Feb., I g I j
T H E J O U R N A L OF I N D U S T R I A L .4ND E W G I N E E H N G C H E M I S T R Y
119
LABORATORY AND PLANT THE USE OF HYDROMETALLURGICAL APPARATUS IN CHEMICAL ENGINEERING’ By
JOHN
V. N. DORR
This paper, although t h e title is somewhat general, will be devoted principally t o a description of t h e apparatus with which m y name is connected. It was prepared a t t h e suggestion of your president who felt t h a t i t would interest chemical engineers t o know of methods which have, proved of definite value in metallurgical work where large quantities of material are handled with low costs. G E X E R A L 31 E T A L L U R G I C A L T E K D E X C I E S
Lletallurgical engineering is really one department of chemical engineering in its broadest sense. Comparing metallurgical operations of to-day with t h e work of thirty years ago, we note t h a t nearly everything is done on a much larger scale. One reason for this is t h e general development of engineering knowledge with improved materials a n d t h e substitution of exact scientific methods for the old “rule of t h u m b ” ways. Another is t h e greatly increased demand for metallurgical products, t h e consumption of all t h e common metals having increased several times as fast as t h e growth of our population. A third a n d perhaps more important reason is t h a t most of t h e smaller rich ore deposits have been worked o u t so t h a t we have been forced t o a t t a c k t h e large low-grade deposits which twenty years ago were considered of no value. This tendency is well shown b y t h e treatment of t h e disseminated porphyry copper ores a n d t h e handling on a much larger scale of t h e lower grade ores a t B u t t e a n d in t h e Lake Superior district. The introduction of mechanical power in place of h a n d labor has, of course, been as important a factor in metallurgical progress as i n all other lines of endeavor. T h e introduction of continuous methods in t h e different steps of metallurgical processes in place of intermittent methods formerly used, has led t o great advances. Continuous work invariably nieans lower labor costs and a high load factor, t h u s assisting in lower power costs a n d interest charges. Where the change does not sacrifice control of t h e operation i t results usually in allowing a large output of uniform nature. As illustrations of this tendency towards continuous work in metallurgy may be noted t h e use of p a n conveyors a n d rotary cylinder feeders in delivering ore t o primary crushers, automatic weighing machines, a n d belt conveyors for t h e transport of dry material around a mill. The belt conveyor is also in use for stacking tailings from leaching t a n k s a n d has been used t o stack slime tails from a n Oliver filter a n d dewatered sand tails from a Dorr classifier. I n smelting plants we see t h e use of mechanical roasters a n d sintering machines. such as t h e Dwight-Lloyd, with continuous removal of slag by granulation. 1 Presented a t t h e 7th Annual Meeting of t h e American Institute of Chemical Engineers, Philadelphia, December 2 t o 5 , 1914.
The change in cyanide practice has been quite marked. Where leaching was practiced t h e original means of removing slime was t o collect t h e sand in t a n k s while overflowing t h e slime, and then shovel o r otherwise transfer t h e sand so collected t o other vats for leaching Continuous separation, whether b y cones or mechanical classifiers, has done away with this, a n d t h e sand usually passes directly to leaching tanks without coming t o rest. I n slime treatment, whethcr of classified slime or the whole mill feed reground, Dorr continuous thickeners have made continuous settling feasible and continuous agitation has replaced t h e charge system for t h e dissolution of precious metals, with a very definite saving in power, attention and first cost of the installation required. The Oliver continuous filter has come into extensive use for t h a t step in t h e process. I t is interesting t o note t h a t t h e decantation process, t h e first method used for t h e recovery of dissolved gold from slimes, which was abandoned everywhere except in South Africa on t h e introduction of t h e modern t y p e of filter, when made continuous by t h e use of t h e Dorr thickener, is coming into extensive use again and displacing in some instances intermittent types of filters. Zinc dust precipitation, when first introduced as a n intermittent process, could make n o headway against continuous precipitation with zinc shavings, b u t since being made a continuous process by Merrill has come into extensive use in modern mills. I K T E R C H A N G E O F A P P A R A T U S IiY I K D U S T R I E S
One characteristic of technological work in t h e last few years t h a t was not i n evidence in t h e centuries preceding, is t h e exchange of information among workers in different fields. Although t h e total knowledge in t h e world has increased so enormously, and this has been called very properly a n age of specialization, t h e specialist is expected t o have a general knowledge of progress outside his own field t o such a n extent t h a t he can profit by i t in advancing his own work. This has been made easier in metallurgy t h a n in most other industries, I should say, by t h e greater freedom t h a t exists among metallurgists in t h e exchange of data. A valid reason for this is t h a t t h e metallurgist competes in a much greater degree with natural conditions t h a n with other producers of t h e same product. The price of gold is essentially constant whether more or fewer mines are operating, and, while t h e prices of silver and t h e base metals vary with t h e supply a n d demand, t h e profits made depend so greatly on other factors, such as richness of t h e ore and costs of mining a n d milling, t h a t I doubt if a n y producer would hope t o get a n advantage over t h e other by secrecy. I n fact, there is a general understanding among t h e larger copper companies to-day for a n exchange of information which gives t h e m all a n advantage in the combat with Dame Nature. I n manufacturing, on t h e other hand, there is com-
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T H E J O U R N A L OF I N D Y S T R I A L A N D EYGI-VEERIXG C H E M I S T R Y
petition for t h e purchase of t h e raw material a n d t h e sale of t h e finished product so t h a t manufacturing costs a n d profits must come between the two, a n d any advance t h a t can be made in reducing t h e former seems of no real benefit unless kept from competitors. The cyanide process has been advanced b y the adoption Of methods and machinery f r o m Other industries a n d has made its own contribution in return. Fine grinding has been aided by the adopOf the tube from cement grinding, and when dry crushing and On Creek Ores were in vogue t h e inclined traveling screens used in clay working were of great assistance as were t h e dust collectors taken from Minneapolis flour mills. Our early paddle arm agitators came from chemical works a n d t h e air-lift agitator or “Pachuca” t a n k which succeeded them, was first used, I a m told, in a powder plant in California. The plate and frame filter press so extensively used in ilustralian metallurgical practice was another adoption from chemical industries, and t h e rotary d r u m filter was used in Syracuse as a dewaterer of chemical precipitates before t h e cyanide process was known. As developed by Oliver with means for washing a n d blowing off t h e cake formed, i t has been returned by t h e cyanide operator to several industrial uses. Various forms of vacuum and pressure “cake” filters, such as the LIoore, Butters, Kelly and Sweetland, might be said t o have their prototype in t h e potters’ method of making pitchers which is done by pouring “slip” into a plaster of Paris mould and when a solid deposit has been formed by t h e absorption of moisture, pouring out t h e remainder of t h e liquid “slip” a n d drying the vessel thus formed in t h e mould. From cyaniding they have found their way extensively into sugar refining, alkali works a n d many other lines. The Merrill sluicing press is being installed in a large aluminum plant in t h e South. I n closing this general discussion I will say t h a t , while t h e basic principles for the mechanical treatment of analogous materials are t h e same, great caution should be used in proposing the substitution of a n y method or apparatus successful in some other field for t h a t now used. It is safe t o assume t h a t t h e latter is in use as a result of much study a n d gradual development, and its replacement should be preceded by a most careful detailed study of all t h e conditions of operation a n d the purposes t o be accomplished. Suggestions as t o such substitutions may be advanced tentatively, b u t a n actual demonstration is worth a lot of theorizing. I n introducing new methods we usually find troubles come where they are not expected, a n d apparent difficulties do not prove real. T H E DORR A P P A R A T U S
The apparatus t o which I wish t o call your attention has been developed in the last ten years in the course of operating cyanide mills in the West. Each machine performs a certain useful function satisfactorily and economically, and I feel t h a t you will agree with me t h a t where similar work is t o be done they are worthy of consideration. I do not mean, of course, t h a t a machine adapted for handling a sandy pulp is neces-
V O ~7. , NO.
