Modern Gasworks Practice

REFRACTORIES FOR GAS WORKS PURPOSES 195 portant feature in connection. with impurities is that their influence is usually more marked in the presence of silica th.an of alumina ; hence deletion, as far as possible, from the highly siliceous clays is desirable. Calcium compounds are particularly to be guarded against, owing to their tendency to act as an extremely rapid and powerful flux. The ability of the various compounds classified as impurities to act as fluxes is in. direct ratio to their equivalent combining weights. The most desirable means of determining the suitability of a material for high-temperature work is undoubtedly by a combination of the ultimate, rational, and mechanical analyses. THE WINNING AND WORKING-UP OF FIRECLAYS By far the largest deposita of aluminous 1 fireclays in this country occur in the Stourbridge, Yorkshire, and Newcastle districts. The clay is generally found. at a considerable distance below the surface, and shafts are frequently sunk to depths of 500 or 600 feet, or more. In wirming the clay, care has to be taken to avoid mixing the produots from distinct seams ; for although the seams occur iii close proximity to one another, their Chemical constituents and properties are by no means similar. Tlius a highly desirable substance may be found adjacent to one containing an abnormal proportion of deleterious salts. After being brought to the surface the majority of clays imdergo treatment known as “ weathering,” which. has an important influence on. their suitability for future use. This process is essential, m that the clay (staoked in shallow heaps for some considerable period) is subjected to thorough lixiviation, with consequent reduction of the harmful alkalis, which are usually decreased by at least 40 per cent. Once “ weathering ” is complete the requisite quantity of “ green ” clay is made up by tlioroughly grinding together definite proportions of the varying types ; that is to say, the clay from one particular seam. is frequently not used by itself, but, if of the “ strong ” or siliceous type, it is mixed with a certain proportion of the “ mild ” quality until the desired material is obtained. The material having been moulded and dried would then be ready for burning in the kilns but for the faet that the “ green ” clay treated in this manner is subject to considerable shrinkage, and would in addition pro vide a fin.ish.ed article of too fine a grain. for high-temperature purposes. In consequence, not the least important consideration in the manufacture of fireclay articles is the preparation and admixture of “ chamotte ”—a substance known in manufacturing circles as Srog> ’—which reduces the shrinkage to reasonable limits and opens out the texture of the material, rendering it of greater porosity. It is beoause contraction takes place on burning that fireclay goods are rarely absolutely consistent as regards size, for it is impossible to foretell with perfect accuracy what the shrinkage, or “ sink,” will amount to. For ordinary Stourbridge flrebricks th.e contraction on burning usually amounts to 1 inch to 1J inches per foot, the length. of a 9-inch brick being 9f inches before burning ; whereas with. gas retorts, the amount is about | inch per Exception is frequently taken to the use of the term “ aluminous clays.” The author em-ploys it merely as a convenient means of distinguishing between such clays as the well-known Stourbridge variety in contrast with silica or siliceous materials containing less than 72 per cent, of silica.