158 



DRESSING OF ORES 



stuff; r, discharge launder; o, shoot delivering water and raff to launder n; K, 

 cistern receiving slime from trommel. 



Slime Pits. In the several operations of cleansing ores from mud, in grinding, and 

 washing, where a stream of water is used it is impossible to prevent some of the finely 

 divided portions floating in the water from being carried off with it. Slime pits or 

 labyrinths, called btcddle holes in Derbyshire, are employed to collect fine sand and 

 slime at a little distance from the place of agitation. 



Settling Pits. The slime pits shown in section fig. 768 and plan fig. 769 are con- 

 structed on the principle of the Classifier. The viscid water from the jiggers and 

 buddies, takes the direction of the arrows. The stuff deposited in such a system of 

 pits, will gradually vary from a maximum degree of coarseness at the head of the first 

 pit, to the least degree of fineness at the end of the fourth pit. Should the water 

 flowing into the pits be moderately charged with ore, the latter will be found mostly 

 at the narrow ends, and may therefore be readily collected for mechanical enrichment. 

 For the purpose of running off the pits, a side gullet is constructed which is in com- 

 munication by means of iron slides, one to each pit. The aggregate length of the pits 

 is 40 feet, width 31 feet, width of dividing walls 2 feet, depth of pit 4 feet. The 



content of each pit is about 400 cubic feet. Settling pits may also be constructed on 

 the principle of the long classifier ; the object in this, as well as in the case illustrated, 

 being to effect an uniform distribution and classification of the slimes. 



Dry Separation of Ores. Considerable attention has been given by various engineers 

 both in Europe and America to the subject of separating ores by the dry or pneumatic 

 method theoretically the separation of grains of equal volume and of different 

 densities would not seem to be a difficult matter. But practically there are two or 

 three conditions which militate against success ; as, for instance, the grains require to 

 be accurately sized, and free from moisture, whilst if air jigging be attempted the 

 feed to the sieve must bo altogether uniform, and the bed such as to offer an equal 

 resistance throughout the whole of its surface. In countries where the climate is 

 both dry and hot, water scarce, carriage heavy, and mineral abundant, it is just 

 probable that pneumatic separation might bo resorted to with partial satisfaction. If 

 the ores were crushed and sized, then exposed to the effects of a current of air, the 

 lighter grains would naturally bo ejected to a greater distance than the heavier ones ; 

 but it may be assumed beforehand that most of the grains composed of gangue 

 and ore would be grouped together, and that the loss on such a mode of concentration 

 would be very heavy. 



Magnetic Separation of Ores. When ores and vein-stones are nearly of the same 

 density, their separation by water dressing is almost impracticable or necessarily very 

 slow. Further, the value of such ores is greatly diminished both by the cost of en- 

 richment and the comparatively low percentage of the dressed product. 



Recently Mr. F. J. King of London has found that carbonate and sulphide of iron, 

 as well as copper pyrites are rendered magnetic by simply heating these ores in 

 a retort or calciner. Carbonate of iron, parting with carbonic acid, is converted into a 

 magnetic oxide of iron, whilst iron and copper pyrites, e.-ich yielding up one atom of 

 sulphur becomes magnetic pyrites. In order to apply this important fact to tha 



