Analysis of sodium polyacrylate as a rheological modifier for kaolin suspensions in seawater

Applied Clay Science 183 (2019) 105328

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Analysis of sodium polyacrylate as a rheological modifier for kaolin suspensions in seawater

T

⁎

Pedro Roblesa, , Eder Picerosb, Williams H. Leivac,d, Julio Valenzuelae, Norman Torof,g, Ricardo I. Jeldresc a

Escuela de Ingeniería Química, Pontificia Universidad Católica de Valparaíso, Valparaíso, Chile Faculty of Engineering and Architecture, Universidad Arturo Pratt, PO Box 121, Iquique, Chile c Departamento de Ingeniería Química y Procesos de Minerales, Facultad de Ingeniería, Universidad de Antofagasta, Av. Angamos 601, Antofagasta, Chile d CSIRO Chile, International Center of Excellence, Las Condes, Santiago, Chile e Centro de Investigación Tecnológica del Agua en el Desierto, Universidad Católica del Norte, CEITASAZA, Antofagasta, Chile f Department of Metallurgical and Mining Engineering, Universidad Católica del Norte, Chile g Department of Mining, Geological and Cartographic Department, Universidad Politécnica de Cartagena, Spain b

A R T I C LE I N FO

A B S T R A C T

Keywords: Kaolin Seawater Sodium polyacrylate Steric stabilisation Rheology Viscoelasticity

This research aims to analyse the effect of sodium polyacrylate on the rheological behaviour of kaolin pulps in seawater by means of rheograms and dynamic oscillatory assays. Then, the rheological properties were associated with zeta potential and particle aggregation/dispersion phenomena. Seawater raised the rheological properties compared to distilled water, generating an evident non-Newtonian behaviour, characterised by the appearance of yield stress, followed by a shear-thinning behaviour. This occurred because the high concentration of electrolytes compresses the ionic cloud that surrounds the particles' surfaces, overcoming the electrostatic repulsions, but besides, the seawater counterions (like Mg and Na) contribute to forming cationic bridges between the anionic particles. The addition of sodium polyacrylate did not induce significant alterations on the zeta potential; however, this formed a steric stabilisation where chord length measurements showed a greater presence of fine particles and fewer kaolin aggregates. The yield stress significantly diminished after polymer addition, while the viscoelastic modules and complex viscosity indicate that sodium polyacrylate reduces the strength of the particle networks that make up the slurry, but in turn, the phase angle indicates increase in its solid-like character.

1. Introduction

trapped inside particles aggregates, with little compaction rate. For this reason, there is a growing interest in using disposal methods for paste tailings, where pulps can reach solids percentages over than 70 wt%. This can significantly benefit the activities of pulp deposits in tailings dams (de Kretser et al., 1997; Wang et al., 2014). Given that concentrated suspensions usually acquire a non-Newtonian flow behaviour, a good knowledge of the rheological properties is important to establish the optimum conditions for the handling and transport of the tailings (Boger, 2013; Nguyen and Boger, 1998). This topic is nearly associated to environmental aspects, that is, the seek of implementing basic principles of rheology to reduce risks, recover more water, and reduce the footprint of the residues produced in the mineral processing (Boger, 2013; Sofrá and Boger, 2002). The rheological parameters are affected by many factors, including ore mineralogy and water conditions such as salinity and pH. The presence of clays is of special interest because they are associated with a wide range of minerals and cause major problems

Technological advancement in the mineral processing has enhanced the recovery of low-grade minerals, but this progression and the continuous depletion of ore grade have led to the production of large volumes of tailings that usually must be transported for long distances, before reaching the tailings deposits. Mine tailings management involves two main steps. First, the solid must be separated from the liquid to recover as much water as possible and then recirculated for upstream operations. This is usually carried out in large sedimentation tanks, or thickeners, with an intense application of flocculants to improve the mud settling rate and overflow phase clarity. In the next step, the thickened tailings are pumped from the bottom of thickeners to the tailings storage facilities, where the flocculated solid can continue to settle and consolidate over the time (de Kretser et al., 1997). Once the underflow is discharged out, it still contains high volumes of liquid ⁎

Corresponding author. E-mail address: [email protected] (P. Robles).

https://doi.org/10.1016/j.clay.2019.105328 Received 24 May 2019; Received in revised form 5 October 2019; Accepted 8 October 2019 0169-1317/ © 2019 Elsevier B.V. All rights reserved.

