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THE INFLUENCE OF TEMPERATURE ON THE ADSORPTION OF POTASSIUM ETHYL XANTHATE ON GALENA
Frank F. Apian
22 00 4
A Thesis Submitted to the Department of Mineral Dressing in Partial Fulfillment of the Requirements for the Degree of Master of Science in Mineral Dressing Engineering
MONTANA SCHOOL OF MINES Butte, Montana June 9, 1950
UMI Number: EP33305
All rights reserved INFORMATION TO ALL USERS The quality of this reproduction is dependent on the quality of the copy submitted. In the unlikely event that the author did not send a complete manuscript and there are missing pages, these will be noted. Also, if material had to be removed, a note will indicate the deletion.
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TABLE OF CONTENTS INTRODUCTION
RESUME OF ADSORPTION PHENOMENA
Adsorption in the Liquid-Solid Phase Kolthof f' s Views of Adsorption Review of the Theories of Flotation Dean and Ambrose Detergent Theory Taggart's Chemical Theory Theory Proposed by Wark andHis Co-Workers Recent- Work by Gaudin on Adsorption Significance of the Contact Angle EXPERIMENTAL PROCEDURE Contact Angle Apparatus Modifications Contact Angle Measurement Procedure Specimen Preparation and Handling Solution Preparation Pre-Conditioning of Specimens and Solutions Bubble Considerations Bubble Tube Dimensions Bubble Equilibrium Magnitude of the Contact Angle Skewing of the Bubble pH Measurement Summary of Measurement Procedure EXPERIMENTAL DATA Factors Influencing Data Bubble Stabilization Xanthate Deterioration Mechanism of Xanthate Deterioration Presentation of Results and Discussion Data for 16° - 18° C. and 32 C Data for 40° C Transition Zones Data for 50° C Effect of Xanthate Concentration Changes Theoretical Explanations of Observed Phenomena Alkaline Range Acid Range Weakly Acid Range
8 8 12 12 12 12 13 13 15 15 15 19 19 20 20 21 21 22 22 24 24 26 27 27 28 30 32 34 34 38 38 43 47 48 49 50 51
REC0ML1ENDATI0NS FOR FUTURE INVESTIGATIONS
LIST OF ILLUSTRATIONS PLATES Page Plate 1.
FIGURES Figure 1.
Equilibrium Relationships for a Bubble of Air in Contact With a Mineral Surface Under Water
Figure 2. Bubble Contact at a Galena Surface Figure 3.
C o n t a c t Angle Measurements on Galena v s . pH a t 32° C
Contact Angle Measurements on Galena vs. pH at 40° C
Contact Angle Measurements on Galena vs. pH at 16 - 18° C
C o n t a c t Angle Measurements on Galena v s . pH a t 50° C
Effect of Temperature on Hydrogen Ion Concentration
Observations at 40
C. Showing Change of Contact
Angle With Bubble Contact Stabilization Time Table 3.
Observations at 50° C. Showing Change of Contact Angle With Bubble Contact Stabilization Time
Decomposition of Potassium Ethyl Xanthate
THE INFLUENCE OF TEMPERATURE ON THE ADSORPTION OF POTASSIUM ETHYL XANTHATE ON GALENA INTRODUCTION In striving to ever-improve flotation practice and thus increase milling recoveries, the mineral dressing engineer has sadly neglected to follow through on one of the fundamental variables, that of temperature. This research project was undertaken in order to add in some measure to the fundamental knowledge of the effect of temperature on adsorption at the liquid-solid interface, with special reference to its influence on the floatability of galena by potassium ethyl xanthate, HISTORY The concept of the use of temperature changes and control in 26 flotation practices is by no means a new one. T. J. Hoover has described In some detail the Bradford, Hyde, Minerals Separation, and Potter-Delprat acid flotation processes. Many of these processes date from about 1900 and all involve the use of elevated temperatures, many up to 70° or 80° C.
texts list heat as one of the important variables to be given serious consideration in any ore testing procedure.26' 65 » Wiard
described a simple qualitative flotation test which in-
volved heating the pulp to 75
Theodora"Simons, former Profes-
sor of Mining at the Mdhtana School of Mines, 3et forth the generally accepted idea cn'*the u3© of tempera'ture effects in the acid flotation circuit when he stated:
"The preferential selection of oil and air for the metallic sulphides may and probably does increase with in(1)
creasing temperature, but a maximum is reached (about 70° C.) above which no further advantage is to be gained." With the introduction of the alkaline flotation circuit, Griswald's use of cyanide as a depressant and new surface active agents (I.e., xanthstes), the Timber Butte mill became one of the chief exponents of selective flotation through use of temperature 1 ft 27 ^V 64 control. -LO' c"' w ' It can be recalled that separation of lead-zinc ores of the Butte district has posed a special milling problem because the frequently present copper ions tended to activate the sphalerite and cause it to float with the lead concentrate. The Timber Butte mill accomplished the separation by producing a bulk lead-zinc concentrate using xanthate, Cleveland Cliff's oil and lime.
The concentrate was thickened to 70 per cent solids,
cyanide and zinc sulfate depressants added, and the pulp heated to 140 F. for one-half hour.
The pulp was then diluted to 50 per cent
solids and a lead concentrate was produced in pneumatic cells. The tailings from the Callow cells were then treated with copper sulfate, Barrett oil and an Aerofloat to produce a zinc concentrate. Over a period of years, several changes in the process were made, but the important features of the process remained essentially unchanged, . Many other mills throughout the country used similar methods for selective flotation of lead-zinc ores, though a lower temperature was found to suffice.
The Anaconda Copper Mining Company used the method and, for a while, dispensed with the heating. 27 But, it was found that without heating the conditioning time became excessively long (about three hours). Present-day Anaconda (2)
practice Involves intentional activation with copper sulfate to produce a bulk lead-zinc concentrate, followed by thickening and conditioning the thickened pulp at about 70° F.
Cyanide and zinc
sulfate depressants are then added and a separation Is made in Callow cells. The float product is the lead concentrate, and the underflow Is further cleaned to produce zinc concentrate. Consolidated Mining and Smelting Company of Canada has also made use of other than ordinary temperatures as an aid to flotation.
