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The effect of amine type, pH, and size range in the flotation of quartz
Minerals Engineering 20 (2007) 1008–1013 This article is also available online at: www.elsevier.com/locate/mineng
The effect of amine type, pH, and size range in the flotation of quartz Ana M. Vieira, Antonio E.C. Peres
*
Escola de Engenharia da UFMG, Rua Espı´rito Santo, 35/2006, 30160-030 Belo Horizonte, MG, Brazil Received 14 December 2006; accepted 25 March 2007 Available online 11 May 2007
Abstract Cationic reverse flotation of quartz is the most important technique for the concentration of itabirite iron ores for the production of pellet feed. Mineral processors dealing with this industrial system face two major challenges: (i) coarse quartz particles do not respond well to the action of the amine collectors; (ii) fine iron oxide particles do not respond well to the depressant action of the gelatinised corn starch. The first challenge is addressed in this paper. Bench scale experiments were performed in a conventional mechanical machine aiming at investigating the flotation performance of quartz particles of different size ranges in the presence of four dosages of ether monoamine and ether diamine, at three pH levels. It was observed that ether diamine was more effective in the flotation of medium and coarse quartz, while ether monoamine performs better in the case of fine quartz, and also that the flotation of coarse particles is enhanced by the presence of fine particles in the system. Ó 2007 Elsevier Ltd. All rights reserved. Keywords: Flotation reagents; Flotation collectors; Froth flotation; Quartz flotation; Quartz collectors; Particle size
1. Introduction Mineral particles in a broad size range are present in flotation pulps. The importance of size in the sequence of events that leads to the flotation of a mineral particle has been extensively recognized a long time ago (Glembotskii et al., 1972; Trahar, 1981; Bazin and Proulx, 2001). The use of flotation recovery versus particle size curves became usual practice for determining the maximum size for adequate flotation. Several equations have been developed and presented correlating maximum particle size with flotation parameters, such as particle density, medium density, liquid surface tension, bubbles radium, detachment angle, contact angle (Drzymala, 1994; Schulze, 1982; Yoon and Luttrell, 1989; Diaz-Penafiel and Soto, 1994). Results of investigations aiming at extending the range of coarse particles that respond adequately to flotation were also reported in the literature (Hall, 1996; Ahmed *
Corresponding author. Tel.: +55 31 32381717; fax: +55 31 32381815. E-mail address: [email protected] (A.E.C. Peres).
0892-6875/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.mineng.2007.03.013
and Jameson, 1989). Ahmed and Jameson (1989) studied the effect of agitation (100, 300, and 600 rpm), in a mechanical cell, on the collection kinetic constant, for different quartz particle sizes (5, 10, 20, and 40 lm) and different bubble sizes (75, 165, 360, and 655 lm). At 300 rpm, bubbles of all sizes were effective, 75 lm bubbles yielding the largest kinetic constant. At 600 rpm, the flotation rates for the three larger bubble sizes are smaller than those achieved with minimum agitation (100 rpm). Trahar (1981) reported that the flotation recovery of coarse particles of different minerals is more sensitive to the chemical medium, in comparison with finer particles. ¨ teyaka and Soto (1994), investigating the effect of O coarse particles of quartz and calcite in flotation columns, concluded that the kinetics of coarse particles flotation is controlled by the stability of the aggregates formed. This stability is enhanced by stronger attractive forces and weaker rupture forces. The first effect relies on the choice of a selective collector at an optimum dosage. Klimpel (1988) claimed that high collector dosages are required for the economical flotation of coarse particles, but alerted that excessive dosages may cause deleterious
A.M. Vieira, A.E.C. Peres / Minerals Engineering 20 (2007) 1008–1013
effects in the flotation system. Fundamental studies, performed by Crawford and Ralston (1988) in a modified Hallimond tube, with quartz particles in the size range 15–125 lm, indicated that 71 lm particles required only 35% surface coverage with collector for achieving 80% recovery. Larger particles (121 lm) required 60% coverage for reaching the same recovery. Fine particles exhibit much higher specific surface area (cm2/g) than coarse particles, so the collector consumption required for a certain coverage degree is much larger for fine particles than for coarse particles. When a combination of coarse and fine particles are exposed to a large collector dosage, most of the collector will be consumed by the fines that require less coverage for efficient flotation. Not enough collector species are left for effective flotation of the coarse particles. Particles can report to the froth product by true flotation or entrainment. Gaudin (1957) presented one of the first references to entrainment, even without mentioning the word: ‘‘bubbles exert a sort of filtering action and enmesh a number of gangue particles in the comparatively dry bubble walls. Small particles are retained in the interbubble water. The recovery of fine gangue particles is usually greater than the corresponding water recovery’’. Smith and Warren (1989) define ‘‘entrainment as the process by which particles enter the base of the froth and are transferred up and out