chloride precipitate about 50 pounds of lime and 10 pounds of coal dust are required, also fuel is needed for heating the melting pot or furnace used. This should yield 180 pounds of lead, or a 90 per cent extraction.

For a 100-ton mill, the cost of salt, coal dust, and lime would be approximately as follows:

7.5 tons salt, at $5..........

7.0 tons coal dust, at $3.
2.5 tons lime, at $4.

Total.......

Cost per 100
tons of ore.

$37.50

21.00

10.00

68.00

The operation of such a mill, as can be seen in figure 13, is very simple. The ore after being crushed in the dry state to about 10-mesh size, is mixed with the proper proportions of salt and coal dust or other fuel, and sufficiently moistened with water to ball slightly. This mixture is fed onto the grates of a Dwight-Lloyd sintering machine, or similar down-draft roaster. The lead volatilizes as lead chloride and the sintered residue is discharged to the tailing dump. The gases pass through an electrical precipitator, leaving their suspended burden of lead chloride. The precipitated lead chloride is mixed with lime and a small amount of coal dust and melted down in a pot to metallic lead and calcium chloride slag. Any lead chloride volatilizing in the melting operation is caught by the electrical precipitator again.

The list of machines and products, as represented on the sketch shown in figure 13, follows:

List of parts and products shown in figure 13.

1. Crushing department, containing jaw crusher, rolls, etc., or equivalent equipment.

2. Distributing belt.

3. Ore bin.

4. Coal-dust bin.

5. Salt bin.

6. Mixing belt.

7. Christensen mixer.

8. Mixed-ore bin.

9. Dwight-Lloyd or equivalent down-draft roaster.

10. Suction fan or blower, 24,000 cubic feet per minute. 11. Electrical precipitator.

12. Lead-chloride bin.

13. Lime bin.

14. Melting pot or small reverberatory reduction furnace. 15. Water tank.

(a) 100 tons of ore, containing 10 per cent Pb, per day.

(b) 100 tons of ore, containing 10 per cent Pb, passing a 10-mesh screen.

(c) Coal, passing 10-mesh screen, 5 tons per day.

(d) Salt, 7.5 tons, granulated.

(e) Water, 6 tons.

(f) Sintered tailing, 85 tons.

(g) Gases from electrical precipitator, 24,000 cubic feet per minute. (h) Lead chloride fume, 12.85 tons containing 70 per cent Pb.

(i) Lime, equivalent to 2.5 tons CaO.

(j) Bars of lead, 9 tons.

(k) Calcium chloride, 5 tons.

The 5 tons of calcium chloride obtained from melting the lead chloride is equivalent to 2.6 tons of sodium chloride and can be used in the mix that passes to the down-draft roaster 9. This would reduce the cost for sodium chloride $12 and make the total costs for reagents $56 instead of $68.

3

15

FIGURE 13.-Flow sheet of mill for volatilization of lead.

The other operating costs per ton of ore treated will be approximately as follows: Crushing, $0.20; mixing, $0.05; roasting and precipitation, $0.35; melting and casting, $0.25; total, $0.90.

The first cost of such a plant is estimated at about $50,000 and its life as five years. Hence a depreciation charge of 33 cents per ton of ore treated in the plant during its life should be made. Interest at 10 per cent per annum on the investment amounts to 16 cents per ton of ore treated.

Summing up, the metallurgical costs per ton of ore would be: Chemicals $0.56,

For sign figures of numbers and letters, see table on and operation of plant $0.90,

opposite page.

total $1.46.

At $2.50 per ton for mining, the entire cost per ton of ore mined and treated would be as follows: Mining, $2.50; metallurgical treatment, $1.46; overhead expense, $0.49, or a total cost of $4.45 per ton of ore treated.

The value of the 180 pounds of lead recovered, at 2.75 cents net (4 cents gross) per pound, would be $4.95, yielding a profit of $0.50 per ton of ore treated, even under the somewhat high costs assumed. The lead is very pure and does not need to be refined or desilverized,

so the deduction of 1.25 cents per pound of lead from the market quotation of 4 cents is somewhat excessive. With each increase of 0.25 cent per pound in the price of lead, the corresponding saving on a ton of 10 per cent ore would be 45 cents.

The volatilization process seems to have some advantage over the leaching process, but requires a greater initial outlay and is hence less adapted to the treatment of small deposits of ore.

If material from a mill dump is to be treated, the cost of recovering such material would be much less than mining, and a dump of 10 per cent lead content would yield a handsome profit on the same basis of operation. With a cost of $2 per ton, and 90 per cent recovery, a 10 per cent lead ore in normal times would yield a profit of $2.95 per ton in a 100-ton mill working 300 days per year, and the return on the investment in one year would be 177 per cent.

REQUIREMENTS COMPARED WITH OTHER METHODS.

To summarize, the requirements of this process in comparison with the two leaching methods before described are as follows:

1. The volatilization process, when a down-draft sintering machine is used as a roaster, is adapted to the treatment of an ore containing only lead. Silver, zinc, copper, and other metals are not extracted from an ore by this method.