In 1931 the following temperatures were used: Lead Rougher Zinc Rougher Lead Cleaner
28° C, 30° C. 30 C.
Zinc Cleaner Lead Recleaner Zinc Recleaner
30° C, 30° C. 35 C.
These were said to have given greatly Improved mill results and were the result of a considerable amount of study.
crease in temperature enabled the use of a relatively high per cent of pulp solids, approximately 37 per cent. The cognizance of the effect of temperature and its control continued to receive considerations in milling operations of Consolidated, While temperature was being utilized in the selective separation of lead-zinc ores, investigations showed that temperature could be used to an advantage In other parts of the Industry, The Grand Central mine at Fairbanks, Arizona used a temperature of 50 C. in a flotation In which sulfidization was practiced. 34 Gaudin and others showed that for fatty acid collection of galenaJ 17 " . . . there is a marked increase in recovery with a given amount of reagent with increase in temperature, particularly in the vicinity of the melting point of the reagent." These studies with fatty acids were also made on the collection (3)
of other minerals, particularly the non-me tallies, and the same generalization with regard to the melting point was found to be true.
Many advantages have been claimed for the use of elevated temperatures in flotation.
Some of these may be enumerated as
Increased fluidity of the water making the oil more soluble, so that a given amount of oil will go farther, 36 » 4 2 It will be recalled that early-day oil flotation utilized many heavy, viscous oils,
Less gangue will rise through the more fluid pulp, and therefore a better grade concentrate could be obtained, 36 » 4 1
Increased number of air bubbles formed by the air released from solution.42' 4 6 A summary of the Taggart-Gaudin bubble attachment controversy is given in Reference 20,
Decreased surface tension and the consequent stability of the froth. 4l> 4 2
Increased rate of reaction between dissolved chemicals and the surface of the minerals, ^®
In the differential lead-zinc flotation, pulp heating increases the effectiveness of the zincinhibiting reagents and therefore brings about a decreased reagent consumption, °
Because of the increased reaction rates, conditioning time may be decreased, with a resulting saving in equipment and plant space. 2 "
Increased recovery. 6 6 Many operating mills report better mineral dressing results during the summer months. 4 6
Change In the solubility between collector and mineral surface, 3 4 which in turn probably influences adsorption at the solid-liquid interface (See section on adsorption),
A saving in reagent cost, as a consequence of statements number 1, 4, 5, 6, 8, and 9,
Greater selectivity of reagents as a result of statements number 1, 5, 6, and 9,
Statement number five, and its corollary, number nine, is of such definite fundamental importance to the study of the solidliquid interface that it deserves special consideration. It was early noted in differential lead-zinc flotation that the heating fi/ and time of conditioning were interchangeable. P7 Wark 4-9 states
as a rough rule of physical chemistry that " . . . the speed of a chemical reaction is increased two-fold for each rise in temperature of 10° C." Some German investigators In the early 1930's believed that:35 "At 40° C. flotation time is only about half that at 6°. With rising temperature the total yield Increases until a maximum is reached and then begins to fall. Each ore and method used has its optimum temperature, generally 23° to 40°, Decrease in yield at higher temperatures is due to increased oxidation of the mineral surface and increased solubility of the compounds formed at its surface,M The concept of mineral surface oxidization with increased temperature appeared to be a popular one with many German mineral dressing scientists at this period.
Their results, which seem reason-
able enough, will have to be accepted at face value, since apparently no other workers studied this phase of the problem, Meyer and Schranz
called attention to the detrimental
effect of mineral surface oxidization resulting from the heating of a flotation pulp. With too great an increase in conditioning time at higher temperatures, a net decrease in the flotabllity of galena was noted, ostensibly due to surface oxidization. Surface oxidization also has advantages in aiding selective flotation. For example, in the separation of copper sulfide-pyrite ores, the pyrlte is more readily oxidized so that by elevating the temperature (5)
a marked difference in ability to float is obtained, A study of the available literature on the effect of temperature on flotation, reveals that many investigators pointed the way, and yet little definite Information is available, especially from a fundamental standpoint. Using the contact angle method for the study of xanthate adsorption on galena surfaces, Wark and Cox concluded that: "Over a comparatively limited range, it has been found that temperature has no effect on the value of the contact angle at a galena surface." The same authors also conducted captive bubble studies to show the Influence of temperature on effect of copper sulfate, alkalies 63 and sodium cyanide on adsorption of xanthate at mineral surfaces. While this was one of the most comprehensive searches for fundamental information in the realm of temperature studies, only ranges of 10° and 35° C. were investigated.
From their work it would be dif-
ficult in many cases to say exactly what effect temperature has on each of the several variables. In an attempt to discover the exact fundamental importance of other than ordinary temperatures on adsorption at the solid-liquid interface, the Mineral Dressing Laboratories of the Montana School of Mines undertook a program of investigation in 1947. This work was done Initially by Mr. Robert E. Baarson, and his results are embodied in a thesis submitted in 1949.
Baarson was assisted in
his investigations during the 1948-49 school year by the author, so that the present study is but a continuation of the original problem. It is particularly appropriate that the Montana School of Mines (6)
undertook such an investigation because the Butte district was the scene of many early developments in the use of elevated temperatures In flotation, RESUME OF ADSORPTION PHENOMENA The literature contains an immense amount of study and conjecture on the adsorption by solids. Adam 32 Langmuir
all present a discussion of the subject and furnish
references for a more comprehensive review. Adam sums up the theory of adsorption by solids as follows: "Gases are adsorbed on solids by Interaction of the unsatisfied fields of force of the surface atoms of the solid, with the fields of force of the molecules striking the solid surface from any gas or liquid, in contact with the solid." He goes on further to explain the existence of two types of adsorp tion, molecular adsorption and chemi-sorption. molecular adsorption is the type wherein van der Waals forces cause the adsorption, and the chemi-sorption occurs when the adsorbed atoms are
held by valence forces to the underlying solid.
