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Comparing the effect of salts and frother (MIBC) on gas dispersion and froth properties
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Minerals Engineering 20 (2007) 1296–1302 This article is also available online at: www.elsevier.com/locate/mineng
Comparing the effect of salts and frother (MIBC) on gas dispersion and froth properties J.J. Quinn a, W. Kracht a, C.O. Gomez a, C. Gagnon b, J.A. Finch a
a,*
Department of Mining and Materials Engineering, McGill University, 3610 University Street, Montreal, QC, Canada H3A 2B2 b COREM, 1180 rue de la Mine´ralogie, Que´bec City, Canada G1N 1X7 Received 18 April 2007; accepted 27 July 2007 Available online 11 September 2007
Abstract The Raglan concentrator (Xstrata Nickel) does not employ frother. It was considered this might be the result of the high salt content in the process water (ca. 30 000 ppm). Two-phase (solution–air) and three-phase (slurry–air) tests were undertaken in a laboratory column to quantify the effect of inorganic ions present in the water (a range of polyvalent ions). The measurements focused on gas dispersion (bubble size and gas holdup) and froth overflow rate. The results were compared to a typical frother (MIBC) system. The two-phase tests revealed reduced bubble size, increased gas holdup and limited froth formation in salt solutions. The gas holdup correlated with ionic strength. At an ionic strength ca. 0.4 (=0.4 M NaCl) the increase in gas holdup was comparable to ca.10 ppm MIBC. In three-phase tests on a sulphide ore, bubble size and froth overflow rate were again comparable between 0.4 M NaCl and 10 ppm MIBC. The observations help explain why the Raglan plant can operate without frother addition. Ó 2007 Elsevier Ltd. All rights reserved. Keywords: Froth flotation; Flotation frothers; Flotation bubbles; Flotation froths
1. Introduction Several flotation plants around the world use process water with high inorganic salt content. This arises from a combination of soluble components of the ore, increasing use of recycle water and the use of sea or well water in some locations. Extreme examples include the Mt Keith operation in Western Australia with salt concentrations around 60 000–80 000 ppm (George, 1996) and the processing of potash in saturated brine (Strathdee, 2000). The example of interest here is Xstrata Nickel’s (formerly Falconbridge) Raglan mine in northern Quebec where salt levels range from ca. 20 000 to 35 000 ppm, winter to summer. An apparent consequence of the high salt content is that the flotation circuit is able to operate without addition of frother. This observation corresponds to previous evidence
*
Corresponding author. Fax: +1 514 398 4492. E-mail address: jim.fi[email protected] (J.A. Finch).
0892-6875/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.mineng.2007.07.007
suggesting that flotation in saline water results in lower reagent consumption (Haig-Smillie, 1974; Yoon and Sabey, 1989). Since the role of frother is to decrease bubble size and increase froth stability, salts must have some similar capability. Generally, the action of frothers is believed to result from retarding coalescence (Harris, 1982). It is well documented that many salts inhibit bubble coalescence (Marrucci and Nicodemo, 1967; Lessard and Zieminski, 1971; Craig et al., 1993; Hofmeier et al., 1995; Laskowski et al., 2003; Craig, 2004). Inorganic ions appear to slow inter-bubble film drainage. There are, however, significant differences between inorganic salts and frothers. High salt concentrations are required for coalescence inhibition (Lessard and Zieminski, 1971; Craig et al., 1993; Zahradnik et al., 1999) compared to a few parts per million of frother (Harris, 1982). Inorganic salts generally tend to increase surface tension while frothers reduce it; and inorganic ions are usually not able to form froth (foam) in two-phase (water–gas) systems. Lekki and Laskowski (1975) observed that only in the
J.J. Quinn et al. / Minerals Engineering 20 (2007) 1296–1302
presence of hydrophobic particles would salt solutions form a stable froth. They noted that inorganic electrolytes fall into the category of surface inactive agents while frothers are surface active. Researchers have attempted to determine a transition concentration at which salts inhibit bubble coalescence (Lessard and Zieminski, 1971; Craig et al., 1993; Zahradnik et al., 1999). Zieminski and Whittemore (1971) have shown that many ions of high valence have a greater effect hindering bubble coalescence than monovalent ions. This valence effect will be included here. The major ions present in Raglan process water are 2 Na+, Cl, SO2 4 , and S2 O3 . Tests were designed to examine the impact of these ions, initially in a two-phase (solution–gas) system. The salts employed were NaCl, Na2SO4, CaCl2 and Na2S2O3 to cover the ions present at Raglan with Al2(SO4)3 included to extend the range of ion charge, i.e., examples of 1–1, j1–2, 2–1j and 3–2 (cation–anion charge) salts were tested. As gas dispersion properties, gas holdup and bubble size distribution were measured. As a froth property, depth was considered. However, the salts gave virtually no froth in the absence of solids (cf. Lekki and Laskowski, 1975). Instead overflow rate was selected in tests on ore from Brunswick Mine (Xstrata Zinc) conducted at a pilot plant facility (i.e., three-phase tests). Both two- and three-phase test-work includes comparison with a common frother, methyl-isobutyl carbinol (MIBC).
1297 Camera
BV
v
Light source
PT
Feed tank
PT pump
Fig. 1. Schematic of set-up.
2. Methodology Both the two- and three-phase tests employed a column with the general features shown in Fig. 1. 2.1. Two-phase tests The column in this case was 7.6 cm (internal) diameter and 34 cm high, operated batch. A cylindrical porous spar-
ger was located at the base of the column (vertical orientation). A Bailey differential pressure transmitter was located between 162.5 cm and 288 cm to determine gas holdup. Another pressure transmitter was located at the base of the column to record the pressure at the sparger. This pressure was used to correct the volumetric airflow rate to the test conditions. Airflow rate is reported as a superficial velocity, Jg (cm/s), by dividing the volumetric flowrate
14 12
water
Gas Holdup (%)
10 8
0.05M NaCl
6
0.10M NaCl
4
0.25M NaCl
2 0 0.0
0.5
1.0
1.5
2.0
Superficial Gas Velocity (cm/s) Fig. 2. Gas holdup vs. superficial gas velocity as a function of sodium chloride concentration.
