J. Cent. South Univ. (2014) 21: 1461−1469 DOI: 10.1007/s1177101420861
Effect of solids on pulp and froth properties in flotation ZHANG Wei(张炜) 1, 2 , James A. Finch 2 1. Mining & Mineral Resources Division, Department of Business Administration, Chinalco China Copper Corporation Limited, Beijing 100082, China; 2. Department of Mining and Materials Engineering, McGill University, Montreal H3A 0C5, Canada © Central South University Press and SpringerVerlag Berlin Heidelberg 2014 Abstract: Froth flotation is a widely used process of particle separation exploiting differences in surface properties. It is important to point out that overall flotation performance (grade and recovery) is a consequence of the quality and quantity of the solid particles collected from the pulp phase, transported into the froth phase, and surviving as bubbleparticle aggregates into the overflow. This work will focus on studying these phenomena and will incorporate the effects of particle hydrophobicities in the 3phase system. Solids are classed as either hydrophilic nonsulphide gangue (e.g. silica, talc), hydrophilic sulphide (e.g. pyrite), or hydrophobic sulphide (e.g. sphalerite). Talc is a surfaceactive species of gangue that has been shown to behave differently from silica (frother adsorbs on the surface of talc particles). Both are common components of ores and will be studied in detail. The focus of this work is to investigate the role of solids on pulp hydrodynamics, froth bubble coalescence intensity, water overflow rate with solids present, and in particular, the interactions between solids, frother and gas on the gas dispersion parameters. The results show that in the pulp zone there is no effect of solids on bubble size and gas holdup; in the froth zone, although hydrophilic particles solely do not effect on the water overflow rate, hydrophobic particles produce higher intensity of rates on water overflow and bubble coalescence, and many be attributed to the water reattachment. Key words: flotation; frothers; bubble size; coalescence; gas holdup; hydrophobicity
1 Introduction Conceptually, flotation systems may be viewed as consisting of two zones: pulp and froth [1−4]. Collection of hydrophobic particles occurs in the pulp zone, while entrained gangue drains from the froth phase resulting in increased grade of the valuable particles in the concentrate. The pulp zone is characterized by intermediate gas holdup (typ. 5%−25%, volume fraction) and regions of turbulence designed to promote bubble particle interaction, while the froth zone is characterized by high gas holdup (typ. 85%−95%, volume fraction) and a relatively quiescent regime of upward moving and coalescing bubbles laden with hydrophobic particles, and downward flowing excess liquid carrying gangue particles. The presence of solids on the bubbles serves to help stabilize the froth. The froth and pulp zones are separated by a narrow region, the pulpfroth interface, over which gas holdup experiences a dramatic transition, and where particles drained from the froth can be recirculated back into the froth phase. Since flotation is primarily a surface area (of gas) driven process, the size and behavior of bubbles in both pulp and froth phases are of paramount significance. In addition, according to the penetration theory proposed by LEJA and SCHULMAN [5], frother molecules accumulate preferentially at the
gas/liquid interface due to their heteropolar structure, and actively interact with collector molecules adsorbed onto mineral particles. It is clear that the efficiency of flotation will depend on the use of frother to control bubble size, and hence particle collection efficiency in the pulp, and also to stabilize the bubbles that are required to successfully exit the froth zone [1, 3, 6]. This process has important implications in terms of overall flotation performance impacting both the quality (the grade) and quantity (the recovery) of the particles delivered from the pulp and froth zones to the concentrate [7]. The difficulty of characterizing frothers in a 3phase system (liquidgas solid) has been recognized [7] – one reason being that the presence of particles may interact with frothers (e.g. adsorb) thus complicating the interpretation of results. One of the objectives of the proposed work is to determine how solids and frothers interact to affect the subprocesses that are occurring in the pulp and froth phases. Understanding and characterizing the role of frother type and concentration in 2phase (air and water) systems in flotation has been extensively studied over the past few decades [8−9]. Similar efforts have not been attempted comprehensively in 3phase (air, water and solids) systems, largely because of system complexity and of the discovery that solids often dominate the
Received date: 2012−12−09; Accepted date: 2013−05−10 Corresponding author: ZHANG Wei, PhD; Tel: +86−13521508736; Email:
[email protected]
