620 



NA TURE 



[Al'KIL 30, 1908 



to conceive that some method of detecting" concealed mineral 

 deposits by these means may be devised. Indeed, for one 

 particular class of minerals such a method has long been 

 in existence ; in Scandinavia there are many deposits of 

 magnetite, and many others of which magnetite forms a 

 constituent, so that all such deposits distinctly affect a 

 magnetic needle. The .Swedish prospector has long used 

 Ihe so-called mining compass, which consists essentially 

 of a small magnetic needle so suspended as to be able to 

 move both horizontally and vertically. When this compass 

 is brought over ground in which such deposits of magnetic 

 mineral exist, the needle indicates their presence by its 

 change of dip, so much so that it has been customary for 

 years past in Sweden to buy and sell mineral properties 

 by their "compass-drag," or their effect on the miners' 

 compass. 



When, by any means, some indication is obtained of 

 the approximate position of a mineral deposit, it has to 

 be more precisely located by boring. Boring is of but 

 little value for tracing mineral veins, owing to their going 

 down so nearly vertically and to their great irregularity, 

 but it is often used to locate irregular masses of ore ; for 

 example, bore-holes have recently been employed success- 

 fully in Cumberland for proving deposits of red hsematite 

 in the Carboniferous limestone, even where this is over- 

 lain by Triassic rocks. Obviously bore-holes are most 

 valuable when stratified deposits have to be tested, and 

 everyone will remember the conspicuous success that 

 attended their use in proving the permanence in depth of 

 the auriferous banket beds of the Witwatersrand. 



The deepest bore-hole put down up to the present is one 

 at Paruschovvitz, in Upper Silesia, which attained a depth 

 "f 6573 feet; it commenced at a diameter of 12-6 inches 

 and finished at 2-7 inches, and it is easy to imagine the 

 difficulties that attend the boring of so small a hole to 

 the depth of i^ miles. The engineers in charge stated 

 that they could not have reached this depth had not 

 Mannesmann weldless steel tubes been available for the 

 boring rods : I mention this fact as illustrating the depend- 

 ence of mining upon the allied arts, for at first sight few 

 would imagine that an improvement in special rolling-mill 

 practice could increase our knowledge of the deeper por- 

 tions of the earth's crust. 



Bore-holes such as these are now always m.ade by means 

 of the well-known diamond drill, which brings up a core 

 of the rocks passed through, and thus aftords positive in- 

 formation respecting them. Unfortunately, the only kind 

 of diamonds suitable for this purpose, ihe dark opaque 

 stones, showing no distinct cleavage, known in the tr.ade 

 as "carbons," are very scarce and proportionately dear, 

 so that diamond-drilling is now a very costly operation ; I 

 have, however, good grounds for saying that we are within 

 measurable distance of seeing such " carbons," or at anv 

 rate " boot." produced artificially. For rocks of moderate 

 hardness, these diamonds have of late years been reolaced 

 to some extent bv shot made of speciallv hard chilled iron, 

 but these are of little use in the harder rocks. One of our 

 greatest needs at the present moment is a metal that shall 

 be strong, tough, and very considerably harder than quartz ; 

 the production of such a material would conduce more to 

 the technical advancement of several branches of mining 

 than almost any other discoverv that could be named. 



Mineral deoosits may be distinguished as superficial, 

 shallow, or deep-seated in the earth's crust; the first of 

 these require no opening up, pronerlv soeakinff ; the second 

 ran mostly be opened un by adit levels, whilst the third 

 class ran only be reached by means of shafts. The deepest 

 shafts in the world are in the copper-mining district of 

 Lake Superior, where there ■ are at least two close upon 

 ■jooo feet in depth ; with the exception of this district, of 

 a few shafts in the Bendigo district of Victoria, a few at 

 lohannesburg. and some in the Przibram mines in 

 Bohemia, itniay be said that there are practically no shafts 

 in ^ metal-mines more than ^ooo feet deep, so that the 

 ability to reach considerably greater denths than have 

 hitherto been attained in most mineral fields mav be taken 

 for granted. Indeed, so far as the actual sinking is con- 

 cerned, there would probablv be no serious difficultv in 

 sinking a shaft in. 000 feet deeo, provided that it could be 

 known wnth certaintv that a deposit of ore would be met 

 with of sufficient value to recoup the outlav incurred in 

 NO. 2009, VOL. 77] 



such a sinking ; in other words, the main problems con- 

 nected with deep sinking are economic rather than 

 technical. 



