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Evaluation of flocculation characteristics of kaolinite dispersion system using guar gum: A green flocculant
International Journal of Mining Science and Technology 29 (2019) 745–755
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Evaluation of flocculation characteristics of kaolinite dispersion system using guar gum: A green flocculant R.K. Dwari a,⇑, B.K. Mishra b a b
Mineral Processing Department, CSIR-Institute of Minerals and Materials Technology, Bhubaneswar 751013, India Indian Institute of Technology, Goa 403401, India
a r t i c l e
i n f o
Article history: Received 5 February 2018 Received in revised form 17 January 2019 Accepted 11 June 2019 Available online 18 June 2019 Keywords: Guar gum Dewatering Kaolinite Light scattering Morphology Flocculation
a b s t r a c t This paper reports the systematic investigation on the flocculation, sedimentation and consolidation characteristics of kaolinite using guar gum as a green flocculant. In-situ flocculation behavior of kaolinite at various pH, guar gum dosages, and ionic strength were studied using a light scattering technique. The effect of these parameters on the settling rate, solid consolidation, and supernatant liquid clarity was recorded. The morphology of kaolinite and flocculated kaolinite aggregates were analyzed using FESEM. The morphology studies suggest that it is poorly crystalline with multiple steps on edge, broken edge; laminar with high aspect ratio and have rough basal surface. The complex irregularity on the basal surface and the presence of multiple steps in the edges, broken edges (hydroxyl groups) have facilitated the guar gum adsorption. The isoelectric point of kaolinite is pH 3.96. The pH, ionic strength and flocculant dosage have a significant effect on the kaolinite settling rate. The guar gum has exhibited excellent turbidity removal efficiency at pH 5. The turbidity removal is inefficient at pH 10. However, guar gum has shown high turbidity removal with 80% transmission at pH 10 in the presence of a KNO3 electrolyte. Ó 2019 Published by Elsevier B.V. on behalf of China University of Mining & Technology. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
1. Introduction The removal of water from mineral tailings by flocculation is one of the major mineral processing activities. Recently, synthetic flocculants like polyacrylamide are in use for the sedimentation of minerals from the process tailings and wastewater. However, their environmental compatibility is a matter of great concern. Presently, the demand for green flocculants is increasing due to their environmental compatibility. Guar gum is also known as guaran. It is a natural non-ionic polysaccharide composed of galactose and mannose. It is a chain of (1 ? 4)-linked b-D-mannopyranose units with a-Dgalactopyranose units connected to the mannose backbone through (1 ? 6) glycosidic linkages. The poly-mannose chain randomly substituted with galactose units at mannose-to-galactose ratio of 1.8–1.0 [1,2]. The molecular weight of guar gum is 1 105 to 20 105 [1,2]. Ma and Pawlik [3] studied the adsorption behavior of guar gum on kaolinite and other oxide minerals at different pH, background electrolyte, and ionic strength. One of the major conclusions of their study is that the adsorption density of this polysaccharide
⇑ Corresponding author.
does not depend on the pH. However, there is a significant increase in adsorption density on kaolinite surface in the presence of chaotropic K+ ion. The hydrogen bonding is the primary mechanism of guar gum adsorption onto the kaolinite surface. The polymer adsorbed onto the mineral surface through hydrophobic interaction, hydrogen bonding, chemical and electrostatic interaction [4–8]. Wang et al. [9] concluded that the hydrogen bonding is the principal force for the adsorption of guar gum onto the talc surface. Mhlanga et al. [10] also reported the guar gum adsorption on several pure minerals such as talc, pyroxene, plagioclase, chromite, and chalcopyrite. The above studies show that guar gum has a natural affinity towards these mineral surfaces. The majority of these works were carried out to understand its application as a depressant in flotation and selective flocculation system. However, not many reports are available regarding its possible use as a flocculant. The effect of guar gum as flocculant on the flocculation characteristics of kaolinite particle regarding floc growth, colloidal stability and, solid consolidation are not well understood. Moreover, the flocculation characteristic method is poorly studied. Guar gum has many applications in mining and mineral processing industries. Many researchers have reported its use in froth flotation and flocculation process [9,11]. In the froth flotation process, guar gum is used as a depressant for talceous gangue minerals for the flotation of base metals and platinum group metals bearing
E-mail address: [email protected] (R.K. Dwari). https://doi.org/10.1016/j.ijmst.2019.06.001 2095-2686/Ó 2019 Published by Elsevier B.V. on behalf of China University of Mining & Technology. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
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ores [9,11]. Guar gum adsorbs onto the talc surface to make it hydrophilic. It also has application as a binder for waterinsoluble slimes in the potash flotation process [12]. Guar gum and grafted derivatives of guar gum have been explored as flocculants for dewatering applications [13–15]. Nasim et al. [13] have reported a high settling rate of kaolinite using guar gum and polyvinyl alcohol grafted guar gum as flocculants. Apart from guar gum other polysaccharides and their grafted derivative such as chitosan [16,17], starch [18], xanthan gum [19] and tamarind kernel [20] have also been explored as flocculants. The Indian iron ore contains a significant amount of gangue minerals such as silicates and aluminium silicates. These minerals liberate at finer sizes. Therefore, the tailings generated in an iron ore processing plant are fine in size. One of our earlier studies suggests that d80 of the tailings is 12 mm. The tailings contain 5%-10% solid (w/w) and the Al2O3, SiO2 content in the tailings are 18% and 14%, respectively [4]. The fine particle causes a serious problem in the dewatering process. Thus, the consolidated solid in the thickener contains high moisture, which in turn creates technical and economic difficulties in handling, disposal, and water removal. The basal faces of 1:1 tetra-octahedral aluminosilicate consists of tetrahedral siloxane (-Si-O-Si-) species and octahedral, alumina (Al2O3) sheet [21]. Van Olphen [21] showed that the basal faces having a siloxane structure carry a permanent negative charge due to the isomorphous substitution of Si4+ by Al3+ groups. Also, aluminum (Al-OH) and silanol (Si-OH) groups occur at the edges. Therefore, the edge faces are charged by protonation and deprotonation of hydroxyl groups depending on the pH [21,22]. These particles are highly charged at neutral and alkaline pH and shows stable dispersion behavior. Therefore, authors choose kaolinite an aluminium silicate to study its flocculation characteristics in the presence of guar gum. The design of a dewatering system requires a thorough analysis of the solid-liquid system and slimes stability behavior. The stability of particle can be changed by controlling the pH, ionic strength, addition of flocculant and coagulant [4,23–28]. In flocculation, the fine particles interact with the flocculating agent and aggregate to form flocs. The flocs settle rapidly under the influence of gravity. The characteristics of floc decide the rate of settling and moisture contents of consolidated tailings [29]. The floc characteristics depend on several factors such as particle surface chemistry, size distribution, density, shape; viscosity and dielectric constant of suspension; chemical nature, molecular weight, charge and charge density of the flocculants [30,31]. The environmental policies have enforced legal rules for the disposal of waste by the mineral industries. It is essential to redesign the dewatering operation to meet the current challenges. The present work has been undertaken to study the sedimentation behavior of kaolinite using guar gum as a flocculant. The spectroscopic techniques are used to understand the influence of different process parameter on the flocculation and consolidation characteristics.
2. Experimental 2.1. Materials and procedure The kaolinite sample was supplied by M/s. Ashapura Minechem Ltd., Trivandrum, India for the sedimentation studies. The particle size of the kaolinite sample was measured using Malvern particle size analyzer (Mastersizer 2000). The surface area and density of the kaolinite sample were determined by using smart BET surface analyzer (SORB 92/93) and Helium-Mercury pycno meter (Smart Pycno 30), respectively. Guar gum was used as a flocculant for settling studies of kaolinite dispersions. It was procured from M/s. Otto Chemicals Ltd,
India. The polysaccharide solution was prepared by dissolving 0.1 g of guar gum in 100 ml of distilled water. The guar gum powder was poured into continuously stirred distilled water using a magnetic stirrer. It was further stirred for 6 h to ensure complete dissolution of the guar gum in water. The stirring rate was maintained at 1100 rpm during the preparation. The flocculant solution was prepared every day to maintain its freshness. The ionic strength of the solution was maintained using potassium nitrate, and the pH was regulated by using analytical grade sodium hydroxide and hydrochloric acid solutions. 2.2. Kaolinite suspension The kaolinite dispersions of 5% solids (w/w) were prepared in distilled water for the flocculation characteristic studies. The slurry was agitated at 800 rpm for two hours to obtain the homogeneity. The pH of slurry was maintained at pH 5 and pH 10. For each experiment, 20 ml of the slurry was pipetted out from the homogeneous slurry and put into the glass cell for the light scattering analysis. The suspension was prepared with a known volume of KNO3 to study the effect of electrolyte on kaolinite flocculation and sedimentation at pH 10. 2.3. Zeta potential studies The zeta potential (f) of kaolinite suspension was measured by potentiometric titration using a Zeta probe (Colloidal Dynamics Ltd., USA) instruments. The measurement was carried out on kaolinite suspensions of 5% solids (w/w) dispersed in 103 mol KNO3 electrolyte background solution. The electrolyte background was used to avoid any effects of inconsistent surface conductance on zeta potential [25]. The complete dispersion of the slurry suspension was ensured using ultrasonic vibration. The zeta potential of dispersed kaolinite suspension was measured at the slurry agitation speed maintained at 250 rpm. The zeta potential measurements were carried out in the pH range of 3–12. The detail zeta potential measurement procedure has been discussed in our earlier work [4]. 2.4. Multiple light scattering measurements The sedimentation/suspension stability of kaolinite dispersion was studied by using optical analyzers (TUBISCAN LABexpert) based on multiple light scattering technique. The stability of the dispersion was monitored by measuring the transmittance and backscattering of the pulsed near-infrared light (k = 880 nm). Turbiscan measurements were carried out for the entire vertical length of the glass cell at regular time intervals. The transmittance detector received the light that passed through the dispersion at an angle of 180° concerning the source while the backscattering detector received backscattered light by the dispersion at an angle of 45°. The technique is used to measure the transmittance and backscattering of suspension. The method helped to monitor the particle in the clarifying zone and the bed consolidation. Approximately 20 ml of 5% (w/w) kaolinite dispersions was taken in the glass cell of the turbiscan equipment. The height of the cell filled with the suspension was 40 mm. The temperature was maintained at 30 ± 2 °C during the analysis. A known volume of freshly prepared starch solution was mixed into the kaolinite dispersion, to study the effect of guar gum on the flocculation behavior of kaolinite dispersion. Then the cell was closed with a cap and was shaken by making it up and down for 20 times for better mixing of both. The vertical glass cell height was scanned 100 times in 50 min at an interval of 30 s. After the scanning, the cell was removed from TURBISCAN LABexpert and kept for 24 h and then the height was measured. After the scanning the rate of settling,
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backscattering, transmission, floc growth, were calculated using TURBISCAN LABexpert software. The diffuse reflectance in terms of back scattering is considered to obey Eq. (1) [32]:
(
BS ¼ p1ffiffiIffi 2d I ðd; /Þ ¼ 3/ð1g ÞQ s
ð1Þ
where, BS is the diffuse reflectance at 45° detector, %; I* is the photon transport mean free path; d is the particle diameter, lm; / is particle volume fraction, %; g and Qs are optical parameters according to Lorenz and Mie theory [33]. Hence, backscattering measurement made by the Turbiscan directly depends on the particle mean diameter (d) and volume fractions (/). Therefore, Turbiscan LAbExpert enables the computation of I*, which represents the dispersion state of the product. The computation of the particle mean diameter (d) is possible by knowing the volume fraction (/) and the refractive indices of disperse and the continuous phase. The Lambert-beer law gives an analytical expression of the transmitted flux T (%) measured by the turbiscan as a function of the photon mean free path (I) and expressed as Eq. (2) [33].
