The method of treatment used in this mill was worked out by Mr. Philip Argall after many years' experience with the cyanide process and Cripple Creek ores. Based on experiments made at a much earlier date, the plant finally was put in operation in April, 1909, after some unavoidable delays and in spite of the unfavorable criticism of doubting Thomases.
The problem was to treat at a profit a large dump of low-grade ore, amounting to almost 1,000,000 tons, which had accumulated for many years. The average value of the dump was determined by extensive sampling to be $3.80 per ton. This figure is much lower than the cost of treatment at some of the earlier mills and is even less than the lowest treatment rate now offered by custom mills on low grade Cripple Creek ores, viz., $4 per ton. That the cost of treating this dump in the Independence mill is about $1.50 per ton is evidence of the general improvement in milling methods and of the ability of the technical staff in charge of the work.
The mill was the first successful cyanide plant treating sulpho-telluride ores in the Cripple Creek district, although several successes had been scored previously in cyaniding the oxidized ores. It has been steadily enlarged since beginning operations from an original capacity of 4500 tons per month to practically more than 9000 tons per month at the present time.
Crushing and Grinding.—The dump ore is mined by electric shovel, loaded into 4-ton cars and hauled up an incline to the coarse crushing plant. Two cars are in use, one being loaded while the other is dumping. In this way a car is loaded and dumped about every three minutes, which permits crushing a day's run of 300 tons for the mill in about four hours.
The cars are dumped directly over a 7½ D Gates gyratory crusher which reduces the ore to 4-in. size. The crushed ore is discharged over a grizzly with 2-in. openings, the oversize falling onto a picking belt where pieces of steel and wood are removed. The belt discharges into a 5 K Gates gyratory which crushes the ore to 1½-in. size.
The product of the second gyratory is combined with the undersize of the grizzly and elevated by a belt conveyor to the storage bin in the concentrator building.
The design of the storage bin and feeder insures a positive steady feed of ore. The bin is circular with conical bottom, the cone terminating in an opening 6 ft. in diameter, through which the ore feeds into an open cone about 12 ft. in diameter at the top and placed with its upper openings in the same horizontal plane with the 6-ft opening of the main bin. The lower cone terminates in an opening about 12 in. in diameter, through which the ore is discharged by a revolving table feeder. By this construction the ore flows freely and the bin may be completely emptied without extra attention.
The method of adding lime for protective alkalinity differs from the usual practice of adding it in a solution form. As the ore leaves the storage bin it is moistened with a small quantity of mill solution, which settles dust and slakes the solid lime, which is added continuously at the rate of about 1000 lb. for the day's run of 300 tons of ore. This gives a protective alkalinity in the solution of .002 per cent. The mixed ore and lime arc then elevated to bins above the rolls, and further reduced in two successive sets of rolls to ⅜-in. size.
This material, which constitutes the Chilean mill feed, is then elevated and distributed to three bins, one for each mill. The Chileans are of the fast running type, 6 ft. in diameter, making 33 r.p.m. They grind in mill solution which is kept at a strength of about ½ lb. KCN per ton and discharge through 12-mesh screens. The capacity is about 100 tons of ore each per 24 hours, 60 per cent of the pulp passing 150-mesh screen.
Classification.—Sand and slime are classified prior to concentration in two spiral classifiers, each of 300 tons daily capacity. The sand is delivered 5 ft. above the feeding point, carrying from 4 per cent to 7 per cent slime, depending on the varying nature of the ore. The slime emerges 6 in. below the feeding point, carrying about 3 per cent of sand coarser than 150-mesh.
Sand Concentration.—Mill solution is added to the sand as it leaves the classifier, and the pulp is distributed to 20 Card tables. Two grades of concentrate are obtained, the first carrying about 5 oz. gold per ton, which is shipped to the smelter; the second with about 0.75 oz. gold, which is sold to chlorination works. The Card table is preferred for this work as it gives a cleaner division between the two classes of concentrate and a consequent financial advantage in their disposition. The sand tailing is laundered to a sump and delivered by centrifugal pump to the cyanide plant for further treatment.
The coarser and heavier sand is separated by spitzlutten and reground in a tube mill. The reground pulp is classified by cones into sand and slime, the former being treated on two Card tables and the latter on four Deister slimers. All of these tables produce first and second grade concentrates.
Slime Concentration.—The slime from the classifiers is thickened in 16 8-ft cones. The underflow is distributed to 10 Deister slime tables and four vanners. Like the Card tables, the Deisters make two grades of concentrate, the first carrying about 7 oz. gold per ton and the second from 0.50 oz. to 0.70 oz. The total concentrate removed by sand and slime treatment amounts to about 3 per cent by weight of the ore treated and contains 40 per cent of its value.
The tailing from the Deisters and vanners is laundered to a sump which also receives the overflow from the thickening cones and the tailing from the tables treating reground sand. A centrifugal pump returns the slime to the cyanide plant for further treatment.
Cyanide Treatment.—The regular cyanide treatment is supplemented by the use of bromo-cyanogen. The sand and slime pumped from the concentrator are delivered to the head of the cyanide mill where they are reclassified. The sand is discharged to a reciprocating conveyer which distributes to six sand filter bins, each of 250 tons capacity. The sand enters the bins with an assay value of 0.06 oz. to 0.08 oz. gold per ton and is discharged, after treatment, with 0.03 oz. to 0.04 oz. The treatment is as follows:
After a bin is filled and drained it is leached with solution strengthened by the addition of 25 lb. KCN to each bin. When this solution has filtered the sand is given successive washes with barren solution and water and sluiced out.