It will be noted that the above discussion of the theory of adsorption was developed from studies in the gas-solid phase. Direct application of this knowledge to the liquid-solid phase would probably lead to at least some erroneous results, A diligent search was made in the literature of surface chemistry In an attempt to find if the theory of gas-solid adsorption was applicable to that of the liquid-solid phase. Most authors appeared unwilling to mention this, and the only connection that could be Q
found was a conjecture by Bikerman: "Just as gases are condensed to a more liquid-like state at the interfaces between solids and gases, a (7)
layer of compressed liquid may be expected to exist a t solid-liquid interfaces." For those Interested in the liquid-solid interphase, i t i s unfortunate t h a t much of the work, and particularly that of Langmuir, has been done in the gas-solid phase. Adsorption in the Liquid-Solid Phase The idea of the Guoy-Freundlich double diffuse layer has, if
with some reservations, been generally accepted as correct. However, the mechanism and action of the adsorption process is a subject of much discussion, some of which is quite controversial. One of the first formulations of a guide to adsorption was that of the Paneth-Fajan-Hahn rule 3 1 which states: "Those ions whose compounds with the oppositely charged constituent of the lattice are slightly soluble in the solution In question are well adsorbed by the ionic lattice." Originally Paneth thought that adsorbability increases with decreasing solubility of the compound formed.
This is approximately
true as a general rule, but exceptions occur. Kolthoff's Views of Adsorption:
' has considered
five different types of adsorption: I.
Adsorption of a salt having an ion in common with the lattice; adsorption of potential-determining ions, Kolthoff gives the following example: The precipitate formed
by mixing equivalent amounts of a sliver salt and potassium iodide is found to contain a slight excess of iodide ions while the solution has an excess of silver ions.
The precipitate, which is neg-
atively charged, will be surrounded by an ionic atmosphere of silver and potassium ions, known as a double layer.
On continued washing
all silver and potassium ions are removed and are replaced by (8)
hydrogen ions, called counter or gegen ions.
Adsorption of this
type takes place In accordance with the previously stated PanethFajan-Hahn rule. An example of this would be the adsorption of silver acetate and silver nitrate by silver Iodide. Here the less soluble acetate is the more strongly adsorbed. 3 1
has thus been found to increase with decreasing electrolytic dissociation of the adsorbed compound as well as with its decreasing solubility. II.
Exchange adsorption between ions in the surface and foreign ions from the solution. An example of this very common type of adsorption may be il-
lustrated by shaking barium sulfate with lead chloride, in which case lead ions, but no chloride ions, are removed from solution. BaS0 4
PbS0 4 Surface
In this case, the foreign ion fits in the lattice of the adsorbent and is of about the same size as the replaced Ion. That this is not an absolute qualification is brought out by both Kolthoff
. Wark has illustrated this by noting that
xanthate is adsorbed on such varied surfaces as copper sulfide, lead sulfate, lead carbonate, and lead chromate where the anions are not nearly of the same size as the xanthate, and hence could not fit into the same volume of the crystal lattice. In a general way, this type of adsorption also follows the Paneth-Fajan-Hahn adsorption rule. For a clear interpretation of this kind of adsorption, it would perhaps be best to distinguish it from ion exchange adsorption, which is typified by the watersoftening action of the zeolites.
Exchange between adsorbed counter ions and foreign ions in the solution. Colloidal solutions of the suspensoid type owe their stabil-
ity to a primary adsorption of lattice ions, an equivalent amount of foreign ions of opposite charge being adsorbed as counter ions in the mobile part of the diffuse double layer. An example of exchange adsorption of "gegen" or counter ions is: Agll" ; H+ +
> Agll" ; £pb++ +
These counter ions may then be replaced by other Ions present in the solution.
The exchange ability of an ion increases enormously
with increase In valency according to the Schulze-Hardy rule and this controls the exchange between counter ions of the same electrical sign. IV.
Molecular adsorption of non-electrolytes and true adsorption of salts. Such substances as water and other dipoles, such as alcohol,
can be adsorbed by ionic lattices and may be firmly held. For example, even in high vacuum CaFg holds water adsorbed from the atmosphere. 400° C.
The water layer cannot be removed even by heating to
Instead this reaction takes place: CaFg +
>- CaF(OH) +
It is perhaps possible that ions as well as dipoles may be adsorbed, due to the strong electrical field around the cations and anions In the lattice surface. Much more study needs to be done on this sort of adsorption.
Activated a d s o r p t i o n . Activated adsorption may best be illustrated by showing the
difference between molecular and activated adsorption.
-c-c— OH OH
I I 0 0 H H
I I -c-cI I 0
F F Ca Ca F F Alizarin
Molecular Adsorption Below 400° C.
Ca F + 2HF
Activated Adsorption Above 400° C,
Activated adsorption takes place when the adsorption energy is great enough to overcome the dissociation energy of the adsorbate in which case It will be adsorbed in the ionized form, whereas in molecular adsorption the undissociated molecules are adsorbed.
The temperature required to bring about activated adsorp-
tion, if any, will of course depend upon the nature of the substance in question. Adam
gives a possible explanation of activated adsorption,
if it is possible to directly correlate the gas-solid and liquidsolid phases in this specific case. At low temperatures the smaller activation energy would cause van der Waal's adsorption, while at some higher temperature adsorption takes the form of chemisorption, where the molecules have gained enough energy of Q
activation to combine covalently with the surface. Bikerman Hassler ^
also consider the possibility of adsorption at higher
temperatures being attributed to chemical or activated adsorption. This kind of adsorption might well be of a special significance to the study of the effect of temperature on flotation.
Review of the Theories of Flotation. 13 Dean and Ambrose Detergent Theory: Dean and Ambrose have proposed the detergent theory in which they postulated that: " . . . surface-active reagents preserve, enhance, create, and sometimes destroy water-repellent films." Rogers and Sutherland
have demonstrated the inability of
these reagents to destroy water-repellent films. They believe that Dean and Ambrose have attempted to over-simplify the underlying physical chemical principles of adsorption which does not require a special theory, Taggart!s Chemical Theory:
postulated that: "All dissolved reagents which, in flotation pulps, either by action on the to-be-floated or on the notto-be-floated particles affect their flotability, function by reason of chemical reactions of wellrecognized types between the reagent and the particles affected." This applies, according to Taggart, to activators and depressants as well as to collectors.
It will be noted that this says essen-
tially the same thing as the Paneth-Fajan-Hahn rule. A very lively discussion between Taggart and Wark has centered around this theory.