1298
J.J. Quinn et al. / Minerals Engineering 20 (2007) 1296–1302
(cm3/s) by the column cross sectional area (cm2). For selected conditions bubble size distribution (BSD) was determined using the McGill Bubble Size Analyzer (or Bubble Viewer, BV) (Hernandez et al., 2002; HernandezAguilar et al., 2004). Typically over 2000 bubble images were processed. The BSD data are presented as number frequency. Solutions were prepared using Montre´al tap water with varying concentrations of the following salts: NaCl (Windsor Select Plus), CaCl2, Na2SO4, Na2S2O3 and Al2(SO4)3 (supplied by Fisher and reagent grade unless otherwise noted), and MIBC (methyl-isobutyl carbinol), a common industrial frother (supplied by Flottec). The tests were conducted at natural pH (6–7) except for aluminum sulphate where the solution was adjusted to ca. pH 3 with sulphuric
acid to avoid aluminum hydroxide formation. Prior testing indicated pH adjustment itself did not influence gas holdup. Once pressure signals became constant (i.e., steady state, taking 5–10 min) readings were recorded. The column was emptied and cleaned between each test.
2.2. Three-phase tests These were conducted at the COREM pilot plant facility (Que´bec City). A modular column (10.16 cm by 320 cm) was assembled on-site and adapted to run continuously. A cylindrical porous sparger was located near the base of the column (horizontal orientation). Operation was that
30
Relative Frequecy (%)
25 20
Water
0.05M NaCl
0.1M NaCl
0.25M NaCl
15 10
5 0 0
1
2
4
3
5
6
8
7
Bubble Diameter (mm) Fig. 3. Bubble size distributions in sodium chloride solutions.
16
Salt
Jg (cm/s)
14
Gas Holdup (%)
12
Jg = 1.7 cm/s
10
8
NaCl
0.7 cm/s
NaCl
1.7 cm/s
Na2SO4
0.7 cm/s
Na2SO4
1.7 cm/s
CaCl2
0.7 cm/s
CaCl2
1.7 cm/s
Na2S2O3 0.7 cm/s
6
Na2S2O3 1.7 cm/s 4
Al2(SO4)3 0.7 cm/s
Jg = 0.7 cm/s
Al2(SO4)3 1.7 cm/s
2
0 0.0
0.1
0.2
0.3
0.4
0. 5
Salt Concentration (M) Fig. 4. Gas holdup vs. salt concentration (Jg = 0.7, 1.7 cm/s).
J.J. Quinn et al. / Minerals Engineering 20 (2007) 1296–1302
1299
(tailings) was discharged via a pump and overflow (concentrate) flowed by gravity through a flexible pipe that could be directed for sampling. Superficial slurry velocity, Jsl, was maintained at 1 cm/s. Overflow rate (l/min) was determined manually by collecting overflow in a graduated cylinder over given time intervals. Airflow rate was controlled manually using a Matheson rotameter. To complement the overflow data, images of the top of the froth were recorded and bubble size measured in the pulp below the froth using the Bubble Viewer. Bubble size
of a typical laboratory flotation column (Finch and Dobby, 1990), except no wash water was added. The ore, ground to 85%–85 lm, was prepared following Brunswick Mine protocol for Cu/Pb bulk rougher flotation. The slurry (water was Que´bec City tap) was brought to the target solution conditions (salt or frother concentration) in the feed tank. The feed (30 wt% solids) was introduced about 40 cm below the column lip. Froth depth was estimated from the bottom pressure and controlled automatically (5 ± 2 cm) by manipulating feed rate. Underflow
14
NaCl NaCl NaCl NaCl Na2SO4 Na2SO4 Na2SO4 Na2SO4 CaCl2 CaCl2 CaCl2 CaCl2 Na2S2O3 Na2S2O3 Na2S2O3 Na2S2O3 Al2(SO4)3 Al2(SO4)3 Al2(SO4)3 Al2(SO4)3
12
Gas Holdup (%)
10
8
6
4
2
0.7 cm/s 1.0 cm/s 1.4 cm/s 1.7 cm/s 0.7 cm/s 1.0 cm/s 1.4 cm/s 1.7 cm/s 0.7 cm/s 1.0 cm/s 1.4 cm/s 1.7 cm/s 0.7 cm/s 1.0 cm/s 1.4 cm/s 1.7 cm/s 0.7 cm/s 1.0 cm/s 1.4 cm/s 1.7 cm/s
0 0.0
0.1
0.2
0.3
0.4
0.5
Ionic Strength Fig. 5. Gas holdup vs. ionic strength for four superficial gas velocities (in order from bottom to top: 0.7, 1.0, 1.4 and 1.7 cm/s) for all salts tested.
[MIBC] (ppm) 0
2
4
6
8
10
12
14
16
18
20
15 14
Gas Holdup (%)
13 12
MIBC MIBC 11
NaCl NaCl 10 9 8 7 6 0.0
0.1
0.3
0.2
0.4
[NaCl] (M) Fig. 6. Determining equivalent frother (MIBC) concentration (Jg = 0.7 cm/s).
0.5
1300
J.J. Quinn et al. / Minerals Engineering 20 (2007) 1296–1302
was used as the gas dispersion measure in this case as using pressure to estimate gas holdup is no longer accurate unless slurry density is continuously monitored. 3. Results and discussion 3.1. Two-phase 3.1.1. Gas holdup and bubble size Gas holdup is an indirect measure of bubble size, increasing as bubble size decreases (for a given air flow rate). This reflects that smaller bubbles rise at lower velocities and thus gas residence time (holdup) in the column increases. Bubble size is dictated by sparger characteristics, airflow rate and the system chemistry. Fig. 2 shows the expected near linear response in gas holdup with superficial gas velocity (obtained up to a limiting Jg) (Finch and Dobby, 1990). Increasing sodium chloride concentration increased gas holdup indicative of salt’s ability to reduce bubble size. Bubble size distribution was measured at three sodium chloride concentrations (0.05, 0.1 and 0.25 M) at a superficial gas velocity of 0.7 cm/s (Fig. 3). An effect is not notable until 0.25 M when the initially dominant mode in water alone at ca. 4–5 mm shifts to ca. 1 mm. This observation correlates with the increase in gas holdup when salt concentration increases to 0.25 M (Fig. 2). Interestingly, the water-only case always showed evidence of fine bubbles (<1 mm) suggesting they either survive the coalescence process or perhaps result from coalescence induced bubble break-up (Tse et al., 2003). As a result of this demonstrated relationship between gas holdup and bubble size and the fact that the former is easier to measure on-line, gas holdup is used in the remaining two-phase tests.