P
sarily the best type t o handle an extremely light chemical precipitate, but I feel t h a t , where the operations of washing, settling or agitation are required, they may be of service.
As a generalization, I believe I a m safe in saying t h a t machinery successful in operations where costs per ton are studied t o a fraction of cent, have passibilities where materials are handled on which costs per poupzd are watched. I t is true on the other hand t h a t t h e economies of continuous work decrease with a reduction in tonnage, so t h a t each case has to be studied by itself. In discussing the apparatus I have of necessity quoted freely from m y paper on the Same read a t t h e Salt Lake hleeting of &4merican Institute of nIining Engineers i n August, 1 9 r q . CLASSIFIER
This
devised in r 9 0 q t o replace hydraulic f o r the purpose of making a separation of sand ~ ~ D~~~ d & b~ y ~ i l ~~ ~ ~~ a n d slime at the mill of ~ at T ~ s o u t~h ~ ~ ~~ ~h~, kmethod~ of cyaniding ~ ~ . t h e gold ore in use at that mill w,as crushing t o pass
FIG. I-THE DORRCLASSIFIER (PATENTED) MODEL“ C ”
a 20 mesh screen, separating the sand from the slime, leaching t h e sand in vats by percolation and treating t h e slime after agitation and thickening by t h e Moore filter process. The machine proved such a n improvement over the cones in use a t our plant a n d elsewhere t h a t i t was rapidly adopted and has come i t o wide use. I t has been improved from time t o time t o meet new conditions and as now manufactured is shown in Fig. I . which illustrates t h e standard duplex machine. It consists of a settling box, in the form of an inclined trough with the upper end open, in which are placed mechanically operated rakes or scrapers for t h e purpose of removing the quick-settling material from t h e open end, t h e liquid a n d slime overflowing a t t h e closed end. Each rake is carried by two hangers: one a t the sand-discharge end suspended from a n arm attachment t o a rocker a r m or lever, which terminates in a roller: t h e other depended from a bell crank connected b y a rod t o the same rocker. The roller is kept pressed against a cam on t h e crank shaft by the weight of t h e rakes. The rakes are lifted and lowered a t opposite ends of the stroke by the action of t h e cams
Feb., I g I j
T H E JOCR,VAL O F I N D U S T R I A L A N D E N G I N E E R I N G C H E M I S T R Y
transmitted through t h e rocker arms a n d bell cranks, a n d t h e horizontal motion is obtained directly from t h e crank. T h e bell cranks at t h e slime end are carried by a second larger bell crank held in position b y a chain attached t o a spool on a worm gear at t h e head end of t h e classifier. By this means t h e rakes can be raised I O inches a t t h e lower end a n d operated in t h a t position or a n y intermediate one. This allows t h e classifier to be started readily when nearly filled with sand after
I21
This latter feature will probably be of more interest from t h e chemical engineer's standpoint as enabling him t o discharge finely divided sandy material a n d give i t a complete wash t o free i t from a n y dissolved material which adheres t o it. CAPACITY-The capacity of t h e Dorr classifier depends upon t h e dilution of t h e feed a n d t h e point of separation which i t is desired t o make. The rakes of t h e duplex machine, operating at normal speed, are able t o discharge a t least 1 5 0 tons of solids in 24 hours.
TABLEI-OPERATING DATA O N DORR CLASSIFIERS FEED SAXD
--
Tons per 24 hrs. 145(a) 151(a) 1$4(b) /3(c) 125(a) 85(c) 264(d,b)
SLIME
,---
Dilution -100 -200 -200 +200 -200 -200 +lo0 +ZOO -200 ~IILL Portland, Colorado . . . . , . . , , . , . , . . . . . . . . 5 to 1 38.3 13.4 47.1 75.3 16.2 7.4 0.9 5.8 94.7 3.5 to 1 45.0 19.0 36.0 68.0 23.0 9.0 11.0 89.0 Golden Cycle, Colorado.. . , . . , . . , . . . . . , . . 4 to 1 42 7 25.1 32.5 43.6 40. I 16.8 O:i 3.8 96.8 Cia Benef. de Pachuca, Mexico . . . . . . . , . , , 65.0 11.1 23.9 6to1 70.0 18.3 11.7 0.6 8.5 90.9 Tonopah Extension, Nevada Amparo Mining Co., Mexico 7 . 5 to 1 52.4 18.5 29.1 71.4 27.4 1.2 .., 6.5 93.5 Tonopah-Liberty, N e v a d a . . . . . . . , , . . . . . . . i to 1 30.5 41.9 26.3 i5.i 24.0 0.1 2.5 14.3 81.4 Goldfield Consolidated, S e v a d a . _ . . . . . , , . 3 to 1 40.0 12.0 48.0 74.0 19.0 7.0 2.6 19.5 79.1 Tonopah-Belmont, Nevada. . . . . . . . . . . . . . . iO(6) ... 65.6 2.7 28.9 ... 0.4 6.6 93.0 Alaska-Treadwell, Alaska(ej . . . . , . . . . , . . 89(c) ... 28.t 41.5 9.8 48.7 4i.5 9.8 .., 2.0(e) 98.0 Nipissing, C a n a d a . . . . , . . . . . . . , . . . . . . . , , .. , . . 18.7 3.6 16.8 85.8 f.2 6.9 ... 0.5 99,5(f) (a) S e w ore. ( b ) Includes tube-mill return. Under normal conditions, with tube-mill in closed circuit with a Dorr classifier, 35 t o 50 per cent of original feed returned. (cj New ore; tube-mill product returned, b u t amount not stated. (dl 168 tons come from the stamos. the rest beine returned from the tube mill. ( e j Classifiers used in connection 'with the regrind7ng of concentrates: rakes operated 24 strokes per minute: 100 per cent of concentrates pass 200 mesh; the 2 per cent is silica from tube-mill pebbles. (f) Of this 200 mesh product 15 t o 20 per cent. is sand: crushing is done in two sets of tube mills: t h e sand from the first classifiers being reground in closed circuit with t u b e mills and other classifiers, the combined slime overflows giving the screen test shown.
....
~~
a n unexpected shutdown, a n d t h e regulation of t h e depth of t h e settling box when in operation t o vary t h e products being made. T h e pulp is fed across t h e settling box as shown a n d a uniform flow t o t h e lip a t t h e end is maintained while t h e sand settles t o t h e b o t t o m a n d is advanced b y t h e rakes until it emerges from t h e liquid a n d is discharged with from 20 t o 30 per cent moisture. T h e agitation near t h e bottom of t h e t a n k , caused b y t h e reciprocating motion of t h e rakes, assists in keeping t h e slime in suspension, b u t is not normally sufficient t o cause fine sand t o overflow. R E G U L A T I O K O F PRODUCTS-The machine is intended of course t o make only two products, a n d t h e point of separation can be varied b y t h e following means:
Table I, showing results a t different plants, will give t h e best idea of what may be expected. C O S T O F OPERATION-Experience has shown t h a t repair costs on these machines are extremely small. I know of one classifier which operated nearly four years with repair costs of less t h a n $ j . o o , and half a mill per t o n seems a safe figure. While t h e power taken %ill vary of course with t h e load, i t appears t h a t ' / 4 H. P. is ample in most cases. With normal feed t h e attendance required is practically nothing, being limited only t o an occasional oiling. All parts in reciprocal motion are removed from contact with t h e liquid so t h a t there is no opportunity for wear starting a t a n y point. nsEs-Besides being used t o separate a leachable
~~
-9
S I D E ELEVATION FIG. 2-ARRANGEMENT
OF
CLASSIFIERS
AND
ELEVATION OF FEED E N D
TUBEh51LI.S OPERATING
I--The use of t h e baffle shown, which allows a reduction of t h e cross section of flow, so as t o overflow more fine sand. 2--Raising t h e rakes a n d t h u s obtaining t h e effect of operating t h e m in a shallower t a n k . 3-Increasing t h e speed of t h e rakes until t h e agitation keeps fine sand in suspension. 4-The attachment of perfqrated spray pipes t o t h e rakes a t a point where t h e y remove t h e sand from t h e liquid, allowing a rewashing of t h e sand a n d removal of a n y slime t h a t m a y be carried down with it.