For a presentation of the various arguments,
the reader is referred to the literature cited. Theory Proposed by Wark and His Co-Workera:
is no definite statement of this theory, but rather an outline of general principles dealing with collection, depression and activation.
Th© reader is urged to consult the reference cited for a
statement of these views. Wark and Cox
have stated that the Paneth-Fajan-Hahn rule
approximates the adsorbability of a sulfur containing collector at (12)
a mineral surface.
They do believe, however, that extension of
the theory to other flotation collectors, activators, and depressants leads to erroneous results, since frequently no compound between collector and mineral ions is known to exist. Here again, the reader is referred to the literature for a complete discussion. Recent Work by Gaudin on Adsorption:
has shown the
lack of a complete monomolecular layer of collector film on mineral surfaces, using the BET nitrogen adsorption method of surface measurement. More recent work at Massachusetts Institute of Tech21 22 nology,
using radioactive tracers to study the adsorption of
dodecylamine on quartz, Indicates that high recoveries may be achieved with as little as five per cent of the total surface area covered by the collector.
It also appears that the attachment of
the amine to quartz is a reversible adsorption phenomenon where the Insecurely anchored amine chains are constantly leaving and arriving at the surface.
If this be the case, It would seem likely that
a similar situation would hold true for many other flotation collecting agents. Significance of the Contact Angle. *•
When the three phases, water, air, mineral, come in contact a condition of equilibrium will be established when the following condition, is fulfilled: T
where T ^ , T^
and T A W are the surface tensions, measured in dynes
per cm., at the interfaces of the air-mineral, mineral-water and water-air respectively.
This may be seen from the accompanying
drawing, and provided that the solid phase is rigid. (13)
Figure 1. Equilibrium Relationships For a Bubble Of Air In Contact With A Mineral Surface Under Water. To further aid understanding of the basic formula, surface energies can be substituted for the surface tensions, for it is as Wark says: 50 "It Is shown In text-books of physics that these values are numerically 2 equal to the surface energies, measured in ergs per cm. " Dupre developed an equation for the work of adhesion: W
E MW ~ T MA + % A ~ T MW 9» ^ 2 ) However, this was developed for the spreading of oil on a surface
and not for an air bubble on the surface. Thus, it is necessary to modify the Dupre equation to fit the flotation application where it is necessary to destroy equal areas of water-air and mineral-liquid interfaces to create the air-mineral Interface. Therefore, W
Substituting for the value of T ^ from equation (1) W
T M (l-Cos 6)
This is a measure of the work that must be done to destroy the airmineral contact.
The equation in this form is more understandable
than is equation (1), since the less comprehendable 1 ^ and Tj^ are removed and only the measureable quantity T % . remains. (14)
has shown that if particles are tested in pure
water, the value of xV* remains constant, and if equal surface areas are compared, then the work W^j. becomes directly proportional to tJtio quantity (1-Cos 6).
Thus, for tests conducted under
uniform conditions, the contacu angle is an indication of the ability to create an air-mineral contact. The close correlation between the contact angle and wetting, EC
adhesion, adsorption and flotation have been shown,
has called attention to the fact that the contact angle should be independent of any factors other than pressure and temperature. As will be noted in a later section, great care was taken in this study to minimize the effect of pressure.
The investigation of
the effect of temperature is intimately concerned with its effect on the contact angle, and thus on the adsorption of a collector at a mineral surface. EXPERIMENTAL PROCEDURE Contact Angle Apparatus. The contact angle apparatus used in the Mineral Dressing Laboratories of the Montana School of Mines uses the same general arrangement as reported by previous investigators, with several modifications. by Baarson,
The original apparatus, as constructed
has the added features of a constant-temperature
jacket, a constant-bubble manometer, and provision for insertion of the specimen,mineral side down. For a complete discussion of the apparatus, its advantages and modifications, the reader is referred to Baarson's thesis. Modifications: A number of modifications of the original (15)
apparatus were necessary.
Plate I shows a picture of the present
apparatus, which may be contrasted with the photograph of Baarson's original equipment. 1.
The following changes were made;
The pyrex storage tank was mounted on a wall bracket sep-
arate from the apparatus bench to prevent vibrations from the agitating unit being transmitted to the contact angle apparatus. 2.
A drain spout was inserted in the lower part of the
constant-temperature jacket to facilitate the removal of the heating solution prior to the removal of the pyrex cell.
This was es-
pecially advantageous when changing from the cell used to check for specimen cleanliness (no contact) to the cell containing the surface active agent. 3.
While work was in progress on the 40° C. curve, it was
deemed advantageous to replace the monocular body tube and metellographic camera with the more convenient Leitz 35 mm. camera and Micro-Ibso attachment. This new addition has several decided advantages over the old apparatus. It offers a more convenient way of taking and developing pictures, since 20 or more pictures may be taken on a roll of film, which is sharply contrasted with the use of sheet film in the metallographic camera. Secondly, it allows the exposure time to be sharply reduced from 20 seconds to l/5 of a second.
This assures that the picture taken is essenti-
ally the same as that viewed in the Ibso attachment side-arm. Perhaps the most important advantage of the new photographic apparatus is that it greatly reduces the vibrations transmitted to the cell.
It is essential that no vibrations are allowed to dis-
rupt the bubbles which often cling to the mineral surface rather feebly in the so-called transition zones. With the former apparatus (16)
Plate I A - Photograph of Former Apparatus
Plate I B - Photograph of Present Apparatus Showing Modifications (17)
it was frequently impossible to obtain a bubble picture because the shutter vibration would be sufficient to destroy the bubble. The Leica camera and Micro-Ibso attachment were made available through use of funds provided by the philanthropy of the Research Corporation, and thus materially aided this research undertaking. In addition to the above changes, the following modifications of the initial apparatus were necessitated by the higher temperature work: 1.
A new heating bath was obtained so that temperatures
above the range used by Baarson (32° C.) could be realized.
unit was a Fisher Unitized Constant Temperature Bath which included a 500 and 750 watt heating element, an electric stirrer, and a vapor pressure thermostat to control the temperature within a fraction of a degree. The former heating bath element was used in the sump tank to enable the elevated temperature to be more easily achieved. 2.
A pre-conditioning bath was inserted in the heating solu-
tion line just following the constant-temperature jacket. It was found necessary to pre-condition both the sample and solution prior to making measurements.
