Cyaniding the Sands

The tailings from the vanners pass to 13 steel cyanide vats, 22 feet 6 inches diameter, 10 being 10 foot deep and 3 of them 8 foot deep. Butters and Mein’s distributors are employed, the vat first being filled with water. The overflowing slime is pumped to spitzkasten by a Tangye 4 inch centrifugal pump having a capacity of 200 gallons per minute, a second pump being kept in reserve. A slotted tube "rod sample" is taken during filling, and the assay of this should be comparable with the corresponding "plate head" sample. The treatment for any vat is not posted until these assays are to hand - the ore being variable in grade and silver-gold ratio.

In consequence of the high silver values, strong solutions are used, the bulk being kept low, about 0.35 ton per ton of ore during contact, and about 1 ton of wash solutions used afterwards. The general outline of the method of treatment adopted by Mr. Deane for a charge of 140 tons of average grade is as follows:-

1. Allow to drain completely.
2. Displace the retained water by an alkaline wash of 4 tons medium solution + 200 lb normal soda carbonate (soda ash).
3. Continue displacement by adding the working solution of 8 tons strong solution + 270lb. cyanide (NaCN), following this by repeated lots of 10 tons of strong solution, until the effluent shows 1.1% total cyanides (equal to 0.7% free KCN).
4. Close the percolation cock and start the injector, i.e., a small air-jet at the bottom of a vertical stand-pipe leading from under the filter. This little air-lift raises the solution to the top of the vat and keeps it in continuous circulation during "contact."
5. After 4 or 5 days "on contact" a rod sample is taken and generally shows the values to be sufficiently reduced to proceed.
6. Drain off the strong solution, and then wash by 5 tons medium solution, followed by 80 to 120 tons weak solution.
7. Drain well, take a rod sample and sluice the residues out into river, if value is satisfactory. When the vat has been half emptied a check sample is taken from top to bottom.

If the stock of week solution is small, a water wash is given to displace the last of it, but not otherwise. The effluent solutions are tested by titration with silver nitrate in the presence of KOH and KI indicator, the total cyanide indicated being expressed in terms of KCN. (Potassium cyanide):-

When titrated without the indicator, the end point is very difficult to determine but the amount of free cyanide is approximately:-

Owing to the variable nature of the silver contents of the ore, the strength of solution used for "contact" has to be varied, and the following examples give a good idea of the practice on rich, medium and poor ore since tube-milling started. The solutions formerly used were stronger, but finer crushing has had the effect of simplifying the treatment. The solutions are cleaner and work more effectively so that less cyanide is required.

Table F.

When a vat filters slowly, washing and draining are assisted by vacuum. There are two pumps with cylinder 6 inches diameter by 6 inch stroke running at 40 r,p.m. The vacuum solutions go to a receiver and then to a settler before passing to the zinc boxes.

Normal soda carbonate (soda ash) was adopted for alkali wash after trials had been made with it and lime, the former being the more efficient. It is preferred to caustic soda, as being less objectionable to handle.

The solutions pass to settling tanks and thence to six 6-compartment zinc-boxes, each having 360 cubic feet zinc space. Three of these are used for week solutions showing from a trace up to 0.3% total cyanide, one for medium solutions with 0.3 to 1.0% and two for strong solutions with more than 1.0%. After passing the boxes the solutions show from a trace to 4 grains of gold and from 8 to 22 grains of silver per ton.

During the author’s stay at Karangahake, Mr. Deane gave one vat special treatment to illustrate the necessity for strong solutions to attack the silver. The charge was in a vat 8 feet deep and consisted of 106 long tons, assaying 16.33 dwt. Gold and 8.02 ounces silver.

After draining and sampling, percolation was begun with weak solution, the effluent showing 0.2% free cyanide. After 24 hours the sand was sampled, and showed an extraction of 27.5% of the gold and 4.3% of silver. Week solution was continued for another day and then the vat was drained, having had 58 tons of week solution. The effect of 0.4% solution was next tried by running on weak solution in 10 ton lots, to each of which was added 38.5lb. of sodium cyanide. When the effluent showed 0.4% free cyanide (KCN) "contact" was established, there being 53 tons of solution. The sand was sampled each day, and after three days contact the gold extraction had risen to 90%, but the silver only to 30.8%, the residue being worth 11 shillings per ton in silver alone. The free cyanide had fallen to 0.36%, and to raise it to 0.5% an additional 126lb. of cyanide was therefore added (21lb. every 4 hours). After three more days the extractions were, gold 92.5%, and silver, 67.6%. The vat was then drained, washed with 30 tons strong solution, showing 0.49% free (0.96% total) cyanide, then 5 tons medium and finally 160 tons weak solution. After this it was drained, sampled and discharged, the residues carrying 0.79 dwt gold and 1.63 ounces silver, indicating extractions of 95% of the gold and 79.5% of the silver. The total cyanide added was 192.5 + 126= 318.5lb., and if the whole of this had been added to the contact solution in the first instance better extractions would have resulted.