Outline of the Chloridizing Leaching Process: The Mines Operating Co.’s plant at Park City, Utah, was designed to treat the low-grade fillings in the old stopes of the Ontario mine. These fillings carry 6 to 14 oz. of silver, 1 to 2 lb. copper, 0.01 to 0.015 oz. of gold, and a small percentage of lead and zinc. The treatment consists in mixing the crushed ore with coal dust and salt and then roasting in a new type of furnace by combustion of the contained fuel. The roasted ore is leached with an acid salt solution to dissolve the silver, gold, copper, and lead. At present these metals are precipitated together on scrap iron and the product sold to a refinery.

The only new feature of importance in this scheme of treatment is the roasting process, which makes possible the chloridizing of ores without any loss of the valuable metals, and at a very low cost.

Development of the Chloridizing Leaching Process

The roasting process had its beginning in connection with research work at the Utah State School of Mines. N. C. Christensen, Jr., who held one of the School of Mines’ fellowships, was engaged in some experimental work on blast roasting. At the same time I was doing some work on the insoluble gold in Mercur base ores. Mr. Christensen roasted some of this base gold ore in his pot furnace, and found that when he mixed a sufficiently low percentage of fuel with the ore, it did not sinter but roasted to a leachable product.

We considered this a new application of blast roasting, and proceeded to test out a large number of ores in several different types of small roasters which we constructed.

In August, the Consolidated Mercur Gold Mines Co. installed a small roaster of our design, which we proposed to operate on the countercurrent principle, feeding mixed ore continuously on top, and at the same time drawing off the roasted ore from the bottom. By proper adjustment we hoped to maintain a permanent roasting zone near the center of the column, the ore moving downward as roasted. However, we experienced difficulty in getting the moving mass to roast properly, and also other troubles of a mechanical nature developed. Mr. Christensen at this time disposed of his interest in the process to G. H. Dern.

We continued experimental work on a number of machines of varying design. Up to the time construction work began on the Mines Operating Co. mill no satisfactory continuous roaster had been developed, so we decided to install an intermittent roaster, which our experience had demonstrated would give a satisfactory product for subsequent leaching operations. Accordingly, we installed roasters similar to the one shown in Fig. 2, the construction and operation of which will be described in its proper place in the description of the plant.

The Plant

The Mines Operating Co.’s plant is installed in the old concentrator of the Ontario Silver Mining Co. An outline of the present scheme of treatment may be followed by reference to the accompanying flow sheet
(Fig. 1).

A second-motion electric hoist, operating 1.2-ton skips in balance delivers the ore from the haulage tunnel of the mine to a bin above the crusher. From here it is fed by a Stephens-Adamson apron feeder to a trommel with 1½-in, openings. The undersize of the trommel passes through a revolving drier, where the moisture is reduced to about 5 per cent. The oversize of the trommel falls upon a picking belt, where mine wood and waste are removed. It then passes through a No. 5 Gates gyratory crusher and, joining the drier fines, is elevated to the crushed-ore storage bin. This bin has a capacity of 400 tons of ore. Sections of the same bin are respectively used for salt and coal storage. From these bins the ore, salt, and coal are fed, in the required proportions, upon a belt conveyor and delivered to the rolls for final crushing. The feeders by means of which the proportioning and mixing are done are of the plunger type, the feed being varied by changing the eccentric throw. This type of feeder works very well for the salt and coal dust, but is not very satisfactory for ore, on account of rapid wear. The mixed ore is crushed in two sets of 15 by 36 in. rolls and is elevated to the mixed-ore bins above the roasters.

There are eight shaft roasters of the intermittent type, similar to the one shown in Fig. 2. The roaster shown is 10 by 10 ft. inside the walls. The walls are reinforced concrete 10 in. thick. About 2 ft. above the bottom is a wood grating, supporting a layer of coarsely crushed rock. The space below this grate forms an air chamber. The grate supports the roasting charge, and also distributes the air blast.

