DRESSING OF ORES 



73 



As the "bodies on starting are supposed to have tho samo forms and densities, it 

 follows that the resistance they experience whilst descending through water will be 

 in proportion to tho surface exposed, and as the volumes of bodies vary according to 

 the cubes of their corresponding dimensions, whilst the surfaces only vary in accord- 

 ance with the squares of the same measurements, it will be seen that the force of 

 movement animating them is regulated by their cubes, whilst their resistance is in pro- 

 portion to their squares. 



If, lastly, it be imagined that all the fragments have the same volume .and density, 

 but are of various forms, it follows that those possessing the largest amount of surface 

 will arrive at the bottom last, and consequently the upper part of the deposit will 

 consist of the thinnest, pieces. 



It is evidently then of the greatest importance that the grains of ore which are to 

 bo concentrated by washing should be as nearly as possible of the same size, or other- 

 wise the smaller surface of one fragment, in proportion to its weight, will in a measure 

 compensate for the greater density of another, and thus cause it to assume a position 

 in the series to which by its constitution it is not entitled. 



This difficulty is constantly found to occur in practice, and, in order as much as 

 possible to obviate it, care is taken to separate by the use of sieves and trommels 

 into distinct parcels, the fragments which have respectively nearly the same size. 

 Although by this means the grains of ore may, to a certain extent, be classified according 

 to their regular dimensions, it is impossible by any mechanical contrivance to regulate 

 their forms, which must greatly depend on the natural cleavages of the substances 

 operated on, and hence this circumstance must always in some degree affect the results 

 obtained. 



Each of the broken fragments of ore must necessarily belong to one of the three 

 following classes : the first class consists of those which are composed of tho mineral 

 sought without admixture of earthy matter ; the second will comprehend the frag- 

 ments which are made up of a mixture of mineral ore and earthy substances ; whilst 

 the third division may be wholly composed of earthy gangue without the presence of 

 metallic ore. By a successful washing these three classes should be separated from 

 each other. 



The most difficult and expensive vein-stuff for the dressing floors is that in which 

 the constituents have nearly an uniform aggregation, and where the specific gravities of 

 the several substances approximate closely to each other. In such cases the ore is 

 only separated from the waste after much care and labour, and often at the loss of a 

 considerable portion of the ore itself. When, however, the ore is massive and distinct 

 from the gangue, and the specific gravity of the latter much less than the former, then 

 the operation of cleaning is usually very simple, effected cheaply, and with but little 

 loss on the ore originally present. 



Some of the more friable ores, when simply exposed to the influence of water, exhibit 

 a large mechanical loss, so much so, that it is considered oftentimes more profitable to 

 sell them without attempting their enrichment. Now it may be laid down as an 

 axiom that water will always steal ore, and the longer it is exposed to its influence, 

 and the more complicated the manipulation, the greater will be the loss incurred. In 

 addition, the constitution of certain ores is so peculiar and delicate, that any attempt 

 to concentrate them beyond a given standard, by varying the treatment, is seen to 

 lead to an enormous loss, as will be apparent by inspecting the following memoranda 

 of practical results : 



(A.) The ore operated upon was sulphide of lead, associated with finely-dissemi- 

 nated iron pyrites, oxide of iron, quartz, and a small portion of clay slate. In each 

 case the vein-stuff assayed 17 per cent, of metal. 



Quantity Quantity 



by weight by weight " 



1 -washed and concentrated to -25 



Assayed 



17 

 per cent. 







1 burnt, roasted, and do. 

 2*4 washed and do. 

 1-56 



20 loss by roasting 

 1*36 washed and concentrated to '40 

 '8 roasted, washed, do, 

 8 



1 The quantity o ore by Weight is, iu the first instance, 1 : this unit 1 which might have been a 

 tonwas washed and dressed ; then weighed, when it was found that 75 parts had disappeared as 

 tailings, The dressed ore, 25 parts, was then assayed, when it appeared that the metallic loss was 

 Gl per cent. In other words, say 1 ton afforded 17 units of metal, the dressed ore only contained 

 6 T " 5 units. (17x61=10-37. Then 17 units-10'37=6^). 