T ðI; ri Þ ¼ T o e
2r i i
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backscattering percentage in the thickening zone increases as more particles settled to the bottom. 2.6. Particle and floc morphology The surface morphology of kaolinite particles and their flocs formed at pH 5 and 800 g guar gum/tonne of solid dosage was studied using the scanning electron microscope (FESEM), Supra 55, Carl Zeiss, Germany. The kaolinite particles were dispersed in an aqueous solution and a drop of this dispersion was carefully placed on aluminium foil. The suspension drop was dried at room temperature and put on an FESEM stub to determine its morphology. In case of flocs, they were carefully collected from the consolidated sediment and placed on aluminium foil. After drying, the floc was placed on a FESEM stub for studying floc morphology and external surface structure. The consolidated sediment was also sheared using a magnetic stirrer rotating at 150 rpm. The flocs were collected at a different interval of time and subjected to morphology investigation as mentioned above. 3. Results and discussion
ð2Þ
where To is the transmittance of continuous phase and ri is the measurement cell internal diameter. 2.5. Flocculation studies The flocculation studies were carried out using the multiple light scattering techniques with 5% (w/w) kaolinite dispersions. The experiments were conducted at pH 5 and pH 10 using different dosages of flocculant in the range of 0–1000 g guar gum/tonne of solid, i.e., 0, 50, 100, 200, 400, 600, 800, 1000 g/tonne of solid. The Turbiscan light scattering data (both transmission and backscattering) obtained for kaolinite dispersions in distilled water at pH 5 without starch dosage is shown in Fig. 1. The source infrared light cannot transmit through a stable dispersion and get scattered by fine suspended particles. The backscattered detector detects the scattered light. A high percentage of backscattering suggests stable dispersions. However, as the particles get aggregate, the sedimentation occurs. The particulates present in the suspension decreases which allows the source light to pass through the medium and detected by the transmittance detector. Fig. 1 suggests that the transmission percentage in the clarified zone and
3.1. Material characterization The particle size distribution of kaolinite sample is shown in Fig. 2. The figure shows that the average particle size (d50) of the kaolinite is 3.69 mm whereas d90 is 24 mm (Fig. 2). The BET surface area and density of kaolinite sample is 17.75 m2/g and 2520 kg/m,
Fig. 2. Particle size distribution of kaolinite.
Fig. 1. Transmission and backscattering data of kaolinite dispersion at pH 5.
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respectively. The surface morphology of kaolinite particles is shown in Fig. 3. The SEM images in Fig. 3a demonstrates that the kaolinite has a complex surface structure on the extensive basal surface. The dimension of the particles is in the range from 100 nm to 2000 nm. The high magnification of particle shows that the particles are poorly crystallized with individual crystallite, micro-islands, microcrystallite on the extensive basal surface (Fig. 3b). The multiple steps in the edge and broken edges on the kaolinite particle are also evident (Fig. 3c). Investigation on many areas of the particle shows that it has lamellate plates with high aspect ratio, edge to face platelet orientation and rugged basal face (Fig. 3d). Fig. 4. Zeta potential of 5% (w/w) kaolinite dispersion in 0.001 mol KNO3 background electrolyte.
3.2. Zeta potential studies The zeta potential of kaolinite dispersions at 0.001 mol KNO3 background electrolyte solution is shown in Fig. 4. The figure shows that the isoelectric point of kaolinite is pH 3.96. This value is close to the value observed by other workers [34–36]. Fig. 4 shows that the zeta potential of kaolinite at pH 5 and 10 are 20 mV and 79 mV, respectively. The result suggests that the colloidal kaolinite particulates are highly negatively charged at alkaline pH indicating strong electrostatic repulsion existing between them. 3.3. Effect of starch dosages on the rate of settling The effect of starch dosages on the particulate migration velocity or rate of settling at pH 5 and 10 are calculated from the light scattering data and is shown in Fig. 5. Fig. 5a shows the rate of settling of kaolinite at pH 5 at different dosages of guar gum. The figure demonstrates that there is a time lag on the settling of particles
in the absence of starch and they start to settle at 186 s. The time lag period is attributed to the time taken by the particulates to form aggregate. Further, there is an exponential increase in the rate of settling. At 248 s, the particulates reached the maximum settling rate of 0.36 mm/min and further maintained a steady state of settling. The time lag decreases with the addition of guar gum to the kaolinite suspension. The time lag observed for 50, and 100 g guar gum/tonne of solid dosages is 62 s while it is 31 s for starch dosages between 200 and 800 g guar gum/tonne of solid. The figure shows that the particle settling rate increases with the increase in starch dosage, and maximum settling rate is observed at 800 g guar gum/tonne of solid dosage. The maximum settling rates observed at 50, 100, 200, 400, 600 and 800 g guar gum/tonne of solid dosages are 0.77, 1.09, 2.25, 2.82, 4.44 and 4.78 mm/min, respectively. It is fascinating to observe that the maximum settling rate is obtained at 93 s, and a curve peak is observed at this point. A steady-state settling rate is observed after reaching the maximum
Fig. 3. High resolution FESEM images of kaolinite colloidal fraction (a) kaolinite particulates; (b) poorly crystallised with individual crystallite, microislands, micro crystallite on the extensive basal surface; (c) multiple steps in the edge, broken edges; (d) laminated plates with high aspect ratio, edge to face platelet orientation, rugged basal surface.
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Fig. 5. Effect of guar gum dosages on the rate of settling of kaolinite dispersions at (a) pH 5 and (b) pH 10 in distilled water.
settling velocity at lower starch dosages, i.e., 50–100 g guar gum/ tonne of solid. Fig. 5b shows the rate of settling of kaolinite particle at different dosages of starch at pH 10. The result shows that the rate of settling increases with the increase in starch dosages. The maximum settling rate is obtained at 31 s for each dosage, and at this point, a peak curve is also observed. The maximum settling rate occurred at 0, 50, 100, 600, 800, 1000 g guar gum/tonne of solid dosages are 0.35, 1.01, 1.4, 2.26, 3.74 and 3.69 mm/min, respectively. The settling rate of kaolinite particles decreases continuously after attaining the maximum settling velocity. The stability of kaolinite suspension at different dosages of starch at pH 5 and pH 10 after 24 h are shown in Fig. 6a and b, respectively. Although a settling rate is obtained at pH 10, unlike pH 5, no phase separation is observed at lower dosages. However, a phase separation occurs at the higher starch dosage (1000 g guar gum/tonne of solid). The consolidated sediment height increases with the increase in the starch dosage. Figs. 5a and 6a reveals that particle aggregation occurs without the addition of flocculating reagent at pH 5, which resulted in precise solid and liquid phase separation. The rate of solid-liquid phase separation expedites with the increase in starch dosages. Several researchers reported that the isoelectric points of Al-OH and Si-OH edge sites are between pH 5.0 to 7.0 [35,37,38]. The edge-face kaolinite structures exist because of the electrostatic attraction of positively charged edges and negatively charged basal faces before flocculation at pH below the isoelectric point of edge hydroxyls. The pH of kaolinite slurry is 6.38. It is in between the isoelectric point of hydroxyl edge sites. The edge face attraction and edge face structure are expected at this pH. The FESEM analysis of floc structure confirmed the presence of the edge-face struc-
ture. At pH above the isoelectric point of edge hydroxyls, both the edge and basal surface potential are negative which make the system deflocculated [39]. Du et al. [29] reported that self-aggregation might also occur due to hydrogen bonding between SiAOH and AlAOH edge sites at alkaline pH. The step sites of the poorly crystallized kaolinite may produce some hydrogen bonded edge face and face-face structures. Therefore, at pH 10 the kaolinite particles were highly negatively charged and due to electrostatic repulsion very fine colloidal particle remained in the suspension. However, with the addition of starch dosages the –OH sites available on the poorly crystallized edge and basal faces facilitate the adsorption of organic polymer flocculants by hydrogen bonding [29]. Ma and Pawlik [3] carried out guar gum adsorption studies on various types of mineral surfaces such as alumina, rutile, hematite, quartz, and kaolinite as a function of pH, ionic strength and background electrolyte. They showed that adsorption density of guar gum on these minerals surface is very much pH independent and reported that hydrogen bonding is the primary mechanism of guar gum adsorption onto the mineral surface. In the present investigation, it is observed that the settling rate of kaolinite particles increased with the increase in starch dosages at pH 5 and pH 10. The results confirmed the guar gum adsorption onto the kaolinite surface at both acidic and alkaline pH and induced higher flocculation at former pH than the later. The observations commensurated well with the earlier study done by Ma and Pawlik [3]. A significant amount of highly charged kaolinite particles are not settled at alkaline pH 10 and remain suspended in the solution. After one hour of settling studies, the supernatant solution was decanted and dried. The amount of kaolinite present in the supernatant at different starch dosages is shown in Fig. 7. The figure shows that the kaolinite particle in the clarifying zone decreases with the increase in
Fig. 6. View of kaolinite dispersions stability at 24 h (a) pH 5 and (b) pH 10 after addition of guar gum dosages.