IN
CLOSED ClRCCIT
I N THE
TONAPAH-BELYONT XILI,
sand product from slime, Dorr classifiers are in general used to-day in connection with fine grinding. Fig. z shows t h e arrangement of classifiers a n d tube mills operating in closed circuit in t h e TonopahBelmont Uill. I t is considered much more economical in power a n d equipment v h e n a product of perhaps I j o mesh or finer is desired, t o pass a large tonnage continually through a t u b e mill. return the discharge t o a Dorr classifier and overflow t h e part which is ground fine enough, returning t h e balance t o t h e same machines for further grinding together with t h e sand
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T H E JOUR.V..IL
OF I N D C S T R I A L 4 N D ENGINEERING C H E M I S T R Y
coming from t h e new feed. This is considered much more efficient t h a n t o a t t e m p t t o reduce t h e product t o t h e size required in one passage through t h e t u b e mill. It seems more t h a n probable t h a t it will prove equally advantageous in connection with industrial pulverization of quartz, flint, barytes, cement material a n d other mineral products t h a t require fine grinding. Some work has already been done on these lines, especially in connection with pigment grinding, b u t I have no d a t a t h a t are a t present available. Dorr classifiers are now in successful use as dewaterers of concentrates, both magnetic iron concentrates a n d sulfides, a n d at one plant are discharging t h e former with only 1 2 per cent moisture.
FIG.
~ - T A ~ L I NDISCHARGE G FROM
TRE
LASTMACHINE OF
A SERIES OF
ACID-PROOFC L A S S I F I E R S U S E D F O R L E A C H I N G C O P P E R O R E . BY THE BUTTE-DULCTH MININGCo.
LEACHING-The use of a series of classifiers for t h e purpose of leaching a n d washing was first suggested b y Rlr. Utley Wedge and some experiments were made along t h a t line several years ago b u t not carried t o a conclusion. Last year Captain Wolvin, of t h e ButteDuluth Company, a t Butte, who was leaching oxidized copper ores in vats there. brought up t h e same idea a n d we furnished him with five classifiers for trial, all parts subject t o contact with t h e acid being made of hard wood. These machines proved very successful, increasing t h e extraction over t h e former leaching in vats a t least 20 per cent, a n d we have since then furnished this Company with more machines for t h e
Vol. 7 , No.
2
same purpose. Fig. 3 shows t h e discharge of tailing from t h e last machine of a series. I have been unable t o get reliable figures on.these operations as they have TABLE 11-DATA O N C L A S S I F I E R S U S E D F O R LEACHING 6 Classifiers in series; five 15 in. X 41/r ip. and one 30 in. X 41/n in. Ore crushed through 0.12 in., opening in inclined impact screens, giving a product about -10 mesh Value of heads
H. P. (Motor 80 per cent efficient) T o t a l . . . .
.. . , , . . ..
4.6
been changing details from time t o time, b u t Table 11 shows d a t a obtained last summer from t h e ButteDuluth Company, when t h e first installation consisting of six small classifiers was in use, a n d will give a n idea of t h e operations a t t h a t time. T h e average daily capacity for t h e month in which t h e above figures were obtained was about 150 tons. T h e limit t o t h e capacity was t h e lack of electrolytic cell room. Spent electrolyte after strengthening was added t o t h e first two classifiers a n d water into S o . 6. M o s t of t h e overflow from t h e latter went t o scrap iron precipitation a n d a n amount equal t o t h e acid liquor brought out from No. 5 with t h e ore was carried through Nos. 5 a n d 4 t o No. 3, t h u s effecting a counter-current washing. The overflows from Nos. I , 2 a n d 3, after clarifying, were sent t o electrolytic precipitation. The ore received about 40 minutes' contact while passing through t h e whole series. TRIPLE-WASHIKG CLASSIFIER-Fig. 4 Shows a machine which we have designed t o wash a sandy material absolutely free from a n y trace of slime or to wash t h e same kind of material entirely free from a n y liquid in which i t may be suspended. It will be noted t h a t there are a number of opportunities t o get rid of t h e liquid or slime. As t h e sand is raked u p above the water level in each box i t is sprayed with a n y solution, or water, a n d as i t is washed into t h e following box it is diluted with t h e same liquid, SO t h a t in all i t gets a very thorough treatment. POSSIBLE U S E S O F T H E cLAssIFIERs--Several of m y classifiers are now in operation a t one of t h e midwestern chemical works, used, I believe, for draining 'a granular chemical precipitate a n d washing i t . Its use in t h e preparation of a high-grade kaolin is now being considered in place of t h e settling boxes which require a great deal of labor for removing t h e sand from them, a n d its use for dewate'ring granular material from any liquid will be apparent. C 0 iYTI iX U 0 US T H I C K E N E R
This apparatus was developed in 1906, when, in remodeling a mill in t h e Black Hills, I desired t o thicken t h e slime pulp from classification, continuously, in large units a n d avoid t h e use of t h e large cones which accumulated solid slime on t h e sides a n d gave much trouble. T h e trial machine was operated in a t a n k 3 j Et. in diameter b y 1 2 ft. deep at a speed of 1/20 R. P. M. a n d gave excellent results, continuing in operation until t h e mill burned five years later.
Feb., 191j
T H E J O U R N A L OF I N D U S T R I A L A N D ENGINEERING C H E M I S T R Y
DESCRIPTIOT-The Dorr continuous thickener, Fig. j, consists of a slow-moving mechanism placed in a suitable t a n k , b y means of which t h e operation of settling may be made continuous through t h e removal of t h e settled material t o a point of discharge a n d t h e prevention of i t s accumulation as a solid in t h e t a n k . As usually furnished, i t consists of a central vertical shaft with radial arms equipped with scrapers t o bring
resistance unless a comparatively high speed is maintained, while colloidal slime will give a slightly thicker underflow, at a very slow speed. It will be found when handling sandy material t h a t if t h e sand accumulates so t h a t i t is being moved around t h e t a n k by t h e channel arms as well as being advanced toward t h e center b y t h e plows the resistance increases rapidly a n d t h e speed should be increased.
.8 s: 2
3
v
~~
.orom
FIG.4 - T R I P L E - W
t h e thickened material t o a discharge opening a t t h e center b y t h e slow rotation of t h e mechanism. T h e thick material m a y be discharged b y gravity a t this point into a launder, or piped t o t h e side of t h e t a n k a n d raised b y air lift or p u m p t o t h e level of t h e overflow or higher. T h e machine is arranged for raising t h e shaft so t h a t t h e arms will not be embedded i n t h e thick material if t h e power should be s h u t off for a n y length of time. The shaft can be lowered again gradually while running. Shaft a n d gear bracket are supported by a bridge over t h e t a n k or suspended from t h e roof trusses. At t h e Liberty Bell Mill, thickeners have been installed driven from below t h e t a n k through a mercury bearing, a n d have proved efficient although t h e y cannot be raised. T h e thin pulp is delivered near t h e center of t h e t a n k in a suitable well with a float t o cause minim u m disturbance, a n d t h e overflow is taken off by a peripheral launder. The thickened pulp can be accumulated a n d withdrawn a t intervals or a continuous discharge maintained as desired. Nozzle discharge is in use in some concentrating mills where a comparatively thin pulp is desired, a n d also in one case where a product of only 30 per cent moist u r e is being obtained. M a n y are using diaphragm pumps for this purpose. T h e y have t h e advantage of ready regulation a n d require little attention. Having a positive displacement they tend t o regulate automatically t h e amount of solids withdrawn, for, when t h e pulp becomes thicker, more solids are pumped with each stroke. A pet cock to admit air into t h e suction has proved most satisfactory as a means of regulation. SPEED-The thickener has been operated a t speeds ranging from I revolution in 2 minutes t o I in 40 minutes. A quick-settling sandy material will offer great
ASHING
~-
CLASSIFIER
P O W E R varies with t h e size of the t a n k a n d t h e nature of the feed. Of course t h e motor input is much larger t h a n t h e actual power consumed, owing t o t h e low load factor commonly used, as i t is essential t o have power enough t o meet a n unusual strain. Spring measurements have shown approximately 1 / 2 0 H. P. being transmitted t o t h e worm shaft of a 44-ft. thickener handling a classified slime. It would not be advisable, however, t o instal less t h a n a I H. P.
