678 PYRITES 



Although it has been long known to those engaged in copper- extraction that Spanish 

 pyrites contains a notable quantity of silver, together with distinct traces of gold, no 

 successful attempt to render it available had been made up to the year 1870, when 

 Mr. F. Claudet patented a process for the separation of tkis metal from ordinary copper- 

 liquors by the addition of a soluble iodide. 



The amount of silver contained in burnt ore seldom exceeds 18 dwts. per ton, but as 

 the whole of this is never obtained in solution, it follows, that in order to obtain satis- 

 factory commercial results, the process adopted should be both economical and expe- 

 ditious. 



The tanks in which burnt ore, roasted with common salt, is lixiviated, usually 

 receive from eight to ten successive washings, and the liquors from the first three of 

 these contain nearly 95 per cent, of the total amount dissolved ; the first three 

 washings are therefore alone treated for the precious metals. For the purpose of 

 removing the soluble salts from the roasted ore, hot water is first employed , and, as a 

 large proportion of the sodium chloride used remains undecomposed, it acts as a solvent 

 for the silver chloride produced during the operation of furnacing. The analysis of a 

 first washing from a lixiviating tank gave Mr. Claudet the following results : 



Analysis of Strong Liquors. 



Sp. Gr. =1-240 Contents per gallon 



Sodium sulphate 10-092 



chloride 4-474 



Chlorine, combined with metals .... 4-630 



Copper 3-700 



Zinc . 480 



Lead . 40 



Iron 32 



Calcium 52 



Silver 3-06 



Arsenic, antimony, bismuth, &c., not estimated . . trace 

 Proportion of copper to silver, 10,000 : 8-2. 



The several operations for the recovery of the silver are conducted in the following 

 manner, and, as the first three washings contain nearly the whole of that metal, these 

 are alone operated on : 



These liquors are first run into suitable wooden cisterns, each of a capacity of about 

 2,700 gallons, where they are allowed to settle. 



The yield of silver per gallon of the solution is now ascertained by taking a measured 

 quantity, to which are added hydrochloric acid, potassium iodide, and a solution of 

 acetate of lead. The precipitate thus formed is thrown upon a filter, and, after being 

 dried, is fused with a suitable flux. Argentiferous lead is thus obtained in the form 

 of a button of convenient size, which is passed to the cupel, and from the weight of the 

 resulting silver the amount of that metal, in a gallon of the copper-liquors, is readily 

 calculated. 



After thus determining the amount of silver contained in a gallon of the liquor to 

 be operated on, it is drawn off into another vat, of slightly larger capacity than the 

 settler, and, at the same time, the exact amount of some soluble iodide necessary to 

 precipitate the silver present (a solution of kelp being employed by preference) is run 

 into it from a graduated tank, together with a quantity of water equal to about one- 

 tenth the volume of the copper-liquor. During the filling of the second tank its 

 contents are constantly stirred, and, when filled, they are allowed to settle during 

 48 hours. The liquors are then run off, and the tank is again filled ; the pre- 

 cipitate which is deposited in the bottom of these tanks is, about once a fortnight, 

 washed into a vessel prepared for its reception. 



This precipitate is chiefly composed of a mixture of lead sulphate, lead chloride, 

 silver iodide, and subsalts of copper, from which the latter are removed by washing 

 with water acidulated by hydrochloric acid. Thus freed from copper, the precipitate 

 is decomposed by metallic zinc, which reduces the silver iodide completely, and also 

 the lead chloride. The results of this decomposition are : 1st. A precipitate, rich in 

 silver, containing a valuable amount of gold. 2nd. Zinc iodide, which, after being 

 standardised, is employed in subsequent operations for the precipitation of silver. 



The argentiferous precipitate finally obtained, after the decomposition of the iodides 

 by metallic zinc, contains nearly 60 per cent, of lead, a trace of copper, and a 

 little zinc ; the amount of silver present varies from 1,600 to 2,000 oz. per ton; the 

 yield of gold remains nearly constant at 20 oz. per ton. This process is employed at 

 the Copper Extraction Works of Messrs. Muspratt Brothers and Bentley, Flint ; at 

 those of Messrs. Morris & Co., Stockwith-on-Trent ; and at those of the "VVidnes Metal 

 Co., Widnes. At the latter establishment the results of two years' regular working 