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effects of frothers in both pulp and froth phases. Compared to the wellstudied 2phase (gas/liquid) flotation system, the hydrodynamics of real flotation systems can be considered to effectively involve four phases: liquid, gas, hydrophilic solids and hydrophobic solids. The hydrophobic particles in the systems can change the surface stresses at the gas/liquid interface, and hydrophilic particles can modify the rheology of the interstitial fluid (the Plateau borders) within the froth [10]. Froth is an integral part of the flotation system but the importance, relatively few studies have been conducted to examine the actual impact of froth variables (e.g. bubble size, frother type and concentration, liquid/gas/solid content) on the performance of flotation cells [11−13]. If the froth phase is not sufficiently stable (i.e. excessive bubble coalescence), mineralized bubbles that enter the froth may rupture prematurely causing valuable mineral particles to drop back into the pulp zone. Conversely, too stable a froth may cause nonselective entrainment of hydrophilic gangue particles due to excessive water recovery, thereby reducing concentrate grade. Froth stability can be characterized by several methods including: water overflow rate, dynamic formability index (DFI is defined as the slope of gas retention time vs. frother concentration) [14], the slope of froth height vs. gas holdup [15]. The focus of this work will be to investigate in sufficient detail to determine the role of solids on pulp and froth behavior, in particular, the relationship between solids, frother and gas (i.e. bubbles). The main variable will be type of solids, which will include hydrophilic nonsulphide gangue (silica) and naturally hydrophobic minerals (talc). The reason for considering different minerals when investigating the effect of solids is that in flotation, hydrophilic particles are expected to concentrate in the interstitial water between bubbles (within the froth) while hydrophobic particles will accumulate at the bubble liquidair interface. To determine the effect of solid particles on pulp zone gas dispersion and froth zone stability using a laboratory flotation column system, the relationship between bubble coalescence and frother/solid interaction in the froth zone using a specifically designed froth viewing column as the test beds was established.
2 Experimental 2.1 Continuous labscale flotation column system The setup is based on a 110 cm × 10 cm diameter flotation column for continuous labscale flotation testing. The arrangement comprises a flotation column, holding tank, conditioning tank and feed flowregulator tank configured in a closed loop to achieve steady state (Fig. 1(b)). The column (Fig. 1(a)) is 9 L and volume of
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the entire setup is 56 L. A rigid cylindrical porous sparger is set vertically at the bottom of the column to disperse gas. A calibrated amount of compressed air is delivered to the sparger via an air flow meter. Two 0.5inch diameter tubes, vertically mounted on the side of column, hydraulically connected to the column at different heights, are used for gas holdup measurements. The McGill Bubble Size Analyzer (Fig. 1(a)) is placed on top of the column and measures bubble size in the pulp [16]. Gas holdup and bubble size measurements are measured in the mid section of the column. Feed (water and solids) is held in the holding tank and pumped into the feed flowregulator, which allows a calibrated flow into the conditioning tank while the remainder returns to the holding tank. The conditioning tank is continuously mixed (stirrer at 1000 r/min). Flow from the conditioning tank is fed to the column. A 0.5inch tube mounted at the bottom of the column controls water level and regulates water rate to underflow thereby controlling froth depth (level) in the column. Column overflow and underflow are recombined and pumped to the holding tank to close the loop. The pump is used to ensure a flow above the minimum to maintain the target feed flow rate set at the regulator tank. Samples of both cell products (concentrate and tailings) are taken to measure overflow rate and frother concentration; all unused sample are returned to the holding tank. Solid and frother were added to water as required and the slurry conditioned in the tank using a mechanical stirrer. Timed overflow samples were weighed then filtered and the cake ovendried at 100 °C (2 h). The dry solids were weighed to yield the solids overflow rate, and by difference, the water overflow rate. Air flow rate Jg was set at 0.725 cm/s and froth depth controlled automatically at 1 cm by manipulating underflow rate. This depth was determined from airwater tests that showed 1 cm was the about maximum for overflow with MIBC at the concentrations used. Control on froth depth was better than 0.15 cm. 2.2 “Froth viewing chamber” style flotation column and imaging process A schematic of the “froth viewing chamber” flotation column shown in Fig. 2 was specifically constructed to permit viewing of the froth structure and froth bubble size from the side. The flotation column consists of a rigid cylindrical sparger (to disperse gas) and a stirred vessel (as the collection zone) connected to the froth viewing chamber through a tapered transition piece (as the pulp/froth interface). The transition piece was necessary in order to achieve a formation of deep froth over wide operating conditions. The chamber is made of glass to which bubbles do not adhere, and consists of a camera (to capture the bubble images), a
