CHAPTER VIII.

THE MARKETING, SMELTING, ASSAYING, AND COINING OF

GOLD.

What the miner does with the unrefined product of his stamp-mill and concentrator Processes which the yellow metal must pass through before the world sees it as coin-The chlorination and cyanide operations-Acid baths to separate the baser metals from the treasure-The great smelting furnaces and their daily flood of riches-Among the ingots of pure gold at the mint-The assayer's difficult task-The world's output of gold in four hundred years.

A

FTER what are termed sulphurets have been obtained by the miner in the form of concentrates they must pass through still other processes before the world handles them in gold coin, jewelry, and plate. These other processes are not in this day regarded as a part of the mill man's duty, and the material is usually sent to outside assay and reduction works where it is treated generally by one of two methods, the wet and the smelting.

It is interesting to note here what modern methods have done in the way of improvement in the securing of gold from ores. The Robinson mine in South Africa furnishes an admirable illustration of this as does also the Treadwell mine in Alaska. After the treatment of the Robinson ore by ordinary stamping and amalgamation methods the return of gold in a recent year was $1,885,583. The tailings from the stamp-mill were then passed over Frue vanners by which concentrates amounting to nearly three thousand tons were obtained. This

168

HALF A MILLION SAVED.

product was roasted and treated by chlorination and $219,514 resulted. The tailings from the concentrators were in turn passed through the cyanide works and yielded $289,722, more than half a million dollars being reclaimed, which under old methods would have gone to

waste.

Gold, when it comes from the concentrator, usually contains silver. Some gold ores contain nearly all the metals used in the arts, and it will thus be seen that the material sent to the assayers and smelters is of the most heterogeneous description, and many methods must be used to purify the mass. The most important of these are the nitric acid process, the sulphuric acid and the electrolytic. The latter is little used outside the largest smelting plants.

Acid-parting processes depend for their success upon the solubility of silver, copper, and other metals in a liquid which will not attack the gold. Nitric acid was first used in Venice for this purpose, and for many years no other method was known. The operation consists of assorting and proportioning the bullion, granulation of the same, solution of the outside metals in acid, and treatment of the thus parted materials by washing, drying, and melting.

Thoroughly satisfactory as is the nitric acid process, so far as its effectiveness is concerned and the high grade of gold which it yields, yet the comparative high price of the acid and the necessity for using either platinum or porcelain vessels in the operation led to its

SULPHURIC ACID PROCESS.

169

being superseded in many plants by the sulphuric acid process.

The electrolytic process gives the most satisfactory results when the bullion to be parted has been refined in a cupel furnace until it contains not over two per cent. of impurities, such as lead, copper, bismuth, and the like. The material is cast from the cupel into flat plates about eighteen inches long, ten inches wide, and onehalf inch thick.

These plates form the anodes, and are suspended by three lugs cast on one of the long sides. They, therefore, hang with the greater length horizontal. Tanks of California redwood planks are very carefully made, eleven feet long by two feet wide and twenty inches. deep (inside measure). Six partitions are placed crosswise in a tank so as to give seven separate cells or "baths." In each cell three plates or anodes are suspended, alternating with four cathodes. These latter are thin-rolled sheets of pure silver, thirteen by twenty by one-thirty-second inches thick, weighing fifty troy ounces each. The distance between anode and cathode is about one and three-quarter inches.

Both anodes and cathodes are suspended by conducting wires from copper rods resting on the edges of the tank. Two copper bars traverse these top edges, and are connected with the respective poles of the dynamo. The cross rods supporting the plates rest on these bars, but one end of the rod carrying an anode is insulated by a rubber band, while the opposite end of the rod

170

CONDUCTING THE CURRENT.

carrying a cathode is insulated in like manner. The current must therefore pass from one conductor to the anodes, through the solution and the cathodes, to the return conductor. It will be seen that the current is divided between these seven cells and that we have twenty-one anodes connected in multiple with twentyeight cathodes.

A model plant consists of fourteen such tanks containing seven cells each. Ten of these tanks are constantly in circuit, four being in turn cut out for charging, discharging, and possible repairs. These ten tanks are connected in series. The dynamo furnishes a current of one hundred and eighty amperes, with an electromotive force of ninety volts. Such a current requires twenty-two horse-power. The total cathode surface is ten square feet in each tank. There is, therefore, a current density of eighteen amperes per square foot of cathode surface.

Each anode is inclosed in a muslin bag, that serves to catch the undissolved metals, which fall as a black slime. In this are found all the gold and bismuth, the greater part of the lead as peroxide, together with some silver and copper. Below this system of anodes, cathodes, and bags in the bath, is stretched on a box-like frame a piece of cloth, on which is gathered the deposited silver as it is scraped from the cathodes by wooden "brushes." These brushes straddle the cathodes without touching, and are kept moving to and fro by machinery, and they serve, not only to brush off the silver as fast as it is deposited,

KEEPING THE SOLUTION UNIFORM.

171

thus preventing short circuits, but also to keep the solution uniform by gentle agitation.

The solution is one of silver and copper nitrate, to which about one per cent. of nitric acid is added. The acid tends to prevent the deposition of copper with the silver, and about one pint is added to each bath every twenty-four hours. Three-eights of one volt will decompose silver nitrate, while copper nitrate requires onesixth of a volt more and lead nitrate a still higher voltage.

The chlorination process, which is familiar to all gold workers, was invented in 1848. It depends upon the fact that chlorine has a strong affinity for native gold, and readily combines with it, forming the soluble auric chloride. The solution containing the gold can be filtered off from the residue with ease. The subject to be treated is first properly moistened, in the improved method, and then shoveled into a vat with a double bottom. The upper false bottom is perforated and supports a suitable filter. Chlorine gas is passed into the space below this false bottom, and gradually rises until the vat is full. The lid is then adjusted and the whole allowed to remain until the action is complete, when the soluble chloride of gold is washed out through the filter into other vats, where the gold is precipitated. Various precipitants, such as ferrous sulphate, charcoal, sulphuretted hydrogen, and others are used for this purpose.

The presence of any substance which chlorine attacks necessarily causes a waste of the gas and a hindrance