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Surface properties of red mud particles from potentiometric titration
Colloids and Surfaces A: Physicochemical and Engineering Aspects 182 (2001) 131– 141 www.elsevier.nl/locate/colsurfa
Surface properties of red mud particles from potentiometric titration D. Chvedov a,*, S. Ostap a, T. Le b a
Alcan International Limited, Kingston R&D Centre, P.O. Box 8400, 945 Princess Street, Kingston, Ont., Canada K7L 5L9 b Uni6ersity of Waterloo, Waterloo, Canada Received 24 April 2000; accepted 8 November 2000
Abstract Acid/basic potentiometric titration has been used to obtain data on the surface charge and the amount of surface hydroxyl groups on red mud particles generated from different bauxite sources. It has been demonstrated that this can be used to quantify the surface properties of red mud particles. Red mud particles carry a significant negative charge under the basic conditions that exist in the Bayer process for alumina production due to ionized hydroxyl groups on their surfaces. Hydroxyl groups and corresponding surface charge are primarily due to the silica-containing compounds formed in the Bayer process. Addition of a synthetic, organic flocculant to red mud slurries causes the negative charge on the surface of mud particles to rise, which is reflected in the shift of the point of zero charge (PZC) to lower pH values. Under the conditions studied, red mud slurries have the same PZC after flocculation with synthetic flocculant, independent of the bauxite origin and variation in digestion conditions. In general the PZC was found to be sensitive to the presence of flocculant in the slurry and the mineralogical composition. When compared to sodium ions, potassium ions have a higher affinity to the surface of red mud particles and exhibit preferential adsorption. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Surface; charge; Red mud; Titration; Bauxites
1. Introduction Red mud is the insoluble residue remaining after the caustic digestion of bauxite, the ore used in the production of alumina by the Bayer process. One of the major steps of this process is the separation of red mud solids from caustic-alumi* Corresponding author. Tel.: +1-613-5412089; fax: +1613-5412134. E-mail address: [email protected] (D. Chvedov).
nate solution by means of flocculation and decantation. This process is very sensitive to the chemical and mineralogical composition of the processed bauxite and operating conditions [1]. The rate of formation of flocs and their stability strongly depends on the surface chemistry of the particles. The other important aspect of the flocculation process is clarity of the supernatant liquor, which is governed by the particle size distribution and is also related to the surface properties of red mud particles. The importance
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of the surface chemistry in the Bayer process and particularly in the process of red mud separations has been recently reviewed by Hind et al. [2]. Red muds consist of a mixture of caustic insoluble minerals originally present in the bauxite, a reaction product formed by clay or quartz in the bauxite reacting with the caustic-aluminate solution used to digest the bauxite, and various calcium compounds formed by the charging of lime to bauxite digestion or some other part of the process. The caustic insoluble bauxite minerals are typically hematite (Fe2O3) and aluminin goethite ((Fe, Al)OOH) along with the titanium dioxides, anatase (TiO2) and rutile (TiO2). Some boehmite (AlOOH) may also be present if the process is not designed to extract this form of alumina. The remaining elements in bauxite are present in the muds only in minor or trace quantities. The major caustic insoluble reaction product formed is the sodium aluminum silicate, Bayer-sodalite, (3(Na2O·Al2O3·2SiO2·nH2O)·Na2X) where X may be CO3, SO4, 2OH, 2Cl or a mixture of any or all depending upon the impurity composition of the digesting liquor. The value ‘n’ may range from 0 to 2. Lime charged to process will react to form CaCO3, 3CaO·Al2O3·6H2O and a form of calcium phosphate, i.e. carbonate- or hydroxyapatite. At high bauxite digestion temperatures (230 – 260°C), the lime can also react with the titanium mineral anatase to form perovskite (CaTiO3) and/or kassite (CaTi2O4(OH)2). The definition of red mud particles is essential for understanding the surface properties of red mud slurries. The mineral constituents of red muds do not necessarily occur as discrete particles and may be physically associated with other minerals. Desilication products precipitate as a discrete phase and also on the other constituents present in the mud. The small individual particles exist in the slurry as heterogeneous natural aggregates. Adding flocculant to the slurry, flocculates these natural aggregates rather than individual particles. The acid/base potentiometric technique used in the present study probes the surfaces that are accessible for protons under studied conditions. Taking into account the mobility of protons and long equilibration time in our experiments with red muds it is reasonable to assume that the
surface of the natural aggregates is involved in the proton exchange reaction. Therefore, in the present study heterogeneous natural primary aggregates are referred to as ‘red mud particles’. Sodalites which are zeolite-type compounds with an extremely high ion exchange capacity may have a significant influence on the surface properties of red mud slurries. The surface chemistry of red mud particles is extremely complicated as the composition of red mud slurries varies considerably depending on the type of bauxite and digestion conditions. At the same time there is much uncertainty about the exact chemical composition on the surface due to the difficulties of analyzing the thin surface layer of extremely small particles ranging from 50 A, to 1 mm. However, it is well known that most of the minerals and oxides found in red mud slurries demonstrate acid/base type of behaviour in aqueous solutions [3]. Therefore it is reasonable to expect that the surface of red mud particles will exhibit similar type of behaviour. The surface OH groups responsible for the acid/base properties of the particles can be important for the interaction with flocculant molecules serving as active sites. Surface charge properties of the particles can play a substantial role in the process of flocculation. The similar charge distributed on the surfaces of the particles acts to prevent them from flocculating. Thus, the addition of a long-chained organic bridging species or some mechanism of neutralizing the repulsive electrostatic forces is needed to allow flocculation to take place. The conventional explanation of the effect of the ‘flocculant molecules’ flocculating the mud is based on the concept of long-chained polymers adsorbing on more than one particle and acting to bridge them [4]. However, both carry negative charges and this mechanism is possible only with the assumption that positively charged sodium ions act as an intermediate species in the process of bridging. Surface charge properties can be determined by means of potentiometric (acid/base) titration. This has been widely used to characterize the hydroxyl surface groups of oxides and clay minerals [5–10]. Recently this technique was used to determine the
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surface charge properties of activated and pretreated red mud particles [11,12]. The main objective of the present study was to determine the surface charge and the amount of hydroxyl groups on the surface of red mud particles generated from bauxites of various mineralogical compositions. The other aspect of the present study was to explore the flocculation effect on the surface properties of the red mud solids. Flocculated and unflocculated red mud slurries generated from bauxites from different geographical sources have been studied using acid/base potentiometric titration.
