SMELTING FURNACES FIRED WITH OIL.
Many difficulties attend the use of coal or coke in reverberatory and roasting furnaces used for smelt ing and refining metal ores. If coking coal is used, the volatile matter is quickly driven off, leaving a heavy bed of slow-burning coke. Ordinary coal reduces the atmosphere of the furnace at each charge, producing a period of incomplete combustion, during which no sulphur can be oxidized by the oxygen of the air. This feature is particularly undesirable in reverberatory furnaces in which copper ores are treated. If pulver ized coal is used much difficulty is encountered in reg ulating the proper amount of air. Trouble is also caused by the settling of ash and unburned fuel on top of the charge and by the clogging of the flues.
The use of fuel oil obviates all these difficulties, for when the fires are regulated, the flame is clear and steady, the furnace temperature even, and there is no smoke or soot. This desirable condition may be main tained for weeks, allowing the sulphur in the ores to combine with the oxygen of the air at all times. Other advantages accruing from the use of oil fuel in rever beratory furnaces are its cleanliness, low cost of hand ling, freedom from sulphur, and the increased capacity of production made possible by its use.
The following table shows a record performance at the Steptoe plant, McGill, Nevada, on December 17, 1911: In addition to these many inherent advantages, great economy can be attained by the use of a waste heat boiler in conjunction with oil firing, savings of as much as 50 per cent of the fuel having been recorded by this device.
Fig. 67 shows the plan of such a furnace at Can anea, Mexico ; the records of a typical run in this fur nace show that during a period in which 8140 tons of material were handled, 7003 bbl. of oil were used, of which only 3901 bbl. were chargeable to smelting, the remaining 3102 bbl. being recovered in equivalent steam.
The attached table shows typical operating con ditions of two reverberatory furnaces : The use of fuel oil in blast furnaces presents much more serious difficulties. In the process of smelt
ing with these furnaces, solid carbon plays a very im portant part, both mechanically and chemically. The lumps of coke used serve to keep the charge from be coming too densely packed, thus permitting the hot gases to penetrate more freely ; and the incandescent carbon performs certain chemical functions which need not be discussed here. Thus coke or charcoal cannot be entirely replaced by oil fuel in the blast furnace; and while a few experiments have been conducted, with economical results, in which oil and coke have been used together, much difficulty has been encountered in the slagging of the furnace tuyeres.
These tests were carried out without radical changes in the construction of the furnace ; coke was used in the ordinary way, and the oil was injected at the tuyeres with a hot blast. Other tests have been made with specially constructed furnaces, in which the oil is supposed to be converted into a gas at the mouth of the tuyeres by means of elaborate atomizers, the combustion taking place in the channels leading to the tuyeres. These experiments were carried on with high gravity oils ; and while the results were sat isfactory from the chemist's standpoint, the expense was too great to make the method commercially prac ticable.
Another difficulty in the way of using oil as a fuel for blast furnaces lies in the fact that if the burners are placed in the bottom of the shaft, the burner tips will soon be melted off. The operation at the start may be perfectly satisfactory ; but as the temperature increases the zone of fusion is lowered, the molten metal cannot withstand the great weight of material in the shaft above it, with the result that it is forced down unitl in contact with the burner tips, with the result that the latter are burned off. Even if this extreme difficulty does not arise, the sagging of the charge is soon great enough to close the air channels upon which the oil depends.