FLOTATION OF GALENA
93
(7) MARSHALL, C. E . : Z. Krist. AM, 8 (1935). (8) M ~ L L E RH.: , Kolloid-Beihefte 27, 223 (1928). (9) TUORILA, P.: Kolloid-Beihefte 27, 44 (1928). (IO) WEITESIDE, E. P., AND MARSHALL, C. E. : Proc. Soil Sci. SOC.Am. 4,100 (1939). (11) WIEGNER, G., AND MARSHALL, C. E.: Z. physik. Chem. A l a , 1 (1929). (12) WIEGNER,G., AND RUSSELL, E. W . : Kolloid-Z. 62, 1 (1930). (13) ZOCHER, H . : Z. physik. Chem. 98, 293 (1921).
SURFACE CHEMISTRY I N T H E FLOTATION OF GALEXJA HAROLD H. HERD
AND
WILLIAM URE
Department of Chemistry, The University of British CoEumbia, Vancouver, Canada Receitled June 4, 1940 0
Sulfide minerals are readily amenable to flotation, and the successful utilization of the complex sulfide ores of the northwestern part of the American continent has been made possible by the development of methods of selective flotation. Natural minerals, however, are not readily floated until the surface has been treated in some way to produce adherence of the mineral particle to the air bubble. To attain this result certain reagents known as collectors are added to the pulp, and of these, compounds of the xanthate type have wide application in dealing with sulfide ores. Potassium ethyl xanthate has the formula
/OGHs
s=c \SK and is an excellent collector for galena and similar minerals. It is known that the surface of natural sulfides is largely altered by oxidation, so that a layer of sulfate and other compounds exists on the mineral surface as it enters the flotation circuit. The behavior of this oxidized layer with the collector has been a subject of very considerable investigation, without, however, the appearance of any complete explanation. Two questions at once present themselves: ( 1 ) If the oxidized coating were cleaned off, would the mineral possess natural floatability? and ( 2 ) is the collector a cleaning agent or is its main function that of producing a water-repellent layer by adsorption on the mineral particle? Associated with these questions is the query as to whether a pure sulfide surface will have any reaction a t all with a collector. Wark (7) in his recent book concludes that the experimental evidence is against inherent floatability for sulfide minerals. On the other hand,
94
HAROLD H. HERD AND WILLIAM URE
Ravitz and Porter (5) have shown that galena washed with a solution of ammonium acetate shows a high flotation recovery without the use of a collector. The behavior of minerals at air-liquid interfaces has been extensively studied by Wark and Cox (8), making use of a method of measuring contact angles between air bubbles and solid surfaces. Their results are definitely in favor of the concept of an oriented layer of collector at the mineral surface. Wark (7) concludes that oxidation of the surface is not a necessary prerequisite to adsorption of a collector. The present work deals with a study of galena, its floatability in the native state and after surface cleaning, and the interaction of potassium ethyl xanthate with this mineral. The conclusions reached are ( I ) that a clean galena surface has a high degree of inherent floatability, (2) that xanthate exerts a cleaning action on the surface, accompanied by the formation of an adsorbed layer, and (3) that cleaned galena does not react with xanthate. While this work was in progress, Ravitz (4) publi3hed further data confirming the native floatability of clean galena. Our results are in complete agreement with those of this worker. EXPERIMENTAL
Materials The galena used in this work was from a single crystalline lump weighing some 30 to 40 pounds. Analysis showed 83.8 per cent of lead, with some antimony and a trace of silver. The metals of the iron group were negligible in amount and copper was absent. Bright crystalline portions were selected for all tests. Potassium ethyl xanthate prepared in the usual way from potassium hydroxide, ethyl alcohol, and carbon disulfide was washed with ether and precipitated from acetone solution by the addition of benzene. The product was redissolved in acetone, and precipitated three times with petroleum ether. After drying a t 35°C. it was stored in a vacuum over concentrated sulfuric acid. The product was white with only a slight yellow tint.
