Table of Contents
Since the introduction of the improved method of precipitating gold from chlorine solution by SO2 and H2S at the Golden Reward chlorination-works, Deadwood, S. D., this modern method has been further adopted in the chlorination-works. It has proved practically most successful in the handling of large quantities of gold solution, and constitutes up to date, the most important improvement, namely, barrel-leaching under hydraulic pressure, the principal progress made in gold chlorination.
The method of refining the gold sulphides which result from this method of precipitation. It has not suffered any changes during the last three years, and is substantially as described below.
Drying and Roasting
The gold sulphides collected are dried already as far as practicable in the filter-press by passing compressed air through them, and are then transferred (care being taken to avoid loss in handling) to light sheet-iron pans, 20 inches wide, 36 inches long and 4 inches high.
Precipitate and filter-cloth are kept separate as much as possible. If dried well in the press, the precipitate is easily detached from the filter-cloth in pretty hard black-brown cakes. The pans, with precipitate and filter-cloth, are now introduced into the muffles of the roasting-furnace.
Fig. 1 is a sketch of the muffle roasting- and melting-furnace at the Golden Reward works. It gives the arrangement and front elevation, and makes a detailed description unnecessary. The parts are lettered as follows:
- C, cast-iron frame;
- D, brick floor of the refinery;
- E, ash-pit;
- H, dampers;
- I, braces;
- K, Dixon’s plumbago crucible, No. 100;
- L, flue to large chimney;
- M, muffle;
- N, firebox;
- O, mould.
The muffle is of cast-iron, 2 by 4 feet in plan, 12 inches high and 1 inch thick. The melting-furnace is 8 feet square, and its ash-pit connects with the outside of the building through an underground tunnel. The roasting-furnace is fired from the back side.
The later plant, constructed by the writer in 1892 for the Black Hills Company, is shown in Fig. 2, in which the parts are lettered as follows:
- A, chimney;
- B, cast-iron roasting-pans;
- C, cast-iron front;
- D, brick floor of the refinery;
- E, ash-pit;
- F, staircase;
- G, basement;
- H, damper;
- I, braces;
- K, Dixon’s plumbago crucible, No. 100;
- L, flue.
The roasting-pans are 2 by 4 feet in area, with sides 4 inches high. The melting-furnace is round, and 3 feet in diameter.
At these works, the dried precipitate and the filter-cloth are transferred to the open cast-iron pans, B; the muffle is done away with, and the furnace is, in consequence, much simplified. Here the roasting-gases escape by the telescope-shaped stack or gas-catcher through the roof. The gas-catcher is balanced by counterweights. Its lower part is enlarged, and is lowered to about 4 inches from the top of the roasting-pans as soon as the furnace has been charged, to allow sufficient free access of air from all sides. If the draft is too strong, the gas-catcher is moved higher and can thus be regulated to get the best condition for roasting.
When the muffle or roasting-pans are charged, the heat in the furnace is kept gentle at first to drive off the moisture, and is raised little by little to a dark-red heat. Sulphur, arsenic and antimony are oxidized and driven off, and the filter-cloth quickly burns to ashes when detached from the precipitate. The whole treatment of a charge can be conducted within two to three hours. The mass now invariably presents a red-brown or yellow appearance, and only a very small percentage of arsenic and sulphur ought to be left. Some care has to be exercised during the whole operation of roasting, not to lose any of the fine precipitate. The draft must be well regulated and stirring almost completely avoided. Steam-drying before roasting is unnecessary, and only adds to cost and labor.
Pulverizing and Fluxing
When sufficiently cooled, the roasted sulphides are carefully transferred by means of a hand-scoop and brush from the muffle or the pans into the pulverizing-drum (see Fig. 1), a cylindrical sheet-iron barrel, 3 feet in diameter and 4 feet long, running on trunnions, provided with an air-tight man-hole and revolved by means of a crank or pulley. Some good-sized cobblestones put into it greatly assist pulverization.
A little borax, soda and niter is now added, according to the composition of the sulphides. Sometimes the solution from leaching has not been quite clear, and a perceptible amount of slimes (ore) has accumulated with the sulphides in the precipitating-tank; or the roasting before chlorination has not been as thorough as it might have been, and considerable quantities of arsenic and antimony have gone into solution with the gold in chlorination, and have been precipitated with SO2 and H2S, and not all eliminated in roasting the sulphides. All such circumstances have to be considered, and the necessary fluxes added, to secure a fusible slag of light specific gravity that will render possible the collection of the gold and a clear slag. If the ore treated in the mill is siliceous, the flux will have to be in general an alkaline one, such as soda, potash, etc. If, on the contrary, the ore is a basic one, a siliceous flux, such as glass or sand, etc., has to be added. If sulphur, arsenic or antimony has remained, niter or metallic iron may be added. Niter, however, must always be employed with caution, as it occasions violent action during fusion. As the conditions are always varying, the fluxing in each case has to be left to the discretion of the chemist or manipulator.