Introduction of the bath following the
constant-temperature jacket allowed pre-conditioning at a temperature within one degree of that temperature being investigated. The bath was constructed of l/64 inch copper sheet and is 4^ inches square in top cross-sec tion and 6 inches deep. Warm solution enters at the top, while the cooler solution is removed from the bottom of the bath.
The bath is of sufficient size to conven-
iently hold two pyrex cells, (18)
Contact Angle Measurement Procedure. Since this work is a continuation of Baarson's investigation, a complete presentation of the fundamentals may be found in that work.
(The reader Is referred to that work for a basic understand-
ing of the procedure used.)
The following presentation is con-
cerned chiefly with modifications and additions to the technique. Specimen Preparation and Handling:
The specimens are mounted
in leucite and polished in the manner described by Baarson. Even though cubic galena was used, every attempt was made to mount the specimens as closely parallel to the crystal face as possible, though a few degrees variance probably resulted. Mounting the specimens in this manner was done so that the effect of preferential adsorption on different crystal faces, as noted by Adam, could be minimized. After polishing, a fast-running stream of cold water was played upon the mineral surface. Baarson's premise, that this had the effect of cooling the mineral, has some basis in fact since polishing occurs only If the melting point of the polisher Is higher than that of the substance being polished.
However, it is believed by
the present writer that the chief value of the water stream is to remove the nuisance factor caused by the thin film of tin oxide (SnOg) that forms over the mineral surface. Even with a thin visible layer of the polishing agent, the specimen passed the cleanliness tests, i.e., it was water-wetted and showed no contact angle when tested in the bubble apparatus. In testing a mineral covered with the stannic oxide layer in the xanthate solution, no contact angle could be achieved until this film had been removed. This was done by forcing a bubble against the mineral surface and (19)
subsequently removing the bubble. Where the bubble had encountered
the mineral surface, a definite film removal could be detected,
and a new bubble placed on the surface assumed the normal contact
angle. Solution Preparation:
In accordance with previous investiga-
tions by Wark and Cox, 6 5 a xanthate solution containing 25 mg. per liter was selected.
In the preliminary tests considerable dif!
ficulty was encountered because the elevated temperature caused air to be ejected from the solution and this air would precipitate in small bubbles on the leucite and mineral surface. To partially
alleviate this condition, freshly boiled distilled water was used
to prepare the solutions.
One liter of 25 mg. per liter xanthate
solution was freshly prepared every other day using Eastman Kodak
quality potassium ethyl xanthate. Aliquot 250 cc. portions of the original solution were mixed with appropriate amounts of 0.1 N
hydrochloric acid or 0.1 N sodium hydroxide to give the desired pH, To insure uniformity of composition and to minimize solution of
air all solution mixing was done by allowing sufficient time to elapse (about 12 hours) for diffusion, with occasional gentle
Laboratory technique and experience justified this pro-
cedure. All glassware was kept clean by use of standard laboratory dichromate cleaning solution.
To prevent contamination, this
solution was used only once.
! •I P ' >1
Pre-Conditioning of Specimens and Solutions:
of the polishing, the specimens were given a preliminary check for cleanliness by observing their ability to be water-wetted.
they passed this test, they were inserted in a cell containing
air-free distilled water and brought up to the proper temperature In the pre-cunui uioning bath.
The specimens then were fin-
ally checked for surface cleanliness using the bubble test. If they showed no contact, the mineral surface was assumed to be free from contamination and the specimens were deemed satisfactory for test work. With the introduction of the 40° C. temperature, air precipitation on lucite and specimen became a major difficulty even with the precautions taken.
This difficulty was overcome to a consid-
erable extent by inserting a lucite strip in the cell containing the xanthate which was being pre-conditioned in the bath. The lucite strip was removed periodically and the attached air bubbles wiped off.
For solutions at or about pH 7, the solution condition-
ing also served the double purpose of stabilizing the pH of the solution. Bubble Considerations:
Throughout the investigational work,
efforts were made to critically examine technique previously developed and to evaluate the apparatus utilized.
the technique did not require any major changes. Bubble Tube Dimensions:
It will be noted that in the previous
investigation the bubble tube was made roughly four-tenths of a millimeter in diameter. For the present study, every effort was made to hold more exact dimensions.
This was achieved through use
of a binocular microscope calibrated eyepiece. A very fine metallographic sand paper was used to reduce the bubble tube tip to the desired size.
This control was used to eliminate a possible varla-
tion in the test work, even though Wark and Cox small size variations are important. (21)
do not believe
Several methods were used to assure
that the bubble was in equilibrium.
Tapping of the cell, as men-
tioned by other investigators, 5 7 helped somewhat, but it had to be done very gently or the bubble would "spread" or be lost completely.
Tapping could not be used at all in certain pH ranges,
subsequently known as transition zones, because the slightest vibration would cause the bubble to be lost from the mineral surface.
This, incidentally, was one of the major factors contrib-
uting to the use of the Micro-lbso attachment to replace the monocular body tube. Baarson1s more accurate methods of attaining equilibrium are the circular highlight method and the test wherein slight changes in pressure achieve gentle back-and-forth vibrations. These latter two tests, coupled with the operator's visual appreciation of a bubble in near equilibrium, enabled satisfactory pictures to be taken. Magnitude of the Contact Angle: As might be expected, it was necessary to somewhat modify the technique used with the Microlbso attachment from that used with the monocular body tube. While the monocular body tube was still in use, it was found that closer bubble equilibrium conditions could be achieved by use of a calibrated micrometer eyepiece inserted in the sidearm. This scale, when used in conjunction with a chart showing average dimension ratios between bubble diameter, bubble base, and bubble height of numerous well-formed bubbles, aided materially in accomplishing satisfactory bubble equilibrium results. This is attested by the fact that the data for 40° C , presented in Figure 3, shows a closer grouping than here-to-fore obtained at 16° - 18° C. or 32° C. With the inclusion of the Micro-lbso attachment, the use of (22)
the calibrated eyepiece was not feasible, so the other methods of checking for bubble equilibrium became of even greater importance.
