198
MODERN GASWORKS PRACTICE
(6) Bonnybridge (Type B).
Silica.
Alumina
Ferrio oxide
Lime
Titanic oxide Magnesia . Alkalis
61-10 per cent.
33-64 „
2-40 „
072 „
0-89 „
0-80 „
0-45 „
The following is an analysis of typical German retort material:—
Silica. Alumina Oxide of iron Lime Magnesia . Alkalis
• 71'48 per cent.
• 24-79 „
• 2-08 „
• 0-28 „
• 0-61 „
• 0-76 „ .
SILICA AS A REFRACTORY
A valuable report indicating the character and extent of refractory material deposita in the United Kingdom has been issued by the Geological Survey.
The Yorkshire deposits yield the best varieties of ganisters, and there are not only ampie reserves in the areas at present worked near Sheffield, but other deposits which are still untouclied. Sheffield may indeed be regarded as the centre of the silica brick industry, and Leeds, Huddersfield, and Halifax of firebricks. Ganister is also obtained from the fireclays of the millstone grit of Lanarkshire and Stirling-shire, and it seems likely that new deposits which have been opened up near the base of the limestone series along the bordets of Edinburghshire may also yield refractory materials of high quality, although. it is doubtful whether any of these are likely to prove equal to the best English ganister.
Other materials have been investigated, includmg quartzites, sands, and siliceous clays. All occur as beds in the series of sedimentary formations and exist in great quantity and variety, several of th.em in one part or another of the carboniferous system. With. the exception of dolomite tliey all fall within the dass of acid refractories.
At the present time in the gas industry the tendency towards the use of silica or siliceous materials is gradually asserting itself, and it is to be noted that in American gas un.dertakin.gs silica is almost universally used in the construction of retort settings.
THE INVERSIONS OF SILICA
In studying the use and vagaries of silica it is important to note that it is presumed to exist in, probably, seven crystalline modifications. The most important of these are a-quartz and /3-quartz, various forms of tridymite, and cristobalite. The ability of th.e material to transform at different temperatures into one or other of the above modifications is closely associated with. its behaviour in practice. The ordinary silica material is generally understood to contain a proportion of silica amonnting to over 92 per cent., although many of the well-known varieties such as Dinas and