Flotation in concentrated electrolyte solutions
Janusz Laskowski1* and Sergio Castro2
1. N.B. Keevil Institute of Mining Engineering, University of British Columbia, Canada
2. Department of Metallurgical Engineering, University of Concepcion, Chile
DOI 10.1016/j.minpro.2015.09.017
ABSTRACT
In the flotation in highly concentrated electrolyte solutions not only ionic strength but also
chemical composition, flotation pH and mineral properties play an important role. As a result an
improvement or depression may be observed. A strong improvement in the floatability of
hydrophobic bituminous coals and other inherently hydrophobic minerals is characteristic for the
“salt flotation process”, which is carried out in concentrated electrolyte solutions at natural pH and
without addition of any flotation reagents. However, pH is a key factor when saline waters with
hydrolyzing metallic ions are used as a process water. In the case of flotation of Cu-Mo sulfide
ores in seawater, molybdenite is strongly depressed by the Mg2+ hydrolysis products when pH is
raised to depress pyrite. The process waters with a high content of Mg2+ and Ca2+ ions, and this
also includes seawater, have deleterious effect on the anionic flotation of industrial minerals with
fatty acids due to the precipitation of Ca/Mg fatty acid salts. The flotation of potash ores is carried
out in NaCl-KCl saturated brine (6-7 mole/L of NaCl and KCl) with the use of long-chain primary
amines. Such a high electrolyte concentration changes dramatically the amine Krafft point making
the collector insoluble in brine; its mode of action entirely differs from that in conventional
flotation processes.
Keywords: Salt flotation, Seawater, Seawater flotation, Cu-Mo ores flotation, Fatty acids, Longchain amines, Potash ore flotation.
This paper is based on the keynote presented at the 27th International Mineral Processing
Congress, Santiago, October 20-24, 2014.
*Corresponding author:
E-mail address: [email protected]
INTRODUCTION
Mineral processing unit operations are commonly grouped into four distinct clusters:
comminution-classification, separation, product dewatering and water clarification. In the closed
circuits of modern mineral processing plants water after use is recirculated and thus becomes a
concentrated electrolyte solution. Its concentration may be as high as 1 M NaCl (e.g. Mt. Keith
plant in Australia); flotation of potash ores is carried out in NaCl-KCl saturated brine that is at
these salts concentration exceeding6 mole/L (e.g. plants in Saskatchewan in Canada). The use of
seawater in flotation has been extensively studied and it is already used in several plants (e.g. Las
Luces concentrator at Taltal in Chile, Esperanza concentrator at Sierra Gorda in Chile).
Distinct unit operations require the use of different chemical additives. For instance, while
dispersing agents may be needed in grinding, collectors and modifying agents are utilized in
flotation,solid-liquid separation processes (thickening and filtration) commonly require the use of
flocculants. These are high molecular weight polymers and it cannot be expected that the chemical
compounds so different from collectors will respond similarly to an increased ionic strength of the
pulp.
The topics considered in this paper are limited to the flotation and flotation reagents.
Polymeric compounds referred to as flocculants are typical lyophilic colloids and they are quite
different from flotation reagents. Quite different is also the effect of electrolyte concentration on
their properties from the effect of electrolyte concentration on flotation collectors. The effect of
ionic strength on flocculation is dealt with in a different paper (Huang P. et al, 2013].
For the sake of discussion, in the present work the systems in which flotation is carried out
in concentrated electrolyte solutions will be classified into the following groups:
1.
High ionic strength systems;
(a) Without pH adjustment; example: “salt flotation” of inherently
hydrophobic solids, e.g. flotation of a bituminous coal in NaCl solution.
(b) With pH adjustment; example: flotation of Cu-Mo sulfide ores in seawater
when pH is raised to depress pyrite.
2.
The systems in which collector precipitates with ions present in the pulp;
example: anionic flotation of phosphate ores with fatty acids in hard/seawater.
3.
Flotation in saturated brine; example: separation of sylvite (KCl) from halite
(NaCl) inpotash ore flotation.
1. HIGH IONIC STRENGTH SYSTEMS
(a) Salt Flotation (high ionic strength systems without pH adjustment). Flotation of
inherently hydrophobic minerals in concentrated solutions of electrolytes does not require
the use of any other reagents. In Klassen’s publications this is referred to as “salt flotation”
(Klassen and Mokrousow, 1963).