The operator's judgement of a well-formed bubble became
increasingly critical, and it was necessary to study pictures of numerous bubbles in equilibrium. While the data obtained by use of the two different picturetaking devices differs somewhat, it will be noticed In the nearneutral pH range of the 40° C. and 50° C. curves that the contact angle average with the former apparatus is roughly 62°, while that of the current apparatus is about 58°. These values fall within 55 the previously reported value of 60 ± 2° by Wark. Since Vnerk did not make an intensive study of the exact value of the angle of contact, but rather was interested primarily in whether or not good contact was achieved, the experimental results obtained herein were thought sufficiently satisfactory. Pictures taken by the two different methods are included below for comparison purposes.
Comparison may also be made be-
tween the accompanying pictures and those on page 30 of Baarson1s thesis
or those of Wark
Taken With Monocular Body Tube Figure 2.
or del Giudice.15
Taken miith Micro-lbso
Bubble Contact At A Galena Surface.
Upper half of photograph is a reflection from the mineral surface. "Skewing" of the Bubble:
During the course of this investi-
gation, there were discovered at least two causes for the previously reported bubble "skewing".
In all probability, many of the
causes of "skewing" reported by Baarson were due to misalignment of the monocular body tube.
If the monocular body tube does not
move back exactly in alignment with the light source, an oblique angle is made with the bubble, and thus the bubble appears to become distorted.
This source of bubble asymmetry was eliminated
with the Installation of the Micro-lbso apparatus. Another source of bubble "skewing" or lop-sidedness is found to exist in the transitional zones, where the bubble contact angle became less than its normal value. Here the bubble did not cling tenaciously to the mineral surface and so could readily become misshapen under the slightest vibrational or pressure change conditions.
Reasons for this will be conjectured upon in the discussion
of the experimental data. pH Measurement:
All pH measurements for this investigation
were made with a Beckman industrial Model "M" Hydrogen-Ion Electrometer.
A special glass electrode was used for readings at other
than room temperatures. Results were checked before and after each test using a LaMotte Roulette Comparator. When recording pH measurements at higher than room temperatures, it was found advantageous to pre-condition the electrodes In a solution of approximately the same pH and temperature as that of the solution to be tested.
This reduced ''drifting" of the meter
needle and increased the rapidity with which readings could be made. (24)
A study was made on the effect of 40° C. temperatures on the pH of the s o l u t i o n s .
The r e s u l t s of t h i s experiment are tabulated
below: Table 1.
Effect of Temperature on Hydrogen Ion Concentration pH pH pH Temperature Solution A Solution B Solution C Time 0 Min. 30 60 90 120
20° C 40 40 40 40 20*
4.15 4.10 4.10 4.05 4.05 4.15*
6.40 6.90 7.00 7.10 7.20 7.10*
9.25 9.00 8.85 8.60 8.40 8.15*
* Final solution measurement conducted at room temperature. Test "A" in Table 1 shows that for acidic solutions the pH at the beginning and at the finish of the test remains the same. Slight changes in the solution pH after a conditioning period at an elevated temperature are probably due to the increased activity of ions at that temperature. Much difficulty in maintaining pH stability was encountered with solution "B" andother similar solutions near neutrality.
is conjectured that the abrupt initial change in the pH of solution "B" was due to the elimination of adsorbed carbon dioxide. The fact that the final pH was close to that of the solution during the test seems to give weight to this supposition.
It must be re-
membered that near neutrality a very small change in the hydrogen ion concentration will give a marked change in the pH reading. It is indeed fortunate that the pH of a solution "B" remains fairly constant during the test. The alkaline solutions, especially those having an initial value of pH 8 and above, exhibit very erratic behavior when heatect. In all cases, there is a downward revision of the solution pH, and this change may or may not be abrupt Initially.
The cause of this
is probably intimately tied in with xanthate deterioration or molecular rearrangement. Because the pH in the alkaline range would often change several tenths of a pH unit during a test, it was often difficult to assign an exact pH value to the test. In this respect continuous recording pH instrumentation would be of great value. The alkaline pH readings are further complicated because quite frequently' the pH meter response was slow in this range. pH readings of solutions at 50° C. showed the same general features as those exhibited by the solutions at 40° C. Summary of Measurement Procedure:
The mineral specimen was
polished with stannic oxide on a polishing lap covered with canton flannel, using an ample amount of distilled water.
It was washed
In a fast-running stream of cool water, and a check was made for water adherence.
If It passed this cleanliness test, the sample
was then placed in distilled water and brought up to the proper test temperature in the pre-conditioning bath. When the proper temperature had been attained, the mineral was transferred to the contact angle apparatus and a bubble placed on its surface.
The bubble showed no contact if the mineral was
free from surface contamination.
Upon completion of this final
cleanliness test, the specimen was returned to the pre-conditioning bath. In the meantime, the xanthate solution had been pre-conditioned in the bath and most of the soluble air removed by use of the lucite strip.
The cell containing the xanthate solution was then placed
in the constant-temperature jacket and when the appropriate temperature had been achieved, the mineral specimen was transferred to the cell.
A short conditioning period was allowed to assure that (26)
the proper collector coating had forned on the surface. A bubble was then placed on the mineral surface and given the various tests to assure equilibrium had been attained, following which the stabilization period was begun.
This bubble sta-
bilization period (to be discussed in the data section) was found necessary to enable the bubble to fully attain its normal contact angle. During the stabilization period, it was found necessary to make frequent changes in the bubble pressure so that the conditions of equilibrium could be maintained. Upon completion of the stabilization period, the bubble picture was taken. The pictures were developed and the negatives projected with an Omega photographic enlarger so that the contact angle could be accurately measured, pH measurements were made on the solution at the beginning and at the end of each test. EXPERIMENTAL DATA The experimental data Is interpreted on the basis of physical and chemical principles, though it is recognized that it is not altogether possible to directly correlate the known with the unknown.
Throughout the investigation each phenomemon observed and
thought to be new was carefully evaluated to the extent permitted by instrumentation used. the discussion.
Appraisal of these factors is a part of
Essentially then an attempt Is made to appraise
the experimental data and from this appraisal approximate the influence of elevated temperatures on the adsorption of potassium ethyl xanthate at a galena surface. Factors Influencing Data. In addition to the other previously mentioned factors
influencing the data, the effect of the bubble stabilization period and the effect of xanthate deterioration are of sufficient importance to be given mention. Bubble Stabilization:
Wark and Cox
development of the contact angle.
have noted a rate of
They observed that at 10 min-
utes the bubble closely approached equilibrium, but that 30 minutes was perhaps the average time needed to achieve the non-changing contact angle.
