the petroleum purified. Many of the flotation oils can be caused to take up sulphur and lose hydrogen, by distilling or boiling the oil with elemental sulphur. If the sulphur content in the oil is sufficient to from a film of lead sulphide over the lead carbonate particles, the flotation oil is evidently in the most advantageous position to "wet" the freshly formed artificial sulphide. In actual tests the writers were unable to get satisfactory extractions by this method. However, the results with sulphureted flotation oil and a sample that had previously been sulphidized with sodium sulphide were slightly better than those with nonsulphureted oil.

COLLOIDAL SULPHUR.

Attempts to sulphidize oxidized ores with colloidal sulphur, obtained by the interaction of hydrogen sulphide and sulphur dioxide, also failed, as colloidal sulphur does not sulphidize lead carbonate. If the ore has first been sulphidized with hydrogen sulphide, and the excess hydrogen sulphide is neutralized with sulphur dioxide to form colloidal sulphur, no froth is formed on the addition of a flotation oil, as all the froth seems to be "killed" by the sulphur dioxide or compounds formed in the action on the ore. Further, the writers have not found that colloidal sulphur can be successfully substituted for the flotation oil, although this point was given little attention after one or two failures.

“METALLIZING" THE LEAD.

Still another method of chemically altering an ore of lead in order to make it amenable to flotation is that of "metallizing" the lead. This was suggested by W. H. Coghill, who had tried the aluminium desulphurizing method, as practiced at the Nipissing mine, in Cobalt, Ontario, on an ore containing both silver and lead. He informed the writers that he had noted the formation of sponge lead during the

tests.

In tests at the Salt Lake City station solutions of caustic soda and pieces of aluminium were agitated with the ore. The nascent hydrogen was found to desulphurize silver and lead, or reduce them to metallic form if they were present in oxidized forms. It was hoped that the metallized lead and silver could be floated. No difficulty in carrying out the metallizing operation with oxidized ores of lead was experienced, as sponge lead was visible, but this lead could not be floated. The presence of flotation oils of widely different types during the metallizing failed to improve the results, although it was possible to perceive small masses of sponge lead in the pulp. By rolling the pulp for three days in a bottle the sponge lead collected in small round balls which could be screened out of the pulp. The extraction of lead was high, but that of the silver was low.

As this method would be very expensive for lead alone, and was designed for the treatment of lead-silver ores that were refractory to other methods of treatment, the experiments were regarded as failures. One pound of aluminium can reduce 11 pounds of lead; hence the use of aluminium would be rather expensive.

Silicon or ferrosilicon will likewise react slowly with caustic soda solutions, reducing the lead compounds and also some of the silver. One pound of silicon will reduce about 17 pounds of lead. Silicon would be cheaper to use than aluminium, as it would reduce the metallizing cost per pound of lead to less than 1 cent. However, the method was considered of no value, as the extraction of silver was too low.

CONCLUSIONS AS TO RESULTS OF SULPHIDIZING TESTS.

The conclusions drawn from a comparison of the various methods of sulphidizing are as follows:

1. Hydrogen sulphide is the most active of the sulphidizing agents tested, but its poisonous and offensive character makes its use undesirable in ore-dressing mills.

2. Sodium sulphide and the other sulpho compounds of sodium are among the best alkaline sulphides for sulphidizing lead carbonate ores. The sulphydrate of sodium is the most active, with the normal sulphide a close second. The cheapness of commercial normal sulphide and the small quantity required per ton of ore treated make it one of the most desirable agents for sulphidizing.

3. Polysulphide of calcium is the best calcium salt for commercial sulphidizing of lead carbonate ores, because it can be prepared cheaply from elemental sulphur and lime, and is readily soluble. It is less active in sulphidizing than the sodium compounds.

4. Other methods of sulphidizing, especially the use of elemental sulphur vapor, sulphuretted flotation oils, or colloidal sulphur, are not a success in flotation of the lead carbonates.

5. Metallizing a lead-silver ore by the use of caustic soda and aluminium (or ferrosilicon), followed by flotation, likewise failed, owing to inability to float the sponge lead and silver.

PRACTICAL APPLICATION OF RESULTS OF TESTS.

A number of alternative methods have been worked out for the treatment of low-grade oxidized ores of lead, and these methods vary widely in the conditions under which they can find application. If an ore is not suited, because of its physical or chemical properties, to treatment by a certain one of these methods, it is often possible to use some other method. The argentiferous lead carbonate ores require different treatment from the ores containing only lead.

LEACHING METHOD FOR LEAD CARBONATE ORES.

In the treatment of ores containing lead carbonate and no other valuable mineral, one of the simplest and cheapest methods of extracting the lead is that of leaching it out with acidified saturated brine. This method, however, can be used only for ores that contain small amounts of acid-soluble gangue material. If much alumina or lime is present, these constituents will consume the acid intended for the lead compounds, and hence one of the other methods will be required. In any event, the cost of the acid and the salt for getting the lead into solution has been found to be the greatest expense,

FIGURE 12.-Flow sheet of proposed mill for leaching oxidized ores of lead. 1, ore bins, capacity 100 tons per 24 hours; 2, crusher; 3, roughing rolls; 4, finishing rolls; 5, triple-washing Dorr classifier; 6, sulphuric acid storage; 7, brine storage; 8, Dorr thickener to remove slimes; 9, Dorr thickener to remove lead hydrate precipitate; 10, continuous vaccum filter; 11, revolving dryer; 12, pump; 13, lime storage bin; 14, lime slaking box; 15, Dorr agitator for making milk of lime. Clear solutions. Pulps. For significance of letters see table on page 109.

the cost of the lime or the electric current used for precipitation being small. An example of the calculated costs for treating one of the ores tested is shown on page 110. The estimates are based on the actual requirements as determined by tests of the ore.

