T H E ACTION OF POTASSIUM n-AMYL XANTHATE ON CHALCOCITE A. M. GAUDIN AND REINHARDT SCHUHMANN, JR. Ore Dressing Laboratories, Montana School of Mines, Butte, Montana
Received July 66, 1996
In the flotation concentration of ores, water-soluble substances such as xanthates, generally termed collecting agents, are used to prepare the surface of the minerals to be floated so that attachment will take place between air bubbles and the mineral particles. Very small quantities of xanthates (of the order of 0.05 lb. per ton of ore) are found entirely adequate to impart to the mineral particles a surface a t which air will displace water to such an extent that efficient flotation is possible. At the present time the nature of the mechanism of this action is for the most part an unsettled matter. In previous work with potassium n-amyl xanthate and chalcocite, Dewey (1, 3) found that cuprous xanthate and several other organic copper compounds could be leached from the surface of chalcocite which had been treated with potassium n-amyl xanthate under various conditions. He explained the formation of the organic copper compounds other than cuprous xanthate by “decomposition” and “association” hypotheses. In long-time grinding tests of potassium n-amyl xanthate and chalcocite nothing could be leached from the mineral. No effort was made to find the cause of this phenomenon and its relation to other results. Dewey also made experiments which indicated that oxygen is necessary for the reaction between xanthate and chalcocite, and unnecessary if dixanthogen is substituted for the potassium xanthate. In Dewey’s work as a whole, the experimental variables, time of grind, time of xanthate treatment, and amount of xanthate, had values far above those encountered in flotation practice. Also the desirability of working out and applying a quantitative technique to the determination of the reaction products of amyl xanthate and chalcocite was strongly indicated. I n the present investigation the primary aim has been to establish a sound experimental basis for the explanation of the mechanism of the collecting action of xanthates on chalcocite. For the xanthate treatment of chalcocite an experimental technique has been used which allows reasonably close laboratory reproduction of actual flotation operation procedure and conditions, and a t the same time makes possible an accurate 257
258
A. M. GAUDIN AND REIKHARDT SCHUHX4NN, JR.
accounting of all the reaction products. The influence of several experimental variables on the reaction product relationships was found, and the results were correlated with the flotative properties of the mineral. A group of tests was conducted to ascertain whether cuprous xanthate is adsorbed from benzene solution by chalcocite. For comparative purposes a few of the experiments were repeated using the oxidized mineral malachite instead of chalcocite. EXPERIMENTAL
Materials Chalcocite (Cuss) was obtained from a coarse, relatively pure Kennecott jig concentrate by consecutive hand picking, crushing, table concentration, sizing (-28 +65 mesh), then finally chemical cleaning with 1:l hydrochloric acid, then concentrated aqua ammonia. This “cleaned chalcocite” was found to be quite pure, containing minute quantities of bornite (Cu4FeSs),chalcopyrite (CuFeSz), and somewhat more covellite (CuS). A small batch of “cleaned chalcocite” was melted in a crucible, crushed, and sized (-28 +65 mesh) to give a product suitable for use. This artificial chalcocite has been termed “furnace chalcocite.” Potassium n-amyl xanthate was prepared from n-amyl alcohol (Sharples Solvents Corporation), carbon disulfide, and potassium hydroxide. The crude preparation was purified by recrystallization from an acetone-ether mixture, followed by a thorough washing with ether. Prepared in this way the xanthate is very slightly yellowish in color, practically odorless, and very voluminous. Iodometric titration of an aqueous solution showed the purity to be 99.5 per cent. Cuprous n-amyl xanthate was prepared from potassium xanthate and cupric chloride in water or alcoholic solution according to the following reaction:
4KX
+ 2C~Clz
-+
CUZXZ + 4KC1 + Xz
(In this and other equations in this paper, X is used to denote the amyl xanthate radical, -SC(S)OC5H11.) It was found that the pure substance could be prepared by washing the yellow precipitate very carefully with alcohol and ether to remove the dixanthogen (Xz, also known as amyl thioformate disulfide) also formed by the reaction. The cuprous n-amyl xanthate prepared in this way appeared non-crystalline, even under the microscope, but it was found that very small yellow prismatic crystals could be crystallized from pyridine. General experimental procedure The procedure used may be summarized as follows: (1) A 200-g. charge of chalcocite was ground in a 500-g. capacity Abbe porcelain pebble mill