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Fig. 1 9 L labscale flotation column assembly and MBSA (a) and setup for continuous closedloop testing (b)
Fig. 2 “Froth viewing chamber” flotation column to measure bubble size in froth (a) and schematic of “froth viewing chamber” column with front and side view, and dimensions (b)
background light, and a diffuser paper (stick on the back window). The dimensions are 1 cm (to ensure the bubbles are spread into the chamber as the monolayer) for the width, 150 cm and 40 cm for the height and
length, respectively (Fig. 2(b)). The flotation column was operated in a continuous mode where the feed was pumped into the column and a constant flow rate of slurry through the column was
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achieved. The tailing stream was passed through a gravity overflow. The bubble size was determined by a photographic technique. Bubbles near the column wall in the froth zone were photographed using a camera after the concentrate reached a steady state. A movable frame supporting the camera could be moved up and down easily to allow bubbles to be photographed at various levels in the froth. The superficial gas rate (i.e., Jg) was kept constant as 0.725 cm/s and frother MIBC, at a constant concentration of 20×10 −6 (equivalent to the CCC (critical coalescene concentration) of MIBC) was added to the column with 3% (mass fraction) solids (i.e., talc and silica, respectively, and the particle sizes are ca. 10 µm). The sparger was selected as the same as the one used previously. The bubble images captured at three different locations were selected according to the different froth levels (i.e., 150 mm, 250 mm and 350 mm above pulp/ froth interface, respectively). The height of pulp/froth interface was changeless as 80 cm above the bottom of the froth viewing chamber, for all experiments. At least 500 bubbles (ca. 10−25 images were processed for each test) were sized in each position. Figures 3(a), (b) and (c)
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show the images captured at three different locations, respectively, on 20×10 −6 MIBC and silica particles (and Fig. 3(d), (e) and (f) for talc particles). A bubble image analysis routine was developed by applying CLEMEX Vision PE software. The image analysis procedure is shown in Fig. 4. The original froth bubble image is shown in Fig. 4(a). The image needs to be loaded into the computer (software) and the delineate filter is applied in order to enhance bubble edges. The routine performs an autothreshold according to the grey levels to separate or extract the silhouettes of the bubbles from the background and assigns a bitplane color, as shown in Fig. 4(b). Figure 4(c) shows the product for filling holes – eliminate open areas in the silhouettes of the bubbles due to the surface reflections. Finally, the diameter, area and volume of each individual bubble is automatically determined and then printed in the EXCEL file. 2.3 Materials The type of frother used in the experiments was MIBC (purity>99.9%), provided by AldrichSigma. Frother concentrations were selected with respect to
Fig. 3 Example images of froth bubbles in presence of solids with different heights: (a) 150 mm (above pulp/froth interface), 1% silica; (b) 250 mm, 1% silica; (c) 350 mm, 1% silica; (d) 150 mm, 1% talc; (e) 250 mm, 1% talc; (f) 350 mm, 1% talc
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CCC. To detect loss of frother due to adsorption on the solids TOC (total organic carbon) analysis was employed [8, 17]. Talc and silica were supplied by Fisher Scientific and the average particle size of the sample (d50), determined using a Malvern particle size analyzer (Model: MasterSizer 2000) were 10 µm, respectively.
3 Results and discussion
Fig. 4 Image processing procedure: (a) Image input; (b) Threshold; (c) Binary transformations
3.1 Selection of column sparger porosity Gas holdup is influenced by air flow rate and the (rigid) sparger porosity. Therefore, in order to produce a sensitive response in gas holdup, the choice of sparger porosity needs to be examined first. There are four candidate spagers of varying porosity. Figure 5(a) presents a plot of volumetric air flow rate (Q) versus pressure drop (ΔP) for each of the four spargers. According to Darcy’s law, the curves should result in straight lines if the spargers are operating under “good conditions” (“good condition” means the sparger contains enough active pores for bubble formation). All spargers tested respected this relationship. This figure also shows that Sparger 1 has the largest equivalent nominal pore sizes since it has the smallest ΔP/Q. According to DAHLKE et al [18], with sufficient
