DRESSING OF ORES 



107 



Diameter of Grains having equal Velocity 

 of Fall 



Copper P^ites Jj-l-jg Then j = 1 . 3,, {$} 1 dian,. Blende 1-03 dia m . 



Copper Pyrites 4-1- 1=3-11 ^_i.q / Copper 1, Quartz 1-9 



Quartz . .2'6-l = l-6J 1'6~ "\PyritesJ 



Galena 1 Quartz 4-0 



Galena 

 Quarte 



. 7'5- 1=6-5) 



~ 



As the latter rule will allow of a wider difference between the minimum and 

 maximum diameter of the grains to be treated than the former, and is used by many 

 practical engineers, it will, perhaps, be best to employ it, particularly in connection 

 with ores and gangue, of a simple character ; but apart from the rule itself, it will bo 

 desirable to bear in mind the following observations : (1) It will be impossible to 

 separate by any hydraulic jigger a mixture composed of grains having an equal 

 velocity of fall. (2) Separation of grains can only be effected by bringing the diameter 

 of one set within such limits that the other will pass it within the descending range 

 of the process. (3) If galena of 1 diameter and blende of 3 diameters are treated, 

 the blende will be found at the bottom; galena of 1 diameter and blende of 2 

 diameters will be associated more or less together; whilst galena and blende of 

 1 diameter will fall with such a wide difference in their respective velocities as to 

 leave each other at a very early period. The necessity of sizing stuff into equal 

 volumes, so as to take advantage of the difference of fall due to their respective 

 densities is therefore evident, as well as the fact that the nearer grains of stuff of 

 equal volume approximate to each other in density, the greater length of fall will be 

 requisite for their absolute solution. 



Hand Sieve. This apparatus, fig. 680, is formed of a circular hoop of oak, fths of an 

 inch thick and 6 inches deep. Its diameter ranges from 18 to 20 inches. The bottom 

 is made of copper or iron wire meshes, of various sizes. Sometimes perforated copper 

 plate is employed, when the sieve is termed 

 a copper bottom. The sieve is shaken 

 with the two hands in a cistern or tub of 

 water, an ore vat is, however, sometimes 

 employed, and either fixed horizontally or 

 in an inclined position. In using this sieve 

 the workman (fig. 680) shakes it in the vat 

 with much rapidity and a dexterous toss 

 till he has separated the totally sterile por- 

 tions from the mingled as well as from the pure ore. He then removes these several 

 qualities with a sheet-iron scraper, called a limp, and finds beneath them a certain 

 portion of enriched ore. 



Deluing Sieve. This sieve A, fig. 681, is either constructed with a hair or canvas 

 bottom ; the former is more expensive, but more durable. Its peculiar application is 

 chiefly for the final treatment of ores previous to being put to pile, such ores having 

 first passed through the finest jigging sieves, yet still maintaining a certain degree of 

 coarseness and bearing a high specific gravity. 



In the separation of ores from light waste, or 

 such minerals as approach one another somewhat 

 closely in their densities, this form of sieve is 

 both good and effective, but to use it properly 

 a considerable amount of dexterity and practice 

 is requisite. 



There are two principal methods of using it ; by one a motion is given, wh*ereby 

 the waste is being constantly projected and carried over the rim into the kieve by a 

 current of water forced through its bottom. This mode of treatment is adapted for 

 poor ores. In the second case, when the ore is nearly pure, but still associated with 

 a heavy gangue, a motion is given to the sieve whereby the water is forced through 

 the ore, and made to traverse the surface of the mineral in concentric circles. This 

 motion collects the waste into the middle of the clean result. By the first method 

 about six tons per day may be passed through by each workman and enriched for 

 the second operation. The weight of the sieve varies from four to five pounds, its 

 diameter is twenty-six inches, depth four inches, and cost from 2s. 3d. to 2s. 6d. 



A jigging sieve, constructed as shown in fig. 682, is sometimes employed on the 

 Continent. A represents the table on which the mineral is placed ; B is a large kieve 

 of water, in which the sieve is suspended by the iron rod D, set in motion by means of 



681 



