Bulletin of the American Institute of Mining Engineers …library.aimehq.org/library/books/Bulletin of the AIME...t,reatment. In addition 'to this saving, the cyanide-tailings would

SUBJECT TO REVISION.

[TRANSACTIONS OF THE AMERICAN INSTITUTE O F BIINING EKGINEERS.]

T h e Cyanide-Plant at the Treadwell Mines, Alaska.

BY W. P. LASS, TREADWELL, ALASKA."

(San Francisco Meeting, October, 1911.)

THE purpose of this article is not only to describe the plant and method of cyaniding the Treadwell concentrates, but to present some of the results of the experimental work obtained in the past three years for the Alaska-Treadwell Gold Mining Co., at Douglas Island, Alaska, under the direction of F. W, Bradley, Consulting Engineer, and Robert A. Einzie, General Superintendent, of the afliliated companies.

A t the time the experimental work was undertaken the con- centrates were being shipped to the smeltery at Tacoma, Wash., and the cost for treatment of 3-oz. (gold) concentrates was $11.95 per t,on, divided as follows : ,

Smelting-charges, . . . . . . . . . . . $4.00 Loading, freight, insurance, etc., . . . 2.89 Interest due to time lost in transit and in settlement, . . . 0.05 Loss due to settlement for 95 per ceut. of the gold a t $20 per ounce, 5.01 -

Total, . . $11.95

From the experimental work described later, i t mas estimated that 96 per cent. extraction could be made by treatment on the ground, and that the cost, when treating 80 tons per day, would be $3.25 per ton, divided as follows:

, Per Day. Per Ton. Labor, . . . $66.16 $0.827 Chemicals,. . 76.60 . 0.960 Power and steam-heat, . 67.60 0.845 Marketing-, refining- and other charges, . 49.36 0.617 - -

Totals, . . $259.72 $3.250

Adding to this total the 4 per cent. treatment-loss, which on 3-02. concentrates amounts to $2.48, gives a total cost of $5.73 per ton. Comparing this with $11.95, the cost when shipping to the smelter, leaves a net gain of $6.22 per ton by the local --

* Cyanide Superintendent of the Alaska-Treadwell Gold Mining Co. r 1 1

t,reatment. I n addition 'to this saving, the cyanide-tailings would have an economic value due to the sulphnr- and iron- content, as well as the value of the residual gold after oxi- dation.

I. LABORATORY-WORK.

1. Character of the Concentrates.-The coucentrates, amount- iug to 1.8 per cent. of the original ore, coutain : Fe, 40; S, 1 ; SiO,, 11 per cent., and carry from 2.5 to 4 oz. of gold and 0.75 oz. of silver per ton. The gold- and silver-values amount to about 37 per cent. of the values contained in the original ore from the mine. The figures in Table I. are assays and aver- ages of sizing-tests 011 coilcentrates from the various mills.

TABLE I.-Assay Sizing-Tests of Tre'rendzoell Gold- an.d Silver-Ores.

Value in Size of Material. IVeigbl. Value. / One Ton of Per Ton. 1

I , Original.

~ h r o u i l l 100; on 120 .... ... 13.37 39.27 8.14 1 5.21 T h r o ~ ~ g l l 120, on 150 .......I 7.69 1 26.61 1 3.17 2.05 Through 150 .................. 36.46 17.10 9.65 6.23

O n 2.0-mesh sereen .... . Through 20, 011 40 .... ... Through 40, on GO ...... Through 60, on 80 .... ... Tl~rounh 80. on 100 .... ...

(In this paper, all figures, unless otherwise stated, are based on the dry ton of 2,000 lb., with gold at $20.67 per oz. Silver- value is not included. Screen-mesh is expressed in openings per linear inch.)

On account of the decrepitatiou of the pyritic crystals during the process of drying, as well as the tendency of the particles to adhere to one another, all sizing-tests were made in water witl~out previous drying of the sample. Results show that the values vary directly with the degree of comminution. I t being understood that the concentrates are derived from pulp after amalgamation at the mills, i t seemed evident that the gold was present as metallics incased within the pyritc. Work done in

a the laboratory previous to the year 1909 confirmed this view, and iildicated that a satisfactory extraction could -be obtained by regrinding, followed by amalgamation and cyanidation.

[ 2 1

Per Cent. 0.44 1 $70.35 8.23 . 203.96

10.96 143.89 12.45 94.88 10.38 60.85

Per cent. ~ $0.31

26.05 16.83 24.39 15.76 18.34 11.85 9.78 6.32

CYANIDE-PLANT AT THE TREADWELL MINES, ALASKA. 185

2. Preliminary Tests.-For the preliminary tests ordinary quart-size glass jars were used, and agitated by placing them on the distribatiug-boxes of the Frue vanners. In each case ail excess of lime and a small amount of lead acetate mere added to the solution. Sizing and assaying of the residues showed the gold to have been removecl from the finely-ground particles, while the large percentage ot value remained in the coarse particles.

The next step was with 50-lb. composite samples from all the mills. A clean-up barrel was fitted with iron balls and used to grind the concentrates to a 200-mesh product, which was passed over a 2- by 4-ft. amalgamated copper plate, the pulp

. collected and cyanided in small agitation-vats, built on the plan of " Brown " or Pachuca " tanke. These were 14 in. in diam- eter and 4 ft. high, with a 1.25-in. pipe suspended through the center. A t the apex of the cone a needle-valve regulated the supply of air.

The 50-lb. samples were treated in these small tanks, the results given in Table 11. being a fair average from one of these tests.

TABLE 11.-Results Obtained from Treatment of 50-Lb. composite Samples from Treadwell Mills.

Assay-value of original concentrates, . . . . . . Amalgamation-extraction based upon head- and tail-assays, per

cent., . . . . . . . . . . . Proportion of ground product passing 200-mesh screen, per cent., Assay-value of cyanide heads, . . . . . . . Assay-value of cyanide tails, . . . . . . . Cyanide-extraction based upon head- and tail-assays, per cent., . Cyanide-extraction based upon solution-assays, per cent., . . Total extraction by amalgamation and cyanide, per cent., . . Time of dyanide-treatment, hours, . . . . . . Strength of cyanide solution (1 lb. per ton), per cent., . . Cyanide-consumption per ton of concentrates, pounds, . . Lime-consumption per ton of concentrates, pounds, . . .

The tests in Table.11. show that '75 per cent.. of the gold could be obtained by fine grinding and amalgamating, or 96 per cent. by fine grinding and amalgamating followecl by cyaniding.

