Dry Crushing of Ore. 365 



and economy must be obser\ed in even,- department and full ad- 

 vantage taken of even- assistance that mechanical and chemical 

 science can afford. 



A saving of one shilling per ton means an addition of ;^45o,ooo 

 to the annual profit from the Mines on the Rand. 



Out of the 9.81 dwts. won from each ton of ore, cyanide claims 

 credit for at least 3.17 dwts., and it is very probable that the whole 

 could have been obtained by the latter process. 



It is surely worth consideration whether the system of mechanical 

 preparation of the ore which for centuries has only enabled the miner 

 to extract as a rule less than two-thirds of the gold from his ore 

 should not be modified or even changed in order to suit the important 

 additional means with which science has furnished him in the last 

 13 years, a system which has increased the quantity of gold won by 

 nearly one-half. 



It is evident that the ideal crushing machinery or system will be 

 one which can be adjusted s(j as to reduce the bulk of ore to such a 

 size as will enable an economically efficient extraction to be obtained, 

 the percentage of particles below this size being as low as possible in 

 order to avoid loss of power and the production of the objectionable 

 material known on the Rand as " Slimes." 



It is hardly necessarv- to say that the finer the ore is crushed the 

 greater will be the expense. 



When treating comparatively low grade ores such as those of the 

 Rand, or, say, containing I ounce per ton (worth, say, 40s.), great 

 oare must be exercised to avoid that the extra cost incurred in crush- 

 ing and extracting an additional five per cent, (worth, say, 2 s.) does 

 not exceed, say, is. 6d. (one shilling and sixpence), otherwise the 

 operation would be unprofitable. 



On the other hand, ores such as are often mined in Kalgoorlie 

 containing 2 ounces of gold per ton could well afford an increased 

 cost of several shillings, as 5 per cent, of that value would equal 8s. 



This Paper deals principally with ores of the lower grades, in 

 •dealing with which strict economy is necessary. 



A visit to a modern stamj) batter}' in w'ork is necessary in order 

 to realise the enormous effect produced by a number of heavy masses 

 of iron exceeding half a ton in weight being raised .six to nine inches 

 at least 90 times ever}' minute and falling on to solid foundations or 

 anvils. 



There is no possibility of arresting the action of one of these 

 pestles when in motion, and the ore beneath them stands as much 

 chance of being crushed too fine as of being reduced to the required 

 size. 



To crush one pound of Rand ore an effort equivalent to from 

 7,000 to 8,000 foot pounds is demanded from the engines, not in- 

 cluding loss of power from friction, &c. 



The impossibility of checking the crushing action of a stamp 

 battery is the cause of the formation of the large percentage of 

 slimes bv that method. 