Apparatus The mneral was either ground in an agate mortar under distilled water or first broken up in a porcelain mortar and ground in a laboratory pebble mill with porcelain jar and flint pebbles. The ground material was sized in a water elutriator, which consisted of a large glass cylinder set in a porcelain dish. Water entered the cylinder at constant head through a glass tube extending to the bottom, and overflow was obtained by a glass siphon. The rate of flow was determined from the dimensions of the
95
FLOTATION OF GALENA
cylinder and the volume of overflow. Samples of galena with settling rates up to 30 cm. per minute could be obtained. Flotation tests were carried out in the flotation cell shown in figure 1. This was designed and constructed by Mr. R. L. Bennett, who carried out preliminary work on this project, and consists of a glass tube 20 cm. long and 5 cm. in diameter attached to a fritted-glass filter funnel. The top of the cell is cut off at 45' and is fitted with a sheet-metal overflow lip and trough. In operation, air or nitrogen at 0.5 atm. pressure is blown through from the bottom, and concentrates are collected by displacing the froth with a stream of water from the top.
n
I
U
I--
FIG. 1. Flotation cell
FIQ.2. Decantation f l a k
For treatment under nitrogen the decantation flask shown in figure 2 was used. This was constructed with side tubes for admitting nitrogen and reagents and could be emptied by tipping so that the contents could be completely expelled through the siphon shown. Connections to reagent reservoirs were made by interchangeable ground-glass joints. Since other workers have stressed the importance of small amounts of grease in flotation tests, the mineral samples used in the experiments were not touched by hand. RESULTS
1 . The inherent floatability of galena ( 1 ) Presence of the oxidized layer. While galena in bulk has the proper-
ties of lead sulfide, the readiness with which lead sulfide is oxidized in air to the sulfate makes it highly improbable that a freshly prepared surface
96
HAROLD H . HERD AND WILLIAM URE
of this mineral could long remain as sulfide. The presence of an oxidized layer on the natural ore surface can easily be demonstrated, and it will obviously exert a profound influence upon the action which collecting agents may have upon the mineral. Samples of galena were ground under distilled water in an agate mortar, and portions with settling rates of from 9 to 25 cm. per min. were taken from the elutriator. These were extracted with boiling solutions of ammonium acetate or sodium chloride, both in air and under nitrogen. The extract was tested for lead ion and sulfate ion, and both were readily detected. Upon repeated extraction with these solvents under nitrogen, lead ion and sulfate ion were found to become progressively less, until the extract gave only a slight darkening when tested for lead with hydrogen sulfide, and sulfate could not be detected with barium acetate. (2) Treatment with ammonium acetate. Galena was ground in an agate mortar and samples with settling rates of 9 to 25 cm. per minute were withdrawn from the elutriator. One part was transferred to the flotation cell with 250 ml. of water containing 5 ml. of terpineol solution (250 mg. per liter) as frother, the cell was blown with air, and concentrates and tails were collected to determine the floatability of the untreated ore. A second portion was boiled with 25 ml. of nearly saturated ammonium acetate solu ion and subsequently washed five times with 50-ml. portions of boiling distilled water. This mineral was transferred in suspension in water to the flotation cell with the minimum exposure to air. The flotation yield was measured under the same conditions as before. Another sized sample was divided into four parts, each of which was extracted with a more concentrated solution of ammonium acetate than the preceding. Flotation yields were again measured and were observed to parallel the vigor of the extraction treatment. The results of these tests, which are in agreement with the work of Ravitz and Porter (5), are summarized in table 1. That the function of the ammonium acetate is that of cleaning the mineral surface is shown by the following experiment: Approximately equal samples of ground and sized galena were treated with ammonium acetate in 50 ml. of distilled water, brought just to boiling, and then washed by decantation through a filter, the solution passing into volumetric flasks. The ammonium acetate extract and washings were collected together and analyzed for lead by colorimetric comparison with standard lead sulfide colloids, using a photoelectric colorimeter. Some of the results of these experiments are given in table 2, which shows that lead ion is removed from the surface in proportion to the vigor of the ammonium acetate treatment and the consequent change in floatability. Wark (7)has raised the objection to the work of Ravitr and Porter with ammonium acetate that collecting action may take place owing to traces of impurities or decomposition products, particularly acetamide. We have
97
FLOTATION OF GALENA
found that acetamide has practically no action as a collector for galena. In experiments using concentrations of acetamide up to 20 g. per liter, recovery was less than 10 per cent. (3) Treatment with sodium chloride. The above objection will obviously not apply to the use of sodium chloride in place of ammonium acetate, and a hot concentrated solution of sodium chloride is an excellent solvent for lead sulfate. Hence a series of experiments was made using sodium chloride as the cleaning agent. TABLE 1 Influence of washing with ammonium acetate on the flotation of galena WEIQHT OF BAMPLE
FLOTATION YIELD
A M M O M U M ACETATE UBED
per cent
grams
2.62 2.35 2.08 2.35 2.61 2.92 2.69 3.92
5.82 91.5 21.8 82.3 6.3 25.5 55.5 91.5
0 Saturated solution 0 Saturated solution 0 1 g. in 50 ml. 2 g. in 50 ml. Washed twice with above solution TABLE 2 Removal of lead ion from surface b y ammonium acetate
W E l Q E T OF SAMPLE
AMMONIUM ACETATE USED
mme
omma in 60 ml.
