SOME NOTES ON TREATMENT OF SANDS FOR STOPE FILLING. 20/ 



" cyanicides " were substituted, in view of the dangers which 

 would arise from the introduction of free cyanide into the mine, 

 the " cyanicide " employed would require to have the power to 

 destroy the cyanide almost instantly ; and further, in order to 

 avoid hig-h costs, it would require either to do its work in com- 

 paratively slight excess or have the virtue of extreme cheapness. 



The other method of dealing with the mixture of sands and 

 water was described by Mr. O. P. Powell at the August meeting 

 of the Society already referred to. It is of special interest 

 because current sands involving the use of " cyanicides " are 

 daily being dealt with by it. The main point of difference be- 

 tween it and the first method, lies in the use of " Caldecott " 

 de-watering cones. Before passing to the borehole the pulp is 

 pumped into these cones. There the greater proportion of the 

 water is removed and only a thick pulp passes from the bottom 

 of the cones into the borehole. The value of this arrangement 

 in saving the pumping of so much water from the mine has 

 already been mentioned, but the point of most importance to us 

 at the moment is that the sands take something like three to four 

 hours to pass through the cones, so that the " cyanicide " in 

 use, which, it should be said, is added at the mixing chamber, 

 has a considerable time to get in its work before the sands actu- 

 ally pass into the mine. Further, the bulk of the excess water 

 separated off in the cones is led back to the mixing chamber and 

 used over again, thereby effecting economy of the "cyanicide" 

 by utilising any excess of that body present in the water. These 

 less exacting conditions allow of more scope in the selection of a 

 "cyanicide," and at the same time the danger of evolution of 

 prussic acid in the mine is reduced. Of course, this arrangement 

 calls for considerable initial outlay for plant where some thousands 

 of tons of sands have to be dealt with daily, but as the cones do 

 their work without the help of mechanical power, and require 

 very little attention, the running costs are low. 



I propose now to review shortly the various means proposed 

 for the destruction of cyanide in current sands. Various well- 

 known chemicals, nearly all oxidising agents, have been pro- 

 posed for this purpose. In June, iqio. Dr. Moir and Mr. J. 

 Gray read a paper before the Chemical, Metallurgical and Mining 

 Society giving results of their experiments with ferrous sulphate. 

 Under suitable conditions, in contact with cyanide, this body 

 forms ferrocyanide, which is harmless. The principal conclusions 

 arrived at by those gentlemen were that (i) in no case was com- 

 plete conversion of the cyanide effected, although under favour- 

 able conditions the quantity left was negligible ; (2) rise of tem- 

 perature about 2o°C. was harmful to the reaction ; (3) results 

 were entirely independent of dilution ; (4) the reaction was as 

 complete in 5 or 10 seconds as after long standing. 



At the same meeting Mr. H. A. White gave details of labora- 

 tory trials with alkaline permanganates, aldehydes, ketones and 

 alkaline picrates. Of these bodies Mr. White seemed to prefer 

 permanganate. For it he claimed (i) complete destruction of 