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in thickening and tailings management (Ndlovu et al., 2013). Kaolinite is a non-swelling clay with a laminar structure whose general composition is Al2Si2O5(OH)4. The crystallographic structure suggests that kaolinite particles consist of a tetrahedral silica surface and aluminium hydroxide octahedral surface corresponding to the 001 basal plane. The kaolinite particles have edge surfaces which are generated as a result of broken covalent bonds. The basal faces of kaolinite are ionizable in aqueous solutions. They have a charge that is always anionic, but the number of hydroxyls which lose H+ and become negatively charged is controlled by solution pH. Higher pH values give rise to more negative charges. The edges change the sign from cationic to anionic, depending on the pH as it arises from protonation or deprotonation of aluminol (AleOH) and silanol (SieOH) groups at the exposed hydroxyl-terminated planes (Ma and Eggleton, 1999; Zhou and Gunter, 1992). Due to its anisotropic structure and charge properties, the particles can be associated in three different ways: face-face (FF), edge-edge (EE), and edge-face (EF). This is a key aspect for rheology since it is connected with the strength and number of particles bonds (Hong et al., 2016; Ndlovu et al., 2011). Another feature is the ionic strength of process water that is especially relevant for mining companies that use seawater in their operations (Cisternas and Gálvez, 2018; Jeldres et al., 2019, 2016), where a highly saline environment alters the nature of interparticular forces through a reduction of electrostatic interactions and/ or changes in the structure that water molecules organise themselves through hydrogen bonds (Colic et al., 1997; Quezada et al., 2017). For example, Jeldres et al. (2017) analysed the effect of salinity on the yield stress of kaolin pulp flocculated with a polyelectrolyte of high molecular weight. After the addition of NaCl, the yield stress increased until reaching a maximum, which then decreased progressively with increasing salinity. Recently, Reyes et al. (2019) showed that seawater would increase the rheological properties of magnetite tail without flocculation. By mixing freshwater with seawater, the authors showed the critical role of electrostatic forces, since they directly related zeta potential with the rheological parameters. The manipulation of the rheological properties of concentrated pulps requires changes in the physical-chemistry of the particles' surfaces (de Kretser et al., 1998). This can be done by rheology modifying reagents, which has been widely addressed in the literature (Basnayaka et al., 2017; Konan et al., 2008). For example, Li et al. (2014) synthetised polycarboxylate copolymers that were then used to disperse kaolin suspensions. The results showed a remarkable ability of the polymers to reduce the viscosity, due to the dispersion of the particles caused by steric and electrostatic effects. Abu-Jdayil et al. (2016) investigated the effect of two surfactants on the rheological properties of bentonite suspensions, sodium dodecyl sulfate (SDS) and cetyltrimethylammonium bromide (CTAB). SDS was effective in modifying the rheological behaviour of bentonite dispersions in the concentration range corresponding to the critical micelle concentration (CMC) and the critical coagulation concentration (CCC). The addition of CTAB significantly reduced the pulp viscosity and changed the behaviour from a shear thinning material with yield stress to a Newtonian fluid. Du et al. (2019) used multiple-charge phosphate-based additives to increase the electrostatic repulsion of bentonite surfaces, achieving an impressive reduction of the yield stress to zero when the additive charge was four or higher. Labanda and Llorens (2005) showed that sodium polyacrylate can adhere to the Laponite surface and produce a steric barrier that prevents its agglomeration. Simultaneously, this anionic polyelectrolyte increased the net anionic charge of particles generating a greater electrostatic dispersion. The rheological consequence was a reduction of viscosity until an optimum polymer concentration at 0.5 wt%. In this sense, several reports analysed the rheological behaviour of clay pulps in saline water (Abu-Jdayil, 2011; Jeldres et al., 2018, 2017; Kameda and Morisaki, 2017; Montoro and Francisca, 2019), but to date, no systematic study analysing the efficiency of a rheology modifier in seawater has been published. In this work, the influence of sodium polyacrylate, an anionic polymer of low molecular

Fig. 1. XRD for kaolin powder.