of the flotation cell suspended in the water between bubbles’’. Besides Smith and Warren (1989) and Trahar (1981) addressed the subject in the 1980s. In the past 10 years more scientific approaches have been developed to investigate the entrainment process, bringing light to the understanding of its role in plant practice. Relevant contributions came from the JKMRC and the University of Queensland. Savassi (1998) proposed and validated a model for the direct estimation of the degree of entrainment and the froth recovery of attached particles in industrial flotation cells. In his model entrainment was considered as directly related with water recovery. This approach was also reported in earlier contributions (Engelbrecht and Woodburn, 1975; Johnson, 1972; Bisshop and White, 1976; Kirjavainen, 1992). Zheng et al. (2005) developed an innovative model of entrainment in industrial flotation cells taking into account the two simultaneous entrainment mechanisms in the flotation process: transfer of the suspended solids in the pulp phase to the froth phase and transfer of the entrained particles in the froth phase to the concentrate. The two steps in the entrainment process are combined to calculate the overall entrainment recovery. In the present investigation, the flotation of quartz particles in different size ranges is addressed aiming at bringing light to the so-called reverse cationic flotation of itabirite iron ores. Quartz particles are removed in the froth phase leaving hematite as the non floated fraction. In this particular system, the removal of quartz particles either by true flotation or entrained in the froth is beneficial to yield a clean concentrate. In the reverse flotation of iron ores, the entrainment of iron oxide particles in the froth phase
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represents a serious problem, for it results lower iron recovery. The other factor contributing to the loss of iron in the froth phase is the lack of efficiency of the depressants towards fine particles of iron oxides (Turrer et al., 2007). The entrainment in the cationic flotation an iron ore was investigated by Borges and Araujo (1994), regarding the entrainment of iron oxides, with no mention to quartz. A direct correlation between the solids content in the froth and the entrainment degree was observed. The entrained weight was not proportional to the water recovery in the froth product. 2. Methodology A very pure quartz sample from Minas Gerais state, Brazil, was carefully comminuted to avoid contamination yielding three fractions designated as F – fine (mainly in the size range 74 lm to +38 lm), M – medium (mainly in the size range 150 lm to +74 lm), and C – coarse (mainly in the size range 297 lm to +150 lm). These fractions were submitted to chemical and size analyses. Flotation experiments were performed in a laboratory Denver, model D12, machine, using a 2L cell. The test conditions were: 1300 rpm, 20% solids, collector conditioning 4 min, froth removal until the froth was barren. It is worthwhile mentioning that all experiments were performed in triplicate and the reproducibility of the experiments was excellent. Two classes of amines are used in plant practice of cationic reverse flotation of iron ores: ether monoamine and ether diamine. For the present investigation Flotigam EDA 3 (monoamine, neutralisation degree 29.8%) and Flotigan 2835-2L (diamine, neutralisation degree 20.6%), manufactured by Clariant, were selected. Ether monoamine is designation, in the jargon of mineral processors, of N-alkyloxipropylamine (R–O–CH2– CH2–C–NH2), produced in two stages from the reaction of a fatty alcohol with acrynitrile, producing ether nitrile that is then catalytically hydrogenated under high pressure (Shapiro, 1968). Ether diamine is designation, in the jargon of mineral processors, of N-alkyloxipropyl-1,3-diaminepropane (R– O–(CH2)3–NH–(CH2)3–NH2), produced in two stages from the reaction of the ether monoamines with acrylonitrile followed by hydrogenation. The tests were performed at three pH levels: 9.0, 10.0, and 10.5. Collector dosages were: 20 g/t, 40 g/t, 60 g/t, and 80 g/t. 3. Results and discussion Results of chemical analyses are presented in Table 1. Results of size analyses are presented in Table 2. Figs. 1–3 illustrate, respectively, the recoveries of fine, medium, and coarse quartz achieved by varying the dosage of ether monoamine and the pH.
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A.M. Vieira, A.E.C. Peres / Minerals Engineering 20 (2007) 1008–1013
Table 1 Results of chemical analyses Fraction
Species SiO2, %
Fe2O3, %
Al2O3, %
Loss on ignition, %
C coarse M medium F fine
99.71 99.63 99.43
0.03 0.06 0.15
0.04 0.06 0.12
0.20 0.23 0.25
Table 2 Results of size analyses and proposed retention Fraction
297 to +150 150 to +74 74 to +38
Analysed retention, %
Proposed retention, %
98.57 92.92 89.29
100.00 100.00 100.00
Fine quartz recovery (%)
60 40 20 0 30
40
50
60
70
80
90
Ether monoamine dosage (g/t) pH = 9.0
pH = 10.0
pH = 10.5
Fig. 1. Fine quartz recovery as a function of ether monoamine dosage and pH.
Medium quartz recovery (%)
100
80
60
40
20
0 0
10
20
20
20
40
60
80
100
30
40
50
60
70
80
90
Ether monoamine dosage (g/t) pH = 9.0
pH = 10.0
pH = 9.0
pH = 10.0
pH = 10.5
Fig. 3. Coarse quartz recovery as a function of ether monoamine dosage and pH.
80
20
40
0
100
10
60
Ether monoamine dosage (g/t)
Size range, lm
0
80
0
Species
C coarse M medium F fine
Coarse quartz recovery (%)
100
pH = 10.5
Fig. 2. Medium quartz recovery as a function of ether monomine dosage and pH.
