It has become pretty general to use a small proportion (say 1 to 2 per cent.) of sodium with the mercury employed for amalgamation. The object of this is to prevent the surface of the mercury being coated with a film of sulphide, by which it is rendered partially inactive and divided into little particles, an occurrence technically known as "flouring" or "sickening"; the sodium acts as a preventive by decomposing the mercury sulphide with consequent formation of sodium sulphide, and leasing the mercury free. The reader interested in this subject will do well to refer to Lock's ' Gold,' where the use of sodium-amalgam and of many other substances for economizing the mercury is discussed at length.
Having obtained an amalgam (or mercury-silver compound) by any of the methods just described, the next step is to effect the separation of the two metals without losing either. This is performed by subjecting the amalgam to heat in a retort which admits of the collection of the mercury thereby distilled over, leaving the metallic silver behind in a free state, and ready for melting into ingots.
Before retorting, the liquid amalgam is first relieved as far as possible of the excess of uncombined mercury. This may be done by placing the mass in a narrow wooden cylinder about 8 ft. high, when the liquid mercury forms a stratum at the bottom; but the separation is more generally effected by squeezing the mass in a canvas bag, when the liquid portion strains through.
This liquid portion is not pore mercury, but contains about 20 oz. silver per ton; it is need for amalgnmating a fresh lot of ore. The solid portion remaining in the canvas is a pasty mass consisting of about 80 Per cent. mercury, and 20 Per cent. silver, with email proportions of the various other metals originally present in the ore, especially antimony, bismuth, copper, gold, lead, and zinc. This pasty amalgam, made up into balls, is' submitted to distillation in furnaces of varied construction.
A simple form is shown in Fig. 164. The balls of amalgam are put into iron trays a about 5 in. apart, supported on an iron rod b traversing their centres, and standing in an iron box c filled with water. When the trays a have been charged with amalgam, an iron bell d is dropped over them, with its mouth resting beneath the surface of the water in c, and a fire is made in the compartment e, successively with wood, peat, and charcoal, so that the upper portion of the bell becomes red bot; the mercury is thereby vaporized, and passes down to the water beneath, where it condenses, the water being kept cool by continually renewed supplies entering the trough f in which the iron bot c stands. The distillation of 3 cwt. of amalgam is complete in about 8 hours, when the bell is lifted off, and the spongy masses of impure silver are collected from the trays.
A more modern form of retort is shown in Fie;. 165. The cylindrical retort a is made of l 1/2-in. cast iron, and extending its full length beneath, and a vault above communicating with the chimney by flues k. The amalgam is either charged directly upon the bottom of the retort, or put into trays of suitable shape. The surface of the trays or retort is first covered with a thin wash of clay, battery slime, or whiting, measures 3 to 5 ft. long by I ft. in diameter; the charging end b is cast with a flange into which the door shuts closely, and fastens by means of a bar turning under two projecting lugs. The exit end c of the retort is conoidal i in form, gradually diminishing to 2 1/2 in., from which point the exhaust-pipe d passes to the condensing-tube e; this is made on the principle of a Liebig's condenser, consisting of a pipe / of considerably larger diameter than the exhanst-pipe d (which passes through it), and kept constantly supplied with cold water Sowing in at g, while it escapes (as it becomes heated) through h. The bottom of the exhaust-pipe dips just beneath the surface of water contained in a vessel i placed below to catch the condensing mercury.
The retort is set in a brickwork furnace of Simple construction, r sting either on an arch or on iron bars, with a fireplace or with a layer of wood ashes or paper, to prevent adhesion of the metal. When the amalgam has been introduced, the door is closed and bolted, and the fire is gradually raised too great a heat being avoided at first, as tending surface of the metal in contact with the retort, and thus prevent the liberation of the mercury from the inner portion. The charge is some 1200 lb., and the firing occupies about S hours. When volatilisation of the mercury has ceased, the retort is allowed to cool down, and the bullion is withdrawn.
Fig. 166 illustrates another modern form of distilling apparatus, consisting of a charcoal fire a which heats an iron crucible b measuring 23 in. wide and 11 in. deep; the vaporized mercury is collected in the iron hood c, and passes by the urea-neck d into the condensing tube e, which traverses a cistern of water f, escaping at g into a suitable receptacle. The inside of the crucible in coated with lime wash, and an iron plate similarly coated is fitted into the crucible, and withdrawn with the silver adhering to it at the close of the operation. The charge is 4 cwt., and the duration is 5 hours.
The impure metal left after the distillation of the mercury is melted down, commonly in crucibles made of a mixture of fire-clay and graphite. During the melting, the mass is briskly stirred with an iron rod, to facilitate the escape of fumes of antimony, bismuth, and zinc oxides, and the formation of a dross containing copper and lead oxide', which is skimmed off as long as any appears, after which, the metal is cast into ingots. Where very large quantities are dealt with, a furnace holding several egg-shaped retorts is employed for the melting.