FIG. 5 - T H E
D O R R COXTINCOUS
THICKESER (PATENTED)
motor on a single machine, b u t 1 / 2 H. P. each can be allowed when several are driven from one lineshaft. REPAIRS-Normal operation of t h e thickener causes no wear except on t h e worm, a n d many machines are running to-day t h a t have not cost a cent for several years. On t h e other hand, if started after a shutdown without raising, a strain of a n y amount may be given so t h a t t h e “weakest point” would have to yield.
T H E J O U R i V A L O F I N D l j - S T R I A L A N D EIVGINEERILVG C H E M I S T R Y TABLE111-OPERATING
D A T AO N DORR THICKEKERS
Sq. f t . settling area per 7
T o n of solid: thickened per 24 hrs. ,,, ,,,. , 4.5 ,. , ,,., 15.0
MILL San Rafael, Mexico. , . . . . . . . , . Liberty Bell, Colorado. . . , . . . . , .
. .. . .
Mogul, South D a k o t a . .
.
. . , .. . . . . . . .. .. . .
,
Batopilas, Mexico, , , . . , , , . , . , , , , , . , . , , , , , Zambona, Mexico. , , . . . , , . . . . , . . , , , . , , , , , Dominion, Ontario. , , . . . , . . . , , . . , , , , , , , , , Porcupine-Crown, Ontario El Palmarito, Mexico..
,
0.6 t o 0.9 3.1 5.4
4.25
.., .. . . . . .. . . . . . . .
Amparo, Jalisco, Mexico. , . . . . . . .
,, ,
Veta Colorado, Parral, M e x . , , , , . , , ,
..
-
Gallon overflowed per minute
REMARKS Tube-mill product, 75 per cent -200 mesh, discharge 45.5 per cent solids. 12.6 Tube-mill product, much light argillaceous slime. Discharge 33 per cent solids: + l o o , 17 per cent; +ZOO, 13 per cent; -200, 70 per cent. Feed 9: 1. Solution fed a t capacity; solids not. Large area per gallon overflowed per minute due to density of underflow a n d nature of the slime. . . . Tube-mill product; ore silicious: +60. 0.6 per cent; + l o o . 7.8 per cent; , 1 2 0 0 , 26 per cent; -200, 6 5 . 6 per cent. Discharge 56 to 59 per cent solids. Continuous decantation. . . . 40 mesh product; 90 per cent passing 100 mesh. ... Tube-mill product. Discharge 40 per cent solids. . . . Tube-mill product, 88 per cent -200 mesh, ore diabase. Discharge 40 per cent solids. Feed 6: 1. . . . Tube-mill product. 7 5 per cent -200 mesh. Discharge 65 per cent solids. Quartz ore. Continuous decantation. With 5.1 sq. ft. settling area Der ton settles to 71 to 8 3 Der cent solids. ... Tube-mill product: pure quirtzite, 9 7 p e r - c e n t -200 mesh. Feed 7 . I . Discharge 65 t o 70 per cent solids. Continuous decantation. 1.4 Tube-mill product, silicious: 93.5 per cent -200 mesh. Feed 24.5: 1. Discharge 23.5 per cent solids: used t o feed vanners. mesh. Feed 3I/a(a) Tube-mill product, rather argillaceous: 71 per cent -200 11: 1. Discharge 33 per cent solids for agitator. Have settled to 65 per cent solids. Very clayey slime with classified sand. Screen test: +40, 1.48 per cent; + 6 0 . 7.27 Der cent: 1100. 14.81 Der cent: 1 2 0 0 . 11.63 Der cent: -200. 65.81 per cent. 26.0 Settling from cold water, slightly alkaline. Feed 8 : 1. Discharge 50 per cent solids, 1.429 sp. gr. .. Settling from cyanide solution. Feed, 2 . 5 : 1. Discharge 40 per cent solids, 1.316 sp. gr. 8.11 Considerable argillaceous slime. Feed 10.4 per cent solids. Discharge 25.3 per cent solids. 2.48 Thickening ahead of vanner concentration. Feed 2.8 per cent solids. Discharge 10 6 per cent solids. Overflow 0.4 per cent solids, extremely fine, which does not interfere with using water again. 1.25 “Each 17-ft. thickener supplies wash water for 20 Wilfley tables a n d occasionally for wash on vanners. One thickener has a greater capacity than twelve 8-ft. cones.” Area of 17-ft. tank is 226 sq. f t . ; of the twelve 8-ft. cones, 525 sq. f t . 1.80 Dewatering slime from lead-zinc concentration mill. Feed 100: 1. Discharze 55 Der cent solids. 5.95 D e w a t k g -&ne from concentrator Forty 4-deck thickeners, each 28 f t . in diameter by 3 f t . 3 in. deep, handle a b o u t 26,000,000 gal. of pulp per d a y which contains approximately 2 per cent solids. A clear overflow obtained, the underflow containing about 15 per cent solids, which is fed to buddles
....
~
3.92
4.5
, , , ,
4.9
,, ,, ,,
5.0
Val. 7 , NO. 9
~~
~~
Smuggler-Union, Telluride, Colo
., , .
... 30.0 10.0
A large copper company, Arizona. . . . . ,
..,. P a . . . ... . , . , .
11.6
Pennsylvania Steel, Lebanon,
14.2
Nevada Consolidated, Ely, Nev..
. ... . .
,,.,. ,, , .. . . . . . . . . , . . . , . .
Broken Hill, Proprietory, Australia. Anaconda Copper, M o n t .
(a) N o t u p t o capacity of overflow.
.
T h e Overload Alarm, Fig. 6. is arranged t o indicate t h e resistance offered b y t h e mechanism as shown b y t h e t h r u s t on t h e worm shaft a n d t o ring a n alarm when t h e load becomes excessive. A solenoid or other means can be used t o correct automatically t h e conditions causing t h e excessive load, by reducing t h e feed or increasing t h e underflow. T h e alarm has proved very valuable. especially on quick-settling pulp, when it is desired t o obtain t h e thickest discharge. LABoR-The attendance required varies n-ith t h e regularity of conditions maintained a n d is usually confined t o lubrication once a shift, so t h a t t h e care of t h e thickeners is included in t h e duties of some m a n employed principally on other work. CAPACITY-The capacity of m y t h i c k e m r on any pulp has been found t o be primarily a function of area, although t h e depth of t h e t a n k has a n influence depending on t h e dilution of t h e feed a n d t h e dilution of t h e underflow desired. With a given area a n d depth a n d a very dilute feed a n d underflow t h e capacity depends on t h e a m o u n t of liquid t h a t can be clarified, i. e., additional solids, b u t no additional liquid can be added t o a t a n k already fed t o capacity, without overflowing slime. On t h e other hand, n-ith a feed of perhaps 8 of liquid t o I of solids a n d a thick discharge of 2 t o I or less, i t will be found usually t h a t additional liquid can be added t o a thickener operating a t capacity without overloading it, b u t any addition of solids will cause slime t o overflow, If a plant requires more settling capacity, raising t h e temperature of t h e solutions m a y prove a n economical way t o a d d I O t o 20 per cent. The capacity of a n y filter can also be increased b y this means.
