COAL-TAR. PRODUCTS.
In the distillation of coal for the production of illuminating gas, there passes over with the gases into the condensers, a variety of solid and liquid substances, the former being dissolved or suspended in the latter, and the whole forming a thick or unctuous mixture. When the mass is allowcfl to settle, it separates into two distinct layers: generally, the upper portion consiats of water, holding in solution ammonia and ammoniacal salta, with more or less of the soluble con stituenta of the oily portion of the distillate; the lower portion is a mixture principally of oily hydrocarbons, and forma the article known as " coal-tar " or "gas-tar." The mean average yield per ton of coal carbonized for gas in the various London gasworks, in 1878, is stated as follows:—Coke, 34•12 bush. ; breeze, 3•17 bush.; tar, 10-40 gall.; ammoniacal liquor, 29.44 ; gas, 10,183 ft.
The composition of tar varies with the description of coal submitted to distillation, and with the degree of heat employed ; though, probably, the degree of refrigeration to which the gas itself is submitted, after leaving the hydraulic main, haa more to do with enriching tbe tar than all the other eonditions combined. The specific gravity of tar ranges from 0-95 to 1•25 ; that of samples containing much naphthalene will fluctuate between 1•10 and 1-25 : and such tars, being heavier than the " ammonia liquor," will sink in tbe tanks; the density of tars obtained from Scotch cannel coal approximates that of water, being sometimes a little above it, at others a little below—in such tars the naphthalene is replaced by paraffin.
London tar is always heavier than country tar ; high heat inereasea the quantity of naphtbslette. The larger gas companies generally dispose of their tar by contract; and, though there are most important differences in the quality and quantity of the distillate from taro of variou3 origin; and produced under varying conditions of temperature, the distiller is nnable to exercise any control over the character of the tar supplied to him. The tar is fried from the ammonia liquor, as far as is practicable, by allowing it to settle. It is then removcd from the gasworks, in tanka fitted to
railway trucks, or waggons, or in iron barges. On arrival at the works, the tar is pumpcd into large tanks of masonry or iron, sunk into the ground. Here it is left to settle for some days, to enable the watery portion to separate out. When the tar is very thick, tbe separation of the watery portion is facilitated by means of heat; through coils of copper or iron pipe placed in the tanks; steam or hot water is passed, until the tar is rendered fairly fluid. Over-heating should be avoided, 043 it might drive off the more valuable conatituents of the tar. Too much care cannot be taken to free the tar from water, eapeeially if the tar happen to be very thick, as is always the ease in cold weather. The presence of water causes the tar to froth when being distilled, so that it boils over through the worms ; it may even lift the still head, when contact with tho firea will produce disastrous results. Tars which approximate tbe sp. gr. of water are not easily dehydrated ; their distillation ia best conducted on the Scotch or French aystems. The inflammability- of the tar pro ducts necessitates every precaution against fire. The tanks in which the liquida are collected are placed as far as possible from the furnaces, and are kept securely covered. As fires occur most frequently at the stilla, it is best to if3olate them from the general plant altogether ; and, since it is impossible to extinguish burning tar, it is a good plan to supply each still with an outlet, at or near the bottom, communicating with an empty boiler or still, so as to run off the tar from the burning matter as quickly poaaible. By taking this precaution, the opening of the outlet relieves the pressure due to frothing, and renders an accident from thia cause much lees liable to occur. There are three methods of treating tar—English, Scotch, and French ; it will be necessary only to give those points of difference which render one method better suited than another in particulitr cases.