The results shown in Fig. 1 indicate that the performances of ether monoamine are the same for pH levels
9.0 and 10.0, irrespectively of the collector dosage. In the case of pH 10.5, quartz recovery was impaired for lower collector dosages (20 g/t and 40 g/t), the curves superimposing for higher dosages (60 g/t and 80 g/t). The dissociation of ether monoamines (Leja, 1982) explains the results. At pH 9.0 the ionised species prevails over the molecular species. At pH 10.0 there is an equilibrium between the concentrations of both species and at pH 10.5 the molecular species prevails. The electrostatic adsorption followed by hemimicelles formation, proposed by Gaudin and Fuerstenau (1955), requires at least 50% dissociation for enhanced floatability. The coincidence between the recovery curves for the lower pH levels and the fact that the curve for pH 10.5 is not too far away from the other curves are due to the easiness of fine the light particles of fine quartz to be carried to the froth phase. Entrainment might be a factor contributing to this transport. The results shown in Fig. 2 indicate a wide dispersion of the recovery curves for different pH levels. The results stress the role of the coulombic mechanism. At pH 9.0 (prevalence of ionic species) the response is similar to that of fine quartz. As far as the pH increases, the effectiveness of ether monoamine towards medium quartz decreases sharply. The recoveries of coarse quartz are illustrated in Fig. 3. The results strongly emphasise the comments regarding Fig. 2. Floatability at pH 10.5 and pH 10.0 is negligible at all dosages, reaching almost 30% in the case of a very high dosage (80 g/t). Fig. 4 illustrates the recovery of fine quartz with ether diamine. Recoveries are sharply enhanced when amine dosages increase from 60 g/t to 80 g/t. The trend of higher recovery corresponding to lower pH was again observed. Ether diamine is not an efficient collector for fine quartz at pH levels 10.0 and 10.5, even at dosages as high as 80 g/t. A comparison between Figs. 1 and 4 provides an interesting hint to plant practice: monoamine is more efficient than diamine in fine quartz flotation. Fine quartz particles are light enough to be carried by the bubbles even after hydrophobisation by a weaker collector. There is no
A.M. Vieira, A.E.C. Peres / Minerals Engineering 20 (2007) 1008–1013 100
Coarse quartz recovery (%)
100
Fine quartz recovery (%)
1011
80
60
40
80 60 40 20
20 0 0
10
20
30
0
10
20
30
40
50
60
70
80
50
60
70
80
90
90
Ether diamine dosage (g/t) pH = 9.0
pH = 10.0
pH = 9.0
pH = 10.5
Fig. 4. Fine quartz recovery as a function of ether diamine dosage and pH.
plausible explanation for the inferior performance of diamine in this size range. Recovery values of medium quartz are presented in Fig. 5. Sharp increases in recovery are observed when the dosage is increased from 40 g/t to 60 g/t. The effect of pH was not as significant as that observed in other sets of experiments. A comparison between Figs. 2 and 5 indicates that diamine performs better than monoamine in the flotation of medium quartz at pH 9.0, but diamine is more effective at higher pH values. A similar trend is observed in Fig. 6, reporting recoveries of coarse quartz, the curves being more spaced than in the case of Fig. 5. A comparison between Figs. 3 and 6 shows that both amines are effective in the flotation of coarse quartz at pH 9.0, monoamine presenting even a better performance at the dosage 40 g/t, but diamine is by far a superior collector at higher pH levels. The presence in the diamine species of two polar groups capable of adsorption onto quartz particles surfaces explains the enhanced performance of this type of collector with respect to coarser particles. References to the use of ether diamine as a collector for quartz are scarce in the literature. Papini et al. (2001) inves-
100
pH = 10.0
pH = 10.5
Fig. 6. Coarse quartz recovery as a function of ether diamine dosage and pH.
tigated the flotation of an itabirite iron with different amine type collectors, at pH 10.0. Higher levels of quartz removal in the froth were achieved with diamines in comparison with those reached by the monoamine of each manufacturer. This investigation did not address the effect of particle size on the floatability. A large concentrator of iron ore, in Brazil, operates its flotation circuit using as quartz collector a combination of ether monoamine and ether diamine. The proportion of diamine in the collector blend varies from 25% to 50%, depending on the ore type and also on the specification of the product (pellet feed for blast furnace or direct reduction). The presence of amine in the collector blend is essential for the flotation performance in this plant. In general, higher quartz recoveries in all size ranges under investigation were achieved at pH 9.0. This enhanced floatability is in agreement with determinations of zeta potential of quartz, illustrated in Fig. 7, showing that the zeta potential becomes less negative above and below this pH. Another explanation for the high performance at pH near 9 is given by Sirkeci (2000), who reported that pH 9.3 represents the equilibrium between ionised and molecular amine species. Most iron ore concentrators operate their flotation circuits at pH 10.0 or even slightly higher. The dosage of 80 g/t of amine in the case pure quartz corresponds roughly to 35 g/t of amine used in the flotation of
80
10 60
Zeta potential (mV)
Medium quartz recovery (%)
40
Ether diamine dosage (g/t)
0
40
20
0 0
10
20
30
40
50
60
70
80
0
-10
-20
90
Ether diamine dosage (g/t) pH = 9.0
pH = 10.0
pH = 10.5
Fig. 5. Medium quartz recovery as a function of ether diamine dosage and pH.