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starch dosage. About 22% of kaolinite particles remain unsettled without any starch. However, at 1000 g guar gum/tonne of solid dosage, only 7% of kaolinite stay in the suspension. The rest of the particles are flocculated and get settled at the bottom. 3.4. Effect of starch dosages on transmission and backscattering
Fig. 7. Effect of guar gum dosages on the kaolinite concentration in the supernatant after 1 h of addition of the starch at pH 10.
The multiple light scattering measurements were carried out on a 5% (w/w) kaolinite suspension using different dosages of guar gum at pH 5 and 10. The scanning process monitors the downward particle movement in the suspension. The particle moves vertically downward by gravity and consolidated at the bottom and formed a clarifying and thickening zone at the top and bottom, respectively. The effect of guar gum on the percentage transmission and backscattering as a function of time is shown in Figs. 8 and 9 at pH 5 and 10, respectively. The backscattering (%) is analyzed at different starch dosages in the entire length of the cell, clarifying zone and the thickening zone at pH 5 (Fig. 8a, c and d) and pH 10 (Fig. 9b, c, and d), respectively. The transmission (%) is analyzed
Fig. 8. Effect of guar gum dosages on the (a) backscattering (%) in entire length of cell; (b) Transmission (%) in clarification zone; (c) backscattering (%) in clarification zone; (d) backscattering (%) in thickening zone at pH 5.
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for the clarification zone of the cell at pH 5 (Fig. 8b) and pH 10 (Fig. 9a) at different starch dosages. Fig. 8a shows that the backscattering (%) decreases with time in the entire length of the cell. The backscattering (%) also decreases with the increase in guar gum dosage. Similar observations are also found in the clarifying zone (Fig. 8c). The backscattering (%) decreases with time, and the fastest decrease is observed at higher starch dosages. The backscattering is decreased from an initial value of 50% to 15% at higher starch dosages. On the other hand, the backscattering (%) is increased with time in the thickening zone (Fig. 8d). The particle consolidation and compaction took place with time in the thickening zone making it more opaque for the source light to penetrate through the bed. Therefore, the backscattering (%) increases with time at each flocculant dosage. Also, it is observed that the backscattering curves shifted towards left with the increase in starch dosages. The particle flocculates and settle fast at higher guar gum dosage and attended a steady state compaction. The steady-state backscattering (%) are attained in 3000, 2883, 2635, 1395, 450, and 450 s, at 0, 50, 100, 200, 400, 600 and 800 g guar gum/tonne of solid dosage, respectively. The corresponding steady backscattering (%) at these dosages are 57.21, 61.86, 64.88, 63.89, 62.2, 61.55 and 61.55, respectively. The highest backscattering of 65% is observed at 100 g guar gum/tonne of solid flocculent dosage. However, with further increase in the starch
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dosages the backscattering (%) decreases. The floc size increases with the increase in starch dosage. The consolidated bed with large flocs increases the bed height as well as porosity. Therefore, bed opaqueness to source light decreases and backscattering (%) decreases at a higher dosage. Fig. 8b shows that the transmission (%) in the clarification zone increases with time and with the increase in starch dosages. About 85% transmission is observed at 800 g guar gum/tonne of solid dosage with no particle in suspension at this zone. The transmission (%) in the entire length of the cell, backscattering (%) in the entire length of the cell, clarifying zone and thickening zone are shown in Fig. 9a–d, respectively at pH 10 and different guar gum dosages. No transmission is observed with time at various starch dosages at pH 10 (Fig. 9a). The zeta potential of kaolinite is 79 mV at pH 10. That means the electrostatic repulsion between the particles is high. Therefore, their stability in water is high. Even at the high starch dosage (1000 g guar gum/tonne of solid), there are 7% of the particle remained in the suspension which obstructs the transmission of source light through the medium. However, the backscattering (%) decreases in the entire cell length with time (Fig. 9b). The backscattering (%) decreases at a faster rate with the increase in starch dosages. The backscattering is reduced from 49% to 38% at 1000 g guar gum/tonne of solid starch dosage. The backscattering (%) in the clarification zone
Fig. 9. Effect of guar gum dosages on the (a) transmission (%) in the entire length of the cell; (b) backscattering (%) in the entire length of the cell; (c) backscattering (%) in clarification zone; (d) backscattering (%) thickening zone at pH 10.
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shows that it decreases with the increase in time and starch dosage (Fig. 9c). It is reduced from 49% to 24% at 1000 g guar gum/tonne of solid dosage. However, the backscattering in the thickening zone shows that it increases with time (Fig. 9d). The higher backscattering (%) in this zone without the addition of starch is related to the size of the aggregated settled mass as explained earlier. 3.5. Effect of starch dosages on floc growth, surface area per volume The effect of starch dosages on the rate of floc growth and surface area per volume of floc were calculated using Turbiscan lab expert software and are shown in Figs. 10 and 11 at pH 5 and pH 10, respectively. Fig. 10a shows the floc growth at different dosages (0–800 g guar gum/tonne of solid) at pH 5. The figure shows that the initial kaolinite particle size is 5–6 mm. The results indicate that at 500 s, the particles start to aggregate, and the size increases up to 348 mm at 0 g guar gum/tonne of solid dosage. With further increase in time, there is an exponential increase in kaolinite particle growth. The time lag exists at the start of aggregation. After the addition of starch (50 g guar gum/tonne of solid), the time lag in the floc formation is reduced to 186 s. The floc formation is instantaneous at higher dosages of starch. The highest rate of floc formation is observed at 800 g guar gum/tonne of solid dosage.
At this dosage, the rate of sedimentation is also highest. Fig. 10b shows that the particle surface area per volume decreases with time. Also at a particular time, it decreases with the increase in guar gum dosage. Fig. 11a shows the rate of floc formation at different dosages (0–1000 g guar gum/tonne of solid) at pH 10. The result shows that the floc growth is slow at a lower concentration of starch and increases with the increase in starch dosage. The particle size is increased from 5 mm to 9.5 mm at a very high dosage of starch (1000 g/t). The surface area per volume of the kaolinite particle decreases with the increase in starch dosage (Fig. 11b) which is corresponding to the observation made in Fig. 11a. According to the DLVO (Dejaguin-Landau-Verwey-Overbeek) theory [40,41], the stability of colloidal dispersions is due to the existence of the potential energy barrier between the particles. The barrier arises as a result of the interactions of the electric double layer and Van der Waals force of attraction. The van der Waals forces are very strong at a short separation distance. The rate of aggregation of these particulates depends on the resultant net forces. The aggregation will reduce at higher net resultant force. Rao et al. [42] showed that for the kaolinite particles, the resultant net force increases from 10 KBT to 109 KBT in aqueous solution with an increase in pH from 2.5 to 11.5. Verwey and Overbeek [41] and Lu et al. [43] reported that there would not be any
Fig. 10. Effect of guar gum dosages on (a) rate of change of floc growth and (b) rate of change of the surface area per volume of kaolinite dispersions at pH 5.
Fig. 11. Effect of guar gum dosages on (a) rate of change of floc growth and (b) rate of change of surface area per volume of kaolinite dispersions at pH 10.
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aggregation if the energy barrier is more than 15 KBT. Therefore, the particles will aggregate at isoelectric point (kaolinite isoelectric point: pH 3.96) with a smallest potential energy barrier. The electric double layer repulsion increases above the isoelectric point and potential energy barrier increase significantly at higher pH that prevents the particle from aggregation. As mentioned above, in an aqueous solution there was a time lag of 500 s before the particle aggregation starts at pH 5, and the energy barrier slowed down the rate of floc growth. The energy barrier is very high at alkaline pH with high electric double layer repulsion. Therefore, no floc growth is observed at pH 10 with highly stable kaolinite dispersion in aqueous solution. The results are good agreement with the observation made by Rao et al. [42], and kaolinite dispersion stability in aqueous solution corroborated well to the DLVO theory. However, the effect of guar gum on the DLVO forces between kaolinite particles needs to establish further. 3.6. Kaolinite floc morphology Fig. 12 show the SEM micrographs of kaolinite flocs formed at pH 5 and 800 g guar gum/tonne of solid dosage. The SEM images of flocs obtained from the consolidated sediment bed are shown in Fig. 12a and b. The figure shows that the kaolinite particles has formed the aggregate structures during flocculation using guar gum by edge-edge, edge face, and face-face interaction. The edge face interaction leads to form a cavity in the floc aggregates. The sizes of the cavity were varying from 100 nm to 2 mm. The floc aggregates are characterized by the presence of extensive particle
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network and cellular structure known as card house structure. A large conglomeration of particles is also observed in the floc aggregates. The SEM image provides direct insight into the real aggregate structure in an aqueous medium without disturbing the aggregate structure. In an attempt to microscopically investigate the flocculation bridging mechanism, the sediment floc aggregates are sheared. They are sheared by using a magnetic stirrer and aggregates are collected at a different interval of time. Fig. 12c and d shows the SEM images of floc aggregate collected at 15 min. The figure shows the stretch of particles in the floc aggregate by shearing action with a fibrous network of starch chains. It is visible that fibrous network of starch chains are bridging the edge-edge and edge-basal surface of kaolinite particle. The kaolinite particle characteristics suggest that the particles are poorly crystallined with complex nanometer scale irregularity with broken steps, edges, the presence of microcrystallite and microislands on the extensive basal surface. These edges are the sites for the polysaccharide adsorption and bridging formation. The bridge formations are observed between the edge-edge, edge face of kaolinite particle with polysaccharide chains. The high amount of starch adsorption on the edges and the basal surface is visible due to complex irregularity. The bridge is formed between these surface sites as proposed by Du et al. [29] and Nabzar et al. [44]. The polysaccharides adsorbed to the surface sites (hydroxyl group on the surface), oriented to the bulk of the solution and adhered to the other particle thereby facilitate the bridge formation. These polysaccharides are also adsorbed at multiple sites on the basal surface preventing bridge formation.
