cold blast is admitted at D, and passes out heated at B. walls forming the regenerator consist of 5-inch brickwork.

The

The

being about 70 feet high and 25 feet in diameter. A domed top is now used.

The relative merits of fire-brick and iron hot-blast stoves may be seen from the following comparison drawn by Lürmann :

Fire-brick stoves may be heated to very high temperatures without interfering with their working, if the dust contained in the blast-furnace gases contains no basic constituent which has a detrimental action on the fire-brick.

The heating surface of fire-brick stoves must be considerable if the temperature of the blast is not to be altered too rapidly, and if the working is not to be interrupted too often. The cost of erection of these stoves is consequently high. cost of maintenance, on the other hand, is very low.

The

The older forms of fire-brick stoves are not easily cleaned. By rendering cleaning possible, the construction becomes more complicated, and the cost is consequently increased.

Fire-brick stoves require a skilled attendant, as the working is intermittent.

Fire-brick stoves, as compared with those of iron, possess the advantage of effecting a saving in heat by lowering the temperature at which the products of combustion enter the chimney. The quantity of coke consumed when fire-brick stoves are employed being less than when iron stoves are used, the weight of furnace gases is correspondingly affected. Sir Lowthian Bell† assumes that for 20 units of iron made there are 122 units of gas in the former against 129 units in the latter. Allowing 10 per cent. of free atmospheric air in the burnt gases as they leave the stoves, their weight may be taken at 212 and 240 units respectively per 20 units of iron produced. The two accounts may be stated as follows:

* Stahl und Eisen, 1882, p. 361.

+ Journ. Iron and Steel Inst., 1883, p. 119.

Whether heated in brick or in iron stoves, the blast is conveyed

to the furnace in mains lined with fire-brick.

CHAPTER IX.

TYPICAL METALLURGICAL PROCESSES.

Classification of Processes.-The methods that are adopted in extracting metals from their ores may conveniently be grouped in the following mannèr :—

A. DRY PROCESSES.

I. a. By Simple Fusion with Suitable Fluxes.

II. By Reduction of Oxide by Carbon.

(a) In blast furnaces 1. Copper

or, more

rarely, in 2. Lead

hearths or crucibles.

(b) In

furnace.

3. Antimony
4. Nickel

5. Iron

6. Nickel

7. Manganese

reverberatory 8. Tin.

(c) In retort furnaces. 10. Zinc

9. Bismuth.

II. Arsenic

:{

12. Sodium.

Usually after previous roasting of sulphide or arsenide;

Usually with simultaneous carburisation of the liberated metal.

Always after roasting the ore; and with volatilisation of the liberated metal.

(d) In furnaces heated 13. Aluminium.

electrically.

'III. By Concentration as Sulphide;
usually followed by reaction between sulphide and oxide,
or between sulphide and sulphate.

(a) In reverberatory 1. Copper

furnaces.

2. Lead

(b) In liquation fur- 3. Antimony

nace.

Usually after partial roasting; sometimes, in the case of lead and antimony, the removal of the sulphur and precipitation of the metals is aided by a cheaper metal, iron.

IV. By Concentration in Sulphide, or Arsenide,
and subsequent smelting as in II. or III.

V. By Concentration in Other Metals, by Fusion.

Usually in blast fur- I. Gold.

naces; but often in re

verberatory furnaces.

2. Silver.

3. Platinum.

The metal is subsequently concentrated or extracted by (1) cupellation, (2) crystallisation (Pattinson process), (3) liquation, (4) recourse to wet methods, (5) superior affinity of a third metal (zinc in Parkes' process), (6) distillation (treatment of gold amalgam).

VI. By Reduction of a Haloid Salt.

In reverberatory fur

naces or in crucibles.

1. Aluminium. ( The halogen is removed 2. Magnesium. 1 by sodium.

B. PROCESSES INVOLVING THE USE OF MERCURY.

(a) In cradles, flumes, stamp) batteries, concentrators

(b) In pans and appliances

of varied form.

1. Gold

As part of the operation of dressing the ore.

(Patio process.

Old Freiberg process.
Cazo process.

Modern pan-amalgamation.

C. WET PROCESSES.

a. Methods of Solution.

These processes are usually conducted in earthenware vessels, or cast-iron pans, or wooden vats, usually lined.