ARTICLE pubs.acs.org/IECR
Enhancement of Hematite Flocculation in the Hematite Starch (Low-Molecular-Weight) Poly(acrylic acid) System Mark Ma* CSIRO Process Science and Engineering, Box 312, Clayton, Victoria, Australia 3168 ABSTRACT: In the iron ore industry, the flocculation of hematite with starch in the presence of a dispersant for gangue minerals is important for the production of high-grade concentrates. The sequence of reagent addition in this process has been considered negligible as starch was reported to have no significant effect on the adsorption of other reagents in the literature. In this work, the colloid stability of hematite with different reagent addition sequences was studied. The results demonstrate that, when hematite is preconditioned with starch, the subsequent addition of a low-molecular-weight poly(acrylic acid) does not show any dispersive effect on hematite at starch concentrations greater than or equal to 4 ppm, because the adsorption of poly(acrylic acid) is completely blocked by the starch molecules coated on the hematite surfaces. When hematite is preconditioned with poly(acrylic acid) , the flocculation effect of the subsequently added starch is significantly affected, with strong steric stabilization observed because of the reduced number of adsorption sites available to starch molecules.
1. INTRODUCTION Hematite is a principal iron ore consumed by the steel industry and the predominant iron-bearing mineral in the largest iron ore deposits on the Earth, such as the Pilbara district of Western Australia and the Carajas Mineral Province in Brazil. In hematite ore, the Al2O3-containing gangue minerals are detrimental to blast furnace and sinter plant operations. In the literature, selective flocculation and flotation have both been investigated for the removal of Al2O3-containing minerals from hematite.1 9 Kaolinite is a major Al2O3-containing mineral in hematite ore. It is of very fine particle size in nature. In kaolinite hematite separation, the hematite particles selectively flocculated with starch quickly settle to the bottom, whereas the effectively dispersed kaolinite particles remain stable in the suspension and can then be removed from the hematite.7 9 In the iron ore industry, sodium silicate is a widely used dispersant of gangue minerals. In the author’s recent studies, sodium silicate was found to be inadequate for dispersing kaolinite under iron ore processing conditions, whereas a low-molecular-weight poly(acrylic acid) (PAA) was found to be an effective dispersant of kaolinite.9 However, PAA of low molecular weight can also disperse hematite, and thus, its presence in flotation pulp might cause difficulties for the flocculation of hematite with starch. Therefore, the flocculation of hematite with starch in the presence of PAA, which has never been studied in the literature, is of critical importance for kaolinite hematite separation. In this work, the flocculation of hematite with starch in the presence of a low-molecular-weight PAA was investigated. 2. EXPERIMENTAL SECTION 2.1. Materials. The hematite sample (99.9% pure) was supplied by Alfa Aesar. The particle size distribution of the hematite sample was determined using a Malvern Mastersizer 2000, and its d80 value was found to be 1.5 μm. The BET (Brunauer Emmett Teller) specific surface area for the sample, determined using a r 2011 American Chemical Society
Micromeritics Tristar 3000 instrument, was 9.92 m2/g. Natural corn starch was obtained from Wintersun Chemical (Ontario, CA). The starch was gelatinized using sodium hydroxide at room temperature. PAA (MW 1800) was supplied by Sigma. 2.2. Methods. The colloid stability of hematite particles was investigated through turbidity measurements using a Hach 2100AN turbidimeter. Turbidity values are expressed in nephelometric turbidity units (NTUs). In the turbidity tests, 0.1 g of hematite was first conditioned with 100 mL of distilled water, starch, or PAA for 30 min. Then, 100 mL of starch or PAA solution was added, and the entire mixture was further conditioned for 30 min. After conditioning, 30 mL of the sample was collected for turbidity measurements. The samples were allowed to settle for 15 min before their turbidity was measured. The rest of the sample was centrifuged, and the clear supernatant was analyzed for the residual concentration of PAA, based on the reaction of PAA with hyamine, as proposed by Crummett and Hummel.10 Zeta potential measurements were performed on a Zetacompact Z8000 model apparatus (CAD Instruments, Les Essartsle-Roi, France). An electric field of 80 V/cm was applied to hematite suspensions. Results were based on an automated video analysis of particles. Zeta potentials were calculated from electrophoretic mobilities using the Smoluchowski equation. In the tests, 0.001 M NaCl was used as the background electrolyte.