The fluxes are added direct to the sulphides in the pulverizing- drum, and become well mixed during pulverization. They should, at all times, be perfectly dry. Moisture in the flux, anywhere, will surely occasion loss during fusion, gold being carried away in the form of fine dust with the steam out of the crucible. Borax-glass is to be recommended instead of common crystallized borax.
The pulverizing-drum has proved to be the very best machine for this purpose; since, if it is carefully closed, no dusting and consequent loss are experienced.
The gold is now metallic, being reduced by heat in roasting;
Au2S3 + heat = 2Au + 3S, and 3S + 60 = 3SO2. In melting the fluxed and roasted sulphides a crucible of good capacity, and yet easily handled, is of great importance. No. 100, Dixon’s plumbago has proved a suitable size. A little borax-glass or slag is first put into the crucibles, and the mixture is then charged from the drum into the crucibles, each of which is filled to about 2 to 4 inches from the top, and a covering is given of borax-glass or rich slag from previous meltings, which will prevent loss by dusting while the contents are fusing. The crucibles are now placed in the furnace with the assistance of a pair of blocks and tackles and a basket-tongs. A lid is placed on the crucible, and the steadily increasing temperature soon fuses the contents without any boiling or violent action. After fusion the heat has to be kept at very high temperature for some time, to effect a complete collection of the smaller gold-globules. The crucible is then taken out and quickly poured into a conical mould of suitable capacity.
Bullion and Slag
The bullion separates from the slag in conical buttons. Each crucible-melt of good sulphides produces from 100 to 150 ounces of bullion, from .800 to .950 fine. Arsenic, antimony, copper, platinum and silver are the principal impurities. The buttons are remelted, as usual, and cast into a bullion-mould to be ready for shipment.
The resulting slags are still comparatively rich in gold. They are crushed and pulverized, and the gold-shots are panned out and added to the next melting. The tailings from this panning are dried and mixed with the slags of assays from the assay-office, or other lead-containing substances. Metallic iron is added, and the mixture is melted in crucibles which have served for sulphide-meltings a good many times already, but are considered not any longer quite as sound as is desired for this most important operation. The resulting lead-bullion is cupelled, yielding the remainder of the gold. The slags resulting from this second melting are too poor to be re-handled.
The losses in refining by this method are almost entirely mechanical, and depend, therefore, to a great extent on the care and skill of the operator. The flue-dust from the roasting- as well as the melting-furnaces has been assayed, at different times, after continued operations, but has shown comparatively little gold. There is, of course, some loss by volatilization in melting; but it is small, and only noticeable when arsenic or antimony is present in large quantity.
I have once observed on the iron cover of the melting-furnace a white sublimate of arsenic, which showed in some places a beautiful pink color. Some of this pink sublimate was carefully gathered and assayed, and proved to contain considerable gold. Experience has shown me that this loss only occurs at very high temperatures (above the melting-point of gold), and in the presence of a large percentage of volatile metals, such as arsenic and antimony, and can be avoided altogether by careful roasting and fluxing.
Method of Refining of Gold Sulphides
The chemical fact that bromine dissolves and extracts gold from ores is in itself nothing new; but that several thousand tons of gold-ore have been treated with technical as well as financial success, and the gold has been extracted not less easily—in fact, better and more cheaply—with bromine than with chlorine-solution, is surely worth knowing.
Last year the Asiatic cholera invaded Europe, and the price of chloride of lime advanced materially in consequence. I then employed bromine instead of chloride of lime and sulphuric acid, and practiced barrel bromination.
The cost of bromine was at first 35 cents per pound, but we received it later from the manufacturer, on contract, for 26 cents per pound. One to one and a half pounds, was all that was needed per ton of ore that had been roasted fairly well. On well-roasted ores, less than one pound was quite sufficient. The outlay of 26 to 40 cents for bromine per ton of ore, compared very favorably with 80 cents to $1 for chloride of lime and sulphuric acid; besides which, the handling was more convenient, and the extraction was from 50 cents to $1 better on ore that was not roasted as well as it ought to have been. We consequently changed altogether over to bromination. Thousands of tons have been brominated since, and I believe we have fully demonstrated, on a large scale, the technical and financial practicability of this branch of metallurgy. The methods of precipitation with the H2S and SO2, and the refining of the gold sulphides, as described above, worked as well on bromine-solution as it had done before on chlorine-solution, and made no changes of plant necessary.