For the flotation to be successful the bubbles:
must be fine enough (the flotation rate constant is proportional to the bubble
surface area flux which increases with decreasing bubble size);
(ii)
and the attachment of the mineral particles when they collide with bubbles must be
possible within the milliseconds of the particle-bubble contact time.
In flotation process the size of bubbles is determined by bubble coalescence that can be
prevented by a frother. Frothers are best characterized by their critical coalescence
concentration (Cho and Laskowski, 2002a, 2002b). As Figure 1a shows, the critical
coalescence concentration of MIBC in water is about 10 p.p.m. At the concentrations
higher than that the bubbles generated in MIBC solutions are stable and do not coalesce.
However, bubble coalescence can also be prevented by increasing electrolyte
concentration. In concentrated electrolyte systems, as shown in Figure 1a, the bubbles are
stable and do not coalesce even in the absence of a frother (Laskowski et al., 2003).
1.6
2.5
Sauter mean bubble diameter, mm
Sauter mean bubble diameter, mm
(i)
Distilled water
50% saturated brine
100% saturated brine
2.0
1.5
1.0
0.5
0.0
0
10
20
30
40
50
MIBC Concentration, ppm
60
0 ppm
2 ppm
4 ppm
6 ppm
8 ppm
10 ppm
15 ppm
30 ppm
50 ppm
100 ppm
1.4
1.2
1.0
0.8
0.6
0
20
40
60
80
100
Seawater, % (v/v)
Figure 1a. Sauter mean bubble diameter as a Figure 1b. Effect of the frother MIBC on
function of MIBC concentration and bubble size in sea water (Castro et al., 2010).
electrolyte concentration (Laskowski et al.,
2003)). The term “brine” used here stands for
saturated solution of KCl +NaCl(about 6
mole/L).
This is further illustrated in Figure 1b. As this figure shows,in distilled water fine bubbles
can be generated only in the presence of a frother. However, the same process when carried out in
seawater leads to the formation of fine bubbles even without any frother (Castro et al. 2010; Castro
et al., 2013). These results then prove that fine bubbles can be generated in a flotation machine
without the use of a frother in concentrated electrolyte solutions.
In the flotation process, mineral particles that are to be floated, are made hydrophobic by
adsorbing collector. Inorganic ions cannot make solid surfaces hydrophobic. The salt flotation then
must depend on initial solid hydrophobicity.Klassen in his Coal Flotation monograph (Klassen,
1962) used Kharlamov’s data to demonstrate that only bituminous coals, the coals which are very
hydrophobic, float well in 0.5 M NaCl solutions. 20 years later this was clearly confirmed (Figure
2). In these tests carried out at the University of California, Berkeley (Fuerstenau et al., 1983)
various coals from different mines were used in the batch flotation experiments and the wettability
of these coal samples werecharacterized by measuring the equilibrium moisture content. As it is
known, while the equilibrium moisture content in bituminous coals is very low (can be lower than
1%), it is much higher for more hydrophilic low rank subbituminous coals and lignites. This was
experimentally confirmed by Fuerstenauat al. (1983) as shown in Figure 2. Salt flotation tests
carried out in 0.5 M solution of NaCl with coals varying in rank clearly demonstrate that the less
hydrophobic sub-bituminous coals (which are characterized by a high moisture content) poorly
float under such conditions while the salt flotation of hydrophobic bituminous coals (which are
characterized by a very low equilibrium moisture content) is very satisfactory.
Figure 2Effect of coal surface wettability (expressed by coal equilibrium moisture content) on
the salt flotation rate in 0.5 M NaCl (Fuerstenau et al., 1983)
As shown by Laskowski and Kitchener (1969), very hydrophobic surfaces (methylated
silica) can carry a substantial negative charge. Existing experimental evidence (Beattie, 2007) is
consistent with the idea that water at the low dielectric surfaces (air, oil, solid hydrocarbons)
acquires a negative charge above pH 3-4 from the preferential adsorption of hydroxide ions.