3. RESULTS In the selective flocculation and reverse cationic flotation of hematite ore, the most widely used flotation route in the iron ore industry, the pulp pH is in the range of 10 10.5.11 Therefore, all of the tests in this work were conducted at pH 10. Figure 1 Received: June 21, 2011 Accepted: September 10, 2011 Revised: August 21, 2011 Published: October 07, 2011 11950
dx.doi.org/10.1021/ie2013373 | Ind. Eng. Chem. Res. 2011, 50, 11950–11953
Industrial & Engineering Chemistry Research
Figure 1. Effects of starch and PAA on the colloid stability and zeta potential of hematite at pH 10.
ARTICLE
Figure 3. Residual concentration of PAA after conditioning with hematite for 5, 10, 20, and 30 min. Hematite was preconditioned with starch from 0 to 4 ppm. pH 10.
Figure 2. Effects of PAA concentration on the zeta potential of hematite preconditioned with starch at pH 10.
presents the effects of starch and PAA on the zeta potential and colloid stability of hematite at pH 10. The more negative surface charge of hematite in the presence of PAA significantly enhances the colloid stability of hematite particles. In contrast, starch renders hematite less negatively charged as the polymer concentration increases. Its effect on hematite aggregation is also different from that of PAA. At low concentration, starch destabilizes hematite by bridging flocculation, but it restabilizes hematite at higher concentration as steric forces induced by the polymer molecules start to grow. For flocculation with a polysaccharide, the turning point (4 ppm in this work) is typically around the polysaccharide’s concentration for a full coverage of the macromolecules on the solid surfaces. It appears that the effect of starch on the colloid stability of hematite is independent of the polymer’s effect on the zeta potential of hematite. Figure 2 shows the effects of the PAA concentration on the zeta potential of hematite preconditioned with starch at pH 10. As the concentration of starch preconditioned with hematite increased, the effect of PAA on the zeta potential of hematite quickly diminished. For sufficiently high concentrations of starch, the zeta potential of hematite was found to become independent of PAA addition.
Figure 4. Effects of dosing sequence on the colloid stability of hematite as a function of starch concentration.
Figure 3 presents the residual concentration of PAA after conditioning with hematite for 5, 10, 20, and 30 min. In the absence of starch, the residual concentration of PAA decreased from 100 to 31 ppm, because of the adsorption of the dispersant on hematite surfaces. After preconditioning with 1 ppm starch, the residual concentration of PAA increased to 58 ppm, indicating a significantly reduced adsorption of PAA on the hematite. As the concentration of starch preconditioned with hematite increased from 1 to 3 ppm, the adsorption of PAA onto the hematite surfaces still occurred but became increasingly difficult. At 4 ppm starch, the adsorption of PAA on hematite eventually ceased. This is also the concentration of starch at which the zeta potential of hematite was found to become independent of PAA addition (Figure 2), and thus, it is of crucial importance for colloid stability in the hematite starch PAA system. Figure 4 illustrates the effects of dosing sequence on the colloid stability of the hematite starch PAA system. In the iron ore industry, a dispersant is normally added in the grinding circuits, and starch, the flocculant, is added at a subsequent 11951
dx.doi.org/10.1021/ie2013373 |Ind. Eng. Chem. Res. 2011, 50, 11950–11953
Industrial & Engineering Chemistry Research
Figure 5. Effects of dosing sequence on the colloid stability of hematite as a function of PAA concentration.
stage.12 As Figure 4 shows, for hematite preconditioned with PAA (the dispersant), the subsequent addition of starch could still induce flocculation of hematite, but only at a very narrow range of starch concentrations. Beyond this narrow range, the flocculation effect of starch was significantly weakened by PAA. For sufficiently high concentrations of preadded PAA (1000 ppm), strong flocculation with the subsequently added starch was impossible at any starch concentration. In contrast, when hematite was conditioned with starch first, the subsequent addition of PAA only slightly interfered with the flocculation effect of starch at very low concentrations of the polymer (1 ppm). At higher concentrations of starch (4 1000 ppm), the flocculation effect of starch was not affected by the addition of PAA. The effects of dosing sequence on the flocculation of hematite as a function of PAA concentration are presented in Figure 5. The turbidity levels of hematite preconditioned with starch were generally constant over the entire concentration range of PAA studied in this work. However, for hematite conditioned with PAA first, the flocculation effect of starch that was added subsequently was significantly weakened.
