80 



DRESSING OF ORES 



648 



the apparatus are of great strength, and the amount of wear and tear is comparatively 

 small. In mines where it is requisite to reduce large quantities of vein-stuff) either 

 for the cobbers, coarse sand jiggers, or crushing mill, this machine will prove an im- 

 portant and valuable auxiliary. 



The jaw pieces of stone-breakers should be formed of double chilled iron : soft and 

 badly-hardened iron is highly objectionable, since with the early inefficiency of the 

 jaw pieces, together with cost and delay attending their re- 

 placement, the cost of reducing stone must be materially 

 increased. A pair of hard jaws 1 2 inches by 9 inches will 

 broak from 2,000 to 3,000 tons of siliceous vein-stono. 

 In large dre.ssing establishments the stone-breaker is 

 driven directly by a steam-engine. Small breakers, with 

 jaws 7 inches deep and 4 inches wide, are also frequently 

 used for reducing orey vein-stone, derived from the 'coarse 

 sand' jiggers. 



Cobbing, fig. 648. This work is generally performed 

 by women, young girls, or boys. It consists of picking 

 the best work from the dradge, and with a peculiarly 

 shaped hammer, detaching from each piece the inferior portions, and thus forming 

 either prill or best dradge ore. An expert cobber will manage to prepare about 

 10 cwts. of tolerably hard stuff" per ten hours. 



Sizing Apparatus, In the various processes of dressing, no point is of greater im- 

 portance than that of correctly sizing the vein-stuff, neither is there one demanding 

 the exercise of a more correct judgment. If the particles of ore be reduced below their 

 natural size a source of loss is immediately created, whilst if they are not brought 

 within the limit of their size a portion of waste will probably adhere to each atom, 

 forming a serious difference in the aggregate quantity of castaways, although such waste 

 may afford but a low percentage of metal. The holes in the sieves or trommels should 

 therefore be proportioned to the nature of the ore, but such apparatus should also be 

 introduced wherever necessary. To the crushing mill, trommels are essential, whilst 

 it will be found highly advantageous to employ them for the purpose of dividing the 

 stuff whenever it may be intermixed. The simplest form of sizing is perhaps by 



649 



the hand riddle,^, 649, which is formed of a circular hoop of oak, |ths of an inch 

 thick and 6 inches deep. Its diameter ranges from 18 to 20 inches. 



The bottom is made of a meshwork of copper or iron wire. The weight of an iron 

 wire riddle is about seven pounds, and its cost 4s. 6d. 



Fig. 650 represents a swing sieve employed in mines 011 the Continent, a, box 

 into which the stuff to be sifted is introduced ; b, regulating door ; c. pendulating rod 

 attaching the sieve frame to the frame e ; /, friction roller carrying the sieve frame g. 

 At h springs are fitted to each side of the frame, in order to give it a vibratory action ; 

 t, rod, giving motion to the apparatus, The width of the sieve frame is about one- 

 third its length, but the sieve bottom only extends from the box, a, two-thirds of the 

 length. The bottom of the sieve frame is subsequently contracted so as to form a 

 shoot. 



Fig. 651 is a swing sieve employed in the Hartz, for sifting the small fragments of 

 the ores of argentiferous lead. Such an apparatus is usually set up on the outside of 

 a stamps or washing mill. The two moveablo chests or boxes A B, of the sieve, are 

 connected together, at their lower ends, with an upright rod, which terminates at one 

 of the arms of a small balance beam, mounted between the driving shaft of the stamps 

 and the sieve, perpendicularly to the length of both. The opposite arm of this bram 

 carries another upright rod, with cams, placed so as to be pushed down by tho 