The method of operating the roaster is as follows: A special starting “mix” of about 1 ton is prepared in front of the roaster. A layer of coal dust mixed with oil is then spread over the gravel floor of the roaster.

This is ignited and sufficient blast admitted to burn it rapidly to glowing coals. The special mix is then shoveled evenly over the surface. By the aid of the air blast, the coals ignite the fuel in the ore, which begins to roast. When the starting layer has roasted through, so that the fire appears on top, the air is shut off for a few minutes, while the first charge of about 5 tons is dropped from the mixed bin gate into the roaster. This is spread out by hand and the air again turned on. The roast is allowed to proceed until it appears on the surface of the charge. Then a second charge of about 10 tons is dropped on and 4 hr. later a final charge of about the same amount. This brings the total depth of charge up to about 7 ft. Under normal conditions this will roast through in 8 to 12 hr.

When the roasting zone has reached, the surface at all points the air blast is shut off. The discharge door is opened and a sluicing apron inserted, to connect with the launders. The hot roasted ore is then sluiced out with mill solution. The sluicing nozzle is made of hard wood and has

a 1 1/8-in. nozzle opening. It operates under a head of 45 ft. The nozzle is suspended in position in front of the discharge door. The operator stands at one side, to avoid the numerous steam explosions, and directs the nozzle with a long rod. Under normal conditions it takes from 1 to 3 hr. to sluice out a roaster charge of 24 tons.

From the roasters the hot ore mixed with mill solution passes by means of launders to the leaching tanks. The leaching tanks are 20 ft. in diameter and 12 ft. deep. There are six of these, each holding 135 tons of ore. In the bottom of each tank is a 12-in. round discharge hole for sluicing out the tailing. This is closed by a turned wood plug, having a 4 by 4 in. stem extending to the top of the tank. For convenience in sluicing out, the discharge hole is surrounded by a box extending to the top of the tank and provided at intervals with 12 by 12 in. sluice gates.

Of the various filter bottoms tried, one made of 4-in. drain tile has given the best service. Four lines of tile are spaced across the bottom of the tank, and cemented in position by a thin slab of concrete on each side. This tile system connects at one side of the tank with a 3-in. wood-pipe solution line.

The solution lines from the leaching vats lead to a distributing box. Here the solution is directed either to the “weak” tank or the “silver” tank, depending upon its metal content. From the weak storage tank the solution is pumped back for sluicing purposes, while from the silver tank it goes to the precipitating boxes.

The iron precipitation boxes are similar in design to the wood zinc boxes sometimes used in cyanide mills. They are larger, however, and so constructed that the iron tie rods are not in contact with the acid-soaked wood. From the precipitation boxes the solution passes to the barren sump and is returned as a leaching solution to the vats.

New Features

Having followed the ore treatment through in a general way, we will now return to discuss more in detail certain features that are more or less new.

Importance of Proper Mixing

Perhaps the most important step in the whole operation is the proper mixing of the ore preparatory to roasting. Roasting for lixiviation requires a very close regulation of temperature. The best results on many ores are obtained between 600° and 700° C. In any case, to sinter parts of the ore is to render it unfit for subsequent lixiviation. The three chief factors which determine the temperature are: (1) the percentage of fuel in the mixture, (2) the percentage of moisture in the mixture, and (3) the amount of air blown through the charge per unit of time. The first of these three is much the most important.

Coal dust is used as, fuel. The present supply costs 25c. per ton f.o.b. cars at the coal mine. Since this class of material takes a cheap freight rate it is the most economical fuel to use under our conditions. The amount required varies from 2.4 to 3.0 per cent, of the weight of the ore being roasted.

The ore as it comes from the mine is very wet. The drier is supposed to reduce the moisture to about 5 per cent., but the variation is considerable. We might suppose that an increase in moisture would lower the temperature of the roast. It has exactly the opposite effect, however, and it is necessary to reduce the percentage of fuel as the moisture in the ore increases.

