CONCLUSIONS ON LEACHING AND ELECTROLYTIC PRECIPITATION OF

LEAD.

The conclusions to be drawn from the results of the tests of leaching and electrolytic precipitation are as follows:

1. It is possible to precipitate lead from brine by electrolysis, yielding sponge lead that can be melted into pure lead.

2. Impurities do not build up in the solution to an excessive extent and can be removed with lime when necessary. Purification of solutions is not so essential as when lime is used to precipitate the lead.

3. Current efficiencies of more than 90 per cent are possible with current densities ranging up to 40 amperes per square foot of cathode surface.

4. Iron anodes can be used, yielding solutions containing both ferrous and ferric chlorides, and cutting down the voltage to a fraction of one volt. This allows the production of as much as 17 pounds of lead per kilowatt-hour.

5. The presence of ferric iron in the solution permits slightly higher extractions of silver, owing to the ferric iron attacking silver sulphide minerals.

6. Many of the materials tested contained considerable amounts of silver in soluble form.

7. Silver can be removed from the solutions by the use of either scrap iron or sponge lead, although the use of iron is preferable because of the greater speed of precipitation. The amount of iron introduced into the solution is negligible.

8. Melting the sponge lead presents few difficulties, although a small proportion of dross is formed. Treatment by careful washing and hydraulic pressing will remove most of the water and most of the salt and will reduce the formation of dross.

CHEMICAL REACTIONS INVOLVED IN BRINE LEACHING.

In order that the chemistry of the brine leaching processes may be more clearly understood, the following reactions are presented: In leaching the chief reaction is as follows:

xNaCl+CaCl2+H2SO1+PbCO2=PbCl2+xNaCl+CaSO1+CO2+H2O

This is the reaction sought as ideal. If not enough calcium chloride is present in the brine returning from precipitation all the sulphates will not be precipitated and some sodium sulphate will be left in the leaching solution. Some of the usual impurities enter the leaching solution by the following reactions:

2NaCl+H2SO, Na2SO,+2 HCI
Fe2O,+6 HCl=2 FeCl2+3 H2O
Al2O3+6 HCl=2 AICI,+3 H2O

The purification of the solution is based on the following reactions:

CaCO3+2 HCl=CaCl2+H2O+CO2

3CaCO3+2 FeCl3+3 H2O=2 Fe(OH)3+3 CaCl2+3CO2
3CaCO3+2 AICI,+3 H2O=2 Al(OH) ̧+3 CaCl2+3CO2
CaCl2+Na2SO, CaSO,+2 NaCl

In precipitation with lime the reaction can be written as follows:

Ca(OH)2+PbCl2+xNaCl=Pb(OH)2+xNaCl+CaCl2

For electrolytic precipitation the reaction is

xNaCl+PbCl2+Fe+Elec.=xNaCl+Pb+FeCl2

ROASTING AND LEACHING OF ARGENTIFEROUS OXIDIZED ORES OF LEAD.

In the treatment of lead ores containing silver, the problem of extracting the silver with the lead complicates matters. Unless the silver is present as silver chloride it will usually not dissolve in an acid brine except under the conditions mentioned on page 53, where ferric chloride in the brine seemed to add slightly to its dissolving power, as far as silver is concerned. Hence, some method of chloridizing the silver is needed,

In former days, before the advent of the cyanide process, the principal hydrometallurgical method of recovering silver consisted of a chloridizing roast to render the silver soluble in either a solution of sodium thiosulphate or brine. The latter constituted what was called the Augustine process. Chloridizing roasting was done in the old style reverberatory furnaces, and was acknowledged to be a delicate operation, as there were volatilization losses when the temperature was too high and losses of insoluble silver when other conditions were not just right. In present-day metallurgy losses from volatilization can be prevented with the use of electrical precipitation apparatus, which utilizes high-tension direct-current discharges through gases, but losses of insoluble silver from improper roasting are still to be feared.

TESTS AT ONTARIO MILL.

A year before the experimental work at the Utah station was begun, a modification of the Augustine process was tried in two mills in Utah. One was the old Ontario mill at Park City, treating stope fillings from the Ontario mine. In this mill a Holt-Dern roaster a was used for the chloridizing roast. The Holt-Dern roaster is a shaft roaster 7 by 9 feet in horizontal section, in which the charge fills the shaft and heat is generated by the burning of fuel mixed with the ore or of sulphides in the ore, a blast of air being supplied from below. Drawings of one form of this roaster are shown in figure 9. The mix

a Dern, G. H., and Holt, T. P., Ore roaster: U. S. patent 1113962, Apr. 9, 1914.

ture of ore and fuel is ignited at the bottom and the roasting zone travels upward. The method proposed by the makers involved removal

of the calcines at the bottom at intervals of about an hour, fresh charge being added at the top, so that the roasting zone would be