ACTION OF POTASSIUM %-AMYL XANTHATE ON CHALCOCITE
259
(4000 g. of pebbles) with 200 cc. of distilled water. (The empty space in mill was about 3 I., containing about 85 milliequivalents of oxygen a t 615 mm. and 2OOC.) (9) The pulp from ( I ) , after dilution to 400-450 cc., was agitated with an accurately weighed amount of potassium n-amyl xanthate in a closed 2.5-1. reagent bottle. (3) The treated pulp was filtered and the mineral washed with water on a Buchner funnel, diluting the filtrate to 500 cc. for analysis. (4) The mineral was leached with acetone, ether, and warm benzene, in the order named. (6) The aqueous filtrate and the leach products were analyzed. Analytical methods The aqueous filtrate from the treatment of the chalcocite with xanthate was analyzed iodometrically for reducing ions and excess xanthate by the procedure of Taylor and Knoll (11). In some of the experiments titrations with 0.02 N hydrochloric acid were made to determine the total hydroxide plus carbonate. The total potassium in solution in several experiments was determined as potassium sulfate (12). Sulfate was determined by precipitation as barium sulfate (9). As cuprous xanthate was found (as described later) to be the major leach product, an iodometric method was developed for its determination in benzene solution, utilizing the following reaction:
cuzxz
+
1 2 -+
Cud2
+ xz
It was found that an accurate determination can be made on a 25-cc. aliquot portion of a benzene leach solution containing cuprous xanthate by titration with a 0.005 N solution of iodine in benzene, using a special technique. As the reaction is too slow to be practicable a t ordinary temperatures, the titrations must be made near the boiling point of benzene, allowing 1-2 minute intervals with occasional shaking between 5-, 2-, or 1-cc. additions of iodine solution, the amount of iodine added depending upon the proximity to the end point as judged by experience. Titrating in this manner, the pink color of excess iodine becomes very noticeable with an excess of 0.5-1.0 cc. of 0.005 N iodine. This small excess may be quite accurately measured by color matching of the solution after removal of the cuprous iodide by filtration. Apparently the above reaction takes place in more than one step, as a dark brown solution is formed on first addition of the iodine, and no precipitate of cuprous iodide is formed until later stages in the titration. REACTION PRODUCTS FOUND WITH THE CHALCOCITE
Leachable products Dewey (1, 3), in his previous work on the reactions of potassium xanthate and chalcocite, was able to leach from the treated mineral a series of
260
A . M. GAUDIN AND REINHARDT SCHUHMANN, JR.
six substances having different solubility properties. In the present work, with experimental conditions maintained as closely as possible within the limits of practical flotation operation, only two substances were leached from chalcocite treated with xanthate, as described in the following: ( I ) cuprous n-amyl xanthate, a yellow substance insoluble in ether, slightly soluble in acetone, and fairly soluble in benzene, forming a yellow solution; and (2) a red substance, soluble in acetone, ether, and benzene, forming orange to red solutions depending on the concentration. This substance was designated as “ether-soluble” to distinguish it from cuprous xanthate. Attempts were made to synthesize the “ether-soluble” compound in several ways from xanthate and monothiocarbonate, but no substance with the same solubility properties was obtained. In all experiments it was found that the major portion of the leach products consisted of cuprous xanthate. From 100 to 1500 cc. of benzene were required for removal of the cuprous xanthate from the mineral, depending upon the amount present. As cuprous xanthate, even in an extremely weak solution, imparts a yellow color to benzene, the leaching was continued until the extract was colorless. The end point was quite definite, and little washing was required to finish the operation after the bulk of the cuprous xanthate had been removed. The acetone leach, containing a little cuprous xanthate and perhaps a large part of the “ether-soluble,” was evaporated (using a fan), and the “ether-soluble” and the cuprous xanthate in the residue separated by means of ether. The “ether-soluble” was combined with the main ether leach solution, evaporated, and the residue taken up with benzene for iodometric titration by the method used for the cuprous xanthate. The cuprous xanthate separated from the acetone extract was combined with the bulk of the cuprous xanthate for determination of the total cuprous xanthate.