decreases. Keitel and Onken (1982) showed a correlation between Sauter mean bubble diameter, D32, and ionic strength (they also noted the possibility of using the diameter of hydrated ions to improve the correlation). As far as is known Fig. 5 is the first time a dependence of gas holdup on ionic strength has been demonstrated. It is argued that the higher the charge on the ionic species the more coalescence is inhibited. Zieminski and Whittemore (1971) noted that coalescence behavior was a function of ion–water interactions. Highly hydrated inorganic ions tend towards the bulk away from the solution/ air interface (i.e., negatively adsorb) and are termed ‘struc-
3.2. Effect of salts Fig. 4 shows the gas holdup results obtained at two gas velocities. The salts fall into groups according to their valence; aluminum sulphate (3–2 salt) showed the strongest response followed by the 2–1, 1–2 salts with the weakest response from monovalent NaCl. The valence effect is incorporated by introducing the ionic strength (l) defined as: X l ¼ 1=2 C i Z 2i ð1Þ where Ci is the molar concentration of the ith species and Zi is the charge of the ith species. The summation is taken over all ionic species in solution. Fig. 5 shows that there appears to be a unique relationship between gas holdup and ionic strength independent of salt type. Zieminski and Whittemore (1971) demonstrated a correlation between total bubble surface area and ionic strength (for 11 salts). Total bubble surface area, like gas holdup, is a function of bubble size, both increasing as bubble size
Fig. 7. Top views of froth and corresponding image of bubbles below the froth for flotation of sample of Brunswick Mine ore (water, Que´bec City tap).
J.J. Quinn et al. / Minerals Engineering 20 (2007) 1296–1302
1301
25
Water
5.10 Water
0.1M NaCl 3.66 0.1M NaCl
Relative Frequency (%)
20
0.2M NaCl 2.84 0.2M NaCl 15
0.4M NaCl 2.45 0.4M NaCl 10ppm MIBC 1.93 10ppm MIBC
10
5
0
0
1
2
3
4
5
6
7
8
Bubble Diameter (mm) Fig. 8. Evolution of bubble size distribution for conditions in Fig. 7 (Jg = 0.9 cm/s).
3.3. Equivalent frother concentration As salt type (among those tested) does not appear to be a factor, NaCl was selected as representative for this part of the work. Being monovalent, ionic strength is numerically equal to the concentration in mol/L (M). Fig. 6 compares gas holdup as a function of the concentration of NaCl (lower axis) and MIBC (upper axis). The trend-lines are polynomial fits. While it is evident that MIBC is capable of greater increases in gas holdup the results for NaCl cover a similar range. A water sample from Raglan showed an ionic strength of ca. 0.4 (vertical line on Fig. 6). From this observation it can be estimated that salts at Raglan have the same effect, with respect to increased gas holdup, as roughly 8–10 ppm MIBC. An MIBC concentration at this level is typical in plant operations (Ge´linas et al., 2005) and helps explain why Raglan is able to operate without frother. The tests were repeated at three other gas velocities (0.4, 1.0 and 1.7 cm/s) and gave the same equivalent frother concentration. 3.4. Three-phase The three-phase tests were conducted to access froth properties but they also gave some further insight into gas dispersion. Fig. 7 shows overhead views of the froth with typical images of the bubble size in the pulp below. It is evident that with increasing NaCl concentration the froth texture and bubble size merge with those obtained with 10 ppm MIBC. The evolution of the bubble size distribution is seen in Fig. 8; the features are similar to those in
the two-phase tests (Fig. 3). Water alone showed a principal mode at 4–5 mm (and, again, a minor one below 1 mm) which shifts progressively to finer size and the distribution narrows with increasing NaCl concentration. The distribution for 0.2 and 0.4 M NaCl becomes comparable to MIBC. This reinforces the conclusion from two-phase tests that gas dispersion properties are similar between salt solutions of ionic strength ca. 0.4 (i.e., 0.4 M NaCl) and ca. 10 ppm MIBC. Comparing Figs. 3 and 8 indicates that the presence of solids has little impact on the bubble size produced, which is dictated by the solution ‘chemistry’. Fig. 9 gives the measured overflow rates for the threephase tests. The overflow rate increases with salt concentration and becomes similar to that achieved by 10 ppm MIBC between 0.2 and 0.4 M NaCl. Recalling that in the two-phase tests salts could not sustain froth it is evident that the hydrophobic particles (i.e., those attached to the 0.8 water 0.1 M NaCl
0.6
O/F Rate (L/min)
ture makers’. It has been shown that both positive and negative adsorption can cause bubble coalescence inhibition (Foulk and Miller, 1931; Machon et al., 1997).
0.2 M NaCl 0.4 M NaCl 10 ppm MIBC
0.4
0.2
0 0
0.5
1
1.5
2
2.5
Gas Velocity (cm/s) Fig. 9. Overflow rates as a function of superficial gas velocity and salt concentration compared to 10 ppm MIBC (corresponding to Fig. 7).