Fig. 5 Several preliminary tests results for sparger selection: (a) Measurements of Q−ΔP data for a set of spargers; (b) Determination of Jg−Eg relationships; (c) Eg as function of MIBC concentrations (Jg=0.725 cm/s); (d) Sauter mean bubble size vs MIBC concentrations (Jg=0.725 cm/s)
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frother concentration (exceeding the CCC) to optimize metallurgical performance, the proper Jg−Eg relationships always indicate that Eg at 2 cm/s Jg is double that at 1 cm/s Jg. Figure 5(b) shows that in 20×10 −6 MIBC concentration (the CCC of MIBC is ca. (14−16)×10 −6 in this situation), only Spargers 1 and 2 reach this criteria (e.g., Eg is 8% at 1 cm/s Jg and 15.5 % at 2 cm/s Jg for Sparger 1). In other words, it has a sufficiently broad operating range to represent an industrial setting. Figure 5(c) shows the gas holdupconcentration relationship for varying porosity spargers at 0.725 cm/s Jg. As expected, while pore size decreases, i.e., bubble size generated decreases, gas holdup increases. A general trend is noticed: a relatively slow increase in Eg at very low concentration (ca. 5×10 −6 or less) then rapidly increasing at higher concentration and eventually (after CCC) a slower steady increase. Although the curves stay in the practical range (0−15%) of E g, it was felt prudent to reject Sparger 1 as its curve has an unusual trend (more linearly than others). Figure 5(d) shows the Sauter mean bubble size versus concentration at 0.725 cm/s Jg. Sparger 3 is rejected in this case because of its slow response of D32 versus Jg. Overall, Sparger 2 is selected as the most suitable for the experiments. 3.2 Pulp zone: Gas holdup and bubble size Figures 6(a) and (b) give the gas holdup (Eg) and bubble size (D32) results, respectively, as a function of MIBC addition in the presence of two different solid types. Compared to the 2 phase airwater system, these two parameters (Eg and D32) did not change appreciably with the presence of either the talc or silica particles. The results of TOC analysis in Fig. 7 indicated no change in frother concentration with the presence of either the talc or silica particles as the different weight percentage added in the flotation system. The results (Fig. 6) of MIBC/talc and MIBC/silica systems show no evidence of solidinduced coalescence from the bubble size data with the presence of either hydrophobic (talc) or hydrophilic (silica) particles; the gas holdup data confirm this observation. Nevertheless, concerning the role of solids on pulp properties, some discussions in the literature reveal the results are not conclusive and sometimes contradictory. For instance, measurements on bubble size (Db) by GRAU and HEISKANEN [19] suggest that at high solids content of hydrophilic particles, there is an increase in Db in a lab scale cell for two different polyglycol frothers. However, QUINN et al [20−22] concluded that the presence of solids appeared to have no significant effect. BANISI et al [23] using DF250 as the frother concluded that the presence of solids (hydrophilic or hydrophobic) decreased
Fig. 6 Gas holdup (a) and bubble size (b) as a function of solids addition for MIBC (Note: mass fraction of solids is 3% including talc and silica)
Fig. 7 Measured MIBC concentration as function of solids addition
gas holdup but inferred bubble size remained constant and discussed possible mechanism–loss of frother by adsorption which may distort gas holdup. They noted the need to account for frother adsorption in tests with coal. Nevertheless, in this research, no adsorption of MIBC is demonstrated (Fig. 7) in either MIBC/talc or MIBC/silica system, revealing that there is no effect of solids on
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bubble size and gas holdup in the pulp zone and no adsorptions exist between the surface of particles and frother molecules. 3.3 Froth zone: Overflow rate and bubble size The water and solids overflow rate and bubble size in the froth were selected to reflect the froth property, with the presence of the two difference types of solids. Figure 8 quantitatively shows the effect of MIBC concentration on water overflow rate, with or without the presence of the talc (i.e., 3% talc, mass fraction) when a froth depth of 1 cm was used. Surprisingly, without changing other conditions, the water overflow rate increased more than doubled before 40×10 −6 MIBC concentration when 3% talc was added to the systems, compared to the results in only 2phase systems. The increase in water overflow can be attributed to an increase in water retention in the froth due to the talc restricting water drainage from the froth back to the pulp. Another observation from Fig. 8 is the trend of water overflow rate (3 phase) mirrors the concentrate overflow; as the result, the solid overflow is relatively constant. MELO and LASKOWSKI [24] reported the similar observations with using MIBC. They revealed that the water recoveries which were obtained by applying coal (i.e., hydrophobic) particles were apparently much larger than those in 2phase systems.