F i l m at mineral surface unleachable by acetone, ether, and benzene Using the leaching agents, acetone, ether, and benzene, a considerable portion of the xanthate abstracted by the chalcocite from an aqueous potassium amyl xanthate solution remained with the mineral in an unleachable state. This unleachable film decomposed, producing a pleasant ester-like odor, when the treated mineral was allowed to stand for a few days in a loosely stoppered bottle. On agitation with water of the treated chalcocite which had developed the characteristic odor, an appreciable frothing was noted, indicating the presence of a soluble surface-active organic compound. It was found that by heating the xanthate-treated and leached chalcocite to about 220OC. in a distillation apparatus suitable for the distillation of very small quantities, a colorless, pleasant-smelling liquid could be
ACTION O F POTASSIUM
n-A.MYL
XANTHATE ON CHALCOCITE
261
distilled off. Potassium amyl xanthate and other xanthate derivatives gave the same distillation product when heated in the presence of finely ground, untreated chalcocite, which presumably contained a little moisture not removed by the drying after grinding. The distillate was not a sulfur compound. The refractive index and boiling point checked those of amyl alcohol quite closely. Attempts were made to remove the unleachable film with most of the common laboratory reactants and solvents, and a few rather uncommon ones. Except for pyridine, all the agents tried met with no success. The pyridine extract of xanthate-treated and leached (acetone, ether, and benzene) chalcocite was yellow. Evaporation of the yellow pyridine solution showed the substance removed to be cuprous xanthate. Thus, the unleachable film appears to be cuprous xanthate so intimately associated with the chalcocite that a chemically active agent is necessary to break it away. THE EFFECT O F QUANTITY O F XANTHATE
The effect of quantity of xanthate on products of treatment A series of experiments was made using the experimental procedure previously described to find the effect of quantity of xanthate added on the products of the action of potassium n-amyl xanthate on chalcocite. The grinding time and xanthate treatment time in this series were kept constant a t one hour and a t fifteen minutes, respectively. Results are given in table 1, and graphically represented in figures 1 and 2. In this group of experiments the “ether-soluble” leach product was quite small in quantity in comparison with the cuprous xanthate, having from 1 to 4 per cent of the iodine equivalent of the cuprous xanthate leached. Enough of the red “ether-soluble” substance for an analysis was not obtained. Since the amount was so small, it was included with the cuprous xanthate in the table of results. As a matter of fact, in most of the tests only the total amount was determined by titration of the combined leach products, as the very small quantity of “ether-soluble” material was not observed and differentiated from cuprous xanthate until some of the tests with larger quantities of xanthate had been conducted. Two experiments were made using “furnace chalcocite” in place of the “cleaned chalcocite.” Under like conditions the proportion of products was roughly the same as for the unmelted mineral. The little difference was probably due to the difference in grinding properties between the natural and artificial copper sulfides. These two experiments showed that porosity or some other property peculiar to the chemically cleaned mineral was not an important determinative factor in the results. In this series of tests it is seen that the xanthate added may be accounted for as excess xanthate in solu:ion, leachable compounds (practically all
262
A. M, GAUDIN AND REINHARDT SCHUHMANN, JR.
TABLE 1 T h e effect of quantity of xanthate added on the reaction products 200 g. of cleaned chalcocite, I-hour grind in 500-g. pebble mill with 4000 g. of pebbles, conditioned 15 minutes on mechanical rolls in 2.5-1. bottle with 0.5 1. of water a n d potassium n-amyl xanthate as indicated. .4mounts expressed as milliequivalents per 200 g. of chalcocite. The symbol X is used for the xanthate radical. KX ADDED
CUBS*
EXCESS
EX-
x IN
IlRACTED
6 0L U T ION
___ __ 0.25 0.007 0.50 0.058 1.10 0.46 1.61 0.81 1.80 2.67 1.65 2.66 2.86 4.21 4.21 2.80 4.64 6.31 6.00 8.55 6.52 10.64 7.02 12.90
0.01 0.01 0.02 0.02 0.02 0.02 0.09 0.95 2.42 4.19
x-
POTAL 8 S TR A C TE I
0.25 0.50 1.09 1.60 2.65 2.64 4.19 4.19 6.22 7.60 8.22 8.71
JNLEACAELE Oh CHALCOCITE’
x
-__
0.24 0.44 0.63 0.79 0.85 1.oo
2.12
1.33 1.39 1.58 1.60 1.70 1.69
5.50 6.35 7.13 8.00
0.20
2.34 6.31 8.34 10 50
0.30
* Obtained by difference.
t Including reducing ions, (&On)--. In most of the tests the reducing ions were negligible, increasing with xanthate added to a maximum of 0.08 milliequivalent for 12.90 milliequivalents of xanthate added.
hl/iequivalen fs Xon fhofe Added FIG.1. The effect of quantity of xanthate added on the reaction products
ACTION OF POTASSIUM ‘/+AMYL XANTHATE ON CHALCOCITE
263
cuprous xanthate), and unleachable xanthate, the proportions of these three being dependent on the quantity of xanthate added. The unleachable portion tends to approach a maximum with increasing amount of xanthate added. All the potassium of the collector remains in solution, and hydroxide, carbonate, and sulfate are thrown into solution to make a total equivalent to the xanthate taken out of solution, within the limits of experimental error. Under the conditions of these tests, reducing ions (S,O,)thrown into solution were practically negligible, in no experiment amounting to over 0.08 milliequivalent per 200 g. of chalcocite.
ao
to
40
6.0
80
loo
tzo
140
MiIliequ~va/enf3 Xan fhafe Added
FIG.2. The effect of quantity of xanthate added on the unleachable xanthate on the chalcocite
Flotation tests were made on leached and unleached treated chalcocite. These tests were made in a 50-g. celluloid cell with no further addition of reagent, either frother or collector. They showed that the removal of leachable products had no appreciable effect on the floatability of the xanthate-treated mineral; 95 to 100 per cent recovery could be obtained in all the tests but that involving the smallest amount of xanthate. In the tests involving larger amounts of xanthate the froth was very dry and the chalcocite seemed to float very peculiarly, more as a “dust” than as a mineralized froth. I n the tests in which the maximum unleachable quantity was closely approached, the mineral floated immediately upon addition to the machine as an apparently dry mass which could be literally blown out of the machine. Ciaudin and Malozemoff (4) observed this phenomenon of an extremely dry froth with both galena and chalcocite under certain conditions. This condition might correspond to an air-
264
A. 51. GAUDIN Ah-D REINHARDT SCHUHMANN, JR.
solution-mineral contact angle in the neighborhood of 90’) the result perhaps of a compleke filming on the mineral particles by the unleachable xanthate.
