Page:The American Cyclopædia (1879) Volume X.djvu/267

 LEAD 261 opening for the removal of the litharge. The test having been previously heated, melted lead is poured into the cavity in the hone ash, and oxidation at once commences. A constant supply of lead is kept up by gradually protru- ding pigs of lead through openings at the back of the furnace. When the lead in the test has become enriched, say from 200 or 250 oz. to 3,000 oz. per ton, it is tapped off, by drilling a hole through the bottom of the test, into an iron pot below, and cast into pigs. After tapping, the hole is plugged up with the same
 * laterial of which the test is composed, and

the. process resumed. The enriched lead is placed on a fresh test, and the operation con- tinued until the silver is fine. The loss of lead in the English process is said to be 5 per cent. The tests and hearth in both processes which have become soaked with litharge are broken up and added in the ore smelting. The amount of silver in lead that will repay extraction by cupellation was formerly considered to be about 8 oz. per ton. Within the last 50 years two processes of concentrating silver in lead have been invented, by means of which silver to the amount of 2 'oz. or less can be profitably extracted. The first of these processes, intro- duced by Pattinson about 1833, was univer- sally employed until replaced in a great mea- sure by Parkes's process, which was perfected about 1866. Pattinson's process depends on the fact that when molten lead containing silver is allowed to cool slowly, crystals of lead near- ly free from silver separate, which can be re- moved by means of a perforated ladle, leaving a lead much enriched in silver. Lead to be submitted to this process should be refined, and contain only traces of antimony or other metals which impede the formation of crystals of poor lead. The following will serve to give a gen- eral idea of the manner of carrying out this process in practice. A battery of seven large iron pots, capable of holding 6 tons of melted lead each, is arranged in a straight line, each pot being provided with a separate fireplace. At one end is a smaller pot holding- 3 tons for the "poor" or marketable lead. Pot No. 4 we will suppose to contain 126 cwts. of molten lead, containing from 7 to 8 oz. of silver per ton of lead. As fast as crystals of lead form on cooling, they are taken out on a perforated ladle, allowed to drain thoroughly, and placed in pot No. 5 ; 90 cwts. of lead, con- taining from 3 to 4 oz. of silver, are thus transferred, and the remaining 36 cwts., con- taining 16 to 8 oz., are ladled into No. 3. To No. 5 is added 36 cwts. of lead, with the same amount of silver as that already in it (16 to 18 oz.) ; and'of this 90 cwts. of crystals, con- taining from 2 to 2 oz., are ladled into No. 6, and the 36 cwts. remaining, holding 7 to 8 oz., transferred to No. 4. No. 6 is treated in the same way as No. 5 ; 90 cwts. of lead crystals, containing 1 to 1 oz., being ladled into No. 7, and 36 cwts. of 3| to 4 oz. into No. 5. From No. 7 crystals containing 9 to 10 dwts. of sil- ver are ladled into the market pot, and then cast into pigs ready for sale. The nature of the operation in the other direction toward pot No. 1 is sufficiently indicated by the above. The operation goes on continuously, market lead being produced at one end, and at the other rich lead which is cupelled. It is found that concentration of the silver beyond 200 to 300 oz. to the ton of lead is not advantageous. From 846 cwts. of lead, of 7 to 8 oz. silver to the ton, there is obtained (in an example given by Pattinson) 36 cwts. of rich lead, containing 160 to 170 oz., and 810 cwts. of poor lead, with 7 to 8 dwts. ; ratio of rich to poor, 1 : 22'5. There are numerous systems of conducting the process, but the above two-thirds system is perhaps the one most generally employed. Parkes's process is founded on the fact that when zinc is thoroughly mixed with melted lead containing silver, and the mixture allowed to cool tranquilly, nearly all the zinc will rise to the surface and bring the greater part of the silver with it. Parkes first introduced his process in England in 1850, but owing to some practical difficulties it was abandoned. Experiments were made on it in Germany under the direc- tion of Karsten in 1851, which did not result favorably. In 1866 the subject was again in- vestigated in Germany, the difficulties previ- ously encountered were overcome, and the pro- cess was made a practical success; it is now rapidly replacing Pattinson's process. The argentiferous lead is melted in a large pot, and zinc added in a perforated iron box and thor- oughly mixed by stirring. The amount of zinc used is from 1 to 1, and sometimes as high as 2 per cent, of the weight of the lead ; it is generally added in three portions. After the removal of the zinc crusts, the lead contains only traces of silver, but retains about 0'75 per cent, of zinc. This must be removed before the lead is marketable. It may be effected by heating in a reverberatory furnace, as in the usual process for softening, by prolonged po- ling, or by steam. The use of steam was intro- duced by Cordurie", and is found to be prompt and efficacious in removing the zinc. The steam is forced through the molten lead in a large kettle provided with a cover. When a sufficient quantity of zinc crusts have accumu- lated, they are liquated at a gentle heat, giv- ing lead, which is returned to the desilvering pots, and a residue containing, according to the amount of lead removed by liquating, from 1 to 3 per cent, of silver. The treatment of this enriched product without loss of silver has been the greatest difficulty in the practical working of the zinc process. It is at present either heated in retorts, by which the zinc is distilled off and a rich lead left, or treated by Cordurie's process by steam, by which the zinc and part of the lead is oxidized, and a lead rich in silver left as before. In Havre, where Cor- durie*'s process was first introduced, the steam in the treatment both of the desilverized lead and the zinc crusts is employed at a pressure