Fig. 12. High resolution SEM images of kaolinite floc at pH 5 and 800 g guar gum/tonne of solid dosage (a and b) porous flocculated kaolinite, face face and edge face flakes structure; (c and d) stretching of flocs by shearing action at 15 min, adsorbed starch onto kaolinite clearly visible, fibrous network of starch chain connecting edge edge, edge and basal face.
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However, edge face and edge-edge bridge formation are more prominent.
3.7. Effect of electrolyte on the kaolinite sedimentation at pH 10 The effect of KNO3 electrolyte concentration on the rate of settling of kaolinite particle at pH 10 and 800 g guar gum/tonne of solid dosage is presented in Fig. 13. The result shows that with the increase in electrolyte concentration the rate of settling increases. Also, the initial 31 s time lag reduced to zero at 0.1 mol KNO3 dosage. The effect of electrolyte concentration on the rate of transmission, backscattering and floc growth at pH 10 and 800 g/t of guar gum dosage is shown in Fig. 14a–c, respectively. Fig. 14a displays the rate of transmission for the entire length of the cell at different dosages of the electrolyte. The result indicates that the transmission percentage increases with the increase in electrolyte concentration. At 0.01 mol KNO3, the transmission of source infrared reaches 80%, signifies that all the particles are flocculated and settled at this electrolyte concentration. The percentages backscattering also decreased with the increase in the electrolyte concentration (Fig. 14b). The rate of floc growth also increased with the increase in electrolyte concentration (Fig. 14c). The floc size increased from 5 mm to 850 mm at a period of 125 s. The zeta potential measurement of kaolinite particles in 0.001 mol KNO3 electrolyte solution shows that the particle potential at pH 10 is 79 mV. Therefore, the electrostatic repulsion between them is high and their stability in water becomes high. The measurements of guar gum adsorption onto kaolinite surface at both pH 5 and pH 10 has confirmed that adsorption is pH independent. Although guar gum adsorbed onto the kaolinite surface,
strong electrostatic repulsion prevented the particles from flocculation. It is clear that the rate of settling of kaolinite particles increases in the presence of electrolyte KNO3. The high affinity of guar gum adsorption onto kaolinite surface in the presence of an electrolyte is realized from the fact that the floc growth and rate of transmission increases significantly at higher electrolyte concentration. The mechanism prevailing the polymer adsorption onto the mineral surfaces includes hydrophobic interaction, hydration forces, hydrogen bonding, chemical and electrostatic interaction [5–8,45]. Wang et al. [9] also suggested that adsorption of guar gum on talc is independent of pH in the presence of 0.1 mol KCl and concluded that electrostatic interaction is not dominant force for its interaction with talc. Similarly, in the present study the electrostatic interaction is not the dominant force for the adsorption of guar gum on the kaolinite surface. The extensive studies carried out by Ma and Pawlik [3] also confirmed the strong affinity of guar gum onto kaolinite surface in the presence of K+ counter ion and independent of pH. The maximum guar gum adsorptions of 1 mg/m2 were reported at K+ counterion concentration in the range of 0.1 mol/L to 1 mol/L [3]. Hiemenz and Rajagopalan [46] showed that monovalent counter ion (Na+ and K+) compressed the electric double layer and consequently reduce the electrophoretic mobility of the kaolinite particle. The reduced mobility may allow the particles to flocculate, and same is observed in the present case where the rate of settling, floc growth and transmission in the clarification zone increases significantly at high ionic strength. Apart from DLVO theory another significant factor that required to be considered for the adsorption of guar gum on kaolinite and their aggregation with or without electrolyte is a non-DLVO force of interaction. Peschel et al. [47] suggested that a short-range repulsive force known as the hydration force is inherently present on quartz/silica surfaces. The hydration force between the quartz surface decreases in the presence of K+ counter ions [48,49]. The quartz surface becomes weakly hydrophobic at the higher ionic strength with K+ counter ion. Similar analogy may also draw for the kaolinite particles that it may become weakly hydrophobic at higher ionic strength at higher K+ ion. The interfacial water layers present on the kaolinite surface get disrupted by poorly hydrated structure breaking ion (K+). The short range hydration force eliminated between the kaolinite surfaces in the presence of chaotropic electrolyte KNO3. The lack of hydration force at higher ionic strength allowed the kaolinite particles to form the floc with higher growth rate. It resulted in a higher rate of settling of the particles.
4. Conclusions Fig. 13. Effect of ionic strength on the rate of settling of kaolinite particles at 800 g/t guar gum dosages.
The present work on the influence of guar gum on the sedimentation behavior of kaolinite is significant since it creates a major
Fig. 14. Effect of ionic strength on the rate of change in (a) transmission (%) (b) backscattering (%) (c) change in floc growth at 800 g guar gum/tonne of solid dosage.
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problem in the tailings dewatering. The scanning electron microscope images of the kaolinite particle show that the particles are poorly crystalline and have micro islands and microcrystallite in the basal surface. The particles have broken edges and multiple steps on the edges. These step sites on the edge and rough basal surface are the sites for guar gum adsorption and promote faceface and edge-face bridging during flocculation. The zeta potential study shows that the isoelectric point of kaolinite particle is pH 3.96. The sedimentation studies were carried out at different pH, ionic strength and flocculant dosage. The pH and ionic strength have a significant effect on the flocculation characteristics of kaolinite using guar gum as a flocculant. Guar gum has shown excellent turbidity removal efficiency with 83% transmission at pH 5 using 200 g guar gum/tonne of solid dosage. However, it has performed poorly at pH 10. Conversely, its turbidity removal efficiency has improved significantly in the presence of potassium nitrate with 80% transmission in the clarifying zone. The opaqueness of consolidated tailing decreases with the increase of guar gum dosage at pH 5 as floc size increases with the increase of dosage. The analysis of the settling rate, supernatant clarity and sediment consolidation suggests that 200 g guar gum/tonne of solid is the optimized dosage for dewatering of the kaolinite dispersions at pH 5. Acknowledgment The authors gratefully acknowledge the financial support from the Ministry of Steel, Government of India, India (GAP 214). References [1] Whistler RL, Hymowitz T. Guar: agronomy, production, industrial use, and nutrition. West Lafayette: Purdue University Press; 1979. [2] Painter TJ, Gonzalez JJ, Hemmer PC. The distribution of D-galactosyl groups in guaran and locust-bean gum: new evidence from periodate. Carbohydr Res 1979;69:217–26. [3] Ma X, Pawlik M. Role of background ions in guar gum adsorption on oxide minerals and kaolinite. J Colloid Interface Sci 2007;313:440–8. [4] Dash M, Dwari RK, Biswal SK, Reddy PSR, Chattopadhyay P, Mishra BK. Studies on the effect of flocculant adsorption on the dewatering of iron ore tailings. Chem Eng J 2011;173:318–25. [5] Mackenzie M. Chemical reagents in the mineral processing industry. Proceeding of D Malhotra, WF Riggs Eds, Colorado: Society of Mineral Engineering, 1986. [6] Pugh RJ. Macromolecular organic depressants in sulfide flotation-a review, 1. Principles, types and applications. Int J Miner Process 1989;25:101–30. [7] Rath RK, Subramanian S. Study on adsorption of guar gum onto biotite mica. Miner Eng 1997;10:1405–20. [8] Rath RK, Subramanian S, Laskowski JS. Adsorption of dextrin and guar gum onto talc. A comparative study. Langmuir 1997;13:6260–6. [9] Wang J, Somasundaran P, Nagaraj DR. Adsorption mechanism of guar gum at solid–liquid interfaces. Miner Eng 2005;18:77–81. [10] Mhlanga SS, O’Connor CT, McFadzean B. A study of the relative adsorption of guar onto pure minerals. Miner Eng 2012;36–38:172–8. [11] Harris PJ, Mapasa K, Canham A, Bradshaw D. Polymers in mineral processing. Proceeding of 3rd UBC–McGill international symposium, Quebec City: Metallurgical Society of CIM, 1999. [12] Ma X, Pawlik M. Effect of alkali metal cations on adsorption of guar gum on to quartz. J Colloid Interface Sci 2005;289:48–55. [13] Nasima T, Pandab AB, Bandyopadhyaya A. Guar gum and guar gum-oligomeric poly (vinyl alcohol) blends as novel flocculants for kaolinated waste water. Int J Biol Macromole 2013;58:140–7. [14] Lapointe M, Barbeau B. Dual starch–polyacrylamide polymer system for improved flocculation. Water Res 2017;124:202–9. [15] Liu Z, Wei H, Li A, Yang H. Evaluation of structural effects on the flocculation performance of a co-graft starch-based flocculant. Water Res 2017;118:160–6. [16] Yang Z, Shang Y, Lu Y, Chen Y, Huang X, Chen A, et al. Flocculation properties of biodegradable amphoteric chitosan-based flocculants. Chem Eng J 2011;172:287–95. [17] Yang Z, Yuan B, Huang X, Zhou J, Cai J, Yang H, et al. Evaluation of the flocculation performance of carboxymethyl chitosan-graft-polyacrylamide, a novel amphoteric chemically bonded composite flocculant. Water Res 2012;46:107–14. [18] Chen Y, Liu S, Wang G. A kinetic investigation of cationic starch adsorption and flocculation in kaolin suspension. Chem Eng J 2007;133:325–33.