2. As the process is an igneous one, little water is required, and the process can be used in arid regions where leaching or flotation would require too much water.

3. The operation of a plant using this process is simple and almost automatic, the labor required being largely for the supervision of machinery, with the exception of melting the lead chloride.

4. Very fine grinding is not necessary, although the ore must be ground dry. Modern dry-grinding ball mills are ideal for this

purpose.

5. The reagents necessary for the process-salt, lime, and coal— are standard articles of commerce and easily available in practically every mining district.

6. The final product is metallic lead ready for market, hence transportation costs are reduced to a minimum. The cars used for bringing salt, lime, and coal to the mill can be loaded out from the mill, which should permit better transportation rates.

7. The machinery used is all standard types for sale on the market, except the Christensen mixer, which is of simple construction, being made from a piece of iron pipe and a piece of shafting to which are attached short cross bars.

8. The first costs of such a plant would be higher than those of a leaching plant, but the total cost of operation would be lower.

ROASTING AND LEACHING.

The chloridizing-leaching process, as developed in Utah by N. C. Christensen and T. P. Holt, at first working together and later independently, consists of roasting the ore with salt in the presence of fuel and a sulphide for obtaining a chloridizing atmosphere. The ore, after being roasted at slightly above 600° C., is leached with brine that is about 75 to 80 per cent saturated with salt and is acidified with sulphuric acid.

Christensen used a down-draft roaster and Holt used a shaft roaster with up-draft through the bed of ore. The temperature was not allowed to rise high enough to volatilize much lead, although it is impossible to prevent considerable volatilization of lead at any roasting temperature. The modification of using fully saturated brine will permit the process being applied to the treatment of argentiferous lead carbonate ores, with recovery of copper and gold as well. The combination plant for volatilization and leaching mentioned below could be used for this purpose, by further reducing the draft so that lower roasting temperatures could be used. In that event the electrical precipitator would be rendered useless, and instead the gases could be passed through an acid tower and scrubbed with mill solution in order to catch any acid formed by the roasting.

The process, as outlined, is well known, except for the fact that the use of saturated brine will permit the recovery of lead. The authors do not believe that the process is so well adapted to such ores as partial volatilization, followed by leaching, for the reason that greater volumes of brine are necessary to dissolve all of the lead from the ore. The total costs are known to be about $2.50 to $3 per ton of ore.

VOLATILIZATION AND LEACHING.

The volatilization process described not being adapted to the treatment of argentiferous lead carbonate ores, a modification of the plant shown in figure 9, page 59, is necessary for recovering the silver. Such a plant would be similar to the plant sketched in figure 9, except that the calcines from the down-draft roaster would be put through a leaching plant resembling that shown in figure 8, page 48. The roaster would have to be set for lower draft and the ore not sintered. Only 75 per cent of the total lead content could be extracted by volatilization; the rest could be leached with a brine solution along with the chloridized silver, copper, and gold.

This modified process has the following characteristics:

1. A small leaching plant can be used for extraction of the residual lead, because a smaller volume of brine will be necessary for extracting the residual lead than when leaching the raw ore.

2. The silver, gold, and copper content of a mixed carbonate ore can be recovered together with the remaining lead.

3. The roasting operation breaks up ore colloids and leaves a product that can be more easily leached, and all slime troubles are eliminated.

4. The operation of both volatilization and leaching plants causes higher costs, which are only justifiable on account of the recovery of metals other than the lead.

5. The addition of sulphides, like pyrite, to the roast in order to help chloridizing of the silver and the copper is an added expense. 6. The necessity of leaching makes this process poorly adapted to arid regions.

VOLATILIZATION OF LEAD ORES CONTAINING PRECIOUS

METALS.

All of the lead, silver, gold, and copper content could be extracted from an oxidized ore by mixing the ore with salt and by volatilization roasting at about 900° C. for a sufficient length of time. The down-draft sintering machine and similar roasters are not applicable for this purpose, because the duration of the roast is too short. Reverberatory or kiln roasters capable of giving the proper temperature for one to two hours would be required. Otherwise the flow sheet of the plant used would be practically similar to that shown in figure 13, page 116. The gold and silver being extracted with the lead and later reduced with it would necessitate sending the pig lead to a refinery; so that the whole $1.25 for each 100 pounds of lead would have to be deducted for shipping and refining charges. The roasting would doubtless be more expensive and would consume more fuel, but otherwise the total costs would be about the same as those previously given. Oxidized ores containing 3 to 8 per cent lead and 1 to 8 ounces of silver, in siliceous or limestone gangue, are fairly common in the intermountain region, and are rarely mined, even for gravity concentration.

The advantages of this process, in comparison with the others mentioned herein, when considered from a commercial standpoint

are:

1. Salt, lime, and fuel are the only materials needed, and these are readily obtainable. Special materials like sulphuric acid are not needed, and hence the process is not dependent on a cheap supply of acid.

2. High extractions of all the desirable metals are possible, and almost any type of gangue is permissible provided that sintering or slagging does not take place at too low a temperature.