Because of this charge, the metastable wetting film is formed on such hydrophobic surfaces (Blake
and Kitchener, 1972) and as a result when such particles collide with bubbles the energy barrier
opposes the attachment. This is consistent with the concept of induction time. Since electrical
double layer is compressed when electrolyte concentration increases, in the case of hydrophobic
particles this reduces the energy barrier facilitating the particle-to-bubble attachment (Laskowski
at al. 1991; Laskowski, 2012).Recent calculations based on the DLVO theory reveal that because
of negative electrical charge of coal particles and bubbles there is a significant energy barrier that
opposes the attachment. Raising electrolyte concentration up to 0.24 M eliminates the energy
barrier and promotes attachment of the coal particles to gas bubbles through attractive hydrophobic
interactions (Nguyen et al., 2007).
As Figure 2 confirms, hydrophobic coal particles float very well indeed in concentrated
electrolyte solutions, but since inorganic ions cannot change solid wettability only very
hydrophobic particles can float under such conditions.The conditions existing in the salt flotation
of hydrophobic particles are thus ideal for the flotation of such particles, at high concentration of
electrolyte the energy barrier opposing bubble-to-particle attachment is reduced, and at the same
time coalescence of bubbles is prevented allowing for generation of fine bubbles.
Figure 3.Effect of NaCl concentration on flotation of bituminous coal with 400 g/t of either fuel
oil (A) or kerosene (B). A and B, results after 12 minutes of flotation; A’ and B’, results after 6
minutes of flotation in 0.1 M NaCl; A” and B” , results after 6 minutes of flotation in 0.2 M NaCl;
A’’’ and B’’’ results after 4.5 min of flotation in 0.3 M NaCl, A’’’’ and B’’’’ results after 4.5 min
of flotation in 0.5 M NaCl (Laskowski, 2001).
It is known that bituminous coals are floated very well with oily collectors (Wojcik and
Al-Taweel, 1984). If the salt flotation of the inherently hydrophobic solids indeed results from the
suppression of the double layers and thus the energy barrier then the flotation with the oily
collector which requires attachment of the oil droplets to solid particles should be very sensitive
to the electrolyte concentration. The use of emulsified oil in concentrated electrolyte solutions
should then be very beneficial as increased ionic strength should facilitate attachment of oil
droplets to coal surface. Figure 3 shows how dramatic is an improvement in the flotation of
bituminous coal with kerosene (or fuel oil) at increased concentrationsof NaCl. While at 400 g/t
of oily collector the concentrate yield was only in the 60-70 % range after 12 minutes of flotation,
the yield jumped to 90% after only 6 minutes of flotation in 0.2 M NaCl solution. One of the
characteristic features of the salt flotation is very high rate of this process and Figure 3 is totally
in line with it.
The question that immediately arises is what is the effect of different ions on salt
flotation?Are these effects different? More than 50 years ago Klassen (1962) pointed out that coal
salt flotation is better in sodium sulfate than in sodium chloride solutions, and in turn sodium chloride is
better than sodium nitrate. These differences were explained by different frothabilities of these
systems. Other researches took into consideration other effects, for instance, the effect of ions on water
structure, and full reviews of such different hypotheses are available (e.g. Bo Wang and YongjunPeng,
2014). Such effects are not discussed further in this paper and the reader is referred to the quoted
original contributions.
(b) Flotation of Cu-Mo sulfide ores in seawater. Flotation of Cu-Mo sulfide ores in seawater has
been studied for many years. Successful pilot plant flotation tests on the use of seawater were
reported by Morales in 1975 (Morales, 1975). In the Las Luces plant at Taltal-Chile seawater is in
use for many years (Monardes, 2009). Esperanza (Chile), a large flotation plant with 95,000 t/day
capacity, has been using seawater for the last three years.
As it has been well documented, copper sulfides float in seawater as well as in fresh water. This
is shownin Figures 4 and 5 (Castro, 2012; Castro, Rioseco and Laskowski, 2012c).
Figure 4. Effect of pH on flotation of copper ore containing mostly chalcopyrite in fresh
water and in seawater
Figure 5 Effect of pH on flotation of copper ore containing mostly chalcocite in fresh and
seawater
Cu-Mo sulfide ores usually also contain pyrite which must be depressed. It is commonly achieved
with the use of lime. This is shown in Figure 6.