-30 0
1
2
3
4
5
6
7
8
9
10
pH
Fig. 7. Zeta potential of quartz as a function of pH.
11
12
Fine, Medium, Coarse quartz recovery (%)
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A.M. Vieira, A.E.C. Peres / Minerals Engineering 20 (2007) 1008–1013 96.4
100 90
86.1
82.5
80 70
79.1
65.3 55.4
60 50 40 30 20
88.3
89.6
41.5 34.9 20.4
10 0 0
6
11.6
15.2
Ether monoamine was not effective in the flotation of coarse quartz at higher pH levels. High recoveries of coarse quartz were achieved with ether diamine at high dosages. The flotation of coarse and medium quartz was enhanced by the presence in the pulps of a defined amount of fine quartz. The presence of fine quartz affects the aspect of the froth, decreasing the bubbles size and improving the froth stability. References
Percentage of fine quartz in the feed Coarse recovery
Medium recovery
Fine recovery
Fig. 8. Fine, medium, and coarse quartz recovery as a function of the percentage of fine quartz in the feed.
low grade itabirites. At this dosage the superiority of diamine as collector for medium and coarse quartz is evident. The influence of the percentage of fine quartz in the flotation feed on the recovery of fine, medium, and coarse quartz is illustrated in Fig. 8. It is observed that in the absence of fine quartz low recoveries of coarse and medium quartz were achieved. The recoveries increased with the increase of the percentage of fine quartz up 11.6% and then decreased. The effect of the presence of fine quartz is more significant in the case of coarse quartz than for medium quartz. At pH 9, the beneficial effect of fine quartz (11.6%) on the flotation of coarse quartz was observed even for a low dosage of ether monoamine (20 g/t). The visual aspect of the froth was also influenced by the presence of fines, rendering the bubbles smaller and more stable, an effect similar to that of frothers. Leja (1982) discussed the effect of solid particles, incorporated within the froth structure, on froth stability. It is mentioned that froth becomes stabilized by hydrophobic solids when they adhere to the air/water interface so closely together that the draining of the liquid is restricted. According to this reasoning, fine particles, due to larger specific surface area, adsorb more collector, developing higher hydrophobicity level, and contribute to froth stability. The fact that amines play the dual role of collector and frother prevents the film rupture that would be caused by excessive hydrophobicity leading to contact angles larger than 90° (Finch and Dobby, 1990). 4. Conclusions The most favourable pH for the flotation of quartz in the size ranges under investigation was pH 9.0 for both types of amine used, ether monoamine and ether diamine. Ether monoamine was more efficient than ether diamine in the flotation of fine quartz. At higher pH levels, ether diamine yielded better results than ether monoamine in the case of medium quartz, higher collector dosages being required.
Ahmed, N., Jameson, G.J., 1989. Flotation kinetics. Mineral Processing and Extractive Metallurgy Review 5, 77–99. Bazin, C., Proulx, M., 2001. Distribution of reagents down a flotation bank to improve the recovery of coarse particles. International Journal of Mineral Processing 61, 1–12. Bisshop, J.P., White, M.E., 1976. Study of particle entrainment in flotation froths. Transactions Institute of Mining and Metallurgy 85, C191–C194. Borges, A.M.B., Araujo, A.C., 1994. Study on entrainment in the reverse cationic flotation of iron ore. In: Proceedings IV Southern Hemisphere Meeting on Mineral Technology, vol. IV. Universidad de Concepcion, pp. 195–207 (in Portuguese). Crawford, R., Ralston, J., 1988. The influence of particle size and contact angle in mineral flotation. International Journal of Mineral Processing 23, 1–24. Diaz-Penafiel, P., Soto, H., 1994. Modelling of negative bias column for particles flotation. Minerals Engineering 7 (4), 465–478. Drzymala, J., 1994. Characterization of materials by Hallimond tube flotation. Part 2: maximum size of floating particles and contact angle. International Journal of Mineral Processing 42, 153–167. Engelbrecht, J.A., Woodburn, E.T., 1975. The effect of froth height, aeration rate and gas precipitation on flotation. Journal of South African Institute of Mining and Metallurgy 76 (Special Issue), 125– 132. Finch, J.A., Dobby, G.S., 1990. Column Flotation. Pergamon Press, New York, p. 179. Gaudin, A.M., 1957. Flotation. McGraw-Hill, New York, pp. 573. Gaudin, A.M., Fuerstenau, D.W., 1955. Transactions of AIME 202, 958. Glembotskii, V.A., Klassen, V.I., Plaksin, I.N., 1972. The effect of mineral particle size on flotation. In: Flotation, pp. 230–250 (Chapter 2). Hall, S., 1996. Froth flotation – the importance of the froth. Mining Magazine, 16–17. Johnson, N.W., 1972. The flotation behaviour of some chalcopyrite ores. Ph.D. Thesis. The University of Queensland, Australia. Kirjavainen, V.M., 1992. Mathematical model for the entrainment of hydrophilic particles in froth flotation. International Journal of Mineral Processing 35, 1–11. Klimpel, R.R., 1988. Considerations for improving the performance of froth flotation systems. Mining Engineering, 1093–1100. Leja, J., 1982. Surface Chemistry of Froth Flotation. Plenum Press, New York, p. 758. ¨ teyaka, B., Soto, H., 1994. Modelling of negative bias column for O particles flotation. Minerals Engineering 8 (1/2), 91–100. Papini, R.M., Branda˜o, P.R.G., Peres, A.E.C., 2001. Cationic flotation of iron ores: amine characterization and performance. Minerals & Metallurgical Processing 18 (1), 5–9. Savassi, O.N., 1998. Direct estimation of the degree of entrainment and the froth recovery of attached particles in industrial flotation cells. Ph.D. Thesis, The University of Queensland, Australia. Schulze, H.J., 1982. Dimensionless number and approximate calculation of the upper particle size of floatability in flotation machines. International Journal of Mineral Processing 9, 321–328. Shapiro, S.H., 1968. Commercial nitrogen derivatives of fatty acids. In: Patisson, E.S. (Ed.), Fatty Acids and their Industrial Applications. Marcel Dekker, New York, pp. 77–154.
