Page:Encyclopædia Britannica, Ninth Edition, v. 16.djvu/70

Rh 60 METALLURGY funnels sift out the pyrites and the blende respectively, so that almost nothing but dross runs off ultimately. The apparatus is said to do its work with a wonderful degree of precision, and of course is susceptible of wider application, but it ceases to work when the raw material is a slime so fine that the particles fall too slowly. Modes of Producing High Temperatures. Most of what is to be said on this topic has already been anticipated in the articles FUEL, FURNACE, and BELLOWS ; but a few notes may be added on specially metallurgic points. Furnace Materials. In a metallurgic furnace the working parts at least must be made of special materials capable of withstanding the very high temperatures to which they are exposed and the action of the fluxes which may be used. No practically available material fully meets both requirements, but there is no lack of merely fire-proof substances. Of native stones, a pure quartzose sandstone, free from marl, may be named as being well adapted for the generality of structures; but such sandstone, or indeed any kind of fire-proof stone, is not always at hand. What is more readily procured, and consequently more widely used, is refractory brick, made from &quot;fire-clay.&quot; The characteristic chemical feature of fire-clays is that in them the clay proper (always some kind of hydrated silicate of alumina) is associated with only small proportions of lime, magnesia, ferrous oxide, or other protoxides. If the percentage of these goes beyond certain limits, the bricks, when strongly heated, melt down into a slag. The presence of free silica, on the other hand, adds to their refrac toriness. In fact the best fire-bricks in existence are the so-called Dinas bricks, which consist substantially of silica, contaminated only with just enough of bases to cause it to frit together on being baked. Dinas bricks, however, on account of their high price, are reserved for special cases involving exceptionally high temperatures. Amongst ordinary fire-bricks those from Stourbridge enjoy the highest reputation. It follows from what has just been said that, in a metallurgic furnace, lime-mortar cannot be used as a cement, but must be replaced by fire-clay paste. In the construction of cupels, reverberatory furnaces, &c., only the general groundwork is, as a rule, made of built bricks, and this froundwork is coated over with some kind of special fire-proof and ux-proof material, such as bone-ash, a mixture of baked fire-clay and cokes or graphite, or of quartz and very highly silicated slags, &c. These beddings are put on in a loose powdery form, and then stamped fast. They offer the advantage that, when worn out, they are easily removed and renewed. The powerful draught which a metallurgic fire needs can be produced by a chimney, where the fuel forms a relatively shallow layer spread over a large grating; but, when closely -packed deep masses of fuel or fuel and ore have to be kept ablaze, a blast becomes indispensable. Chimneys. The efficiency of a chimney is measured by the velocity V with which the air ascends through it, multiplied by its section ; and the former is in roughly approximate accordance with the formula where h stands for the height of the chimney, g for the acceleration of gravity (32 2 feet per second), and T and T for the absolute temperatures (meaning the temperatures counted from - 273 C.) of the air within and the air without the chimney respectively, while k is a factor meant to account for the resistances which the air, in its progress through the furnace, &c. , has to overcome. In practice T is taken as the mean temperature of the chimney gases, which theoretically is not unobjectionable ; but the weakest point in the formula is the smallness and utter inconstancy of the factor k, which, according to Peclet, generally assumes some value of the power, f, &c. Yet the formula is of some use as enabling one to see the way in which V depends on h and (T - T )/T con jointly, to see, for instance, how deficient chimney height may be compensated for by an increase of temperature in the chimney gases, and vice versa. Sloioing- Machines. Of the several kinds of blowers described under BELLOWS (q.v.), the &quot; fans &quot; are the best means for producing large volumes of wind of relatively small but steady pressure ; &quot;bellows&quot; are indicated in the case of work on a relatively small scale requiring moderate wind pressure; while the &quot;cylinder blast&quot; comes in where large masses of high-pressure wind are required. Two highly interesting blowing-machines, however, are omitted in that article, which may be shortly described here. _ The &quot; Water Blast &quot; (Wassertrommelgeblase) is interesting historically, having been used metallurgically in Hungary for many centuries. A mass of water, stored up in a reservoir, is made to fall down continuously through a high narrow vertical shaft having air-holes at its upper end. The vertical column of water sucks in air through these holes and carries it down with it into a kind of inverted tub standing in a reservoir kept at a constant level. Air and water there separate, the former flowing away through a pipe into a wind-box, from which it is led to its destina tion. The &quot;Cagniardelle&quot; (figs. 5, 6), so called from its inventor Cagniard Latour, also utilizes water to carry air, but in quite another way. By means of a round shaft passing through its axis, a cylindrical drum of sheet-metal is suspended slantingly in a mass of water, so that the lower end is fully immersed, while of the upper end the segment above the upper side of the shaft is uncovered. The space between shaft and drum is converted into a very wide screw-shaped canal by a band of sheet-metal hermetically fixed edgeways to the two. Both the top and the bottom end of the drum are partially closed by flat Fia. 5. Cagniardelle. bottoms soldered or riveted to the respective edges ; the lower one leaves a ring-shaped opening between its edge and the shaft, which serves for the introduction of a fixed air-pipe bent so as to reach up to near the top of the drum s air-space ; in the upper bottom three quadrants are closed, the fourth is open. Supposing the screw-canal, traced from below, to go from the left to the right, the drum is made to revolve in the same.sense, and the effect is that, in each revolution, the screw-canal at its top end swallows a certain volume of air which, by the succeeding entrance of the water which, of course, moves relatively to the screw is pushed towards and ultimately into the air-space at the bottom end. The Cagniardelle yields a perfectly continu ous blast, and, as it is not encumbered with any dead resistances except the friction of the shaft against its bearings (which can be reduced to very little) and the very slight friction of the water against the screw-canal, it utilizes a very large percentage of the energy spent on it. This percentage, accord ing to experiments by Schwamkrug, amounts to from 75 to 84 - 5 ; in the case of the cylinder-blast it is 60 to 65 per cent. ; with bellows, about 40 per cent. ; with the &quot;Wassertrommelgeblase&quot; 10 to 15 per cent. Hence the &quot; Wassertrommelgeblase &quot; stands last in relative efficiency ; but we must not forget that it alone directly utilizes native energy, while, in the cylinder blast, for example, 100 units of work done by the steam-engine involve a vastly greater energy spent on the engine as heat. To maintain a desired temperature in a given furnace charged in a certain manner, the introduction of a certain volume of air per unit of time is necessary. But this quantity, in a given blowing- machine, is determined by the over-pressure of the wind, as measured by a manometer, the velocity of the wind being approxi mately proportional to /M/(B + M), where M stands for the height of the mercury-manometer, and B for that of the barometer. Hence the practical metallurgist, in adjusting his blast, has nothing to do but to see that the manometer shows the reading which, by previous trials, has been proved to yield an adequate supply of wind. Fuel. In some isolated cases the ore itself, by its combustion, supplies the necessary heat for the operation to be performed upon it. Thus, for instance, the roasting of blackband iron-stone is effected by simply piling up the ore and setting fire to it, so that the ore is at the same time its own furnace and fuel ; in the Bessemer process of steel-making, the burning carbon of the pig- iron supplies the heat necessary for its own combustion ; and a similar process has been tried experimentally, and not without success, for the working up of certain kinds of pyrites. But, as a rule, the high temperatures required for the working of ores are pro duced by the combustion of extraneous fuel, such as wood, wood- charcoal, coal, coke. Of these four, wood-charcoal is of the widest applicability, but not much used in Britain on account of its high price. High-class coke or pure anthracite, volume for volume,