Another parcel of ore might be found to be deficient in bases, and to require, say, bases to neutralise four parts of silica to every hundred parts of ore. If it contained 35 per cent. iron, its label might be

If there is any considerable proportion of ash from the coke, as is often the case, it can be entered as acid or basic, just as in the case of an ore, thus :

or

Acid, yielding 2 per cent. silica,

Basic, requiring 1 per cent. silica,

and the amount of silica for each part of iron present added (or deducted, as the case may be) to each ore in proportion to the number of parts of iron contained in it.

Any small quantity of sulphur can be considered to be one half its own amount of silica, or a special vertical scale employed, the length of its divisions being as 28: 60.

If, as is usual, several kinds of ores are to be smelted together, they should, for convenience, be subjected to a preliminary mixing, so as to reduce them to three only. Of these, one should have less, and another more iron than is required in the final charge, and one should be acid and another basic after the correction for the ash from the coke has been made. Or one of the three may be a limestone or a silicious flux; it need not necessarily contain iron.

Then let it be required to have n parts of iron per hundred of the charge, and let a,,a,,a,, be percentages of iron in the ores, and bbb, percentages of deficiency (or excess) of silica in the same, and x,y,z, the number of parts required of the component ores per hundred of the charge.

Solving these simple equations, the number of parts of each component required to satisfy the desired conditions of the charge is at once formed.

If it is desired to produce a more acid or a more basic slag, it is only necessary that the scale b be replaced by one the length of whose gradations are one half (for bi-silicate slag) or twice (for bibasic slag) that of the normal scale.

A simpler form of the rule can be made by drawing the lines AB, AC, AD, AE, as before, upon a sheet of card, and then setting out a scale of equal parts upon the datum line AB, and providing a loose strip of card, one edge of which is graduated to the same scale, taking care, in using the scale, to keep it at right angles to the line AB. This is, in fact, the equivalent of a set of Professor Balling's diagrams, reduced to one scale, and brought to

a common centre.

Mr. A. Wingham* has also devised an excellent slide rule for the same purpose as the foregoing appliance, and by it the additions, as well as the multiplications, are mechanically per formed, provision being made for all the ordinary constituents of slags.

Classification of Metallurgical Processes.-Metals may be extracted from their ores by the following methods:

1. By liquation-that is, a separation of the easily fusible metals or metallic compounds from the infusible ones, or the gangue, at a low temperature, by taking advantage of the different melting points. In this way, bismuth and sulphide of anti

* Journ. Iron and Steel Inst. part i. 1892, p. 233.

mony are obtained, and argentiferous lead is separated from

copper.

2. By distillation and sublimation—that is, heating the ore, until the metal is driven off in the form of vapour, and condensed in a liquid or solid state. Mercury is extracted from its ore by distillation, and arsenic is obtained by sublimation.

3. By reduction of metallic oxides at a high temperature. In the case of metals possessing a slight affinity for oxygen, the reduction may be effected by merely heating the oxide, whereby it splits up into metal and oxygen. The reduction is usually effected by heating the oxides with carbon or other substances possessing a greater affinity for oxygen than is possessed by the metal. In addition to solid carbon the substances used are carbonic oxide, marsh-gas, and hydrogen, as well as sulphur in the reverberatory-furnace processes for extracting copper and lead. The reactions are shown by the following equations:

4. By decomposing metallic sulphides by means of iron at a high temperature. Lead and antimony are extracted from their ores in this way, the equation in the case of lead being

5. By extraction by means of molten lead. The precious metals are extracted in this manner.

(Ag + Cu) + Pb

=

(Pb + Ag) + Cu.

6. By extraction by means of molten zinc, as in the extraction of silver and gold from lead by the Parkes process.

7. By extraction by means of mercury in the cold. This process is known as amalgamation. It is used for extracting silver and gold from their ores, the mercury being subsequently driven off by heat.

