The large proportion of nitrogen is brought into the furnace in the blast along with the oxygen, but while the latter performs an essential part of the work in the furnace, the former passes through unchanged, and abstracts a valuable proportion of the heat, without contributing to its production. The amount of oxygen escaping is greater than that introduced by the blast, which increase is obtained from the solid materials in the furnace. The hydrogen, both the free, and that in combination with carbon, is derived from the decomposition of the water vapour of the air. The carbon is chiefly derived from the fuel. By withdrawing the gases at different levels, it is found that the proportion of carbonic acid to carbonic oxide diminishes progressively down the furnace, and that in the upper part of the hearth they consist almost entirely of nitrogen and carbonic oxide. The waste gases carry over with them a considerable amount of dust of a non-combustible nature, which greatly diminishes the heating power, so that the practice of washing the gases by passing them through a water trough, before going to the stoves and boilers, is often adopted.

Scaffolding, bears, etc. Sometimes the charge of a furnace is obstructed in its descent, the lower portion being melted and withdrawn leaves a "scaffold," which, with the increasing weight from above, often gives way suddenly, and falls into the hearth. This is called a "slip," and deranges the working of the furnace.

In some cases large ferruginous masses, called "bears," are formed in the hearth of a blast-furnace, often containing iron, copper, silicon, graphite, manganese, nickel, cobalt, and cyano-nitride of titanium, a compound resembling copper in colour.

QUESTIONS.

1. Give a sketch in vertical section of a modern blast-furnace with a closed top.

2. Explain the working of a blast-furnace for smelting iron, more especially in regard to chemical changes as affecting carbon and phosphorus.

3. State the chief chemical changes taking place inside a blast-furnace using clay-ironstone and hot-blast, with limestone as flux, and coke as fuel.

4. In a blast-furnace it is impossible to make the fuel do its full duty. Why is this?

5. Why is more fuel required to produce Bessemer pig- than white pig-iron from a given ore, under constant conditions as to furnace, etc.?

6. How would you regulate the working of a blast-furnace so as to obtain varieties of pig-iron suitable for (1) casting, (2) for conversion into malleable iron by the puddling process?

7. Enumerate the different fluxes employed in smelting iron and state for which class of ore each is specially suitable.

8. Discuss the merits of charcoal, coke, and coal as fuel for smelting iron in the blast-furnace.

9. State the physical properties of ferro-manganese and how it is made on the large scale.

10. How would you bring into working order a blast-furnace in which the charge had become "chilled," that is, solidified in the neighbourhood of the twyers?

11. What are the relative advantages and disadvantages of "hot-" and "cold-"blast as applied to a blast-furnace for smelting iron?

12. Scaffolds are sometimes formed in a blast furnace, to what causes are they attributed and how may they be avoided? 13. What advantages have followed from increasing the height of blast-furnaces in recent years?

14. Sketch and describe the Rachette furnace.

15. Describe the Cowper-stove and explain how it differs from that of Whitwell.

16. Mention some methods of utilising blast-furnace slag. 17. Give the composition of the gases from the top of a modern iron smelting blast-furnace.

18. Describe different modes of closing the top of a blastfurnace for the purpose of collecting the waste gases.

F

19. Describe some form of hot-blast twyer.

20. Describe some form of lift in common use for raising materials to the top of a blast-furnace.

21. Describe the blowing-cylinder used for producing compressed air for a modern blast-furnace. Why is a regulator sometimes required?

22. What object is gained by washing blast-furnace gases?

CHAPTER VIII.

REFINING PIG-IRON.

Two distinct methods have been adopted for the conversion of pig-iron into wrought-iron, depending on the kind of furnace employed, viz., the open-fire or hearth-finery, and the puddling-furnace, which is of the reverberatory type. The chemical reactions are similar in both cases, being based on the oxidation of the impurities by oxygen, both in the gaseous and in the combined form.

Open-fire or finery.-In former times this hearth. was termed a "bloomery" from the nature of the product which was called a bloom. The hearth is rectangular,

and formed of cast-iron plates lined with charcoal, the bottom being exposed to a current of air in order to keep it cool. Three sides are vertical while the remaining side slopes a little outwards. In the simplest or German forge the flames escape into an open hood, but in the Swedish forge the hearth is covered with a cylindrical roof, and communicates with. the chimney by a lateral flue. The fuel employed is charcoal, the fire being blown by a blast of air issuing from a single twyer. A view of the finery in vertical section is given in Fig. 31.

Fig. 31.

In the open-fire the iron is placed in contact with the fuel. Now this intermixture is objectionable, because the refining is based on oxidation, and the fuel acts as a reducer, so that which the air oxidises, the fuel tends to deoxidise, the purification being retarded thereby. The consumption of fuel is also very great. If the iron is designed to retain some carbon, so as to produce a steely product, then the open-fire is admissible.

The charge of pig-iron, usually in broad thin plates, is placed on a bed of glowing charcoal, which reaches to the level of the twyers. The operation of refining is hastened by the addition of hammer scale and rich slags, which assist in oxidising the impurities. The fusion of the iron is allowed to take place gradually, so as to expose it for a lengthened period to the oxygen of the blast. At the moment of fusion the foreign elements are rapidly oxidised. The silica unites with the oxide of manganese and oxide of iron to form a fusible slag, which, in consequence of the excess of metallic oxides, is called a basic silicate (3FeO.SiO2).

The ferrous oxide FeO, acts as a vehicle for oxygen, absorbing the gas, and being converted into the black oxide FeO. The latter coming in contact with the impure iron, oxidises the impurities, itself being again reduced to FeO. After time the slag becomes neutral, and is in part removed; then fresh basic slag and hammer scale are added. To complete the operation, the masses of iron are lifted up to the twyer level in order to completely oxidise the combined carbon. The white-hot mass of iron, which is now of a spongy texture, is then taken to a heavy hammer and compressed to a slab, termed a "bloom." The subsequent treatment of the bloom varies in different districts. In Italy the partially refined mass is removed from the hearth and cooled with water; then broken up, and re-heated sufficiently to cause the iron and slag to cake together, when it is again removed. In the third stage the iron

is re-heated with rich slags until it is sufficiently refined.

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In South Wales a superior quality of iron sheets for tin-plates is made from the best pig-iron in a charcoalfinery. The pig-iron is first partially refined in a hearth termed a refinery," Fig. 32, which is described on the next page. The charge of 2 to 3 cwts. of refined iron from a coke "refinery" produces a finery ball, weighing about 2 cwts., which is shingled and drawn out to a long bar, 2 inches thick, under a leverhammer. The bar is then nicked and broken into pieces, the best pieces being selected, and made into small bundles or piles. These bundles are re-heated in the

flame of a coke-fire, in a special furnace known as the "hollow-fire," the upper part of which forms a chamber in which the piles are re-heated. The piles are then welded under a hammer and rolled into sheets. Before finishing, the sheets are annealed, pickled in sulphuric acid, and then rolled. cold. Finery-slag is highly basic, containing upwards of 75 per cent. of ferrous oxide FeO.

Fig. 32.

The Swedish-Lancashire finery is arched over at the top, and communicates with the chimney by a horizontal flue, in which the pig-iron undergoes a preliminary heating.

REFINING IN THE REFINERY.

For the better qualities of wrought iron, crude pigiron was formerly submitted to a preliminary operation in a rectangular hearth, termed a refinery, Fig. 32, with