4. DISCUSSION Starch is not soluble in cold water and must be put into solution in a process known as gelatinization. Although the physicochemical fundamentals of starch gelatinization are still not fully understood, it is generally accepted that gelatinized starch is slightly anionic,13 because of either the dissociation of OH groups14 16 or the formation of fatty acids in the reaction of the lipids in amylose with NaOH.17 Starch is actually a mixture of two polymers: the linear polymer “amylose” and the branched polymer “amylopectin”. The dissociation constant of the hydroxyl group in amylose is 12.6 at 25 °C. Based on the assumption that amylose is similar to amylopectin, this value has been used as the dissociation constant of the hydroxyl group of starch in the literature.18 Therefore, the electronegativity of gelatinized starch is relatively low. PAA contains carboxylic acid groups that are capable of dissociation depending on the pH of the solution. The pH at which 50% ionization of PAA occurs was determined to be 4.5 by Michaels and Morelos.19 Thus, at the pH studied in this work, the PAA molecules contain significant numbers of ionized groups.
ARTICLE
Because of its low electronegativity and high molecular weight, starch acts as a flocculant of hematite. In contrast, the high electronegativity and low molecular weight of PAA makes it a dispersant of hematite. The different effects of PAA and starch on the zeta potential of hematite reflect these differences between the two reagents (Figure 1). The adsorption of starch macromolecules in the electrical double layer moves the shear plane farther from the mineral surfaces, decreasing the magnitude of the zeta potential. A similar phenomenon was observed in the author’s previous work on a guar gum quartz system.20 It has been convincingly demonstrated in the literature that both starch5,21 and PAA22 chemically adsorb on hematite. However, the coadsorption of starch and PAA on hematite has not been studied. According to Schulz and Cooke,23 the adsorption of laurylamine acetate (MW 245) on hematite is not appreciably affected by the presence of gum 3502, a modified corn starch. Balajee and Iwasaki24 also reported that British gum 9084, a corn starch modified by pyrolytic degradation, does not affect the adsorption of dodecylammonium chloride (MW 222) on hematite. Partridge and Smith25 reported that starch reduces the adsorption of dodecylamine acetate (MW 245) on hematite in alkaline solutions. Along the same lines, Khosla and Biswas26 also found that the adsorption of myristic acid collector (MW 228) on hematite is reduced as the starch concentration increases. In summary, starch and its derivatives have been reported to have no effect or to reduce the adsorption of surfactants onto hematite. In both cases, significant amounts of surfactants still adsorbed on the hematite surfaces in the presence of starch and its derivatives. However, in this work, starch, when used to precondition hematite, was found to completely block the adsorption of PAA (MW 1800), as shown in Figure 3. It is noteworthy to point out that the molecular weight of the surfactants studied in the literature is only a fraction of that of the PAA studied in this work, which probably makes it easier for the surfactant molecules to penetrate the layer of starch molecules coated on the hematite surfaces. Santhiya et al.27 studied the coadsorption of PAA (MW 5000) and poly(vinyl alcohol) (MW 25000) and found that a variation in the sequence of addition of the two reagents did not affect their adsorption behavior. The molecular weight of poly(vinyl alcohol) is significantly lower than that of starch. Corn starch typically contains 70 75% amylopectin, with a molecular weight in the range of (10 420) 106,28 and 25 30% amylose, with a molecular weight around 1.5 105.29 It appears that this type of phenomenon might be negligible for two chemicals that are both of low molecular weight. However, when a reagent of high molecular weight is involved, the sequence of