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Clay flotation: Effect of TTAB cationic surfactant on foaming and stability of illite clay micro-aggregates foams Julie Chapelain, Sylvain Faure, and Davide Beneventi Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.5b04450 • Publication Date (Web): 02 Feb 2016 Downloaded from http://pubs.acs.org on February 7, 2016

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Industrial & Engineering Chemistry Research

Clay flotation: Effect of TTAB cationic surfactant on foaming and stability of illite clay micro-aggregates foams AUTHORS NAMES Julie C. M. Chapelain*1, 2, Sylvain Faure 1, Davide Beneventi 2, 3, 4

1 CEA, DEN, DTCD, Marcoule, BP 17171, F-30207 Bagnols-sur-Cèze, France, [email protected], [email protected]

2 Univ. Grenoble Alpes, LGP2, F-38000 Grenoble, France, [email protected]

3 CNRS, LGP2, F-38000 Grenoble, France

4 Agefpi, LGP2, F-38000 Grenoble, France

AUTHOR ADDRESS Julie C. M. Chapelain, Tel: +33 6 23 37 03 45, Postal address: CEA Marcoule (Julie Chapelain, Bâtiment 56), BP 17171, 30207 Bagnols-sur-Cèze, France, [email protected]

KEYWORDS Clay, flotation, foam, cationic surfactant, illite

ABSTRACT Particulate foams are of great interest in mineral processing, water treatment and soil remediation. One of the first conditions allowing the capture of a particle by an air bubble is surface hydrophobicity. An alkylammonium bromide surfactant (TTAB) was used to perform in-situ hydrophobication of negatively charged illite clay particles. This treatment causes the neutralization of clay surface charge and the irreversible ACS Paragon Plus Environment

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aggregation of particles. At a fixed amount of TTAB in solution, foams generated from illite clay suspensions present a better foaming ability and stability than bare TTAB solutions. The relative quantity of TTAB necessary to reach the “ideal foaming ability” corresponding to the capture of all injected gas by the foam decreased when clay concentration increased. Capture and collision probability increase may explain this phenomenon. Moreover, selectivity for the smallest clay particle size fraction (3 µm) in foams was observed and is correlated with foam height and drainage time.

1.

Introduction

Since the early 1900’s with Pickering and Ramsden discoveries1, particles have been known to influence the stability of fluid interfaces by adsorption. In certain conditions of particle hydrophobicity, size, concentration and shape, very stable foams and emulsions can be formed in absence of any surfactant2-6. Flotation process is one of the main industrial applications of particulate foams and consists in performing insitu hydrophobication of ore particles by selective surfactant adsorption. Two flotation strategies coexist. The valuable ore is either collected in the foam (direct flotation) or collected in the unfloated fraction (reverse flotation). In this last case, gangue minerals need to be selectively collected and they frequently contain clays7, 8. Clay extraction can also be of interest for soil remediation purposes9. To date, few research studies10-14 were carried out on clay flotation and foam stability. In solution, clays hold a negative intrinsic charge due to isomorphic substitutions (ie: substitution of an ion by another ion with lower valence, for example Si4+ replaced by Al3+) within the mesh. Cationic surfactants are therefore the most employed molecules in flotation. Several Chinese teams worked on the synthesis of selective amine8, 15, 16, amide17, 18 and guanidine19, 20 surfactants for kaolinite, illite and pyrophillite flotation. More recently, Huang et al.21 proposed an amino trisiloxane Gemini surfactant with 2 carbon chains. These studies dealt with surfactant efficiency but did not address foaming and stability aspects. To date, Laponite, a synthetic smectite clay made of circular discs with a diameter of 30 nm, is the only clay studied as foam stabilizer10-13. Zhang et al.10 looked at the stability of CTAB/Laponite foams and found that it reaches a maximum at intermediate CTAB concentration, around 1.7 CEC (cationic exchange capacity). Above 3.0 CEC, particles no longer take part in foam stabilization because they are hydrophilic. It ACS Paragon Plus Environment