weight, on the yielding and viscoelasticity of kaolin pulps in seawater is studied, relating the impact of the reagent with the particles aggregation/dispersion mechanisms. This study is of special interest for mining industries that use seawater in their operations, whose ores have high clay contents. 2. Materials and methods Kaolin particles (from Ward's Science) were used with a density of 2.6 g/cm3. A quantitative XRD analysis indicated that it contained 84 wt% kaolinite (Al2Si2O5(OH)4) and 16% of halloysite (Al2Si2O5(OH)4·2H2O). An X-ray diffractometer Siemens D5000 was used. The data were processed with TOPAS (Total Pattern Analysis Software). The DRX spectrum can be found in Fig. 1. Seawater from the coast of Antofagasta (Chile) were used. A chemical analysis using atomic absorption spectrophotometry determined that the ions concentration was Na+: 0.5 mol/L, Mg+2: 0.057 mol/L; Ca+2: and 0.01 mol/L. By the argentometric method, the concentration of Cl− was 0.56 mol/L. Cytec Chile Ltda. provided the rheological modifier (sodium polyacrylate in aqueous solution with 45 wt%, MW = 8000 aprox). An FTIR spectrum can be found in supplementary material. The pH was controlled with sodium hydroxide and distilled water was used with NaCl 0.001 M for all measurement. 2.1. Rheological tests Kaolin pulp was prepared at pH 8 in a vessel at the appropriate solids and reagent concentration as described below. After 1 h of mixing, a 56 mL aliquot was taken for rheological analysis. An Anton Paar MCR 102 rheometer with the RheoCompass Software operated in controlled-rate, and a sandblasted #CC39 bob-in-cup configuration (gap 1.5 mm) was used to reduce wall slip effects. Each trial lasted 15 min, ensuring that the rheological properties were obtained in steady state. The starting value of shear rate was preset at 1 s−1 for distilled water and 10 s−1 for seawater. The maximum shear rate measured was 500 s−1, but the results presented in this paper are shown up to the shear-rate in which unstable flows appear as Taylor vortices (Taylor, 1923). The temperature of the sample was kept constant at 23 °C. The rheological parameters (yield stress and flow index) were calculated by fitting the experimental data to the constitutive equation of the Herschel-Bulkley model (see Eq. (1)).

τ = τ0 + kγ̇n

(1)

where τ is the shear stress, γ̇ the shear rate, τ0 the yield stress, k the consistency index, and n the flow index. For oscillatory rheology tests, a vane-in-cup geometry was used (ST22-4V-16), with a frequency sweep that considered a strain amplitude of 0.5%, ensuring a linear 2

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viscoelastic regime. 1.0 80

To have the chord length distribution of kaolin particles and aggregates, the focused beam reflectance measurement (FBRM) were used (ParticleTrack G400, Metter-Toledo, Columbus, OH, USA). The instrument consists of a processing unit and a 14-mm diameter probe, which was immersed in the suspension. The probe features a high-speed rotating laser (2 m/s tangential velocity) that backscatters light intensity when the laser beam meets particles or flocs. In all the experiments, the probe was introduced directly into the suspension of 300 g at the required solid concentration and reagent dosage, and aggregates size evolution was recorded.

0.8 60 0.6 40 0.4 20

Flow index, n

Yield stress [Pa], t0

2.2. Chord length distribution (CLD)

0.2 Yield stress Flow index

0 10

20

30

40

0.0

50

Solid concentration [%wt]

2.3. Zeta potential

Fig. 2. Herschel-Bulkley parameters (yield stress and flow index) of kaolin pulp in seawater at pH 8 at varied solid concentrations (10–50 wt%).

A kaolin suspension of 0.1 wt% was prepared at pH 8. The corresponding amount of reagent was added and kept stirring for 10 min before the measurement of zeta potential. The test was performed in a Stabino® (Particle Metrix) equipment.