[ 3 1

11. EXPERIMENTAL PLANT. ' Having proved that a satisfactory extraction could be ob-

tained, the next step was to determine the most economical methocl of handling the material, For this purpose, an adclition was built to one of the mills, in which was installed an Abbk 4- by 12-ft. tube-mill, with the necessary plates for amalgamation. The tube-mill ground 0.5 ton of concel~trates per hour to pass a 200- mesh screen, or 1 ton per hour, 95 per cent. of which woulcl pass a 200-mesh screen. With a cleaner separation of the coarse return-product, the grinding-capacity could have been increased. Various forms of classifiers were tried, the Dorr "drag" classifier proving the most satisfactory, not only making a good separation between the sands autl fines, but acting as a feeder to the tube-mills. I n later practice, with a duplex Dorr classifier treating 125 tons daily of concentrates clischarged from a larger tube-mill, the following results were obtained :

7---Screen BIes11,----- On 100. On 200. Through 200.

Per Cent. Per Cent. Per Cent. Feed to classifier, . . 10.1 2G.4 G3.5 Coarse discharge, . . 51.3 44.0 1.7 Fine overflow, . . . 1.1 29.7 69.2

As ordinarily used, the water is much in excess of the ore, so that the fines are carried over by the rising current from the rakes; but in operating the Dorr to its fullest capacity on con-

' centrates, it is ilecessary to reduce the v o l ~ ~ m e of water used, and depend upon the greater specific gravity of the pulp hold- ing the fines iu snspeusion unt,il carried over with the fine product.

Callom1 cones arranged with suspendccl diaphragms were used for de-watering the sands previo~ls to cyaniding. When cleliveri~lg a clear overflow, one standard 8-ft. cone was found to have an hourly capacity of 1 ton of concentrates with 15 tons of lime-water, making a spigot-product of less than 35 per cent. of moisture.

Grinding in an alkali solution equival$nt to 2 lb. of lime per ton kept the amalgamation-plates i u a clean, bright condition, and materially aided in the settlement of slimes. Without lime the pulp discharged from the tube-mill possessed a latent acidity equivalent to 6 lb. of lime per ton of concentrates,

[ 4 I

which made plate-amalgamation allnost impossible on account of a black surface-deposit completely coating the plates within 10 mia. after being dressed.

Sea-water as a substitute for lime-water 1vas tried, and al- though i t gave better amalgamation-results than fresh water, it was aot as satisfactory as the lime solution. The plates became coated with slime and the solution remained turbid in the tanks.

By fine grinding and amalgamatung in 15-ton lots, an ex- traction of from 75 to 80 per cent. was obtained, the extraction varying dirkctly with the fineness of grinding. On the original ore this amounts to an extraction of 84 per cent, by amalga- mation.

To obtaiu the best results by amalgamation, mercury ~ 1 ~ a s fed into the tube-mill with the concentrates. After having completed the amalg&mation-tests, during which time 7,050 oz. of amalgam were recovered, the mill was emptiecl of its pebbles and the inside thoroughly cleaned,,in order to cleter- mine the amount of mercury or amalgam that remained. No free mercury and only 3 per cent. of the total amalgam m7as recovered from the tube.

Upoii again feeding the concentrates to the tube-mill without either cyanide or mercury, a concentration took place inside the mill, as sho~vn by the daily sampling of the feed aud dis- charge of the mill, Table 111.

TABLE 111.-Results Obtained by l'reatment of Concentrates in the Tube-XZl. -

Tube- Original Fced Feed (Includes c[:!Ee&ds Slirnc Finer

from Bins. Coarse Keturil Tube-lfill. than 200-Mesh. 1 1 Product). 1 1 First 6 hr. grinding. .... ... Second 6 hr. grinding ... ... Third 6 hr. grinding .... .....

--I

$48.00 . 48.00

48.00

I

$95.00 113.00 131.00

$85.00 103.00

$18.00 1G.90

120.00 , 19.20

Cyanide mas then introcluced into the grinding-solution and samples assayed as follons :

The method of grinding proving successful, the next step mas to test. the cyanide process on a larger scale. For this purpose st Brown or Pachuca agitation-tank, 10 ft. in diameter ancl 22 ft. high, with 60° conical bottom, was, erected beside the tube-mill, together with four small redwood tanks. A Merrill precipitation-press was later purchased and a few filter- leaves placed on the suction of the gold-pump for clarifying the solutions. This completed the necessary equipment for cyaniding t.he tnbejmill procluct in 15-ton lots. The gold-values were removed from the pulp by successive washes and clecan-

First 6 hr. gr inding. ...... .. Second ti hr. grinding.. ...

tations.

TABLE IV.-~esults of Zinc-Dust Precipilati~n, Obtained in Experimental Plant.

Per Cent. 0.0.5 $18.00 $96.00 0.05 4s. 00 67.00

Cyanide Per Ton Lime Per To11 Gold Before Gold After of solution. 1 of ~o ln t ion . PrecipiWtioii. Precipitation. (

Third 6 l:i-. g r i ~ d i n g . . 0 . 0 4 1 48.00 68.40

- - -. .~~ Pounds. Pounds.

0.44 1 0.42 1 0. 80 0.46

--

$80.00 62 00

- Per Cent.

8.82

$14.60 11.60

59.20 12.00

CYANIDE-PLANT AT THE TREADWELL MINES, ALASKA. 189

The figures of zinc-dust precipitation presented in Table IV. show the non-precipitation of the values when the lime-con- tent of the solutions fell much below 1 lb. per toil. Wi th solu- tioils high in lime an excess of cyanide was adcled to keep the filter-cloths clear. I n each case an excess of zinc-dust was added. I

The flow-sheet, Fig. 1, shows diagrammatically the method ilsecl for these experiments, with the exception that the filter- box shown mas later superseded by a Kelly filter-press (type 1 B) of 50 tons daily capacity, which clicl away with the nu- merous washes and decantations previously requirecl.

The cycle of operatione of the Kelly press and the time of working, when forming a 1-in. cake of about 4 tons of con- centrates (dry weight), are as follows:

Operation, of Ihe Kelly Press.

Filling press, . . . . Forming cake, . . . . Returning excess pulp; . .

. . . . Washing, Returning excess wash, . . Drying, . . . . . Opening, discharging, and closing,

Total time of one cycle, .

Time. Minutes.

