122 DRESSING OF OEES 



these upper strata arc of course tho lightest, the- heavier part settling upon tho per- 

 forated plates below. 



When from the action of the machine a considerable quantity of material has 

 accumulated upon these plates, tho scrapers are thrown out of gear by means of 

 apparatus attached, HH', and tho stuff raked off, the operation being then continued on 

 fresh supplies. Doors, G o', at tho bottom of tho machines, admit of any fino stuff which 

 may pass through tho perforated plates being removed from time to time as may bo 

 necessary. 



These machines are in use for cleansing coal as well as other mineral substances. 



In such cases the heavier stuff which remains upon the plates consists of hnlo 

 and pyrites, very injurious substances in the manufacture of coke. One machine of 

 two connected cisterns is capable of washing about thirty tons per diem of coal, but 

 the quantity of mineral work will depend upon the amount of ore present in proportion 

 to the waste. The size of the perforations in the screens is adapted to the quality of 

 the material acted upon. 



The introduction of water skimming jiggers into our British mines is probably 

 due to Mr. John Hunt, late of Falmonth. This gentleman used an ore bed, two sieves 

 divided by a low partition, two ore chambers, and a horizontal flow of water. Hunt's 

 Specification, No. 707, is dated March 8, 1866. 



In order to concentrate lead, zinc, copper, and other ores associated with vein-stono, 

 coarse and fine sand jiggers should be employed, the former to receive the stuff from 

 sizing trommels, the latter from trommels or classifiers. In the coarse sand jigger, 

 stuff 10, 7, 5, 3 and 2 millimeters size is usually enriched on a plate, the holes in 

 which are of less diameter than the grains, whilst in the fine sand jigger, sand l, $ 

 millimeter size is separated through the intervention of a bed of coarse grain ore, sup- 

 ported either on a perforated plate or wire work, the openings of which are larger 

 than the grains to be collected. 



For the purpose of effecting a good separation of ore from its gangue it is necessary 

 to give great attention to the length of stroke, number per minute, and volume of 

 flowing water. Every distinct class of vein-stuff will necessitate the observance of 

 specific conditions, so that no absolute data can be offered ; as, however, approximate 

 figures may be useful, the following are given : Coarse sand jiggers, speed from 60 to 

 75 strokes per minute ; flowing or skimming water required, say ten to fifteen gallons 

 each machine per minute. Fine sand jiggers, speed from 90 to 250 strokes per minute ; 

 piston and horizontal flow of water necessary, from fifteen to twenty gallons per minute. 



The approximate length of stroke and piston speed for stuff ranging in size from one- 

 third to ten millimeters, the area of the piston being that of the sieve, may be gathered 

 from the following figures : 



Size of stuff , , , l, 2, 3, 5, 7J-, 10 millimeters 



Approximate length of stroke , f, , 1, lj, 1J, 2, 2J, 2. \ inches 



Piston speed 150, 120, 110, 100, 60 to 75 per minute. 



SEPARATORS OR CLASSIFIERS. 



Of late years apparatus of this class have been steadily coming into operation, not 

 only in lead and copper mines, but also in the dressing of tin ores. Unliko the trommel 

 which groups together grains of equal volume irrespective of their densities, the 

 classifier groups the grains into equivalents, that is to say, grains which differ from 

 each other in their densities, but which have an equal velocity of fall in water. 



To separate such grains in tho after dressing process, advantage is taken of the 

 resistance offered by the surface of a heavier over a lighter grain in passing upon a 

 bed of ore as in the fine sand jigger, or in traversing the surface of a buddle or table. 

 For the purpose of sizing stuff from x to grains of one millimeter diameter, trommels 

 should be employed, but for dividing grains from to 1 millimeter diameter, tho 

 cistern, cone, or expanding launder, will be found suitable. 



Slime Separator. This apparatus devised by Captain Isaac Richards, of Devon 

 Great Consols, is employed for removing tho slime from finely-divided ores which 

 have passed through a series of sieves set in motion by tho crusher. Tho finely- 

 divided ores are for this purpose conveyed by moans of a launder upon a small 

 water-wheel, thereby imparting to it a slow rotatory motion. Whilst this is turning 

 time is allowed for the particles to settle in accordance with their several densities ; 

 the result obtained, is, the heavier and coarser grains arc found at the bottom of 

 tho buckets, whilst the lighter and finer matters held in suspension are poured out of 

 the buckets and flow away through a launder provided for that purpose. Tho stuff 

 remaining in the bottom of tho buckets is washed out by means of jets of water ob- 

 tained from a pressure -column 10 feet in height, and passes directly into the funnel, 

 of a round bmddle. 