to 84 Pa for 50 wt%. The flow index (n) for the seawater pulp was close to 1 when solid concentration is 10 wt%, but then an evident shear thinning behaviour appeared, thus internal changes occured in their particle networks as the shear rate increased, generating a lower apparent viscosity (Table 1). The flow index was monotonically reduced, reaching a value of n = 0.37 for a 50 wt%. A similar trend appeared for the apparent viscosities, for example, this was 1346 mPa·s for seawater at 97.4 s−1, while in distilled water it was only 14 mPa·s (Table 1). After the addition of sodium polyacrylate (Fig. 3), the rheological properties of kaolin pulps in seawater decreased considerably, and the rheograms exhibited a sharp drop of shear stress for a fixed shear rate when low polymer doses were applied. The greatest changes were obtained up to a concentration of 0.3 wt% of NaPA, where the yield stress reduced to 70%. The flow index increased monotonically as reagent concentration increased, raising from n = 0.37 for the slurry without reagent, until n = 0.79 for 1 wt% of NaPA. Fig. 4 presents the viscoelastic behaviour of the kaolin pulp in seawater, formulated with different doses of sodium polyacrylate. The assays were carried out by oscillatory rheology, where Fig. 4a correspond to a frequency sweep with a shear strain of 0.5%. In the frequency range (0.15–60 Hz), elastic modulus was much larger than viscous modulus, and both modules had low dependence of frequency, which is typical of a gel-like behaviour (Lin et al., 2015; Michot et al., 2009). Interestingly, the modules decreased when the reagent dose was increased, indicating reduction of the strength of particle networks that make up the slurry. These results are supported by the complex

2.4. Ion adsorption determination Kaolin pulps were prepared in seawater at 5 and 10 wt% and pH 8 in a cylinder of 41.5 cm in height, 3.5 cm in diameter, and 400 cm3 in volume. After a vigorous agitation to achieve homogeneity of the pulps, they were settled for 24 h. The supernatant was removed and centrifuged to remove the particles in suspension. Then, an aliquot was extracted and cation concentrations were determined by direct-aspiration atomic absorption spectrophotometry (Varian 220 FS Atomic Absorption Spectrophotometer, Varian, Palo Alto, CA, USA). The quantity of cations adsorbed on the kaolin surface was calculated by mass balance, considering the concentration of ions in solution, before and after sedimentation. 3. Results 3.1. Kaolin pulps rheograms In distilled water (NaCl 0.001 M) a Newtonian behaviour was obtained for the entire solid concentration range (10–50%), where a single viscosity value characterised the suspensions (see Table 1, full rheogram in the supplementary material). However, a highly saline environment like seawater reduces electrostatic repulsion between the particles, generating more bonds as a result of the attractive van der Waals interactions, and eventually cationic bridges due to the cations in solution. This phenomenon made rheological changes, appearing a clear non-Newtonian behaviour, characterised by a yield stress followed by shear-thinning behaviour (Fig. 2). The yield stress and flow index, calculated with the Herschel Bulkley model, are presented in Fig. 2, as a function of solid concentration. Yield stress displayed a remarkable growth since the pulp had a concentration higher than 30 wt%, rising from 2.2 Pa for 30 wt%

1.0 80

Table 1 Viscosity of clay slurries in distilled water and seawater (apparent viscosity for a shear rate of 97.4 s−1) at pH 8 and varied solid concentrations (10–50 wt%). Solid concentration (%)

Viscosity [mPas] (distilled water)

Viscosity [mPas] (seawater at 97.4 s−1)

10 20 30 40 50

1.3 1.7 3.3 6.7 14

2.5 8.7 41 273 1346

60 0.6 40

0.4

20

Flow index, n

Yield stress [Pa], t0

0.8

0.2 Yield stress Flow index

0

0.0 0.0

0.2

0.4

0.6

0.8

1.0

NaPA dosage [wt%] Fig. 3. Herschel-Bulkley parameters (yield stress and flow index) of kaolin pulp in seawater at pH 8 with sodium polyacrylate variations (0–1 wt%). 3

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18 Seawater (SW) Distilled water SW + NaPA 0.1 wt%

3000

16 14 12 10

2000 8 6 1000

4

Counts [square weighted]

Counts [unweighted]

4000

2 0

0 1

10

100

Chord lenght [µm], lc Fig. 5. Chord length distribution (CLD) for kaolin suspensions at pH 8 in distilled water, seawater, and seawater with sodium polyacrylate (0.1 wt%): unweighted CLD at left (lines) and square weighted CLD at right (lines and plus symbol).