1302
J.J. Quinn et al. / Minerals Engineering 20 (2007) 1296–1302
bubbles) are responsible for froth formation (Fig. 7) and overflow.1 Water alone (Fig. 9) even showed some overflow. Pugh (2006) recently demonstrated that hydrophobic particles could build froth in the absence of surfactant. However, without the small bubbles provided by frother or salts the overflow (particle recovery) rate is limited, i.e., flotation capacity is limited. The combination of gas dispersion and froth overflow results in both the two- and three-phase systems provides the evidence that salts present in the Raglan water can substitute for frother. The important similarity between frother and salts is the ability to produce fine bubbles. Any plant with process water of ionic strength at least 0.4 should consider the probable impact on bubble size. 4. Conclusions Two-phase results show increasing gas holdup with increasing concentration of NaCl, CaCl2, Na2SO4, Na2S2O3 and Al2(SO4)3. The effect increased with ion valence and a correlation between gas holdup and ionic strength was demonstrated. Gas holdup for salt solutions with ionic strength ca. 0.4, as at the Raglan concentrator, is similar to that of ca. 10 ppm MIBC, which is a typical dosage for this common frother. Continuous three-phase tests with a sulphide ore showed similar froth texture, bubble size distribution and froth overflow rate for 10 ppm MIBC and 0.4 M NaCl (i.e., ionic strength 0.4). The results help interpret why the Raglan plant can operate without frother; in particular, high concentration of salts can substitute for the bubble size reduction normally provided by frother. Acknowledgements The funding was under a Collaborative Research and Development grant from Natural Sciences and Engineering Research Council of Canada (NSERC) with industrial sponsorship from (giving the names at the onset of the sponsorship, 2001): Inco, Teck-Cominco, Noranda, Falconbridge, COREM and SGS-Lakefield (joined 2003). In particular, the authors would also like to acknowledge COREM under project R-116 for additional financial support. Constructive discussions with Colin Hardie and Carmine Ciriello (Raglan operation) are also gratefully acknowledged. References Cappuccitti, F., Finch, J.A. 2007. Development of new frothers through hydrodynamic characterization. In: Proceedings of the 39th Annual Meeting of the Canadian Mineral Processors, pp. 399–412. Craig, V.S.J., 2004. Bubble coalescence and specific-ion effects. Curr. Opin. Colloid Interface Sci. 9, 178–184. Craig, V.S.J., Ninham, B.W., Pashley, R.M., 1993. The effect of electrolytes on bubble coalescence in water. J. Phys. Chem. 97, 10192–10197.
1 It is also the case that 10 ppm MIBC does not produce substantial froth in the absence of floatable particles (Cappuccitti and Finch, 2007).
Finch, J.A., Dobby, G.S., 1990. Column Flotation. Pergamon Press. Foulk, C.W., Miller, J.N., 1931. Experimental evidence in support of the balanced-layer theory of liquid film formation. Ind. Eng. Chem. 23 (11), 1283–1288. Ge´linas, S., Finch, J., Cappuccitti, F. 2005. Frother analysis: procedure and plant experience. In: Proceedings of the 37th Annual Meeting of the Canadian Mineral Processors, pp. 569–576. George, C., 1996. The Mt Keith operation. In: Grimsey, E.J., Neuss, I. (Eds.), Nickel. Australasian Inst. Min. Metall., Melbourne, pp. 9–23. Haig-Smillie, L.D. 1974. Sea water flotation. In: Proceedings of the 6th Annual Meeting of Canadian Mineral Processors, pp. 263–281. Harris, P.J. 1982. Frothing phenomena and frothers. In: King, R.P. (Ed.), Principles of Flotation, S. African Institute of Mining and Metallurgy, Monograph Series No. 3, pp. 237–250. Hernandez-Aguilar, J.R., Coleman, R.G., Gomez, C.O., Finch, J.A., 2004. A comparison between capillary and imaging techniques for sizing bubbles in flotation systems. Miner. Eng. 17 (1), 53–61. Hernandez-Aguilar, J.R., Gomez, C.O., Finch, J.A. 2002. A technique for the direct measurement of bubble size distributions in industrial flotation cells. In: Proceedings of the 34th Annual Meeting of the Canadian Mineral Processors, pp. 389–402. Hofmeier, U., Yaminsky, V.V., Christensen, H.K., 1995. Observations of solute effects on bubble formation, J. Colloid Interface Sci. 174, 199– 210. Keitel, G., Onken, U., 1982. Inhibition of bubble coalescence by solutes in air/water dispersions. Chem. Eng. Sci. 37, 1633–1638. Laskowski, J.S., Cho, Y.S., Ding, K., 2003. Effect of frothers on bubble size and foam stability in potash ore flotation systems. Can. J. Chem. Eng. 81, 63–69. Lekki, J., Laskowski, J. 1975. A new concept of frothing in flotation systems and general classification of flotation frothers. In: Proceedings of the XIth International Mineral Processing Congress, Universita di Cagliari, pp. 429–448. Lessard, R.D., Zieminski, S.A., 1971. Bubble coalescence and gas transfer in aqueous electrolytic solutions. Ind. Eng. Chem. Fund. 10, 260–289. Machon, V., Pacek, A.W., Nienow, A.W., 1997. Bubble sizes in electrolyte and alcohol solutions in a turbulent stirred vessel. Trans. Inst. Chem. Eng. 75(A), 339–348. Marrucci, G., Nicodemo, L., 1967. Coalescence of gas bubbles in aqueous solutions of inorganic electrolytes. Chem. Eng. Sci. 22, 1257–1265. Pugh, R., 2006. Surface chemical studies on particle-stabilized froths. In: ¨ nal, G., Acarkan, N., Celik, M.S., Arslan, F., Atesßok, G., Guney, A., O Sirkeci, A.A., Yuce, A.E., Perek, K.T. (Eds.), Proceedings of the XXIIIth International Mineral Processing Congress, Istanbul, pp. 430–435. Strathdee, G.C., 2000. Milling of potash in Canada – An overview. In: Damjanovic´, B., Goode, J.R. (Eds.), In: Canadian Milling Practice, vol. 49. Canadian Institute of Mining, Metallurgy and Petroleum, pp. 235–238. Tse, K.L., Martin, T., McFarlane, C.M., Nienow, A.W., 2003. Small bubble formation via a coalescence dependent break-up mechanism. Chem. Eng. Sci. 58, 275–286. Yoon, R.H., Sabey, J.B., 1989. Coal flotation in inorganic salt solutions. In: Botsaris, G.D., Glazman, Y.M. (Eds.), Interfacial Phenomena in Coal Technology, In: Surfactant Science Series, vol. 32. Marcel Dekker, pp. 87–114. Zahradnik, J., Fialova´, M., Linek, V., 1999. The effect of surface additives on bubble coalescence in aqueous media. Chem. Eng. Sci. 54, 4757– 4766. Zieminski, S.A., Whittemore, R.C., 1971. Behavior of gas bubbles in aqueous electrolyte solutions. Chem. Eng. Sci. 26, 509–520.