2.34 3.47 2.13 2.66 2.61
0 0.5 1.0 2.0 Twice with final solution
,1
FLOTATION YIELD
p m cent
LEAD REMOVED
milligram per gram of sample
0
0.19 0.28 0.29 0.38
Extreme care was taken to prevent the introduction of any impurities which might possess collector action, and the washing and flotation processes were carried out in an atmosphere of nitrogen. Wark (7) has called attention to the fact that a galena surface, if allowed to dry, attains some air-avidity. In the experiments to be described, the samples were under water from the start of treatment until the concentrates were removed from the flotation cell. Larger samples of galena than those used previously were ground with water in the ball mill. The ore was sized to settling rates of 8.6 to 28.3 cm. per minute and washed under nitrogen with saturated sodium chloride solutions. Ntrogen was
98
HAROLD H. HERD AND WILLIAM URE
bubbled through two absorption bottles containing 250 ml. each of an oxygen-absorbent solution consisting of sodium hydrosulfite, sodium anthraquinone-8-sulfonate, and sodium hydroxide, then through traps containing concentrated sulfuric acid, activated charcoal, and cotton to prevent entrainment of the oxygen-absorbent. Reagent solutions were made up in water, which had been freshly distilled through block tin and saturated with nitrogen, and were transferred from storage by means of nitrogen pressure. The washing and storage apparatus employed was made entirely of glass with suitable ground joints, so that the liquids were never in contact with rubber. However, rubber was permitted in connections in the nitrogen-scrubbing train and in the gas distribution manifold, so that pinch clamps could be used in place of greased stopcocks. The rubber for these connections was boiled for some hours in sodium hydroxide solutions and then in distilled water. TABLE 3 Effect of washing galena with sodium chloride i n the absence of. oxygen .. WEIQET OF BAYPLE TAKEN FOB FLOTARON
grama
22.0 19.40 21 $80 15.39 20.64 17.92 17.57 16.30
pmm
1
1 2 3 4 4 5 7 7
17.86 10.85 9.69 10.56 14.70 16.68 11.39 12.08
FLOTATION YIELD
2 minutes
7 minutes
par cant
per cant
Nil Nil
Nil Nil
12.2 65.8 82.8 78.5 77.9 76.9 87.7
15.2 72.7 86.2 88.0 89.0 88.2 91.6
Samples of the ground, sized ore in suspension in water were drawn into the decantation flask, washed twice with distilled water saturated with nitrogen, and were then boiled with successive 50- to 100-ml. ,portions of saturated sodium chloride solution. After each sodium chloride extract was ejected the ore was washed with 50 to 100 ml. of distilled water and after the final sodium chloride treatment, water washings were continued until little or no chloride ion could be detected with silver nitrate or lead ion was negligible to sodium sulfide. Ore samples were given in this way from one to seven extraction treatments with sodium chlorideand were then ejected by nitrogen pressure into the flotation cell without contact with air. The flotation cell was blown immediately with nitrogen for 7 min., using 5 ml. of 250 mg. per liter terpineol as frother. Concentrates were collected a t 2 min. and at 7 min. Tailing waa also collected and weighed, and the per cent recovery calculated. The results are given in table 3.
