METALLURGICAL METHODS AT BROKEN HILL, 299 
hyposulpite of soda before destroyed. Lead carbonate being in- 
soluble in hyposolution is not again attacked. It was found that 
about ten hours after the hyposulphite solution was run on the 
ore was the best time to put in the carbonate of soda. Some 
experimenting took place to determine the proper hour, and it 
was found that the best silver extraction results were obtained if 
this rule was maintained. 
The solution of hyposulphite of soda, with silver and lead in it, 
is conducted into one of a series of circular precipitating vats, each 
10 feet diameter and 9 feet deep. When the vat is full, the lead 
in solution is precipitated by a saturated solution of carbonate of 
soda, using phenol for the alkaline reaction, which is taken asa 
sign that the lead is all precipitated. Some silver goes down with 
the lead, so that the precipitated carbonate of lead usually assayed 
about 70 oz. of silver per ton. The liquor, with the precipitate 
in suspension, was blown with compressed air for twenty minutes 
or thereabouts, the precipitate settling very rapidly afterwards. 
When the supernatant liquor was quite clear, it was drawn off 
into one of a series of six vats of the same size as the lead pre- 
cipitating vats. Here the silver was precipitated out by sulphide 
of sodium, and on settling, the clear liquid was run through a sand 
filter and elevated back to the hyposulphite tanks in the top of 
the building ready for use again on fresh vats of ore. 
The sludge of carbonate of lead was run into two storage vats, 
each 10 feet x 7 feet, and from these was sucked into a boiler-iron 
pressure-tank with a vacuum created by a steam ejector. When 
the tank was full the suction tube was closed, the steam ejector 
stopped, and a tap opened, which admitted compressed air, so 
forcing the semi-liquid sludge into a Johnson filter-press at a 
pressure of 60 fb. to the square inch. The liquor from this press 
was returned to the silver precipitating vats, and there treated, 
The dry cakes of carbonate of lead, obtained from the filter press, 
were sent to the blast furnaces for further treatment. 
The silver sulphide was filter-pressed in similar manner from 
storage tanks, 7 feet 9 inches diameter by 5 feet 9 inches deep, 
and the dry cakes roasted and melted with poor lead to make a 
rich bullion. 
The roasted ore handled was equal to about 1,000 or 1,100 tons 
weekly, and the average apparent extraction of silver was about 
79 per cent.; the real extraction, after taking into consideration 
the 63 per cent. solubility of the ore, was 80} per cent. The 
roasted ore treated averaged between 12 to 14 oz. per ton of 
raw ore. The total cost of leaching, counted on the raw ore 
originally crushed at the chloridising plant, amounted to about 
6s. 9d. per ton for all costs, including chemicals, labour, super- 
intendence, water, light, stores, repairs, renewals, coal, scrap- 
iron, &c. The water consumed cost about 4s. per 1,000 gallons ; 