A.M. Vieira, A.E.C. Peres / Minerals Engineering 20 (2007) 1008–1013 Sirkeci, A.A., 2000. Electrokinetic properties of pyrite, arsenopyrite and quartz in the absence and presence of cationic collectors and their flotation behaviour. Minerals Engineering 13 (10/11), 1037– 1048. Smith, P.G., Warren, L.J., 1989. Entrainment of particles into flotation froths. Mineral Processing and Extractive Metallurgy Review 5, 123– 145. Trahar, W.J., 1981. A rational interpretation of the role of particle size in flotation. International Journal of Mineral Processing 8, 289– 327.
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Turrer, H.D.G., Araujo, A.C., Papini, R.M., Peres, A.E.C., 2007. Iron ore flotation in the presence of polyacrylamides. Mineral Processing and Extractive Metallurgy, Transactions Institute of Mining and Metallurgy Section C 116 (2). Yoon, R.H., Luttrell, G.H., 1989. The effect of bubble size on fine particle flotation. Mineral Processing and Extractive Metallurgy Review 5, 101–122. Zheng, X., Franzidis, J.-P., Johnson, N.W., Manpalig, E.V., 2005. Modelling of entrainment in industrial flotation cells: the effect of solids suspension. Minerals Engineering 18, 51–58.
The effect of amine type, pH, and size range in the flotation of quartz Ana M. Vieira, Antonio E.C. Peres
*
Escola de Engenharia da UFMG, Rua Espı´rito Santo, 35/2006, 30160-030 Belo Horizonte, MG, Brazil Received 14 December 2006; accepted 25 March 2007 Available online 11 May 2007
Abstract Cationic reverse flotation of quartz is the most important technique for the concentration of itabirite iron ores for the production of pellet feed. Mineral processors dealing with this industrial system face two major challenges: (i) coarse quartz particles do not respond well to the action of the amine collectors; (ii) fine iron oxide particles do not respond well to the depressant action of the gelatinised corn starch. The first challenge is addressed in this paper. Bench scale experiments were performed in a conventional mechanical machine aiming at investigating the flotation performance of quartz particles of different size ranges in the presence of four dosages of ether monoamine and ether diamine, at three pH levels. It was observed that ether diamine was more effective in the flotation of medium and coarse quartz, while ether monoamine performs better in the case of fine quartz, and also that the flotation of coarse particles is enhanced by the presence of fine particles in the system. Ó 2007 Elsevier Ltd. All rights reserved. Keywords: Flotation reagents; Flotation collectors; Froth flotation; Quartz flotation; Quartz collectors; Particle size
1. Introduction Mineral particles in a broad size range are present in flotation pulps. The importance of size in the sequence of events that leads to the flotation of a mineral particle has been extensively recognized a long time ago (Glembotskii et al., 1972; Trahar, 1981; Bazin and Proulx, 2001). The use of flotation recovery versus particle size curves became usual practice for determining the maximum size for adequate flotation. Several equations have been developed and presented correlating maximum particle size with flotation parameters, such as particle density, medium density, liquid surface tension, bubbles radium, detachment angle, contact angle (Drzymala, 1994; Schulze, 1982; Yoon and Luttrell, 1989; Diaz-Penafiel and Soto, 1994). Results of investigations aiming at extending the range of coarse particles that respond adequately to flotation were also reported in the literature (Hall, 1996; Ahmed *
Corresponding author. Tel.: +55 31 32381717; fax: +55 31 32381815. E-mail address: [email protected] (A.E.C. Peres).