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was concluded that stability is strongly influenced by particles hydrophobicity and, to a lesser extent, by particles surface charge. In 2009, Laponite/hexylamine foams were studied by Liu et al11. These long-life foams were able to retain water during several days avoiding collapse. Laser-induced confocal microscopy of foam confirmed the adsorption of Laponite in-situ modified particles on bubbles surface and showed the presence of flocs in surrounding continuous phase. This phenomenon can cause an increase in rigidity, surface viscosity and strength of the foam structure. Laponite was also studied in combination with an anionic surfactant, Sodium Dodecyl Sulphate (SDS)13. Laponite particles were therefore not adsorbed on bubble surface but were present in Plateau borders only where they get jammed, increasing yield stress. It was observed that drainage was blocked at a certain point and eventually restarted after several minutes when Laponite concentration was above 10 g/L. The present study will look at the properties of natural illite clay/TTAB suspensions and their foaming ability and stability. Illite clay is characterized by a non-hydrated interlayer space and is frequent in soils. TTAB is a common cationic surfactant. Natural illite clay has a broad size distribution between 0.5 to about 80.0 µm and tends to form micro-aggregates in presence of TTAB. The influence of this inhomogeneous size distribution on foam properties and the selectivity of certain size classes in foam will be studied.

2.

Experimental procedure 2.1.Materials

Natural illite clay was provided by the company Argile du Velay (Saint Paulien, France). This type of clay is also known as illite Du Puy22. Tetradecyltrimethyl ammonium bromide (TTAB) cationic surfactant was obtained from Fluka with a purity of 98%. Suspensions were prepared in MilliQ water (18.2 MΩ.cm at 25 °C) in which 250 mg/L of CaCl2 from Sigma Aldrich, 96%, was added to reach a level of hardness corresponding to very hard tap water. This deliberated choice was made to allow future scaled-up experiments in industrial water. 2.2.Illite clay characterization Raw illite clay powder was analyzed by Scanning Electronic Microscopy (SEM) on FEI Inspect F50 after graphite coating and by X-ray diffraction on PANalytical X’pert Pro. Specific surface was determined by N2 adsorption on 1 g of raw illite clay powder on Micromeritics ASAP 2020. ACS Paragon Plus Environment

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. 2.3.Suspension preparation and characterization: Four series of suspensions containing respectively 5, 25, 50 and 100 g/L of illite clay and varying TTAB concentration were prepared in MilliQ water with 250 mg/L CaCl2. TTAB concentration was expressed as the following ratio (%): ℎ = ℎ × 100 Suspensions are elaborated 24 h before the first foam experiments or characterizations to reach adsorption equilibrium of TTAB surfactant on clay surface and are kept under continuous magnetic stirring. Suspensions pH was stable for the 4 series at 8.3 ± 0.3. 2.3.1. Particle size distribution Size distribution of illite clay in the absence and presence of surfactant was determined by laser diffraction on CILAS 1090 (CILAS, Orléans, France). 1 g/L illite clay suspensions were prepared in MilliQ water with 250 mg/L CaCl2 and stirred during 10 min before analysis. Ultrasonication (50W) was applied to disperse the clay before and during analysis with a total ultrasonication time of 3 min. Indeed, prior trials showed that the size distribution no longer evolves after 2 min ultrasonication. The influence of CaCl2 concentration (from 0 to 1 g/L) on clay aggregation was also studied on 1 g/L illite clay suspensions in MilliQ water following the same experimental protocol. 2.3.2. Adsorption isotherm (Dosage of TTAB by tensiometry) After centrifugation at 3500 rpm during 5 min of 50 mL suspension aliquots, the supernatant solution was extracted and un-adsorbed TTAB concentration was determined by tensiometry. The un-adsorbed TTAB concentration is called free TTAB concentration. Adsorption isotherm determination was made only on 5 g/L illite clay series because in the presence of TTAB, higher clay concentrations cause the formation of a thin particle layer at liquid/air interface which impairs surface tension measurements. The absence of a thin particle layer after centrifugation was checked by measuring the surface tension of an illite clay suspension with a

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TTAB/illite ratio of 4% after 12 h of sedimentation and filtration on a 0.45 µm cellulose acetate filter. A value similar to the one obtained after centrifugation was found. Surface tension measurements were performed with a Du Noüy ring on Krüss K10ST (Hamburg, Germany). Un-adsorbed TTAB concentration is deduced from calibration with known TTAB concentration solutions23, 24 (see supporting information). This titration method can only be used to determine a surfactant concentration that is below the CMC (Critical Micellar Concentration). In the case of very dilute surfactant concentration, a limitation linked to the high equilibration times needed and the adsorption on vessel walls exists and is estimated at 10-3 mmol/L in the literature24. It was chosen to carry out calibration experiments in MilliQ water with 270 mg/L of CaCl2 instead of 250 mg/L to simulate the salt release of clay when it is highly concentrated (100 g/L). Indeed, salinity is known to have a significant effect on surface tension, especially with divalent cations like Ca2+ 25. The Langmuir-Szyszkowski equation was used to interpret the calibration data: γ = γ − RTΓ ln (1 + c⁄a' ) where γ is the surface tension (mN/m), γ is the surface tension in the absence of surfactant, Γ is the surface excess at the saturation of the interface (mol/m2), c is the concentration of surfactant in solution (mol/L) and a' (mol/L) is the concentration of surfactant in the bulk solution corresponding to half of Γ . Adsorption isotherm of TTAB on clay was fitted with the Langmuir model: )* =