Table I11 presents d a t a given me from time t o time. It shows t h e settling area in use per t o n in different mills which in m a n y cases were not feeding their thickeners a t capacity. T h e figures show t h a t when pulp is carried in cyanide solution a provision of j t o 6 sq. f t . per t o n for a silicious t u b e mill product is ample a n d from 7 t o I j sq. ft. for a clayey material or classified slime product. When very dilute products are handled t h e area required is determined usually by t h e gallons per minute t o be overflowed. DESSITY OF UNDERFLOW-This depends on t h e nature of t h e pulp t o be settled a n d t h e size of t h e particles. An argillaceous pulp, such as t h a t at t h e Liberty Bell in Colorado, although containing a large percentage of reground silicious material, will not settle thicker t h a n 60 per cent moisture, while a finely ground quartz will give as low as 2 7 per cent moisture. Alt t h e Porcupine-Crown plant, handling a quartz produc: of 7 j per cent minus zoo mesh, t h e average final pulp discharged contained 3 0 per cent moisture with t h e feed a t t h e r a t e of I t o n of solids per d a y for each 4.7 sq. ft. of t a n k area. We have no reason t o think t h a t t h e use of t h e Dorr thickener Tvill increase t h e a m o u n t of water t h a t can be clarified in a given settling area at t h e beginning of t h e flow into t h e t a n k , b u t t h e removal of t h e pulp a s rapidly as i t settles certainly avoids t h e decrease in t h e settling r a t e which would follow when a t a n k is filling up in intermittent settling. At one plant where reground ore pulp was thickened from I O t? I t o approxiniately z t o I , actual experience indicated t h a t t h e change from intermittent t o continuous settling enabled three t a n k s equipped with Dorr
F e b . , 191j
T H E .TOC-RS21L O F ILVDC'STRIAL A S D E N G I N E E R I N G C H E M I S T R Y
125
clear water overfloJying from i t joined t h e overflow of t h e shallow thickener t a n k . T h e latter has i t s own feed a n d discharge as well. The drawings shown in Fig. 8 give t h e result of tests made there t o show comparative capacities of t a n k s with a n d without t h e tray. Applying t h e idea t o thickeners already installed, Fig. 9 shows t h e installation of a t r a y in a standard deep thickener It gives in effect two t a n k s each with its own feed a n d discharge a n d a combined overflow, a n d in practice has resulted in enabling us t o a d d 70 t o I O O per cent to t h e capacity of plants already in operation. I t has been found unnecessary t o make t h i s t r a y strong enough t o support these t a n k s filled with liquid if t h e . space below i t is e m p t y as b y means of a n automatic float we have arranged t o make connections between t h e upper t a n k and t h e lower in case t h e level of the liquid drops in either. I t will be evident, I think, t h a t these t r a y s will afford means of obtaining a large a m o u n t of settling capacity with comparatively small floor space, a n d we are working now on their further development. LARGE THICKEKERS-FOr a long time FIG.6-OVERLOAD ALARMA X D RESISTANCE INDICATOR O N THICKENER WITH A R R A N G E - t h e largest thickeners in use were in t a n k s MENT FOR RAISIAG SHAFTWHILE OPERATING 50 f t . in diameter by 2 0 f t . deep at t h e b o t t o m of t h e t a n k . It not infrequently happens Nevada Consolidated Company. Within t h e last t h a t t h e flow of slime t o t h e settling t a n k will vary dur- year, LIr David Cole, t h e constructing engineer ing t h e d a y a n d t h e settling qualities of t h e slime of t h e Arizona Copper Company, designed a n d in vary as u-ell so t h a t if t h e feed is light a large quantity stalled a Dorr thickener in a t a n k 130 ft. in diameter: of clear liquid accumulates on t h e t o p of t h e thickener which, when more slime comes, flows an-ay together with t h e freshly clarified water a n d partly thickened slime takes its place, t h u s giving a great reserve capacity. I n 1913,liowever, I h a d occasion t o go into t h e question of securing large settling capacity with small floor space a n d developed w h a t I have called a t r a y thickener. There are two types of this The first, which mould seem t o he t h e simplest, has been I installed b y t h e Anaconda Copper Company a n d in It consists ot t n o or more shallow ' t h e Coeur d'Alenes t a n k s , each carrying thickening apparatt's attached t o a common shaft a n d acting as a n independent unit with its own feed. clear water overflow and thick slime discharge. Tests made b y t h e Anaconda Copper 1 Company indicated t h a t with their material, t a n k s 28 in. X 3 in. would handle approximately 9 0 per cent of t h e material t h a t t a n k s 2 8 in. X 9 in would handle, so t h a t this gives t h e m a great economy in plant conI struction. Fig. 7 shows this t y p e a n d represents one I of forty t r a y thickeners installed in their settling plant which is handling 26.000,ooo gallons of water a F I G . 7-THE DORRT R A YTHICKENZR ( P A T E N T E D ) : SUPERPOSED TYPE d a y a n d separating from i t 2 , 6 0 0 tons of extremely colloidal slimes. Fig. I O shows t h e machine in operation. It has proved T h e second t y p e of t r a y thickener. Tvhich I have entirely successful a n d indicates t h e possibility of furcalled the submerged t y p c , was tested first a t nishing very large units which will cost less per s q . f t . . t h e Homestake 11ill a t Lead, South Dakota, a n d of settling area t h a n t h e smaller ones. The t a n k in consisted as tried there essentially of a shallon- this case is merely a ring of concrete erected on a slag thickener a n d t a n k entirely submerged in a cone- d u m p , t h e settled slime forming its own bottom. b o t t o m settling t a n k . T h e space below t h e small T a n k s built along these lines with dirt bottom a n d thickener h a d its own feed a n d discharge a n d t h e walls of concrete or wood will aid materially in rethickeners t o do t h e work of five used intermittently. T R A Y THICKEXER-The original thickener v-as installed in a t a n k 1 2 it. deep a n d we adhered t o a p proximately t h a t d e p t h , varying from 6 t o 16 or 2 0 ft., for a number of years. One reason for this was t h a t i t early showed itself t o be a n advantage t o have ample storage capacity for thickened slime a t t h e
T H E JOLTRNAL O F I N D U S T R I A L A N D E N G I N E E R I N G C H E M I S T R Y
126
ducing costs of installation. It is probably a s large as we would care t o build, b u t we anticipate building u p t o I O O ft. using t h e bridge a n d central shaft, which we now use on our s t a n d a r d machine. This large thickener has handled over 1 2 0 0 tons of solids per d a y a n d M r . Cole estimates t h a t it would t a k e care of 2 0 0 0 tons of material 40 mesh a n d finer. I t s overflow has been over ~ ~ , o o o , o ogallons o per d a y a n d its limit of
FIG.S-TEsT
OF
T R A YTHICXENER AT HOMESTAKE MILL, LEAD,S. D.
capacity has not yet been reached. T h e net saving in water b y t h e use of this machine with the mill a t capacity is $120 per d a y . mas-The thickener was originally introduced in cyaniding for thickening slime or reground pulp previous t o agitation a n d filtration a n d came into general use for t h a t purpose. Within t h e last few years it has proved a means for recovering t h e dissolved values as well b y continuous counter-current decantation, which will be spoken of later. I t s use in thickening ahead of slime concentration has become very general, nearly all t h e large copper a n d lead companies having a d o p t e d it within t h e last few years. T h e development of flotation has also found a new use for it a t most of t h e plants t h a t have been installed recently in this c o u n t r y a n d in Australia.
C
.
FIG. THE D O R RTRAYTHICKENER (PATENTED): SUBMERGED TYPE
Ix D us T RI A L u s E s- Th e thicken e r pres en t s great opportunities for the recovery a t a n extremely small expense of all kinds of finely divided residues t h a t go t o waste or are recovered intermittently from settling ponds. T h e expense, including interest on equipment, a m o u n t s in one case t o one mill a n d in another t w o mills per thousand gallons clarified. I n both cases t h e solids removed are colloidal ore slimes. One plant
Vol. 7 , No.
