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Ind. Eng. Chem. Res. 2004, 43, 4422-4427
Effect of Ultrasonication on the Flotation of Talc Dingwu Feng† and Chris Aldrich*,‡ Department of Chemical and Biomolecular Engineering, The University of Melbourne, Victoria 3010, Australia, and Department of Chemical Engineering, University of Stellenbosch, Private Bag X1, Matieland 7602, South Africa
The natural hydrophobicity of talc was depressed by the use of ultrasonic treatment during flotation. The recovery of the talc decreased by nearly 30% after 4 min of ultrasonic preconditioning. In addition, two talc depressants, namely, IMP4 and carboxymethyl cellulose (CMC), were compared during ultrasound treatment. The guar gum IMP4 showed a better depressant ability than the CMC. The effects of these depressants on talc were enhanced by ultrasonication, preferably by adding the depressants before ultrasonic treatment. Microtopographic studies by atomic force microscopy indicated that surface defects generated by ultrasound could serve as active sites for water adsorption, rendering the naturally hydrophobic talc hydrophilic. The enhanced formation of the hydrophilic magnesium and silicon hydroxyl species on talc surfaces slightly affected the natural hydrophobicity of talc in the presence of ultrasound as well. 1. Introduction Talc is a trioctahedral phyllosilicate with no layer charge. In a pure form, talc has natural flotation properties. Much work has been done on the hydrophobicity of talc and its origin.1,2 Mechanical activation can result in the structural changes of talc and surface properties as well.3-5 Talc is a significant constituent of the gangue in platinum-bearing ore bodies in South Africa. The structure of talc makes it hydrophobic and as such causes froth stabilization. This characteristic results in the difficult separation of talc and the gangue minerals from the valuable platinum-bearing minerals in the flotation process. To prevent talc from reporting to the concentrate with these valuable minerals, long-chain polysaccharides are often used as depressants. Polysaccharides can be expensive, and it is not unusual for the depressant costs to exceed the cost of any other reagents added to the flotation system. Polysaccharides and their derivatives are effective depressants for talc and other magnesia-bearing minerals. There are numerous reports on the application of carboxymethyl cellulose (CMC) as a depressant for talc and other magnesia-bearing minerals present as impurities in various sulfide ores.6-9 The mechanisms of depressant action and application of various dispersing and depressing agents have been critically examined by Laskowski and Pugh.10 The important factors governing the effectiveness of the polymeric depressants from a chemical viewpoint have been highlighted by Lin and Burdick,11 while the constitution of talc has been extensively investigated by Pask and Warner.12 Some aspects of the structure and surface properties of talc have been the subject of discussion of a number of papers.13 For example, the role of hydrolyzed cations in the natural hydrophobicity of talc has been examined by Fuerstenau et al.14 The adsorption and depressant * To whom correspondence should be addressed. Fax: +27(21)8082059. E-mail:
[email protected]. † The University of Melbourne. ‡ University of Stellenbosch.
action of poly(oxyethylene alkyl ether) on talc has been studied by Pugh and Tjus,15 while dextrin has also been reported as a depressant for talc. Likewise, Rath et al.16 conducted a comparative study on the adsorption of dextrin and guar gum onto talc, etc. The use of ultrasonic preconditioning has resulted in a significant improvement of the flotation of the Merensky platinum-bearing complex sulfide ore, particularly the flotation rate, selectivity, and overall recovery of sulfides.17 The dosage of the IMP4 depressant could be reduced when the pulp was preconditioned by ultrasound, probably owing to the increased hydrophobicity of the sulfides, as well as an increase in the hydrophilicity of the silicates such as talc. Ultrasonic activation could act as a talc depressant. Nevertheless, the flotation behavior of talc after ultrasonic treatment has not yet been investigated and the ultrasonic activation mechanism on talc is still certain. In the present investigation, the effect of ultrasound pretreatment on the flotation behavior of talc has been conducted. Two natural polysaccharides having different configurations of hydroxyl groups, namely, guar gum and CMC, have been compared with regard to their depressant behavior on talc in the presence of ultrasound. Possible mechanisms of ultrasonic action on talc are discussed. 