reagent addition is worth close attention. The starch concentration required to completely block the adsorption of PAA was found to be 4 ppm; at lower starch concentrations, coadsorption of PAA and starch occurred (Figure 3). Corresponding to such adsorption behavior, when hematite was preconditioned with starch, the subsequent addition of PAA demonstrated weak dispersive effect on hematite at starch concentrations less than 4 ppm; at higher starch concentrations (g4 ppm), the dispersive effect of PAA disappeared (Figures 4 and 5). When hematite was preconditioned with PAA, the flocculation effect of subsequently added starch was significantly weakened. At starch concentration greater than or equal to 4 ppm, steric stabilization was enhanced significantly, probably because 11952
dx.doi.org/10.1021/ie2013373 |Ind. Eng. Chem. Res. 2011, 50, 11950–11953
Industrial & Engineering Chemistry Research PAA occupied a significant amount of adsorption sites on hematite surfaces and consequently reduced the number of adsorption sites available for starch. When the PAA concentration was increased to 1000 ppm, efficient flocculation of hematite became difficult at any starch concentration (Figure 4). When the PAA concentration was less than or equal to 100 ppm, the sequence of reagent addition did not make any difference in an extremely narrow range of starch concentrations (around 4 ppm). In the iron ore industry, the content of hematite in the feed to a processing plant varies from time to time, and in some cases, the feed has to be blended to maintain a relatively stable mineral composition. As the content of hematite in the feed changes, the very narrow range of starch concentrations for efficient flocculation will also change accordingly, which makes operations extremely difficult to control. Therefore, preconditioning hematite with starch before adding PAA can significantly widen the range of starch concentrations for efficient flocculation. Such an approach also allows the use of very high dosages of PAA (1000 ppm), when required to disperse kaolinite particles in iron ore suspensions.
5. CONCLUSIONS In the absence of PAA, starch causes bridging flocculation of hematite when the polymer concentration is less than 4 ppm; at higher concentrations, starch induces weak steric stabilization. In the presence of PAA, the effect of starch on the colloid stability of hematite was found to depend strongly on the sequence of reagent addition. When hematite is preconditioned with starch, the subsequent addition of PAA does not show any dispersive effect on hematite at starch concentrations greater than or equal to 4 ppm, as the adsorption of PAA is completed blocked by starch molecules coated on the hematite surfaces. At lower starch concentrations, coadsorption of PAA and starch occurs, and PAA is able to disperse hematite slightly. When hematite is preconditioned with PAA, the subsequent addition of starch flocculates hematite only in a very narrow concentration range, and steric stabilization at high starch concentration (>4 ppm) is enhanced significantly, because of the reduced number of adsorption sites available to starch molecules. For sufficiently high concentrations of PAA (1000 ppm), efficient flocculation of hematite becomes impossible at any starch concentration. ’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected]. Fax: +61 3 95628919. Tel.: +61 3 95458731.
’ ACKNOWLEDGMENT The author thanks Minerals Down Under, CSIRO National Research Flagship, for support of this research. ’ REFERENCES (1) Gururaj, B.; Sharma, J. P.; Baldawa, A.; Arora, S. C. D.; Prasad, N.; Biswas, A. K. Dispersion flocculation studies on hematite clay systems. Int. J. Miner. Process. 1983, 11, 285. (2) Rao, K.; Narasimhan, K. S. Selective flocculation applied to Barsuan iron ore tailings. Int. J. Miner. Process. 1985, 14, 67. (3) Mahiuddin, S.; Bondyopadhway, S.; Baruah, J. N. A study on the beneficiation of Indian iron-ore fines and slime using chemical additives. Int. J. Miner. Process. 1989, 26, 285.
