82 



DRESSING OF ORES 



sieves. A A', BB' is a strong wooden frame. MM, guides for frame; N N N, basement 

 tipon which the sieve frame rests; p, cistern fitted with perforated plate through which 

 clean water is distributed upon the sieves ; T, hopper supplying the stuff to bo sil'trd ; 

 s s bottom of ditto. The sieves are lifted by the rod I and make from 40 to ,50 drops 

 per minute. The sieves are set about 8 inches apart, and discharge the stuff upon the 

 inclines p p p. 



The holes in No. 1 sieve are ^ inch diameter. 

 2 1 



O 3 



"4 



Ya 



Fig. 655 represents the trommel or sizing sieves formerly in operation at the Devon 

 Great Consols. Although the yield of oro at these mines was extremely largo, it may 



not be generally known that much of it was obtained from stuff yielding no more than 

 from to 1^ per cent, of metal. The product of the lode on arriving at the surface was 

 cobbed and divided into two classes, the first going to market without further elabora- 

 tion, whilst the dradge or inferior portion was treated by various processes of washing. 

 The whole was, however, crushed to such a degree of fineness as to pass through the 

 following holes : 



Trommel A, holes ~$ inch diameter. 



The trommels wore each 6 feet long, 24 inches diameter at the largo end, and 18 inches 

 diameter at the smaller, making 20 revolutions per minute, and altogether affording 

 an area of 6,000 square inches. 



Continuous Trommels. Those are made of various diameters and lengths; the. 

 diameters of the perforated holes vary from two and a half inches to one-twentieth 

 part of an inch. On a suitable wrought-iron shaft A, fig. 656, a cast-iron boss plate 

 u, is fitted, the bosses receive six wrought-iron radial arms c, which pass through a 

 thin but stiff wrought-iron ring n, governing tho diameter of the trommel. Tho arms 

 are fastened to tho ring by moans of outside and inside nuts, tho perforated plates 1, 

 2, 3, and 4, are rivotted to tho several rings, and in this way an excellent trommel is 

 produced. 



Conical Trommel. The trommel fi<j. 657 may be used singly or in combina- 

 tion with others for sizing stuff coming from tho wash jrratc clearing drum or crusher. 

 Tho diameter as well as tho length should bo proportioned to the quantity of stuff to 

 be treated xwi thin a given p< : 



The angle of tho shell with tho horizontal axis must also correspond to tho dia- 

 meter of tho grains. Tho angle varies from three to five degrees, while the speed 

 ranges from fifteen to twenty revolutions per minute. 



Double Conical Trommel, fig. 658. For the purpose of obtaining throe distinct sizes of 

 stuff within tho length almost of a single trommel and, without incurring much loss of 



