GOLD EXTRACTION METHODS. 99 



of 1909 158 tubes were in use. against 9.545 stamps erected, 

 and it must be observed that there are many mines, whose 

 " lives " are now too short for the re-payment of the necessary 

 capital expenditure, where no tube-mills are yet put up, and 

 that the present tendency is in the direction of one tube to 

 every 15 or 20 stamps; it is possible that the future will see 

 even this proportion of tube-mills increased. 



A change of some importance has been made in the method 

 employed for elevating" the pulp leaving the mill for classifica- 

 tion and subsequent treatment. Formerly tailing-wheels were 

 almost imiversally employed, though even seven years ago 

 .plunger pumps were successfully used in one or two cases. 

 The great disadvantage of the tailing-wheel, beside its high 

 capital cost, was total lack of elasticity when the introduction 

 of improved appliances demanded an increase in the total 

 elevation necessary. 



After considerable experimenting, a type of centrifugal 

 pump, with cheap and easily-renewable wearing parts, was 

 found suitable for elevating to any required height the coarsest 

 material yet produced, and many are now in operation. 

 Though the wearing parts may last only 20 or 30 days, the 

 total cost, including capital redemption, of elevating pulp is 

 less than by any other method, and any increase in elevation 

 subsequentlv required is easily arranged by adding a length 

 of pipe to the delivery column and suitablv increasing the 

 speed of the motor. One disadvantage which can, however, 

 be overcome without very great difficulty is the lack of 

 regular flow from the pump if the amount of pulp to be lifted 

 is varied by hanging up stamps or from other causes. This 

 lack of regularity interferes considerably with the effectiveness 

 of any arrangeriient for classifying the pulp and separating 

 the coarser and heavier portions as is required for their 

 different treatments. 



The use of compressed air in the " Pohle lift "' is quite 

 effective for elevating pulp, as at the East Rand Mines, and 

 though the efficiency is low, a similar method is largely used, 

 where the aerating" effect is also of value in the solution of 

 gold, as in "Brown" agitators or similar apparatus. 

 " The method of separation by means of cones has in the last 

 seven vears largely displaced the use of the spitzlutte and 

 spitzkasten. formerly chieflv employed for this purpose, and 

 the introduction, by Dr. Caldecott. of a diaphragm partition 

 near the bottom of the cone enables a much steadier and more 

 uniform product to be returned to the tube-mill plant for re- 

 grinding and increases the efficiencv of the apparatus for other 

 separations required. The underflow may be thus arranged 

 to retain even less water than is found suitable for the best 

 tube-mill work, so that a second set of cones for removing 

 the excess water may now be dispensed with, and any required 

 adjustment may be" accurately made by adding the amount 

 of water experiment has shown to be requisite. _A further 

 advantage is found in the increased size of discharging nozzle 

 that mav be used when diaphragms are suitably employed; this 