. . . 3

. . 2

. . 2

. . 1 2

. . . 2

. . 8 15 . . . -

. . 4 4

Moisture in pulp fed to press, per cent., . . 35 AIoisture in tailings cake discharged, per cent., Y to 10 Pressure of forming cake, . 30 Ib. per sq. in. Amount of mash-water used per ton of concen-

trates, . . . . . . . . . 0.5 ton.

The first 25 test-runs made in the experimental plant are summarized in Table V.

TABLE V.-Results qf first 1luen.ty-Five Experimental Test-'Runs on 700-Pool Mill-Concentrates.

a Assay Per Ton. a Assay Per Ton.

E

- z 5

P;malgamation (0.05 Der cent. eranide:.l

1. 20 $42.16 $19.63 Amalgamation .......... 48 90.0 ii.70 ......... 2 15 54.80 9.00 Amalgamation

3 18 41.00 10.40 Amalgamation ........... 71; 2.97 96.6 4 16 47.20 10.00 Amalgamation 16 1.92 8 97.8 ........... 5 10 32.00 8.40 Amalgamation ........... 24 2.00 2 97.5

0.05 per cent. cyanide.. 0.046 per cent. cyanide 0.018 per cent. cyanide 0.024 Der cent. cvanide

li S 9 - 10

w - 11

n All values are i n gold a t $20 per ounce. Tests 2, 6,8,9,10,16 a n d li given preliminary alkali ag i ta t i~11 ~ l l solutions contained a n excess of lime.

10 S 9

I5 15

12 15 40.80 9.60 76.5 98.0 1 to 2 per cent, Amalgamat'ion 13 15 46.00 9.40 79.6 100.0 1 to 2 0.025 Der cent.

56.80 50.00 30.80 52.00 37.20

9.liO

9.40

9.60 8.EO 9.00

14.60 13.40

2.40

1.70

Amalgamation ........... Amalgamation ........... Amalgamation ........... Amalgamation ...........

Amnlgam~t ion 0,002percent.cyanide,

Amalgamation 75.0

S2.0

83.1 62.4 70.7 71.9 fA.0

0.16

0.15

97.0 98.0 9i.0 90.0 88.0

............... 0.67

1 to S 1 to 3 I t 0 2 1 to 2.1 1 to 2

24

24

9. ti0 8.80 9.00

14.60 13.40

4.46

2.20

1.80 2.30 1 . 0 2.00 9.40 ,

2

2

81.2 '73.8 71.7 87.6 82.0

94.0

96.3

0.07 0.038 0.1 0.15 0.15

............... 1 : O

............... 1.0 1.0

10 1s 12 24 94

2. :10 1.44 1.08 0.90 0.96

:: Y

2 2

9ti.S 95.4

2 9 4 . 5 96.1 93.6

The results of the tests showed that 75 per cent. of the gold could be recovered by grinding and amalgamating, or 96 per cent. by the combinec!method of amalgamating and cyaniding.

[ 9 I

192 CYANIDE-PLANT AT THE TREADWELL ;MINES, ALASKA.

Results also showed that during the process of grinding in 1.5-lb. (0.075 per cent.) cyanide solution, a similar extraction could be obtained without amalgamation. T l ~ u s a satisfactory extraction was obtained either by amalgamating and cyaniding or by cyaniding direct.

A preliminary agitation with an alkali solution was found to shorten the time of cyanide treatment and save 25 per cent. in the cyanide-consumptio11. \

Air-receiver

Discharge for cle'aning

Passing the air used for agitation through a Feceiver filled with a solution of caustic sod& or milk of lime also decreased the cyanide-consumption, presumably by the removal of oil and carbonic oxides from the air. The -pipe-connections illustrat- ing the method of a d d i ~ ~ g the alkali solution are shown in Fig. 2.

When grinding in cyanide solution stronger than 1 lb. per ton (0.05 per cent.), followed by amalgamation, it mas difficult

[ 101

CYANlDE-PLANT AT TIIE TREAD\VEI,L BIINES, ALASKA. 193

to kcep the plates bright, due to a dull white surface-cleposit, which if allowed to remain turned to a dull gray. A Muntz- metal plate nras substituted for a copper plate, but as all the plates were silver coated no variation in the result was noted.

The results obtained from this exteuded period of investiga- tion, lasting over two years and at a cost of $27,794, justified the building of a plant of 100 tons daily capacity. This cost was largely oflset by the ability of the final plant to treat the concentrates without the usual alterations necessary in starting -

, a new mill. I t also formed the nucleus of the final mill-crew. As thc abanclonment of amalgamatioll of high free-gold

values in favor of direct cyaniding seemed a somewhat radical change, the new mill was planned for operating either way, and ultimately nearly 5,000 tons were treated by .each method before deciding to cast out the time-honored amalgam-plate. All of the equipment purchased for the experimental work was used in the permanent plant, which was completed in Septem- ber, 1910.

111. THE 100-TON CYANIDE-PLANT.

The cyanide-plant col~sists of three main buildings located on a hill-side 200 ft. above the stamp-mills. The upper building contains the grinding-and-amalgamating plant, with a lower floor for solution-storage tanks. The lower contains the cya- nide equipment proper, while the refinery is in a concrete building at one side, as shown in Fig. 3.

The five mills on the island contain a total of 900 stamps, and crush approximately 5,000 tons daily. The crushed ore after anlalgamation is concentrated on 360 Frue vanners, yield- ing au average of 90 tons of concentrates daily, of from 2.5 to 4 oz. of gold per ton. A flowsheet of the operations is shown in Fig. 4.

From the canner-boxes the concentrates are shoveled into specially-constructed flat-bottomed steel cars. These cars, each holding 2 tons of concentrates, are maae up into trains at the mills, and brought by locomotives to the foot of the incline below the cyanide-plant. This incline is 900 ft. long with 14O rise. A Union Iron Works geared hoist, driven by a 75-h-p. electric motor, brings the train to a switch above the upper building. Beginning with this switch, the entire plant is in duplicate throughout.

[ 11 I

I. 75-13-P. Electric Hoist. ' 2. Osgood T.rack-Scales.

3. Car-Tipples. 4. Ore-Bins. 5. Dorr Classifiers. 6. AbbF: Tube-Mills. 7. Classifiers. 8. Launder Distributors. 9. Amalgam-Plates for coarse pulp.

