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discharged through five radial openings (f). charging is performed at the top, which is

mounted by

a chimney for regulating the draught. Hoffmann's calciner is

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the fuel, thereby effecting a considerable saving. It dif fers from a barrel furnace with regard to the direction of the gaseous current, and the stationary condition of the solid matter. The combustion is conducted in a horizontal direction in the same order as the gas. Figs. 11 and 12 represent a circular furnace of this kind in plan and section. The calcining space is circular, and is roughly divided into sixteen compartments (MM) by means of projections raised towards the roof. The whole circular space is divided by means of a wrought iron door (p). In Fig. 11 this

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door divides No. 16 from No. 1 compartment.

A lateral opening or doorway (B) is also provided for each chamber, for the purpose of discharging the calcined ore, but it is bricked up during the operation, only two being open at a time-one being charged and the other being discharged. The air for combustion is drawn through these doorways, and passes in the direction of the chambers 3, 4, 5, 6, etc. Each compartment communicates with the central gallery (C), and thus with the chimney by means of inclined flues (n), each of which is provided with a damper (d); all these are kept closed, except that connected with No. 16. Along this flue the products of combustion finally pass into an interior circular space (c), termed the smoke-flue, which communicates with the chimney. The outer wall is double, about 3 feet thick, the two portions being separated by a space filled with sand, in order to close up any cracks produced by heat in the brick-work.

The cold air, as before mentioned, passes into the calciner at Nos. 1 and 2, then circulates through 3, 4, and 5, containing ore already calcined, the air being heated at its expense. The air then arrives at No. 6 where the ore is red-hot. The fire commences here and occupies No. 7 and part of No. 8. This is the zone of greatest heat. The gases then traverse the remaining chambers, giving up their heat to the matter being calcined.

The fuel is charged in small quantities through openings in the roof, when it inflames immediately, and burns without smoke, because of the high temperature of the air employed for its combustion, and the incandescent space into which it arrives. The same openings are only used for charging every 24 hours, a fresh range being opened every 4 hours, working in the direction of the general current. Every 24 hours the operation in one chamber is completed. Then the iron partition (p) is transferred from No. 1 to No. 2, the doorway of No. 1 closed and that of No. 3 opened. The damper of No. 16 is then closed

and that of No. 1 opened. Now the contents of No. 3 are discharged and the chamber No. 2 re-filled. The operation is thus continuous, and only stopped in the case of serious repairs. This furnace is chiefly used for baking bricks and burning limestone. An oval form is also employed in some works.

QUESTIONS.

1. What impurities does pig-iron usually contain?

2. Describe the classification of pig-iron based on the character of its texture as exhibited at a fractured surface.

3. What are the leading differences between forge- and foundry-pigs, and why are they requisite ?

4. Under what furnace conditions are white, grey, and mottled pig-iron respectively produced?

5. What relations have been observed between the character of the slag and the nature of the iron produced in a blast furnace ?

6. What purpose is served in calcining iron ores? Describe some method of calcination with which you are familiar. 7. Describe Gjers' calciner for iron ores.

8. Describe the Swedish kiln employing waste gases as a source of heat.

9. Describe Hoffmann's circular calciner and the method of calcination conducted therein.

CHAPTER VII.

INDIRECT METHOD OF EXTRACTION.

Reduction in the blast-furnace.—The calcined ore is put into the blast-furnace with coal, coke, or charcoal, and a suitable flux, which is usually limestone. The heat is well utilised, and the reduction more perfect than in any other form of furnace. In the blast-furnace there are

two currents travelling in opposite directions, and constantly acting on each other-a "gaseous" ascending current and a "solid" descending one. The former travels at the rate of about three feet per second, the latter at the rate of 3 feet per hour. The effect of the blast on the carbon of the fuel is to produce carbonic acid CO2 at the level of the twyers, with evolution of great heat. This gas ascends, and is reduced by carbon at a very short distance from the twyers, thus,

CO,+C=2CO.

This carbonic oxide is the principal reducing agent in the blast-furnace, the oxide of iron being reduced to the metallic state as a spongy mass, thus—

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At the same time the flux and earthy matter of the ore unite to form slag, which descends with the iron; the latter in contact with highly heated carbon is carburised, then melts and collects in the hearth, where, in combination with other substances, such as silicon, phosphorus, sulphur, and manganese, which have also been reduced, it constitutes pig-iron. On the top of the molten iron floats the liquid slag. The temperature and pressure have a great influence on the reducing action, and as the temperature increases with the temperature and pressure of the blast, it follows that as the reducing energy becomes greater the metal is more impure. When very pure iron is desired rich hæmatite and cold-blast are used, with charcoal as fuel. Great advantage is gained by the use of hot air, as less carbon is required for reduction and fusion. It is also useful to remedy defects, and to regulate the passage of materials in the furnace. If the fusion or reduction is at fault the temperature of the blast is raised, or more carbon is added. The former acts instantly, while the latter often takes several hours to remedy the defect.

The quality of the pig-iron produced from a given

furnace will depend on the temperature, the nature of the charge, and the mode of working. With easily reducible ores and heavy burdens-that is with a large proportion of ore to fuel-the iron will be white, since the metal is kept only the minimum time in contact with incandescent carbon. With a high temperature and a light burden the pig-iron is more siliceous and grey. The same things influence the character of the slag. Blast-furnace slags are mainly double silicates of lime and alumina, and may be represented by the formula

or

3(CaO.SiO2) + Al¿O„.3SiO,

6(2CaO.SiO2) + 2Al2O3.3SiO2.

The former is the kind of slag obtained from charcoalfurnaces, and the latter from furnaces using coke or coal. In both cases the lime is replaced more or less by magnesia, oxide of iron, and oxide of manganese; while the silica is sometimes replaced to a small extent by alumina. The colour varies from white to grey, sometimes with varying shades of yellow, green, blue, and black, according to the metallic oxides present. Generally a white or grey slag accompanies grey iron, and a dark coloured slag, white iron. The former slag often contains excess of lime, which diminishes its fusibility; the latter is more fusible and contains oxide of iron, which, when present in quantity, makes a very liquid "scouring" slag, i.e. one attacking the lining of the furnace. The Scouring" slag sometimes contains as much as 20 per cent. of iron. When forge or mill cinders are added to the charge, the resulting metal is called cinder-pig-iron, and the change produced by the reduction of such slags may be represented by the following equation—

3(2FeO. SiO2) + 4C = (2FeO. 3SiO2) + 4CO + 4Fe. When phosphoric acid P2O, is present in the blastfurnace, it is reduced, and the phosphorus passes into the iron, which can be prevented by allowing much

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