The mill was designed to treat 150 tons per day. It is handling considerably more than this, the average for the month of March being 166 tons. This tonnage is mixed and rolled on one shift. The mixing is in charge of a trustworthy man who checks up the feeders by weighing the output of each frequently.

Holt-Dern Roaster

During 1913, a new continuous roaster was developed, which overcomes the difficulties experienced with previous machines. A full description of the roaster will not be attempted in the present paper. A few of its general characteristics and the results obtained will be noted.

We installed a commercial-size machine at our plant. Except for a few brief delays for changes, it has been in continuous operation. In the Holt and Dern roaster the column of ore moves down at intervals as the roasting zone travels upward. The air blast travels in an opposite direction to the ore. In this way the air passes first up through the hot roasted ore and becomes highly heated. It then passes through the roasting zone where active combustion of the fuel in the ore is taking place, and finally through a layer of moist unroasted ore. Hence when it leaves the roaster it is fairly cool and entirely free from dust.

Our experience at this plant has demonstrated the following advantages in this roaster over the intermittent type already described:

A series of extraction tests for a period of five weeks gave an average of 80.3 per cent, of the silver for the old roasters and 86.4 per cent, for the new roaster. This is on the coarse crushing we have found most economical in the old type of roaster. Following is a screen sizing test on the ore fed to both roasters:

The ore treated is a dense one and the recovery would be materially increased by finer crushing. With the old type of roaster this is not economical, while in the new roaster 10 mesh, and even finer product, may be handled very well.

The salt used in both cases was equal to 7.5 per cent, of the weight of ore roasted. We are of the opinion that this can be economically reduced in the new roaster, but this has not been determined.

When compared with the roasting furnaces commonly employed for preparing ore for leaching purposes the following advantages may be pointed out.

1. There is no perceptible volatilization loss of the valuable metals.
2. There is no dust loss.
3. An ideal product for percolation is furnished even when the raw ore contains much slime.
4. Only a low percentage of an inexpensive fuel is required.
5. A high recovery without fine crushing is possible.
6. The roaster gases are concentrated; cool, and easily condensed for leaching purposes.

Leaching Process

The leaching process is very simple. There is in fact but one mill solution, which is designated as “silver,” “weak,” or “barred,” depending on its value in silver. The solution is made up merely of soluble salts from the roasted ore, carrying in addition about 4 lb. of free acid. The acid is in part supplied from the condensed roaster fumes, the remainder being added as sulphuric acid. At such times as the ore carries a considerable percentage of pyrite no extra acid is required. Of the salts taken up by the solution from the roasted ore, during leaching, NaCl forms the greater part. Chlorides and sulphates of the various other metals are also present.

The sluicing out of the roasters into the leaching vats is done with weak solution. This dissolves the greater part of the silver and copper from the hot roasted ore while conveying it to the leaching vat. The solution passes from the vat as the “silver” solution and most of it is pumped to the precipitation boxes. When the leaching vat is full it is leveled off, and leached with barren solution for 24 hr. It is then washed with water down to from 5° to 10° B. (specific gravity, 1.036 to 1.075) and sluiced out. The washing is regulated so as to keep the solution at the proper density. Experiments have shown this to be about 24° B. (specific gravity, 1.200). The excess barren solution is run to waste from time to time.

The best results are obtained by leaching with warm solution. During the summer months the heat from the roasted ore is sufficient to maintain a temperature of from 30° to 40° C., but in cold weather steam is blown into the solutions. This is best done in the precipitation boxes, as maximum efficiency is obtained by heating the solutions at this point.

Leaching Costs

The cost of leaching varies considerably. The minimum of 17c. was attained during the summer when the heating plant was closed down, and the roasters were making the necessary acid. On the other hand, during the month of March when it was necessary to buy both acid and coal the leaching cost reached the high figure of 43.8c.