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International Journal of Mining Science and Technology journal homepage: www.elsevier.com/locate/ijmst
Evaluation of flocculation characteristics of kaolinite dispersion system using guar gum: A green flocculant R.K. Dwari a,⇑, B.K. Mishra b a b
Mineral Processing Department, CSIR-Institute of Minerals and Materials Technology, Bhubaneswar 751013, India Indian Institute of Technology, Goa 403401, India
a r t i c l e
i n f o
Article history: Received 5 February 2018 Received in revised form 17 January 2019 Accepted 11 June 2019 Available online 18 June 2019 Keywords: Guar gum Dewatering Kaolinite Light scattering Morphology Flocculation
a b s t r a c t This paper reports the systematic investigation on the flocculation, sedimentation and consolidation characteristics of kaolinite using guar gum as a green flocculant. In-situ flocculation behavior of kaolinite at various pH, guar gum dosages, and ionic strength were studied using a light scattering technique. The effect of these parameters on the settling rate, solid consolidation, and supernatant liquid clarity was recorded. The morphology of kaolinite and flocculated kaolinite aggregates were analyzed using FESEM. The morphology studies suggest that it is poorly crystalline with multiple steps on edge, broken edge; laminar with high aspect ratio and have rough basal surface. The complex irregularity on the basal surface and the presence of multiple steps in the edges, broken edges (hydroxyl groups) have facilitated the guar gum adsorption. The isoelectric point of kaolinite is pH 3.96. The pH, ionic strength and flocculant dosage have a significant effect on the kaolinite settling rate. The guar gum has exhibited excellent turbidity removal efficiency at pH 5. The turbidity removal is inefficient at pH 10. However, guar gum has shown high turbidity removal with 80% transmission at pH 10 in the presence of a KNO3 electrolyte. Ó 2019 Published by Elsevier B.V. on behalf of China University of Mining & Technology. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
1. Introduction The removal of water from mineral tailings by flocculation is one of the major mineral processing activities. Recently, synthetic flocculants like polyacrylamide are in use for the sedimentation of minerals from the process tailings and wastewater. However, their environmental compatibility is a matter of great concern. Presently, the demand for green flocculants is increasing due to their environmental compatibility. Guar gum is also known as guaran. It is a natural non-ionic polysaccharide composed of galactose and mannose. It is a chain of (1 ? 4)-linked b-D-mannopyranose units with a-Dgalactopyranose units connected to the mannose backbone through (1 ? 6) glycosidic linkages. The poly-mannose chain randomly substituted with galactose units at mannose-to-galactose ratio of 1.8–1.0 [1,2]. The molecular weight of guar gum is 1 105 to 20 105 [1,2]. Ma and Pawlik [3] studied the adsorption behavior of guar gum on kaolinite and other oxide minerals at different pH, background electrolyte, and ionic strength. One of the major conclusions of their study is that the adsorption density of this polysaccharide
⇑ Corresponding author.
does not depend on the pH. However, there is a significant increase in adsorption density on kaolinite surface in the presence of chaotropic K+ ion. The hydrogen bonding is the primary mechanism of guar gum adsorption onto the kaolinite surface. The polymer adsorbed onto the mineral surface through hydrophobic interaction, hydrogen bonding, chemical and electrostatic interaction [4–8]. Wang et al. [9] concluded that the hydrogen bonding is the principal force for the adsorption of guar gum onto the talc surface. Mhlanga et al. [10] also reported the guar gum adsorption on several pure minerals such as talc, pyroxene, plagioclase, chromite, and chalcopyrite. The above studies show that guar gum has a natural affinity towards these mineral surfaces. The majority of these works were carried out to understand its application as a depressant in flotation and selective flocculation system. However, not many reports are available regarding its possible use as a flocculant. The effect of guar gum as flocculant on the flocculation characteristics of kaolinite particle regarding floc growth, colloidal stability and, solid consolidation are not well understood. Moreover, the flocculation characteristic method is poorly studied. Guar gum has many applications in mining and mineral processing industries. Many researchers have reported its use in froth flotation and flocculation process [9,11]. In the froth flotation process, guar gum is used as a depressant for talceous gangue minerals for the flotation of base metals and platinum group metals bearing
E-mail address: [email protected] (R.K. Dwari). https://doi.org/10.1016/j.ijmst.2019.06.001 2095-2686/Ó 2019 Published by Elsevier B.V. on behalf of China University of Mining & Technology. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
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ores [9,11]. Guar gum adsorbs onto the talc surface to make it hydrophilic. It also has application as a binder for waterinsoluble slimes in the potash flotation process [12]. Guar gum and grafted derivatives of guar gum have been explored as flocculants for dewatering applications [13–15]. Nasim et al. [13] have reported a high settling rate of kaolinite using guar gum and polyvinyl alcohol grafted guar gum as flocculants. Apart from guar gum other polysaccharides and their grafted derivative such as chitosan [16,17], starch [18], xanthan gum [19] and tamarind kernel [20] have also been explored as flocculants. The Indian iron ore contains a significant amount of gangue minerals such as silicates and aluminium silicates. These minerals liberate at finer sizes. Therefore, the tailings generated in an iron ore processing plant are fine in size. One of our earlier studies suggests that d80 of the tailings is 12 mm. The tailings contain 5%-10% solid (w/w) and the Al2O3, SiO2 content in the tailings are 18% and 14%, respectively [4]. The fine particle causes a serious problem in the dewatering process. Thus, the consolidated solid in the thickener contains high moisture, which in turn creates technical and economic difficulties in handling, disposal, and water removal. The basal faces of 1:1 tetra-octahedral aluminosilicate consists of tetrahedral siloxane (-Si-O-Si-) species and octahedral, alumina (Al2O3) sheet [21]. Van Olphen [21] showed that the basal faces having a siloxane structure carry a permanent negative charge due to the isomorphous substitution of Si4+ by Al3+ groups. Also, aluminum (Al-OH) and silanol (Si-OH) groups occur at the edges. Therefore, the edge faces are charged by protonation and deprotonation of hydroxyl groups depending on the pH [21,22]. These particles are highly charged at neutral and alkaline pH and shows stable dispersion behavior. Therefore, authors choose kaolinite an aluminium silicate to study its flocculation characteristics in the presence of guar gum. The design of a dewatering system requires a thorough analysis of the solid-liquid system and slimes stability behavior. The stability of particle can be changed by controlling the pH, ionic strength, addition of flocculant and coagulant [4,23–28]. In flocculation, the fine particles interact with the flocculating agent and aggregate to form flocs. The flocs settle rapidly under the influence of gravity. The characteristics of floc decide the rate of settling and moisture contents of consolidated tailings [29]. The floc characteristics depend on several factors such as particle surface chemistry, size distribution, density, shape; viscosity and dielectric constant of suspension; chemical nature, molecular weight, charge and charge density of the flocculants [30,31]. The environmental policies have enforced legal rules for the disposal of waste by the mineral industries. It is essential to redesign the dewatering operation to meet the current challenges. The present work has been undertaken to study the sedimentation behavior of kaolinite using guar gum as a flocculant. The spectroscopic techniques are used to understand the influence of different process parameter on the flocculation and consolidation characteristics.
2. Experimental 2.1. Materials and procedure The kaolinite sample was supplied by M/s. Ashapura Minechem Ltd., Trivandrum, India for the sedimentation studies. The particle size of the kaolinite sample was measured using Malvern particle size analyzer (Mastersizer 2000). The surface area and density of the kaolinite sample were determined by using smart BET surface analyzer (SORB 92/93) and Helium-Mercury pycno meter (Smart Pycno 30), respectively. Guar gum was used as a flocculant for settling studies of kaolinite dispersions. It was procured from M/s. Otto Chemicals Ltd,
India. The polysaccharide solution was prepared by dissolving 0.1 g of guar gum in 100 ml of distilled water. The guar gum powder was poured into continuously stirred distilled water using a magnetic stirrer. It was further stirred for 6 h to ensure complete dissolution of the guar gum in water. The stirring rate was maintained at 1100 rpm during the preparation. The flocculant solution was prepared every day to maintain its freshness. The ionic strength of the solution was maintained using potassium nitrate, and the pH was regulated by using analytical grade sodium hydroxide and hydrochloric acid solutions. 2.2. Kaolinite suspension The kaolinite dispersions of 5% solids (w/w) were prepared in distilled water for the flocculation characteristic studies. The slurry was agitated at 800 rpm for two hours to obtain the homogeneity. The pH of slurry was maintained at pH 5 and pH 10. For each experiment, 20 ml of the slurry was pipetted out from the homogeneous slurry and put into the glass cell for the light scattering analysis. The suspension was prepared with a known volume of KNO3 to study the effect of electrolyte on kaolinite flocculation and sedimentation at pH 10. 2.3. Zeta potential studies The zeta potential (f) of kaolinite suspension was measured by potentiometric titration using a Zeta probe (Colloidal Dynamics Ltd., USA) instruments. The measurement was carried out on kaolinite suspensions of 5% solids (w/w) dispersed in 103 mol KNO3 electrolyte background solution. The electrolyte background was used to avoid any effects of inconsistent surface conductance on zeta potential [25]. The complete dispersion of the slurry suspension was ensured using ultrasonic vibration. The zeta potential of dispersed kaolinite suspension was measured at the slurry agitation speed maintained at 250 rpm. The zeta potential measurements were carried out in the pH range of 3–12. The detail zeta potential measurement procedure has been discussed in our earlier work [4]. 2.4. Multiple light scattering measurements The sedimentation/suspension stability of kaolinite dispersion was studied by using optical analyzers (TUBISCAN LABexpert) based on multiple light scattering technique. The stability of the dispersion was monitored by measuring the transmittance and backscattering of the pulsed near-infrared light (k = 880 nm). Turbiscan measurements were carried out for the entire vertical length of the glass cell at regular time intervals. The transmittance detector received the light that passed through the dispersion at an angle of 180° concerning the source while the backscattering detector received backscattered light by the dispersion at an angle of 45°. The technique is used to measure the transmittance and backscattering of suspension. The method helped to monitor the particle in the clarifying zone and the bed consolidation. Approximately 20 ml of 5% (w/w) kaolinite dispersions was taken in the glass cell of the turbiscan equipment. The height of the cell filled with the suspension was 40 mm. The temperature was maintained at 30 ± 2 °C during the analysis. A known volume of freshly prepared starch solution was mixed into the kaolinite dispersion, to study the effect of guar gum on the flocculation behavior of kaolinite dispersion. Then the cell was closed with a cap and was shaken by making it up and down for 20 times for better mixing of both. The vertical glass cell height was scanned 100 times in 50 min at an interval of 30 s. After the scanning, the cell was removed from TURBISCAN LABexpert and kept for 24 h and then the height was measured. After the scanning the rate of settling,
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backscattering, transmission, floc growth, were calculated using TURBISCAN LABexpert software. The diffuse reflectance in terms of back scattering is considered to obey Eq. (1) [32]:
(
BS ¼ p1ffiffiIffi 2d I ðd; /Þ ¼ 3/ð1g ÞQ s
ð1Þ
where, BS is the diffuse reflectance at 45° detector, %; I* is the photon transport mean free path; d is the particle diameter, lm; / is particle volume fraction, %; g and Qs are optical parameters according to Lorenz and Mie theory [33]. Hence, backscattering measurement made by the Turbiscan directly depends on the particle mean diameter (d) and volume fractions (/). Therefore, Turbiscan LAbExpert enables the computation of I*, which represents the dispersion state of the product. The computation of the particle mean diameter (d) is possible by knowing the volume fraction (/) and the refractive indices of disperse and the continuous phase. The Lambert-beer law gives an analytical expression of the transmitted flux T (%) measured by the turbiscan as a function of the photon mean free path (I) and expressed as Eq. (2) [33].