2
recovering a finely divided organic material from settling ponds substituted t h e Dorr thickeners for t h e m and, I a m advised, save t h e labor of 1 2 men per day. Its use for removing heavy silt in water purification seems possible with t h e large units now feasible. Its use for sewage sedimentation a n d for t h e prevention of stream pollution looks promising. A plant is now planned for t h e recovery of precipitated ferric h y d r a t e from mine water. I n all industrial work its use t o furnish a much thicker product t o a filter press will mean great increase in t h e capacity of t h e press with diminished costs. C O N T I N U O U S COUNTER-CURRENT D E C A N T A T I O N Continuous counter-current decantation consists of so operating a series of settling vessels t h a t t h e solids t o be treated continuously pass through t h e m in succession, being diluted after each thickening b y solution from t h e overflows passing in t h e opposite direction. T h e objects t o be attained are t h e dissolution under most favorable conditions of a portion of the solids, t h e separation of a solution from a finely divided solid with minimum dilution, or both combined. T h e last result is t h a t usually obtained in metallurgical work as t h e separation of most of t h e dissolved material from the solids always occurs a n d even when apparently adequate means for dissolution are provided ahead of t h e system t h e favorable conditions afforded usually give a n additional extraction HISTORY-The principles of continuous countercurrent decantation are very old a n d have been applied in m a n y ways in chemical technology as well as in metallurgy. T h e first cyanide plant using this method in the United States was erected b y John Randall in t h e Black Hills in 1902,using large cones for settlers, a n d was operated for a year. T h e invention of t h e Dorr thickener in 1906 gave a n opportunity t o avoid the troubles inherent in cones, a n d in 1910 t h e first modern plant was started. Since t h a t t i m e a large number of plants have been installed, several of t h e m replacing vacuum filters, so t h a t it may be regarded to-day as one of t h e well-established methods of cyaniding. Several years ago Dr. R . Gahl successfully operated a n experimental plant with cone settlers leaching copper ore slimes with sulfuric acid, treating seven or eight tons daily, t o which he referred in a paper read before t h e American Electrochemical Society, April, 1914. A test plant of 7 j tons capacity is now starting, using thickeners. T h e following description of a typical flow sheet of a continuous counter-current decantation cyanide plant will undoubtedly explain t h e process further a n d also show t h e method we have used t o determine t h e results t h a t m a y be expected from a n y conditions assumed. The square marked MILL, Fig. 1 1 , represents the grinding machinery, and we assume the ore is crushed in cyanide solution which brings the fine pulp continuously into thickener V from whence the solids pass through the agitators and thickeners W, X, Y and 2 until discharged from 2 as tailings. The following conditions are assumed: I-The thick pulp undedows are thoroughly mixed with the clear overflows before entering the next tank so that the dissolved gold is uniformly distributed through the li,quid.
Feb., 191j
T H E JOL-RLVAL O F I N D U S T R I A L A N D E LVGIATE E RI ;IrG C H E M I S T R Y
11-roo tons of ore are milled per day. 111--$8.00 is dissolved per ton; 7 5 per cent in the mill, the balance in the agitators. IV-The pulp from all thickeners-is discharged a t a ratio of I : I , or 50 per cent moisture. V-300 tons of solution are precipitated to a value of $0.02 per ton. To determine the value of the solution from any thickener, if T‘, W, X, Y and Z equal the value in dollars per ton of the solution leaving each thickener, we get the following equations, as the same amount of gold and solution will enter and leave each tank : 400 1%’ ($6.00 X 100). = 3 0 0 V IOO V (1) That is, the gold in 1%’ plus the additional gold assumed to be dissolved in the mill equals the overflow and underflow of V. IOO 5’ ( $ 2 . 0 0 X 100) 400 X = 400 1.A’ IOO W (2) The ( $ 2 . 0 0 X 100) represents additional gold dissolved in the agitators. 100W 400 Y = 400 X IOO X ( 31 IOO X IOO Z 11300 X $o.oz) = 400 Y IOO Y (4) IOO I’ 100 water (no value) = IOO Z IOO Z (5) In (4) the $ 0 . 0 2 represents the value of the precipitated solution returned to 1’.
+
+
+
+
+
+
+ +
+
+
+
+
127
of solution, instead of 100,making it contain 37 per cent moisture, the amount precipitated can be reduced to 240 tons and the loss will be only $0.76 per ton of ore, although the solution precipitated will assay $3.32 I . I should explain that the reason the precipitated solution is added at Y, instead of Z, where it would obviously be more effective in increasing the extraction of gold, is that we have also a mechanical loss of cyanide to contend with. The IOO tons of water which can be added at Z to replace the liquid discharged with the tails reduces the strength of the solution lost to onehalf when added to 100 tons of cyanide solution, whereas if added to 400 tons of solution it would reduce i t to only fourfifths. A further saving in cyanide can be made by adding the barren solution a t X, but this usually means adding an additional thickener on account of excessive gold losses. It will be noted from these figures t h a t t h e loss i n gold c a n b e regulated a t will b y increasing t h e solution precipitated o r t h e n u m b e r of thickeners i n t h e series, a n d also t h a t pulp settling t o 3-40 per cent moisture c a n b e treated m u c h more efficiently t h a n pulp settling t o 5-60 per cent. If pulp cannot be settled t o 5 0 per cent or thicker i t will usually be profitable t o use a filter merely for dewatering at t h e e n d of t h e series.
IS T A N K130 FEETIN DIAMETER A T ARIZONACOPPERC O . ’ s MILL FIG. ]&THE DORR THICKEKGR
Equation ( j ) , representing a simple dilution with water, means that Z will have one-half the value of Y . Solving these equations we get the values, V = 2.656 W = 1.156 X = 0.286 Y = 0.076 Z = 0.038 which may be checked as follows : T h e gold precipitated (52.656 -$0.02) X 3 0 0 . . . . . . . . . . . . . . $796.80 Lost in tailings ($0.038 :< 100). . . . . . . . . . . . . . . . . . . . . . . . . . . 3.80 TOTAL . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . $800.60 Total assumed dissolved (100 X $8.00). . . . . . . . . . . . . . . . . . . 800.00 Discrepancy due t o decimals.
............................
-_
.60
From the above we deduce: Assay value of solution precipitated. . . . . . . . . . . . . . . . . . ., . . Dissolved loss per ton solution.. . . . . . . . . . . . . . . . . . . . . . . ... Dissolved loss per ton o r e . . . . . . . . . . . . . . . . . . . . . . . . . . . . ... Percentage dissolved gold recovered, . . . . . . . . . . . . . . . . . . , . .
S Z . 656 0.038 0.038 99.6
Similar calculations have shown that if one thickener is omitted from the flow sheet and no other conditions are changed the dissolved loss is increased to $0.133, Using the same four thickeners and precipitating 400 tons, instead of 300, the loss is cut to $0.08, while if the pulp can be discharged with 60 tons
D E T A I L S O F PLANT-Fig. I T a n d w h a t h a s already been said a b o u t Dorr thickeners will give a clear idea of t h e construction of a continuous counter-current decantation p l a n t . T h e t a n k s i n t h e series a r e either arranged o n a level. i n which case b o t h t h e overflow a n d underflow m u s t b e elevated before being t r a n s ferred i n t o t h e next t a n k or i n a series of steps so that t h e overflow goes b y gravity from t a n k t o t a n k and only t h e smaller q u a n t i t y of underflow is raised each time. E a c h m e t h o d h a s i t s advantages. T R A N S F E R A L OF PULP is preferably done by a d i a p h r a g m o r other p u m p of fixed displacement as thereby a certain a m o u n t of a u t o m a t i c regulation is obtained. S T O R A G E TAKKs-The m o r e a flow sheet a n d t h e actual conditions of mill operations a r e studied t h e less additional storage capacity is felt necessary.