10. Air-Lifts. 15 11. Dorr Classifier.

12. Abbe Tube-Mill. 13. Air-Lift.

15. Callow Tanks. 16. Launder Distributors. 17. Amalgam-Plates for fine pulp. 18. Distributor. 19. Callow Tanks. 20. Callow Tanks. 21. Pierce Amalgamators. 22. Preliminary Agitation-Tanks. 23. Aldrich Electric Triplex Pump.

29. Kelly Filter-Presses. 30. Distributor. 31. Clarifying-Tank. 32. Gold-Sumps. 33. Wash-Water Sumps. 34. Aldrich Electric Triplex Pump. 35. filerrill Zinc-Feeder. 36. Aldrich Electric Triplex Pump. 37. Merrill Gold-Precipitation Presses.

taPnchuco tanks. Tailinga to dam.

FIG. ~.--FLOT\'-SHEET O F ~OO-TOX CYAKIDE-PLAXT.

CI'ANIDE-PLANT AT TIIE TREAD\VELJ, MINES, ALASKA. , 195

Leaving the switch by gravity, thc cars are weigheel, sam- pled, and run illto revolving tipples. Upon releasing the brake the tipple rcvolves, turning the car bottom up and dropping the loacl from the car. The chauge in the center of gravity then causes the tipple to right itself, and the empty car is weighed and returned to the main switch.

Most of the water is removed from the concentrates while in tlie vauner-boxes by the aicl of a bumper, which is simply a large air-piston machine nio~illtecl 011 a truck and movecl from box to box. This bumping causes the concentrates to readjust themselves and pack in the bottom of the box, while the water is run OR, leaving about 12 per cent. of moisture in the concen- trates. I t is considerably easier to shovel the concentrates into the cars after the bumping.

The conceutrt~tes are sampled while in the car by mealls of a long ship-auger. With the ordinary long spoon i t was im- possible to obtai~l satisfactory checks in the samples, as the concentrates are usually covered with water. Ulislaked lime is acldecl to each of the empty cars as i t leaves the tipple in orcler to reach the concentrates a t the carliest possible stage. I t also forms a line of cleavage, causing the concentrates to dun111 clean from the bottom.

From the cars the concentrates fall illto 100-ton steel storage- bins, 15 ft. in diameter, with 55' collical bottoms. The con- centrates iu the bins are kept covered with water, which eftectually prevents oxidation of the " sulphurets" while lying ill the bins. From this point nntil the cyanide treatment begins the concentrate is in strong lime solution a t all times.

A t the apex of tlie conical bottom of each bin tight-fitting gates coutrol the outflow of concentrates, which is a t once sluicecl directly into Dorr classifiers, Fig. 5. The sluicing medium is the coarse return-product referred to later. There are three Dorr classifiers driven by one 7.5-h-p. electric motor, one feeding illto each tube-mill and making 24 strokes per minute. This rate of speed, causing greater agitation, was found necessary to separate the large bulk of the fine from the coarse.

The coarse product of the c1assifiel.s falls into the spiral feeders of the tube-mills. These mills are of the Abbi: type, 5 by 22 ft., lap-welded, trunnion bearings, with corrugated sec-

[ 13 I

196 CYANIDE-PLANT AT TBE TREADWELL MINES, A L A S I ~ A .

tional liners; 3-in. Danish flint pebbles are used for the grinding.

Two 75-h-p. motors on three-phase circuit at 2,200 volts are I~elted to an overhead central line-shaft, ~vllicli in turn is belted to the pinion-shaft of the tube-mills. The tubes arc driven from the discharge ends aud make 27 rev. per min. The mills are controlled by" friction-clutch 1bu1leg.s on the central line- shaft.

For the period from May 15 to July 15, 1911, one tube-mill ground at the rate of 88.75 tons of concelltrates per 24 hr. ac- tual running-time, the power-consumption averaging 64 h-p. By replacing each 75-h-p. motor with a 100-h-p. motor, ancl substituting leather for canvas belts on the main drive, the power-consumption was reduced to an average of 59 h-p. for the same tube duty. This was with the tube just half filled with pebbles, the normal runaing-load. By increasing the pebble-load to 6 in. above the center of the tube, the power- consumption rises to 75 h-p., and both thc quantity of tube- feed and the fineness of the product discharged are increased.

The following is an average screen-analysis of the feed and discharge of one 5- by 22-ft. mill, when grinding a11 original feed of' 88.75 tons per 24 hours:

On 100-Mesh. On 200-JIcsh. Througll 200-Mesh Per Cent. Per Cent. Pcr Cent.

Feed, . . . . 48.7 41.5 9.8 Discharge, . . . 10.1 26.4 63.5

The pulp contained 38.5 per cent. of moisture. .

When the concentrates are amalgamated previous to cyanid- ing, the product discharged from the tubes is distributed over 10 copper amalgamating-plates, each 4 ft. 8 in. wide by 10 ft. long, platecl with 2 oz. of silver per square foot.

The pulp flows from the plates into launders built into the floor. No traps are used, as they are quickly clogged by the metallic iron which accumulates iu the conce~~tratcs from the wear of the various machines used in the processes of mining and milling.

This iron, if allowed to accumulate in the coarse return- product, will amount to as much as 15 per cent. of the total. Experiments are now being carried on with a magnetic clevice for removing the iron from the pulp.

[ 14 I

From a sump iu the launder an air-lift elevates the pulp to a spitslutte, from which the coarse rnaterial is continuously dranri~ into a Dorr classifier, the coarse froni which feeds a 4- by 12-ft. ~ b l i : tube-mill, similar to the larger ones described

'

above. The discharge f r o n this mill joins the overflow f'rorn the spitzlutle, and is elevated by air-lifts to two settling-concs, so situated that the spigot-discharge froni them becomes the sluicing medium for the origiual feed referred to above.

Two points mill be observed here: (1) that the , D o n classi- fiers are a t present doing all the classifying for the mill; and (2) that the concentrates are carried arouad in a closed circuit from which there is no escape until the particles h i~ve become fine enough to join the overflow from the back of the Dorr clas- sifiers.

The Dorr overflow, which is the product cyanided, is more than 98 per cent. through 200-mesh. The remaining 2 per cent. is silica from the wear of the pebbles. Of the concen- trates, the entire product will pass a 200-mesh screen.

The overflow of the Dorrs passes into two Callow de-mater- iag-cones, the spigot-product of which is distributed over 10 amalganiati~~g-plates biniilar to the coarse amalgamatiug-plates previously described. From the plates' the pulp flows iuto launders, thence into a 6-in. pipe, 37 ft. long, having a fall of 0.75 in. pcr foot, which conveys the pulp directly to the lower or cyanide building.

