THE CYANIDE PROCESS.

Otago Witness, Issue 1994, 12 May 1892, Page 14

THE CYANIDE PROCESS.

In order to understand the present systems of gold extraction, to properly appreciate the work already done, and to gauge the difficulties to be surmounted, it is necessary for us to look back and trace the connection between the known forms of gold as found in nature and the methods used to separate it from its base environments.

In the earliest ages gold would naturally only be found in nuggets and grains (our word caret is derived from a Persian word meaning a grain), which required no means of separation beyond picking up— happy age when gold had only to be picked up !

From grains the ancient would come down to gold dust, and this was no doubt separated from the sand or earthy matter^with which it was associated by the skillful., use of air currents, which were caused to blow away the sand, leaving the precious metallic dust. This method is still practised in Arabia and the East. Wherever water was plentiful it was found that it did the required separation better and more economically than air,, a method of separation which has been and still is in use. everywhere all over the world. The prospector, digger, explorer, and even the tourist nowadays provides himself with a "pan" in which to wash a sample of "dirt" at the nearest stream. From the pan comes the " cradle," " long torn," and the innumerable mechanical arrangements for the separation of gold from earthy matters. Up to this point we have only mechanical means of separation which depend on the high specific gravity of gold compared with 6and, clay, &c, but at this stage in the evolution of gold extraction we find the first move made toward a chemical method. When there was a lot of rich dirt in his pan, the digger found it very difficult to wash away the last of the sand without losing a large portion of the finely divided gold, so he hit on the expedient of pouring in a little mercury, which formed a heavy, massive, though fluid alloy with the gold, making the separation of the last portions of the light granular sand a very easy matter. This plan is still in world-wide use alike by the solitary digger who lives in a lonely canyou, and by the well organised company that "hydraulics" 1000 tons of " pay dirt " her day. Let us now look at the PRESENT STATE OF THE GOLD INDUSTRY. Let us imagine a digger who has exausted all the gravel and pay dirt in the canyon or gully ; he travels up tho gully looking to the right hand and to the left for traces of the precious metal ; occasionally he finds a piece of goldbearing rock, and is led on and on till he finds the source of these auriferous stones to be a reef. Now begins gold mining proper. A shaft is sunk on the reef or a tunnel driven into it and great masses of rock are brought to the surface, and naturally the miner is led to imitate Nature by crushing this rock to a fine sand, and then he modifies the treatment formerly given to alluvial deposits, to adapt it to the new circumstances. The ore, immediately on being crushed to powder (which is generally done by huge gravitation stamps), is carried over au amalgamated copper plale by a stream of water. In passing over the amalgamated plate the gold is caught by the mercury, while the sand, now called tailings, is washed off. The gold is recovered by scraping the amalgam off the copper plate at stated intervals, generally once a month, when by distillation the mercury is recovered as well as the gold separated. If all the gold the ore contained existed in the heavy metallic form, the recovery by this method would be complete and the loss nil', but the tailings are frequently found to contain a little gold, and on close examination particles of pyrites and sulphides of the various base metals are found diffused through tha mass. When these metalliferous particles are separated from the mass of tailings, ib is found that they principally contain the gold, so that it is now usual in practice to pass the tailings through some form of concentrating machinery, of which the well-known Frue vanner is a good example, whereby the pyrites aud other sulphides are retained in virtue of their higher specific gravity and the sand washed away. The rich proportion now called "concentrates" may contain up to 20 ounces of gold per ton, though four or five ounces is much nearer the average figure. 11' THE CONCKNTRATES AHE EXAMINED

closely, even microscopically, no free gold cm be distinguished, aud if they be treated with mercury, th^y will yield to it little or none of their gold contents. Sometimes a considerable portion of gold mtiy be gofc from them by a long-continued grinding in cast-iron pans in presence of mercury. This grinding beems to force the sulphur and base

metals, as well as the gold, into combination with the mercury, so that the bullion got from the amalgam often contains over 95 per cent, of copper, lead, and other base metals, while there is a corresponding loss of mercury which is carried away partly as sulphide and partly in a "floured state." ' The flouring is caused by the small globules of mercury being coated with a film of sulphide of mercury, which prevents the particles coalescing. Thus there is a double losS-chemical, by the famfon . j sulnhide of mercury, and mechanical, by flouring. E loss is so well known that concentrates are onl? treatedTy this form of amalgamation ?n lS£^ where Nature forbids any more LlaSte Process. Sometimes the amount of foTSS te lessened by roasting, the concentrates before amalgamation, bub this is by no means a perfect remedy, as the roasting reoxide to oxidise and waste their equivalent of mercury. Attempts have also been made to decrease the loss of mercury and increase the yield of gold by adding to the contents of the pan all sorts of chemicals to "doctor" the "sickened "-that is, floured mercury These chemicals are generally mixtures of sulphateof concentrates is limited, and number of mines, it is common for some to deal with the concentrates of the district. I will not take time to describe the various precedes of smelting, nor could ib be done, at any evening 'SeroS; but we may bear m mind S which we are all familiar Because of Se obvious impracticability of smelting reQuiring well-built brick furnaces, coals, fluxes &J?at the typical gold mine high up on the mountain range, possibly near or beyond the CbeaP THE FIRST AND ESSENTIAL OPERATION prior to chlorinationproper is roasting. X i« the sulphur is expelled, and their affinities The concentrates are charged into the fui=nt^:i»«s« cSorinatTon is practised on the most extensive scalethisroasting geneiaUy ta^«*ou* 24^J£ and consumes from a minimum of half a cord to a cord of wood per ton roasted. (A cord of wood gives about as much heat as a ton of coal.) It is a common saying that the success of chlorina:«on depends more on the furnaceman than on the chlorinator, and on looking into hard chemical facts we find that this » really^eoase, for if only one-half per cent, of iron were left unoridised Lit would absorb nearly 1 per cent of chlorine, equal to about 3 per cent, of bleaching powder, and all this before the gold gets a Sngle molecule of chlorine Towards the end ZZZZSft dSL^Se 3 « form chlorides when they get chloride ßented to them in the free state. This chlor dising device is, however, only partially effective, as the chlorides formed are apt to 'mmediately decomposed under the influence of hot »«.««* in the caseof lead the oxide of sul P hate ™" n m fc combine with chlorine in the furnace, but combines with it readily when offered moist and at the ordinary atmosphere temperature.

THE ROASTED AND OXIDISED ORE is now sprinkled with water to make it slightly moist, and is then charged into a wooden vat having a perforated false bottom. The chlorine, generated at an outside source, is led in between the true and false bottom, and gradually permeates upward through the mass of the porous ore. The small amount of water held by the ore then becomes a saturated solution of chlorine, which gradually acts on the gold, so that in the course of one or two days it may be washed out as the soluble chloride by a further amount cf water. This weak solution or chloride of gold is run into a tank, a solution of ferrous sulphate added, precipitate of gold allowed to settle for 48 hours if possible, and the 'supernatent solution allowed to flow off. When sufficient gold precipitate has accumulated, it is collected and run into bars. Besides the Plattner form of chlorination which I have described there are others which vary from Plattner and among themselves in the manner of application of the chlorine— most of them using revolving barrels instead of open vats for the chlorination proper. The best known of these are : The Mear's process, where chlorine is used under its own gaseous pressure. The Newbery-Vautin, where air pressure is used. The Pollok, recently described before this section of the society, where hydarulic pressure is used, and the Tuies, where the chlorination is done in a barrel without any pressure.

(To be continued.)