The efect of quantity of xanthate on Jlotation recovery Flotation tests on 40-g. samples of ground chalcocite were made in a 50-g. capacity celluloid “University of Utah” cell with the addition of terpineol, 0.05 lb. per ton, and varying amounts of potassium n-amyl xanthate. The chalcocite was ground in 200-g. batches according to the TABLE 2 The eflect of quantity of xanthate added on the j o t a t i o n recovery 40-g. samples floated in 50-g. celluloid cell with terpineol, 0.05 lb. per ton, for 6-8 minutes after 2 minutes conditioning with amount of potassium n-amyl xanthate as indicated. LBS. OF X A N T H A T E PER T O N OF CRALCOCITE
PER C E N T
15-minute grind
0.00 0.025 0.14 0.28 0.41
0 00 0.013 0.07 0.14 0.20
13 31 65 a3 85
1-hour grind 0 0 0 0 0 0 0 0 0
00 005 055 11 16 21 25 39 51
0.00 0 0025 0 027 0 053 0 08 0 10 0 12 0 20 0 26
7 10 31 41 54 66 70 75 85
procedure previously described, flotation data being obtained for two grinding times, fifteen minutes and one hour, respectively. Mohr pipettes were used to add a standard xanthate solution directly to the pulp in the cell. Two minutes after the xanthate collector addition the frother was added and the froth raked off for 6 to 8 minutes. Concentrates and tailings were weighed and the recoveries calculated. Results are given in table 2 and graphically represented in figure 3. From the data for the chalcocite ground for one hour it is seen that 0.26 milliequivalent of potassium n-amyl xanthate per 200 g. of chalcocite
ACTIO
# OF POTASSIUM %-AMYL XANTHATE
ON CHALCOCITE
265
is sufficient for substantial flotation. From table 1 it is seen that if this amount of xanthate is added to 200 g. of chalcocite ground for one hour, practically all of the xanthate abstracted by the mineral is unleachable. Aiso, under these conditions the amount of unleachable product on the mineral is only about one-seventh of the maximum possible unleachable amount for the quantity of mineral and time of grind in question, as indicated by the curve of figure 2. If this unleachable entity corresponds to a monomolecular film (as will be shown hereafter), it can thus be said that only a fraction of a monomolecular film is necessary for flotation of a particle. Comparison of the data for the two times of grind confirm this line of reasoning by showing that the quantity of xanthate required for a
000
005
O/O
01.5
020
025
0.30
Mifhequiva/enfs Xantha +e
Per 2009. Chdcocife
FIG.3. The effect of quantity of xanthate on the flotation recovery
given flotation recovery is dependent upon the fineness of grinding, and therefore, on the area of mineral surface. The froth dryness-contact angle analogy previously suggested gives weight to the idea of only a partial monomolecular film with a surface of relatively low contact angle being necessary for high flotation recovery. To use another terminology, the mineral particles do not have to be treated to the maximum degree of non-wettability but only to a certain fraction of that degree, the value of that fraction being dependent upon particle size, shape, and specific gravity. Mathematical calculations of the theoretical order of magnitude of the contact angle necessary for flotation made by Gaudin (2) led to the conclusion that particles are floatable with contact angles under 10'; whereas Wark and Cox (13) experimentally found the maximum contact angle of minerals in contact with amyl xanthate solution to be of the order of 90". TAI JOURNAL OF PEYSICAL CHEMISTRY, VOL. 40, NO. 2
266
A. Y. GAUDIN AND REINHARDT BCHUHMANN, JR. THE EFFECT OF TIME OF XANTHATE TREATMENT
Several experiments were made varying the time of treatment with potassium n-amyl xanthate. The time of grind was one hour in all of this group of experiments. Results are given in table 3. In the experiments involving long-time agitation with xanthate, appreciable amounts of the “ether-soluble” substance in addition to the cuprous xanthate were obtained by leaching. This red substance was apparently the same as the “ether-soluble” leach product observed ir, previous work with fifteen minutes treatment; in long-time agitation tests this substance was present in somewhat greater quantities. It Seems likely that the TABLE 3 T h e effect of t i m e 0.f xanthate treatment on the reaction products Tests using general experiniental procedure, grinding for 1 hour. Amounts expressed in milliequivalents per 200 g. of chalcocite. The symbol X is used for the xanthate radical __.