FLWTATION OF QALENA
99
These results show rather conclusively that cleaned galena possesses a high degree of floatability in the absence of a collector, and that flotation is quite rapid, the major portion of the recovery taking place within a few minutes. Under the conditions of the experiment, the cleaning action is substantially completed after three or four washes. With the cell used, maximum recovery was abcfut 90 per cent even when a collector was added. (4) The e$ect of oxidation. The rate of oxidation of lead sulfide is extremely rapid, and the following experiments show the effect on flotation: Samples of galena were washed four times with sodium chloride solution under nitrogen as before. After thorough washing with water, the decantation flask was connected to a source of oxygen, warmed to 4OoC., and shaken for not more than 5 min. The samples were subjected to flotation exactly as before, using nitrogen in the cell. The yield was now less than 10 per cent and in one case absolutely nil while, as shown in table 3, yields of better than 85 per cent could be obtained after four washes with sodium chloride in the absence of oxygen. The samples used here were from the same grinding lot as those which gave the high yields. 2. Behavior of xanthate at the mineral surface
Although a clean galena surface has been shown to possess inherent floatability, in practice the mineral particles are so oxidized that it is necessary to use a collector to obtain successful flotation. In view of the preceding results, it becomes of great interest to study the action of a collector on the naturally occurring mineral with its oxidized surface and also upon the cleaned material. ( I ) Analytical. w e n natural galena is treated with potassium ethyl xanthate, the formation of insoluble lead xanthate has been demonstrated (3). It therefore became necessary to develop a method by which both potassium xanthate and lead xanthate could be determined. The usual method of determining xanthate is that of iodine titration using a starch indicator, since xanthate ion is quantitatively oxidized to dixanthogen by iodine. This method was found to give erroneous results when lead ion was present. However, if the end points are determined potentiometrically, it is possible to determine both soluble xanthate and suspended lead xanthate in the same solution. Solutions were prepared as follows: potassium ethyl xanthate (KEtX), 1 g. per liter; iodine, 0.8 g. per liter (with 2 g. of sodium iodide); lead nitrate, l g. per liter. The iodine and xanthate solutions were of equivalent concentration. Portions of the xanthate solution (25 ml.) were titrated potentiometrically with iodine, using platinum and calomel electrodes. In the absence of lead ion, normal titration curves were obtained with one sharp maximum slope corresponding to complete oxidation of the xanthate. This is shown by curve a of figure 3. Now to 25-ml. portions of potassium ethyl xanthate measured volumes
100
HAROLD H. HERD AND WILLIAM URE
of lead nitrate solution were added. A precipitate of lead ethyl xanthate appeared, almost in colloidal form. These mixtures of lead ethyl xanthate and excess potassium ethyl xanthate were then titrated as before. In these cases two maxima appeared in the slopes, as shown in curves b and c of figure 3 (curve c being the slope curve), which represent the case where 3 ml. of lead nitrate were added. If it is assumed that the first maximum
3
FIG.3. Potentiometric titration of xanthate with and without lead nitrate
occurs when the excess potassium ethyl xanthate is completely oxidized and the second when total xanthate is removed, then the difference on the volume axis is the iodine equivalent to the lead nitrate added. The agreement between the measured and calculated values is shown in table 4 and is seen to be quite close; the method is found to be sufficiently accurate for the work in hand. (2) Formation of lead xanthate from galena. Samples of sized unwashed
101
FLOTATION O F GALENA
TABLE 4 Determination o j lead ethyl xanthate b y potentiometric titration
mlllili&rs
IODINE EQUAL TO Pb++ (CALCULATED)
SElROR
psr cent
milliliters
mailitera
1 2 3 4 5 6 7
1.0 2.0 3.0 3.9 4.9 5.9
0.968
8
7.7
10
9.7
6.8
1.94
2.90 3.84 4.84 5.81 6.76 7.74 9.68
+3 +3 +3
+1 +1.5 +2
+0.6 -0.6 +0.2
FIQ.4. Potentiometric titration of galena extracts
galena were treated with 25-ml. portions of potassium ethyl xanthate solution and stirred for 5 min. Formation of finely divided lead ethyl xanthate was observed. After settling, the solutions were decanted along
102
HAROLD H. HERD AND WILLIAM URE
W ~ G H Tor E
LEAD ~
T n xLr n A m POBYED
LEAD PFHTL XANTE4TE PBB QBAY O? 8AXPLE
mm
milliaram
tnJliamm
26.84 19.0 13.46 9.88 9.73 4.65 2.46
16.7 12.5 9.0 4.9 6.6 3.6 2.8
0.62 0.66 0.67 0.49 0.68 0.76 1.1
(3) Adeorption of zanthate by galena. It was observed in the preceding experiments that not all of the xanthate added to the mineral was recovered in the titration. That is, the sum of the lead ethyl xanthate and the excess potassium ethyl xanthate differed by a small amount from the total potassium ethyl xanthate added. Apparently this difference corresponds to xanthate which is firmly held by the mineral surface, and the amount so held should be proportional to the extent of the surface. This appears to be borne out by the figures given in table 6, where the loss of xanthate is related to the weight of the galena sample, all samples having approximately the same particle size. The figures shown in the third column of table 6 are reasonably constant, the greatest variation being about twofold, while the weight of the samples is varied tenfold. It would appear, therefore, that we are dealing with a true surface effect. (4) Non-remval of potassium. If xanthate is actually adsorbed at the mineral surface it becomes of interest to find out whether or not potassium ion is simultaneously adsorbed.