I n the lower building the pulp is iuto a wooden distributing-box, from ~vhich i t flows through two Pierce amal- ganlators into four 8-ft. Callo\v cones. The spigot-product from these cones discharges into four similar ones placed lower than the first set.

The spigot-procluct from the lower cones eaters one of four Pachuca tanks, where i t receives a preliminary treatment of 3 hr. agitation in a solution containing 2 111. of lime per ton (0.1 per cent.), after ~vlzich it is allowed to settle and the clear solu- tiou is decauted. The filling, agitating, settling, decanting, and discharging of a 25-ton charge of concentrates, which includes 46 tons of lime solntiou, requires somewhat less than 24 llr. This prelinlinary treatment saves in the subsequent treatment a t least 1 lb. of cyanide per ton of concentrates.

The overflow lime-water from the Callow cones enters the [ = I

same sump with the decanted lime-water from the preliminary treatment, ancl is pumped by an Aldrich triplex 7- by 9-in. electric pump into a reservoir of 75 tons capacity situated in the upper building. The thickened pulp, ranging from 1.8 to 2.2 specific gravity, is dranrn irito one of eight Pachuca tan l~s , where-it is given the cyanide treatment.

All Pachnca tanks in the mill are 10 ft. in cliameter and 30 ft. high, wit11 60' conical bottoms, Fig. 6. When filled to the level found best for agitating (which is 6 in. below the top of the central column), each tank holds a volume equivalent to 51 tons of water. This is equal to the regnlar charge of 30 tons of concct~trates with 40 tons of solulion, altbongh as high as 40 tons of concel1tr:~tes have becu trcatcd as one charge \vitl~- out any cliference in extraction-resnlts. , The floors under the Pachucas, as \veil as all other floors in

the builcling, are of smooth concrete sloping to a central sump, suppliecl with small pumps to re t~ l rn any escapecl solution or pump to the proper tanks.

The first cya~licle treatment consists of 8 hr. agitation in a 2:lb. (0.1 per cent.) cyanide solution; either potassium or the

0

mixed cyanides being successf~illy used. Alkali is kept a t 1.25 lb. (0.063 per cent.) of lime (CaO) per ton of solution. Lime is acldecl dur i l~g the treatment if the titrations show below that figure; 18 hr. is allo~vecl for settlement and decailtatio~l of this solution.

Decantation takes place through a flexible hose, which is made as follonrs: Canvas coatecl with tar is wrapped around pieces of old boiler-tubing 3 in. in diameter and 4 in. long, spaced 0.75 in. apart. The canvas between the short lengths of tubing is wrappecl with wire, making the dianleter of these spaces slightly smaller. thau that of the tubing, thus insurillg flexibility as well as avoicli!~g the shifting of the tubing. At- tached3horizontally to the top of the flexible hose is a 3-in. slotted pipe. I n operation this slotted intake floats by the aid of two adjustable air-cylinders. The arrangement of these cjlinclers is such as to allow. of the vertical adjustment of the intake-pipe to any depth of submergeuce desired.

The long settlement allowed, with the excessively fine condi- tion of the coacentrates, their high specific gravity, from 4.6 to 5.0, ancl the high alkalinity of the solution, leaves a 30-ton

[ 16 I 1

packed mass in the bottom of the Pachuca. This is brought into thorough agitation within 15 mill. by a device designated as the c g spider," which is an adjustable hollow annular casting with radiating fingers, the whole encircling the central agitation- column.

When the charge is to be put into agitation the spider is lowered by a small hand-windlass until it rests on top of the set- tled charge. Air is then turned through the fingers, and at the same time the solution for the next treatment is run iuto the tank. The device rapidly bores its way to the bottom of the Pachuca, leaving a boiling, churning pulp above, ant1 clearing the way to the bottom opening of the central 10-in. agitating- column. As soon as this is opened and air has been admitted to the iuner-pipe the spider is raised from the tank and full agitation of the charge proceeds.

The second cyanide treatment of the charge is with solution drawn from the barren-solution storage-tauks or the wash-solu- tion storage, the cyanide strength being 1.5 lb. (0.075 per cent.) per ton of solotion. After 2 hr. of agitation the air is shut off and almost in~n~ediately decantatioll is started. This decanted solution is pumped directly on to a11 incoming fresh charge, being strengthened in cyanide as i t enters the tank, and becom- ing the first cyanide solution for the new charge.

This cycle in handling solution-barren to wash-solution, then to second cyauide treatment at 0.075 per cent. cyanide, then to first treatment at 0.1 per cent. cyanide, thence to pre- cipitation and back to barren-gives at each step just the con- '

ditions best suited for that step, and is very satisfactory in practical operation.

The settled pulp after the second decantation has a specific 8

gravity of 1.8, and is readily agitated by means of the spider, and then discharged iuto the pulp-storage tank by a Byron- Jackson 4-in. centrifugal pump, from which it is drawn to the Kelly filter-press. This thick pulp holds in suspension the sands which would settle through a lighter medium.

The storage-tank is conical bottomed, 15 ft. in diameter, and situated at such an elevation that a static pressure of 30 lb. per sq. in. is exerted at the filter-presses. The pulp in the tank is kept in constant circulatidn by an air-lift, drawing from the conical bottom and carrying the pulp down close under the

C 24 I

filter-presses and back up again over the top of the tank. The pulp as pumped from the Pachuca tanks enters the bottom of this same line, and the whole is thus kept in suspension and circulation past the presses, into which it is intermittently drawn for filter-treatment. Fig. 7 shows the Kelly filter-presses in- stalled to work under a gravity head as described.

Above the pulp-storage. tank is placed a similar tank for the storage of wush-water, fiom which a hydrostatic pressure of 25 Ib. per sq. in. is obtained at the presses. This solution is kept in circulation, using the same method as applied to the pulp. The higher gravity of the pulp in the lower tank results in a greater pressure a t the presses than that obtained from the wash solution, although the latter carries a higher head.

Filtering is done in two type 1 B Kelly presses. By opening valves in the circulation-lines directly under each press it is filled with either pulp or wash-solution as desired. The excess pulp or wash-solution from the press-cylinder is returned into its proper line by displacillg with compressed air admitted into the cjlinder. The amount of wash given depends upon the comminutiou of the concentrates, the usual pulp being washed with 0.5 tou of solutior~ per ton of concentrates. The cake formed during deca~ltatioll of the first-treatmeut solution, being very fine slime ancl more impervious to wash-solution than the regular pulp, is given 1 ton of wash per ton of concentrates.