Fig. 8 Overflow rates vs MIBC concentrations for 2phase and 3phase systems (Note: Mass fraction of solids is 3% including talc only)
Nevertheless, instead of the talc, 3% (mass fraction) silica particles were applied into the flotation system, and in contrast, the results show no impact of the silica on the water overflow rate (not shown). To explain the overflow results, explore the bubble size in the froth might be necessary. The results are shown in Fig. 9 where bubble size (D32 and D10) is plotted as a function of the froth heights (above pulp/froth interface) for particles of either sole
Fig. 9 Sauter mean (a) and mean number bubble size (b) versus froth height for different hydrophobic (talc) and hydrophilic (silica) particles
hydrophobic (i.e., talc) or hydrophilic (silica). The results of Fig. 9(a) show that at the same froth height, D32 values are larger with the presence of talc than the one with the presence of silica. It reveals that D32 increase with the increase of the hydrophobicity. It is also seen that D32 increases continuously with froth height for both talc and silica. This indicates that there is substantial bubble coalescence with increasing froth height in the presence of either hydrophobic or hydrophilic particles. However, the trend (i.e. bubble growth rate) of talc is similar to that observed with the system in which only silica particles are present. D10 results from Fig. 9(b) shows that D10 values do not have any significant changes between talc and silica, and reveals that the amount of small size of bubbles do not change with varying hydrophobicity. Figures 10(a) and (b) show typical bubble size distributions at various levels (i.e., distance above pulp/froth interface) in the froth zone for hydrophobic and hydrophilic particles, respectively. It can be seen from Fig. 10 that bubble size distributions with the presence of silica for three froth heights are narrower than the one with talc, which indicates that the bubble coalescence occurs with hydrophobic particles are more
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the bubble size distribution (i.e., BSD) for talc is obviously wider than silica (Fig. 10), and the D10 results (Fig. 9(b)) for both talc and silica are similar, suggesting that in the case of talc the larger (compare to silica) bubble emerge, but also that a significant number of small bubbles remain. To understand the effect of hydrophobic particles on flotation performance, the water overflow rate should be considered in conjunction with the bubble size distribution in the froth phase. From Fig. 8, the presence of 3% talc results in significant increase in water overflow rate compared to the 2phase system. It reveals that hydrophobic particles appear to play a greater froth stabilizing effect than hydrophilic particles (i.e., adding talc the water overflow rate is much higher than the rate with the silica) which is in contrast to the suggestions from froth bubble size results in Figs. 9 and 10. The contrast may be attributed as: although the detachment rate of water in the froth may be higher due to higher intensity of bubble coalescence, they might be easily attached again because the life time of the bubbles with the presence of hydrophobic particles appears to be longer than those either in 2phase system or in 3phase system with the presence of hydrophilic particles.
4 Conclusions Fig. 10 Bubble size distribution as a function of froth height for different (a) hydrophobic (talc) and (b) hydrophilic (silica) particles
rapid than the one with hydrophilic particles. Apparently the inclusion of solids influence froth stability. From Figs. 9 and 10, since the rate of bubble growth is higher in the presence of talc than the rate with silica, the study suggests that the hydrophobicity of the particles in the bubble film has a significant effect on bubble coalescence in the froth phase. These results are in partial agreement with the results of DISPPENAAR [25], where in the presence of particles with strong hydrophobicity, bubbles coalesced more rapidly (lower froth stability) than in the presence of hydrophilic particles. One explanation can be that hydrophobic particles penetrate the interface to a greater extent and ruptured the film, thus leading to unstable film bridging and promotion of bubble coalescence. However, although the particle size of both talc and silica are very similar (ca. 10 µm), the shape of the talc particles is “lamellar” and its geometry is irregular. It is not clear whether the particles shape or the hydrophobicity of the particles is the dominant factor to control bubble coalescence in the froth. Hydrophobic sulphide such as Sphalerite (which has “cubic” geometry) may need to be considered to replace talc in the future test plans. Since
1) For pulp phase, bubble size and gas holdup are the two key measured parameters to characterize the pulp properties. The results reveal that there is no effect of solids on bubble size and gas holdup in the pulp zone and no adsorptions exist between the surface of particles and frother molecules, regardless of the particle hydrophobicities. 2) For froth phase, in the column tests, the remarkable increase of water overflow rate in the presence of hydrophobic particles can be attributed to the role of hydrophobic particles which enhances froth stabilization. Adding hydrophilic particles solely do not affect the water overflow rate. In the froth viewing chamber tests, the results of the bubble size as a function of froth height (above pulp/froth interface) show that D32 in the froth increases with increasing froth height, which indicates bubble coalescence occurring at all levels of the froth, regardless of whether hydrophobic or hydrophilic particles are involved. Results also suggest that the hydrophobic particles which exhibit a higher intensity of bubble coalescence rate produce higher water overflow rate may be attributed to the water reattachment in the froth.
Acknowledgement This work was financially supported by the Chair in
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Mineral Processing at McGill University, under the Collaborative Research and Development program of NSERC (Natural Sciences and Engineering Research Council of Canada) with industrial sponsorship from Vale, Teck Cominco, Xstrata Process Support, AgnicoEagle, Shell Canada, Barrick Gold, COREM, SGS Lakefield Research and Flottec
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