+,-. /01 23/01

where Qe (mol/g) is the amount of adsorbed surfactant and cE (mol/L) is the equilibrium concentration of surfactant or free surfactant concentration. Qmax (mol/g) corresponds to the maximum adsorption capacity of surfactants by clay. L (L/mol) is an index of adsorption energy. This model is originally dedicated to the adsorption of a monolayer of gaseous molecules on a homogeneous surface but the choice was made to use it to allow the estimation of Qmax and L like in surfactant adsorption previous studies26, 27.

2.3.3. Zeta potential


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Two different techniques were combined to determine the charge of both the colloidal clay fraction and the coarse clay fraction (>12 µm). Concerning the colloidal fraction, a Malvern Nano ZS (Malvern, UK) was used to measure the zeta potential by electrophoresis of 50 g/L illite clay series suspensions supernatant after 24 h of sedimentation. The Smoluchowski equation was applied to calculate zeta potential from microelectrophoretic mobility. Dynamic Light Scattering measurement on 50 g/L suspension supernatant by Malvern Nano ZS gave a Z-average of 0.8 µm in absence of TTAB. Zeta potential measurement of the coarse fraction was carried out on a Mütek Magendans SZP 06 by streaming current measurement. A 12-25 µm Whatmann 589/1 filter was added to allow the formation of a bed of clay particles from 25 and 50 g/L illite clay suspensions. 2.3.4. Foaming ability and stability (drainage) Foam generation and characterization were performed using a commercial instrument Foamscan (Teclis, Longessaigne, France). Foam is formed by blowing air through a porous glass filter (pores diameter =10-16 µm, thickness: 3 mm) at a flow rate of 63 mL/min into a fixed volume (40 mL) of illite clay with TTAB suspension. The foam volume is monitored by camera and air blowing is automatically stopped when it reaches 30 mL. The time needed to reach a certain foam volume is a characteristic called foaming ability. Several pairs of electrodes placed at different heights measure foam conductivity. The volumetric liquid fraction in the foam is deduced from these values and the reference suspension conductivity using Feitosa28 empirical model. Measurements were repeated at least 3 times. We chose to look at the free drainage behavior of a 1.5 cm high foam portion corresponding to about 9.5 mL situated above electrode 1. This protocol presents the advantage of reduced surfactant depletion from the foaming suspension. Furthermore, drainage modelling is made easier because it will only be impacted by the water flow through the thin 1.5 cm foam portion. Drainage was modelled using the following equation derived from Gorain et al.29 empirical model: 4 = 4 . 5678 + C

(A)

where ε is the liquid fraction (%) i.e.: the volumetric fraction of water in foam, ε0 is the liquid fraction at the beginning of drainage (%), k is the drainage rate constant (s-1), t is the drainage time (s) and C is the residual liquid fraction at infinite time (due to a conductive layer of wet clay on Foamscan column walls after foam breakage).


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The Gorain model was initially developed for froth flotation and it relates foam residence time (FRT, ie: foam age) to water recovery Rw (fraction of the water entering the flotation cell that is recovered in foam) by an exponential law: 9: = 90: . 56< .=>?

(B)