2. Experimental methods
2.1. Preparation of red mud samples The red mud samples were prepared by digesting the bauxites under conditions similar to those
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used in both the low and high temperatures digestion stages of the Bayer process. The digest medium was 3.9 or 4.5 M NaOH –NaAlO2 solution. The digests were done in 6-l, heated, agitated autoclaves fitted with a sliding cup apparatus by which bauxite could be charged to the digestion solution preheated to the desired digestion temperature. Table 1 shows the digestion conditions used to prepare each mud sample. Bauxite, ground to pass 100 mesh (149 mm) was dry mixed with ground quicklime, and charged to the sliding cup arrangement located above the solution surface in the autoclave. Once charged with bauxite and digest solution, the autoclave was closed and the solution was heated to the desired temperature. The cup with the bauxite –lime mix was then lowered into the hot agitated liquor. Upon completion of digestion, the slurry was allowed to cool to atmospheric pressure for 45 min before draining the autoclave. The red mud slurry was split into two parts, with one part being floccu-
Table 1 Bauxite digestion and red mud flocculation conditions Bauxite sample
Bauxite digestion conditions
Flocculation conditions
T (°C)
Time (min)
NaOH (mol l−1)
Lime charge (mol l−1 as CaO)
Type of flocculant
Dosage (ppm)
Claremont
230
30
3.92
0.014
Bauxite mix
143
45
3.90
0.012
Weipa
245
30
4.54
0.031
Mix of Nalco 9779 and 190 Alclar 665 Mix of Alclar 665 and 200 Alclar-663 Nalco 9779 50
Table 2 Properties of dried red muds Type of red mud (from bauxite)
Specific gravity (kg m−3)
Surface area (m2 g−1)
Total pore volume (cm3 g−1)
Claremont (unflocculated) Claremont (flocculated) Bauxite mix (unflocculated) Bauxite mix (flocculated) Weipa (unflocculated) Weipa (flocculated)
3500 3200 3000 3000 2900 3000
48.7 26.8 N.A.a 20.0 22.2 21.9
0.21 0.15 N.A.a 0.18 0.23 0.14
a
Not available.
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lated by addition of flocculant solution. The red muds from the flocculated and non-flocculated slurries were then separated by centrifugation and the last traces of digest liquor removed by five water washes. Portions of the centrifuge cakes were dried for 2 h at 105°C and analysed for the properties shown in Table 2. Bauxites used in the studies originated in the following locations: the Claremont district of Jamaica, Cape York in northern Australia (Weipa), the Boke region of Guinea, Africa and the Amazon basin in northern Brazil (bauxite mix). Bauxites from Jamaica belong to the soillike, terra rosa type with negligible aggregation of any of its submicron mineral components. Gibbsite occurs as platelets less than 0.1 um in size. Typically the bauxites contain about 18% Fe2O3 as a mix of hematite and aluminin goethite and 1 – 2% clay mineral. Weipa bauxites from Australia consist of 0.5 – 3.0 mm loose pisolites in a finer matrix. This bauxite is unique in that its 15% Fe2O3 is present only as hematite. It is high in boehmite and contains up to 2% SiO2 as quartz and up to 10% clay mineral. The bauxites from Guinea and Brazil are hard, highly aggregated, massive aluminous laterites whose mining requires drilling and blasting. Gibbsite forming the matrices of both bauxites is coarsely crystalline. The Guinea bauxite has a high boehmite content, about 15% Fe2O3 as a mix of hematite and aluminin goethite and 2 – 3% clay. The Amazon bauxite contains 10 – 11% Fe2O3 as a goethite – hematite mix, less than 1% boehmite and up to 10% clay. The iron minerals in the bauxites from Guinea, Brazil and Australia are dispersed throughout the gibbsite matrices but also occur as small aggregated masses. In Jamaican bauxites, no aggregation of the fine iron particles takes place. High molecular weight sodium polyacrylates (17 000 000 g mol − 1) obtained from Nalco Chemicals (Nalco 9779) and Allied Colloids (Alclar 665 and 663) were used in the study as flocculants. According to the manufacturer’s description these flocculants consists of coiled nonbranched polymer chains containing charged acidic groups along the chain, balanced by
sodium ions. In strong caustic solution these chains straighten out [4].
2.2. Analysis of red mud slurries The surface charge properties of red mud particles were determined using potentiometric techniques described in the literature [8–10]. The conventional method [10] was slightly modified, as described below, to reflect the specific properties of red mud slurries, i.e. the high viscosity of undiluted slurries and stickiness of muds, especially the non-flocculated slurries. About 0.13 g (on a dry base) of centrifuged red mud solids generated as discussed was placed into preweighed plastic bottles with 70 ml of solution containing electrolyte (NaCl or KNO3) and acid or base. Samples were then equilibrated for approximately 18 h on a reciprocal shaker with a speed of 60 oscillations per min., centrifuged at 2500 rpm, decanted and the pH of the supernatants determined using a Accumet-50 ion/pH meter (Denver Instrument) and polymer body pH combination electrode. The pH meter was calibrated daily with a set of standards over the wide range of pH 4.0, 7.0, 9.0, 11.0 in accordance to the standard procedure. NaCl, NaOH and KNO3 solutions were prepared using nanopure water and ASC grade salts from Fisher Scientific. A 1 N HCl solution (Fisher Scientific, certified) was used in the present study. The exact HCl concentration was confirmed by titration with standard NaOH. Each sample of red mud slurry was titrated by acid and base at least in three different concentrations of indifferent electrolyte (NaCl or KNO3). The effect of the contact time on the pH of solutions was studied for the acid and base titrations of Jamaican red mud slurries. After measuring the pH as described above, the supernatant was re-combined with the corresponding red mud residue and left to shake overnight. After an additional 18–20 h of shaking, the slurry was centrifuged and the pH was determined once more as previously described. The shapes of the acid and base titration curves were the same after an additional day of shaking. However, the pH values shifted slightly (0.2 –0.4
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Fig. 1. Titration curves of flocculated red mud slurry generated from Weipa bauxite.
