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Art. LXIV.—On the Liberation of Hydrogen in a certain Variation of the Apparatus used for cyaniding Gold.
[Read before the Wellington Philosophical Society, 16th February, 1898.]
Two theories have been formulated to explain how gold becomes dissolved in the cyanide process. One is that given by a joint patentee of the process, Mr. MacArthur. This assumes that water is decomposed, its oxygen being used to replace the
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cyanogen necessary to cyanide the gold, while its hydrogen is evolved. Now, this theory is that of the minority, and is still, I believe, held by its author. My own experiments in this matter gave, as I have already stated,* results entirely against it, and I have seen no reason to alter my expressed opinion in this matter—that is, as regards the practical working of the process; but by an apparently trivial and unimportant variation in cyaniding gold we actually do get a liberation of hydrogen. For this it is only necessary to so arrange that the negative gold of the cyanide apparatus (the voltaic cell, as it really is) shall be placed in an acid solution—say, in hydrochloric, sulphuric, or acetic acid. The diagram here given explains what I mean. A is a vessel containing potassic cyanide of any strength, in which is a sheet of gold-leaf (gummed to paper). B is another vessel, containing the acid in water heavily salted to drive out the oxygen that might interfere with the result. In this vessel is a gold wire d, which is con nected with the gold-leaf c. Interpolar connection is completed by the bent glass tube e, that is full of hydrated, but almost solid, gelatine, slightly salted for conducting purposes, if necessary.
Long vert: section
Soon after this arrangement is set up a stream of gas may be observed issuing from the gold or platina wire. This is hydrogen, and the evolution goes on continuously till all the gold-leaf is dissolved. It is necessary to have a small cathode and a large anode to be certain of success in the experiment, as cyaniding is at the best a slow process, and even concentrated solutions of the acids, and salts let in oxygen fast enough to oxidize hydrogen to a notable extent, and thus prevent its liberation being noticed. It is a singular fact that if potash, or any alkaline solution, were substituted for the acid in B I was unable to observe any evolution of gas, yet the currents formed appear as strong as when the acid was used. It is possible, however, that potash, by its great affinity for water, so rapidly determines the oxidation of hydrogen that none is to be seen escaping.
[Footnote] * Mines Report for 1895, p. 88.
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These results, while they clearly show the unexpected fact to many that hydrogen can be liberated in a cyanide process, show just as clearly that in the process as carried on at the mines the gas is never evolved, but is, so to speak, only transferred in a liquid form to the oxygen present at the anode. The process demands that the oxygen shall be present there and in quantity, while the alkalinity of the solutions around them favours the rapid union of this gas with oxygen before it can assume the gaseous form, or even before it can in reality become free.
It should be stated that a brisk evolution of hydrogen is afforded by cyaniding copper, even when the anode is in an alkaline solution. This branch of the investigation requires further researches.