Surface Chemistry. Proceedings of Second Scandinavian Symposium on Surface Activity, Stockholm, November 18–19, 1964

310 Pages·1965·7.911 MB·English

by  Per Ekwall, Kjell Groth and Vera Runnström-Reio (Eds.)

Surface Chemistry
Proceedings of the Second Scandinavian Symposium
on Surface Activity
Stockholm, November 18-19, 1964
Sponsored by
The Royal Swedish Academy of Engineering Sciences
Editors
PER EKWALL KJELL GROTH
VERA RUNNSTRÖM-REIO
1965
ACADEMIC PRESS
NEW YORK and LONDON
ACADEMIC PRESS INC.
Ill FIFTH AVENUE
NEW YORK, NEW YORK 10003
Published simultaneously in the USA by
Munksgaard, Copenhagen
© 1965 by
Munksgaard, Copenhagen, Denmark
All rights reserved
No part of this book may be reproduced in any form,
by photostat, microfilm, or any other means, without written
permission from the publishers
Library of Congress Catalog Card Number: 65-28458
Planned and produced by Munksgaard, Copenhagen
PRINTED IN DENMARK BY
VALD. PEDERSENS BOGTRYKKERI
COPENHAGEN
PREFACE
Surface chemistry has important applications in many problems of
both fundamental and applied research. The Royal Swedish Academy
of Engineering Sciences has attempted to promote this branch of chem-
istry in Sweden by organizing a laboratory for surface chemistry. This
laboratory will serve as a basis for fundamental research and for appli-
cations in industrial and other fields. It is necessary, however, that
close contacts are established between surface chemists in different
spheres. Already at the First Scandinavian Symposium on Surface Ac-
tivity in Âbo, 1962, it was proposed that a symposium on surface chem-
istry should be hold in Stockholm about two years later. The Academy
and its Committee for Surface Chemistry were therefore glad to have
the opportunity to arrange the Second Scandinavian Symposium on
Surface Activity on the 18th and 19th November, 1964. The number of
papers was so large that it was difficult to find place for them in the
two days, and the discussions were so lively that they seriously dis-
turbed the time schedule.
We hope that the publication of these proceedings will further stim-
ulate the interest in surface chemistry and will show still more chem-
ists the advantages of using the methods of surface chemistry, so that
there will be a still larger number of participants at the third sympo-
sium, which will probably be arranged in Denmark.
SVEN BROHULT PER EKWALL
Surface Chemistry, 1965, p. 9-18.
Henry Krumb School of Mines, Stanley-Thompson Laboratory,
Columbia University, New York 27, New York, U.S.A.
OPPOSITELY CHARGED MIXED COLLECTORS IN FLOTATION
JACK H. SCHULMAN
SURFACE ENERGIES
In mineral flotation the most important parameter is the work necessary
to remove the layer of water attached to the surface of the solid to leave
a vapor covered solid. If the surface energy is below that of the co-
hesional energy of the water (liquid), a static deposited air bubble will
adhere to the solid surface, e.g., the surface energy must be below (2γ1)
twice the surface tension of the liquid. The total surface energy E of
s
a solid is related to the surface tension (y ) of the solid and the surface
s
entropy (AS ) and at constant pressure and very small volume change
S
is equal to the enthalpy (ΔΗ) or heat of wetting of the solid in the
presence of a liquid for unit area (ε)
A E s - T s -T (|^) ρ,ε = Ys - TASs
Es/v — Es/1 = Ys/v — Ys/1 — T SYS/V , _ ( SYS/I ΔΗ
8T ; ρ,ε ^ ST
(ys/v equals surface tension of the solid
in the presence of the vapor of the liquid)
The work of adhesion ω (S/L) solid to liquid is given by the equation
Α
ω (SL) = γ - Y -l· Yi (1 + Cos Θ)
Α 8 s/v
This is related to the contact angle and the spreading pressure (τΓ ) =
β
γ s-ys/v of the vapor on the solid surface and these terms relate to the
chemistry of the solid surface and to the liquid.
Now whenever a solid it treated to grinding or oppositely charged
ionic surfactants are added together and precipitated in aqueous solu-
tions a considerable increase in surface entropy is obtained for each
system together with a change in total surface energy as compared to
the crystalline state of the materials. When the particles are placed in
10
an aqueous environment the energy is dissipated by an increase of the
heats of wetting and diminution of the surface area by coalescing or
adhesion of the amorphous particles to give smaller interfacial area, and
the entropy term is overcome by lattice energy forces. Both the surface
tension of the solid and its surface entropy can be shown experimentally
to be very important factors in many processes besides flotation, such
as adhesion and membrane permeability of particles, silicosis, in sinter-
ing, and in powder metallurgy and coalescence of particles in general.
Papers on some of the other aspects will be presented for publication
elsewhere, but this paper deals with influence of the surface tension
of the solids and the structures of disordered precipitates of the collector
agents in aqueous environment on flotation.
