(1) Cupellation And Equation

The process known as cupellation is performed with the object of separating silver from lead by the oxidation of the latter. In this country, it is mostly employed on the highly argentiferous lead obtained by the Pattinson desilverizing method, as described under Lead, p. 35.. The chief object is generally the recovery of the metallic silver in a purer condition, but sometimes the lead oxide (litharge) is the article mainly sought, and then the purification of the silver takes a secondary place. The details of the cupellation process are subject to a variety of modifications, but there are only two essentially different plans, known respectively an the English and the German.

(1) English Process

The operation consists in exposing the silver-lead to the action of air at a nigh temperature on a porous hearth, by which the lead oxide is partly removed in a liquid state and partly absorbed by the hearth, the silver remaining unchanged. The English "cupel" or hearth (Fig. 167) is a wrooght-iron elliptic fiame a, 5 ft. by 2 1/2 ft., crossed by 5 iron bars b 3 1/4 in. wide, and filled with bone-ash c moist oned with water containing a little potash carbonate; this bone-ash filling is well beaten down and dished out till it is only 3/4 in. thick over the bars, but leaving a rim d 2 in. wide all round, except at the end e, where it is 5 in. wide, and through which (the "breast " of the cupel a channel f is cut to allow the liquid lead oxide to flow away into a receptacle without coming into contact with the iron rim, which would be attacked by it. The prepared cupel is placed on a truck, to facilitate its removal into the space it is to occupy in a reverberatory furnace (Fig. 168), the hearth a of which it forma. Here it is 10 arranged that the flame from a coal fire b passes completely across it, and into the two flues c opening into a 40-ft. chimney d.

An air blast measuring about 200 cub. ft. a minute is projected over the whole surface of the metal by means of a twyer e introduced into the cupel at the opposite end to that from which the lead oxide escapes, and provided with a hood and pipe / for conveying the fumes into the chimney. The silver-lead is first melted in an iron pot g heated by a separate file h, and from this it is ladled into the cupel to the amount of 500 to 600 lb., nearly filling it after it (the cupel) has been very gradually heated almost to redness. As soon as the surface .of the metal becomes covered with melted lead oxide, the blast is turned on, and the lead oxide is driven off in waves through a channel cut for it,and into a cast-iron pot placed outside to catch it. As one channel becomes corroded, it is closed, and another is cut; and as the lead disappears from the cupel it is replaced by additions from the melting-pot g, so as to maintain an even level. This goes on for some 16 or 18 hours, during which period 4 or 5 tons of lead will have been introduced, leaving an alloy holding about 8 Per cent. of silver in the cupel; this is let out through a hole made in the bottom, and cast into ingots.

The aperture is closed with fresh bone-ash, and a new charge is supplied, till the cupel is worn out; the life of a cupel does not often exceed 48 hours, with a capacity for treating 5 cwt. an hour. The ingots of metal from the first cupellation, to the amount of about 3 tons, are cupelled anew in the same furnace, but on a hearth provided with a depression in the bottom, capable of containing the mass of residual silver, weighing perhaps 500 lb. The reason for dividing the cnpellation into 2 stages is to be sought in the fact that the lead oxide driven off from the highly argentiferous mass carries away an appreciable quantity of silver with it, therefore the lead obtained by smelting it and the cupel is reserved for treatment with the object of recovering this silver,amounting perhaps to 40 oz. per ton. Sometimes a steam-jet Is used instead of a blowing machine to supply air for the oxidation of the lead, and this plan is said to afford a better product in a snorter time. Much lead and silver is in any case driven away as vapour, by the force of the blast, and partially condenses as oxide in the flues, whence it can be recovered.

The lead oxide is reduced to metallic lead by smelting in a reverberatory furnace with a hearth measuring 8 ft. by 7 ft., lined with bituminous coal, which soon forms a porous coke, capable of protecting the hearth from corrosion while permitting the fluid metallic lead to escape through its pores. The action of the furnace is very simple, the carbon of the burning fuel uniting with the oxygen of the lead oxide and leaving the lead in a metallic state. The charge for a furnace of the dimensions given is composed of 3 tons lead oxide and 6 cwt. small coal.

Fig.167.

(2) German Process

This is adapted for treating leads of low quality, while the English process is only suited to those containing but little antimony and copper. The main difference in the furnaces is that the German (Fig. 168) has a fixed circular hearth a. which is covered by an iron dome b lined with clay and capable of being removed by a crane c; the cupel is about 10 ft, in diameter, and is formed of damp wood ashes, or a mixture of clay and lime, which is dished out and shaped with a central hollow about 1/2 in. deeper than the rest and 2G in. across, where the silver accumulates. Air is introduced by 2 twyers d, and heat is supplied from the fireplace e, in which wood is preferably used as fuel. The lead pigs are piled on a straw bed, and the fire is gradually increased (without any blast for the first 3 hours) till the metal is completely fused, when the supernatant scum (abzug) is skimmed off through the working door f, and the twyers are brought into play to aid the oxidation. The mass is kept at a low-red heat till the impurities cease to separate, and impure litharge (abstich) commences to form.