pH units). In all cases, the pH shifted towards more basic values for both base and acid titrations. The increase in pH values for the base titrations was more significant at higher pH than at lower pH values. This evidence supports the conclusion that changes in pH with increase of contact time are caused by the dissolution of red mud particles in the process of shaking. The overall shift in pH induced by the increase in contact time was negligible and one day of shaking was found to be enough to reach equilibrium. All analysed data are based on 18 h of contact time. Surface area and porosity of red mud particles were determined by means of an Autosorb1 analyzer according to the standard procedure for the B.E.T. analysis using nitrogen as adsorbant. Samples of red mud slurries were dried at 90, 105 and 110°C for several hours in a convection oven and then outgassed before the analysis for 30 min at the same temperatures. The temperature of 90°C was found to be insufficient in certain cases to expel adsorbed water and resulted in random B.E.T. results for the surface area. Surface areas obtained for the samples dried and outgassed at 110°C were used in subsequent calculations. The results are pre-
sented in Table 2 along with the total pore volumes. Almost all of the main constituents in red mud particles are soluble in acidic mediums and some of them in strong caustic. However these compounds are practically insoluble at neutral pH. Dissolution of the surface causing possible changes in the surface properties was considered because of the long equilibration time at constant shaking. Selected supernatants were analysed by ICP-AS for elemental content to determine any red mud dissolution.
3. Results and discussion
3.1. Acid/base beha6iour of red mud slurries Typical acid/base titration curves for flocculated and unflocculated red mud slurries are shown in Figs. 1 and 2. Titration curves exhibit the response of the system to the addition of acid or base as they show changes in pH of the bulk of the slurry after addition of acid or base. The amount of HCl added to the solution is expressed in moles. It was assumed that species
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trapped by red mud particles from caustic solution after digestion other than hydroxyl ions did not affect the titration results to any significant extent. The main constituents of the caustic solution used in the digestion that could consume
protons in the process of titration were organics containing carboxylic groups, carbonate and aluminate ions. However, the washing process resulted in dilution of the red mud slurry after digestion by at least 10 000 times as indicated
Fig. 2. Titration curves of unflocculated red mud slurry generated from Claremont bauxite.
Fig. 3. Generic titration curves of red mud slurry (dotted line) and caustic solution (solid line).
D. Ch6edo6 et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 182 (2001) 131–141 Table 3 Estimate of the surface hydroxyl groups on red mud particles Red mud sample from bauxite
Mols of hydroxyl groups on dry red mud basis Mol kg−1
Claremont (unflocculated) Claremont (flocculated) Bauxite mix (flocculated) Weipa (unflocculated) Weipa (flocculated)
104 Mol m−2 of mud surface
6.1
1.2
7.4
2.7
1.92
9.6
2.61
11.9
2.72
12.4
by the drop in caustic concentration of the slurry. Fig. 3 illustrates the schematic acid titration curves of red mud suspensions (dotted line) and a caustic solution (solid line). The pH of the initial point for all titration curves reflects the amount of free caustic in the bulk of solution at the beginning of titration. In case of red mud slurries this pH (on average around 10.5) will correspond to the amount of caustic trapped by the slurry and released in the process of equilibration during shaking. Addition of red mud particles to the caustic solution changes the shape of the acid/base titration curve dramatically. In pure caustic solution the drop in pH is due only to the consumption of protons due to neutralization of free OH− ions in the bulk of solution. Red mud particles in basic aqueous solutions carry ionized surface hydroxyl groups (SO−) and can consume protons in accordance to the reaction: SO− +H+ =SOH, where SO− is an ionized, exposed oxygen atom on the surface of the particle. Therefore, caustic solutions of red mud slurries have an additional way of consuming protons without changing the pH of the suspension. Because free hydroxide ions in solution are more mobile and therefore more accessible, they will be titrated first and only small amount of SO− groups will be titrated at this stage (Zone I in Fig. 3). The
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ionization constant for the reaction SOH =S– O− + H+ (Ka) is much higher than the ionization constant of water. In the present study it was not possible to accurately determine the ionization constants of the surface hydroxyl groups from the titration curves because of the lack of data points. At neutral pH all of the free hydroxide ions will be consumed during the titration. The classic titration curve of pure caustic shows a sharp drop to an acidic pH. The titration curves of red mud slurries at neutral pH flatten out to different extents depending on the types of slurry (Zone II, Fig. 3). At this point only surface hydroxyl groups (SO− and SOH) are titrated to form SOH and SOH+. Almost all of the protons added to the slurry are consumed by surface hydroxyl groups which results in nearly a constant pH in the bulk of the solution. At the end of Zone II, all of the surface groups have been titrated and the titration curve for red mud slurries and caustic solutions meet in Zone III where the drop in pH is due to the accumulation of protons in solution. With the assumption that only a minor amount of SO− groups has been titrated before the slurry has reached neutral pH (Zone I/II), the first bend in the titration curve corresponds to the beginning of the titration of the surface hydroxyl groups. The end of the titration of the surface groups occurs at about a neutral pH and is marked by the second point of inflection in the titration curve (Zone II/III). From the two inflection points on the titration curves the amounts of surface hydroxyl groups were calculated for the red mud slurries and the values are summarized in Table 3. The amount of hydroxyl groups on the surface determined in the present study are one to two orders of magnitude higher than the average values obtained for metal oxides 1–3·10 − 5 mol m − 2 [13]. This difference can be attributed to the high sodalite content of the red muds studied, especially in the case of the Weipa mud. As a zeolite-type compound, with 2–3A, diameter channels, sodalite has a high internal surface area accessible for protons with exposed oxygen atoms that might serve as an adsorption site for
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protons. Sodalites also contain a considerable amount of water molecules trapped in their caged structure that may interact with protons. Mud generated from Jamaican bauxite consists mostly of fine iron oxides and minor amounts (less than 1%) of Bayer-sodalite. Weipa red mud, on the other hand, is one third Bayer-sodalite. Both types of mud will contain calcium in the form of calcium aluminate, titanates and hydroxyapatite. The amount of the surface hydroxyl groups is roughly proportional to the reactive silica content in the original bauxites, which suggests that sodalite is one of the major constituents on the surface of red mud particles. This conclusion is in agreement with previously reported correlation between the sodalite content and ion exchange capacity of red mud particles [14]. As indicated in Table 3 addition of the flocculant to red mud slurry results in an increase in the amount of acid required to neutralize surface groups, especially for Jamaican red mud. For Weipa red mud it is slightly above the experimental error. This increase is possibly due to the fact that flocculant molecules present in the slurry, or adsorbed on red mud particles, carry a large number of ionised carboxyl groups (2105 mol mol − 1). The flocculant dosage for red mud generated from Jamaican bauxite was significantly higher than in muds from Weipa or mix bauxites. The increase in the amount of surface hydroxyl groups followed the same pattern. However, acid titration of the flocculant dissolved in caustic solution, at concentrations close to ones used in the present study (100 ppm), did not demonstrate the shoulder on the titration curve observed for red mud slurries (Zone II, Fig. 3). At around a neutral pH most of the carboxyl groups of the flocculant molecules are inaccessible for titration because of the high degree of flocculant coiling. The molecules behave almost as inert molecules with respect to acid/base titration.