where Rcw is the recovery of the water from the pulp phase to the froth phase and β is a constant. In our case, as only small foam volumes are studied, the assumption was made that foam residence time (FRT) equals to the time elapsed since the beginning of free drainage (end of gas blowing) because foam formation time is very short (about 10 seconds). Rw and ε are proportional because Rw is the ratio of two water volume flows entering and leaving the froth. Attempts were made to fit drainage data with power law models developed for foams that do not contain particles30 but the exponential model was preferred because fitting error was smaller (see supporting information). In parallel, the Foamscan column was used to generate and collect foams by overflowing. To this purpose, 63 mL/min of air was blown through 80 mL of illite clay with TTAB suspension. Collect started when foam reached the top of the column (i.e. total foam height = 27 cm) and lasted 3 min. Collected samples were characterized by laser granulometry on CILAS 1090 and dried at 50 °C during 24 h to determine the mass ratio of clay compared to water. Pictures of foam bubbles were taken using the same starting volume of illite clay suspension (ie: 80 mL) but bubbling was stopped when foam volume attained 30 mL. This allowed getting pictures at foam height similar to the one of the electrode 1. To validate these last experiments, another column with a diameter of 8 cm was used. It permitted to increase the quantity of collected foam from few tens of milliliters for Foamscan device to several liters for the 8 cm column. Air flow rate was fixed at 1 L/min. This corresponds to a gas superficial velocity of 20 cm/min which is identical to Foamscan experiment. Foam was collected at 5 different heights with a vacuum cone from a 50 g/L illite clay suspension containing a TTAB/illite ratio of 0.7%.

3.

Results and discussion 3.1. TTAB-illite interactions ACS Paragon Plus Environment

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3.1.1. Illite clay characterization SEM pictures of illite clay show irregular shaped particles with a broad size distribution (see figure 1a) between few micrometers to about 80 µm. This is confirmed by laser granulometry analysis on figure 1b. There are two main size fractions: the first one is centered on 8 µm and the second one on 30 µm on de-convoluted size distribution. Minor mineral phases made of kaolinite and halloysite were detected by X-Ray diffraction (see supporting information). Illite is the main mineral phase (2θ = 8.64 ° d = 9.98 Å).

Figure 1. (a) SEM pictures of illite clay particles - On the left: magnification x500 and on the right: magnification x5000 (b) Size distribution of 1 g/L illite clay suspension in MilliQ water with 250 mg/L CaCl2 – Triangles: Not deconvoluted, Diamonds: Deconvoluted Specific area of illite clay was measured at 95.5 m2/g. 3.1.2. TTAB adsorption isotherm The following Langmuir-Szyszkowski equation parameters (see supporting information) were used to calculate free TTAB concentration from surface tension: Γ = 2.21x10-6 mol/m2 and aL = 6.02x10-6 mol/m3. The Critical Micellar Concentration (CMC) of TTAB in MilliQ water with 270 mg/L of CaCl2 was graphically determined at 1.25 ± 0.2 mmol/L (see supporting information). Above that concentration, surface tension reached a plateau centered on 38.0 ± 0.5 mN/m. These values are in good agreement with previous literature works performed in deionized water31. ACS Paragon Plus Environment

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10-3

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Figure 2. Adsorption isotherm of TTAB on illite clay 5 g/L in distilled water with 250 mg/L CaCl2 – Error bars are combined with data points – Squares: experimental data; Dashed line: Langmuir fitting Figure 2 shows the evolution of adsorbed TTAB per gram of clay with free TTAB concentration in log-log representation. This adsorption isotherm belongs to subgroup 2 of H-type Giles classification32 characterized by a strong adsorption at low free TTAB concentrations due to high affinity between adsorbate and adsorbent and a plateau corresponding to maximum adsorption. The Langmuir model was used to fit the data with the maximum sorption capacity Qmax = 3.48x10-4 mol/g and L= 2.02x104 L/mol.


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Regression coefficient r was 0.94. Similar results were found in the past with illite and a C18 alkyl ammonium bromide surfactant with longer chain length than TTAB26. Nevertheless, the maximum adsorption was reached below the CMC for C18 whereas in our case maximum adsorption corresponds to CMC. Several teams that studied the adsorption of cationic surfactants on negative surfaces (alumina33, quartz34) showed that adsorption mechanism is a 4 steps process. First, surfactant ions classically adsorbed themselves as individual ions (step 1).

Some authors34 mentioned associations into patches or hemimicelles when “surface

concentration” is close to CMC (step 2). After the surface charge neutralization point, adsorption via hydrophobic chain interaction starts (step 3). CMC is the onset of step 4 corresponding to the adsorption maximum (plateau). These steps are characterized by a change in the slope of the adsorption isotherm but are sometimes not easily detectable for clays35. In our case, we can only note that the slope of the adsorption isotherm is strong at low free TTAB concentration before CMC (1.25± 0.2 mmol/L). That could correspond to a combination of adsorption steps 1 and 2 via electrostatic bonding. Above free TTAB concentration of 0.07 mmol/L (corresponding to a ratio of 3-5%), the slope changed and became smaller. That corresponds to the beginning of adsorption through hydrophobic interaction (step 3). 3.1.3. Zeta potential 60 40

