Shepherd. — Treatment of Tailings. 559 



held in suspension, passed off with the overflow water. When 

 the tank was filled the percolation-tap was opened and the 

 tailings washed with water until no trace of acid was to be found 

 in the washings. I found that a very reliable and convenient 

 method of determining when the tailings had been sufficiently 

 washed was to take a little working cyanide-solution in a beaker 

 and to add to it some of the water to be tested. A resulting 

 blue or brown colour indicated that more washing was required. 

 The washing must always be continued until the test gives no 

 colour. 



When the tailings had been sufficiently washed they were 

 transferred to another tank. First a layer of tailings 2 in. 

 thick was placed on the filter-cloth, then a layer of shell or 

 stone lime at the rate of about 10 lb. to the ton of tailings, 

 then a layer of from 6 in. to 12 in. of tailings followed by an- 

 other layer of lime, and so on until the tank was filled, in every 

 case finishing with a layer of lime on the top. By this method 

 of working heavily mineralised decomposing tailings were suc- 

 cessfuly treated with a 0'4-per-cent. solution of cyanide. 



The causes which led me to put the lime in layers were 

 the low bullion- extractions and the high cyanide and zinc con- 

 sumption when the sands were treated in the ordinary way. 

 Besides, the reaction going on in the extractor-boxes was so 

 strong as to cause the zinc to become brittle, thereby tending 

 to form an undue proportion of zinc-fines. 



The bullion- slimes from the extractors were very low-grade, 

 and on investigation this proved to be chiefly due to the presence 

 of iron and alumina. The oxides of these metals left in the 

 tailings after washing in some way passed into solution, and 

 became deposited in the extractors on the zinc. I concluded 

 that the presence of these oxides was the cause of the whole 

 trouble, for the following reasons : — (a.) A small quantity 

 of iron and alumina would saturate a - 4-per-cent. cyanide 

 solution and thus render it incapable of dissolving the gold. 

 (b.) In dissolving the oxides the consumption of cyanide would 

 be increased, (c.) In precipitating the iron from the solution 

 a strong reaction would be set up in the boxes, thus causing 

 the zinc to become brittle and broken. 



It was to overcome these difficulties that I put the lime 

 in layers through the tailings. I found that the solutions in 

 passing through the tailings became partly saturated, and, 

 when in contact with the layer of lime, deposited the iron and 

 alumina, being thereby regenerated and made capable of again 

 dissolving bullion. Thus the solutions passing through the 

 alternate layers of tailings and lime were alternately exhausted 

 and regenerated. Moreover, the layer of lime near the bottom 