Minerals Engineering 20 (2007) 1296–1302 This article is also available online at: www.elsevier.com/locate/mineng
Comparing the effect of salts and frother (MIBC) on gas dispersion and froth properties J.J. Quinn a, W. Kracht a, C.O. Gomez a, C. Gagnon b, J.A. Finch a
a,*
Department of Mining and Materials Engineering, McGill University, 3610 University Street, Montreal, QC, Canada H3A 2B2 b COREM, 1180 rue de la Mine´ralogie, Que´bec City, Canada G1N 1X7 Received 18 April 2007; accepted 27 July 2007 Available online 11 September 2007
Abstract The Raglan concentrator (Xstrata Nickel) does not employ frother. It was considered this might be the result of the high salt content in the process water (ca. 30 000 ppm). Two-phase (solution–air) and three-phase (slurry–air) tests were undertaken in a laboratory column to quantify the effect of inorganic ions present in the water (a range of polyvalent ions). The measurements focused on gas dispersion (bubble size and gas holdup) and froth overflow rate. The results were compared to a typical frother (MIBC) system. The two-phase tests revealed reduced bubble size, increased gas holdup and limited froth formation in salt solutions. The gas holdup correlated with ionic strength. At an ionic strength ca. 0.4 (=0.4 M NaCl) the increase in gas holdup was comparable to ca.10 ppm MIBC. In three-phase tests on a sulphide ore, bubble size and froth overflow rate were again comparable between 0.4 M NaCl and 10 ppm MIBC. The observations help explain why the Raglan plant can operate without frother addition. Ó 2007 Elsevier Ltd. All rights reserved. Keywords: Froth flotation; Flotation frothers; Flotation bubbles; Flotation froths
1. Introduction Several flotation plants around the world use process water with high inorganic salt content. This arises from a combination of soluble components of the ore, increasing use of recycle water and the use of sea or well water in some locations. Extreme examples include the Mt Keith operation in Western Australia with salt concentrations around 60 000–80 000 ppm (George, 1996) and the processing of potash in saturated brine (Strathdee, 2000). The example of interest here is Xstrata Nickel’s (formerly Falconbridge) Raglan mine in northern Quebec where salt levels range from ca. 20 000 to 35 000 ppm, winter to summer. An apparent consequence of the high salt content is that the flotation circuit is able to operate without addition of frother. This observation corresponds to previous evidence
*
Corresponding author. Fax: +1 514 398 4492. E-mail address: jim.fi[email protected] (J.A. Finch).
0892-6875/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.mineng.2007.07.007
suggesting that flotation in saline water results in lower reagent consumption (Haig-Smillie, 1974; Yoon and Sabey, 1989). Since the role of frother is to decrease bubble size and increase froth stability, salts must have some similar capability. Generally, the action of frothers is believed to result from retarding coalescence (Harris, 1982). It is well documented that many salts inhibit bubble coalescence (Marrucci and Nicodemo, 1967; Lessard and Zieminski, 1971; Craig et al., 1993; Hofmeier et al., 1995; Laskowski et al., 2003; Craig, 2004). Inorganic ions appear to slow inter-bubble film drainage. There are, however, significant differences between inorganic salts and frothers. High salt concentrations are required for coalescence inhibition (Lessard and Zieminski, 1971; Craig et al., 1993; Zahradnik et al., 1999) compared to a few parts per million of frother (Harris, 1982). Inorganic salts generally tend to increase surface tension while frothers reduce it; and inorganic ions are usually not able to form froth (foam) in two-phase (water–gas) systems. Lekki and Laskowski (1975) observed that only in the
J.J. Quinn et al. / Minerals Engineering 20 (2007) 1296–1302
presence of hydrophobic particles would salt solutions form a stable froth. They noted that inorganic electrolytes fall into the category of surface inactive agents while frothers are surface active. Researchers have attempted to determine a transition concentration at which salts inhibit bubble coalescence (Lessard and Zieminski, 1971; Craig et al., 1993; Zahradnik et al., 1999). Zieminski and Whittemore (1971) have shown that many ions of high valence have a greater effect hindering bubble coalescence than monovalent ions. This valence effect will be included here. The major ions present in Raglan process water are 2 Na+, Cl, SO2 4 , and S2 O3 . Tests were designed to examine the impact of these ions, initially in a two-phase (solution–gas) system. The salts employed were NaCl, Na2SO4, CaCl2 and Na2S2O3 to cover the ions present at Raglan with Al2(SO4)3 included to extend the range of ion charge, i.e., examples of 1–1, j1–2, 2–1j and 3–2 (cation–anion charge) salts were tested. As gas dispersion properties, gas holdup and bubble size distribution were measured. As a froth property, depth was considered. However, the salts gave virtually no froth in the absence of solids (cf. Lekki and Laskowski, 1975). Instead overflow rate was selected in tests on ore from Brunswick Mine (Xstrata Zinc) conducted at a pilot plant facility (i.e., three-phase tests). Both two- and three-phase test-work includes comparison with a common frother, methyl-isobutyl carbinol (MIBC).
1297 Camera
BV
v
Light source
PT
Feed tank
PT pump
Fig. 1. Schematic of set-up.
2. Methodology Both the two- and three-phase tests employed a column with the general features shown in Fig. 1. 2.1. Two-phase tests The column in this case was 7.6 cm (internal) diameter and 34 cm high, operated batch. A cylindrical porous spar-
ger was located at the base of the column (vertical orientation). A Bailey differential pressure transmitter was located between 162.5 cm and 288 cm to determine gas holdup. Another pressure transmitter was located at the base of the column to record the pressure at the sparger. This pressure was used to correct the volumetric airflow rate to the test conditions. Airflow rate is reported as a superficial velocity, Jg (cm/s), by dividing the volumetric flowrate
14 12
water
Gas Holdup (%)
10 8
0.05M NaCl
6
0.10M NaCl
4
0.25M NaCl
2 0 0.0
0.5
1.0
1.5
2.0
Superficial Gas Velocity (cm/s) Fig. 2. Gas holdup vs. superficial gas velocity as a function of sodium chloride concentration.
