76 DRESSING OF ORES 



castaways obtained at each operation, and tho quantity of middles or stuff to be 

 re-worked Trill bo diminished. 



Seventh. Wo may further consider as a great improvement in dressing operations 

 such apparatus or plan of working as will allow, without a disproportionate increase 

 in the cost, of the equally perfect separation of fine and coarse stuff. This will be of 

 especial benefit in the case of finely-disseminated ore, v hich is necessarily obliged to 

 bo reduced to a great degree of fineness. 



WASHING AND SEPARATING ORES. 



The vein-stuff on arriving at the surface, is not only associated with a large amount 

 of gangue, but is frequently much intermixed with clay, rock, and siliceous matter. 



In order to get rid of the latter substances, it is usually washed and picked. Tho 

 washing apparatus ought to be so contrived as to allow the cleansing to be effected both 

 cheaply and expeditiously, and for this purpose a good volume and fall of water is 

 always desirable. In accordance with the character of the ore the apparatus will have 

 to be varied, but for lead, certain varieties of copper ore, as well as iron, or other 

 abundant ores, the kiln is well adapted. In many mines rectangular grates are fitted 

 to the bottom of the kilns, but a perforated plate would be found to furnish better 

 results, since tho former allows of the passage of flat irregular masses of stone, ren- 

 dering the treatment in the jigging sieves less successful. The holes in the perforated 

 plate should be conical, the largest diameter underneath, so that the stones may have 

 iinobstructed passage. In connection with the kiln-plate a sizing trommel should be 

 used, and in order to economise both time and expenditure it would be judicious to 

 introduce the vein-stuff and discharge the castaways by means of railways. 



Tho picking of the stuff is a highly important operation. As a rule all picked ore 

 should be selected, and the dradge deprived of the largest possible amount of waste 

 before it is sent to the crusher. It is fallacious to suppose, because machinery will 

 deal with large quantities expeditiously, that it is cheaper to subject the mass to its 

 action ; on the contrary, if correct calculations are made of the losses which will 

 ensue on the initial quantity of ore before the residue is ready for the pile, tho 

 cost of the several intricate manipulations requisite to get rid of the castaways, tho 

 wear, tear, and maintenance of machinery, it will appear in the majority of cases that 

 the most profitable method is to incur an extra first charge in order to reject the sterile 

 portions by means of hand labour. The ragging hammer should therefore be brought 

 into free requisition, and all worthless stones at once rejected ; then in spalling such 

 portions as have been ragged, an additional quantity of refuse should be excluded ; 

 whilst in the process of cobbing, either ragged or spalled work, the greatest care and 

 attention should be given in order to bring the dradgo to a maximum degree of 

 richness. 



Among the siftings and washings which ores are made to undergo, we would notice 

 those practised on the Continent, grilles anglaises, and step-was 'kings of Austria, 

 lavcries a gradins. These methods of freeing the ores from pulverulent matters, 

 consist in placing them, at their out-put from the mine, upon gratings, and bringing 

 over them a stream of water, which merely takes down through the bars the small 

 fragments, but carries off the finer portions. Tho latter are received in cisterns, 

 where they are allowed to rest long enough to settle to tho bottom. The washing by 

 steps is an extension of the preceding plan. To form an idea of this, let us imagine a 

 scries of grates placed successively at different levels, so that the water, arriving on 

 the highest, where the ore for washing lies, carries off a portion of it, through this 

 first grate upon a second closer in its bars, thence to a third, &c., and finally into 

 labyrinths or cisterns of deposition. 



The grilles anglaises are similar to the sleeping tables used at Idria. The system of 



these gradins is represented \nfig. 642. Sucli system is resorted to for the purpose 

 of sorting the small fragments of quicksilver oro intended for the stamping milL 



