THE USEFUL METALS AND THEIR ALLOYS
MINING VENTILATION, MINING JURISPRUDENCE
AND METALLUEGIC CHEMISTRY
The working of the apparatus is extremely simple. The admission-valve is opened, and the blast passing through the pipe, presses against the closed end of the tube, with a force proportional to its density, and the horizontal area exposed to its action. Assuming the tube to be 5 feet in its interior diameter, and the pressure of the blast 3 Ibs. to the square inch, it will rise with a force of 8481 Ibs. On arriving at the required elevation, the further admission of blast is shut off, and by its elasticity, the portion in the interior buoys up the tube until the barrows ore emptied into the furnace, when the descent is accomplished, by opening, more or less, the outlet-valve on the tube ; the escape of the confined blast, by diminishing the resistance, allows it to
descend. The rate of descent is regulated at pleasure by the escape valve, which is made of such a size, relatively to the tube, that when full open, the uninterrupted flow of blast will not be sufficient to produce an injurious velocity.
The " blast" for urging the blast-furnace is commonly generated by powerful steam-engines, working metal cylinders, fitted with the requisite pistons and valves for the admission of the air and delivery of the blast. In the United States, and some parts of Germany, wooden blowing-machines, driven by water-power, are very general, though greatly inferior in point of efficiency to the steam-engine and metal cylinder. Water-power was formerly much employed in England ; but the frequent deficiency of water, in summer, has caused it to be superseded by steam, even in the most favorable localities. The simplest form of engine, with due regard to regularity of working, is the beam high-pressure, having a heavy fly-wheel to carry it over center. The proportions of the parts belonging to the steam-end are made considerably heavier and stronger than for ordinary steam engines, with a view of removing all risk of accident from weakness : the blowing cylinder is usually about twice the area
of the steam cylinder. With these proportions, and high-pressure steam, the engine will be perfectly able to compress the blast to a pressure of (i or 7 Ibs. to the square inch, though in practice 3 lbs. is a more common pressure. In erecting a blowing-engine, however, it is best to have a surplus of power on the steam side, in case a high pressure of blast should at any time seem advisable.
The size of the blowing-engine will depend on the number of blast-furnaces in operation, and the quantity of iron which it is sought to obtain from them. If the furnaces are small, and working on gray crude iron, a blowing-engine of the largest class, with a twelve-feet blowing-cylinder, will suffice for ten or twelve ; with larger furnaces, working on white crude iron, the same engine will suffice for seven or eight furnaces, and the usual complement of blast refineries. To blow so many, however, it is necessary that the engine should be driven at a minimum speed of 400 feet per minute, and be furnished with sufficiently large mains, and numerous and sufficiently large blast and delivery valves. "
Where quantity is sought irrespective of quality, a modern invention for Elevation of High-pressure Blowing-engine. heating the blast is very generally adopted. Over a fire-place a series of cast-iron pipes of an arched form are mounted on two horizontal pipes, in the one of which cold air enters, and, passing through the arched pipes, absorbs from the heated metal a large accession of temperature before entering the furnace By placing over the fire-place, the requisite number of arched pipes, a sufficiently large surface is exposed to the action of the fire underneath, to heat the blast, in its rapid passage, to a temperature equal to the melting-point of zinc. In the earlier furnaces to which the invention was applied, a large pipe was exposed to the fire, several yards in length ; but the low temperature to which the blast was heated led ultimately to the adoption of a number of smaller pipes of a sectional form, offering a large surface to the heat ; through these pipes the blast is forced in a number of thin streams. With this provision, from the rapid rate at which the blast is propelled, namely, from 3000 to 6000 feet per minute, or 36 to 70 miles per hour; it is apparent that atmospheric air is capable of receiving an accession of nearly 700 degrees of temperature in less than one second of timea rapidity perfectly marvelous, compared with the general slowness of natural phenomena.
The arrangements followed in the production of crude iron with the high- blast furnace, consist in filling into the throat at top ; ore, fuel, and flux, and supplying blast through the tuyere orifices at bottom. The combustion of the fuel generates a sufficiently high temperature, to fuse the adjacent pieces of ore, and flux, thus liberating metallic iron, which descends into the lower hearth, from whence it is allowed to flow away through the tapping-hole at stated tunes in the day. The lighter portions of the liquid matter, comprising the cinder, floats on the metal, and flows over a notch in the dam-stone into a suitable receptacle, so as to solidify, previous to removal. The attendants at top have to fill regularly and evenly, overall the material in the furnace, as fast as it descends by fusion ; while those at the bottom are occupied in withdrawing the metal, attending to the cinder, and clearing the tuyere orifices, so that no obstruction may be opposed to the entrance of the blast.