205 SOME NOTES ON TREATMENT OF SANDS FOR STOPE FILLING. 



Great as the mechanical difficulties are in connection with the 

 handling- of the pulp, they are being- gradually overcome, and 

 the main interest is certainly chemical. All the banket ores con- 

 tain pyrites, some greater and others lesser amounts, but all 

 contain some. Now in the hot humid atmosphere of the mine 

 the pyrites oxidises, producing sulphuric acid, so all mine water 

 is acid. This factor of itself leads to considerable expenditure on 

 renewals to pipes and pumps. Then the tailings, as has already 

 been said, also contain pyrites, and oxidise, so that accumulated 

 tailings are very strongly acid. For this reason, in the early 

 experiments, the wear and tear of the pipes was terrible. This 

 has been overcome by the addition of alkali in the form of lime, 

 but the amount required makes the process somewhat costly. 

 Current tailings or sands, i.e., the product as it comes from the 

 cyanide leaching vats, is always slightly alkaline, due to the 

 addition of lime at a certain stage of the metallurgical process ; 

 consequently, if they are used, the difficulty of acid is overcome, 

 but another one is substituted through the presence of cyanide — 

 for they always contain a certain amount of this bodv. One can 

 easily realise what might happen if acid mine water were to 

 come into contact with tailings containing fixed cyanide. Given 

 enough of both, every person in the mine would be poisoned by 

 prussic acid. The problem to be solved on the Rand at the 

 moment is, how to fix this cyanide so as to render it innocuous, 

 and the problem is far from being a simple one. There are then 

 two sets of raw materials available for stope filling ; firstly, accu- 

 mulated sands containing no cyanide worth speaking of, but a 

 great deal of acid ; and secondly, current sands containing no 

 acid but a good deal of cyanide. 



A good argument in favour of the use of current sands is that 

 they can be taken from the treatment tanks direct to the chamber 

 in which the mixing with water is carried out, whereas the accu- 

 mulated sands have, at considerable cost, to be shovelled from 

 the dump into trucks, and conveyed sometimes over considerable 

 distances to the mixing chamber. 



Taking all the points mentioned into consideration, one is 

 inclined to think that, although both classes of sands are in use 

 to-day for stope filling, as the practice develops, current sands 

 will almost exclusively be used, provided a cheap and suitable 

 means of destroying the cyanide in them can be found. Mention 

 has already been made of the two methods of stope filling in use 

 on the Rand to-day, and in view of what has just been said, it 

 will not be, out of place briefly to compare these methods as to 

 their suitability for dealing with current sands. 



The first method referred to was described by Mr. E. Pam 

 at the June meeting of the Chemical, Metallurgical and Mining 

 Society of South Africa. By it, the pulp passes direct from the 

 mixing chamber into the mine, so that only a few minutes elapse 

 between the time of mixing with water and the moment when 

 the sands reach their final resting place in the stope. There can 

 be no objection to this so long as accumulated sands free from 

 cyanide arc used, but if current sands, involving the use of 



