170 ANNUAL OF SCIENTIFIC DISCOVERY. 



ALKALI MANUFACTURE IN GREAT BRITAIN. 



In 1861, it was estimated that the total quantity of salt decora- 

 posed in Great Britain, for the production of soda, was 260,000 

 tons. According to the returns of the Alkali Manufacturers' As- 

 sociation for the year 1869, the total quantity decomposed was 

 326,000 tons, showing an increase of 25 per cent. 



CLAUDET'S PROCESS FOR THE EXTRACTION OF SILVER. 



The amount of pyrites annually burned in Great Britain in the 

 manufacture of sulphuric acid reaches 350,000 tons, of which at 

 least 250,000 tons contain a sufficient amount of copper to render its 

 treatment for that metal commercially advantageous. For several 

 years past a large proportion of the " burnt ore " produced in the 

 various chemical works of the country has been worked by what 

 is known as the ivet process of extraction. By the process of liquid 

 extraction at present usually employed, the burnt ore is first 

 finely ground and sifted, and subsequently roasted with common 

 salt until by the oxidation of the metallic sulphides present a 

 portion of the alkaline salt is converted into sulphate of sodium, 

 whilst the copper is, on the contrary, converted into a soluble 

 chloride. The copper salt is subsequently removed by repeated 

 washings and the copper precipitated by iron in the metallic 

 state. 



This precipitated copper contains a notable quantity of silver 

 as well as a distinct trace of gold. Thus in 9 successive wash- 

 ings of one tank of ore there were found respectively 4.06, 3.25, 

 1.05, 0.19, 0.12, 0.06, 0.03, 0.06, 0.04 grains of silverto the gallon 

 of liquor. This silver comes from the ore which has been roasted 

 with common salt as chloride held in solution by the large amount 

 of undecomposed chloride of sodium. Of these various washings 

 the first 3 alone contain enough of the precious metal to pay 

 for working. The treatment is as follows : 



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 is now ascertained by tak- 

 ing a measured quantity, to which are added chlorhj^dric acid, 

 iodide of potassium, and acetate of lead in solution. The precip- 

 itate obtained is thrown upon a filter, and after being dried is 

 fused with a flux, consisting of a mixture of carbonate of so- 

 dium, borax, and lamp-black. The resulting argentiferous lead is 

 passed to the cupel, and from the weight of the button of silver 

 obtained the amount of that metal in a gallon of the liquor is 

 estimated. 



The liquor from the settling-vat is now allowed to flow into 

 another of slightly larger capacity, whilst at the same time the 

 exact amount of a soluble iodide necessary to precipitate the sil- 

 ver present 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 con- 