FLOW-SHEET OF PROPOSED PLANT.

A flow sheet of a mill suitable for treating this ore is shown in figure 12.

CRUSHING.

In the proposed mill the crushing machinery consists of a crusher and two sets of rolls. Ore crushed to pass through a 10-mesh sieve was used in many of the tests, but probably ore considerably coarser

than 10 mesh could be successfully leached in the large plant. As most of these oxidized lead ores are soft and friable, it is probable that the crushing equipment could be simplified. Hence, all of the accessory apparatus, like elevators, are not shown.

LEACHING.

The tests have shown that less than a half-hour of contact of the acid-brine solution is necessary to get a 93 per cent extraction of the lead. Hence, it is proposed to use a number of Dorr classifiers in series for leaching the ore. Such machines were used during the life of the Butte and Duluth mill at Butte, Mont., with considerable satisfaction. They permit agitation of a coarse sand in contact with an acid solution, maple-wood rakes being used in their construction. The sand would pass through six of the classifiers in series in about 35 minutes. As shown in figure 12, most of the brine and all of the acid would be added near the head of the second classifier and the overflow into the first where the fresh ore enters. This would give a countercurrent of acid and ore. The third classifier would receive neutral brine for washing out the acid and lead-bearing brine. Water can not be used for washing, because saturated brine is necessary for good leaching of the lead. This condition is the main mechanical drawback to the proposed process.

SAND AND SALT.

The sand overflow, at b in the flow sheet, can be sent to the tailing heap or dropped into a sand tank to drain more thoroughly, in order to recover as much of the brine as possible. Where the cost of common salt is high it might pay to install apparatus for final washing of the sands with water and for evaporating part of the washings to saturation. A part of the waste water could be made into saturated solution direct by adding more salt to make up for mechanical losses. The moisture content of 1.65-mm. (10-mesh) sand can usually be reduced by draining to about 16 per cent. As the saturated solution contains 26.8 per cent sodium chloride, by weight, about 4 tons of that salt in 16 tons of solution would be retained by every 100 tons of sand. Hence 100 pounds of salt would be lost per ton of ore. The price of salt would not exceed $5 per ton, except in the more isolated localities. The loss from salt in the tailing would rarely be more than 25 cents per ton of ore, and would usually be less, as such a leaching plant could probably get good rates from the salt manufacturers. In the vicinity of Great Salt Lake the cost of making salt from the lake brine is probably less than $1.50 per ton, and the salt can be bought in large amounts for about $2.

SLIME.

The slime, overflowing with the pregnant liquid, passes to a Dorr thickener, from which a product containing 60 per cent solids is run to waste. As many of the lead carbonate ores are known to produce about 20 per cent of slime when crushed to 10-mesh size, 100 tons of sand would yield 20 tons of slime containing 8 tons of liquid or 2 tons of salt-that is, the total salt loss would be 5 tons per 100 tons of ore. If the pregnant liquid entrained in the slime is washed out with barren brine, there should be recovered from 20 tons of slime 160 pounds of dissolved lead, which is worth 2.5 cents per pound at the smelter to the producer in the Rocky Mountain States. If the cost per day of operating a Dorr thickener is less than the value of the lead, it would be well to put in another thickener to recover the liquid. A large proportion of this liquid could also be recovered with a vacuum filter, which would have to be acid proof as the liquid is still slightly acid.

SOLUTION.

The clarified overflow solution from the Dorr thickener is passed into another Dorr thickener along with the proper amount of milk of lime, or rather of lime slaked in brine and suspended in brine. This neutralizes any remaining acid and precipitates the lead as a basic hydrate which settles out of solution fairly quickly. The precipitated solution is returned to the solution storage tank. The thickened sludge can be sent to an ordinary vacuum filter without further treatment and be washed free of solution with a small jet of water or of steam. The filter cake could be sent directly to a rotary drier. This filter need not be of acid-proof construction, as there is no acid left in the solution and no dissolved lead to be replaced by metal from metal parts of the filter.

CAPACITY OF PLANT AND COST OF OPERATION.

For those interested in the working of such a plant, the tonnage of every product at the different steps of the process is given in the following list. The letters in figure 12 refer to the letters in the following list and the accompanying weights of materials involved.

Significance of letters in figure 12.

(a) 100 tons of ore, containing 10 per cent lead, of which 90 per cent can be extracted. (b) 67 tons of sand tailing, containing 1 per cent lead; 12 tons of solution, containing 3 tons of sodium chloride.

(e) 18 tons of slime; 882 tons of solution, containing 236 tons of sodium chloride. (d) 8.5 tons of sulphuric acid, specific gravity 1.84.

(e) 800 tons of solution.

(f) 100 tons of solution.

(g) 18 tons of slime tailing; 8 tons solution, containing 2 tons of sodium chloride, (h) 874 tons of overflow pregnant solution; 9 tons of lead.