0892-6875/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.mineng.2007.03.013
and Jameson, 1989). Ahmed and Jameson (1989) studied the effect of agitation (100, 300, and 600 rpm), in a mechanical cell, on the collection kinetic constant, for different quartz particle sizes (5, 10, 20, and 40 lm) and different bubble sizes (75, 165, 360, and 655 lm). At 300 rpm, bubbles of all sizes were effective, 75 lm bubbles yielding the largest kinetic constant. At 600 rpm, the flotation rates for the three larger bubble sizes are smaller than those achieved with minimum agitation (100 rpm). Trahar (1981) reported that the flotation recovery of coarse particles of different minerals is more sensitive to the chemical medium, in comparison with finer particles. ¨ teyaka and Soto (1994), investigating the effect of O coarse particles of quartz and calcite in flotation columns, concluded that the kinetics of coarse particles flotation is controlled by the stability of the aggregates formed. This stability is enhanced by stronger attractive forces and weaker rupture forces. The first effect relies on the choice of a selective collector at an optimum dosage. Klimpel (1988) claimed that high collector dosages are required for the economical flotation of coarse particles, but alerted that excessive dosages may cause deleterious
A.M. Vieira, A.E.C. Peres / Minerals Engineering 20 (2007) 1008–1013
effects in the flotation system. Fundamental studies, performed by Crawford and Ralston (1988) in a modified Hallimond tube, with quartz particles in the size range 15–125 lm, indicated that 71 lm particles required only 35% surface coverage with collector for achieving 80% recovery. Larger particles (121 lm) required 60% coverage for reaching the same recovery. Fine particles exhibit much higher specific surface area (cm2/g) than coarse particles, so the collector consumption required for a certain coverage degree is much larger for fine particles than for coarse particles. When a combination of coarse and fine particles are exposed to a large collector dosage, most of the collector will be consumed by the fines that require less coverage for efficient flotation. Not enough collector species are left for effective flotation of the coarse particles. Particles can report to the froth product by true flotation or entrainment. Gaudin (1957) presented one of the first references to entrainment, even without mentioning the word: ‘‘bubbles exert a sort of filtering action and enmesh a number of gangue particles in the comparatively dry bubble walls. Small particles are retained in the interbubble water. The recovery of fine gangue particles is usually greater than the corresponding water recovery’’. Smith and Warren (1989) define ‘‘entrainment as the process by which particles enter the base of the froth and are transferred up and out of the flotation cell suspended in the water between bubbles’’. Besides Smith and Warren (1989) and Trahar (1981) addressed the subject in the 1980s. In the past 10 years more scientific approaches have been developed to investigate the entrainment process, bringing light to the understanding of its role in plant practice. Relevant contributions came from the JKMRC and the University of Queensland. Savassi (1998) proposed and validated a model for the direct estimation of the degree of entrainment and the froth recovery of attached particles in industrial flotation cells. In his model entrainment was considered as directly related with water recovery. This approach was also reported in earlier contributions (Engelbrecht and Woodburn, 1975; Johnson, 1972; Bisshop and White, 1976; Kirjavainen, 1992). Zheng et al. (2005) developed an innovative model of entrainment in industrial flotation cells taking into account the two simultaneous entrainment mechanisms in the flotation process: transfer of the suspended solids in the pulp phase to the froth phase and transfer of the entrained particles in the froth phase to the concentrate. The two steps in the entrainment process are combined to calculate the overall entrainment recovery. In the present investigation, the flotation of quartz particles in different size ranges is addressed aiming at bringing light to the so-called reverse cationic flotation of itabirite iron ores. Quartz particles are removed in the froth phase leaving hematite as the non floated fraction. In this particular system, the removal of quartz particles either by true flotation or entrained in the froth is beneficial to yield a clean concentrate. In the reverse flotation of iron ores, the entrainment of iron oxide particles in the froth phase
1009
represents a serious problem, for it results lower iron recovery. The other factor contributing to the loss of iron in the froth phase is the lack of efficiency of the depressants towards fine particles of iron oxides (Turrer et al., 2007). The entrainment in the cationic flotation an iron ore was investigated by Borges and Araujo (1994), regarding the entrainment of iron oxides, with no mention to quartz. A direct correlation between the solids content in the froth and the entrainment degree was observed. The entrained weight was not proportional to the water recovery in the froth product. 2. Methodology A very pure quartz sample from Minas Gerais state, Brazil, was carefully comminuted to avoid contamination yielding three fractions designated as F – fine (mainly in the size range 74 lm to +38 lm), M – medium (mainly in the size range 150 lm to +74 lm), and C – coarse (mainly in the size range 297 lm to +150 lm). These fractions were submitted to chemical and size analyses. Flotation experiments were performed in a laboratory Denver, model D12, machine, using a 2L cell. The test conditions were: 1300 rpm, 20% solids, collector conditioning 4 min, froth removal until the froth was barren. It is worthwhile mentioning that all experiments were performed in triplicate and the reproducibility of the experiments was excellent. Two classes of amines are used in plant practice of cationic reverse flotation of iron ores: ether monoamine and ether diamine. For the present investigation Flotigam EDA 3 (monoamine, neutralisation degree 29.8%) and Flotigan 2835-2L (diamine, neutralisation degree 20.6%), manufactured by Clariant, were selected. Ether monoamine is designation, in the jargon of mineral processors, of N-alkyloxipropylamine (R–O–CH2– CH2–C–NH2), produced in two stages from the reaction of a fatty alcohol with acrynitrile, producing ether nitrile that is then catalytically hydrogenated under high pressure (Shapiro, 1968). Ether diamine is designation, in the jargon of mineral processors, of N-alkyloxipropyl-1,3-diaminepropane (R– O–(CH2)3–NH–(CH2)3–NH2), produced in two stages from the reaction of the ether monoamines with acrylonitrile followed by hydrogenation. The tests were performed at three pH levels: 9.0, 10.0, and 10.5. Collector dosages were: 20 g/t, 40 g/t, 60 g/t, and 80 g/t. 3. Results and discussion Results of chemical analyses are presented in Table 1. Results of size analyses are presented in Table 2. Figs. 1–3 illustrate, respectively, the recoveries of fine, medium, and coarse quartz achieved by varying the dosage of ether monoamine and the pH.