3.2. Surface charge of red mud particles The sources of the charge on the surfaces of oxides and clays are discussed in detail in the literature [3,5 – 10]. The charge can be derived from pH measurements because it is usually gov-
erned by the proton’s adsorption/desorption. The difference between the amounts of acid/base added and increases of the acid/base concentration in solution correlate to the difference between the surface charge before and after this addition. The surface charge of red mud particles was calculated by the procedure described in the literature [3,10,15,16]. The effect of the activity coefficients on the values of the surface charge was checked in selected cases, using mean activity coefficients for NaCl calculated from the Pitzer’s equation [17]. The correction was found to be negligible compared to the experimental error and, consequently, unit activity coefficients were used. A commonly accepted characteristic of the surface charge properties is the point of zero charge (PZC) defined as the pH at which the net charge on the surface is zero [15,16]. Potentiometric titration provides the values of the point zero salt effect: the intersection of the surface charge –pH curves obtained at various concentrations of indifferent electrolyte. It is equal to PZC with certain limitations: the only exchange reaction between the liquid and surface is proton adsorption/desorption, an absence of a permanent charge and surface dissolution/precipitation reactions [3,10,15,16]. For red mud particles the PZC was taken as being equal to the point zero salt effect because most of the red mud mineral constituents under the experimental conditions used in the present study (pH, temperature) satisfied the aforementioned limitations. A series of surface charge vs. pH curves was derived as described in the literature [8]. Curves shown in Figs. 4 and 5 represent the actual charge on the surface. The experimental results presented in Figs. 4 and 5 demonstrate that the overall pH dependence of the surface charge (|) has a similar shape for red muds generated from different bauxites and are the same for flocculated or nonflocculated muds. Sharp changes in | occur at basic, acidic and neutral pH. Under basic and acidic conditions the steps are the results of partial dissolution of some component of the red mud particles (sodalite in particular), which was confirmed by ICP analysis. The step at neutral pH is obviously related to acid/base titration of water.
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A summary of calculated PZC values is shown in Table 4. For most aluminium and iron oxides and hydroxides the PZC is around 7 – 8 [18]. Silica and titanium oxides have lower values of PZC at about 2–6 [18], which suggests that sodalite structures possibly will have an acidic PZC. Therefore, red mud generated from Weipa bauxite, rich in silica, should have a lower PZC than Jamaican red mud with a low silica content, which is consistent with the data (6.5 vs. 7.8, Table 4).
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As shown in Table 4 the PZC is decreased by the mud flocculation process. This means that the negative charge on the surface of the red mud particles increases in the presence of the flocculant molecules. The same conclusion can be drawn by comparison of the pH dependence of the surface charge of flocculated and non-flocculated samples. Polyacrylate groups of flocculant molecules carry a negative charge due to the ionization of carboxylic groups in caustic solution. It can be spec-
Fig. 4. Surface charge of unflocculated red mud slurry generated from Claremont bauxite.
Fig. 5. Surface charge of flocculated red mud slurry generated from Weipa bauxite.
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Table 4 PZC of red mud slurries Source of red mud slurry
Bauxite mix Weipa Claremont Claremont (in the presence of KNO3)
PZC of Jamaican flocculated red mud increases and gets closer to the reported value (7.7 vs. 8.5).
PZC of unflocculated slurry
PZC of flocculated slurry
– 6.5 7.8 –
6.0 6.0 6.0 7.7
ulated that these groups can contribute to the increase of negative charge of flocculated slurry versus non-flocculated slurry. However, attempts to determine the surface charge of the flocculant molecules by using acid/base potentiometric techniques were not successful as discussed above. All tested flocculated red mud slurries have the same PZC = 6.0 based on the titration in the presence of NaCl. The same PZC suggests a strong similarity in the surface charge properties. This observation can not be adequately explained without carrying out additional tests with different flocculants to determine whether the PZC of the flocculated slurry can be related to the type of flocculant. Potentiometric titrations in the presence of two different 1– 1 electrolytes NaCl and KNO3 were performed. The measurements were done at a basic pH where the charge of the red mud surface is negative to compare the relative affinities of the positive ions (Na+ and K+). The PZC of the Jamaican red mud slurry titrated in KNO3 solution was higher than in NaCl solution (7.7 vs. 6.0, Table 4). These results suggest that the specific adsorption of K+ ions on the surface of red mud particles make the surface less negative. PZC values obtained in the present study are different from a PZC value of 8.5 reported in the literature [11,12] due to differences in sample preparation and indifferent electrolyte used in the potentiometric titration. In the studies [11,12], samples of red mud were subjected to extensive pre-treatment leading to the partial dissolution of red muds and KNO3 was used as the indifferent electrolyte in the potentiometric titration. As the present study shows, in the presence of KNO3, the .
4. Conclusions It has been shown that a potentiometric technique can be used as a tool to characterise the surface properties of different red muds. Hydroxyl groups and the corresponding surface charge on red mud particles are primarily due to the silicacontaining compounds formed in the Bayer process and precipitated on the surface of the particles. Addition of a synthetic, organic flocculant to red mud slurries causes the negative charge on the surface of the mud particles to rise. The PZC was found to be sensitive to the presence of flocculant in the slurry and the mineralogical composition of the red mud.