For centuries the only property made use of to effect the 

 separation of minerals was the difference in their densities ; 

 in 1858, however, an entirely new property was brought 

 into play for the purpose, namely, the difference in their 

 magnetic susceptibilities. This idea was due to a famous 

 Italian engineer. Sella, whose name is well known in con- 

 nection with the Mont Cenis tunnel. He was called upon 

 10 treat the iron ores of Traversella, in Piedmont, which 

 consist of magnetite containing a certain proportion of 

 copper pyrites (the mass carrying 2 per cent, to 4 per cent, 

 of copper), which interfered with the use of the ore for 

 iron smelting. Sella devised a machine carrying rotating 

 electromagnets, by which the magnetic iron ore was 

 separated from the non-magnetic copper ore, so that both 

 could be utilised. 



Other machines on similar principles were subsequently 

 devised, and, naturally enough, they emanated from 

 countries rich in deposits of magnetite, such as Scandinavia 

 and some of the eastern States of .America. Sweden 

 especially took a prominent part in the development of the 

 magnetic system of separation, and the Wenstrom machine, 

 patented in 1884, which was one of the first practical 

 machines brought out, is still largely used, as it is well 

 adapted to the separation of lump ore. Other machines, 

 more particularly designed for the treatment of finely 

 crushed ore, were brought out in rapid succession, and 

 to-day one of the main difficulties that beset the mining 

 engineer lies in the selection of the most suitable machine 

 for any given purpose out of the vast number with which 

 the market is flooded. All these machines work either by 

 means of a moving magnetic field, produced by travelling 

 pole-pieces, passing through the mass of crushed ore, or 

 by causing a stream of the ore to traverse a stationary 

 field, these results being obtained either by travelling belts 

 or revolving drums, or, as in the case of Edison's machine, 

 by the deflection of a falling stream. It soon became 

 apparent that, where very clean concentrates were required, 

 the best results could onlv be obtained by applying mag- 

 netic separation to a pulp of mineral suspended in water, 

 and wet magnetic separators were soon introduced, and 

 are to-day preferred wherever possible ; they avoid the 

 necessity for artificial drying, which is, moreover, in the 

 case of minerals that contain iron pyrites, apt to affect the 

 magnetic susceptibility of this mineral sufficiently to inter- 

 fere seriously with the success of the operation. Attempts 

 have been made to devise magnetic separators without 

 moving parts, by the use of polyphase rotating fields, but 

 although the idea looks promising, no satisfactory machine 

 on this principle has yet been constructed. 



At first magnetic separation was only applied to the 

 naturally magnetic ores, magnetite and magnetic pyrites ; 

 it was soon, however, extended to certain other minerals 

 that can be rendered magnetic by heating, such as spathic 

 iron ore, brown haematite, iron pyrites, &c. As early as 

 1875 a magnetic separator was used at Przibram for 

 separating roasted spathic ore from zinc blende, this form- 

 ing an excellent example of the value of magnetic separa- 

 tion. The presence of spathic iron ore causes great difficul- 

 ties in smelting zinc ores, as it forms a readily fusible 

 silicate of iron which destroys the zinc retorts ; at the 

 same time, the densities of the two minerals are so nearly 

 the same that separation by ordinary dressing is impossible. 

 The application of magnetic separation has solved the 

 difficultv, and has rendered available for the smelter 

 ninnerous ferriferous ;^inc ores that were previously useless. 

 The process is receiving an extended application in America 

 for treating argentiferous galena and zinc blende, finely 

 divided, and intimately mixed with a large proportion of 

 iron pyrites, in which the proportion of zinc is too high 

 to admit of the ore being smelted direct, whilst the large 

 amount of iron pyrites present interferes with ordinary wet 

 dressing. This ore is crushed and then gently heated, 

 which renders the pyrites magnetic, so that it can be re- 

 moved bv a magnetic separator ; the dressing of the residual 

 mixture of zinc and lend ores by the ordinary methods then 

 offers no particular difficulties. 



Whilst the ordinary methods of magnetic separation were 

 thus extending the sphere of their applicability, another 