3.3. Zeta potential and ion adsorption The zeta potential helps to understand the role of electrostatic forces on aggregation/dispersion modes. While in distilled water was −42 mV, this considerably decreased (its absolute value) in seawater to −4 mV. The reduction of the electrostatic forces was nearly entire (Fig. 6); therefore the particle aggregation at pH 8 would be produced mainly by the van der Waals interaction and potential cationic bridges exerted by the counterions contained in seawater. In this sense, the chemical analysis suggests that the adsorption of the main cation per gram of clay was in the following order, sodium: 1.9 mg/g, magnesium: 0.7 mg/g, calcium: 0.03 mg/g (see Table 2). It is expected that the cations are attached to the anionic sites of the clay's surface by electrostatic attraction. After the addition of NaPA (0.1 wt%), the zeta potential was −5 mV, not implying a significant change, so it is ruled out that an electrostatic repulsion can be responsible for promoting the clay dispersion, as past studies refer for water with low ionic strength (Huang et al., 2018; Labanda et al., 2007). In this way, a steric dispersion is inferred to be the only mechanism that acts to disperse clay particles in seawater.

Fig. 4. Viscoelastic behaviour of kaolin slurries in seawater at varied sodium polyacrylate concentrations. a) frequency sweep (fixed strain amplitude 0.5%), b) phase angle and complex viscosity (fixed frequency: 5.8 Hz). Pulp prepared at pH 8 and solid concentration of 50 wt%.

viscosity (Fig. 4b), which also decreased progressively with increasing reagent dose, however, the difference between G′ and G″ was lower. The phase angle, defined as tanδ = G′′/G′, decreased with the presence of reagent, indicating that the suspension increased its elastic proportion, that is, the structure of the suspension was altered to generate a higher prevalence of its solid-like character (Fig. 4b).

3.2. Chord length distribution (CLD)

4. Discussion

The aggregation/dispersion phenomena were evaluated through the chord length distribution (CLD) by using the FBRM probe in primary mode (Fig. 5). Seawater lowered the number of fine particles because the ionic cloud that surrounds mineral surfaces was compressed by the high concentration of electrolytes in solution. Then, the unweighted CLD that achieved the maximum count shifted slightly to the right, rising from 7.2 μm in distilled water to 10 μm in seawater, while the count was reduced from 3700 s−1 to 2700 s−1, respectively. Meanwhile, the square-weighted CLD (improves the resolution of coarse particles and aggregates), showed an increase of kaolin aggregates in seawater, where the chord length with the highest count (the peak) increased from 18.5 μm in distilled water to 23 μm in seawater. In turn, the square-weighted chord length rose from 8.1 in distilled water to 12.2 in seawater. After adding the reagent (NaPA 0.1 wt%), the proportion of fine particles increased showing a peak of 2700 s−1 without reagent and 2900 s−1 with the polymer. The chord length for the maximum count was shifted slightly to the left, from 10 μm to 7.5 μm. While the squareweighted CLD gave a reduction of the coarse particles (mainly agglomerated clay), reducing the maximum count from 12.3 to 11.3. Overall, after applying sodium polyacrylate the amount of fines increased and the number of kaolin aggregates decreased.

Kaolin pulps at pH 8 exhibited a Newtonian behaviour within the range of solids concentration analysed in this study (up to 50 wt%). Measurements of zeta potential show that the electrostatic repulsions caused by the clay faces and possibly face-edge attraction (Gupta et al., 2011) generate that the inter-particle bonds are weak, producing a stable slurry with low rheological properties. However, the presence of salt suppresses electrostatic forces. In this study, the portion of cations that are going to the clay surface was calculated by using a kaolin pulp in seawater at pH 8 (10 wt% of solid concentration). This reveals the eventual dominance of the attractive van der Waals interactions, which together with new cationic bridges (of sodium and magnesium) endorse the strengthening of particle-particle bonds. A simplified estimation of the interaction energy, considering the DLVO theory for two spherical kaolinite particles (Israelachvili, 2011), shows that in distilled water an energy barrier close to 1 × 10−17 J (Fig. 6a) must be overcome to get spontaneous agglomeration, while in seawater, the barrier energy disappears (Fig. 6b). This generates two relevant phenomena: i) greater particle aggregation, observed by means of the FBRM probe. The unweighted CLD revealed a lower presence of fine particles, and the square-weighted CLD gave a higher number of aggregates; and ii) 4

