144 



DRESSING OF ORES 



found necessary, allow the accumulations in the bottom of each trunk to discharge 

 themselves into separate troughs, The rocking frame and rakes D D constantly stir 



740 



741 



up the sediment so as to bring it under the action of the water. To produce the 

 oscillation of the rakes, two spade-shaped plates E E are exposed to the action of the 

 falling water discharged from the end of the box which rocks the frame to and fro. 

 The ore in the first trunk is fit for smelting ; the ore passing off from the bottom of 

 the other trunks is in a very favourable condition for framing, or it may be sifted to 

 remove the larger particles of less specific gravity. 



The Back or Hand-frame. This is composed of a frame c, fig. 742, which carries 



a sloping board or table susceptible of turning to the right or left upon two pivots K K. 

 The head of the table is the inclined plane T. A small board p, which is attached by 

 a strip of leather L, forms a communication with the lower table c, whose slope is 

 generally 5 inches in its whole length of 9 feet, but this may vary with the nature of 

 the ore, being somewhat less when it is iliiuly pulverised. 



In operating with the table, the slimy ore, to the extent of 15 or 20 pounds, is 

 placed on the head T, and washed over L and p on to the table ; then the operator 

 with a toothless rake distributes it equally over the head, the richest particles remain 

 on the highest part of the table by virtue of their greater specific gravity, whilst the 

 muddy water falls through a cross slit at the bottom into a receptacle n. "When the 

 charge of ore has been thoroughly racked, the table is turned on its axes K K until it 

 is brought into a vertical position, and the deposit on its surface is washed into boxes 

 B' B". The box B' will contain an impure slime, which must be again framed, whilst 

 B'' will probably contain a slime sufficiently enriched to be finished by the dolly tub. 



The slope of the rack table for washing tin stuff is 7| inches in 9 feet. The width 

 is about 4 feet. 



The average quantity of lead slime which can bo washed per day of ten hours, is 

 about 30 cwts., and the water necessary, say 600 gallons. 



The general appearance of the rack is shown in the illustration, fig. 743, A, table ; 



