GOLD AND SILVER REFINING 



721 



which weigh 323*40 Iroy ounces, and contain, if filled to the neck, 8 gallons of water. 

 A, the retort or boiler ; B, the head, provided with a tube of platinum, D, to which is 

 joined at the time of use a long tube of lead, c is a tube terminating on the shoulder 

 of the boiler, and provided with a lid, and is of service to allow of the occasional 

 stirring of the silver during solution, and of the addition of the small quantity of 

 acid at the termination of the chemical action. The vessels became much coated with 

 gold, which was removed with difficulty and at great risk of attacking the platinum. 



The sketches (Jigs. 1121, 1122, and 1123) are on a scale of 1 inch to a foot. 



According to convenience and requirements, the retort or boilers may be multiplied 

 as to number, but about 5 or 6 would seem to be a convenient set for operations. In- 

 dependently of the smaller prime cost of cast-iron retorts or boilers (now used in place 



1121 



1122 



of platinum), there is the advantage of being able to use acid which is not free from 

 impurities, because the cost of the retorts is practically not worth consideration, if 

 taken in relation to the extra price which must be paid for pure acid. Besides these 

 facts, it is found that owing to some influence (is it chemical or catalytic ?) which the 

 iron exerts, less acid is required to be used in proportion to the precious metals than 

 was used when platinum vessels were believed to be necessary. 



A charge for one boiler varies from 1,130 to 1,300 troy ounces of the granulated 

 mixed precious metals, and is heated with about twice or twice and a half times its 

 weight of sulphuric acid of sp. gr. 17047. The heat is gradually raised until effer- 

 vescence takes place, and it is then regulated with care, while at last, the temperature 

 is raised nearly to the boiling point. As in the case of mercury so in the case of 

 silver, it is better not to rise quite to the boiling point, else sulphuric acid distils off 

 with the escaping sulphurous acid. According to the care with which the granulating 

 has been effected, each charge is heated from 3 to 4 hours. When the elimination of 

 sulphurous acid ceases, the operation is known to be terminated, and chemical examina- 

 tion shows that exactly equivalent quantities of sulphate of silver and sulphate of 

 copper are formed to account for the sulphuric acid. In practice the sulphurous acid 

 is frequently lost, although in all refineries it should be used for the re-composition of 

 sulphuric acid. 



Leading from the top of the boiler or retort is a horizontal leaden tube from 8 to 

 10 yards long, terminating in a leaden chamber, in which sulphuric and sulphurous 

 acids accumulate with some sulphate of silver, mechanically carried over by the 

 violence of the chemical action. It is found that the acid which accumulates in this 

 leaden chamber has a sp. gr. of from 1*3804 to 1*4493. The reduced strength of the 

 acid from 1*7047 to this point is readily understood if the fact be remembered that 

 sulphuric acid is really a compound of anhydrous sulphuric acid and water, and that 

 only the anhydrous sulphuric acid is concerned, although the water performs the 

 friendly part of leading it into action on the silver ; the action having commenced, the 

 water is done with, and passes off with the sulphurous acid as it is eliminated ; but 

 independently of this cause, it is found that sulphuric acid, by boiling, parts with 

 water, and concentrates itself, until by and by the anhydrous acid itself distils off, 

 and when this is seen, it is at once known that the operation is carried rather too far. 

 When the action has quite terminated, it is customary to add to each boiler or retort 

 from 60 to 80 troy ounces of sulphuric acid of sp. gr. T6656, procured from the 

 liquor which has deposited sulphate of copper (presently described) ; then to pour the 

 whole into a leaden boiler, and boil it for a few minutes : afterwards withdraw the fire, 



VOL.IL 3 A 