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potential analyses showed that sodium polyacrylate, an anionic polymer that has been used to disperse different types of slurries (Bernhardt et al., 1999; Huang et al., 2018; Labanda et al., 2007), did not give a significant difference in seawater, although the polymer can adhere onto the clay's surface through hydrogen bonds with carboxylate ions (Labanda and Llorens, 2005). Despite this, the number of particle-particle bonds was reduced by steric stabilisation when the separation distances between particles were close to 10 nm (Fig. 6b). By the FBRM technique, the unweighted CLD showed that the reagent generated a larger number of fine particles while the square-weighted CLD showed a loss in the quantity and size of large aggregates. The rheological assays displayed a notable reduction in the slurry yield stress and systematic rise in flow index concerning the reagent dosage. Additionally, oscillatory trials revealed that viscoelastic modules and the complex viscosity before the yielding were considerably reduced, but on the other hand, phase angle diminished. These findings suggest that the addition of sodium polyacrylate reduces the strength of the particle networks that make up the pulp, though it causes to increase its solid-like character.

5. Conclusions The mechanisms of aggregation/dispersion of clay pulps in seawater were studied, evaluating the implication of adding sodium polyacrylate of low molecular weight. The pulps prepared in distilled water at pH 8 exhibit a Newtonian behaviour; however, the particles bonds in seawater became stronger by the compression of the ionic cloud surrounding colloids and new cationic bridges. This was evidenced by a significant drop in the magnitude of zeta potential, which had a value of −45 mV in distilled water and −4 mV in seawater. This conferred a non-Newtonian behaviour to the clays slurries, characterised by yield stress followed by a shear thinning behaviour. The addition of sodium polyacrylate did not mean a significant influence on electrostatic forces since its zeta potential remained at a low value, but the particle dispersion enhanced by a steric repulsion mechanism. Chord length distribution revealed a clear increase in fine particles after the reagent application, but at the same time reducing the number and size of coarse aggregates. In this way, the polymer adsorption diminished the strength of particles bonds, causing a significant reduction in rheological parameters such as yield stress, viscoelastic modules, and complex viscosity. Otherwise, phase angle showed that slurries increase its solidlike character after polymer addition. The exposed results are of special interest for mining companies that use sea water in their operations. The generation of thickened tailings allows increasing recycled water, but a rise in the rheological properties can involve vast energy consumption for slurry transportation from thickeners to the tailings storage facilities. In this research, it was shown that sodium polyacrylate of low molecular weight may reduce the bonds' strength between particles resulting from steric stabilisation, with attractive rheological consequences.

Fig. 6. Estimation of total interaction energy for kaolinite particles (radius 1 μm) at pH 8 in a) distilled water with NaCl 0.001 M, and b) seawater. Dotted line for seawater marks the range in which the steric repulsion by sodium polyacrylate could appear. Mathematical equations can be found in supplementary material. Table 2 Cation concentration in seawater and supernatant liquid of a kaolin suspension in seawater (Solids concentration: 10 wt%; pH 8; settling time: 2 days). Sample

Calcium [mol/L]

Magnesium [mol/L]

Sodium [mol/L]

Seawater Kaolin suspension

0.010 0.097

0.057 0.053

0.503 0.494

considerable increase of the rheological properties. This characteristic manifests the critical consequence of transporting a clayey pulp in seawater since the energy required by pumping may be huge. For this reason, it is likely that under certain circumstances chemical reagents capable of improving flow properties might be imperatively required, which is not easy in seawater because an electrostatic repulsion of particles cannot be opted for. The archetypal reagents employed to diminish the rheological properties of clay slurries usually suggest raising the electrostatic repulsion between particles, promoting their dispersion. However, in a highly saline environment like seawater, the electrolyte concentration overcomes the electrostatic forces, which would play an insignificant role in the aggregation/dispersion mechanisms. In this sense, zeta

Acknowledgement Pedro Robles thanks the Pontificia Universidad Católica de Valparaíso for the support provided. RIJ thanks the financial support of CONICYT Fondecyt 11171036 and Centro CRHIAM through Project Conicyt/Fondap/15130015.

Declaration of Competing Interests The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. 5

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Appendix A. Supplementary data

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