KX ADDED
_-2 62 2 66 2 66 2 68 10.70 10 64 10 63
___
TIME O F TREATYBNT
____ 1 min. 6 min. 15 min. 24 hrs. 1 niin. 15 min. 20 hrs.
SOLbTION
____1 58 0 02
FOTAL XBBTRACTEI
2.60 2.64 2.64 2.66 6.70 8.22 10.48
12 EQUIV:,LENT O F ETHER~OLUBLE”
UNLEACHLBLE ON BALCOCITE’
x
1.02 1 .06 1 .oo 0.28
t 1.57 1 70
0.30
t
2.16 2.12 2.12 1.63 5.88 7.13 9.23
* Obtained by difference.
t The formula of the “ether-soluble” product is unknown, hence there is no basis for a calculation of the unleachable quantity when an appreciable quantity of “ether-soluble” is found.
“ether-soluble” material is a reaction product, possibly containing the monothiocarbonate radical, formed slowly in long-time treatment from cuprous xanthate, free hydroxide, and chalcocite, as suggested by Dewey in his “decomposition” and “association” hypotheses. Oxygen may also enter the reaction. Evidently the reactions to produce the unleachable film and the cuprous xanthate occur immediately on mixing the reagent with the pulp. However, from the results with the larger quantity of xanthate it seems that the cuprous xanthate formation slov7s up as more cuprous xanthate coating is produced on the mineral, probably owing to difficulty of diffusion of free xanthate ions through the coating to react with the mineral.
267
ACTION OF POTASSIUM %AMYL XANTHATE ON CHALCOCITE RELATION OF EFFECTIVE MINERAL SURFACE AREA TO PRODUCTS
Effect of time of grinding A series of experiments was made to determine the effect of time of grinding on the reaction-product relationships. The usual experimental TABLE 4
The effect of time of grinding on the reaction products Tests using general experimental procedure, treating with 5.4 milliequivalents of potassium n-amyl xanthate for 15 minutes. Amounts expressed as milliequivalents per 200 g. of chalcocite. The symbol X is used for the xanthate radical. EOURSOQ QRIND
~
11ECUIV-
CU&,
ALENTOF
"ETHER-
EXTRACTED
SOLUBLE"
1
1
w1
I
EXCE8SXI N SOLUTION
3.80 0.06 From curve of figure 2 3.52 0.11 2.67 0.18 0.19 1.09 0.03t
1
2 4 12 48
~
WNLEACBABLE ON CBAWOCITE'
x
1
YILLIEPUIV.
HCl TO YIELD A pH OF 7
0.80 1.48 1.81 2.63 4.13 4.46
1.04 0.02 0.02 0.04 0.01
.
REDZ'CING IONB IN SOLUTION
2.96
0.03
3.59 3.44 2.97 0.37
0.11 0.22
* Obtained
by difference. of "ether-soluble" in this run did not justify its separate determination, and i t was added to the cuprous xanthate solution before titration.
t The small quantity
0
8
/6
24 Hour3
32
40
48
56
2 Grind
FIG.4. The effect of time of grinding on the reaction products
procedure was used, the quantity of xanthate being kept constant a t 5.4 milliequivalents and the xanthate treatment time being kept constant at
268
A. M. GAUDIN AND REINHARDT SCHUHMBNN, JR.
fifteen minutes. Experimental results are given in table 4 and plotted in figure 4. In the longer grinding tests a considerable vacuum was noted on opening the pebble mill, indicating reaction of oxygen with the mineral during grinding. Increasing difficulty in filtration of the treated pulp was encountered with increasing fineness of grinding. Silicate in the ground pulp, worn off the pebbles, increased to about 10 to 15per cent of the weight of the mineral for the longest grinding time. The data from these runs show that the unleachable portion definitely increases with increase in surface of the mineral particles with longer grinding, and in long grinds approaches the total amount of xanthate TABLE 5 Size analysis of deslinied ground chalcocite TEEORETICAL SPECIFIC SCRFhCEt
SIZE
WEIGHT, PER CENT
TEEORETICAL SURFACE:
5 0
11 184 637 66
~
microns
-74 (+200 mesh) -74, 4-37 (-200, -37, 4- 15 7.5 -15, -7.5
+
microns
+ 400 mesh:
cm 2 per gmm
80
140
50
220 520
21 * 10*
2t
1100 5500
Total.,. , ., . .. . . , , . . . . .. . , , . . . , , , , , . , .., , . , . . . , , . . , ..
25 4 57 9 1.2 100.0
1
899
* The accuracy of the average size figures from sedimentation analysis is prohably of the order of 10 per cent. The figures are calculated assuming Stokes' law for spheres falling in a liquid; the irregularity in shape of ground mineral particles limits the accuracy of the method. t This is a n estimate, as no further separation was made of the - 7 . 5 ~portion. The figure is purposely low, to be on the safe side. The weight per cent of this portion is so small that the soundness of the estimate has no great effect on the accuracy of the total surface figure. $ Assuming all particles t o be cubes. added. The titration figures on the filtrate show that the hydroxide decreases, while the reducing ions (S,O,)- - increase with increase in surface. Although no determinations were made, it is expected that the sulfate in the filtrate also increases with increase in time of grinding.
