Here is an article from the Australian Town and Country (NSW: 1870-1907) dated Saturday, Jan. 31, 1891. It describes a process almost identical to the process described in my last post. The chlorine, in this case, is produced by adding dilute sulphuric acid to sodium chloride or chloride of lime (calcium hypochlorite). When dissolved in water, the chlorine gas again makes a "chlorine solution" which is, of course, hypochlorous acid.
While they again use a lead lined iron cylinder tightly sealed for the process, they do not attempt to displace all of the air in the cylinder or pressurize to 60 psi as in the other two processes I described. Rather, ground ore and water are first added to the cylinder and then sulphuric acid and chloride of lime, at which point the cylinder is tightly sealed. It is assumed the resultant production of excess chlorine gas will elevate the air pressure inside of the cylinder to a level at which hypochlorous acid cannot lose its oxygen atom and stabilizes.
Chlorination. I
The chlorination process of extracting gold from its matrix, and from from concentrates, is often freely alluded to in newspapers and amongst miners; and yet, as a rule, the principle is but vaguely understood, even by men who make mining their business. Our explanation will be principally for persons who understand but little, if anything, of chemistry. To the scientific man the great affinity of chlorine for the precious metals has been known almost ever since the distinguished German chemist, Schule, first discovered chlorine, about the year 1774. Chlorine, then, is an element, that is a simple substance, out of which nothing can be produced but itself, chlorine. It is a gas, and has many remarkable properties- bleaching almost all colored substances, and being very heavy as compared with common air, and being also very deleterious to breathe. It is largely consumed by man and the lower animals in food as common salt, which contains a little more than half of pure chlorine. It conies into use in mining in consequence chiefly of its great affinity or liking for gold and silver. If it can get sufficiently near it will seize hold of them, appropriating a certain portion to itself, making what is called chloride of gold ot silver. It being so abundant in nature, salt being so common, it is made cheaply-a little dilute sul- phuric acid put on salt liberates chlorine gas, and its attraction for silver can be witnessed by the curious any day in the Sydney Mint by watching the chlorine gas being put into the bottom of the molten gold in the pot, and as it bubbles up to top seeing it seize hold of the silver, carrying it to the top of the gold as a chloride of silver. In using chlorine to extract gold you simply crush the stone or matrix so fine that it can get at the gold and take hold of it as a chloride. The easiest way to do this is to put the chlorine in solution in water, and so powerful is its action on gold that it dissolves that metal, holding it in solution as water holds sugar. There is no difficulty in making a solution of chlorine, as at ordinary temperatures water dissolves about twice its volume of chlorine. Thus then having liberated the gold from the stone by crushing, and having made chlorine gas, as can be done cheaply, say from common salt, and having passed the chlorine so made through water, and thus made a solution of chlorine, and bringing it and the crushed stone together, in a short time you have your chlorine solution, containing all the gold in solution that was in the crushed stone. Having then the auriferous chlorine solution decanted or run off from the sludge or crushed stone or pyrites, you put something into it which will deposit or throw down the gold. Either one of two things, both of them very cheap and easily obtained, will do this effectuality, viz., a solution of sulphate of iron, common green vitrol, or pieces of common wood charcoal. The first will throw all the gold down as a powder; the other, wood charcoal, will take to itself the gold out of the solution as a brown film, and upon the charcoal being burned away the pure gold remains. This is the rationale or reason of what is called the chlorination process, and any person of ordinary capacity, upon reflecting upon the foregoing, will not fail to comprehend the method used through the aid of certain machinery, which we will presently describe. It may help to understand this subject if we remind the reader that each of the three chief processes of extracting gold from its matrix or stone or pyritous matter consists in bringing it into contact with some substance which has so great an affinity or liking for it that it is taken up or amalgamated, and then the amalgamating substance got rid of or the gold taken from it. Thus in the common stamper battery or grinding mill mercury is the amalgamating material, as all miners know, which takes up the gold. In smelting, matter containing gold is put by intense heat, often assisted by fluxes of various kinds, into a state of fusion, and then brought in contact with molten lead, which, like mercury, has an intense affinity for gold, and it is thus extracted-the lead in the one case and the mercury in the other being ultimately taken from the gold by evaporation, or as regards the lead by cupellation, which is in effect evaporation. As regards the third great process, viz., extracting by chlorine, we have explained upon what principle that is carried out, being, in fact, like the other two, merely making use of a substance which, by its great affinity, will seize hold of the gold and collect it, so to speak.
Of the three processes, the crushing and amalgamating with mercury, the smelting and amalgamating with lead, and the crushing and absorption by a solution of chlorine, each is suitable according to the way the gold is found in the stone or auriferous material. Rich pyrites or black sand, or concentrates, require smelting as a rule, because all the grinding, to whatever degree of fineness it may be carried, will not liberate all the gold from the iron and metals, so that mercury can act on it. Again, crushing by stampers or grinding in mills will be suitable where much of the gold is fairly coarse, and, as it is called, free in the stone. Arid chlorination comes in and is useful where gold is free in the stone, but is so very fine that in crushing in the ordinary way the mercury can't take hold of it but is carried away in the sludge or tailings. Chlorination also is useful in extracting the precious metal from concentrates, though in this case it may be found more suitable to sell them to the smelting works than to treat them by chlorine at the mine.
Having thus described what may be termed the principles of chlorination, we now describe the appliances by which they are carried into effect. First, the stone is crushed dry, arid, as in the case of the iron-clad reef at Cargo where chlorination is carried on, by heavy steel rollers driven by steam power. Next the crushed stone is calcined in an ordinary reverberatory calcining furnace to eliminate all the sulphur and arsenic which it is advisable to get rid of before treatment by chlorine. Then the calcined crushed stone is put into a large iron barrel about 8 ft long by about 3 ft in diameter and lined with lead, because chlorine acts powerfully on iron as it does on gold and silver, but will not touch lead.
On page 25
A sufficient quantity of water is put in the barrel to make a thin sludge, then a quantity of chloride lime-that stuff much used as a disinfectant, and which is simply common lime saturated with chlorine gas-is put to the sludge, and upon that a small quantity of sulphuric acid. The barrel is then closed down air-tight, and made to
revolve very slowly. The acid acting on the lime evolves the chlorine, which is taken up by the water, and the chlorine solution takes up the gold in the stone. After some hours of the barrel revolving, the whole contents are turned out into a cask or receptacle and allowed to settle. The clear liquor is then drawn off, and made to pass through a cask or vessel partly packed with common wood charcoal, which, as the auriferous chlorine solution passes through it, takes the gold
to itself; and upon the charcoal being burned away, say in an iron pot, the pure gold remains. This, then, is the chlorination process, which, we think; our readers-even those quite unacquainted with chemistry-will clearly comprehend. Whether this process, admirable as it is in many cases, should be used at any particular gold mine is a
question for serious consideration. As a rule, the stamper battery and amalgamation with mercury and concentrating the pyrites is the best. In
some cases, however, such as the celebrated Mount Morgan stone, chlorination is of the greatest advantage on account chiefly of the extreme fineness of much of the gold.