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(cm3/s) by the column cross sectional area (cm2). For selected conditions bubble size distribution (BSD) was determined using the McGill Bubble Size Analyzer (or Bubble Viewer, BV) (Hernandez et al., 2002; HernandezAguilar et al., 2004). Typically over 2000 bubble images were processed. The BSD data are presented as number frequency. Solutions were prepared using Montre´al tap water with varying concentrations of the following salts: NaCl (Windsor Select Plus), CaCl2, Na2SO4, Na2S2O3 and Al2(SO4)3 (supplied by Fisher and reagent grade unless otherwise noted), and MIBC (methyl-isobutyl carbinol), a common industrial frother (supplied by Flottec). The tests were conducted at natural pH (6–7) except for aluminum sulphate where the solution was adjusted to ca. pH 3 with sulphuric
acid to avoid aluminum hydroxide formation. Prior testing indicated pH adjustment itself did not influence gas holdup. Once pressure signals became constant (i.e., steady state, taking 5–10 min) readings were recorded. The column was emptied and cleaned between each test.
2.2. Three-phase tests These were conducted at the COREM pilot plant facility (Que´bec City). A modular column (10.16 cm by 320 cm) was assembled on-site and adapted to run continuously. A cylindrical porous sparger was located near the base of the column (horizontal orientation). Operation was that
30
Relative Frequecy (%)
25 20
Water
0.05M NaCl
0.1M NaCl
0.25M NaCl
15 10
5 0 0
1
2
4
3
5
6
8
7
Bubble Diameter (mm) Fig. 3. Bubble size distributions in sodium chloride solutions.
16
Salt
Jg (cm/s)
14
Gas Holdup (%)
12
Jg = 1.7 cm/s
10
8
NaCl
0.7 cm/s
NaCl
1.7 cm/s
Na2SO4
0.7 cm/s
Na2SO4
1.7 cm/s
CaCl2
0.7 cm/s
CaCl2
1.7 cm/s
Na2S2O3 0.7 cm/s
6
Na2S2O3 1.7 cm/s 4
Al2(SO4)3 0.7 cm/s
Jg = 0.7 cm/s
Al2(SO4)3 1.7 cm/s
2
0 0.0
0.1
0.2
0.3
0.4
0. 5
Salt Concentration (M) Fig. 4. Gas holdup vs. salt concentration (Jg = 0.7, 1.7 cm/s).
J.J. Quinn et al. / Minerals Engineering 20 (2007) 1296–1302
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(tailings) was discharged via a pump and overflow (concentrate) flowed by gravity through a flexible pipe that could be directed for sampling. Superficial slurry velocity, Jsl, was maintained at 1 cm/s. Overflow rate (l/min) was determined manually by collecting overflow in a graduated cylinder over given time intervals. Airflow rate was controlled manually using a Matheson rotameter. To complement the overflow data, images of the top of the froth were recorded and bubble size measured in the pulp below the froth using the Bubble Viewer. Bubble size
of a typical laboratory flotation column (Finch and Dobby, 1990), except no wash water was added. The ore, ground to 85%–85 lm, was prepared following Brunswick Mine protocol for Cu/Pb bulk rougher flotation. The slurry (water was Que´bec City tap) was brought to the target solution conditions (salt or frother concentration) in the feed tank. The feed (30 wt% solids) was introduced about 40 cm below the column lip. Froth depth was estimated from the bottom pressure and controlled automatically (5 ± 2 cm) by manipulating feed rate. Underflow
14
NaCl NaCl NaCl NaCl Na2SO4 Na2SO4 Na2SO4 Na2SO4 CaCl2 CaCl2 CaCl2 CaCl2 Na2S2O3 Na2S2O3 Na2S2O3 Na2S2O3 Al2(SO4)3 Al2(SO4)3 Al2(SO4)3 Al2(SO4)3
12
Gas Holdup (%)
10
8
6
4
2
0.7 cm/s 1.0 cm/s 1.4 cm/s 1.7 cm/s 0.7 cm/s 1.0 cm/s 1.4 cm/s 1.7 cm/s 0.7 cm/s 1.0 cm/s 1.4 cm/s 1.7 cm/s 0.7 cm/s 1.0 cm/s 1.4 cm/s 1.7 cm/s 0.7 cm/s 1.0 cm/s 1.4 cm/s 1.7 cm/s
0 0.0
0.1
0.2
0.3
0.4
0.5
Ionic Strength Fig. 5. Gas holdup vs. ionic strength for four superficial gas velocities (in order from bottom to top: 0.7, 1.0, 1.4 and 1.7 cm/s) for all salts tested.
[MIBC] (ppm) 0
2
4
6
8
10
12
14
16
18
20
15 14
Gas Holdup (%)
13 12
MIBC MIBC 11
NaCl NaCl 10 9 8 7 6 0.0
0.1
0.3
0.2
0.4
[NaCl] (M) Fig. 6. Determining equivalent frother (MIBC) concentration (Jg = 0.7 cm/s).
0.5
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was used as the gas dispersion measure in this case as using pressure to estimate gas holdup is no longer accurate unless slurry density is continuously monitored. 3. Results and discussion 3.1. Two-phase 3.1.1. Gas holdup and bubble size Gas holdup is an indirect measure of bubble size, increasing as bubble size decreases (for a given air flow rate). This reflects that smaller bubbles rise at lower velocities and thus gas residence time (holdup) in the column increases. Bubble size is dictated by sparger characteristics, airflow rate and the system chemistry. Fig. 2 shows the expected near linear response in gas holdup with superficial gas velocity (obtained up to a limiting Jg) (Finch and Dobby, 1990). Increasing sodium chloride concentration increased gas holdup indicative of salt’s ability to reduce bubble size. Bubble size distribution was measured at three sodium chloride concentrations (0.05, 0.1 and 0.25 M) at a superficial gas velocity of 0.7 cm/s (Fig. 3). An effect is not notable until 0.25 M when the initially dominant mode in water alone at ca. 4–5 mm shifts to ca. 1 mm. This observation correlates with the increase in gas holdup when salt concentration increases to 0.25 M (Fig. 2). Interestingly, the water-only case always showed evidence of fine bubbles (<1 mm) suggesting they either survive the coalescence process or perhaps result from coalescence induced bubble break-up (Tse et al., 2003). As a result of this demonstrated relationship between gas holdup and bubble size and the fact that the former is easier to measure on-line, gas holdup is used in the remaining two-phase tests.