The surface tension and surface entropy terms in flotation, both of
the adsorbent and adsorbate, have been neglected ; but obviously become
very important especially with fines. The surface tension of the liquid
phase at melting point can be determined in various ways and has been
well given with many examples in the Russian symposium on the role
of surface phenomena in metallurgy (4). The surface tension of the
solid is related to the atomic number of the element ( 11 ). Some approxi-
mate values are given for the following metals in dyn/cm Pb 450,
Zn 810, Cu 1350, Ni 1770 and Fe 1815. For the sulphides of these metals
it appears that sulphur is extremely surface active in copper (1, 10)
and the following order for them in surface tension can be assessed.
PbS<CuS<ZnS<Si0 <NiS<FeS
2
In this surface tension series silica (350) comes between ZnS and NiS.
ADSORPTION OF XANTHATES ON TO DIFFERENT MINERALS
It has recently been shown by Le ja and Poling (6) and Le ja and Pomia-
nowski (7) by infrared adsorption techniques that xanthates absorb
on to the surface of the above mentioned sulphide mineral series but
flotation only occurs in the CuS and PbS minerals. The explanation
usually given is that dixanthogen is necessary for flotation and that the
necessary oxidation-reduction potential for its formation only occurs
with the lead and copper systems (12) in the presence of oxygen. Thus
the oily dixanthogen would spread on the metal ethyl xanthate mono-
layer on the mineral surface and produce marked contact angles with
captive air bubbles. Dixanthogen does not form with the other metal ion
systems consequently no flotation occurs.
The purpose of this paper is to emphasize the work of Buckenham
and Schulman (3) that chemisorbed xanthate molecules with short
11
alkyl chains do not lower the tension of the mineral surface of the metal
sulphides enough to reduce the wettability (ΔΗ) sufficiently to enable
bubble attachment to take place. In order to decrease the wettability of
the mineral surface sufficiently, either much longer alkyl chains should
be used or the chemisorbed xanthate molecule should be used to sen-
sitize the surface to enable other molecules to absorb by complexing
with the xanthate. These molecules need not necessarily adsorb on the
solid on their own, but if they do, they also, like the xanthate molecules
are not flotation collectors by themselves. A good example of this latter
process is given by ethyl xanthate for silica flotation. If a short chain
alkyl substituted amine is adsorbed on silica below Cs no flotation
occurs, the addition of ethyl xanthate produces instant flotation. This
can be prevented by charging the silica surface positive by adsorption of
Al3+ ions in the presence of weak AICI3 solutions. This prevents the
adsorption of alkyl amine+ ions.
It is further shown that although the two oppositedly charged agents
on their own are only slightly surface active, the 1:1 combination is
extremely surface active and can be used consequently as a wetting
agent for low energy solids, by orientating with their polar groups out
towards the water and the non polar portion adhering to the solid
surface, exactly opposite to that found for the flotation process.
We are now faced with three types of systems in the flotation process
1) solid surface can vary greatly in surface energy with different ele-
ments or their sulphides, oxides, or other chemical compositions either
by surface tension change or surface entropy change (TAS). 2) Flota-
tion reagents can chemisorb individually or by sensitisation one after
the other with dual component systems, and change the surface energy
of the solid or its wetting to water by its arrangement of the non polar
portions of the molecule. 3) Flotation reagents of the dual component
systems which in equimolecular form are insoluble but, when copreci-
pitated form high entropy aggregates which are very water dispersible.
These can grow in size at controllable rates to the insoluble crystalline
state. These disordered or high entropy aggregates spread very actively
on solid or aqueous surfaces, under conditions where the crystalline
states do not.
Thus care must be taken to diffentiate the dual component collector
systems in the way, the single components are added to the aqueous
solution (5) a) either, sensitization agent followed by the complexing
agent, or b) together, in equimolecular combination from aqueous
solutions, or c) as a crystalline adjunct association compound to the
aqueous system. In the case of the xanthates this is further complicated
by the presence of the dixanthogen oil phase.
12
The above are some of the factors involved in the chemical process
of flotation and an understanding of them can be used to give selective
flotation of minerals in general. Preliminary attempts to do this are
described in the following section.
MEASUREMENTS OF ASSOCIATIONS BETWEEN OPPOSITEDLY
CHARGED SURFACTANTS
It has been shown (2) by monolayer penetration studies that associa-
tions exist between alkyl xanthates and alkyl alcohols and cholesterol
(Fig. 1). Mixed monolayers of the latter are more stable against oxida-
tion as measured by changing the substrate to acid pHs when the
xanthate decomposes to an alkyl alcohol. It is interesting that on their
own no monolayers of the xanthates can be found at the air/water
interface since oxidation takes place immediately. The alkyl xanthate-
alkyl amine mixed films are somewhat more stable than with the
alcohol monolayers, especially with the mixed films of the substituted
amines. Surface pressures up to 70 dyn/cm for the 1:1 mixed film
were registered and the mixed film is a strong gel film, see Fig. 2.