Zeta potential (mV)

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Figure 3. Evolution of zeta potential of illite clay particles with TTAB/illite ratio measured by two techniques: Mütek and Malvern Nano ZS; Diamonds: 50 g/L illite suspension by Nano ZS; Squares: 50 g/L illite suspension by Mütek; Triangles: 25 g/L illite suspension by Mütek

In figure 3, zeta potential evolution as a function of TTAB/illite ratio is represented for coarse (>12 µm) and colloidal particles. At 50 g/L of illite clay, zeta potential increases with TTAB/illite ratio and reverses at 3.5% ACS Paragon Plus Environment

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TTAB/illite ratio. This value corresponds to the onset of TTAB adsorption via hydrophobic interactions (adsorption step 3)34 and is in line with the change in the slope of the adsorption isotherm. This ratio of 3.5% was also compared to the Illite Cationic Exchange Capacity values given in the literature with the hypothesis that all available anionic sites had been neutralized by TTAB. The found value fitted well with literature (10.4 meq/100g for 12.7-15 meq/100g in the literature22, 26). Both techniques used to determine clay zeta potential give the same trends and zeta potential values and are in agreement with previous literature studies36. Colloidal and coarse clay particles have therefore similar surface charge. 3.1.4. Particle size distribution Zeta potential neutralization of clay particles can cause an aggregation phenomenon because electrostatic repulsion forces are getting weak when approaching the zeta potential reversal point. It is empirically accepted that particle aggregation is marked when zeta potential is between -30 and +30 mV37. Generally, outside this zeta potential range, particles tend to disperse again as surfactant is adsorbed via hydrophobic bonding and the resulting surface charge is positive.

Density distr (µm-1) Density distr (µm-1)

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Figure 4. Evolution of size distribution of illite clay particles in MilliQ water at different TTAB/illite ratios (a: not deconvoluted b: deconvoluted)

In figure 4, de-convoluted particle size distribution at different TTAB/illite clay ratios is presented. Size distribution is bimodal and two main size classes are detected in all samples: 10-15 µm and 30-35 µm. TTAB addition causes a shift of the smallest size class from 8 µm to 13 µm combined with an intensity decrease and an increase of the 30-35 µm peak intensity. From a mass ratio of 2.0%, particles in the smallest size class ACS Paragon Plus Environment

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aggregate and their population decreases in favor of the largest size class. Nevertheless, aggregation phenomenon no longer evolves beyond a 3.0-3.5% TTAB/illite ratio. This result is consistent with zeta potential data because this TTAB/illite ratio corresponds to zeta potential reversal point and maximum aggregation. Zeta potential data in figure 3 show that zeta potential is below 30 mV until a TTAB/illite ratio of 4.5-5%. A dispersion phenomenon was therefore expected above this mass ratio. Nevertheless, the invariant particle size distributions for TTAB/illite ratios up to 10% (figure 4) was interpreted as reflecting the irreversible agglomeration of clay particles. In parallel of this measurement, the influence of CaCl2 addition on clay aggregation was studied. No aggregation was observed between a CaCl2 concentration of 0 to 1 g/L.

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Figure 5. (a) Evolution of foam volume with time during foaming step for 0 g/L illite clay suspensions (b) Evolution of foam volume with time during foaming step for 5 g/L illite clay suspensions. In (a) and (b), black continuous lines represent the foam volume evolution in the hypothesis that all gas is captured (c) Evolution of liquid fraction ε during drainage step for 0 g/L illite clay suspensions (d) Evolution of liquid fraction ε during drainage step for 5 g/L illite clay suspensions

Figure 5 represents foam volume and liquid fraction evolution with time for 0 and 5 g/L of illite suspensions. The black continuous line in the foam volume vs. time plots corresponds to “ideal” foaming ability considering that all gas is captured by the dry foam. From a certain free TTAB concentration, this line is reached and even crossed because foams contain a not negligible volume of water which contributes to the total foam volume. At 0 g/L of illite clay, the ideal foaming line is overtaken between 0.30 and 0.59 mmol/L of free TTAB corresponding only to 0.24 CMC and 0.47 CMC. Whereas, at 5 g/L of illite clay: ideal foaming line is overtaken between 0.07 and 0.18 mmol/L of free TTAB, well below what is observed in the absence of illite. For the same free TTAB concentration (ex: 0.59 mmol/L), the illite suspension exhibits superior foaming ability. These observations confirm that clay particles covered with TTAB participate to foam formation, probably in combination with free TTAB molecules. The evolution of the liquid fraction with time at the end of air blowing is represented in figure 5 right column. Drainage was quick and about 80% of the water drained after 500 seconds for the most stable foams from 0 and 5 g/L series. In the case of foam collapse, the liquid fraction recording was stopped. Foam volume was constant overtime in other cases. ε0 [illite]=5 g/L