Thus, the necessity for bubble stabilization stud-
ies became of prime Importance before measurements were made at a given temperature.
The results of these stabilization studies,
conducted at a nearly neutral pH, are shown in Table 2. Table 2.
Observations at 40° C. Showing Change of Contact Angle With Bubble Contact Stabilization Time. Observation Series 1
Observation Series 2
Stabilization Contact Angle Time - Minutes Degrees
Stabilization Contact Angle Degrees Time - Minutes
Accordingly, 20 minutes was selected as the stabilization time for 40° C.
The results obtained with a 20-minute stabiliza-
tion period corresponded favorably with the contact angle value of approximately 63 degrees, found in the neutral pH range from previous studies with this bubble apparatus. Since there was no apparent need, no further bubble stabilization studies were conducted at 40° C. when the Micro-lbso attachment had been installed. (28)
Bubble stabilization studies conducted at 50 C. are summarized in Table 3.
Here again, it is seen that a 20 minute stabili-
zation period is sufficient.
The pH for both observation series
was nearly seven. Table 3.
Observations at 50° C. Showing Change of Contact Angle With Bubble Contact Stabilization Time, (Using Micro-lbso Attachment) Observation Series 2
Observation Series 1 Stabilization Contact Angle Time - Minutes Degrees
Stabilization Time - Minutes
Contact Angle Degrees
In some instances at both 40° and 50° C. where the bubble did not cling very tenaciously to the mineral surface and the value of Its contact angle was invariably low (i.e.'the transition zone'), the stabilization period occasionally had to be reduced from 20 minutes.
If this was not done, the bubble would readily become
lopsided and be lost from the galena surface. This was not always the case In the transitional zones, but did occur a sufficient number of times to warrant mentionIt was observed that the bubbles would undergo either compression or tension pressure changes during the 20 minutes bubble stabilization period.
Unless proper pressure adjustments were made
with the constant-bubble manometer, the bubble would be lost. Pressure changes were especially critical during the first few minutes of the stabilization period. (29)
When the contact angle value ivas small, it was frequently necessary to keep the bubble under a slight compression in order to enable the bubble to stay on the surface. Because of the slight compression, the recorded value of the contact angle for these small angles (20° - 40°) is, in several cases, perhaps a few degrees greater than It should be. Xanthate Deterioration:
The possibility that higher tempera-
tures would result in xanthate deterioration could not be overlooked.
In an effort to determine exactly how much the xanthate
deteriorates at elevated temperatures, a series of tests were made, and these results are consolidated in Table 4. The study was carried out by Wilbur J. Guay, Graduate Assistant in Mineral Dressing, using the Iodine Titration method of A. B. Cox 1 2 and Taylor and Knoll.
The titrations are based on the reaction of xanthate with iodine to form dixanthogen, and It is further assumed that this is the only iodine consuming reaction.
However, it is a distinct pos-
sibility that alcohol, one of the decomposition products of xanthate, may also react with Iodine.
While some doubt may exist re-
garding the formation of an alcohol at certain alkaline pH values, If it does form, the possibility of the Iodoform Reaction consuming iodine becomes feasible: CgHgOH +- 4Ig f 6K0H
* 5KI + 5Hg0 +• HCOOK + HCI 3
Other possibilities of iodine consumption are reactions with such reducing substances as hydrogen sulfide or the SO3" ion. The formation of both of these substances as the result of xanthate deterioration Is likely.
Any iodine consuming reaction, other than the
basic dixanthogen forming reaction, would lead to faulty xanthate (30)
Decomposition of Potassium Ethyl Xanthate at pH 3.9**
20° C 50 50 50 50 50
0 Min. 0 10 30 60 120
14.6 cc. 14.2 14.2 13.8 13.4 13.4
Decompo si tion 8.1 % * 10.7 10.7 13.2 15.7 15.7
Decomposition of Potassium Ethyl Xanthate at pH 3.8**
90° C 90 90 90
30 Min. 60 90 120
11.9 cc. 10.4 9.9 9.6
25.1 $ 34.6 37.7 39.6
Decomposition of Potassium Ethyl Xanthate at pH 7.0**
Temperature 20° C. 90 90 90 90
0 Min. 0 40 60 120
15.9 cc. 14.6 13.2 12.5 9.6
0.0 % 8.1 17.0 20.1 39.6
Decomposition of Potassium Ethyl Xanthate at pH 10.1**
90° C 90 90
15 Min. 60 120
14.9 cc. 14.3 13.0
6.3$ 10.6 18.2
N. B. Decomposition due to acidic solution, Initial pH of the solution. (31)
deterioration data, and this must be kept in mind when evaluating the data of Table 4. An evaluation of the tests compiled in Table 4 can serve only as a very weak first approximation, since much more experimental work needs to be conducted.
It is interesting to note, however,
that there is nearly 40 per cent xanthate "decomposition" at 90° C. in the neutral and acidic pH ranges, whereas only 18 per cent "decomposition" is shown in the alkaline range.
If a reaction, such
as the iodoform reaction, takes place In this basic xanthate solution, It would most certainly require additional titrating iodine and thus indicate the presence of more remaining xanthate than is o actually present. At 50 C. only 16 per cent of the xanthate Is "decomposed" even in acid solution, which is favorable to xanthate deterioration. Mechanism of Xanthate Deterioration:
Several reactions have
been proposed for the decomposition of xanthic acid and its alkaline salts. Perhaps the most popular is that attributed to Holban Art
who further qualified the reaction by stating that
the temperature must be above 5° C. and the pH below 5. CgH50CSS""+ H + — * CSg + C 2 H 5 0H
The products CS g and CgHgOH have been identified, and it is definitely known that the decomposition is catalyzed by hydrogen ions. 25
Wark and Cox 5 9 say that: "Acids rapidly destroy (bubble) contact In xanthate solutions, provided the pH value Is approaching unity."
