70 



LEAD 



Parked Proaess of Desilverising Lead by Zine. When lead and zinc are melted 

 together and allowed to cool slowly, an almost complete separation occurs, the zinc, 

 owing to its lower specific gravity and higher melting-point, cools first and may be 



1353 



removed in a solid crust, while the lead is still liquid underneath. Mr. Alexander 

 Parkes of Birmingham, found that argentiferous lead might be freed from its silver 

 by melting with it zinc ; and in the process of cooling the zinc crystallises and takes 

 out the silver ; the silver may afterwards be removed from the zinc by dissolving in 

 hydrochloric or in dilute sulphuric acid, and the zinc may be used again and again for 

 melting with the lead. Mr. Parkes first patented his process in 1850 ; he improved it 

 in 1851 ; and in 1852, he was granted a third patent, entitled ' Improvements in sepa- 

 rating Silver from other Metals.' The process is as follows : There are two vessels 

 over separate fires, one a Pattinson pot of cast iron, which will hold 7 tons of molten 

 metal this is for the lead and a smaller one for melting the charge of zinc. When 

 the 7 tons of lead to be desilverised is melted, it is skimmed, the temperature is raised 

 to that of molten zinc, and the charge of zinc (about 1 cwt. to every ton of lead, or 

 If Ib. to every ounce of silver) which has been previously melted is laded into the 

 lead, and the whole well stirred with a perforated rabble to insure perfect intermix- 

 ture ; the mass is then left to gradually cool. When the surface has hardened, it is 

 skimmed off with a perforated ladle, and the surface of the liquid lead is skimmed off 

 as well. The lead thus treated contains about 10 dwts. of silver per ton. The 

 solidified zinc is placed in a retort, which is oval on bore, and placed a little on the 

 incline over a fire for the purpose of liquation ; it is heated above the melting point 

 of lead, but decidedly below that of zinc, and so the lead that is taken up with the 

 zinc drains off at the lower end of the retort into an iron pot placed convenient to it. 

 The zinc, which has been thus drained of the lead, but still contains the silver, is dis- 

 tilled in a Belgian retort with twice its bulk of lime and its own bulk of coal ; the 

 zinc, being much more volatile than the silver, is distilled over, and the residue is very 

 rich in silver. The partially-desilverised lead is submitted to a softening process for 

 the separation of any remaining zinc. It is kept melted at a good red heat for a 

 period of from 9 to 12 hours, during which time it is skimmed twice, first, after the 

 first three hours, and again half an hour before tapping. The lead is tapped into a 

 cast-iron receiver, and therein subjected to ' boiling ; ' that is, green wood is kept 

 submerged in it, held down by a lever. Lead treated in this manner is said to contain 

 a small quantity of silver, but no zinc. 



These processes were supposed to have been improved by Flach, who patented 

 his processes in this country in 1866. Although Flach's process has been used, it is 

 said, with success on the Continent, it has never been permanently adopted in this 

 country. 



The difficulty with Parkes' process has been the separation of the zinc from the 

 desilverised lead. A process for effecting this was patented by Cordurie of Toulouse 

 in 1866. Lead to be desilverised was treated by him in the usual manner with zinc. 

 After the removal of the crust of argentiferous zinc, superheated steam is passed into 

 the desilverised lead, whereby the zinc retained by the lead is oxidised by the oxygen 

 of the steam; this oxide rises to the_ surface in the state of powder, and is skimmed 

 off. According to Gruner, who wrote on his process in the Annalcs dcs Mines, the 

 result at Kothschild's works at Havre was most satisfactory. The desilverised zinci- 