T ðI; ri Þ ¼ T o e
2r i i
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backscattering percentage in the thickening zone increases as more particles settled to the bottom. 2.6. Particle and floc morphology The surface morphology of kaolinite particles and their flocs formed at pH 5 and 800 g guar gum/tonne of solid dosage was studied using the scanning electron microscope (FESEM), Supra 55, Carl Zeiss, Germany. The kaolinite particles were dispersed in an aqueous solution and a drop of this dispersion was carefully placed on aluminium foil. The suspension drop was dried at room temperature and put on an FESEM stub to determine its morphology. In case of flocs, they were carefully collected from the consolidated sediment and placed on aluminium foil. After drying, the floc was placed on a FESEM stub for studying floc morphology and external surface structure. The consolidated sediment was also sheared using a magnetic stirrer rotating at 150 rpm. The flocs were collected at a different interval of time and subjected to morphology investigation as mentioned above. 3. Results and discussion
ð2Þ
where To is the transmittance of continuous phase and ri is the measurement cell internal diameter. 2.5. Flocculation studies The flocculation studies were carried out using the multiple light scattering techniques with 5% (w/w) kaolinite dispersions. The experiments were conducted at pH 5 and pH 10 using different dosages of flocculant in the range of 0–1000 g guar gum/tonne of solid, i.e., 0, 50, 100, 200, 400, 600, 800, 1000 g/tonne of solid. The Turbiscan light scattering data (both transmission and backscattering) obtained for kaolinite dispersions in distilled water at pH 5 without starch dosage is shown in Fig. 1. The source infrared light cannot transmit through a stable dispersion and get scattered by fine suspended particles. The backscattered detector detects the scattered light. A high percentage of backscattering suggests stable dispersions. However, as the particles get aggregate, the sedimentation occurs. The particulates present in the suspension decreases which allows the source light to pass through the medium and detected by the transmittance detector. Fig. 1 suggests that the transmission percentage in the clarified zone and
3.1. Material characterization The particle size distribution of kaolinite sample is shown in Fig. 2. The figure shows that the average particle size (d50) of the kaolinite is 3.69 mm whereas d90 is 24 mm (Fig. 2). The BET surface area and density of kaolinite sample is 17.75 m2/g and 2520 kg/m,
Fig. 2. Particle size distribution of kaolinite.
Fig. 1. Transmission and backscattering data of kaolinite dispersion at pH 5.
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respectively. The surface morphology of kaolinite particles is shown in Fig. 3. The SEM images in Fig. 3a demonstrates that the kaolinite has a complex surface structure on the extensive basal surface. The dimension of the particles is in the range from 100 nm to 2000 nm. The high magnification of particle shows that the particles are poorly crystallized with individual crystallite, micro-islands, microcrystallite on the extensive basal surface (Fig. 3b). The multiple steps in the edge and broken edges on the kaolinite particle are also evident (Fig. 3c). Investigation on many areas of the particle shows that it has lamellate plates with high aspect ratio, edge to face platelet orientation and rugged basal face (Fig. 3d). Fig. 4. Zeta potential of 5% (w/w) kaolinite dispersion in 0.001 mol KNO3 background electrolyte.
3.2. Zeta potential studies The zeta potential of kaolinite dispersions at 0.001 mol KNO3 background electrolyte solution is shown in Fig. 4. The figure shows that the isoelectric point of kaolinite is pH 3.96. This value is close to the value observed by other workers [34–36]. Fig. 4 shows that the zeta potential of kaolinite at pH 5 and 10 are 20 mV and 79 mV, respectively. The result suggests that the colloidal kaolinite particulates are highly negatively charged at alkaline pH indicating strong electrostatic repulsion existing between them. 3.3. Effect of starch dosages on the rate of settling The effect of starch dosages on the particulate migration velocity or rate of settling at pH 5 and 10 are calculated from the light scattering data and is shown in Fig. 5. Fig. 5a shows the rate of settling of kaolinite at pH 5 at different dosages of guar gum. The figure demonstrates that there is a time lag on the settling of particles
in the absence of starch and they start to settle at 186 s. The time lag period is attributed to the time taken by the particulates to form aggregate. Further, there is an exponential increase in the rate of settling. At 248 s, the particulates reached the maximum settling rate of 0.36 mm/min and further maintained a steady state of settling. The time lag decreases with the addition of guar gum to the kaolinite suspension. The time lag observed for 50, and 100 g guar gum/tonne of solid dosages is 62 s while it is 31 s for starch dosages between 200 and 800 g guar gum/tonne of solid. The figure shows that the particle settling rate increases with the increase in starch dosage, and maximum settling rate is observed at 800 g guar gum/tonne of solid dosage. The maximum settling rates observed at 50, 100, 200, 400, 600 and 800 g guar gum/tonne of solid dosages are 0.77, 1.09, 2.25, 2.82, 4.44 and 4.78 mm/min, respectively. It is fascinating to observe that the maximum settling rate is obtained at 93 s, and a curve peak is observed at this point. A steady-state settling rate is observed after reaching the maximum
Fig. 3. High resolution FESEM images of kaolinite colloidal fraction (a) kaolinite particulates; (b) poorly crystallised with individual crystallite, microislands, micro crystallite on the extensive basal surface; (c) multiple steps in the edge, broken edges; (d) laminated plates with high aspect ratio, edge to face platelet orientation, rugged basal surface.
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Fig. 5. Effect of guar gum dosages on the rate of settling of kaolinite dispersions at (a) pH 5 and (b) pH 10 in distilled water.
settling velocity at lower starch dosages, i.e., 50–100 g guar gum/ tonne of solid. Fig. 5b shows the rate of settling of kaolinite particle at different dosages of starch at pH 10. The result shows that the rate of settling increases with the increase in starch dosages. The maximum settling rate is obtained at 31 s for each dosage, and at this point, a peak curve is also observed. The maximum settling rate occurred at 0, 50, 100, 600, 800, 1000 g guar gum/tonne of solid dosages are 0.35, 1.01, 1.4, 2.26, 3.74 and 3.69 mm/min, respectively. The settling rate of kaolinite particles decreases continuously after attaining the maximum settling velocity. The stability of kaolinite suspension at different dosages of starch at pH 5 and pH 10 after 24 h are shown in Fig. 6a and b, respectively. Although a settling rate is obtained at pH 10, unlike pH 5, no phase separation is observed at lower dosages. However, a phase separation occurs at the higher starch dosage (1000 g guar gum/tonne of solid). The consolidated sediment height increases with the increase in the starch dosage. Figs. 5a and 6a reveals that particle aggregation occurs without the addition of flocculating reagent at pH 5, which resulted in precise solid and liquid phase separation. The rate of solid-liquid phase separation expedites with the increase in starch dosages. Several researchers reported that the isoelectric points of Al-OH and Si-OH edge sites are between pH 5.0 to 7.0 [35,37,38]. The edge-face kaolinite structures exist because of the electrostatic attraction of positively charged edges and negatively charged basal faces before flocculation at pH below the isoelectric point of edge hydroxyls. The pH of kaolinite slurry is 6.38. It is in between the isoelectric point of hydroxyl edge sites. The edge face attraction and edge face structure are expected at this pH. The FESEM analysis of floc structure confirmed the presence of the edge-face struc-
ture. At pH above the isoelectric point of edge hydroxyls, both the edge and basal surface potential are negative which make the system deflocculated [39]. Du et al. [29] reported that self-aggregation might also occur due to hydrogen bonding between SiAOH and AlAOH edge sites at alkaline pH. The step sites of the poorly crystallized kaolinite may produce some hydrogen bonded edge face and face-face structures. Therefore, at pH 10 the kaolinite particles were highly negatively charged and due to electrostatic repulsion very fine colloidal particle remained in the suspension. However, with the addition of starch dosages the –OH sites available on the poorly crystallized edge and basal faces facilitate the adsorption of organic polymer flocculants by hydrogen bonding [29]. Ma and Pawlik [3] carried out guar gum adsorption studies on various types of mineral surfaces such as alumina, rutile, hematite, quartz, and kaolinite as a function of pH, ionic strength and background electrolyte. They showed that adsorption density of guar gum on these minerals surface is very much pH independent and reported that hydrogen bonding is the primary mechanism of guar gum adsorption onto the mineral surface. In the present investigation, it is observed that the settling rate of kaolinite particles increased with the increase in starch dosages at pH 5 and pH 10. The results confirmed the guar gum adsorption onto the kaolinite surface at both acidic and alkaline pH and induced higher flocculation at former pH than the later. The observations commensurated well with the earlier study done by Ma and Pawlik [3]. A significant amount of highly charged kaolinite particles are not settled at alkaline pH 10 and remain suspended in the solution. After one hour of settling studies, the supernatant solution was decanted and dried. The amount of kaolinite present in the supernatant at different starch dosages is shown in Fig. 7. The figure shows that the kaolinite particle in the clarifying zone decreases with the increase in
Fig. 6. View of kaolinite dispersions stability at 24 h (a) pH 5 and (b) pH 10 after addition of guar gum dosages.