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Table 1 Results of chemical analyses Fraction
Species SiO2, %
Fe2O3, %
Al2O3, %
Loss on ignition, %
C coarse M medium F fine
99.71 99.63 99.43
0.03 0.06 0.15
0.04 0.06 0.12
0.20 0.23 0.25
Table 2 Results of size analyses and proposed retention Fraction
297 to +150 150 to +74 74 to +38
Analysed retention, %
Proposed retention, %
98.57 92.92 89.29
100.00 100.00 100.00
Fine quartz recovery (%)
60 40 20 0 30
40
50
60
70
80
90
Ether monoamine dosage (g/t) pH = 9.0
pH = 10.0
pH = 10.5
Fig. 1. Fine quartz recovery as a function of ether monoamine dosage and pH.
Medium quartz recovery (%)
100
80
60
40
20
0 0
10
20
20
20
40
60
80
100
30
40
50
60
70
80
90
Ether monoamine dosage (g/t) pH = 9.0
pH = 10.0
pH = 9.0
pH = 10.0
pH = 10.5
Fig. 3. Coarse quartz recovery as a function of ether monoamine dosage and pH.
80
20
40
0
100
10
60
Ether monoamine dosage (g/t)
Size range, lm
0
80
0
Species
C coarse M medium F fine
Coarse quartz recovery (%)
100
pH = 10.5
Fig. 2. Medium quartz recovery as a function of ether monomine dosage and pH.
The results shown in Fig. 1 indicate that the performances of ether monoamine are the same for pH levels
9.0 and 10.0, irrespectively of the collector dosage. In the case of pH 10.5, quartz recovery was impaired for lower collector dosages (20 g/t and 40 g/t), the curves superimposing for higher dosages (60 g/t and 80 g/t). The dissociation of ether monoamines (Leja, 1982) explains the results. At pH 9.0 the ionised species prevails over the molecular species. At pH 10.0 there is an equilibrium between the concentrations of both species and at pH 10.5 the molecular species prevails. The electrostatic adsorption followed by hemimicelles formation, proposed by Gaudin and Fuerstenau (1955), requires at least 50% dissociation for enhanced floatability. The coincidence between the recovery curves for the lower pH levels and the fact that the curve for pH 10.5 is not too far away from the other curves are due to the easiness of fine the light particles of fine quartz to be carried to the froth phase. Entrainment might be a factor contributing to this transport. The results shown in Fig. 2 indicate a wide dispersion of the recovery curves for different pH levels. The results stress the role of the coulombic mechanism. At pH 9.0 (prevalence of ionic species) the response is similar to that of fine quartz. As far as the pH increases, the effectiveness of ether monoamine towards medium quartz decreases sharply. The recoveries of coarse quartz are illustrated in Fig. 3. The results strongly emphasise the comments regarding Fig. 2. Floatability at pH 10.5 and pH 10.0 is negligible at all dosages, reaching almost 30% in the case of a very high dosage (80 g/t). Fig. 4 illustrates the recovery of fine quartz with ether diamine. Recoveries are sharply enhanced when amine dosages increase from 60 g/t to 80 g/t. The trend of higher recovery corresponding to lower pH was again observed. Ether diamine is not an efficient collector for fine quartz at pH levels 10.0 and 10.5, even at dosages as high as 80 g/t. A comparison between Figs. 1 and 4 provides an interesting hint to plant practice: monoamine is more efficient than diamine in fine quartz flotation. Fine quartz particles are light enough to be carried by the bubbles even after hydrophobisation by a weaker collector. There is no
A.M. Vieira, A.E.C. Peres / Minerals Engineering 20 (2007) 1008–1013 100
Coarse quartz recovery (%)
100
Fine quartz recovery (%)
1011
80
60
40
80 60 40 20
20 0 0
10
20
30
0
10
20
30
40
50
60
70
80
50
60
70
80
90
90
Ether diamine dosage (g/t) pH = 9.0
pH = 10.0
pH = 9.0
pH = 10.5
Fig. 4. Fine quartz recovery as a function of ether diamine dosage and pH.