2. Experimental Work 2.1. Materials. A pure mineral sample of talc was obtained from Masala Talc Mine in South Africa. Mineralogical analysis by X-ray diffraction confirmed that the talc sample was of high purity with a trace amount of quartz. The sample was dry ground using a porcelain ball mill. The ground sample was then dry screened into two size fractions of 53-75 and 25-53 µm for the flotation tests. The Brunauer-Emmett-Teller nitrogen-specific surface areas of the two size fractions of 53-75 and 25-53 µm talc were found to be 1.87 and 2.94 m2/g, respectively. The sample of CMC with a molecular weight of 250 000 was obtained from Aldrich. The guar gum depressant of IMP4 that was used in this study had a degree of substitution of approximately 0.1
10.1021/ie034057g CCC: $27.50 © 2004 American Chemical Society Published on Web 06/17/2004
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(molecular weight of 200 000; Trohall). The depressant solutions were prepared by dispersing a known weight of depressant in cold distilled water and then dissolving it with boiling distilled water. The solutions were prepared fresh each day. Distilled water was used in all flotation tests. 2.2. Methods. Flotation tests were carried out using a flotation column with a natural solution pH. The test flotation column was made of glass having a diameter and height of 35 and 300 mm, respectively. The sparged air was distributed through a sintered glass with pore size 4 (10-15 µm). A 4 g talc sample was conditioned with distilled water for 5 min using a magnetic stirrer, after which the conditioned slurry was subjected to sonication with ultrasound. The depressant solution was added and allowed to condition for an additional 4 min. After pretreatment, the sample was transferred to the flotation column and made up to 1.6% solids concentration by distilled water. Nitrogen gas was passed through the slurry at a rate of 250 cm3/min. The experiments were conducted at an ambient temperature of 298 K, and the froth was overflowed automatically. Three samples were taken after 4, 8, and 12 min of flotation. These samples were separately filtered, dried, and weighed, and the recoveries were expressed on a weight basis. Great care was taken to clean the column between runs. The ion concentrations of Mg and Si in solution were determined by a Varian inductively coupled plasma (Liberty Series II, Axially-viewed Sequential, ICP Expert Software Package). The microtopography of the solid talc sample was studied with an atomic force microscope. The ultrasonic experiments were conducted in a thermally controlled water bath at 298 K, with a high-tensile titanium alloy ultrasonic probe with a diameter of 3 mm (IKA Sonic U50-3 Sonotool, IKA Labortechnik). The frequency of the probe was fixed at 30 kHz, but the amplitude was adjustable. An acoustic power density of 90 W/cm2 was used in all of the experiments. The ζ potential was measured in a Colloidal Dynamics (model Acoustosizer II) zetameter. Samples were ground to a particle size of -20 µm. A total of 10 g of cleaned samples was added to 200 cm3 0.01 M NaCl electrolyte solutions prepared with deionized water. Readings started from about pH 12 to 2 with the addition of NaOH or HCl. The point of zero charge (PZC) could shift because of the coagulation of particles after the initial several measurements. 3. Results and Discussion 3.1. Effect of the Pulp Concentration. Three pulp concentrations were investigated with the 25-53 µm talc sample, while the conditioning temperature was maintained at 298 K and the conditioning time at 4 min. An 8% solids concentration was optimal with respect to the lowest recovery of talc, as indicated in Figure 1. Higher pulp concentrations had a detrimental effect on flotation. 3.2. Effect of Ultrasonic Preconditioning Time. The pulp concentration was maintained at 8% solids and the conditioning temperature at 298 K. The changes of pulp pH values and Mg and Si ion concentrations in 50 cm3 of pulp with different ultrasonic conditioning times are shown in Figure 2. The pulp pH increased to a maximum of 9.45 from 9.37 after 2 min and then decreased to 9.35 after 4 min, after which it decreased gradually. The magnesium ion
Figure 1. Effect of ultrasonic preconditioning with pulp concentrations of 4%, 8%, and 12% on the recovery of talc. Flotation without ultrasound refers to a pulp concentration of 8% solids.
Figure 2. Variation of the pH and Mg and Si ion concentrations with different sonication times.
