Ore Treatment

Concentration.

In ore treatment the primary object is con centration. The metal occurs in the product of the mine fre quently in a widely dispersed or "diluted" form and it is essen tial to reduce the weight and bulk of the material to the lowest possible amount. This necessity arises first from the need of keeping down the quantity of material which is to be treated in the later stages of metal production, whether in furnaces or in the wet way. There is, however, a further consideration. Mines are often placed in distant parts of the world, far away from the centres of industry where the finished metal is required and this involves relatively costly transport. It is therefore necessary to reduce the weight and bulk of the material to be transported and, in the last resort, this is achieved by carrying the treatment of the ore at or near the mine to the point of the final produc tion of the metal. In a few cases this is done, but in others, where the refining processes are elaborate, the reduction opera tions are not carried to the final stages but only to a certain degree of concentration. There are, for instance, cases in which it would not be profitable to export, say from the Congo or even from Arizona, the crude sulphide ores. On the other hand it might not be worth while to instal electrolytic copper refineries in those places, partly on account of difficulties of power supply, lack of skilled labour, etc. The intermediate course is frequently adopted and a crude variety of copper, or even the sulphur bearing intermediate product known as a "matte" is produced at the mine and sent forward to refineries situated near the centres of industry or near the sea. It is obvious that an inter mediate product containing, for example, two or three per cent of impurities, will cost very little more for transport than the fully refined metal. On the other hand, it is always the removal of the last traces of undesirable impurities which is most diffi cult and demands the greatest skill and care. Purely economic factors, such as cost of labour and commercial organisation also play an important part in such matters.

The refining operations which can be applied to ores after they have undergone preliminary concentration processes, depend en tirely upon the nature of the concentrate, and upon the facilities available in the locality where the work is to be done. Although the changes to be brought about are essentially of a chemical na ture, the methods which can be applied depend to some extent on the physical characteristics of the substance in question. Thus a finely-divided material cannot be treated in blast furnaces, while a strongly sintered product is not so well suited as a finely divided material to treatment with aqueous solvents or "leaching." Reduction Methods.—Broadly the reduction methods appli cable to ores may be divided into two groups involving respec tively furnace treatment and wet processes. Processes involving the use of mercury for "amalgamation" are allied to the true "wet" methods. The furnace methods depend upon chemical changes which occur at high temperatures ; these may involve the oxidation of impurities which it is desired to eliminate and the oxygen for this purpose may be derived from the air or from oxidised compounds which may be added to the ore or slag dur ing or prior to the treatment. There are also reduction processes,

in which oxygen is removed from the ore either partially or com pletely. Here the oxygen is removed, i.e., the oxides in the ore are "reduced," by the action of such agents as carbon, hydrogen, hydrocarbons or even other metals. It is a frequent sequence to find that at an early stage of a process the actions which take place involve oxidation and that reducing actions, which finally result in the production of the metal itself, are subsequently applied. The heat required for these operations may either be supplied by the combustion of fuel which may be mixed with the ore or burnt separately, or the heat may be generated by the combustion of such substances as sulphur or carbon already present in the ore or intermediate product.

Another method of metal production which is intermediate between the typical furnace processes and the wet-way electrolytic methods is that of "fusion electrolysis." Here the purified metal liferous substance is brought into a state of fusion, usually by admixture with some other substance which serves as flux—i.e., has the function of rendering the mixture much more fusible- while itself remaining as nearly as possible neutral. The "solu tion" of oxide of aluminium in molten cryolite is perhaps the most important example of this kind. The fused mixture then serves as electrolyte and the pure metal is separated at the cathode exactly as in wet-way electrolysis, except that the re duced metal is frequently formed in the liquid state and is run off periodically into suitable moulds. In other cases—as in the production of magnesium, calcium and other light metals the deposited metal is solid and is formed as a stick on a cathode rod which is steadily withdrawn from the bath. These processes find their application to metals which it is difficult or impossible to reduce satisfactorily by ordinary furnace methods and which cannot be produced by wet-way electrolysis because they react vigorously with water.

The wet methods of ore reduction afford a similarly large variety. In some few cases it is possible to extract the desired mineral direct from the mine by a leaching process. In the majority of cases, however, the solvent solution is applied to the ore after it has been removed from the mine and subjected to preliminary concentration, as, for example, by flotation. In other cases preliminary roasting, either purely oxidising or "chlor idising"—in which chlorides are added to the ore with a view to converting the mineral into an easily-attacked chloride—is em ployed before leaching is applied. In some cases, in fact, the ore is reduced to a crude form of the metal before it is subjected to wet-way (electrolytic) refining.