This is in spite of the fact that the two substances on their own do not
form insoluble monolayers. Oxidation of the stable mixed film with h
injected into the substrate produced instant collapse and disappearance
of the mixed monolayer. It has recently been shown by Buckenham and
10 20 10 20 30
Minutes Minutes
Fig. i.
(a) The penetration of a cholesterol monolayer, pH 6.8, 20° C. (b) Cetyl alcohol
monolayer penetrated by KLX at pH 6.8. After ten minutes the pH was changed to
2.8, 20° C. (After Bowcott).
13
70
60
50
S«
Φ
|> 30
20
10
30 40 50
A2per molecule
Fig. 2.
Equimolecular mixed monolayer of cetyl trimethyl ammonium iodide and potassium
cetyl xanthate on 0.005 M NaHG0 pH 7.4, 20° G. (After Bowcott).
3
Schulman (3) that associations between oppositedly charged alkyl
xanthate and alkyl substituted amines molecules can be measured with
their soluble short chain homologues. This is done by comparing the
surface tension concentration (γ/c) curves for the single components,
which register negligible surface tensions, to those of mixed solutions.
These latter are made by additions of one reagent solution to the other,
the greatest tension lowering being measured for the 1: 1 equimolecular
concentration. The equimolecular solutions of potassium hexyl xanthate
and octyl trimethyl ammonium iodide lower the surface tension of
water over 50 dyn/cm at 2000 mg/1, see Fig. 3. Calculation of the surface
area of the mixed film by the Gibbs equation gives 48 A 2 this compares
to 55 A2 for the calculated model and 65 A 2 for those found for the
insoluble long chain homologoues in mixed monolayer. The different
association of water molecules with the soluble and insoluble mixed
films could explain the difference (9).
14
400 800 1200 1600
Concentration, mg/I
Fig. 3.
Changes in surface tension of potassium hexyl xanthate (KHX), octyl trimethyl
ammoniumiodide (OTAI) and KHX/OTAI solutions with concentration. Inset:
Changes in surface tension of solutions with increasing KHX/OTAI molar ratios
(OTAI 250 mg per 1). (After Buckenham and Schulman).
FLOTATION OF OPPOSITEDLY CHARGED MIXED
REAGENT SOLUTION (3)
It can be seen in Fig. 4 that flotation of the different minerals by the
mixed reagent solution (300 mg/1) follows an increasing number of
carbons per mixed solution with increasing surface energy of the
minerals as indicated in the surface tension series given in the intro-
duction. Further, it appears that the addition of the number of carbons
in the alkyl chains can be distributed between the two alkyl groups.
There is no need for the two alkyl chains to be equal in length for the
two molecules. Furthermore they can be added singly to the system.
Thus sphalerite can be sensitized by ethyl or propylxanthate, the system
washed and flotation takes place readily with the addition of octyl tri-
methyl ammonium chloride to the aqueous mineral suspension. In this
case this sensitization technique can be reversed when the substituted
15
• p—
100
·-
L·/ /<È
80 h
1° ! m 1
N
L-
60 h-
Ï5
o
Φ 40
l·—
Qz
1 €>
D
20 h 1 /
1 1
4 8 12 16 20
Number of carbon atoms
Fig. k.
Influence of number of carbon atoms per association complex or single component
on flotation recovery. (Note that recoveries for a particular mineral with different
collectors are distiguished by method of plotting.) The number of carbon atoms is
given by the combination in the equimolecular association of single molecules of
the two compounds with varying hydrocarbon chain length. O Sphalerite, Δ Quarts,
□ Pentlandite, ■ Pyrrhotite. (After Buckenham and Schulman).
amine is added first. Similarly flotation systems can be used for pent-
landite and pyrrhotite only the carbon number in this case must add
up to at least 14 and 16 carbons, respectively. The flotation of quartz
is different to the sulphide minerals in that the reagents can be readily
washed off surface. In the former case the reagents are adsorbed by an
ionic exchange process, whereas with the latter the positive substituted
amine ion is precipitated on to the surface by the negatively charged
quartz surface. This process is well demonstrated by the brine flotation
system where alkyl amines effect ionic exchange with a KC1 crystal sur-
face whereas the substituted amine ionic head group is too large to
effect an ionic exchange and cannot float the KC1 crystals (8). Here,
both positively charged alkyl amines can float quartz at the appropriate
chain length. It is interesting that flotation can only occur in those
oppositedly charged mixed alkyl ionic reagents, if they are composed
of salts of a weak acid and strong base and can thus dissociate. If they
are composed of both, strong acids and strong bases, no flotation takes
place. Furthermore if one of the fully ionized compounds is adsorbed
on to a mineral and the oppositedly fully ionized charged alkyl com-
pound is added, the former is deterged from the surface and flotation
ceases. This phenomenon is possibly related to the structure of the