When filling the press, the co~~ta ined air is allonrecl to escape . through an overhead pipe attached to the highest point of the press-cylinder. The change in souud of thc exhaust indicates to the pressman when the press is full. After drying the cake with compressed air until it contains not more than 1 0 per cent. of moisture, the press is openecl and the cakes shaken off with wooden paddles, and then sluiced with water to the tail- ings-dam.

A distributor below the press-launder sends the gold-solution to two gold-sumps and the wash-solution to the two wash-solu- tion storage-tanks. These four tanlts, as well as a clarifying- tank which is in the same group, are built of 3-in. redwood, 1 5 ft. in diameter by 16 ft. deep, and each holds 75 tons of solution.

The wash-solution is pumped to a Pachuca tank as needed, becoming a second-treatment solution. From the gold-tank

[ 25 I

the solution is drawn into the clarifying-tank, in which are suspended vertically six canvas filter-leaves, all connected to the suction of a triplex 7- by 9-in. Aldrich electric pump, used exclu- sively for pumping gold-solution through the precipitation-

A traveling-belt, driven by ratchet-gears and a pair of eccentrics connected to the pump-drive, feeds zinc-dust into a cone. Here the dust is enlulsified with a small stream of gold-solution tapped from the discharge-coluniu of the same pump, and is then drawn into the suction-line. A n automatic float in the cone prevents the introduction of air into the pump- suction.

The pump raises the solution with the zinc-dust to the upper part of the builcling and forces i t through two 36-in. trian- gular, 16-frame Mcrrill presses, Fig. 8. A n average of 145 tons of solution is precipitated daily, with a consumption of -$ Ib. of zinc-dustlper ton of solution, equivalent to 0.86 lb. of zinc-dust per ton of concentrates. The average strength of solution before precipitation is 1.25 lb. (0.0625 per cent.) of cyanide; 1 Ib. (0.05 per cent.) of lime, and $9.50. (9.2 dwt.) gold. The barren or precipitated solutions are kept a t 10 cents (2.3 grains), or less, gold per ton, and are used for mash- solution or returned to the Pachuca tanks, as desired.

Th'e Merrill presses are opened when filled or when the pressure exceeds 25 lb. per sq. in. Forcing the solution

. through at higher pressures caused a mechanical loss of pre- cipitate through the canvas. The precipitate is dropped from the press-frames into steel pans and lowered 1)y an electric ele- vator to the floor below, and thence conveyed by trucks through a concrete passage illto the refining-rooin.

On account of the work required to look after and collect the amalgam, as well as the greater dauger of amalgam-loss from the pipe-lines, launders, etc., the plates nrere removed after the first three months' run, and the whole procluct is now being cyanided directly without amalgamation.

I n order to handle the larger amount of solution made neces- sary when grinding in cyanicle solution, two 1,800-ton steel tanks have been erected, one above ancl one below the plant. All the precipitated or barren solutioi~ flows by gravity from

I 26 I

CYANIDE-PLANT AT THE TREADWELL MINES, ALASKA. 209 a-

the precipitation-presses to the loiver tank. This solution, having an average value of $0.08 in gold, 1.14 Ib. of cyanide, and 1.70 1L. of lime per tou, is pnmpecl to the second of these tanks, which is situated 25 ft. above the mill-bins, and acts as the mill-reservoir.

Thus a t no time is there any cyanide solution run to waste, the solutioii clischarged as nioisture in the tailings, plils that absorbed or evaporated in the mill, compensating for that re- ceived as n l o i s t u ~ in the concentrates delivered to the bins.

All the solution used in grincling ant1 classif~ing is drawn directly from the mill-reservoir. The overflow of fine pulp from the back of the Dorr classitier flo\vs at once to the Callow tanks in the lower builcling, the spigot-product of which empties into one of the 12 Pachuctl tanks for treatment.

The s1,ecific gravity of the pulp as i t enters the Pachucas is 1.5, or a ratio of 1 of collcentrates to 1.18 of solution. The charge is agitated for 8 hr., the necessary c ~ a n i d e ai~cl lime being aclded to bring the cyanide-content of the solr~tion to 1.5 Ib. (0.075 per cent.) and the lime-content to 2 Ib. (0.1 per cent.) per ton. After agitation and settlement, the clear solu- tion is decauted to the gold-tank through the clarifying-press described later, and a fresh charge of barren solution, the same as that used in the grinding, is drawn from the mill- reservoir, brought to the same strength as the previous treat- ment, and the charge agitated for 4 hr. This is then settled and the solution is decanted. Both solutions decanted from the agitators, together with the overflow from the call om^ set- tling-tanks previously mentioned, are drawn by gravity through the clarifying-press before eml~tying into the gold-tanlis.

The settled pulp in the bottom of the Pachuca tanks, having a specific gravity of t~i-o, is then agitated by llleans of the spider and primped to the pulp-storage tank, from which i t is drawn to the Kelly presses for filter-treatment.

This n~ethod of operation, depending upon the one barren solutioll for all purposes, keeps the gold-couteut of the solution to the lowest possiLle value, which, althoagh contrary to the usual practice, is the ol?jcct sought in this mill.

The solution overflowing from thc Callow settling-tanlts (containing gold, $10; cyanide, 1 Ib.; and lime, 2 lb. per ton) florvs 11y gravity tllrouqli ii s l w ~ i i ~ l cli~rifjing-press built in the

[ 27 ]

210 CYANIDE-PLANT AT THE TREAD\I'ELIJ MINES, ALASKA.

Treadivell shops, the same as receives the decalltecl solution. This press is of the ordinary plate-and-frame type, yet with a series of ports or channels so arranged as to allow of discharg- ing or sluicing-out a cake without the necessity of opening the press. This sluicing-out press consists of 20 square frames, each 3 in. thick, with the corresponding plates 1 in. thick. The upper and two side channel{ extending through the press have small holes opening into the frame side of the leaf. The upper small channel allows the introductio~l of compressed air behind the leaves. The lorvcr triangular channel connects with a 6-in. sluicing-out pipe. The press, with connections, is shown in Fig. 9, with one of the plates standing to thc left.

To discharge a cttlte, water is introduced a t the baclc end of the press through the large triangular opening on the bottoni, ancl flows through the underside to the discharge end, ~vhcre i t empties into the launder leading to the tailings-dam. Wi th this passage-way clear, compressed air is introducecl through the port-holes on the plate side of the leaf. The plate corru- gations being depressecl 0.5 in. leaves a concave surface, in which the cake forms. The air now being introduced behind the leaves by a series of separate knocks or bumps causes the cakes to drop off into the sluicing-out channel, where they are carried away by the stream of water.

