Lead is now refined in four large plants by electrolyzing in fluosilicate solution. The main advantage is the recovery of bismuth, but, even with the recovery of bismuth, it is questionable if it is the more economical process. Parkes' process is the method commonly used.
Parkes' process depends on the fact that, if zinc is dissolved in molten precious-metal bearing lead and the mass is allowed to cool, the crystals first separating carry nearly all the noble metals together with much zinc and lead.
The process is executed by melting the lead in large kettles and by heating with zinc to about 1 per cent of the weight of the lead until the zinc, which has a little higher melting point, is dissolved. The lead is cooled slowly, and the crystals which eventually begin to form throughout the mass are skimmed off and placed to one side. This is continued until the lead begins to chill as a mass. The lead is heated again and the process is repeated. After the second skimming the lead is heated and run out into refining furnaces where the dissolved zinc, 0.65 per cent, is eliminated by standing in the hot furnace or by blowing air or steam through. The lead then is run out and is molded into 100-pound bars for the market lead of commerce.
Fig. 44. Fiber da Faur Tilting Furance.
There are, of course, many details that cannot be enumerated here; it ought to be said that the lead as it comes from the blast furnace usually requires a preliminary oxidation, or softening, to get rid of all its dissolved copper and antimony; the zinc must be stirred most thoroughly into the molten lead, usually by mechanical means; the crusts are best squeezed to rid of as much lead as possible; the molding may be by hand, by siphoning off into a circle of molds with a movable trough, or by pouring several molds at once on a conveyor.
Fig. 45. English Cupellation Furuace.
The zinc crust which is removed from the desilverizing kettles is taken to a retorting furnace like that shown in Fig. 44. It is called a Faber du Faur tilting furnace; inside is a retort which is heated by the oil burner seen at the side near the bottom. The zinc is distilled off and collects in the condenser which is kited on in front* This condenser on front is nothing more than a retort which has become unsafe to use further for holding the heavy charge inside. It is seen that the furnace is mounted on trunnions so that the rich lead can be poured out after the zinc has been eliminated. The zinc is volatilized from the retort and condenses in the outside receiver from which it is tapped at intervals into the cast-iron mold seen ready on the carriage.
The rich lead from the retort next is taken to the cupel furnace in which the lead is eliminated and the precious metals are left by themselves as dore silver. The cupel furnace is operated as indicated by Fig. 45. It consists of a shallow oblong hearth in which is melted the rich lead; it is supported on wheels so as to be drawn out; it is adjustable so as to tilt up or down by the operation of the screw wheel seen in front; it is surrounded by brick walls with a good draft up the flue; from one side the flame from coal or gas plays over the metal while from behind a blast of air squirts onto the bath to oxidize the lead. The litharge formed by this oxidation floats to the front and dribbles over the breast to fall into the little pot in front of the man. The adjustment is kept so that only the very uppermost or surface layer - the newly formed litharge - can flow out; the metal is kept behind on the hearth. In this way the operation proceeds until all the lead has left the bath and only silver remains.
The final bullion left on the cupel hearth is the final concentration of all the precious metals originally in the ore. For electrolytic refining, it is taken to a silver refinery as described in the section under Copper; or, the silver may be dissolved out with sulphuric acid, the metal precipitated with copper, and, after collecting and drying, may be melted to fine silver.