Estimation o j thickness o j unleachable film As the phenomena associated with the formation and presence of the unleachable entity on the mineral surface seemed to be indicative of an irreversible adsorption, it seemed desirable to make a quantitative comparison of the extent of mineral surface and the maxinium amount of unleachable att'ainable with that surface. The mineral which had been
ACTIOX O F POTASSIUM %-AMYL XANTHATE ON CHALCOCITE
269
ground in the pebble mill contained too large a percentage of extremely fine material for accurate surface measurement; accordingly for this work the mineral, after grinding, was carefully “deslimed” by water sedimentation in buckets. A sample of the deslimed mineral was then sized by combined screening and acetone sedimentation, and from the sizing analysis the surface was estimated. Results of this operation are given in table 5. In other work with the determination of surface area by sizing (5) it has been shown that a correction factor should be applied to the theoretical surface figure to correct for irregularity inshape of particles, cracks, etc. A factor of 1.5 was used, giving an actual surface figure of 1350 cm.2 per gram (1.5 X 899). The factor used is possibly accurate to within 10 per cent; and as the theoretical surface figure is estimated to be accurate to about 10 per cent, the accuracy of the actual surface figure may be roughly 15 per cent (1/0.102 0.102 = 0.141). Two samples of the sized chalcocite were agitated with a large excess of xanthate (2.5 and 1.5 milliequivalents per 150 g., respectively) and the products determined, using the general experimental procedure, The unleachable xanthate figures were 0.16 and 0.13 milliequivalents per 150 g., respectively. Using the average figure, 0.145 milliequivalent,
+
0.145 X 10-8 X 6.06 X 1023 = 8.8 X l O I 9 molecules of xanthate per 150 g. of chalcocite 1350 X 150 = 2.02 X lo6 cm.2 of surface per 150 g. of chalcocite Assuming a monomolecular film, 2’02 lo5= 23 X 8.8 X loxg
10-16
cm.2 = 23 A . U.’, the area occupied by one xanthate group
Adam and Muller (lo), by two independent methods, estimated the area of a hydrocarbon chain to be 20.5 A.U.*,which agrees well with the figure of 23 A.U.2 found here for the area occupied by one xanthate radical. The difference between these figures is well within the limits of experimental error of the method used in this work for the estimation of the surface of the chalcocite. Also the polar group of the xanthate radical may have some effect on the area which it occupies. Thus the result of the calculation justifies the assumption of a monomolecular film, and it can be said that the maximum amount of unleachable xanthate attainable on a chalcocite surface is equivalent to a complete monomolecular film. ADSORPTION OF CUPROUS XANTHATE FROM BENZENE
Tests were run to determine if cuprous xanthate is adsorbed from benzene solution by chalcocite. 200-g. samples of chalcocite were ground
270
A. M. GAUDIN AND REINHARDT SCHUHMANN, JR.
MILLIEQ~IIVALEIIT6OF ADDED
CUzXl
MILLIFIQLIVALENTS OF CUiYi LEFT I N SOLUTION
MILLIEQUIVALENT6 OF ADSORBED
0 40
0 00
0 40
0 89
0 23
0 66
CUzX,
COMPARATIVE TESTS WITH MALACHITE
A few semiquantitative experiments of the same nature as some of the chalcocite experiments were repeated with malachite, (CuCO3.CU(OH)~). Two tests were made on 200-g. samples, using the general experimental procedure for grinding, treatment, and determination of reaction products. It was found that malachite abstracts xanthate from aqueous potassium n-amyl xanthate solution, forming cuprous xanthate and dixanthogen. The malachite after the leaching procedure, contrary t o experience with chalcocite, was entirely non-floatable. Thus, with malachite, an unleachable water-repellent coating is not formed in an aqueous xanthate solution. Several tests with a benzene solution of cuprous xanthate showed that malachite does not appreciably adsorb the cuprous xanthate from benzene solution under the conditions under which chalcocite exhibits a marked adsorption. DISCUSS ION
On the basis of the results presented in this paper it seems that the collecting action of xanthate is essentially different for chalcocite and for malachite. iipparently, the flotation of the sulfide mineral is caused by the formation of an oriented monomolecular film tightly “anchored” to the mineral; whereas flotation of the oxidized mineral seems t o be due to
ACTION OF POTASSIUM %-AMYL XANTHATE ON CHALCOCITE
271
the formation of a much thicker coating of base-metal xanthate (and dixanthogen), not “anchored” to the mineral, but merely adjacent to the mineral surface. This comparison offers a rational explanation for the fact that in actual flotation operation the quantity of xanthate required for flotation of malachite is many times that required for flotation of chalcocite. The “anchored” monomolecular film formed on chalcocite does not possess the same solubility properties as any known xanthate or xanthate derivatives. Although, as evidenced by its complete insolubility in benzene, the film does not consist of cuprous xanthate, cuprous xanthate may be removed from the surface by means of pyridine. On the whole, all the observed properties of this film seem to be analogous in many ways to the properties of films of adsorbed gases on solids, which were observed and clarified by Langmuir (6). Quoting from his discussion of the adsorption of oxygen on tungsten: “It is very evident from this work that the oxygen layer has totally different properties from those we should expect, c either with a layer of oxide or a film of highly compressed gas. The facts are in good accord, however, with the theory that oxygen atoms are chemically combined with the tungsten atoms.” On the basis of these and other similar results Langmuir advanced the theory that in the adsorption of gases on solids, the adsorptive forces are chemical forces of the primary valence type. Carrying this conception over to the present work, it appears that the “unleachable” film consists of xanthategroups chemically attached to copper atoms of the chalcocite by primary valence bonds. The adsorbed film is formed on the chalcocite from potassium xanthate solution by reaction of the xanthate ions with the surface of the mineral. As suggested by Frumkin (8) for adsorption of electrolytes by charcoal, hydroxide ions are produced a t the surface from adsorbed oxygen; these hydroxide ions are replaced by xanthate ions and enter the solution, as
Carbonate ions may be formed from hydroxide and the carbon dioxide present in the system. During grinding and other treatment, a certain amount of adsorbed or “anchored” reducing ions and sulfate ions are produced a t the mineral surface by oxidation reactions, as suggested by Taylor and Knoll (11) for galena. These ions are also replaced by xanthate and enter the solution, as follows: (A denotes adsorbed anion, sulfate ion, reducing ion, etc.)