128
T H E J O U R . N A L OF I N D U S T R I A L A N D E N G I N E E R I N G C H E M I S T R Y
RESULTS-I realize t h a t details of t h e technical results obtained in cyaniding will not be of great interest t o chemical engineers so will say only t h a t when working on a h a r d , quick-settling ore ground t o pass a n 80 mesh assayers’ screen or finer, which can be settled t o a thick product containing 3 j t o j o per cent moisture, it has proved a most efficient means for t h e
Vol. 7 , A-0.
2
plants operating indicates t h a t on quick-settling ore from which $ j . o o t o $ 2 0 . 0 0 is recovered t h e mechanical loss in gold varies between 4 a n d I O cents per t o n of ore. GEKERAL COXSIDERATIOKS A N D RECOVERY-From t h e nature of t h e process i t is a p p a r e n t t h a t i t s use for dissolution is antagonistic t o its use for recovery of t h e dissolved
DISSOLUTION
I
FIG. 1 E EXAMPLE
OF
CONTINUOUS COUNTER-CURRENT DECANTATION FOR CYANIDE PLANT Figures Refer t o Solution Tonnages
recovery of dissolved gold a n d silver from t h e pulp. Compared with t h e usual filtration methods it has shown lower operating costs in spite of a higher mechanical loss in cyanide, a n d a higher recovery d u e t o a lower loss of dissolved values a n d often t o a n additional extraction being made t h a t could not be made commercially b y direct agitation. T h e following quotation from a n article on continuous counter-current decantation in t h e Engineering and Mining Journal of October 31, 1914,sums u p t h e conclusions reached a t t h e Gold R o a d Mill where continuous counter-current decantation was installed t o replace a v a c u u m p l a n t : “The operators state that the principal disadvantage they found with the vacuum-filtration system upon their ores was that it was almost impossible to wash satisfactorily the cakes on the leaves. Cakes of uniform resistance could not be formed. They would crack badly and large sections would fall from them, making satisfactory washing impossible. Besides, the expense of labor, repairs and power is considerably more than is necessary in following the counter-current decantation system. They find practically no difference between the two systems in the amount of cyanide and lime consumed; perhaps a slightly greater quan\ I+O I tity, amounting to about 0.1 lb. lO’ALU+-&ck’cOrr of cyanide, is being consumed FIG. I2.-EXAMPLE OF C O N T I N U O U S in the decantation system. “An advantage found with counter-current decantation is that the dissolution of gold is so perfect, due to the longer contact between the solutions and the pulp. The difference, considering all things, between leaf filtration and counter-current decantation is a t least 60 cents per ton of ore treated in favor of the latter.” T h i s figure is higher t h a n could be expected under usual conditions, b u t we have also h a d definite statements of savings from t h e other plants t h a t have changed. Information received from a n u m b e r of
material, a n d although some have proposed t h e alternation of agitators a n d thickeners I have always advocated, where a high per cent of recovery in a concentrated form was required, as in cyaniding, t h a t all dissolution possible should be made ahead of t h e system. I n other cases, as in t h a t described b y Dr. Gahl, where it was essential t o make t h e dissolution in stages so as t o e n d with a nearly neutralized liquid a n d have t h e copper pulp when nearly finished meet fresh acid, alternation is of course necessary, b u t i t calls for one series on dissolution a n d a n o t h e r on washing. S C A L E OF OPERATIoNs-It will be seen t h a t a n y continuous method of t r e a t m e n t of materials has greater relative a d v a n t a g e s with an increase in size of operations a n d a continuous counter-current decantation plant with 50-ft. t a n k s will require no more labor, and, in fact, less t h a n one with io-ft. t a n k s , although it will have 2 5 times t h e capacity. As t h e actual labor cost for operating a series of thickeners is less t h a n one man a shift, it will
COCNTER-CURR€NT
DECANTATION WITH
WASHING
LIQUIDN O T REUSED
be seen t h a t if a n y washing or leaching operation is requiring t h e full tirne of one or men this m e t h o d can be considered if other conditions are favorable. O T H E R APPLICATIONS-Fig. 1 2 is intended t o show a copper problem which I think will correspond t o m a n y problems t h a t m a y occur in chemical industries, ~ l i z . , t h e complete washing of finely suspended solids a n d t h e obtaining of t h e dissolved material in as concent r a t e d a form as possible from which i t is t o be re-
Feb., 1915
T H E J O U R N A L OF I N D l i S T R I A L A N D EA-GI.IrEERISG
NUMBER OF THICXENERS USED Tons of solution sent to precipitation.. ............................
3 140
C H E M I S 1 R 1.’
4
3
129
4
Lbs. per Per ton cent Copper in the solution for precipitation.. .......................... 27.5 1.37 0.13 Copper in the solution wasted with the residues.. . . . . . . . . . . . . . . . . . . . 2 . 7 1.48 Lbs. of dissolved copper lost per ton of slimes.. . . . . . . . . . . . . . . . . . . . . 96.3 Per cent of dissolved copper sent t o precipitation. . . . . . . . . . . . . . . . . .
covered b y evaporation or complete precipitation or b y a n y other m e t h o d , so t h a t none of t h e liquid h a s t o be returned t o t h e system as in cyaniding.
t r u e in Cyaniding until i t r a s demonstrated t h a t continuous agitation 11-ith a series of agitators was better.
EXAMPLE
This machine. Fig. 1 3 : T designed in 1 9 1 0 , b u t i t did n o t come i n t o actual use until 1 9 1 2 , when a b o u t a half dozen machines were p u t o u t for trial purposes on a n-orking scale a t several different plants. T h e results obtained were so good t h a t last year m a n y more were installed in this country a n d abroad. D u r ing 1914,although mining in general h a s lieen very quiet, t h e use of t h e m has increased greatly a n d from all installations made v e have had nothing b u t satisfactory reports. T h e illustration will give a good idea of t h e machine, which is very much like t h e Dorr thickener, t h e central shaft being replaced hy a pipe a t t h e t o p of which distributing launders are arranged t o distribute t h e pulp which is raised through t h e pipe b y means of air. ;1 worm a n d sheaves pro-
OF
CONTINUOUS
COUNTER-CURRENT
D€:CAKT.4TIOK
WITH WASHING LIQUID NOT REUSED
The following conditions are assumed in Fig. 12 : 1-100 tons of oxidized copper ore slimes treated per day. II--Slimes contain z per cent soluble copper or 40 lbs. of copper per ton. 111-Pulp is discharged from all thickeners with 41.2 per cent moisture. IV-The i d o w t o each thickener is thoroughly mixed before entering same. V--210 tons of wash water added t o Thickener “2.” The total overflow from Thickener “X,” 140 tons, sent t o precipitation and 7 0 tons sent to waste with the discharged residues from Thickener “2.” VI-Let X, Y and Z represent the pounds of copper per ton of solution discharged from the respective thickeners. If we equate the inflow and outflow of each thickener in terms of tonnage and copper content we have: 70X 140 X = 2 1 0 Y (100 X 40 lbs.) (1) 7 0 Y +21oY = 7 0 X + z 1 0 2 (2 1 70 2 210 2 = 7 0 Y 2 1 0 tons of water (3) Simplifying and solving we obtain: X = 27.5132 lbs. copper per ton of solution. Y = 8.4656 lbs. copper per ton of solution. 2 = 2.7164 lbs. copper per ton of solution. From the above the following results are deduced: I-Overflow to be precipitated, “X”, contains 1.37 per cent copper or 27.5 Ibs. per ton of solution. 2-Solution wasted with the residues, “Z,” contains 0.13 per cent copper or 2 . 7 Ibs. per ton of solution. 3-Copper sent t o precipitation = 96.3 per cent of that dissolved. 4-Copper lost per ton of slimes = 1.48 lbs. Varying the number of thickeners used and solution tonnage sent to precipitation and assuming conditions I to IV t o remain constant, we obtain the results given a t the top of this page. I t is apparent from this that the enriched solutions, which contain 96.3 per cent of the soluble copper, will carry 1.37 per cent copper per ton and that you get no weak liquors a t all for further handling. The additional tabulations above show the results of changing various factors. You can see that, precipitating 140 tons of solution, but adding a fourth thickener, increases the percentage recovered from 96.3 to 98.8 per cent with an enrichment of the solution from 1 . 3 7 t o 1.41 per cent. Precipitating 2 I O tons of solution, on the other hand, shows a solution t o be precipitated carrying 0.93 per cent copper with three thickeners and 0.94 per cent with four thickeners, while the percentage of recovery is 98.5 and 99.6 per cent. In other words, i t is easy t o see t h a t b y using t h e proper n u m b e r of thickeners you can m a k e practically a n y recovery desired a n d get a n y concentration you wish. This m e t h o d could be applied a s readily t o t h e manufacture of caustic from soda ash a n d lime, a n d I c a n s a y t h a t a p l a n t t o use i t t o some e x t e n t h a s been purchased a b r o a d , b u t i t s installation h a s been delayed b y t h e war. As t h e reaction between t h e soda a n d t h e lime requires a definite t i m e i t is possible t h a t it might be necessary t o d o i t in charges, b u t t h e s a m e was believed