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starch dosage. About 22% of kaolinite particles remain unsettled without any starch. However, at 1000 g guar gum/tonne of solid dosage, only 7% of kaolinite stay in the suspension. The rest of the particles are flocculated and get settled at the bottom. 3.4. Effect of starch dosages on transmission and backscattering
Fig. 7. Effect of guar gum dosages on the kaolinite concentration in the supernatant after 1 h of addition of the starch at pH 10.
The multiple light scattering measurements were carried out on a 5% (w/w) kaolinite suspension using different dosages of guar gum at pH 5 and 10. The scanning process monitors the downward particle movement in the suspension. The particle moves vertically downward by gravity and consolidated at the bottom and formed a clarifying and thickening zone at the top and bottom, respectively. The effect of guar gum on the percentage transmission and backscattering as a function of time is shown in Figs. 8 and 9 at pH 5 and 10, respectively. The backscattering (%) is analyzed at different starch dosages in the entire length of the cell, clarifying zone and the thickening zone at pH 5 (Fig. 8a, c and d) and pH 10 (Fig. 9b, c, and d), respectively. The transmission (%) is analyzed
Fig. 8. Effect of guar gum dosages on the (a) backscattering (%) in entire length of cell; (b) Transmission (%) in clarification zone; (c) backscattering (%) in clarification zone; (d) backscattering (%) in thickening zone at pH 5.
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for the clarification zone of the cell at pH 5 (Fig. 8b) and pH 10 (Fig. 9a) at different starch dosages. Fig. 8a shows that the backscattering (%) decreases with time in the entire length of the cell. The backscattering (%) also decreases with the increase in guar gum dosage. Similar observations are also found in the clarifying zone (Fig. 8c). The backscattering (%) decreases with time, and the fastest decrease is observed at higher starch dosages. The backscattering is decreased from an initial value of 50% to 15% at higher starch dosages. On the other hand, the backscattering (%) is increased with time in the thickening zone (Fig. 8d). The particle consolidation and compaction took place with time in the thickening zone making it more opaque for the source light to penetrate through the bed. Therefore, the backscattering (%) increases with time at each flocculant dosage. Also, it is observed that the backscattering curves shifted towards left with the increase in starch dosages. The particle flocculates and settle fast at higher guar gum dosage and attended a steady state compaction. The steady-state backscattering (%) are attained in 3000, 2883, 2635, 1395, 450, and 450 s, at 0, 50, 100, 200, 400, 600 and 800 g guar gum/tonne of solid dosage, respectively. The corresponding steady backscattering (%) at these dosages are 57.21, 61.86, 64.88, 63.89, 62.2, 61.55 and 61.55, respectively. The highest backscattering of 65% is observed at 100 g guar gum/tonne of solid flocculent dosage. However, with further increase in the starch
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dosages the backscattering (%) decreases. The floc size increases with the increase in starch dosage. The consolidated bed with large flocs increases the bed height as well as porosity. Therefore, bed opaqueness to source light decreases and backscattering (%) decreases at a higher dosage. Fig. 8b shows that the transmission (%) in the clarification zone increases with time and with the increase in starch dosages. About 85% transmission is observed at 800 g guar gum/tonne of solid dosage with no particle in suspension at this zone. The transmission (%) in the entire length of the cell, backscattering (%) in the entire length of the cell, clarifying zone and thickening zone are shown in Fig. 9a–d, respectively at pH 10 and different guar gum dosages. No transmission is observed with time at various starch dosages at pH 10 (Fig. 9a). The zeta potential of kaolinite is 79 mV at pH 10. That means the electrostatic repulsion between the particles is high. Therefore, their stability in water is high. Even at the high starch dosage (1000 g guar gum/tonne of solid), there are 7% of the particle remained in the suspension which obstructs the transmission of source light through the medium. However, the backscattering (%) decreases in the entire cell length with time (Fig. 9b). The backscattering (%) decreases at a faster rate with the increase in starch dosages. The backscattering is reduced from 49% to 38% at 1000 g guar gum/tonne of solid starch dosage. The backscattering (%) in the clarification zone
Fig. 9. Effect of guar gum dosages on the (a) transmission (%) in the entire length of the cell; (b) backscattering (%) in the entire length of the cell; (c) backscattering (%) in clarification zone; (d) backscattering (%) thickening zone at pH 10.
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shows that it decreases with the increase in time and starch dosage (Fig. 9c). It is reduced from 49% to 24% at 1000 g guar gum/tonne of solid dosage. However, the backscattering in the thickening zone shows that it increases with time (Fig. 9d). The higher backscattering (%) in this zone without the addition of starch is related to the size of the aggregated settled mass as explained earlier. 3.5. Effect of starch dosages on floc growth, surface area per volume The effect of starch dosages on the rate of floc growth and surface area per volume of floc were calculated using Turbiscan lab expert software and are shown in Figs. 10 and 11 at pH 5 and pH 10, respectively. Fig. 10a shows the floc growth at different dosages (0–800 g guar gum/tonne of solid) at pH 5. The figure shows that the initial kaolinite particle size is 5–6 mm. The results indicate that at 500 s, the particles start to aggregate, and the size increases up to 348 mm at 0 g guar gum/tonne of solid dosage. With further increase in time, there is an exponential increase in kaolinite particle growth. The time lag exists at the start of aggregation. After the addition of starch (50 g guar gum/tonne of solid), the time lag in the floc formation is reduced to 186 s. The floc formation is instantaneous at higher dosages of starch. The highest rate of floc formation is observed at 800 g guar gum/tonne of solid dosage.
At this dosage, the rate of sedimentation is also highest. Fig. 10b shows that the particle surface area per volume decreases with time. Also at a particular time, it decreases with the increase in guar gum dosage. Fig. 11a shows the rate of floc formation at different dosages (0–1000 g guar gum/tonne of solid) at pH 10. The result shows that the floc growth is slow at a lower concentration of starch and increases with the increase in starch dosage. The particle size is increased from 5 mm to 9.5 mm at a very high dosage of starch (1000 g/t). The surface area per volume of the kaolinite particle decreases with the increase in starch dosage (Fig. 11b) which is corresponding to the observation made in Fig. 11a. According to the DLVO (Dejaguin-Landau-Verwey-Overbeek) theory [40,41], the stability of colloidal dispersions is due to the existence of the potential energy barrier between the particles. The barrier arises as a result of the interactions of the electric double layer and Van der Waals force of attraction. The van der Waals forces are very strong at a short separation distance. The rate of aggregation of these particulates depends on the resultant net forces. The aggregation will reduce at higher net resultant force. Rao et al. [42] showed that for the kaolinite particles, the resultant net force increases from 10 KBT to 109 KBT in aqueous solution with an increase in pH from 2.5 to 11.5. Verwey and Overbeek [41] and Lu et al. [43] reported that there would not be any
Fig. 10. Effect of guar gum dosages on (a) rate of change of floc growth and (b) rate of change of the surface area per volume of kaolinite dispersions at pH 5.
Fig. 11. Effect of guar gum dosages on (a) rate of change of floc growth and (b) rate of change of surface area per volume of kaolinite dispersions at pH 10.
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aggregation if the energy barrier is more than 15 KBT. Therefore, the particles will aggregate at isoelectric point (kaolinite isoelectric point: pH 3.96) with a smallest potential energy barrier. The electric double layer repulsion increases above the isoelectric point and potential energy barrier increase significantly at higher pH that prevents the particle from aggregation. As mentioned above, in an aqueous solution there was a time lag of 500 s before the particle aggregation starts at pH 5, and the energy barrier slowed down the rate of floc growth. The energy barrier is very high at alkaline pH with high electric double layer repulsion. Therefore, no floc growth is observed at pH 10 with highly stable kaolinite dispersion in aqueous solution. The results are good agreement with the observation made by Rao et al. [42], and kaolinite dispersion stability in aqueous solution corroborated well to the DLVO theory. However, the effect of guar gum on the DLVO forces between kaolinite particles needs to establish further. 3.6. Kaolinite floc morphology Fig. 12 show the SEM micrographs of kaolinite flocs formed at pH 5 and 800 g guar gum/tonne of solid dosage. The SEM images of flocs obtained from the consolidated sediment bed are shown in Fig. 12a and b. The figure shows that the kaolinite particles has formed the aggregate structures during flocculation using guar gum by edge-edge, edge face, and face-face interaction. The edge face interaction leads to form a cavity in the floc aggregates. The sizes of the cavity were varying from 100 nm to 2 mm. The floc aggregates are characterized by the presence of extensive particle
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network and cellular structure known as card house structure. A large conglomeration of particles is also observed in the floc aggregates. The SEM image provides direct insight into the real aggregate structure in an aqueous medium without disturbing the aggregate structure. In an attempt to microscopically investigate the flocculation bridging mechanism, the sediment floc aggregates are sheared. They are sheared by using a magnetic stirrer and aggregates are collected at a different interval of time. Fig. 12c and d shows the SEM images of floc aggregate collected at 15 min. The figure shows the stretch of particles in the floc aggregate by shearing action with a fibrous network of starch chains. It is visible that fibrous network of starch chains are bridging the edge-edge and edge-basal surface of kaolinite particle. The kaolinite particle characteristics suggest that the particles are poorly crystallined with complex nanometer scale irregularity with broken steps, edges, the presence of microcrystallite and microislands on the extensive basal surface. These edges are the sites for the polysaccharide adsorption and bridging formation. The bridge formations are observed between the edge-edge, edge face of kaolinite particle with polysaccharide chains. The high amount of starch adsorption on the edges and the basal surface is visible due to complex irregularity. The bridge is formed between these surface sites as proposed by Du et al. [29] and Nabzar et al. [44]. The polysaccharides adsorbed to the surface sites (hydroxyl group on the surface), oriented to the bulk of the solution and adhered to the other particle thereby facilitate the bridge formation. These polysaccharides are also adsorbed at multiple sites on the basal surface preventing bridge formation.