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Figure 6. Influence of clay presence on liquid fraction ε0 at t=0 (beginning of drainage) and drainage constant k ACS Paragon Plus Environment

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The plot of the initial liquid fraction ε0 and of the drainage constant k (figure 6) shows that ε0 increases with free TTAB concentration and reaches a maximum value around 12 % when free TTAB concentration is 2.98 mmol/L in suspensions without clay and above 1.23 mmol/L in 5 g/L illite clay suspensions. High liquid fractions led to a prolonged drainage time when looking at drainage constant values. In both series, the drainage rate constant decreased with the free TTAB concentration and levelled off at ca. 0.4 and 1.2 mmol/L of free TTAB in 5 and 0 g/L illite suspensions, respectively. Adsorption data (figure 2) reveal that clay approached its maximum adsorption capacity when k starts to stabilize. Thereafter, the drainage rate (k) of 0 and 5 g/l series has similar values when the free TTAB concentration is above 1.0 mmol/L. This corresponds to TTAB critical micellar concentration (CMC) where it was shown that maximum illite adsorption capacity is reached (figure 2). All surfactant molecules added above this concentration form micelles and therefore the drainage kinetics is no longer depending on free TTAB concentration. It was shown in the literature that foam ability and stability of pure highly concentrated surfactant solutions (ex: 1 to 5 CMC of CTAB) are independent from the surfactant concentration38. This is confirmed here for TTAB concentration between 1 and 3 CMC (free TTAB concentration between 1.49 and 2.98 mmol/L) because foaming ability lines (figure 5) are surimposed and ε0 and k are stable. 3.2. Role of TTAB/illite mass ratio on suspension foaming ability and foam drainage During the flotation process, two processes contribute to the transfer of a hydrophobic particle or aggregate from the aerated suspension to the foam, i.e.: particle capture and attachment on the surface of air bubbles (flotation) and particle entrainment in the wake of rising air bubbles39. Thus, provided that all hydro dynamical parameters that can affect the intensity of particle flotation and entrainment (i.e. air flow, turbulence, suspension volume…) are fixed, among those the illite clay particles coverage by TTAB (fixed TTAB/illite ratio), the particle capture/attachment and entrainment probabilities increase with clay concentration. As a consequence, particles load in the foam increases with their concentration in the suspension thus modifying foam properties and dynamics.


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In figure 7, foam volume and liquid fraction evolution with time for 25, 50 and 100 g/L of illite suspensions are presented. On the left column that shows foaming ability, it is worth noting that TTAB/illite ratio range for which the “ideal foaming” line is overtaken decreases when the illite clay concentration increases, i.e. 4.0-4.8 %, 3.6-4.0 % and 2.7-2.9% at 25, 50 and 100 g/L, respectively. This observation is also confirmed at 5 g/L of illite clay where TTAB/illite ratio range was 6-10 %. The quantity of adsorbate per gram of adsorbent is independent of the quantity of adsorbent40 and the surface coverage can be considered identical in all series at a given mass ratio. Since in TTAB/illite systems all negatively charged sites are neutralized at 3.5% ratio and adsorption attains its maximum at a mass ratio of 10%, suspensions with high content of particles are able to form stable foams even if the adsorption has not reached yet its maximum (25 and 50 g/L of illite) and negative sites neutralization either in some cases (100 g/L of illite). The greater the clay concentration, the smaller the TTAB/ illite ratio to attain ideal foaming. Liquid fraction vs. time plots show that all the foams that exceeded “ideal” foaming ability have a longer stability with time. Indeed, their volume was stable overtime (minimum: 2000 s) with high initial liquid fraction around 12% in average. In a summary, at 5, 25 and 50 g/L of clay, foaming starts to be ideal over the TTAB/illite ratio enough to surface neutralization. It means that foams are stabilized by particles and free TTAB in solution as well because TTAB adsorption on clay via hydrophobic interaction is less favored than adsorption by electrostatic interaction. On the contrary, at 100 g/L of clay, the role of particles in foam stabilization is increased.