Fo r the final washing of the leaves, water is introduced through the three upper channels, and, passing through the tapered holes, is sprayecl on the two filter-cloths, ~vhich bag together by reason of the compressed air introclucecl froni the plate side.

The method of feeding the zinc-clust has been changed some- what from that originally installed. The reasons t'or these changes were to create a more even feed of zinc, to do away with the air previously usecl in the emulsion-conc, and not only to break up any lumps, but to brighten the zinc and grind i t even finer. To clo this, the drive from the zinc-belt was taken

. from the Aldrich pump to a smdl couuter-shaft, which was, in turn, belted. to a worm-gear for the drive of thc zinc-belt, the belt discharging its zinc directly into a small tube-mill 6 ft. long, made from 10-in. pipe, the cast-iron caps of which were turned to run in rollers. This tube is filled with rods of cast zinc 2 in. in diameter. These rods not only grind the zinc to

[ 28

CYANIDE-PLANT AT TIIE TREADWELL MINES, ALASKA. 211

a more uniform product, but may themselves aid precipitation to a slight extent.

Considerable annoyance is occasioned by the clogging of the cloths in the Merrill gold-presses and by the accumu- lation of precipitate in the entire line from the zinc-feeder and pump to the presses. Filter-cloths of several kinds- heavy duck at 31 cents per yard, various grades of drilling a t from 9 to 15 cents, and muslin sheeting at 7 cents-have been tried. The lightest and cheapest rnuslin is now in use, with results no worse than obtained with the more expensive grades.

From the moment of contact of the zinc-dnst with the gold- solution trouble is caused by the slimes or precipitate incrust- ing everything touched. The interior of the pipes graclually becomes smaller in area, even though the solution is driveu through a t a constantly increasing velocity. After three months' use a 6-in. pipe of 28 sq. in. area was so fillcd with caked precipitate that only a triangular opening of 4 sq. in. remained. From 80 ft. of this pipe, $25,898 was recovered.

Being desirous of operating the Merrill presses more or less i~~termi t ten t ly without the necessity of each time closing the cocks to retain the solution, which if allowed to clrain not only oxidizes the zinc, but causes the precipitate when the pressure is renloved to settle in a mass at the bottom of the press-frames, consequently not allowing the greatest anlount of solution to pass through the unoxidizecl zinc, the discharge-coclts were re- moved from the plates, and -open pipes discharging into a launder on top of the presses were substituted, as sho~vn in Fig. 8.

The result of the several changes is a !nore uniformly low tail solution, with the consumption of less zinc, while the gold- value of the precipitate hw been raised from $15 to $25 per pound; hence a correspondin'g lo\vering of refining-charges.

V. THE REPINERY.

The refinery acljoining the mill is 30 by 76 ft. in area; con- structed of reinforced concrete with steel-truss roof covered with corrugated iron, shown in Fig. 10. The precipitate enter- ing the refinery is crushed through 0.5-in. screen, made up into lots of from 1,000 to 1,200 lb., ~veighed, sampled, and charged into one of two red\;roocl tanks, 8 ft. in diameter and 9 ft. deep,

[ 29 I

212 CYANIDE-PLANT AT THE TREADWELL MINES, ALASKA.

conical bottomed and lined with sheet lead. The tanlcs are built on the plan of a Pachuca tank, with a central colunln of wood fitted with lead pipes carrying steam and conlpressed air for heating and agitating the solutions.

I n these tanks the precipitate is treated with acid to dissolve out the zinc, lime, etc. About 1 lb. of 66" sulphuric acid is reqnired per pound of precip~tate, and is added in the follow- ing niai~ner : About 2 tons of water is introduced illto the tank, steanl turned on, and the water brought t o the boiling-point. A i r is turned on in the central air-lift, and the acid-valve opened. The acid flo\vs in by gravity, while the precipitate is shoveled in a t the rate of 2 lb. of precipitate to each pound of acid. TVhen all the precipitate and from 50 to 60 per cent. of the acid hare beer1 aclded, the acid-valve is closed allel the charge agitated until the acid is entirely aeutralizecl, which generally occurs within 30 mill. The tanlc is t11cn filled with water, ant1 the charge allonred to settle for about 2 hr., after which the clear solution is siphoned off into a filter-tanlr. The latter is 8.ft. in diameter and 4 ft. deep, having a false bottom of 1-in. strips, placecl 1 2 in. from the bottom of the tank and 1.5 in. apart. The strips arc covered with heavy iron screen, 1-in. mesh, on which is a bed of burlap 1 in. thick, one thick- ness of mill blanliet, one thickness of light canvas, anel :t bed 1 in. thick of quartz sand screened between 20- aud 80-mesh. The sallcl is divided into sections of 8 by 10 in. by a light wooden frame, covered by a single thickness of drilling, the lgtter forming the working-surface of the filter. The solutions filter freely through this mecliulll, the clear filtrate being run into one of three storage-tanks, where i t is held until a sample has been assayed, and the11 run to waste through a series of zinc-boxes. All solutio~ls and wash-waters from tlle refinery are clieposed of in this way.

After deca~lting the first acid the precipitate in the tallli is give11 tu70 washes of boiling water. Jus t enough water to en- able the charge to be agitateel is then added, aud the renlaiodcr of the acid rut1 in rapidly. This gives a solution cou ta in i~~g from 15 to 1 8 per cent. of acicl, agitation being continuecl until the acidity ceases to decrease, whicl~ usually leaves about 1 per ccnt. of free acid. The tank is the11 fillel! wit11 water, scttlecl

CYANIDE-PLANT AT THE TItEADTVELL MINES, ALASKA. 213

and decanted as before. This solution, containing from 50 to 75 lb. of free acid, is a t present run to waste.

The charge now reccives three or four washes of boiling water followed by washes a t about 30' C. temperature, uutil the wash-water gives no reactioii for sulphates with barium chlo- ride, vhich is generally after 1 5 ~ ~ a s h e s . After clecanting the last wash, the charge is sluiced through a valve in the bottom of the tank on to the filter, which has been thinly covered with silica sand to aid filtration, where the excess water is removed by nleans of a vacuum-pump. The slimes are removed to a large wrought-iron pan, placed upon a 4- by 8-ft. steam-table, i~iclosed by a sheet-iron hood. When nearly dry but still damp enough to prevent dusting, the slimes are rubbed through a 0.5-in. screen, weighed and sanlplecl, the weight of the acid-treated product being from 25 to 33 per cent. that of the original precipita.te. Each lot of precipitate is analyzed before and after the acid treatment, which enables a close calculation to be made of the amounts of fluxes required for the nlouthly ~ne l t i l~g .