103
FLOTATION OF GALENA
Samples of sized galena weighing from 40 to 50 g. were treated with 50-ml. portions of potassium ethyl xanthate solution in the manner adopted for all previous tests. The galena was filtered out in a weighed Gooch crucible, dried, and weighed, and potassium assays by the chloroplatinate method (2) were made on the filtrate. These were compared with blanks in which no galena was used. The results of these tests are given in table 7. TABLE 6 Adsorption of zanthate b y galena WFJQHT OF Q h L I N A
rnm 26.M 19.0 13.46 9.86 9.73 4.66 2.46
LO- OF POTAMIVY
mmn
XANTXATI
miU*nn
13.14 4.25 5.24 3.76 4.25 1.77 0.79
Average.. . . . . . . . . . . . . . , . . . . . . . . . . . . . . . . . . . . . . . . . . . .
POTIULUVY amcovmam
millirnna
miUimnn
70.8 70.8 70.5 74.7' 72.3'
11.35 11.35 11.25 12.1' 11.63'
POTAMIVY m T L XAHTBATI B D X O V I D PEE Q R A Y OF I A X P L I
milliummd
0.490 0.224 0.390 0.381 0.437 0.382 0.322 0.375
W I I Q H T OF QALINA
Pmm
33.76 43.00 43.14
* Blanks. From the data of table 6,40 g. of galena should abstract xanthate corresponding to about 16 mg. of potassium ethyl xanthate. That is, if both xanthate and potassium ions are adsorbed, about 4 mg. of potassium should be lost. Taking the average of the blanks as recoverable potassium, losses of only 0.5, 0.5, and 0.6 mg. of potassium were observed. This is substantially no loss within the limit of error in the analysis, which involved the recovery of 12 mg. of potassium from a large mass of original material. Other workers have noticed that potassium is not removed, but they were dealing with the total loss of collector, including that due to double decomposition.
104
HAROLD H. HERD AND WILLIAM URE
( 5 ) The effect ojxanthate on cleaned galena: When the galena was washed with sodium chloride solution before being treated with potassium ethyl xanthate, only a trace of the xanthate was abstracted. The apparatus used previously for washing galena under nitrogen was modified by the addition of an opening with a ground-glass joint through which the xanthate solution could be admitted. The ore was washed by boiling with sodium chloride solution as before. Twenty-five milliliters of potassium ethyl xanthate solution were then introduced, the flask shaken for 5 min., the solution removed by nitrogen pressure, and the residue washed. Solution and washings were titrated immediately in the usual manner. The results are shown in table 8. TABLE 8 The eflect of xanthate on cleaned galena. WEIQHT 06 QALENA
NUMBER OF WABHlEB WITE SODIUM CHLORIDE SOLUTION
TOTY uNTHATE
milligmma
15.74 28.46 24.89
5 5 6
0.62 0.10
0.30
Reference to table 6 will show that a sample of the same galena at the same particle size weighing 26.84 g. caused a loss of 13.1 mg. of xanthate, in addition to xanthate used up in forming lead xanthate. Thus it would appear that surface oxidation to a degree is necessary for the adsorption of xanthate. DISCUSSION
It is customary to refer to two main viewpoints with regard to the action of collectors a t mineral surfaces,-the Adsorption Theory and the Chemical Theory. The adherents of the Adsorption Theory (7) postulate an adsorbed layer of collector on the surface of the mineral particle. Since the majority of collectors are heteropolar compounds, an orientation is assumed which effectively produces an organic non-wetted surface. Perhaps the most conclusive evidence for such layers comes from the extended investigations of Wark and Cox (8) on contact angles between air bubbles and mineral surfaces. This work shows that for different minerals any collector produces a constant and characteristic angle of contact, and that, as the hydrocarbon chain lengthens, so the angle approaches 105', the angle produced at the surface of a pure hydrocarbon. These workers further state that no contact takes place between galena and air in the absence of a collector, even if the surface is cleaned so that it is presumably lead sulfide.