C U ~ SA/ + 2X-
__ -+
+ 2Ax
‘CUZSI X
‘
_ . .
272
A. M. GAUDIN AND REINHARDT SCHUHMANX, J R .
Cuprous xanthate is formed by reaction of xanthate ions with that part of the chalcocite surface already covered with an adsorbed xanthate film, as follows : XCUZSX X
+ $02 + HzO + 2X-
-+
20H-
+ CU& +
(X - ~)CUZS, X iX 2CuS
The cuprous amyl xanthate so formed is leachable with benzene, and its removal does not appreciably affect the flotative properties of the mineral. With the amounts of xanthate used in flotation practice only a relatively small fraction of the amount of reagent may be accounted for as cuprous xanthate. THEORETICAL FORMULATIOX
Applying the kinetic theory of reaction to the suggested reaction mechanism, a mathematical interpretation of the reaction-product relationships given in table 1 may be obtained. The following symbols are used in the derivation : A = milliequivalents of xanthate ions in solution a t any time, A = total milliequivalents of xanthate extracted by the mineral from solution in a given reaction, B = milliequivalents of adsorbed xanthate on mineral a t any time, Bo = total adsorbed xanthate formed in a given reaction, C = milliequivalents of cuprous xanthate on mineral a t any time, t = time, S = effective mineral surface, expressed 3s milliequivalents of ‘‘elementary spaces’’ (see Langmuir (7)), z = fraction of elementary spaces on which xanthate is adsorbed a t any time, zo = fraction of elementary spaces on which xanthate is adsorbed after completion of the given reaction, and k l , k z , k8 = constants. First, the rate of decrease in free xanthate ions is equal to the sum of the rates of formation of adsorbed xanthate and of cuprous xanthate, as the loss in free xanthate in solution must be accounted for either as cuprous xanthate or as adsorbed xanthate.
dA
_ - = -
dt
dB dC dt -I-
The rate of formation of adsorbed xanthate is proportional to the area of bare, unreacted surface and to the concentration of xanthate. dt
= kl(1
- z)SA
ACTION OF POTASSIUM n-,niyL SANTHATE ON CHALCOCITE
253
The rate of formation of cuprous xanthate is proportional to the covered surface and to the concentration of xanthate.
Substituting equations 2 and 3 in equation 1,
- dA - = kl(l dt
00
20
- x ) S A + kpxSA
(4)
60 80 /O,O Z O Xan ihafe Abs tracfed Per 2OO9 Chdcocrfe
10
4-Milhequiva/enfs
FIG.5. Adsorption curve, A O = -1.25Bo - 9.1 log (1 - B~/1.75). Experimental points from table 1 plotted as circles
The rate of increase in adsorbed xanthate is equal to the rate of increase in adsorbed surface, expressed in the same units. dB - = Sdx -. dt dt From eqiiations 2 and 5,
Dividing equation 4 by equation 6 ,
(5)
274
A . If. GAUDIN AND REINHARDT SCHUHMANN, JR.