Figure 3. Schematic diagram of the face and edge views of talc, Mg12Si16O40(OH)8.1
concentration increased from 2.55 to 2.99 mg/dm3 after 2 min and then decreased to 2.66 mg/dm3, after which it increased gradually, while it slightly decreased after 15 min. The Si ion concentration followed the same trend as the Mg ion concentration but more pronounced, as indicated by Figure 2. Talc crystals are composed of two-dimensional sheet structures, consisting of two layers of silica tetrahedra held together with brucite [Mg(OH)2]. This sandwichlike structure extends indefinitely in two-dimensional layers, which are stacked one on top of the other, as shown in Figure 3.1 The atoms within the layers are held together by ionic bonds, while oxygen-oxygen interlayer atoms are held only by a weak dispersion force.18 As a result, easy cleavage takes place between the talc layers. When talc particles are broken, two different kinds of surfaces are formed, one resulting from the easy cleavage of the layers and the other resulting from the rupture of ionic bonds within these layers. The former are termed faces and the latter edges.19 The faces, made up of fully compensated oxygen atoms, have a very low electric charge (ideally zero free charge) and are nonpolar in
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Figure 5. Effect of IMP4 and CMC on the flotation of talc with and without ultrasonic pretreatment. Solid markers indicate IMP4, while open markers indicate CMC. S represents 4 min of sonication.
Figure 4. Effect of the sonication time on the recovery of talc.
water, whereas the edges, composed of hydroxyl ions, silicon, oxygen, and magnesium ions (all of which undergo hydrolysis), have a relatively high electric charge and are polar in water. The surfaces of these edges are ultimately composed of ionized and neutral hydroxyl groups resembling the surfaces of oxide minerals. The dissolution of the edges of the talc crystals was speeded up with sonication, as the concentrations of Si and Mg ions in solution increased, while the solution pH increased as well. With the increase of the Mg and Si ion concentrations owing to prolonged sonication, Mg and Si ions hydrolyzed to magnesium and silicon hydroxyl species, respectively. The speciation diagram of magnesium (MINEQL+ software, version 4.0, Environmental Research Software, Hallowell, ME, 1998) showed that Mg2+ was present at about pH 8.5. Likewise, the speciation diagram showed that Si4+ hydrolyzed to Si(OH)4, Si(OH)3-, SiO2(OH)22-, and SiO3(OH)3-. The hydrolysis of Mg and Si ions reduced the solution pH, as can be seen in Figure 2. The hydrolyzed products of Mg and Si ions could attach to the particle surfaces, which would explain the observed reduction in the concentrations of Si and Mg ions in solution after 4 min of sonication. The formation of magnesium and silicon hydroxyl species at the talc surfaces was also verified by a slight shift of the PZC of talc from about pH 3.7 to 3.8 after 4 min of sonication. However, with prolonged sonication, the PZC of talc shifted back to about pH 3.7 again, likely because of the hydrolyzed products of Mg and Si ions attached to particle surfaces being stripped off the particle surfaces. The hydrolyzed products of Mg and Si ions are naturally hydrophilic. The observed recovery of talc was therefore lowest after 4 min of sonication. The variation of talc recovery with sonication time is shown in Figure 4. No depressants were added for these runs, and the size fraction was 25-53 µm. The talc suspension was unstable and the talc particles coagulated without ultrasonic pretreatment, owing to the heterocoagulation of talc (heterocoagulation means that the hydrolyzed products of Mg and Si ions adhere to talc particle surface to form larger particles, while coagulation notes that ultrafine talc particles stick together and form large particles). This is in agreement with the observation of