The slime from the classifiers flows to four large settling tanks, the overflow from which is usually returned to mill solution storage. It is the intention to keep the mill solution at a strength of ½ lb. KCN and at a value not exceeding $0.50 per ton. When this is exceeded the tanks are not allowed to overflow, but are decanted to the precipitating department.
The thick settled slime is drawn from the bottom of any one or all of the tanks and pumped to the agitator for 10 hours agitation, with air at 30 lb. pressure.
A charge consists of from 75 to 100 tons of dry slime and from one and one-half to two times that amount of solution. Thirty lb. KCN are added to each charge. The agitation tanks are modified from the standard construction in air agitators by shortening the central tube one-half, thereby securing the full benefit of the air in agitating the pulp in the upper half of the tank.
After 10 hours' agitation the charge is pumped to a second agitator for special chemical treatment.
Filtration.—The vacuum filter is of peculiar construction, generally according to the Cassell type, and consists of no leaves 7.5 ft x 6 ft. The capacity is sufficient to handle the contents of an agitation tank in seven or eight charges in 24 hours, three hours being the time required for a complete cycle of loading, washing and discharging. Vacuum is maintained at 10-12 in., which is practically the best obtainable in slime filtration at the altitude of 10,000 ft., where the barometer normally stands at about 20 in.
Precipitation.—The filtered gold solution is clarified in two Johnson presses and flows to the zinc boxes for precipitation. The daily tonnage of solution precipitated is about 350 tons, or little more than the tonnage of ore treated. Occasionally, when the mill solution rises too high in value, several hundred tons may be precipitated in addition to the regular quantity.
The gold precipitate receives the usual acid treatment and is discharged to a pressure tank connected with a small filter press. The press cake is dried, roasted and ground in a small tube mill using pieces of shafting for the grinding medium. The object of the last step is to put the precipitate into as uniform condition as possible so that an accurate sample can be taken. Prior to the introduction of this practice a great deal of time was consumed in making settlement with the smelter on account of the variation of the sample taken for assay.
The Independence mill was erected at a cost of over a quarter of a million dollars. The preliminary experiments indicated that a fair profit could be made by the treatment of 10,000 tons per month, and it is interesting to know that the actual working costs have checked the estimates remarkably close. No official figures have yet been given by the company, so that details of costs and profits are not actually known to the public, but it is stated that operations are being conducted at a satisfactory profit.
The mill is equipped with duplicate pieces of machinery at those points where a break would interfere seriously with continuous operation. Cement floors, launders and sumps effectually prevent the loss of valuable cyanide solution and contribute to the cleanliness of the plant.
Crushing and concentrating in cyanide solution, which was one of the features concerning which skeptics expressed their doubts, has not given rise to any trouble, either by serious loss of solution or by ill effects on the workmen. Altogether the plant has been a success on which the company and staff are to be congratulated.
Following is a condensed statement of the flow of ore through the mill:
Ore from dump in 4-ton cars, to 1.
1—7½ D Gates gyratory, discharging to 2.
2—Grizzly; oversize to picking belt, thence to 3; undersize to 4.
3—5 K G Gates gyratory, discharging to 4.
4—Incline belt conveyer to 5.
5—Mill storage tank; lime and a small quantity of mill solution are added to the discharge and the mixture elevated to bins, thence to 6.
6—Two sets of rolls; product elevated to bins, thence to 7.
7—Three 6-ft. Chilean mills, 12-mesh screen; discharge to 8.
8—Screw classifiers; sand to 9, slime to 14.
9—Distributor to 20 Card tables; high grade concentrate to smelter and low grade to chlorination mill; tailing to 10.
10—Sand tailing sump; centrifugal pump to cyanide department, 17. Coarsest sand reground in tube mill; product to 11.
11—Two classifying cones; underflow to 12, overflow to 13.
12—Two Card tables; concentrate shipped, tailing to 16.
13—Four Desiter slime tables; concentrate shipped, tailing to 16.
14—Sixteen thickening cones; underflow to 15, overflow to 16.
15—Ten Deister tables and four vanners; concentrate shipped and tailing to 16.
16—Slime tailing sump; centrifugal pump to cyanide department, 17.
17—Cyanide mill; screw classifiers; sand to 18, slime to 19.
18—Sand leaching tanks; filtrate to 23, sand discharged.
19—Four settling tanks; overflow to mill solution or to 23; thickened pulp to 20.
20—Agitating tank for 10 hours; thence to second agitator for special chemical treatment for six hours, thence to 21.
21—Storage tank for filters, to 22.
22—Vacuum filter; gold solution to 23, slime to discharge
23—Two Johnson clarifying presses, to 24.
24—Seven zinc boxes; barren solution to mill storage; precipitate to acid tank and filter press. Press cake dried, roasted, pulverized, sampled and shipped to smelter.
This is the 1908 Sanborn Fire Insurance Map for the Stratton's Independence Mine and Mill on Battle Mountain. This was a full sheet, and is the first edition of this mill that would change over time.
I added in the Sanborn Key and cropped the sheet down as far as I could without losing info, but did not do any straighting.