decreases. Keitel and Onken (1982) showed a correlation between Sauter mean bubble diameter, D32, and ionic strength (they also noted the possibility of using the diameter of hydrated ions to improve the correlation). As far as is known Fig. 5 is the first time a dependence of gas holdup on ionic strength has been demonstrated. It is argued that the higher the charge on the ionic species the more coalescence is inhibited. Zieminski and Whittemore (1971) noted that coalescence behavior was a function of ion–water interactions. Highly hydrated inorganic ions tend towards the bulk away from the solution/ air interface (i.e., negatively adsorb) and are termed ‘struc-
3.2. Effect of salts Fig. 4 shows the gas holdup results obtained at two gas velocities. The salts fall into groups according to their valence; aluminum sulphate (3–2 salt) showed the strongest response followed by the 2–1, 1–2 salts with the weakest response from monovalent NaCl. The valence effect is incorporated by introducing the ionic strength (l) defined as: X l ¼ 1=2 C i Z 2i ð1Þ where Ci is the molar concentration of the ith species and Zi is the charge of the ith species. The summation is taken over all ionic species in solution. Fig. 5 shows that there appears to be a unique relationship between gas holdup and ionic strength independent of salt type. Zieminski and Whittemore (1971) demonstrated a correlation between total bubble surface area and ionic strength (for 11 salts). Total bubble surface area, like gas holdup, is a function of bubble size, both increasing as bubble size
Fig. 7. Top views of froth and corresponding image of bubbles below the froth for flotation of sample of Brunswick Mine ore (water, Que´bec City tap).
J.J. Quinn et al. / Minerals Engineering 20 (2007) 1296–1302
1301
25
Water
5.10 Water
0.1M NaCl 3.66 0.1M NaCl
Relative Frequency (%)
20
0.2M NaCl 2.84 0.2M NaCl 15
0.4M NaCl 2.45 0.4M NaCl 10ppm MIBC 1.93 10ppm MIBC
10
5
0
0
1
2
3
4
5
6
7
8
Bubble Diameter (mm) Fig. 8. Evolution of bubble size distribution for conditions in Fig. 7 (Jg = 0.9 cm/s).
3.3. Equivalent frother concentration As salt type (among those tested) does not appear to be a factor, NaCl was selected as representative for this part of the work. Being monovalent, ionic strength is numerically equal to the concentration in mol/L (M). Fig. 6 compares gas holdup as a function of the concentration of NaCl (lower axis) and MIBC (upper axis). The trend-lines are polynomial fits. While it is evident that MIBC is capable of greater increases in gas holdup the results for NaCl cover a similar range. A water sample from Raglan showed an ionic strength of ca. 0.4 (vertical line on Fig. 6). From this observation it can be estimated that salts at Raglan have the same effect, with respect to increased gas holdup, as roughly 8–10 ppm MIBC. An MIBC concentration at this level is typical in plant operations (Ge´linas et al., 2005) and helps explain why Raglan is able to operate without frother. The tests were repeated at three other gas velocities (0.4, 1.0 and 1.7 cm/s) and gave the same equivalent frother concentration. 3.4. Three-phase The three-phase tests were conducted to access froth properties but they also gave some further insight into gas dispersion. Fig. 7 shows overhead views of the froth with typical images of the bubble size in the pulp below. It is evident that with increasing NaCl concentration the froth texture and bubble size merge with those obtained with 10 ppm MIBC. The evolution of the bubble size distribution is seen in Fig. 8; the features are similar to those in
the two-phase tests (Fig. 3). Water alone showed a principal mode at 4–5 mm (and, again, a minor one below 1 mm) which shifts progressively to finer size and the distribution narrows with increasing NaCl concentration. The distribution for 0.2 and 0.4 M NaCl becomes comparable to MIBC. This reinforces the conclusion from two-phase tests that gas dispersion properties are similar between salt solutions of ionic strength ca. 0.4 (i.e., 0.4 M NaCl) and ca. 10 ppm MIBC. Comparing Figs. 3 and 8 indicates that the presence of solids has little impact on the bubble size produced, which is dictated by the solution ‘chemistry’. Fig. 9 gives the measured overflow rates for the threephase tests. The overflow rate increases with salt concentration and becomes similar to that achieved by 10 ppm MIBC between 0.2 and 0.4 M NaCl. Recalling that in the two-phase tests salts could not sustain froth it is evident that the hydrophobic particles (i.e., those attached to the 0.8 water 0.1 M NaCl
0.6
O/F Rate (L/min)
ture makers’. It has been shown that both positive and negative adsorption can cause bubble coalescence inhibition (Foulk and Miller, 1931; Machon et al., 1997).
0.2 M NaCl 0.4 M NaCl 10 ppm MIBC
0.4
0.2
0 0
0.5
1
1.5
2
2.5
Gas Velocity (cm/s) Fig. 9. Overflow rates as a function of superficial gas velocity and salt concentration compared to 10 ppm MIBC (corresponding to Fig. 7).