ARTICLE
(4) Ravishankar, S. A.; Pradip; Khosla, N. K. Selective flocculation of iron oxide from its synthetic mixtures with clays: A comparison of polyacrylic acid and starch polymers. Int. J. Miner. Process. 1995, 43, 235. (5) Weissenborn, P. K.; Warren, L. J.; Dunn, J. G. Selective flocculation of ultrafine iron ore. Part 1. Mechanism of adsorption of starch onto hematite. Colloids Surf. A 1995, 99, 11. (6) Ma, X.; Bruckard, W. J.; Holmes, R. Effect of collector, pH and ionic strength on the cationic flotation of kaolinite. Int. J. Miner. Process. 2009, 93, 54. (7) Ma, X.; Bruckard, W. Effect of pH and ionic strength on starch koalinite interactions. Int. J. Miner. Process. 2010, 94, 111. (8) Ma, X. Role of hydrolyzable metal cations in starch kaolinite interactions. Int. J. Miner. Process. 2010, 97, 100. (9) Ma, X. Effect of a low-molecular-weight polyacrylic acid on the coagulation of kaolinite particles. Int. J. Miner. Process. 2011, 99, 17. (10) Crummett, W. B.; Hummel, R. A. The determination of tracers of polyacrylamides in water. J. Am. Water Works Assoc. 1963, 55, 209. (11) Ma, X. Iron Ore Flotation; Nova Science: New York, 2011. (12) DeVaney, F. D. Iron Ore. In SME Mineral Processing Handbook; Weiss, N. L., Ed.; American Institute of Mining, Metallurgical and Petroleum Engineers: New York, 1985; pp 20-1 20-34. (13) Ma, X. Role of solvation energy in starch adsorption on oxide surfaces. Colloids Surf. A 2008, 320, 36. (14) Whistler, R. L.; Bemiller, J. N.; Paschall, E. F. Starch: Chemistry and Technology, Academic Press: London, 1984. (15) Chen, J.; Jane, J. Properties of granular cold-water-soluble starches prepared by alcoholic alkaline treatments. Cereal Chem. 1994, 71, 623. (16) Bertuzzi, M. A.; Armada, M.; Gottifredi, J. C. Physicochemical characterization of starch based films. J. Food Eng. 2007, 82, 17. (17) Yamamoto, H.; Makita, E.; Oki, Y.; Otani, M. Flow characteristics and gelatinization kinetics of rice starch under strong alkali conditions. Food Hydrocolloids 2006 (18) Oosten, B. J. Interactions between starch and electrolytes. Starch 1990, 42, 327. (19) Michaels, A. S.; Morelos, O. Polyelectrolyte adsorption by kaolinite. Ind. Eng. Chem. 1955, 47, 1801. (20) Ma, X.; Pawlik, M. Effect of alkali metal cations on adsorption of guar gum onto quartz. J. Colloid Interface Sci. 2005, 289, 48. (21) Khosla, N. K.; Bhagat, R. P.; Gandhi, K. S.; Biswas, A. K. Calorimetric and other interaction studies on mineral starch adsorption systems. Colloids Surf. 1984, 8, 321. (22) Jones, F.; Farrow, J. B.; Bronswijk, W. An infrared study of a polyacrylate flocculant adsorbed on hematite. Langmuir 1998, 14, 6512. (23) Schulz, N. F.; Cooke, S. R. B. Froth flotation of iron ores: Adsorption of starch products and laurylamine acetate. Ind. Eng. Chem. 1953, 45, 2767. (24) Balajee, S. R.; Iwasaki, I. Interaction of British gum and dodecylammonium chloride at quartz and hematite surfaces. Trans. AIME 1969, 244, 407. (25) Partridge, A. C.; Smith, G. W. Flotation and adsorption characteristics of the hematite dodecylamine starch system. Can. Metall. Q. 1971, 10, 229. (26) Khosla, N. K.; Biswas, A. K. Mineral collector starch constituent interactions. Colloids Surf. 1984, 9, 219. (27) Santhiya, D.; Subramanian, S.; Natarajan, K. A.; Malghan, S. G. Surface chemical studies on the competitive adsorption of poly(acrylic acid) and poly(vinyl alcohol) onto alumina. J. Colloid Interface Sci. 1999, 216, 143. (28) Stacy, C. J.; Foster, J. F. A light scattering study of corn amylopectin and its β-amylase limit dextrin. J. Polym. Sci. 1956, 20, 57. (29) Ring, S. G.; Colonna, P. C.; I’Anson, K. J.; Kalichevsky, M. T.; Miles, M. J.; Morris, V. J.; Orford, P. D. The gelation and crystallisation of amylopectin. Carbohydr. Res. 1987, 162, 277.
11953
dx.doi.org/10.1021/ie2013373 |Ind. Eng. Chem. Res. 2011, 50, 11950–11953
