Art. LXII.—On the Treatment of Partially Decomposed Pyritic Tailings by the Cyanide Process.
[Read before the Otago Institute, 14th November, 1905.]
The first stage in the weathering of tailings or sands containing sulphides of iron is the decomposition of the sulphides into sulphates and free sulphuric acid. When clayey matter is present in the sands sulphate of alumina is formed. The next stage in the decomposition is the oxidation of the sulphates into oxides. Consequently, when mineralised tailings have been exposed to the weather for any length of time they will be found to contain sulphates, oxides, and sulphides in various stages of decomposition.
Soluble sulphates and freshly formed oxides possess a much more injurious effect on solutions of potassium-cyanide than fresh, clean sulphides. Hence, in the treatment of piles of mineralised tailings the first thing to be considered is the best means of getting rid of the sulphates and oxides.
The method of handling and successfully working old pyritic tailings at our plant in Coromandel was as follows: With a good jet of water the tailings were broken up and washed through a ¼ in. or ⅜ in. screen, then lifted with an ejector and run into the tanks through a distributor. This broke up the material and removed the sulphates, while most of the oxides, being
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 another 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 successfully 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 consumption 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 0·4-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
of the tank rendered the solution in good condition for the extractor-boxes.
By this method of treatment the following results were obtained: (a) The reaction in the extractors was rendered normal; (b) the bullion-extractions were all that could be desired; (c) the cyanide-consumption was reduced from 4s. to 1s. 6d. per ton; (d) the zinc-consumption was reduced to about one-quarter; besides, there was a much less quantity of slimes to handle.