Their tests indicated that xanthate Is easily destroyed by N/lO acid and that later addition of alkali is unable to regenerate the xanthate. (32)
In studying the ripening mechanism of viscose, W. Klaudltz
was lead to believe that in aqueous solution the following hydrolysis takes place slowly: 6C HgOCSSK + 3H20 — - 2KgCS 3 + KgC0 3 + 6CgHgOH + 3CSg
and in strong alkali solutions the following is the guiding reaction: CgHgOCSSK + 5K0H — * 2KgS -+• KgCOg + CgHgOH + 2H20
Klauditz identified the products K g S and K ? CS 3 and was lead to the conclusion that the older interpretations of alkaline decomposition of xanthate are incorrect. As the alkaline solutions used in the current investigation were only weakly alkaline (pH 8 - 9.5), it would be difficult to judge the applicability of the latter reaction. Ragg 4 ^ has indicated that xanthates hydrolyze to carbonates on boiling with water, especially in the presence of hydroxyl ion. This may be explained on the basis of reactions (2) and (3), or it may be due to the hydrolysis of carbon disulfide: CSg+- 40H" — * COg" + 2HS" + HgO
A presentation of a possible reaction mechanism for the formation of the carbonate may be given as follows:
R-O-C-S-Na ZZZ R-S-C-S-Na
0 n Na-S/-C-S-Na Na/OH
0 n HOH -f- Na-O-C-^S-Na
•*• NagS + H^O-C-S-Na
0 n -*• NagS + Na-0-C-OH
The bicarbonate formed may be further hydrolyzed to the carbonate. It is recognized, of course, that NaSH may also be formed in several of the above intermediate reactions.
The fact that the
smell of hydrogen sulfide may be detected in a decomposing xanthate solution lends some weight to the proposed reaction. The various possible xanthate decomposition reactions shed some light upon the action of various xanthate solutions used In the experimental work.
It has been previously observed that there
was a definite lowering of the alkaline pH values in contradistinction to the near constant value obtained in acid solution.
downward shift in alkalinity might be explained on the basis that the sodium hydroxide pH regulating agent was used in xanthate saponification, and the weaker carbonate ions were made available in the solution. The same sort of mechanism might also explain the pH change near neutrality, since only a slight change in the hydroxyl ion concentration is needed to effect a sizeable pH change. Presentation of Results and Discussion. Data for 16° - 18° C. and 32° C:
The results of Baarson's
previous work are summarized in figures 3 and 4. Both of these sets of data have been statistically analyzed by the method of least squares. For the curve at 16
- 18° C , the resulting par-
abola has a sigma (tr) value of ±7.06 and an epsilon (£) value of ±4.76.
The data obtained at 32° C. yielded another parabolic
curve, though somewhat different in shape than the first, whose sigma quantity is±4.94 and whose epsilon quantity is ±3.33.
statistical analysis was made by Dr. Adam J. Smith of the Mathematics Department, Montana School of Mines. (34)
__ .. _
' ' 0 ' 0 0
50 _l CD Z «
O g H Z
25 MG PER L I T E R
PH FIGURE 3 CONTACT ANGLE MEASUREMENTS ON GALENA VS
ft rf o
< ui **
l-s ( J UJ
20 OBSERVATIONS AT 32°C 10
Mb . r t K
LI 1 t K
Kb 1 A l
FIGURE 4 CONTACT ANGLE MEASUREMENTS
It was previously thought that the lower sigma and epsilon quantities obtained at 32° C. were due to a more accurate temperature control. 6
While this may well be a factor, it is now be-
lieved that perhaps the foremost factor in lowering the sigma and epsilon values is an improved technique resulting from a greater familiarity with the bubble machine. There is a close agreement of the two original curves, figures 3 and 4, with the upper critical (alkaline) pH value previ53 ously obtained by another investigator.
Using the captive bub-
ble method, Wark found that at room temperature galena would not float above a value of pH 10.4 In a solution containing 25 mg. per liter of potassium ethyl xanthate.
Figure 3 shows there is a low
value of contact at pH 10.3 and no contact at 10.5. Similarly, the ucper critical pH value for 35° C. Is 9.7, which compares favorably with the value of about 9.6 at 32 C , shown in figure 4. In the light of subsequent results, the data obtained by oaarson and shown in these two curves will be critically discussed, since this discussion has the advantage of hindsight, no reflection is inferred upon that original work, while note was previously made of the so-called transition zones or zones of "difficulty of contact , nothing else was said in regard to them.
Was it that the rate of contact phenomenon was
absent, or was it not recognized?
This question is difficult to
answer on the basis of the previously collected data, but the phenomenon was recognized at the higher temperatures. The scattering of data, especially at 16° - 18° C , is perhaps a little great, e.g. contact angles in the neighborhood of 70° and 50°.
The author believes that with the old apparatus it was very (37)
easy to obtain other than the normal contact angle because the bubble was not in perfect equilibrium.
This is particularly true
for the higher values where a condition of tension exists. Considerable doubt may also be cast upon the high contact values obtained at 32° C. for pH 7.
That the argument has at least some
basis in fact may be noticed from the data presented for 40° and 50° C. which does not show a contact angle value greater than 66 , and even these are probably excessive. Data for 40° C.:
Attention has already been calleo. to the
circumstance that the data average using the former apparatus
somewhat higher than the average readings obtained with the modified instrumentation. A comparison of the two may be seen from figure 5, though an accurate comparison of results may properly be made only in the near neutral range (pH 6.5 - 7.5) or by comparison with the data presented for 50° C , figure 6, Transition Zones: The existance of several transition zones, or regions of bubble abnormality, were discovered at 40° C.
differed so markedly from the previous studies that discussion of these several regions is necessary to an understanding of the role of temperature in this problem.
There were three localities in
which some abnormality was noticed:
(1) the alkaline range (pH
7.5 - 9.0 ), (2) the acid range (pH 1.7 - 5.0), and (3) the weakly acid range (pH 5.0 - 6.0), The alkaline transition zone presented the most unique of the three regions of bubble anomaly.
The value of the contact angle
began to decrease almost immediately as the hydroxyl ion concentration was increased, and soon it was impossible to achieve contact at all.
Ordinarily contact woald be achieved within 30 seconds or one (381
70 • • •
a ft] „ 0
o c o oo
o n-tt —