94 DRESSING OF ORES 



they are so worn as to become unserviceable in the course of ton or twelve weeks ; the 

 bottom, however, is only replaced once in twelve months. 



This apparatus is well suited to patio amalgamation, but it affords bad results for 

 the power expended. 



Edge-mill. This machine is employed for the purpose of reducing gold and silver 

 ores to an impalpable powder. It is also used extensively in grinding flints, stonos 

 slags, and a variety of other products. However much the details of this apparatus 

 may vary, its principle is the same in all cases. Two vertical runners rotate on 

 the outer circumference of a flat or slightly conical basin, and afford a friction.il 

 or grinding area equal to the difference of distance performed by the inner and outer 

 edges. 



The subjoined woodcut,/^. 671, represents a mill constructed at the Sandycroft Foun- 

 dry, Flintshire. A, rotating pan, resting upon frictional wheels u ; c, vertical shaft firmly 

 keyed to pan A, to which motion is communicated by wheel gearing D. The runners 

 E E revolve on arm F, and may be of cast iron or of stone bound with a ring of iron. 

 These runners have no progressive motion, but have free play to rise or fall on the 

 axis c, and in the stay slots G G. 



The following dimensions and particulars are -derived from one of the edge-mills 

 recently working at the Fabrica La Constante, in the province of Guadalajara, Spain : 



Diameter of edge-runner 6 feet. 



Width of do. do. centre 20 in. edge 16 in. 



Weight of do. do. 3 tons 15 cwt. 



Speed of runner _ 200 feet per minute. 



Diameter of interior circle of runner . . .4 feet. 



Gauge of stuff previous to its being ground . . .10 holes to the lineal inch. 



Do. after it leaves the mill .... 60 



Quantity of stuff reduced per 10 hours . . .350 Ibs. 

 Horse-power employed 7. 



In some machines erected at the Keal-del-Monte mines in Mexico the stones were 

 6 feet in diameter and 12 inches wide. They were fitted with a ring of wrought iron 

 3 inches thick. Each pair of runners revolved round a centre on its own axis, in a 

 cast-iron basin of which the bottom was 7 inches thick. At first good results wore 

 obtained ; each mill if kept constantly at work ground nearly 10 tons per week ; but as 

 their axles, and particularly the wrought-iron rings and cast-iron bottoms, began to 

 wear hollow and to lose an oven surface, the grinding rapidly diminished, and with 

 one year's work they were completely worn out. 



The chief advantage of this machine is its simplicity of construction and consequent 

 small first cost ; but all its parts require to be made of great strength, and therefore of 

 proportionate weight: hence, in addition to the rapid wear to which it is liable, this 

 apparatus becomes objectionable for countries where transit of heavy machinery is 

 more than ordinarily difficult and expensive. 



Horizontal mill. For the purpose of reducing auriferous and argentiferous ores to 

 an exceedingly fine powder, and where dry grinding is essential, no apparatus has 

 been found more effectual than the horizontal mill. It affords the largest area of 

 frictional surface for the least wear and tear, and accomplishes equal results at a cost 

 not exceeding one-fourth of that incident to the edge-mill. 



The construction of the horizontal mill will bo rendered intelligible by the aid of 

 the following illustration, Jiff. 672, in which one pair of stones is shown in section. A 

 is a circular hopper, into which the stutf to be ground is introduced ; B B, small pipes 

 of sheet iron, for delivering the stuff between the surfaces of the runner c and bed- 

 stone c' ; D, casing enclosing the runner into which the ground material is delivered ; 

 E, hole in centre of runner; F, driving-shaft, with continuation shaft o, for giving 

 motion to a Jacob's ladder if requisite ; H H', regulating screw for elevating runner c ; 

 j, driving-wheel ; K, crown-wheel ; L, wheel giving motion to pinions M M 7 ; and N, 

 vertical shaft, to drive any supplementary apparatus which may bo required, such as 

 sizing sieve, &c. Four pairs of stones are usually driven by the wheel L. The 

 surface of the runner is in contact with the bed-stone, from tho periphery to within 

 one third of its diameter. Tho lino of tho runner then feathers upwards, in ordi-r to 

 receive the stuff freely and to equalise the resistance throughout tho area of the bed- 

 stone. 



Tho following particulars will convey much practical information relative to this 

 machine : 



Diameter of stones 4 feet 2 inches. 



Thickness of bod-stone 12 inches. 



Ditto runner U inches. 