Acknowledgements The authors are grateful to Alcan International Limited for permission to publish this work and Dr G.D. Fulford for valuable discussions and comments. The comments made by anonymous referees are also greatly appreciated.
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Surface properties of red mud particles from potentiometric titration D. Chvedov a,*, S. Ostap a, T. Le b a
Alcan International Limited, Kingston R&D Centre, P.O. Box 8400, 945 Princess Street, Kingston, Ont., Canada K7L 5L9 b Uni6ersity of Waterloo, Waterloo, Canada Received 24 April 2000; accepted 8 November 2000
Abstract Acid/basic potentiometric titration has been used to obtain data on the surface charge and the amount of surface hydroxyl groups on red mud particles generated from different bauxite sources. It has been demonstrated that this can be used to quantify the surface properties of red mud particles. Red mud particles carry a significant negative charge under the basic conditions that exist in the Bayer process for alumina production due to ionized hydroxyl groups on their surfaces. Hydroxyl groups and corresponding surface charge are primarily due to the silica-containing compounds formed in the Bayer process. Addition of a synthetic, organic flocculant to red mud slurries causes the negative charge on the surface of mud particles to rise, which is reflected in the shift of the point of zero charge (PZC) to lower pH values. Under the conditions studied, red mud slurries have the same PZC after flocculation with synthetic flocculant, independent of the bauxite origin and variation in digestion conditions. In general the PZC was found to be sensitive to the presence of flocculant in the slurry and the mineralogical composition. When compared to sodium ions, potassium ions have a higher affinity to the surface of red mud particles and exhibit preferential adsorption. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Surface; charge; Red mud; Titration; Bauxites
1. Introduction Red mud is the insoluble residue remaining after the caustic digestion of bauxite, the ore used in the production of alumina by the Bayer process. One of the major steps of this process is the separation of red mud solids from caustic-alumi* Corresponding author. Tel.: +1-613-5412089; fax: +1613-5412134. E-mail address: [email protected] (D. Chvedov).
nate solution by means of flocculation and decantation. This process is very sensitive to the chemical and mineralogical composition of the processed bauxite and operating conditions [1]. The rate of formation of flocs and their stability strongly depends on the surface chemistry of the particles. The other important aspect of the flocculation process is clarity of the supernatant liquor, which is governed by the particle size distribution and is also related to the surface properties of red mud particles. The importance
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of the surface chemistry in the Bayer process and particularly in the process of red mud separations has been recently reviewed by Hind et al. [2]. Red muds consist of a mixture of caustic insoluble minerals originally present in the bauxite, a reaction product formed by clay or quartz in the bauxite reacting with the caustic-aluminate solution used to digest the bauxite, and various calcium compounds formed by the charging of lime to bauxite digestion or some other part of the process. The caustic insoluble bauxite minerals are typically hematite (Fe2O3) and aluminin goethite ((Fe, Al)OOH) along with the titanium dioxides, anatase (TiO2) and rutile (TiO2). Some boehmite (AlOOH) may also be present if the process is not designed to extract this form of alumina. The remaining elements in bauxite are present in the muds only in minor or trace quantities. The major caustic insoluble reaction product formed is the sodium aluminum silicate, Bayer-sodalite, (3(Na2O·Al2O3·2SiO2·nH2O)·Na2X) where X may be CO3, SO4, 2OH, 2Cl or a mixture of any or all depending upon the impurity composition of the digesting liquor. The value ‘n’ may range from 0 to 2. Lime charged to process will react to form CaCO3, 3CaO·Al2O3·6H2O and a form of calcium phosphate, i.e. carbonate- or hydroxyapatite. At high bauxite digestion temperatures (230 – 260°C), the lime can also react with the titanium mineral anatase to form perovskite (CaTiO3) and/or kassite (CaTi2O4(OH)2). The definition of red mud particles is essential for understanding the surface properties of red mud slurries. The mineral constituents of red muds do not necessarily occur as discrete particles and may be physically associated with other minerals. Desilication products precipitate as a discrete phase and also on the other constituents present in the mud. The small individual particles exist in the slurry as heterogeneous natural aggregates. Adding flocculant to the slurry, flocculates these natural aggregates rather than individual particles. The acid/base potentiometric technique used in the present study probes the surfaces that are accessible for protons under studied conditions. Taking into account the mobility of protons and long equilibration time in our experiments with red muds it is reasonable to assume that the
surface of the natural aggregates is involved in the proton exchange reaction. Therefore, in the present study heterogeneous natural primary aggregates are referred to as ‘red mud particles’. Sodalites which are zeolite-type compounds with an extremely high ion exchange capacity may have a significant influence on the surface properties of red mud slurries. The surface chemistry of red mud particles is extremely complicated as the composition of red mud slurries varies considerably depending on the type of bauxite and digestion conditions. At the same time there is much uncertainty about the exact chemical composition on the surface due to the difficulties of analyzing the thin surface layer of extremely small particles ranging from 50 A, to 1 mm. However, it is well known that most of the minerals and oxides found in red mud slurries demonstrate acid/base type of behaviour in aqueous solutions [3]. Therefore it is reasonable to expect that the surface of red mud particles will exhibit similar type of behaviour. The surface OH groups responsible for the acid/base properties of the particles can be important for the interaction with flocculant molecules serving as active sites. Surface charge properties of the particles can play a substantial role in the process of flocculation. The similar charge distributed on the surfaces of the particles acts to prevent them from flocculating. Thus, the addition of a long-chained organic bridging species or some mechanism of neutralizing the repulsive electrostatic forces is needed to allow flocculation to take place. The conventional explanation of the effect of the ‘flocculant molecules’ flocculating the mud is based on the concept of long-chained polymers adsorbing on more than one particle and acting to bridge them [4]. However, both carry negative charges and this mechanism is possible only with the assumption that positively charged sodium ions act as an intermediate species in the process of bridging. Surface charge properties can be determined by means of potentiometric (acid/base) titration. This has been widely used to characterize the hydroxyl surface groups of oxides and clay minerals [5–10]. Recently this technique was used to determine the
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surface charge properties of activated and pretreated red mud particles [11,12]. The main objective of the present study was to determine the surface charge and the amount of hydroxyl groups on the surface of red mud particles generated from bauxites of various mineralogical compositions. The other aspect of the present study was to explore the flocculation effect on the surface properties of the red mud solids. Flocculated and unflocculated red mud slurries generated from bauxites from different geographical sources have been studied using acid/base potentiometric titration.
