Modern Gasworks Practice

REFRACTORIES FOR GAS WORKS PURPOSES 199 Oughtibridge contain considerably more. The material from whicli silica brick is made is a rock very rich in a-quartz, and during buming in the kiln this changes to the ß modification and partly to tridymite ; on cooling again, liowever, the p-quartz reverts to the a form. It has been shown that during the time the brick is in the furnace the quartz gradually undergoes conversion, so tliat the material ultimately con sist,s of tridymite or cristobalite. The chief point of interest is, liowever, the actual temperatures at which the various inversions from one modification to another take place, and. here. it would appear, there is room for considerable further research ; for, from the point of view of actual working conditions, the inversions are accom-panied by changes in volume. For instance, it is generally understood that as tridymite has the lowest co-efficient of expansion of the various forms, it is a most desirable constituent; but, as Scott has found, few silica bricks made in this country contain much tridymite at the end of the kiln-burning. The total expansion of a brick when subjected to heating may be classified under three distinct headings, in eaeh case a different factor coming into play. For instance, there is first tlie ordinary thermal expansion, which is unavoidable and also reversible. This involves not only the ordinary thermal expansion, but also the volume changes due to a — ß transformations, which, again, are reversible. Secondly, there is the permanent expansion, that is, the volume change during the inversion, of the original quartz to the ultimate tridymite. According to Scott, this, by sufficiently prolonged firing before use, con Id be overcome, but the question of the feasibility of tlie operation is mainly an. economic one. The third factor is the increase of bulk, which is in excess of that calculated from the increase of the specific gravity of the finely ground material. In America, D. W. Boss has found tliat on heating a silica brick to 1,400° C. the volume expansion on an average amounts to 2-8 per cent. ; at the same time the diminution in specific gravity is slightly under 2 per cent., while on heating up to 1,500° the corresponding figures are 9-7 and 2-8 per cent, respectively. Acquaintance with the technical modifications associated with silica whicli have been briefly discussed. above will permit the ordinary practical engineer to appreciate tlie part tli.ey play when refractories are actually at work. Fearnsides1 has stated that little is really known. about the inwardness of the texture of the various materials utilized. Quartz is a mineral found everywliere, and represents the survival of the least soluble portions of granite. It can be obtained in millions of tons together, and in great pieces having a purity up to 98 per cent. With a rise of temperature it expands—though. not so rapidly as many otter minerals until, when a temperature of 575° C. is reached, it quite suddenly changes from the a to the ß variety. Tlie chief point to consider, then, in dealing with the texture of bricks is tlie faet that there is present a constituent which in five minutes can. come up from the ß to the a variety, or vice versa. If, in the ordinary works, a silica brick were doing this it might look quite clean at one moment and immediately afterwards be covered with dust, owing to electrical disturbances. Every time a retort or coke- 1 Proc. Midland Assoen. Cole Oven Managers, 1918.