The percentages of thc priucipal substances contai~led in an average analysis of the precipitate before and after ricid treatment are :

A11 . . . . . . . . Zn . . . . . . . . Pb . . . . . . . . Cn . . . . . . . . Cao . . . . . . . . F e . . . a . . . . S . . . . . . . . Itlsoluble, . . . . - . .

Before. Per Cent.

5.08 42.93

8.08 6.19

10.51 1.10 1.41 3.4s

After. Per Cent.

17.34 5.15

20.09 14.28 1.89 0.52 7.62

22.85

The high percentage of insoluble after treatment is due to the silica aclded to the lot just before filtering.

A t the end of the month the various lots of acid-treated pre- cipitate are united and the various fluxes addecl. The melting is clone in a speciallj-constructed oil-burning furnace (Fig. 11). F o r melting purposes the furnace is fired with a reducing flame. The crucible or hearth used for the melting is 4 by 3.5 ft., lined with either yagnesite brick or fire-clay, accorcling to the fluxes used. This hearth is placed on a steel car and run

r 31 I

uuder the furnace. Jack-screws, operated by hancl-wheels at the four corners of the car, allow of raising the hearth to form the furnace-bottom.

From the fire-box at one eud of ' the furnace the heat is drawn across the top of the charge, being reflected dowllrvard by the arch roof ancl the doivn-clraft to a dust-condensing chamber. The furnace is charged with precipitate hourly, the slag and lead-bullion being tapped off intermittently from oppo- site sides of the hearth. The month's clean-up, amounting to 1,450 lb. of acid-treatccl precipitate, or a total charge, iucludiag fluxes, of 2,600 Ib., is n~elt.ecl on this hearth iu 36 hr., ant1 re- q u i ~ s the attention of but one mall per shift.

A typical mixture of fluxes is :

Pounds. Acid-treatedpreci~)itatc, . . 100 Borax glass, . . . . 22 Sodium carbonate, . . . 26 Oldslag, . . . . . . . . . . SO Iron-tnrnings, . . . . . . . . . 15 Powdered graphite (old retorts), . 3

Such a charge will produce about 35 lb. of metal, from 10 t o 1 5 Ib. of matte, and from 160 to 180 lb. of slag.

From 150 to 300 lb. of high-grade coPper-matte are produced each month. This matte is roasted ancl alloived to accunlulate until there is sufficient to make up a charge, ~ v h e u i t is mixed with litharge, fluxed, and melted to prociuce lead-bullion, which is the worli-lead used for the removal of copper in cupellation.

After melting either precipitate or matte, the slag is tapped into conical pots holding about 200 'lb., with a tap 4 in. from the bottom, through which the molten core is drawn off. The shells, contailling most of the metallic values, are clumpecl, crushed, and used in fluxing a later charge. The cores, consti- tuting 75 per cent. of the total slag, are sampled, sackecl, and stored for shipment to the smeltery..

The cupellation is c l o ~ ~ e 011 a limestone test the same size as the melting-hearth, i t being run ulicler the furnace 011 the car previously described. Fo r cupellation the furnace is fired with ail oxidizing flame, while free air is introduced over the test by means of a connection. from a, compressed-air main through a neeclle-valve discharging into the open end of a

[ 32 I

4-in. pipe. This produces low-pressure air, which is iutro- duced into the furnace on the opposite side from which the molten litharge is tapped off.

The fine bullion resulting from this cupellation is drawn off and remelted in Faber du Faur tilting-fur~~nces into bars of 1,000 oz. each. The average fineness of the cupellcd gold is 880.

The retorts of the Faber du Faur furnaces are supported on two 1.5-in. iron pipes built into the furnace, through which cold water is kept circulating. These pipes have proved very satisfactory.

VI. COSTS.

In conclusion, the cyanide-plant has now been in operation one year, using the machines and equipment originally in- stalled, with the exceptiou of the abandoned am~lgamation- plates, the substitution of larger tube-mill motors, and the addi- '

tion of the " Treadwell " clarifying-press, with results summar- ized in Table VI.

For the last month, endil~g August 15, aud not included in cost-sheet, 2,010 tons were treated, a t a cost of $2.8764 per ton, ancl an estimated extraction of 97.025 per cent., as compared with the experimental estimates of $3.25 per ton and 96 per cent. extraction.

TABLE V1.- Costs of Qnnicle- T~eutment, Aluslcn- ilreuclzoell Gold Minin.9 Co.

-- 1 Costs Per Ton Treated. Costs Per 1'011 Treated. / I Costs Per Ton Treated. -

(a) During the first period the refinery had not hecn comp!cted and thc precipitate was shipped to a smelter, cost of w!~ich is not given. (a) Assays arc nrade a t thc central assay office of the combtned cornpallies a t Treadwell, and chargcd ngnlllst the eya~lrde plant at a proportionate rate per ahsay. (c ) Electricity, cornpressed air, and steam are porchased from central power planls, a t cost of generation. (a) Labor first period 20 men a t 03.75 average wa e. ( 6 ) ~ a b o r : second and third periods, 22 men a t *3.$64 average mage., (18 men in mill, 4men in refinery.)

Regrindingin cynnidesolution,followed by agi- tation i n cyanide solution. (No amalgama- tion.)

Regrinding in cyanidesolution, follolved by agi- tation i n cyanide solution. (NQ amalgama- tion.)

Treatment ............... Regrinding and amalgamation, followed by agi- tation in cyanide solutiotl.

Period covered ......... Web. 16 to May 15, 1911. (Plant idle Dec. 2 1 , Sept. 16 to Dec. 20,1910; tris~l rnn and first ad- justmetlts of the plmlt.

Average head value : Average tailing value : S54.6li4. $1.7589.

Estimar.ed extraction : Tons treated : 06.80 per cerit. 4906.1.

bIay 16 to July 15, 1911. 1910, to Feb. 13,1911.)

Average head value: Average tailing value : $46.0360. $I ,7990. $1.8892.

Estimated extraction : Tons treated : 96.17 per Cent. 4620.6. 9G.GO per cetlt. 4080.