8. By extraction in the wet way and subsequent precipitation, as metal or as oxide or sulphide. In some cases the com pound is dissolved in acids, as in the separation of silver from gold, and in the extraction of copper and nickel. In other cases,

the metal is first converted into a chloride, which is dissolved in a solution of salt or of sodium hyposulphite, as in the extraction of silver by Augustin's process, or the metal is converted into a sulphate, which is dissolved in water, as in Ziervogel's process. The extraction may also be effected by treating oxides with salts, as in the Hunt and Douglas method of extracting copper. 3CuO 2FeCl, Fe,O,+ CuCl + Cu,Cl,

=

9. By electrolysis, as in the deposition of copper.

10. By the action of highly oxidisable substances on metallic salts, as in the precipitation of gold from its solution as chloride by ferrous sulphate, or of aluminium from its chlorides by sodium.

11. By crystallisation, as in Pattinson's method of extracting silver.

Roasting and Calcination.-These terms are frequently used indiscriminately to indicate the operations of expelling water, carbonic anhydride, or sulphur from the ore, by heating the substance to a temperature below its melting point. The term calcination should be confined to the application of heat in the expulsion of carbonic anhydride or volatile matters, whilst the term roasting should be used in all cases where a chemical change is involved, resulting in the addition of an element.

Roasting is one of the most important of metallurgical processes. In the treatment of almost all ores it is required as a preliminary process to render them more porous and more suitable for the subsequent smelting, the success of which is dependent on the manner in which the roasting has been effected.

There are several kinds of roasting. An oxidising roasting is one in which the metalliferous substance is oxidised by heating it in contact with air. A reducing roasting consists in heating the substance in a reducing atmosphere. When it is required to obtain chlorides instead of oxides, the material is roasted with salt. This operation is termed a chloridising roasting. It is especially important in the treatment of silver ores. Under the action of a high temperature the salt acts on the sulphates present, and forms chlorides. Its action may also be due to the formation of chlorine by the action of silica and sulphuric anhydride on the salt, or to the evolution of hydrochloric acid, due to the presence of moisture in the atmosphere in which the material is being roasted. In some cases, as a preliminary operation for hydrometallurgical processes, it is necessary to conduct the roasting in such a manner that sulphates are formed. It necessitates a low

temperature and a limited supply of air. This operation is termed sulphating roasting.

Oxidising Agents.-Atmospheric air is the simplest source of oxygen. The carbonic anhydride it, contains may be neglected. Attention, however, must be paid to the aqueous vapour. Air, at the ordinary temperature, contains an amount of moisture equal to 0.0062 of the weight of dry air, or since water contains ths of its weight of oxygen, while dry air contains less than a quarter, it follows that the moister the air is the more oxidising it will be. The cooling action of water-vapour, however, counteracts the advantage derived from its excess of oxygen. Moist air is consequently useful only when the hydrogen it contains is set at liberty during the reactions and is free to combine with sulphur or certain other metalloids. Air is employed at the ordinary pressure or at the pressure of several atmospheres, for the temperature of combustion increases with the pressure.

Water is an active oxidising agent in the case of certain metals which have a strong affinity for oxygen; but in other cases its oxidising action is sensible only at a high temperature. The decomposition of water absorbs much heat, and aqueous vapour can therefore be employed only as an oxidising agent when its cooling effect is desirable, as in the refining of iron. Aqueous vapour is also used for separating zinc from lead. In passing steam over sulphides of oxidisable metals at a red heat the sulphur is eliminated as sulphuretted hydrogen, while the metal is oxidised. This reaction is employed in the roasting of sulphides of iron, and in the treatment of sulphides, arseno-sulphides and antimoniosulphides of copper and silver.

Metallic oxides are frequently used as oxidising agents. Oxides of manganese, iron and lead are those most generally employed. Oxide of copper is used more rarely. Manganese peroxide is occasionally useful in the oxidation of impurities in metals. Ferrous oxide, or ferrous silicate, acts as a carrier of atmospheric oxygen. When ferrous silicate, containing less than 30 per cent. of silica, is exposed in a molten state to the action of air, the iron in excess gradually becomes oxidised, and crystals of magnetic oxide are formed. These render the silicate less fusible, and fall by their superior density to the bottom of the mass. Sometimes, as when such a slag covers a bath of impure metal, the magnetic oxide thus formed gives up its excess of oxygen to the elements in the metal to be refined, and again becomes ferrous oxide, and thus acts as a carrier of atmospheric oxygen to the carbon which has to be oxidised. An illustration of this is afforded by a stage