Huang and Fuerstenau.20 The heterocoagulation of talc increased the flotation recovery of talc. In contrast, the talc suspensions after ultrasonic pretreatment appeared to be much more stable, resulting in lower flotation recoveries. However, this effect became less pronounced after prolonged ultrasonication. As can be seen from Figure 4, both the flotation kinetics and recoveries of talc decreased with the pretreatment by ultrasound. Ultrasonic pretreatment had an obvious depression effect on talc flotation. The talc recovery decreased by nearly 30% after 4 min of ultrasonic preconditioning, and the flotation kinetics was the lowest as well. Prolonged conditioning resulted in a lesser extent of decrease in the talc recovery. The initial reduction in talc could be attributed to the formation of the hydrophilic magnesium and silicon hydroxyl species on the particle surfaces as discussed above. In contrast, prolonged ultrasonication time could have removed the hydrophilic precipitates, resulting in a lesser extent of decrease in the talc recovery. It is noteworthy that the particle size decreased with an increase in the duration of sonication. The particle size remained almost the same after 8 min of sonication, while the size decreased with sonication after more than 8 min. The specific surface area increased from 2.94 to 3.08 m2/g after 15 min of sonication and to 3.27 m2/g after 30 min of sonication. The fine particles generated by sonication could form a stable froth and reported to the froth products, which contributed to the increased talc recovery with prolonged sonication. 3.3. Effect of Depressant Types on the Flotation of Ultrasonically Pretreated Talc. Two types of depressants, viz., CMC and IMP4, were compared for talc flotation without or with (denoted by CMC + S or IMP4 + S) 4 min of ultrasonic pretreatment. The particle size fraction was 25-53 µm. The flotation results are shown in Figure 5. As can be seen from Figure 5, CMC had a less depressing effect on the talc flotation at the same dosages, while the ultrasonic pretreatment could enhance the effects of the depressants. The talc flotation recoveries decreased by about 15% at lower IMP4 and CMC dosages and by about 8% at higher IMP4 and CMC dosages with ultrasonic pretreatment. The possible mechanisms of adsorption of CMC and IMP4 onto talc can be explained as follows:16 (a) The dissolution of talc releases magnesium ions, which could form neutral or charged magnesium hydroxo complexes. The hydroxo complexes could react with the polymer both at the hydroxylated talc surface and in the bulk solution by hydrogen bonding and chemical interaction.
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Figure 6. Effect of the depressant addition point on the flotation of talc with ultrasound treatment. BS indicates IMP4 addition before sonication. AS indicates IMP4 addition after sonication.
(b) The polymer could interact directly with the hydroxylated talc surface by hydrogen bonding, especially at alkaline pH values. At alkaline pH values, the metal hydroxide precipitate on the talc faces could interact with water molecules through hydrogen bonding. (c) The adsorption process could also be brought about by chemical interaction with the magnesium ions released. The better depression of IMP4 can be attributed to the differences in the molecular weights of the two reagents, as well as the more favorable cis configuration of the hydroxyl groups as opposed to the trans configuration in CMC. The pretreatment of talc with ultrasound expedited the release of magnesium ions. The manganese hydroxo complexes could react with the polymer at the hydroxylated talc surface, as well as in the bulk solution by hydrogen bonding and chemical interaction. Ultrasound could generate defects on the talc crystal faces, which could serve as active sites, whereby the adsorption of depressants could be enhanced. In addition, ultrasound could clean the talc crystal edges and expose the magnesium sites for depressant adsorption. Consequently, the effects of depressants on talc were enhanced with ultrasonic pretreatment. 3.4. Effect of Sonication on Depressant. The IMP4 was used as the depressant, the sonication time was 4 min, and the particle size fraction was 25-53 µm. Two cases were investigated, viz., where the depressant was added before sonication (denoted as BS) and after sonication (denoted as AS). The effect of the depressant addition point on the flotation recovery of talc is shown in Figure 6. As can be seen from Figure 6, the depressant preferably has to be added prior to ultrasonic treatment. The difficult-to-dissolve IMP4 could be activated, making it ready to attach to the talc surface apart from the depressing effects of ultrasound on talc itself. 3.5. Results of Factorially Designed Experiments. The influence of three factors on the flotation recovery and kinetics of talc was quantified with regression analysis, viz., particle size (53-75 and 25-53 µm), ultrasound (with or without 4 min of ultrasonic pretreatment), and IMP4 (with or without 4 mg/L of IMP4 added after ultrasonic pretreatment of the talc). The results of the factorially designed experiments are shown in Table 1. It is interesting that the coarser particle size fraction had a higher recovery than the finer one in the absence of ultrasound and depressant, while it had a lower recovery than the finer one at the same conditions of