Figure 6. Effect of pH on recovery in rougher flotation of Cu-Mo sufide ore in fresh water
As these results confirm, while flotation of copper minerals in fresh water and seawater is very
similar, pyrite is depressed with lime when the flotation is carried in alkaline solutions. However
when such an ore is floated in seawater, and lime is used to depress pyrite, molybdenite flotation
is also depressed (Figure 7). The question then arises how to depress pyrite in the flotation of such
sulfide ores in seawater without loosing molybdenum. In order to find the answer to this question
the reasons for molybdenite depression have been studied. Seawater is a solution with about 0.6
M concentration of NaCl but also with a high concentration of Mg2+ ions (about 1,300 ppm) and
Ca2+ (about 400 ppm). It has been shown that over the pH range of 9-10, that is at the pH range
over which magnesium ions hydrolyse to form hydroxy-complexes and magnesium
hydroxide,molybdenite is depressed. This has been discussed in another paper presented at the 27th
Int. Mineral Processing Congress in Satiago(Castro et al., 2014).
Figure 7. Effect of pH, adjusted with lime, on Mo recovery in the flotation Cu-Mo sulfide ore
2. THE SYSTEMS IN WHICH COLLECTORS PRECIPITATE WITH IONS PRESENT
IN THE PULP
Flotation with fatty acids in process waters with high concentration of Ca2+ and Mg2+ions.
At the Kapuskasing Phosphate Operationsin Ontario, Canada, while the ore from their current
operation is relatively easy to upgrade by flotation, the marginal ore, which was stockpiled at the
site for about two years turned out to be extremely difficult to process. The process water was
found to contain more than 1,000 ppm Ca2+. Nanthakumar et al. (2009) showed that by using
soda ash (1,800 g/t) in the desliming circuit it was possible to drastically improve flotation
recovery of P 2 O 5 and concentrate grades. This example illustrates the general problem in the
flotation with anionic collectors (fatty acids) in the process waters at a high content of Ca2+ and
Mg2+. Yousef et al. (2003) showed how to use soda ash and water glass in the flotation of a
phosphate ore in seawater. A high ability of sodium silicate to remove Ca2+/Mg2+ ions from
process water was proven by Li et al. (2005).
3. FLOTATION IN SATURATED BRINE
Flotation of potash ores is carried out with the use of cationic collectors. Onoda and
Fuerstenau (1964) tested the flotation of quartz with dodecylamine in NaCl solutions. They
showed that at low collector concentrations, when collector adsorbs in the form of individual
ions in the electrical double layer at the solid/liquid interface,quartz flotation is depressed in
NaCl solutions. But at high collector concentrations where the collector is strongly adsorbed
through hydrocarbon chain interactions, inorganic ions have little effect on flotation. Because of
the more than ten times lower electrolyte concentration the quartz-amine system studied by
Onoda and Fuerstanu is, however, very different from the potash ore flotation system which will
be discussed next.
Separation of sylvite (KCl) from halite (NaCl) in potash ore flotation.Since two major
components of sylvinite ores, sylvite (KCl) and halite (NaCl), are water-soluble theflotation
process must be carried out in saturated brine (6-7 mole/L). Thus, this electrolyte concentration
is ten times higher than the concentration of seawater.
Figure 8 Effect of concentration of collector (sodium alkyl sulfonates) on flotation of barite:
empty circles, sodium tetradecyl sulfonate (C 14 ); filled circles, sodium dodecyl sulfonate (C 12 );
empty squares, sodium decyl sulfonate (C 10 ). Vertical arrows indicate the critical micelle
concentration for the three studied alkyl-sulfonates (Dobias, 1986)
Figure 9. Relationship between KCl recovery and amine concentration (Roman et al., 1968)
The most fundamental difference between common flotation and potash ore flotation is
best seen when the effect of collector concentration in these two different cases is compared. As
Figure 8indicates, in the case of barite flotation carried out with sodium alkyl sulfonate collectors,
the flotation ceases when collector concentration approaches the critical micelle concentration of
the collector. Around this concentration the micelles appear in the system and since these are
hydrophilic species the flotation dramatically drops to zero. Such curves for sylvite flotation with
amines in brine are entirely different, the flotation starts only when the amine solubility is exceeded
(Roman et al, 1968). In other words, flotation of sylvite takes place only when solid amine particles
(hydrated crystals) are present in the system. These two figures (Figs. 8 and 9) indicate that the
critical micelle concentration and solubility limit are two entirely different phenomena and should
not be confused.