2. Experimental methods
2.1. Preparation of red mud samples The red mud samples were prepared by digesting the bauxites under conditions similar to those
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used in both the low and high temperatures digestion stages of the Bayer process. The digest medium was 3.9 or 4.5 M NaOH –NaAlO2 solution. The digests were done in 6-l, heated, agitated autoclaves fitted with a sliding cup apparatus by which bauxite could be charged to the digestion solution preheated to the desired digestion temperature. Table 1 shows the digestion conditions used to prepare each mud sample. Bauxite, ground to pass 100 mesh (149 mm) was dry mixed with ground quicklime, and charged to the sliding cup arrangement located above the solution surface in the autoclave. Once charged with bauxite and digest solution, the autoclave was closed and the solution was heated to the desired temperature. The cup with the bauxite –lime mix was then lowered into the hot agitated liquor. Upon completion of digestion, the slurry was allowed to cool to atmospheric pressure for 45 min before draining the autoclave. The red mud slurry was split into two parts, with one part being floccu-
Table 1 Bauxite digestion and red mud flocculation conditions Bauxite sample
Bauxite digestion conditions
Flocculation conditions
T (°C)
Time (min)
NaOH (mol l−1)
Lime charge (mol l−1 as CaO)
Type of flocculant
Dosage (ppm)
Claremont
230
30
3.92
0.014
Bauxite mix
143
45
3.90
0.012
Weipa
245
30
4.54
0.031
Mix of Nalco 9779 and 190 Alclar 665 Mix of Alclar 665 and 200 Alclar-663 Nalco 9779 50
Table 2 Properties of dried red muds Type of red mud (from bauxite)
Specific gravity (kg m−3)
Surface area (m2 g−1)
Total pore volume (cm3 g−1)
Claremont (unflocculated) Claremont (flocculated) Bauxite mix (unflocculated) Bauxite mix (flocculated) Weipa (unflocculated) Weipa (flocculated)
3500 3200 3000 3000 2900 3000
48.7 26.8 N.A.a 20.0 22.2 21.9
0.21 0.15 N.A.a 0.18 0.23 0.14
a
Not available.
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lated by addition of flocculant solution. The red muds from the flocculated and non-flocculated slurries were then separated by centrifugation and the last traces of digest liquor removed by five water washes. Portions of the centrifuge cakes were dried for 2 h at 105°C and analysed for the properties shown in Table 2. Bauxites used in the studies originated in the following locations: the Claremont district of Jamaica, Cape York in northern Australia (Weipa), the Boke region of Guinea, Africa and the Amazon basin in northern Brazil (bauxite mix). Bauxites from Jamaica belong to the soillike, terra rosa type with negligible aggregation of any of its submicron mineral components. Gibbsite occurs as platelets less than 0.1 um in size. Typically the bauxites contain about 18% Fe2O3 as a mix of hematite and aluminin goethite and 1 – 2% clay mineral. Weipa bauxites from Australia consist of 0.5 – 3.0 mm loose pisolites in a finer matrix. This bauxite is unique in that its 15% Fe2O3 is present only as hematite. It is high in boehmite and contains up to 2% SiO2 as quartz and up to 10% clay mineral. The bauxites from Guinea and Brazil are hard, highly aggregated, massive aluminous laterites whose mining requires drilling and blasting. Gibbsite forming the matrices of both bauxites is coarsely crystalline. The Guinea bauxite has a high boehmite content, about 15% Fe2O3 as a mix of hematite and aluminin goethite and 2 – 3% clay. The Amazon bauxite contains 10 – 11% Fe2O3 as a goethite – hematite mix, less than 1% boehmite and up to 10% clay. The iron minerals in the bauxites from Guinea, Brazil and Australia are dispersed throughout the gibbsite matrices but also occur as small aggregated masses. In Jamaican bauxites, no aggregation of the fine iron particles takes place. High molecular weight sodium polyacrylates (17 000 000 g mol − 1) obtained from Nalco Chemicals (Nalco 9779) and Allied Colloids (Alclar 665 and 663) were used in the study as flocculants. According to the manufacturer’s description these flocculants consists of coiled nonbranched polymer chains containing charged acidic groups along the chain, balanced by
sodium ions. In strong caustic solution these chains straighten out [4].
2.2. Analysis of red mud slurries The surface charge properties of red mud particles were determined using potentiometric techniques described in the literature [8–10]. The conventional method [10] was slightly modified, as described below, to reflect the specific properties of red mud slurries, i.e. the high viscosity of undiluted slurries and stickiness of muds, especially the non-flocculated slurries. About 0.13 g (on a dry base) of centrifuged red mud solids generated as discussed was placed into preweighed plastic bottles with 70 ml of solution containing electrolyte (NaCl or KNO3) and acid or base. Samples were then equilibrated for approximately 18 h on a reciprocal shaker with a speed of 60 oscillations per min., centrifuged at 2500 rpm, decanted and the pH of the supernatants determined using a Accumet-50 ion/pH meter (Denver Instrument) and polymer body pH combination electrode. The pH meter was calibrated daily with a set of standards over the wide range of pH 4.0, 7.0, 9.0, 11.0 in accordance to the standard procedure. NaCl, NaOH and KNO3 solutions were prepared using nanopure water and ASC grade salts from Fisher Scientific. A 1 N HCl solution (Fisher Scientific, certified) was used in the present study. The exact HCl concentration was confirmed by titration with standard NaOH. Each sample of red mud slurry was titrated by acid and base at least in three different concentrations of indifferent electrolyte (NaCl or KNO3). The effect of the contact time on the pH of solutions was studied for the acid and base titrations of Jamaican red mud slurries. After measuring the pH as described above, the supernatant was re-combined with the corresponding red mud residue and left to shake overnight. After an additional 18–20 h of shaking, the slurry was centrifuged and the pH was determined once more as previously described. The shapes of the acid and base titration curves were the same after an additional day of shaking. However, the pH values shifted slightly (0.2 –0.4
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Fig. 1. Titration curves of flocculated red mud slurry generated from Weipa bauxite.