FLOTATION OF GALENA
105
The Chemical Theory (6), on the other hand, is based upon the known formation of such compounds as lead xanthate when natural galena is treated with xanthate, and an attempt is made to account for collector action in terms merely of double decomposition and solubility product. According to Knoll (3), the surface of galena which has been exposed to air is a mixture of sulfoxides, hydroxide, and carbonate. In the present work we have tested only qualitatively for sulfate, which is present in considerable quantity. Double decomposition can take place when the lead compound with the collector ion is less soluble than the oxidized lead compound, and this is true in the case of lead ethyl xanthate with respect to lead sulfate. At the same time it is obviously necessary to have an adherent layer on the mineral particle, and the lead xanthate which we have found suspended in the liquid can play no part in producing floatability. Gaudin and Schuhmann (1) have studied the products of reaction between chalcocite and xanthate, and find that of the total xanthate abstracted by the mineral a certain amount is not recovered on extraction with acetone and apparently represents a basic layer. The present results agree with this conclusion in the case of galena, and we can draw the following picture of the collector action of xanthate. A considerable proportion of the oxidized surface is changed into lead xanthate, which readily washes off the surface. An adherent layer of xanthate is left behind, which is produced by exchange adsorption between some oxidized layer and the potassium xanthate, potassium ion remaining in solution. Further, a t least some degree of oxidation is necessary for the formation of this basic layer and hence for collector action, since thoroughly cleaned galena does not adsorb xanthate to any extent. It is possible from the data of table 6 to make an estimate of the depth of the adsorbed xanthate layer. The mineral used in that series of tests was ground to a settling rate of from 8.6 to 28.3 cm. per minute. From Stokes’ law the average particle size corresponds to a diameter of 1.48 X IO+ cm. or a surface area of 4070 sq. cm. per cubic centimeter of galena. The applicability of Stokes’ law to the conditions in the elutriator was checked by measurement of a large number of particles under the microscope, the agreement being within about 10 per cent. Fortunately galena fractures into nearly perfect cubes, so that an estimate of the surface can have some precision. Taking the average loss of xanthate (table 6) as 0.375 mg. per gram of galena, 2.8 mg. of potassium ethyl xanthate are spread over 4070 sq. cm. of surface. This corresponds to 2.6 X 1OI6xanthate ions per square centimeter of galena surface. The number of lead ions per square centimeter of surface may be estimated from the molecular volume of lead sulfide. This number is 0.7 X 10l6. If lead xanthate is formed as a surface compound, two xanthate ions will be attached to each lead ion. These calcu-
106
HAROLD H. HERD AND WILLIAM URE
lations would therefore indicate a layer of xanthate between one and two ions in thickness. Since the errors in these calculations are large, we may conclude that the adsorbed layer of xanthate is probably a unimolecular layer. $'.Sucha layer with orientation such that the alkyl groups are outermost would confer upon the particle its non-wettability and account for the collector action. SUMMARY
1. A study has been made of the surface conditions of galena in its natural state and of the effect of various reagents on the surface in regard to flotation. 2. Galena cleaned with solutions of ammonium acetate or sodium chloride exhibits a high degree of floatability in the absence of a collector. 3. Acetamide has no appreciable collecting action on galena. 4. A cleaned lead sulfide surface is rapidly oxidized by oxygen, and flotation recovery is reduced to a low value. 5. Potassium ethyl xanthate reacts with natural galena to produce mainly lead xanthate, but a portion of the xanthate is adsorbed on the mineral surface. Potassium is not abstracted by the mineral, so that the process appears to be exchange adsorption. 6. Cleaned galena does not adsorb xanthate. 7. Evidence is presented to show that the adsorbed layer of xanthate is probably unimolecular. REFERENCES (1) GAUDINAND SCHUHMANN: J. Phys. Chem. 40,257 (1936). (2) HILLEBRAND AND LUNDELL: Applied Inorganic Analysis, p. 518. John Wiley and Sons, Inc., New York (1929). (3) KNOLL:Dissertation, Columbia University, 1932. (4) RAVITZ:Am. Inst. Mining Met. Engrs., Tech. Pub. No. 1147 (1940). (5) R A ~ I TAND Z PORTER:Am. Inst. Mining Met. Engrs., Tech. Pub. No. 513 (1933). (6) TAYLOR, AND KNOLL: Am. Inst. Mining- Met. Enars., . Tech. Pub. No. , , TAGQART. 312 (1930). ' (7) WARK: Principle8 of Flotation. Australasian Institute of Mining and Metallurgy (1938). (8) WARK AND Cox: Trans. Am. Inst. Mining Met. Engrs., 112 (Milling Methods) 189 (1934).
.