Fig. 12. High resolution SEM images of kaolinite floc at pH 5 and 800 g guar gum/tonne of solid dosage (a and b) porous flocculated kaolinite, face face and edge face flakes structure; (c and d) stretching of flocs by shearing action at 15 min, adsorbed starch onto kaolinite clearly visible, fibrous network of starch chain connecting edge edge, edge and basal face.
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However, edge face and edge-edge bridge formation are more prominent.
3.7. Effect of electrolyte on the kaolinite sedimentation at pH 10 The effect of KNO3 electrolyte concentration on the rate of settling of kaolinite particle at pH 10 and 800 g guar gum/tonne of solid dosage is presented in Fig. 13. The result shows that with the increase in electrolyte concentration the rate of settling increases. Also, the initial 31 s time lag reduced to zero at 0.1 mol KNO3 dosage. The effect of electrolyte concentration on the rate of transmission, backscattering and floc growth at pH 10 and 800 g/t of guar gum dosage is shown in Fig. 14a–c, respectively. Fig. 14a displays the rate of transmission for the entire length of the cell at different dosages of the electrolyte. The result indicates that the transmission percentage increases with the increase in electrolyte concentration. At 0.01 mol KNO3, the transmission of source infrared reaches 80%, signifies that all the particles are flocculated and settled at this electrolyte concentration. The percentages backscattering also decreased with the increase in the electrolyte concentration (Fig. 14b). The rate of floc growth also increased with the increase in electrolyte concentration (Fig. 14c). The floc size increased from 5 mm to 850 mm at a period of 125 s. The zeta potential measurement of kaolinite particles in 0.001 mol KNO3 electrolyte solution shows that the particle potential at pH 10 is 79 mV. Therefore, the electrostatic repulsion between them is high and their stability in water becomes high. The measurements of guar gum adsorption onto kaolinite surface at both pH 5 and pH 10 has confirmed that adsorption is pH independent. Although guar gum adsorbed onto the kaolinite surface,
strong electrostatic repulsion prevented the particles from flocculation. It is clear that the rate of settling of kaolinite particles increases in the presence of electrolyte KNO3. The high affinity of guar gum adsorption onto kaolinite surface in the presence of an electrolyte is realized from the fact that the floc growth and rate of transmission increases significantly at higher electrolyte concentration. The mechanism prevailing the polymer adsorption onto the mineral surfaces includes hydrophobic interaction, hydration forces, hydrogen bonding, chemical and electrostatic interaction [5–8,45]. Wang et al. [9] also suggested that adsorption of guar gum on talc is independent of pH in the presence of 0.1 mol KCl and concluded that electrostatic interaction is not dominant force for its interaction with talc. Similarly, in the present study the electrostatic interaction is not the dominant force for the adsorption of guar gum on the kaolinite surface. The extensive studies carried out by Ma and Pawlik [3] also confirmed the strong affinity of guar gum onto kaolinite surface in the presence of K+ counter ion and independent of pH. The maximum guar gum adsorptions of 1 mg/m2 were reported at K+ counterion concentration in the range of 0.1 mol/L to 1 mol/L [3]. Hiemenz and Rajagopalan [46] showed that monovalent counter ion (Na+ and K+) compressed the electric double layer and consequently reduce the electrophoretic mobility of the kaolinite particle. The reduced mobility may allow the particles to flocculate, and same is observed in the present case where the rate of settling, floc growth and transmission in the clarification zone increases significantly at high ionic strength. Apart from DLVO theory another significant factor that required to be considered for the adsorption of guar gum on kaolinite and their aggregation with or without electrolyte is a non-DLVO force of interaction. Peschel et al. [47] suggested that a short-range repulsive force known as the hydration force is inherently present on quartz/silica surfaces. The hydration force between the quartz surface decreases in the presence of K+ counter ions [48,49]. The quartz surface becomes weakly hydrophobic at the higher ionic strength with K+ counter ion. Similar analogy may also draw for the kaolinite particles that it may become weakly hydrophobic at higher ionic strength at higher K+ ion. The interfacial water layers present on the kaolinite surface get disrupted by poorly hydrated structure breaking ion (K+). The short range hydration force eliminated between the kaolinite surfaces in the presence of chaotropic electrolyte KNO3. The lack of hydration force at higher ionic strength allowed the kaolinite particles to form the floc with higher growth rate. It resulted in a higher rate of settling of the particles.
4. Conclusions Fig. 13. Effect of ionic strength on the rate of settling of kaolinite particles at 800 g/t guar gum dosages.
The present work on the influence of guar gum on the sedimentation behavior of kaolinite is significant since it creates a major
Fig. 14. Effect of ionic strength on the rate of change in (a) transmission (%) (b) backscattering (%) (c) change in floc growth at 800 g guar gum/tonne of solid dosage.
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problem in the tailings dewatering. The scanning electron microscope images of the kaolinite particle show that the particles are poorly crystalline and have micro islands and microcrystallite in the basal surface. The particles have broken edges and multiple steps on the edges. These step sites on the edge and rough basal surface are the sites for guar gum adsorption and promote faceface and edge-face bridging during flocculation. The zeta potential study shows that the isoelectric point of kaolinite particle is pH 3.96. The sedimentation studies were carried out at different pH, ionic strength and flocculant dosage. The pH and ionic strength have a significant effect on the flocculation characteristics of kaolinite using guar gum as a flocculant. Guar gum has shown excellent turbidity removal efficiency with 83% transmission at pH 5 using 200 g guar gum/tonne of solid dosage. However, it has performed poorly at pH 10. Conversely, its turbidity removal efficiency has improved significantly in the presence of potassium nitrate with 80% transmission in the clarifying zone. The opaqueness of consolidated tailing decreases with the increase of guar gum dosage at pH 5 as floc size increases with the increase of dosage. The analysis of the settling rate, supernatant clarity and sediment consolidation suggests that 200 g guar gum/tonne of solid is the optimized dosage for dewatering of the kaolinite dispersions at pH 5. Acknowledgment The authors gratefully acknowledge the financial support from the Ministry of Steel, Government of India, India (GAP 214). References [1] Whistler RL, Hymowitz T. Guar: agronomy, production, industrial use, and nutrition. West Lafayette: Purdue University Press; 1979. [2] Painter TJ, Gonzalez JJ, Hemmer PC. The distribution of D-galactosyl groups in guaran and locust-bean gum: new evidence from periodate. Carbohydr Res 1979;69:217–26. [3] Ma X, Pawlik M. Role of background ions in guar gum adsorption on oxide minerals and kaolinite. J Colloid Interface Sci 2007;313:440–8. [4] Dash M, Dwari RK, Biswal SK, Reddy PSR, Chattopadhyay P, Mishra BK. Studies on the effect of flocculant adsorption on the dewatering of iron ore tailings. Chem Eng J 2011;173:318–25. [5] Mackenzie M. Chemical reagents in the mineral processing industry. Proceeding of D Malhotra, WF Riggs Eds, Colorado: Society of Mineral Engineering, 1986. [6] Pugh RJ. Macromolecular organic depressants in sulfide flotation-a review, 1. Principles, types and applications. Int J Miner Process 1989;25:101–30. [7] Rath RK, Subramanian S. Study on adsorption of guar gum onto biotite mica. Miner Eng 1997;10:1405–20. [8] Rath RK, Subramanian S, Laskowski JS. Adsorption of dextrin and guar gum onto talc. A comparative study. Langmuir 1997;13:6260–6. [9] Wang J, Somasundaran P, Nagaraj DR. Adsorption mechanism of guar gum at solid–liquid interfaces. Miner Eng 2005;18:77–81. [10] Mhlanga SS, O’Connor CT, McFadzean B. A study of the relative adsorption of guar onto pure minerals. Miner Eng 2012;36–38:172–8. [11] Harris PJ, Mapasa K, Canham A, Bradshaw D. Polymers in mineral processing. Proceeding of 3rd UBC–McGill international symposium, Quebec City: Metallurgical Society of CIM, 1999. [12] Ma X, Pawlik M. Effect of alkali metal cations on adsorption of guar gum on to quartz. J Colloid Interface Sci 2005;289:48–55. [13] Nasima T, Pandab AB, Bandyopadhyaya A. Guar gum and guar gum-oligomeric poly (vinyl alcohol) blends as novel flocculants for kaolinated waste water. Int J Biol Macromole 2013;58:140–7. [14] Lapointe M, Barbeau B. Dual starch–polyacrylamide polymer system for improved flocculation. Water Res 2017;124:202–9. [15] Liu Z, Wei H, Li A, Yang H. Evaluation of structural effects on the flocculation performance of a co-graft starch-based flocculant. Water Res 2017;118:160–6. [16] Yang Z, Shang Y, Lu Y, Chen Y, Huang X, Chen A, et al. Flocculation properties of biodegradable amphoteric chitosan-based flocculants. Chem Eng J 2011;172:287–95. [17] Yang Z, Yuan B, Huang X, Zhou J, Cai J, Yang H, et al. Evaluation of the flocculation performance of carboxymethyl chitosan-graft-polyacrylamide, a novel amphoteric chemically bonded composite flocculant. Water Res 2012;46:107–14. [18] Chen Y, Liu S, Wang G. A kinetic investigation of cationic starch adsorption and flocculation in kaolin suspension. Chem Eng J 2007;133:325–33.
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