There is still some divergence of opinion whether flotation phenomena are the result of purely "surface tension" forces or whether electrical effects play the most important part. The operations consist of three steps which may, however, overlap and merge into one another. The first is the "oiling" process and consists in adding to the wet pulp or mixture of water and finely-ground ore some substance of a more or less oily nature. This may be either an essential oil—oil of eucalyptus is largely used—or one of a great range of organic substances The choice of the most suitable "oiling" reagent for a given kind of ore is a matter of great importance and often requires extensive pre liminary investigation. Only a very small amount of the "oiling" reagent is used. Its effect is to produce some change in the sur faces of those minerals having a metallic or semi-metallic char acter. Zinc-blende is a typical example of such a mineral. The surface of the mineral thus "oiled" becomes less easily "wetted" by the water of the pulp. The gangue of the ore, on the other hand, is either not affected at all or affected to a much smaller extent than the mineral, and this brings about the different be haviour of the two kinds of material towards the froth bubbles. In some cases, however, it is necessary artificially to increase the difference in this respect between ore and gangue by the intro duction into the pulp of some reagent which renders the gangue particles more readily wetted by the liquor. These "gangue modi fiers" are usually either mineral acids or alkalies.
The next step consists in adding to the pulp some substance which assists in the formation of a stiff and lasting froth when the liquid is aerated. The introduction of air may be brought about either by simply agitating the liquid or by blowing air through it. Frequently the "oiling reagent" serves also as froth producer. When the pulp thus prepared is treated so as to pro duce a froth, the oiled mineral particles adhere very strongly to the froth bubbles. The combined buoyancy of the air-bubble with its mineral burden is sufficient to cause it to float to the surface, where the bubbles accumulate to form a very stiff min eralised froth which can be separated in various ways. There is also flocculation of the mineral particles by the oil emulsion in contact with the air-bubbles. The finest mineral particles thus become attached to the air-bubbles in large and heavy agglomer ations—a circumstance which contributes to efficiency.
The flotation process is applied to sulphide ores, such as those of copper, lead and zinc, and in this connection alone it has at tained great industrial importance. Its application to other types of minerals is not quite so easy but considerable success has been attained. Minerals having a more or less metallic surface are particularly well adapted to flotation, so that the process is readily applicable to the treatment of ores contain ing finely divided metallic gold. None the less, for the treatment of ores of this nature at the Rand in South Africa, the older cyaniding processes hold their own, although flotation is used in some cases in combination with cyaniding.
mentioned above relate to processes of concentration which are "mechanical" in the sense that the chemical composition of the metalliferous minerals is not changed by the treatment. Follow ing on such mechanical treatment, and in many cases preceding it, are a whole series of operations which act upon the chemical composition of the mineral. Some of these are, like the mechani cal processes, of an essentially preliminary nature, but they include the processes by which the metal is finally reduced and refined. Among the preliminary treatments, those which exert an oxidising influence on the mineral may be first considered. The action of atmospheric agencies on sulphide minerals near the surface produces enrichment by converting them into car bonates and oxides. Artificial methods for producing similar effects are widely used. One of the simplest consists in "bed ding' the ore, after crushing to a suitable size, in such a way as to expose it to air and water—a process practised among others at the copper mines of Rio Tinto. The sulphide ore thus be comes gradually oxidised but the process occupies many months.
More frequently, oxidation is brought about by the applica tion of heat in the "roasting" process. This constitutes an im portant part of the treatment of many sulphide and arsenical ores. Appliances for this purpose vary from simple "heap roasting" to the elaborate sintering and roasting machine of the Dwight-Lloyd type in which the ore is fed on to a long con tinuous chain, each link of which is in reality a furnace grate. The link or plate only comes into action as a furnace, however, after passing first through an igniting chamber in which a flame plays upon the ore and heats it sufficiently to ignite, when it passes into the next chamber where air is drawn through the mass. Here the greater proportion of the sulphur in the ore is burnt, sulphur dioxide and trioxide being formed. The ore itself, thus purified, is sintered into a porous mass and, as the chain passes on, is tipped out while still hot. The porous mass thus produced is particularly suited for further reduction and purifi cation in the blast-furnace (see below).
In other roasting processes, such as that of Huntington and Heberlein, the ore is roasted in a large pot where air is blown through it. In other processes, again, the ore is roasted in fur naces provided with a series of shelves which are heated by a flame passing under them while a separate stream of air is drawn over the layers of ore. The ore is raked down successively from one shelf to the next lower one and the roasting process is complete when the ore reaches the lowest shelf and is raked out of the furnace. In many of these roasting processes the sul phurous gases are utilised for the production of sulphuric acid. This utilisation of one of the waste products of a process is typical of modern economic metallurgical operation. The use of blast-furnace gases for the production of heat and power is another example of the same kind. In many processes the value of such "by-products" is an important factor in determining whether the process can be profitably worked.