Precipitation

The precipitation boxes are packed with scrap iron of every description. Thin sheet scrap, old screens, etc., are desirable on account of the large surface presented. The metals in solution are deposited on the iron in accordance with their position in the electromotive series; that is, the metals of greatest potential difference are deposited first: This segregation of the metals, however, is not sufficiently clear cut to be of value in their separation.

A general cleanup of the precipitation boxes is made once each month. An intermediate is sometimes necessary on account of the boxes clogging.

The method of cleanup is rather crude, due to the present layout. The loosely adhering precipitate is brushed from the scrap iron, put into filter boxes and washed. It is then dried and sacked for shipment.

A partial analysis of the product is as follows: Au, 1.78 oz.; Ag, 6,868.5 oz.; Pb, 13.2 per cent.; Cu, 36.94. per cent.; insoluble, 7.5 per cent.; Fe, 2.5 per cent.; S, 1.6 per cent.; Zn, nil.

Our original plan was to precipitate the gold and silver on cement copper in an acid-proof press. The copper used could then be recovered on scrap iron as cement copper and returned to the precipitator. Two difficulties presented themselves. In the first place, it was hard to form and maintain a uniform cake of cement copper on the filter leaves. Secondly, the filter cloth and cake soon became impervious to the solution.

We are at present developing a method of handling the copper precipitation of the gold and silver, which promises to be both simple and efficient. However, as this is still in the experimental stage it will not be considered here.

The consumption of scrap iron amounts to about 1 lb. per pound of product recovered. The grade of scrap used costs $5 per ton delivered at the mill or 6c. per ton of ore treated. The total cost of cleanup, including drying and sacking, amounts to 6.5c. per ton of ore milled.

Pumps

Our practice of sluicing out the roasters makes it necessary to handle large volumes of solution. The mill solutions are corrosive. In addition to acids they carry free chlorine, and chlorides and sulphates of the various metals. No metal, commercially available, can withstand this combination, and pumps on the market, of suitable material, are either too small or too expensive to be considered. Our sluicing alone requires a continuous supply of 140 gal. per minute, which must be elevated 89 ft.

After considerable experience with pumps that work for brief periods, and others that work not at all, we decided to install the Pohle air-lift system. These air lifts are constructed of wood pipe and a rubber air

hose as shown in Fig. 3. Wood stave pipe, with bands well protected with asphalt or similar material, withstands the solutions very well.

The highest lift of 89 ft. is made in three steps. The highest single lift is 37 ft. and this determines the air pressure used, which is about 27 lb. The air-lift pumps have been in continuous operation for over a year and have given complete satisfaction.

Every department of the mill suffers the inconvenience of being housed in an old plant not designed for the process. Furthermore, several features did not work out as anticipated, and in the consequent changes and reconstruction, convenience and economy have suffered. Our experience has taught us many things that would add to the efficiency of a plant we might now design.

Conclusions

The roasting process is not limited to chloridizing but may be advantageously applied to oxidizing gold ores for cyanide treatment, and oxidizing and sulphatizing copper ores for subsequent leaching.

However, it is our opinion that the most favorable field of application is in chloridizing roasting. In this class of roasting we not only have the benefit of cheaper operating cost, but, in addition, have overcome the volatilization loss, which in the past has been a serious difficulty.

We have tested out samples of ore from 38 different mines. These have given us a large variety of combinations. In many cases the recovery of gold, silver, and copper has been well above 90 per cent. The ideal combination, we may suggest, is a siliceous copper-silver ore carrying 6 to 10 per cent, pyrite, and favorably located with respect to a salt supply. Under such favorable conditions, the entire cost of milling in a well-designed plant will be less than $1 per ton, on, the basis of 150 tons per day. When the ore carries gold it is necessary, to carry free chlorine in the leaching solutions and this adds a few cents to the cost of treatment.

Ores high in silica, iron, and manganese chloridize readily. A small percentage of lime does no harm as it is readily neutralized in the roast. A large percentage, however, is detrimental.