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Figure 8. For 5 (diamonds), 25 (squares), 50 (triangles) and 100 (crosses) g/L illite clay suspensions: (a) Evolution of liquid fraction at t=0 (beginning of drainage) with TTAB/illite ratio (b) Evolution of drainage rate constant k with TTAB/illite ratio – Vertical grey line corresponds to the TTAB/illite ratio of zeta potential neutralization

Figure 8 shows the initial liquid fraction ε0 and drainage constant k for the 4 suspensions series. 5 g/L clay series points are shifted toward high TTAB/illite ratio compared to other series. At 5 g/L of illite clay, it was not possible to form stable foam with small TTAB/illite ratios because clay concentration was too small. As seen in figure 6, this illustrates the determining part played by clay particles in foam stabilization. On the right hand column, zoomed view enables to distinguish 25, 50 and 100 g/L series from each other. When looking at ε0, a shared curve shape is visible. ε0 drops slightly when increasing the TTAB/illite ratio and then it sharply increases from a certain TTAB/illite ratio threshold. About the same trend is observed for k at this TTAB/illite threshold value but k decreases afterwards (drainage gets slower, the foam is more stable). Foam is therefore destabilized around this threshold value that corresponds also to a foam bubble diameter increase (see supporting information). For 50 g/L series, this threshold value corresponds to the zeta potential reversal point ACS Paragon Plus Environment

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(measured as well at 50 g/L of clay). The stability drop close to the zeta potential reversal point is a well-known phenomenon. Indeed, it was shown that foams can only be formed at an intermediate surface hydrophobicity6. Thus, the best foam stability in 50 g/L series was obtained when particle zeta potential is slightly positive due to TTAB adsorption via hydrophobic chain affinity34. Besides, zeta potential reversal point should, in theory, be similar for all series40 and this was confirmed at 5 g/L by adsorption isotherm slope change and at 25 g/L by Mütek zetameter. Nevertheless, it appeared here that “corresponding” threshold value shifted toward small TTAB/illite ratio values when the clay concentration increased. Collision and capture probability rise at high clay concentration seems to be responsible for increased stability. Error bars were quite important for both ε0 and k even though up to 5 repetitions were performed in some cases. k error is especially high for unstable foams (k over 0.03 s-1). Two of the most stable foams from each series were generated once again and collected to determine clay loading and size distribution. 5 g/L suspensions series led to the formation of average 36.1 g/L of clay foams whereas 25 g/L, 50 g/L and 100 g/L series gave respectively 132.2, 111.2 and 127.6 g/L of clay foams. This result explains why rather similar ε0 and k were found for 25, 50 and 100 g/L series and shows that from 25 g/L, a maximal clay particles loading is attained. Clay suspension viscosity A at high clay concentration was estimated by using Einstein relation developed for diluted suspensions (volumetric fraction of particle BC below 0.1) of rigid spheres in a Newtonian fluid (water). ADEDF*GDHIG = A:J8*K × (1 + 2.5 × BN )

A viscosity of 1.09 mPa.s was found at 100g/l of illite clay (corresponding to BC = 0.0345) and 1.1 mPa.s at 130 g/L. Thus, we can conclude that the viscosity in foam films is close to water viscosity (1.00 mPa.s) and that its impact on foam drainage is negligible.41


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Concerning particle size distribution of clay in foam, it can be seen in figure 9 that some size classes are preferentially collected. Reference curves correspond to illite clay size distribution in absence of TTAB and are composed of 2 main size classes: 10-15 µm and 30-35 µm. In foams, peaks are shifted toward small particle diameters: 4 and 12 µm. 50 and 100 g/L suspensions series gave similar size distributions.


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As shown in figure 10, this result was confirmed with the 8 cm diameter column at 50 g/L of illite clay where the 4 µm size class is more present in the foam at 100 seconds than in the starting suspension. Furthermore, mean particle diameter decreases with foam retention time and becomes constant above a retention time of 30 seconds corresponding to a foam height of 10 cm. Thus, one question remains unclear: why are these fine particles selectively collected whereas we saw that size distribution in the suspension tends to increase with TTAB? Particles can be transported from suspension by two competing phenomena: attachment to bubble, which is favored with in situ hydrophobic particles and entrainment. This last phenomenon is totally unselective and collects by several mechanisms both hydrophobic and hydrophilic particles42. Small particles are known to be more subject to entrainment than bigger particles and it was shown that entrainment is amplified at high liquid fractions in foams and high solid concentration in suspension42, 43. This could explain the presence of fine particles in clay foams. Nevertheless, clay content in foams is stable when increasing clay concentration from 25 to 100 g/L in suspension while it should have increased in case of strong entrainment so the hypothesis of attachment to bubble is more probable. Several research teams studied the influence of particle size44 and particle volume fraction45 on drainage. They showed that when particle size is of the same order of magnitude than Plateau borders diameter, these particles are expulsed from foam films and go to Plateau borders where they can impact foam drainage44. These unattached coarse particles or aggregates start to sediment through the foam network as the experiment made in the 8 cm ACS Paragon Plus Environment