For ionic surfactants the solubility curve plotted as a function of temperature reveals two
large domains (Fig. 10). At temperature below the Krafft point (T K ) the solubility curve describes
the saturation concentration of a hydrated crystal in equilibrium with monomers (single surfactant
molecules) in solution. At T < T K , when concentration of the surfactant molecules increases (over
solubility limit) the hydrated crystals start appearing in the solution. At T > T K when the surfactant
concentration is increased, the monomers associate to form micellar aggregates; the concentration
at which micelles first occur is referred to as a critical micelle concentration. In Zone I only
individual surfactant molecules (monomers) appear; in Zone II hydrated crystals in equilibrium
with monomers; and in Zone III micelles in equilibrium with monomers. At temperature lower
than the Krafft point, the solubility is too low for micellisation.
The effect of electrolyte concentration on the Krafft point of dodecylamine was studied in one of
our projects (Laskowski et al., 2007) and it was demonstrated (Laskowski et al, 2007) that the
Krafft point of dodecylamineis strongly affected by electrolyte concentration. For longer chain
amines this temperature can be as high as 100 oC.
Figure 10 Schematic representation of the solubility of ionic surfactant plotted versus
temperature
If the Krafft temperature for long chain primary amines (n>16) is indeed in the range of 80-100 oC
then all potash flotation tests (lab tests and the commercial processes) are carried out well below
the Krafft temperature of the collector of the used collector. Under these conditions there are no
micelles in the system.
As Jan Leja demonstrated in 1983 (Leja, 1983) “in quiescent environment no contact angle
or pick-up of sylvite particles was observed even after deposition of amine-alcohol paste on the
surface of the unstirred brine. However, after thorough stirring for a few minutes, KCl particles
were picked up and contact angle was developed on KCl discs”. Since long-chain amines are
practically insoluble in brine they cannot make selected solids hydrophobic by diffusion and
adsorption. This indicated that the collector is transported to the point of bubble-particle collision
on the surface of bubbles.
In commercial potash flotation plants long-chain primary amines utilized as a collector are
melted by heating up to 70-90 oC, then they are emulsified in acidified water (hydrochloric or
acetic acids) and such a hot emulsion/dispersion is introduced into the flotation pulp (which is at
room temperature). According to all reported observations a white precipitate immediately appears
when the hot emulsion of amine is added to the potash flotation pulp and accumulates on the
surface of bubbles. Long chain amines when placed at the liquid/gas interface spread into
molecular films (Arsentiev and Leja, 1976; Leja, 1983) and thus this is how the collector in this
process is transported to the point of bubble to particle collision.
SUMMARY
It is now well established that hydrophobic surfaces usually carry negative electrical charge
(Beattie, 2007) and this creates energy barrier when hydrophobic particles collide with bubbles
which are also negatively charged. In such cases the compression of the electrical double layer
facilitates the attachment. This is the case of the salt flotation of inherently hydrophobic minerals
(Laskowski et al., 1991; Laskowski, 2012). As it is now known also the formation of fine bubbles
is promoted in electrolyte solutions (Laskowski et al., 2003).
Seawater is also a high electrolyte concentration aqueous system, and there is no reason to
expect that the salt flotation of bituminous coal could not be carried out in seawater. However, the
case of flotation of Cu-Mo sulfide ores in seawater is totally different. The difference results from
the use of lime needed to depress pyrite. Change of pH in such a well equilibrated system results
in formation of magnesium hydroxy-complexes (pH>9.5) and hydroxides, and calcium hydroxides
at higher pH.These hydrophilic colloidal species strongly depress molybdenite flotation.
Calcium and magnesium ions form insoluble salts with fatty acids and this is the origin of
the problems when apatite ores are floated with fatty acids at a high concentration of Ca2+ and
Mg2+ ions in the pulp. The Krafft points of sodium salts of anionic surfactants are below room
temperature but this can be as high as 100 oC for calcium salts of these surfactants. This explains
very well precipitation of calcium salts when fatty acids are used in hard/seawater. Removal of
Ca2+ and Mg2+ ions, for example, with the use of soda ash or water glass can make flotation process
possible.
In potash ore flotation, the flotation process carried out in 6-7 mol/L solution of NaCl and
KCl, extremely high electrolyte concentration rises the Krafft point of the amine collector so much
that this entirely changes amine mode of action. This insoluble in brine amphipaticcompound
forms molecular films at the brine/gas interface and is brought to the point of particle-to-bubble
collision on the surface of bubbles.
ACKNOWLEDGEMENTS
The authors wish to thank
CONICYT/FONDAP-15130015 project.
CRHIAM
for
financingthis
work
through
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