Strong Nitric Acid
Many attempts were made to further purify bullion slimes before smelting. The impurities present, as well as the gold, are in a state of fine division, and, therefore, adapted for solution. It might be assumed that copper and silver present could be removed by means of nitric acid. This acid was tried on many samples of base bullion, but in no case did it give the results anticipated. A large amount of the copper, and a notable percentage of silver, would be dissolved, but even when silver was present to the extent of ten times the gold, the solution was not complete. It is possible that some of the metals are precipitated as alloys, not capable of being attacked by single acids, but more likely that the insolubility of the silver is caused by the formation of such insoluble compounds as thiocyanate, AgCNS. The method was wasteful of nitric acid, much trouble is occasioned by the violent frothing which often takes place suddenly, and the uncertainty as to the solution of free gold, due to the action of nitrous acid, or the inter-action of any chlorides which happen to be present with the nitric acid. Although nitric acid may be used to a slight extent on some favourable class of slimes with advantage, yet there are too many objections to its use even as a partial refining agent.
Strong Sulphuric Acid
The next method of purification, suggested to me by Mr. Watson, of Southern Cross, was refining by means of strong sulphuric acid. The strong acid will convert all the metals usually present in the slimes from which the zinc has been removed into sulphates.
2Ag + 3H2SO4=2HAgSO4 + SO2 + 2H2O.
Cu + 2H2S04 = CuSO4 + SO2 + 2H2O.
It was proposed to use this method on the raw slimes and short zinc, but in this case the consumption of acid would be twice as great as if dilute acid were used. The action with dilute acid being:
Zn +H2SO4 = Zn SO4 + H2.
And with strong acid
Zn + 2H2SO4 — ZnSO4 + SO2 + 2H2O.
If the zinc is at all coarse then sulphate of zinc, insoluble in strong sulphuric, forms, and the action is not complete. Further the strong reducing action of the zinc also causes the breaking down of the hot concentrated sulphuric into sulphuretted hydrogen, and this, re-acting with the sulphur dioxide also evolved, produces sulphur. Sulphides are also produced in the precipitate, so that this method can not be looked upon as suitable when zinc shavings are present. Both antimony and arsenic dissolve in strong sulphuric acid, but the former is precipitated on dilution—arsenic acid remaining in solution.
This method will effect a cleaner separation than nitric acid, and has the advantage of being made use of in iron pots, yet the objections to it are too serious to allow of its being generally adopted. In spite of the strong oxidising action of strong sulphuric acid, there are compounds or alloys of silver which resist its action. On testing some Waihi, N.Z., slimes by this method it was found that the whole of the silver would not dissolve, even after boiling, yet the silver was present in the proportion of 3 to 1 of gold. Another objection is the large amount of acid which must be employed in order to get a satisfactory settlement of the slimes. The high density of the acid retards settlement of fine gold, and if the solution is dilute, silver sulphate when present in quantity is precipitated, and gives much trouble to remove by washing with water.
Dilute Sulphuric Acid and Air
When copper is present one of the simplest methods of removing it is by means of dilute sulphuric acid and air.
Cu + H2SO4 + O = CuSO4 + H2O.
Only half the amount of acid is required as compared with that in the preceding method. The air may be blown in with an injector, or even from a compressor. The sediment should be stirred by the air current from the bottom of the vat. Other metals, as lead, are also converted into sulphates, and when these are all transformed some silver may also pass into solution. The operation requires some hours for completion, but, of course, a stream of air might be passed into a vat of slimes while the latter are under lock and key.
Dissolving Out Gold with Chlorine
Attempts were made to dissolve out the gold from admixed slimes with chlorine. For such a process to be economical the whole of the gold should pass into solution readily, and be capable of removal from the remaining slimes. It was found that the presence of silver hindered, and even prevented, the solution of the gold by forming almost impervious clots of chloride of silver, and that certain salts, such as cyanide of gold, which appears to be present, were not readily attacked. In no case was such an attempt satisfactory. In order to overcome the difficulty with regard to the organic compounds and sulphides, samples of slimes were mixed with oxidising agents, and strongly heated. In one experiment:
10 grammes slimes
1 gramme NaNO3
3 grammes NaCl
1 gramme MnO2
were intimately mixed and heated until a coherent cake formed. The product was then treated with sulphuric acid of such strength to evolve chlorine. After agitation the clear yellow solution containing auric chloride was filtered off. Filtration was exceedingly difficult, owing to the formation of silicic acid, due, probably, to the action of the nitre on the siliceous impurities present. After washing until the filtrate ran free from gold it was found that all the gold had not dissolved, even though a considerable excess of chlorine was present. The silver chloride formed even in the slimes protected the gold admixed with it. Experiments with other oxidising agents and acids were also tried, but the results were not satisfactory, so long as much silver was present in the original slimes.