The purification of such alluvial gold alloys is a simple matter. The high specific gravity of the metal allows of separation from most admixed materials by some process of elutriation, or water concentration, or, in arid regions, by dry blowing. The coarser particles are readily picked out, but the finer are usually admixed with heavy sands, consisting of crushed zircons, garnets, cassiterite, wolfram, titanite, menaccanite, and in fact, almost any heavy oxidised compound or mineral. In a few cases platinum and osmiridium also occur. The separation of the sand is effected either by fluxing it when melting the gold, or amalgamating the gold, and thereby separating it from the admixed sands. By smelting the gold freed from sand in a graphite pot with borax glass, the gold is obtained free from any metal, excepting traces, save silver. Should there be any oxides of heavy metals, such as oxide of tin, present the gold should be smelted in fireclay pots, so that these oxides may not be reduced, and the gold be contaminated with base metals.
There is no process for removing silver except in very small quantities from ordinary alluvial gold. Solvents only remove a film. If the alluvial gold be in thin flakes resembling bran, such gold being common in river beds, and far removed from the matrix of the gold, appreciable amounts of silver may be separated by a method to be hereafter described.
Refining of Gold Obtained by Other Processes (Amalgamated Gold)
By far the greater portion of the gold won annually is obtained by amalgamation. Mercury is fed into stamper boxes or pans, so that it will have a chance of coming in contact with the gold as soon as the latter has been liberated by the crushing action of the stampers, or the grinding action of the pans. Mercury will first adhere to particles of gold, but in course of time penetrate them. Particles of amalgam adhere to each other, and in course of time every sheltered spot in the triturating appliances will serve as a resting place for the amalgam, which sometimes builds up into pieces weighing several pounds. Other metals also amalgamate; amongst these are native copper, also arsenic, antimony, bismuth, and some lead from compounds as well as silver compounds.
Pan Amalgamation
In pan amalgamation such amalgamating action on the base metals is more pronounced than in a stamper box, mainly on account of the closer trituration of the metallic ores and the mercury, but also on account of the strong reducing action going on within the pan, finely divided iron and solutions re-acting giving rise to hydrogen gas, carburetted as well as arseniuretted hydrogen, all of which may be readily detected.
Copper Plate Amalgamation
In addition to this a considerable quantity of amalgam is caught on copper plates, which have been amalgamated by giving them a coating of mercury. At first a copper amalgam forms, but after a short time, when running with auriferous ores, a film of gold silver amalgam forms—so long as copper amalgam lies on the surface of the plate, oxidation of the copper goes on, and a film of basic carbonate of copper forms, but as soon as the gold silver amalgam forms such oxidation ceases, and the plates remain bright. When amalgam is first collected from copper plates it is contaminated with copper, but subsequently, unless the copper has been scraped up, the amalgam formed will only contain a minute quantity of the base metal.
The impurities introduced by amalgamation processes thus depend on the quality of the ore, and the care taken to exclude amalgamable metals, although it is not possible to altogether exclude copper caps and fragments of brass, which become mixed with the ore from explosives used.
Purifying Mercury
The first condition for obtaining pure bullion by amalgamation is to use pure mercury. Many methods have been recommended for this. They are based on the fact that impure mercury becomes covered with a film which either may be removed mechanically or chemically. One of the first methods is to shake the impure mercury with a little powdered sugar—the sugar will entangle the scum, and retain it—the mercury may be then run through a pin hole in a stout filter paper.
Other methods are to oxidise or dissolve the impurities with dilute nitric acid, by allowing the mercury to trickle in a thin stream into the acid—the mercury is repeatedly agitated in order to bring any base metals into contact with the acid. Sometimes salts are used for the elimination of metals, for instance, ferric chloride solutions are used for the elimination of zinc or tin—the mercury in this case trickling into a deep vessel filled with a ferric chloride solution.
Methods of purification by retorting have also been proposed and carried out; one method consists in retorting with iron filings, the object of the iron filings being to remove the sulphur. This cannot be done at the temperature attained in retorting. Quicklime has also been recommended for the same purpose, and, finally, a layer of cinnabar, the object of which would be to form sulphides of the metals amalgamating with the mercury, the cinnabar itself being decomposed. This method is not free from objection, since arsenic or antimony are not removed, also the sulphide of mercury itself distils over. A simpler and more effective method proposed and carried out successfully by the writer is to place over the mercury a layer of some oxidising compound, and also alkaline compound, to unite with any volatile oxide which might form and thus retain it. Amongst such compounds are oxide of mercury and caustic potash, or nitrate of soda, nitrate of potash, or, better still, sodium peroxide. At a temperature approaching distillation these substances melt, and metals such as zinc are oxidised, and the oxide fixed as zincate of potash or soda. Arsenic, antimony, lead and sulphur, become respectively arseniate, antimoniate, plumbate and sulphate of sodium, while the non-volatile, heavy metals remain practically unaltered in the retort, or are partly oxidised.
Purifying Amalgam
In spite of having pure mercury to start with, a number of impurities enter the gold
silver amalgam in practice. Some of these can be eliminated by triturating the amalgam with mercury, so as to eliminate any adhering impurities, then by immersing the amalgam in mercury the gold-silver amalgam, if rich in gold, will sink, and other amalgams will float. The latter may be partly removed by skimming them off with a piece of sponge. The amalgam left should be squeezed through chamois leather or calico, when mercury containing a small amount of amalgam in solution passes through, and a gold silver amalgam remains. Lead amalgam may be partly removed by heating the mercury by means of steam, and squeezing the amalgam into warm water; part of the lead amalgam passes through.
Refining Retorted Gold
The amalgam so obtained is retorted, and if the same plan is adopted with regard to oxidising agents in this case as with mercury, the base metals present with the gold will be for the most part oxidised, while the mercury will distil away free from them. The gold, if smelted in clay pots, needs only some borax flux to dissolve the base metallic oxides. The gold-silver alloy remaining is almost free from impurities, these being usually less than three parts per thousand. In ordinary practice this precaution is not taken, but the retorted gold is broken up, and melted in graphite crucibles.