Integrating and substituting limit.,
Ao -=
s
To(1
- ha) - ka log,(l
- To)
(8)
Substituting data from table 1, BO = z 0 S = 1.36 when A0 = 4.19, and assuming S , the surface for 1-hour grind, to be 1.75 milliequivalents as indicated by the maximum of the curve of figure 2, solving for the constant k l , and converting logs to base 10, the equation becomes:
Ao Substituting
20 =
=
-
2 . 1 9 ~0 9.110g (1 -
50)
(9)
- Bo/1.75)
(10)
Bo/S A0 = 1.25Bo - 9.1 log (1
The curve of this equation is plotted in figure 5, with the experimental points iron- table 1 shown as circles. It is seen that the experimental points check this theoretical curve quite closely, especially as the experimental results for the adsorbed or “unleachable” entity were determined by difference from three experimental values: namely, the total xanthate added, the cuprous xanthate extracted, and the excess xanthate in solution. SUMMARY AND CONCLUSIONS
In this work the following results have been obtained: 1. In short-time treatment of chalcocite with xanthate the xanthate abstracted by the mineral may be accounted for as cuprous xanthate and as an entity unleachable with ordinary solvents. The formation of these two products occurs practically instantaneously, although with larger quantities of xanthate the reaction to form cuprous xanthate slows up with increase in quantity of cuprous xanthate on the mineral. 2. I n long-time treatment of chalcocite with xanthate, leachable products other than cuprous xanthate are formed relatively slowly, possibly by reaction of cuprous xanthate and hydroxide in solution. 3. ill1 the potassium of the potassium xanthate remains in the solution. 4. Hydroxide, carbonate, sulfate, and reducing ions (S,O,)- - are thrown into the solution in a total amount metathetically equivalent to the xanthate abstracted from the solution. 5 . Leaching off the leachable products from xanthate-treated chalcvcite has no appreciable effect on the flotative properties of the chalcocite. 6. Pyridine extracts cuprous xanthate from xanthate-treated and leached chalcocite, that is, from chalcocite having the “unleachable” film. 7 . The “unleachable” product increases with increase in xanthate added, approaching a maximum in a manner suggestive of adsorption. 8. The maximum amount of unleachable xanthate attainable with a given sample of chalcocite corresponds to a complete monomolecular film on the chaleocite.
ACTION OF POTASSIUM
n-mn
XANTHATE ON CHALCOCITE
275
9. If a quantity of xanthate just sufficient for effective flotation is added, it may be almost completely accounted for as the unleachable product. Only a relatively small fraction of a monomolecular film of “unleachable” is necessary for efficient flotation. The fraction of the surface filmed determines the dryness of the mineralized froth. 10. Increasing the surface of the mineral by longer grinding increases the unleachable portion and decreases the cuprous xanthate portion of the reaction products. 11. The unleachable water-repellent film may be produced by the adsorption of cuprous xanthate from benzene solution by the chalcocite. 12. There is an essential difference between the collecting actions of xanthate on chalcocite and on malachite, respectively. An unleachable, water-repellent film is not formed on malachite by xanthate. From these experimental facts it appears that the collecting action of xanthates in the flotation of chalcocite is a result of the production of an oriented, partial-monomolecular film of xanthate groups, chemically adsorbed in the Langmuirian sense. This film is formed by reaction of xanthate ions with the surface of the mineral which contains various adsorbed entities as a result of oxidation during grinding and other treatment. The formation of the adsorbed film is accompanied by reaction of a portion of the xanthate to form cuprous xanthate and perhaps small amounts of other substances separable from the mineral by means of organic solvents. Application of kinetic theory principles to the suggested reaction mechanism gives an adsorption-reaction curve which checks the experimental data well within the limits of experimental error. This work is being continued with other xanthates and other minerals. REFERENCES (1) DEWEY,F. D.: Reactions of Some Sulfur-bearing Collecting Agents with Certain Copper Mingrals. Thesis, Montana School of Mines, 1933. (2) GAUDIN,A. M.: Trans. Am. Inst. Mining Met. Engrs. 112, 233 (1934). (3) GAUDIN,A. M., DEWEY,F. D., AND OTHERS:Trans. Am. Inst. Mining Met,. Engrs. 112, 319-47 (1934). (4) GAUDIN,A. M., AND MALOZEMOFF, P . : J. Phys. Chem. 37, 597-607 (1933). (5) GROSS,JOIIN: Trans. Am. Inst. Mining Met. Engrs. 112, 120 (1934). I.: J. Am. Chem. Sac. 38, 2267-78 (1916). (6) LANGMUIR, I.: J. Am. Chem. SOC.40,1361 (1916). (7) LANGMUIR, (8) See PEARCE, J. N.: J. Phys. Chem. 30, 1980 (1932). (9) PREGL:Quantitative Organic Microanalysis. P. Blakiston and Sons, Philadelphia (1930). H. 8.:A Treatise on Physical Chemistry, 2nd edition, Val. 11, (10) See TAYLOR, p. 1647. D. Van Nostrand Co., New York (1930). (11) TAYLOR, T. C., AND KNOLL,A. F.: Trans. Am. Inst. Mining Met. Engrs. 112, 382-97 (1934). (12) TREADWELL, F. P., AND HALL,W. T . : Analytical Chemistry, Val. 11, p. 57. John Wiley and Sons, New York (1930). (13) WARK,I. W., AND Cox, A. B.: Trans. Am. Inst. Mining Met. Engrs. 112, 189 (1934).