Figure 7. Typical topographic image of the talc surface without ultrasonic treatment. Table 1. Effect of the Particle Size, Ultrasound, and IMP4 on the Recovery of Talc run
particle size (µm)
ultrasound (off ) 0, on ) 1)
depressant IMP4
recovery (%)
1 2 3 4 5 6 7 8
25-53 53-75 53-75 53-75 25-53 25-53 25-53 53-75
0 0 1 0 0 1 1 1
1 0 0 1 0 0 1 1
35.31 75.61 47.80 22.31 67.79 38.73 20.04 10.76
ultrasound and depressant. In the coarser size, the faceto-edge ratio is higher than that of the finer size. The larger hydrophobic surfaces of the talc crystal in the coarse size fraction could contribute to the higher recovery in the absence of ultrasound and depressant. The faces of the talc crystal are made up of fully compensated oxygen atoms, have a very low electric charge, and are nonpolar in water. It can be expected that the depressant adsorption took place to a greater extent on the faces. On the other hand, the edges were composed of hydroxyl ions, silicon, oxygen, and magnesium ions that are easily hydrolyzable, have a relatively high electric charge, and are polar in water. Consequently, the coarser particles had probably experienced higher a depressant adsorption than the finer ones, resulting in lower flotation recovery. This was well in accordance with the observations of Rath et al.16 3.6. Topographic Studies of Talc Surfaces. Typical topographic images of talc surfaces by atomic force microscopy with and without ultrasonic treatment are shown in Figures 7 and 8. As can be seen from Figures 7 and 8, the talc particle surface became rougher and some defects appeared on the surface after ultrasonic treatment as indicated by Figure 8b. The rougher surface could be attributed to the dissolution of Mg, Si, O, and OH species under ultrasonication. It is likely that the surface defects had been caused by the mechanical effects of the ultrasound. These defects could act as active sites for water and depressant adsorption. When the hydrophobic faces of the talc crystals incurred defects, it meant that the faces could not be zero charged anymore. The polar water molecule could therefore absorb onto the faces and make the faces hydrophilic. 4. Discussion and Conclusions The use of ultrasonic preconditioning has resulted in significant improvement of the flotation of the Merensky platinum-bearing complex sulfide ore, particularly the
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both depressants on talc. The pretreatment of talc with ultrasound appeared to expedite the release of magnesium ions. The resulting manganese hydroxo complexes could react with the polymers both at the hydroxylated talc surfaces and in the bulk solution by hydrogen bonding and chemical interaction. Ultrasound generated defects on the talc crystal faces, which served as active sites, so that the adsorption of depressants could be enhanced. In addition, ultrasound could clean the talc crystal edges and expose the magnesium sites for depressant adsorption. The depressants preferably have to be added before ultrasonic treatment. (iii) Coarse particles had higher recoveries than fine ones in the absence of ultrasound and depressants, owing to the higher surface face-to-edge ratio in coarse particles. In contrast, fine particles had higher recoveries than coarse ones in the presence of ultrasound and depressants, owing to the superior adsorption of depressants onto the faces. (iv) Microtopographic studies indicated that the talc crystal surfaces incurred some point or line defects when subjected to ultrasound. These defects probably served as active sites for water or depressant adsorption, making the naturally hydrophobic talc hydrophilic and enhancing the effects of depressants on the talc. Literature Cited
Figure 8. Typical topographic images of the talc surface after 4 min of ultrasonic treatment showing (a) a roughened surface and (b) the occurrence of large local defects.
