danger of this change, and arrangements must be made to meet tlie inversion. Anotlier important constituent of fireclays is the true clay substance or kaolin. Very little is known of this substance,beyond the faet that it always occurs in particles which are too small to be individualized, even in a high-power microscope. Kaolin, in fact, is not amenable to the theoretical study such, as is applicable in the case of quartz. The reason for admixing kaolin with quartz is that the one may grip th.e other, but there is still a good deal of argument as to whetlier the material should be made up in large or small pieces. As regards the ultimate behaviour of refrac-tories, it is generally understood that no harm will result from the presence of quartz so Jong as the impure remnants of the original mother rocks are eliminated as com-pletely as possible. An important feature in Connection with impurities (i.e., lime, magnesia, iron, etc.) is that iheir influence is more marked in the presence of silica than. of alumina ; lien.ce deletion, as far as possible, from the highly siliceous clays is imperative. As Fearnsides says, experimental evidence shows that quartz can live quite happily with. kaolin at temperatures far higher than those employed in the ■distillation of coal, for up to 1,500° probably no reaction takes place. Undoubtedly a pure material forms the ideal, but the employment of such a material is not in all probability commercially practicable. It is generally known that there is some substance in fireclays which wh.en Associated with two solid materials of high melting point causes its neighbours to react and form a low melting point mixture. This reaction appears to begin just above 900° C. For instance, if alkaliue impurities are present the tridymite expansion may be expected to take place quickly and on each occasion when variation in temperature occurs. Evidence points to the fact that iron com-pounds may possess a similar influence. Iron is present in eitlier the ferrous or the ferric form. The latter form is inert so far as it is possible to judge, but it has been stated that in the presence of the gases the ferric salt cannot remain owing to reducing reactions which will be set up. Thus fireclay users must chiefly be concerned with the ferrous salt which, in conjunction with the alkaline impurities, has the effect of causing the silica to “ grow.” In this direction texture is of importance, for if large pieces of silica are employed the portions of each piece which. are in contact with the fireclay (and it is the fireclay and not the silica which. contains the impurities) form a small portion of the whole. Accord-large pieces are much more likely to survive protracted contact with the impurities present than if a very fine silica is utilized. On the other hånd, certain dificulties are found in practice when a coarse material is employed. Very much must depend upon the proportion of impurities present in the clay, and if a practically pure kaolin admixed with. a high-class silica is obtainable, better results will follow the use of a fine grain, whereas it is advisable to turn to a coarse texture when the clay is such as to be chemically active with respect to silica. Fearnsides suggests that the period during which the shattering of the material takes place, and when pieces an incli or so in thickness come away, is when the quartz is passing from the a to the ß stage at 575° C. In this connection it must be remembered that