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column shows (figure 10). The particle volume fraction φQ is a parameter that also impacts the drainage velocity. It was shown by Haffner et al. that drainage velocity is minimal at a given critical confinement parameter (particle size on Plateau border size ratio) and that it decreases when particle volume fraction increases. This aspect could be the subject of further studies on clay foams. Two complementary and interrelated phenomena are therefore involved: sedimentation whom speed increases with particle size46, 47 and water flow going downward due to drainage that can be slowed down by particle plugging in some cases13, 45. As for fine particles, they remain attached to bubble and that would explain this selectivity. Moreover, research showed that foams tend to be more stable when particle size decreases and that is confirmed for illite clay. Thus, Aktas and al. underlined that maximum foam height doubled when they went from d50= 74 µm suspensions to d50=29 µm suspensions48 but they did not analyze the size distribution of collected foams. 4.

Conclusion

Even though natural illite clay particles present an inhomogeneous distribution of shapes and diameters, we showed that they are able to stabilize foams after in situ hydrophobication by a cationic surfactant. For a same amount of TTAB in solution, clay foams present superior foaming ability and stability compared to pure TTAB foams. Optimum foaming ability and stability at 5, 25 and 50 g/L coincide with the beginning of TTAB adsorption via hydrophobic chains (adsorption step 3), meaning that zeta potential is slightly positive. On the contrary, at 100 g/L of clay, the optimum is reached below the neutralization point at a negative zeta potential value. The role of clay particles in foam stabilization is therefore predominant in this case because free TTAB concentration is very low (~10-5 M). These observations were therefore in agreement with Horozov findings6 that showed that stable particle foams can only be formed at intermediate hydrophobicity. Further studies aimed at measuring the contact angle between TTAB hydrophobized clay particle, air and water could be a research track to confirm this result. Four different clay concentrations (5, 25, 50 and 100 g/L) were studied and it permitted to see how collision and capture probability increase made possible the formation of stable foam from smaller TTAB/illite ratio at high clay concentration. Furthermore, it was shown that attachment to bubbles is the main transport mechanism because entrainment should have caused an increase in clay concentration in foam ACS Paragon Plus Environment

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when increasing clay concentration in suspension. This work is of interest for all flotation processes that require extracting clays in foams from positively charged valuable ore or depressed negatively charged ore. It could be useful as well for soil remediation processes that aim at removing clays from soil, using this way there natural pollutant adsorption properties. On top of that, the selectivity for small particles in foams, explained by both sedimentation and drainage phenomena through foam network, is of great interest because surface exchange area with pollutants is even more important.

AUTHOR INFORMATION Corresponding Author Julie CHAPELAIN, [email protected], Tel: +33 6 23 37 03 45, Postal address: CEA Marcoule, BP 17171, 30207 Bagnols-sur-Cèze, France Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ‡These authors contributed equally: J. C. M CHAPELAIN‡, D. BENEVENTI‡, S. FAURE‡. ACKNOWLEDGEMENTS This work was supported by the French Agence Nationale de la Recherche through DEMETERRES project (especially AREVA partner) (Investissements d’avenir – grant agreement n° ANR-11-RSNR-0005). LGP2 is part of the LabEx Tec 21 (Investissements d’Avenir – grant agreement n° ANR-11-LABX-0030) and of the Energies du Futur and PolyNat Carnot Institutes (Investissements d’Avenir – grant agreements n° ANR-11CARN-007-01 and ANR-11-CARN-030-01).

SUPPORTING INFORMATION XRD diffractogramm of illite clay, calibration curves used to quantify TTAB by tensiometry and experimental error calculation on adsorbed TTAB, an example of comparison of power law and exponential law modeling for ACS Paragon Plus Environment

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drainage and bubble foam diameter evolution with TTAB/illite ratio. This material is available free of charge via the Internet at http://pubs.acs.org

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Effect of TTAB Cationic Surfactant on Foaming and ... - ACS Publications

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