plausible explanation for the inferior performance of diamine in this size range. Recovery values of medium quartz are presented in Fig. 5. Sharp increases in recovery are observed when the dosage is increased from 40 g/t to 60 g/t. The effect of pH was not as significant as that observed in other sets of experiments. A comparison between Figs. 2 and 5 indicates that diamine performs better than monoamine in the flotation of medium quartz at pH 9.0, but diamine is more effective at higher pH values. A similar trend is observed in Fig. 6, reporting recoveries of coarse quartz, the curves being more spaced than in the case of Fig. 5. A comparison between Figs. 3 and 6 shows that both amines are effective in the flotation of coarse quartz at pH 9.0, monoamine presenting even a better performance at the dosage 40 g/t, but diamine is by far a superior collector at higher pH levels. The presence in the diamine species of two polar groups capable of adsorption onto quartz particles surfaces explains the enhanced performance of this type of collector with respect to coarser particles. References to the use of ether diamine as a collector for quartz are scarce in the literature. Papini et al. (2001) inves-
100
pH = 10.0
pH = 10.5
Fig. 6. Coarse quartz recovery as a function of ether diamine dosage and pH.
tigated the flotation of an itabirite iron with different amine type collectors, at pH 10.0. Higher levels of quartz removal in the froth were achieved with diamines in comparison with those reached by the monoamine of each manufacturer. This investigation did not address the effect of particle size on the floatability. A large concentrator of iron ore, in Brazil, operates its flotation circuit using as quartz collector a combination of ether monoamine and ether diamine. The proportion of diamine in the collector blend varies from 25% to 50%, depending on the ore type and also on the specification of the product (pellet feed for blast furnace or direct reduction). The presence of amine in the collector blend is essential for the flotation performance in this plant. In general, higher quartz recoveries in all size ranges under investigation were achieved at pH 9.0. This enhanced floatability is in agreement with determinations of zeta potential of quartz, illustrated in Fig. 7, showing that the zeta potential becomes less negative above and below this pH. Another explanation for the high performance at pH near 9 is given by Sirkeci (2000), who reported that pH 9.3 represents the equilibrium between ionised and molecular amine species. Most iron ore concentrators operate their flotation circuits at pH 10.0 or even slightly higher. The dosage of 80 g/t of amine in the case pure quartz corresponds roughly to 35 g/t of amine used in the flotation of
80
10 60
Zeta potential (mV)
Medium quartz recovery (%)
40
Ether diamine dosage (g/t)
0
40
20
0 0
10
20
30
40
50
60
70
80
0
-10
-20
90
Ether diamine dosage (g/t) pH = 9.0
pH = 10.0
pH = 10.5
Fig. 5. Medium quartz recovery as a function of ether diamine dosage and pH.
-30 0
1
2
3
4
5
6
7
8
9
10
pH
Fig. 7. Zeta potential of quartz as a function of pH.
11
12
Fine, Medium, Coarse quartz recovery (%)
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A.M. Vieira, A.E.C. Peres / Minerals Engineering 20 (2007) 1008–1013 96.4
100 90
86.1
82.5
80 70
79.1
65.3 55.4
60 50 40 30 20
88.3
89.6
41.5 34.9 20.4
10 0 0
6
11.6
15.2
Ether monoamine was not effective in the flotation of coarse quartz at higher pH levels. High recoveries of coarse quartz were achieved with ether diamine at high dosages. The flotation of coarse and medium quartz was enhanced by the presence in the pulps of a defined amount of fine quartz. The presence of fine quartz affects the aspect of the froth, decreasing the bubbles size and improving the froth stability. References
Percentage of fine quartz in the feed Coarse recovery
Medium recovery
Fine recovery
Fig. 8. Fine, medium, and coarse quartz recovery as a function of the percentage of fine quartz in the feed.
low grade itabirites. At this dosage the superiority of diamine as collector for medium and coarse quartz is evident. The influence of the percentage of fine quartz in the flotation feed on the recovery of fine, medium, and coarse quartz is illustrated in Fig. 8. It is observed that in the absence of fine quartz low recoveries of coarse and medium quartz were achieved. The recoveries increased with the increase of the percentage of fine quartz up 11.6% and then decreased. The effect of the presence of fine quartz is more significant in the case of coarse quartz than for medium quartz. At pH 9, the beneficial effect of fine quartz (11.6%) on the flotation of coarse quartz was observed even for a low dosage of ether monoamine (20 g/t). The visual aspect of the froth was also influenced by the presence of fines, rendering the bubbles smaller and more stable, an effect similar to that of frothers. Leja (1982) discussed the effect of solid particles, incorporated within the froth structure, on froth stability. It is mentioned that froth becomes stabilized by hydrophobic solids when they adhere to the air/water interface so closely together that the draining of the liquid is restricted. According to this reasoning, fine particles, due to larger specific surface area, adsorb more collector, developing higher hydrophobicity level, and contribute to froth stability. The fact that amines play the dual role of collector and frother prevents the film rupture that would be caused by excessive hydrophobicity leading to contact angles larger than 90° (Finch and Dobby, 1990). 4. Conclusions The most favourable pH for the flotation of quartz in the size ranges under investigation was pH 9.0 for both types of amine used, ether monoamine and ether diamine. Ether monoamine was more efficient than ether diamine in the flotation of fine quartz. At higher pH levels, ether diamine yielded better results than ether monoamine in the case of medium quartz, higher collector dosages being required.
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