pH units). In all cases, the pH shifted towards more basic values for both base and acid titrations. The increase in pH values for the base titrations was more significant at higher pH than at lower pH values. This evidence supports the conclusion that changes in pH with increase of contact time are caused by the dissolution of red mud particles in the process of shaking. The overall shift in pH induced by the increase in contact time was negligible and one day of shaking was found to be enough to reach equilibrium. All analysed data are based on 18 h of contact time. Surface area and porosity of red mud particles were determined by means of an Autosorb1 analyzer according to the standard procedure for the B.E.T. analysis using nitrogen as adsorbant. Samples of red mud slurries were dried at 90, 105 and 110°C for several hours in a convection oven and then outgassed before the analysis for 30 min at the same temperatures. The temperature of 90°C was found to be insufficient in certain cases to expel adsorbed water and resulted in random B.E.T. results for the surface area. Surface areas obtained for the samples dried and outgassed at 110°C were used in subsequent calculations. The results are pre-
sented in Table 2 along with the total pore volumes. Almost all of the main constituents in red mud particles are soluble in acidic mediums and some of them in strong caustic. However these compounds are practically insoluble at neutral pH. Dissolution of the surface causing possible changes in the surface properties was considered because of the long equilibration time at constant shaking. Selected supernatants were analysed by ICP-AS for elemental content to determine any red mud dissolution.
3. Results and discussion
3.1. Acid/base beha6iour of red mud slurries Typical acid/base titration curves for flocculated and unflocculated red mud slurries are shown in Figs. 1 and 2. Titration curves exhibit the response of the system to the addition of acid or base as they show changes in pH of the bulk of the slurry after addition of acid or base. The amount of HCl added to the solution is expressed in moles. It was assumed that species
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trapped by red mud particles from caustic solution after digestion other than hydroxyl ions did not affect the titration results to any significant extent. The main constituents of the caustic solution used in the digestion that could consume
protons in the process of titration were organics containing carboxylic groups, carbonate and aluminate ions. However, the washing process resulted in dilution of the red mud slurry after digestion by at least 10 000 times as indicated
Fig. 2. Titration curves of unflocculated red mud slurry generated from Claremont bauxite.
Fig. 3. Generic titration curves of red mud slurry (dotted line) and caustic solution (solid line).
D. Ch6edo6 et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 182 (2001) 131–141 Table 3 Estimate of the surface hydroxyl groups on red mud particles Red mud sample from bauxite
Mols of hydroxyl groups on dry red mud basis Mol kg−1
Claremont (unflocculated) Claremont (flocculated) Bauxite mix (flocculated) Weipa (unflocculated) Weipa (flocculated)
104 Mol m−2 of mud surface
6.1
1.2
7.4
2.7
1.92
9.6
2.61
11.9
2.72
12.4
by the drop in caustic concentration of the slurry. Fig. 3 illustrates the schematic acid titration curves of red mud suspensions (dotted line) and a caustic solution (solid line). The pH of the initial point for all titration curves reflects the amount of free caustic in the bulk of solution at the beginning of titration. In case of red mud slurries this pH (on average around 10.5) will correspond to the amount of caustic trapped by the slurry and released in the process of equilibration during shaking. Addition of red mud particles to the caustic solution changes the shape of the acid/base titration curve dramatically. In pure caustic solution the drop in pH is due only to the consumption of protons due to neutralization of free OH− ions in the bulk of solution. Red mud particles in basic aqueous solutions carry ionized surface hydroxyl groups (SO−) and can consume protons in accordance to the reaction: SO− +H+ =SOH, where SO− is an ionized, exposed oxygen atom on the surface of the particle. Therefore, caustic solutions of red mud slurries have an additional way of consuming protons without changing the pH of the suspension. Because free hydroxide ions in solution are more mobile and therefore more accessible, they will be titrated first and only small amount of SO− groups will be titrated at this stage (Zone I in Fig. 3). The
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ionization constant for the reaction SOH =S– O− + H+ (Ka) is much higher than the ionization constant of water. In the present study it was not possible to accurately determine the ionization constants of the surface hydroxyl groups from the titration curves because of the lack of data points. At neutral pH all of the free hydroxide ions will be consumed during the titration. The classic titration curve of pure caustic shows a sharp drop to an acidic pH. The titration curves of red mud slurries at neutral pH flatten out to different extents depending on the types of slurry (Zone II, Fig. 3). At this point only surface hydroxyl groups (SO− and SOH) are titrated to form SOH and SOH+. Almost all of the protons added to the slurry are consumed by surface hydroxyl groups which results in nearly a constant pH in the bulk of the solution. At the end of Zone II, all of the surface groups have been titrated and the titration curve for red mud slurries and caustic solutions meet in Zone III where the drop in pH is due to the accumulation of protons in solution. With the assumption that only a minor amount of SO− groups has been titrated before the slurry has reached neutral pH (Zone I/II), the first bend in the titration curve corresponds to the beginning of the titration of the surface hydroxyl groups. The end of the titration of the surface groups occurs at about a neutral pH and is marked by the second point of inflection in the titration curve (Zone II/III). From the two inflection points on the titration curves the amounts of surface hydroxyl groups were calculated for the red mud slurries and the values are summarized in Table 3. The amount of hydroxyl groups on the surface determined in the present study are one to two orders of magnitude higher than the average values obtained for metal oxides 1–3·10 − 5 mol m − 2 [13]. This difference can be attributed to the high sodalite content of the red muds studied, especially in the case of the Weipa mud. As a zeolite-type compound, with 2–3A, diameter channels, sodalite has a high internal surface area accessible for protons with exposed oxygen atoms that might serve as an adsorption site for
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protons. Sodalites also contain a considerable amount of water molecules trapped in their caged structure that may interact with protons. Mud generated from Jamaican bauxite consists mostly of fine iron oxides and minor amounts (less than 1%) of Bayer-sodalite. Weipa red mud, on the other hand, is one third Bayer-sodalite. Both types of mud will contain calcium in the form of calcium aluminate, titanates and hydroxyapatite. The amount of the surface hydroxyl groups is roughly proportional to the reactive silica content in the original bauxites, which suggests that sodalite is one of the major constituents on the surface of red mud particles. This conclusion is in agreement with previously reported correlation between the sodalite content and ion exchange capacity of red mud particles [14]. As indicated in Table 3 addition of the flocculant to red mud slurry results in an increase in the amount of acid required to neutralize surface groups, especially for Jamaican red mud. For Weipa red mud it is slightly above the experimental error. This increase is possibly due to the fact that flocculant molecules present in the slurry, or adsorbed on red mud particles, carry a large number of ionised carboxyl groups (2105 mol mol − 1). The flocculant dosage for red mud generated from Jamaican bauxite was significantly higher than in muds from Weipa or mix bauxites. The increase in the amount of surface hydroxyl groups followed the same pattern. However, acid titration of the flocculant dissolved in caustic solution, at concentrations close to ones used in the present study (100 ppm), did not demonstrate the shoulder on the titration curve observed for red mud slurries (Zone II, Fig. 3). At around a neutral pH most of the carboxyl groups of the flocculant molecules are inaccessible for titration because of the high degree of flocculant coiling. The molecules behave almost as inert molecules with respect to acid/base titration.