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J.J. Quinn et al. / Minerals Engineering 20 (2007) 1296–1302
bubbles) are responsible for froth formation (Fig. 7) and overflow.1 Water alone (Fig. 9) even showed some overflow. Pugh (2006) recently demonstrated that hydrophobic particles could build froth in the absence of surfactant. However, without the small bubbles provided by frother or salts the overflow (particle recovery) rate is limited, i.e., flotation capacity is limited. The combination of gas dispersion and froth overflow results in both the two- and three-phase systems provides the evidence that salts present in the Raglan water can substitute for frother. The important similarity between frother and salts is the ability to produce fine bubbles. Any plant with process water of ionic strength at least 0.4 should consider the probable impact on bubble size. 4. Conclusions Two-phase results show increasing gas holdup with increasing concentration of NaCl, CaCl2, Na2SO4, Na2S2O3 and Al2(SO4)3. The effect increased with ion valence and a correlation between gas holdup and ionic strength was demonstrated. Gas holdup for salt solutions with ionic strength ca. 0.4, as at the Raglan concentrator, is similar to that of ca. 10 ppm MIBC, which is a typical dosage for this common frother. Continuous three-phase tests with a sulphide ore showed similar froth texture, bubble size distribution and froth overflow rate for 10 ppm MIBC and 0.4 M NaCl (i.e., ionic strength 0.4). The results help interpret why the Raglan plant can operate without frother; in particular, high concentration of salts can substitute for the bubble size reduction normally provided by frother. Acknowledgements The funding was under a Collaborative Research and Development grant from Natural Sciences and Engineering Research Council of Canada (NSERC) with industrial sponsorship from (giving the names at the onset of the sponsorship, 2001): Inco, Teck-Cominco, Noranda, Falconbridge, COREM and SGS-Lakefield (joined 2003). In particular, the authors would also like to acknowledge COREM under project R-116 for additional financial support. Constructive discussions with Colin Hardie and Carmine Ciriello (Raglan operation) are also gratefully acknowledged. References Cappuccitti, F., Finch, J.A. 2007. Development of new frothers through hydrodynamic characterization. In: Proceedings of the 39th Annual Meeting of the Canadian Mineral Processors, pp. 399–412. Craig, V.S.J., 2004. Bubble coalescence and specific-ion effects. Curr. Opin. Colloid Interface Sci. 9, 178–184. Craig, V.S.J., Ninham, B.W., Pashley, R.M., 1993. The effect of electrolytes on bubble coalescence in water. J. Phys. Chem. 97, 10192–10197.
1 It is also the case that 10 ppm MIBC does not produce substantial froth in the absence of floatable particles (Cappuccitti and Finch, 2007).
Finch, J.A., Dobby, G.S., 1990. Column Flotation. Pergamon Press. Foulk, C.W., Miller, J.N., 1931. Experimental evidence in support of the balanced-layer theory of liquid film formation. Ind. Eng. Chem. 23 (11), 1283–1288. Ge´linas, S., Finch, J., Cappuccitti, F. 2005. Frother analysis: procedure and plant experience. In: Proceedings of the 37th Annual Meeting of the Canadian Mineral Processors, pp. 569–576. George, C., 1996. The Mt Keith operation. In: Grimsey, E.J., Neuss, I. (Eds.), Nickel. Australasian Inst. Min. Metall., Melbourne, pp. 9–23. Haig-Smillie, L.D. 1974. Sea water flotation. In: Proceedings of the 6th Annual Meeting of Canadian Mineral Processors, pp. 263–281. Harris, P.J. 1982. Frothing phenomena and frothers. In: King, R.P. (Ed.), Principles of Flotation, S. African Institute of Mining and Metallurgy, Monograph Series No. 3, pp. 237–250. Hernandez-Aguilar, J.R., Coleman, R.G., Gomez, C.O., Finch, J.A., 2004. A comparison between capillary and imaging techniques for sizing bubbles in flotation systems. Miner. Eng. 17 (1), 53–61. Hernandez-Aguilar, J.R., Gomez, C.O., Finch, J.A. 2002. A technique for the direct measurement of bubble size distributions in industrial flotation cells. In: Proceedings of the 34th Annual Meeting of the Canadian Mineral Processors, pp. 389–402. Hofmeier, U., Yaminsky, V.V., Christensen, H.K., 1995. Observations of solute effects on bubble formation, J. Colloid Interface Sci. 174, 199– 210. Keitel, G., Onken, U., 1982. Inhibition of bubble coalescence by solutes in air/water dispersions. Chem. Eng. Sci. 37, 1633–1638. Laskowski, J.S., Cho, Y.S., Ding, K., 2003. Effect of frothers on bubble size and foam stability in potash ore flotation systems. Can. J. Chem. Eng. 81, 63–69. Lekki, J., Laskowski, J. 1975. A new concept of frothing in flotation systems and general classification of flotation frothers. In: Proceedings of the XIth International Mineral Processing Congress, Universita di Cagliari, pp. 429–448. Lessard, R.D., Zieminski, S.A., 1971. Bubble coalescence and gas transfer in aqueous electrolytic solutions. Ind. Eng. Chem. Fund. 10, 260–289. Machon, V., Pacek, A.W., Nienow, A.W., 1997. Bubble sizes in electrolyte and alcohol solutions in a turbulent stirred vessel. Trans. Inst. Chem. Eng. 75(A), 339–348. Marrucci, G., Nicodemo, L., 1967. Coalescence of gas bubbles in aqueous solutions of inorganic electrolytes. Chem. Eng. Sci. 22, 1257–1265. Pugh, R., 2006. Surface chemical studies on particle-stabilized froths. In: ¨ nal, G., Acarkan, N., Celik, M.S., Arslan, F., Atesßok, G., Guney, A., O Sirkeci, A.A., Yuce, A.E., Perek, K.T. (Eds.), Proceedings of the XXIIIth International Mineral Processing Congress, Istanbul, pp. 430–435. Strathdee, G.C., 2000. Milling of potash in Canada – An overview. In: Damjanovic´, B., Goode, J.R. (Eds.), In: Canadian Milling Practice, vol. 49. Canadian Institute of Mining, Metallurgy and Petroleum, pp. 235–238. Tse, K.L., Martin, T., McFarlane, C.M., Nienow, A.W., 2003. Small bubble formation via a coalescence dependent break-up mechanism. Chem. Eng. Sci. 58, 275–286. Yoon, R.H., Sabey, J.B., 1989. Coal flotation in inorganic salt solutions. In: Botsaris, G.D., Glazman, Y.M. (Eds.), Interfacial Phenomena in Coal Technology, In: Surfactant Science Series, vol. 32. Marcel Dekker, pp. 87–114. Zahradnik, J., Fialova´, M., Linek, V., 1999. The effect of surface additives on bubble coalescence in aqueous media. Chem. Eng. Sci. 54, 4757– 4766. Zieminski, S.A., Whittemore, R.C., 1971. Behavior of gas bubbles in aqueous electrolyte solutions. Chem. Eng. Sci. 26, 509–520.