Modern Gasworks Practice

218 MODERN GASWORKS PRACTICE The decomposition of the salts in coals commences at about 900° C., th.e acict radicle usually combiiiing with the ammoaia in the gas to give ammonium Chloride, etc. The salt vapour appears to penetrate into the body of the brick, passing from the cooler face at the interior of the oven to the hotter external face, when it reaches. a zone where the temperature is sufficiently high for interaction to take place with the refractory. The result is the formation of a spongy layer, which, as it disin-tegrates, opens the way for an attack on. the remainder of the wall. The decomposition of the original salts is followed by the formation of alkaline alumino sili-cates, which may appear at first as a glaze on the surface of the brick. Cobb 1 has. pointed out that the refractory is destroyed, not by the removal of successive thin layers by fusion, but a much more drastic process of breaking ofE of layers of con-siderable dimensions. As a consequence of Chemical alterations in that part of the refractory into which the salt has penetrated, there is an accompanying physical change in the coefficient of expansion which may have considerable inflnenne, As to the means of preventing or curtailing corrosion, there are two directions. from which the problem can be attacked, namely :— (®) By attention to the coal before carbonization, particularly the process of washing. In general, too, the drier coals appear to be followed by far less serious. effects than result from the employment of coals containing from 10 to 15 per cent, of moisture. (b) By attention to the Chemical and pBysical properties of the refractories. employed. As regards the latter consideration, it may be said in general that aluminous. refractories are far less prone to attack than are those made of silica. Of primary importance, however, is the question of texture, for with. an open-grained surface the vapours are afforded ampie opportunity for penetration, thus from the salt-stand-point a perfectly non-porous body would provide the ideal. Joints between the bricks are again of importance, and as these provide facilities for penetration, they should be as few as possible in number and as fine as it is practicable to maka them. Coating the interior surfaces with. an impermeable glaze has been suggested as a remedy, but in practice many difficulties arise ; firstly, owing to the faet that. the glaze is quickly removed by the abrasion of the moving charge and ram-head. during discharging ; and, secondly, owing to the faet that cohesion of the glaze is rendered doubtful by reason of the alternate contraction and expansion of the face of the oven. Observations made at a coke-oven works showed that a proportion of 0-143 per cent, of salt in the water employed for washing coals ruins a retort in three years, while one-quarter of this amount renders a refractory substance of little value after five years’ work. Fortunately, the majority of coals in general use in gasworks contain. only small quantities of saline impurities, and the corrosive effeet, accordingly, is not. particularly marked. 1 Coke-Oven Managers’ Assoen., March, 1916.