+ +
A GIT -4T 0 R
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vide means for raising or lowering t h e hinged a r m s at will. I n cyanide metallurgy mechanical sweeps in flat-bottomed t a n k s were first used for agitating t h e pulp, b u t t h e g r e a t difficulty experienced in most cases with these, including breakages a n d t h e power required, made t h e use of a high cone t a n k with a central air lift seem most attractive. a n d n.hen i t was introduced i n t o this c o u n t r y a n d Yexico from Australia a very large percentage of t h e mills installed i t . A t t h i s t i m e agitation was done i n charges, b u t \Then continuous agitation became t h e established practice a fem- years ago i t was found t h a t there was a great tendency for t h e coarser material in t h e pulp being agitated t o build u p on t h e sides of t h e 60° cones used
130
T H E J O U R N A L OF I N D C S T R I A L A N D ENGINEERING C H E M I S T R Y
so t h a t eventually t h e major portion of t h e agitating area became filled u p with solids a n d t h e agitation took place only in a small cylinder in t h e center. It was also found necessary t o give a much more violent agitation t h a n t h e chemical treatment of t h e ore required in order t o keep t h e pulp in suspension as much as possible. T h e Dorr agitator, b y t h e use of mechanical arms with t h e thickener plows on, which prevents settling of solids a n d brings t h e material from t h e b o t t o m t o t h e central uplift pipe, allows t h e regulation of t h e intensity of t h e agitation given t o suit t h e chemical need of t h e ore a n d not t h e necessity for keeping eT7erything in suspension. EmICIExcT-The value of a n agitator depends on t h e cost of making t h e maximum economic extraction b y i t s use. This will be determined b y its cost of agitation per t o n per hour a n d t h e time required t o obtain dissolution. All t h e d a t a I ha\-e been able t o obtain indicates t h a t t h e cost of agitation per hour is extremely low. Extraction tests t h a t have been made indicate t h a t t h e r a t e of dissolution on both gold a n d silver ores in Dorr agitators, with a uniform although less violent agitation, is a t least as rapid as in a n y other agitators with which t h e y have been compared. T h e most favorable conditions for t h e dissolution of gold a n d silver in cyanide solution v a r y with each ore treated. T h e maintenance of a n excess of dissolved oxygen throughout t h e whole mass of pulp a n d t h e free movement of all particles of solids i n t h e liquid immediately adjacent seem t o be t h e only conditions t h a t can be generally specified. Definite work both in milling a n d t h e laboratory indicate t h a t m a n y ores will give a more rapid extraction a n d allow t h e use of a weaker solution when agitated as a dilute pulp of 3 t o I or 4 t o I , a n d also cause less chemical consumption of cyanide. T h e Dorr agitator with i t s combination of air a n d mechanical agitation gives a flexibility t h a t is apparent a n d it insures keeping all t h e solids in suspension all t h e time whether t h e pulp is subjected t o a gentle or a violent movement. This is essential especially in continuous agitation. S E L E C T I V E AGITATION-The question Of selective agitation was discussed by Mark R. Lamb in t h e Transactions of t h e A m e r i c a n Institute of -Mi?zi?zg Engirzeevs I 1 ( r g o g ) , 7 7 3 . As used here t h e expressiqn is taken t o mean continuous agitation under conditions which cause t h e coarser particles of t h e ore t o remain in t h e agitator longer t h a n t h e average time of t r e a t m e n t a n d t h u s give t h e m t h e longer exposure t h e y m a y require t o obtain t h e best extraction. This can be accomplished readily with t h e Dorr agitator b y agitating a t a dilution which allows t h e coarser material t o classify o u t a n d discharging t h e raised pulp near t h e center while t h e outflow is t a k e n from near t h e periphery. It will be seen t h a t if a segregation takes place a n d t h e agitator is fed a pulp carrying IO per cent plus I O O mesh, t h e discharge m a y be only j per cent plus I O O mesh a t first. With twice as much inflow a s outflow of Ioo-mesh material i t will concentrate in t h e agitator while gradually increasing in t h e outflow until a n equilibrium m a y be reached i n which t h e feed a n d dis-
1-01, 7 , S o .
2
charge will both be I O per cent a n d t h e average pulp i n t h e t a n k 20 per cent, so t h a t t h e exposure of coarse s a n d would be approximately double t h e average. While this is theoretical, enough work has been done t o indicate t h a t t h e idea can be p u t t o practical use. Lly agitators have been installed in t a n k s of 7-arious sizes, t h e largest being those in use a t t h e Desert 11ill a t Tonopah, S e v a d a , in t a n k s 36 f t . in diameter by 2 0 f t . deep. T h e Company estimate t h a t t h e total power rcquired t o operate one of these large machines including mechanical horse power a n d t h e air required for maintaining agitation, is not o r e r j H. P . , so you will see t h a t it is extremely small. A recent measurement in a 2 0 ft. by I 2 ft. agitator showed t h a t only I O cu. ft. of free air at I j lbs. pressure were needed. The application of Dorr agitators in t h e chemical industry has n o t been investigated t o a n y extent, b u t i t would seem t h a t with finely divided materials from which i t is desired t o dissolve one constituent t h e y should perform as useful a function as t h e y are now doing in metallurgy. A C I D- P R 0 0 F M A C HI NE R Y As all t h e submerged portions of classifiers, thickeners a n d agitators have no reciprocal motion, i t has not been found difficult t o manufacture t h e m so as t o resist ordinary acid liquors, a n d acid-proof machines of all three kinds are now in use. 30
CHURCH STREET, h-EW Y O R K
A SIMPLE FAT EXTRACTION TUBE By C A. BUTT Received December 22, 1914
T h e cost of fat extraction tubes has long been a source of much expense t o laboratories doing work requiring t h e use of Soxhlet’s apparatus, owing t o their fragileness a n d t h e necessity of importing t h e m . It occurred t o me t h a t in place of t h e rather elaborate tube now in general use, one constructed similar t o t h e ordinary filter t u b e , with t h e stem enlarged so t h a t t h e vapors from t h e solvent m a y pass upward, permitting a t t h e same time t h e downward flow of t h e con dens a t e, wou1d answer the purpose. Accordingly, I have h a d some of these t u b e s made a n d t h e y have proved in se.iera.1 tests, using cottonseed meal a n d extracting with l petrolic ether, t o be even more e efficient t h a n t h e usual form. T h e K R results of these tests will be found , on t h e following page. I t will be noticed t h a t t h e new t u b e shows a more rapid extraction t h a n t h e Smalley tube. T h e illustration shows t h e approxim a t e dimensions of t h e tubes we now have in use. T h e vapors from t h e solvent pass from flask “ A ” through t h e small p a r t of t u b e “B” a n d around thimble
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L-Ci I
“c,”