3.2. Surface charge of red mud particles The sources of the charge on the surfaces of oxides and clays are discussed in detail in the literature [3,5 – 10]. The charge can be derived from pH measurements because it is usually gov-
erned by the proton’s adsorption/desorption. The difference between the amounts of acid/base added and increases of the acid/base concentration in solution correlate to the difference between the surface charge before and after this addition. The surface charge of red mud particles was calculated by the procedure described in the literature [3,10,15,16]. The effect of the activity coefficients on the values of the surface charge was checked in selected cases, using mean activity coefficients for NaCl calculated from the Pitzer’s equation [17]. The correction was found to be negligible compared to the experimental error and, consequently, unit activity coefficients were used. A commonly accepted characteristic of the surface charge properties is the point of zero charge (PZC) defined as the pH at which the net charge on the surface is zero [15,16]. Potentiometric titration provides the values of the point zero salt effect: the intersection of the surface charge –pH curves obtained at various concentrations of indifferent electrolyte. It is equal to PZC with certain limitations: the only exchange reaction between the liquid and surface is proton adsorption/desorption, an absence of a permanent charge and surface dissolution/precipitation reactions [3,10,15,16]. For red mud particles the PZC was taken as being equal to the point zero salt effect because most of the red mud mineral constituents under the experimental conditions used in the present study (pH, temperature) satisfied the aforementioned limitations. A series of surface charge vs. pH curves was derived as described in the literature [8]. Curves shown in Figs. 4 and 5 represent the actual charge on the surface. The experimental results presented in Figs. 4 and 5 demonstrate that the overall pH dependence of the surface charge (|) has a similar shape for red muds generated from different bauxites and are the same for flocculated or nonflocculated muds. Sharp changes in | occur at basic, acidic and neutral pH. Under basic and acidic conditions the steps are the results of partial dissolution of some component of the red mud particles (sodalite in particular), which was confirmed by ICP analysis. The step at neutral pH is obviously related to acid/base titration of water.
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A summary of calculated PZC values is shown in Table 4. For most aluminium and iron oxides and hydroxides the PZC is around 7 – 8 [18]. Silica and titanium oxides have lower values of PZC at about 2–6 [18], which suggests that sodalite structures possibly will have an acidic PZC. Therefore, red mud generated from Weipa bauxite, rich in silica, should have a lower PZC than Jamaican red mud with a low silica content, which is consistent with the data (6.5 vs. 7.8, Table 4).
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As shown in Table 4 the PZC is decreased by the mud flocculation process. This means that the negative charge on the surface of the red mud particles increases in the presence of the flocculant molecules. The same conclusion can be drawn by comparison of the pH dependence of the surface charge of flocculated and non-flocculated samples. Polyacrylate groups of flocculant molecules carry a negative charge due to the ionization of carboxylic groups in caustic solution. It can be spec-
Fig. 4. Surface charge of unflocculated red mud slurry generated from Claremont bauxite.
Fig. 5. Surface charge of flocculated red mud slurry generated from Weipa bauxite.
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Table 4 PZC of red mud slurries Source of red mud slurry
Bauxite mix Weipa Claremont Claremont (in the presence of KNO3)
PZC of Jamaican flocculated red mud increases and gets closer to the reported value (7.7 vs. 8.5).
PZC of unflocculated slurry
PZC of flocculated slurry
– 6.5 7.8 –
6.0 6.0 6.0 7.7
ulated that these groups can contribute to the increase of negative charge of flocculated slurry versus non-flocculated slurry. However, attempts to determine the surface charge of the flocculant molecules by using acid/base potentiometric techniques were not successful as discussed above. All tested flocculated red mud slurries have the same PZC = 6.0 based on the titration in the presence of NaCl. The same PZC suggests a strong similarity in the surface charge properties. This observation can not be adequately explained without carrying out additional tests with different flocculants to determine whether the PZC of the flocculated slurry can be related to the type of flocculant. Potentiometric titrations in the presence of two different 1– 1 electrolytes NaCl and KNO3 were performed. The measurements were done at a basic pH where the charge of the red mud surface is negative to compare the relative affinities of the positive ions (Na+ and K+). The PZC of the Jamaican red mud slurry titrated in KNO3 solution was higher than in NaCl solution (7.7 vs. 6.0, Table 4). These results suggest that the specific adsorption of K+ ions on the surface of red mud particles make the surface less negative. PZC values obtained in the present study are different from a PZC value of 8.5 reported in the literature [11,12] due to differences in sample preparation and indifferent electrolyte used in the potentiometric titration. In the studies [11,12], samples of red mud were subjected to extensive pre-treatment leading to the partial dissolution of red muds and KNO3 was used as the indifferent electrolyte in the potentiometric titration. As the present study shows, in the presence of KNO3, the .
4. Conclusions It has been shown that a potentiometric technique can be used as a tool to characterise the surface properties of different red muds. Hydroxyl groups and the corresponding surface charge on red mud particles are primarily due to the silicacontaining compounds formed in the Bayer process and precipitated on the surface of the particles. Addition of a synthetic, organic flocculant to red mud slurries causes the negative charge on the surface of the mud particles to rise. The PZC was found to be sensitive to the presence of flocculant in the slurry and the mineralogical composition of the red mud.
Acknowledgements The authors are grateful to Alcan International Limited for permission to publish this work and Dr G.D. Fulford for valuable discussions and comments. The comments made by anonymous referees are also greatly appreciated.
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