depression of silicates such as talc.17 The dosage of the IMP4 depressant could be reduced when the pulp was preconditioned by ultrasound. The practical application of ultrasonic pretreatment for the flotation is dependent upon successful scaling up of the laboratory results for industrial use. Further work as far as scaling up of the laboratory results is presently being conducted, and several configurations are being considered. One of the best approaches appears to be a flow system consisting of a flow loop outside a normal batch reactor, which acts as a reservoir within which the sonochemical reactions can occur. Such an arrangement allows the ultrasonic dose of energy entering the reservoir to be controlled by the flow rate (residence time), while dispensing with the need for temperature control. From the results of this investigation, the following conclusions can be drawn: (i) Both the flotation kinetics and recoveries of talc decreased with pretreatment by ultrasound. Ultrasonic pretreatment had an obvious depressant effect on the talc flotation. The talc recovery of the 25-53 µm fraction decreased by nearly 30% after 4 min of ultrasonic preconditioning, from approximately 70% down to 40% recovery. The reduced talc recovery resulting from ultrasound pretreatment could be attributed to the formation of the hydrophilic magnesium and silicon hydroxyl species on the particle surfaces. Prolonged ultrasonication could remove the hydrophilic precipitation layers, resulting in increased talc recovery. (ii) Floatability of talc was depressed to a greater extent by IMP4 than by CMC at the same weight dosages. Ultrasonic treatment enhanced the effects of
(1) Michot, L. J.; Villieras, F.; Francois, M.; Yvon, J.; Dred, R. L.; Cases, J. M. Structural microscopic hydrophilicity of talc. Langmuir 1994, 10, 3765-3773. (2) Drzymala, J. Hydrophobicity and collectorless flotation of inorganic materials. Adv. Colloid Interface Sci. 1994, 50, 143185. (3) Zbik, M.; Morris, G. E. Influence of grinding on talc wettability. 16th Clay Mineral Society Conference, Brisbane, Australia, June 30-July 2, 1998. (4) Wu, W.; Giese, R. F., Jr.; Van Oss, C. J. Change in surface properties of solids caused by grinding. Power Technol. 1996, 89, 129-132. (5) Aglietti, E. F. Effect of dry grinding on the structure of talc. Appl. Clay Sci. 1994, 9, 139-147. (6) Rhodes, M. K. In Proceedings of the 13th International Mineral Processing Congress; Laskowski, J., Ed.; Elsevier: Amsterdam, The Netherlands, 1981; Vol. 2, p 346. (7) Edwards, C. R.; Kipkie, W. B.; Agar, G. E. The effect of slime coatings of the serpentine minerals, chrysotile and lizardite on pentlandite flotation. Int. J. Miner. Process. 1980, 7, 33-42. (8) Barker, C. W.; Featherstone, S. F.; Storey, M. J. Development of flotation practice at Trojan nickel mine concentrator, Zimbabwe. Trans. Inst. Min. Metall., Sect. C 1982, 91, 135-141. (9) Oliveira, J. F.; Gomes, L. M. B. In Proceedings of Hydrophobic Minerals and Fine Coal; Laskowski, J. S., Poling, G. W., Eds.; Canadian Institute of Mining, Metallurgy and Petroleum: Montreal, Canada, 1995; p 341. (10) Laskowski, J. S.; Pugh, R. J. Dispersions stability and dispersing agents. In Colloid Chemistry in Mineral Processing; Laskowski, J. S., Ralston, J., Eds.; Elsevier: Amsterdam, The Netherlands, 1991. (11) Lin, K. F.; Burdick, C. L. In Reagents in Mineral Technology; Moudgil, B. M., Somasundaran, P., Eds.; Surfactants Science Series 27; Dekker: New York, 1988. (12) Pask, J. A.; Warner, M. F. Fundamental studies of talc, I. Constitution of talcs. J. Am. Ceram. Soc. 1954, 37, 118-128. (13) Malhammar, G. Determination of some surface properties of talc. Colloids Surf. 1990, 44, 61-69. (14) Fuerstenau, M. C.; Lopez-Valdieso, A.; Fuerstenau, D. W. Role of hydrolyzed cations in the natural hydrophobicity of talc. Int. J. Miner. Process. 1988, 23, 161-170. (15) Pugh, R. J.; Tjus, K. Flotation depressant action of poly(oxyethylene) alkyl ethers on talc. Colloids Surf. 1990, 47, 179194.
Ind. Eng. Chem. Res., Vol. 43, No. 15, 2004 4427 (16) Rath, R. K.; Subramanian, S.; Laskowski, J. S. Adsorption of dextrin and guar gum onto talc. A comparative study. Langmuir 1997, 13, 6260-6266. (17) Aldrich, C.; Feng, D. Effect of ultrasonic preconditioning of pulp on the flotation of sulphide ores. Miner. Eng. 1999, 12 (6), 701-707. (18) Aplan, F. F.; Fuerstenau, D. W. Principles of nonmetallic mineral flotation. In Froth Flotation; Fuerstenau, D. W., Ed.; AIME: New York, 1962. (19) Chander, S.; Wie, J. M.; Fuerstenau, D. W. On native flotability and the surface properties of naturally hydrophobic
solids. Advances in Interfacial Phenomena; Series No. 150; AIChE: New York, 1975; pp 183-188. (20) Huang, P.; Fuerstenau, D. W. The effect of the adsorption of lead and cadmium ions on the interfacial behaviour of quartz and talc. Colloids Surf., A 2001, 177, 147-156.
Received for review August 8, 2003 Revised manuscript received February 19, 2004 Accepted February 28, 2004 IE034057G
