proceedings of 14 international mineral processing ...

PROCEEDINGS OF 14th INTERNATIONAL MINERAL PROCESSING SYMPOSIUM October 15 - 17, 2014 Turkey

Editors

Turkish Mining Development Foundation

Proceedings of 14th International Mineral Processing Symposium

Turkey, 2014

REMOVAL OF COPPER CATION (II) FROM INDUSTRIAL WASTEWATERS BY CONVENTIONAL AND MULTI BUBBLE ION FLOTATION R. Ahmadi1, a and H. Haghi2 1. Faculty of Engineering and Technology, Mining Engineering Department, Imam Khomeini International University, Qazvin, Iran. 2. School of Mining Engineering, College of Engineering, University of Tehran, Tehran, Iran a. Corresponding author ([email protected])

ABSTRACT: The removal of Copper from a dilute synthetic wastewater ( ) was studied by ion flotation at laboratory scale. Anionic sodium dodecyl sulfate (SDS) was used as a collector and ethanol as a frother. Different parameters such as pH, collector and frother concentrations, foam height and bubble size distribution were tested to determine the optimum flotation conditions. In order to check the effect of bubble size distribution, a micro bubble generator was designed. The bubbles that are generated in this way have a dimensional distribution ranging from 10 to 100 microns. Metal removed up to about 93% and water was recovered 13% by using an intermixing method of coalescence phenomena (combined flotation by micro-bubbles and normal size bubbles generated mechanically) in a Denver type flotation machine, at low pH. The flotation time fell considerably at 37.5% when the bubble coalescence phenomenon was used. 1. INTRODUCTION Since 1960 when Felix Saba introduced and set the concept of ion Flotation, a vast variety of research studies have been made to remove different metal ions from solutions (Tomlinson and Sebba 1962). Ion flotation involves the removal of surface inactive ions (colligend) from aqueous solutions by adding surfactants acting as collectors. One of the objectives of this study is to study how to exclude ions of heavy metals such a Copper from dilute wastewater with an emphasis on a metal recovery to water ratio based on ion flotation procedure. The foam height in ion flotation case is of special importance, in particular when reduction of water recovery with foam is in view. Wet foams are made up of excess volumes of entrained water containing unwanted ions. The foam height in this study was examined with change of flotation cell height.

The attachment of a larger bubble to a mineral particle by coalescence with frosting of tiny bubbles on its surface was termed the "coalescent" attachment mechanism by Klassen (Klassen 1960).The feature of this process is that tiny bubbles forming on the particle surface facilitate the attachment of larger bubbles and in this way activate flotation. According to the previous studies, this phenomenon increases flotation rates which results in decrease of flotation time. Low flotation rate is one of the common problems in ion flotation. Therefore, the second aim that is followed in this study is to use bubbles coalescence process (coalescing of microbubbles produced by using hydrodynamic cavitation method and bubbles of normal sized in a mechanical cell flotation) and study the effect of this phenomenon on metal ion recovery and flotation rate (ion flotation time).

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2. MATERIAL Sodium dodecyl sulfate (SDS; MW= 288 M) was used as surfactant. Ethanol was used as frother in all tests. Copper sulfate (CuSO4, 99.3%) supplied and produced by MERKE Co. was used to prepare the simulated waste water solutions. The solution pH was adjusted using NaOH and HCI when time required. The water once distilled by a distiller SDL. 12L. US was used to prepare the stock solutions. 2.1. Micro-bubble generator system To produce micro-bubbles, a microbubbles generator system was designed using hydrodynamic cavitation method. This system consists of a glass cylinder of 20 cm inner diameter and 27 cm length, a static mixer, a peristaltic pump, a Venturi tube, pressure gauges, air flowmeter, Laser particle size analyzer 2000S from Malvern instruments, UK, and a computer set. When using this system, a volume of wastewater was pumped into venturi tube through peristaltic pump. A high liquid flow velocity within the

Venturi tube produces a lower (or negative) static pressure at that point thus small (micro) bubbles form by cavitation. The air was injected up-stream of the pump. One reason for injection of upstream gas is to achieve higher gas saturation rates. Furthermore, saturation rate is further increased by installing static mixers (Zhou 1996). Based on this, two static mixers (6 elements for each static mixer, 100.0 mm long) in-line were used to increase the turbulence and shear effect to further disperse the air. Figure 1 shows micro-bubble generating and sampling system for measuring the bubbles size schematically. Laser particle size analyzer equipment was used to measure the size distribution of bubbles generated by Venturi tube. It uses the principle of laser diffraction to analyze particles from 0.02 to 2000 micrometers in size in a time span of several minutes (Hudson et al. 2009).

Figure 1. Schematic of Micro-bubbles generator. 1. Laser particle size analyzer 2. Denver flotation machine 3. Micro-bubble generator

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3. METHODS A Denver- type flotation machine was used in all experiments. The impeller speed was kept constant at 750 rpm. The concentration of Cu (II) in the flotation cell was 10 mg/l. Flotation tests were carried out in three sections. The first series of tests were made in a glass cylinder flotation cell (type a) of 20 cm inner diameter and 15 cm length while the second series were carried out in a glass cylinder flotation cell (type b) with the inner diameter 20 cm and 27.5 cm length. The third series were carried out in the type (b) cell and using a micro bubble generator (coalescence phenomena). In type (a) cell, some 1.5 liters of solutions with cured copper were made available. The height of water level up to the brim of cell was 30 mm. Half of type (b) cell was also put of copper contained solution of 10 mg/l concentration. In this position, the height of water level up to the brim of cell was approximately 125 mm. The flotation solution was conditioned for 5 minutes and pH of the solution was adjusted to a desired value. Following the addition of the required amount of the collector, the solution was conditioned for an additional 15 minutes to ensure consistent mixing of all reagents. Ethanol was added as the frother and the solution was conditioned for an additional 5 minutes prior to the introduction of air into the flotation cell. At the third series and at the time when micro-bubbles were being used, the processes of solution and collector conditioning lasted 5 and 15 minutes respectively. After the end of 15 minutes, ethanol was added, after a period of 2 minutes, the micro-bubbles generator was started and after 3 minutes that followed, the air was passed into the cell through flotation cell. Amount of air introduced in to the flotation cell and micro-bubble generator was 2 l/min and 0.2 l/min respectively. The froth developed at the surface of the flotation

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cells was removed by hand using a rubber paddle at preset time-intervals for up to 16 (in the type (a) cell) and 40 (in the type (b) cell) minutes. After completion of each experiment, the foams obtained from time intervals were primarily weighted in order to determine water weight (relatively the same as foam weight). Ultimately after foams fractured in some 2 hours, the solutions of these foams and those of remained solution that was left in the cell were sent to Laboratory for determination of copper value by atomic absorption spectrometry (AAS). All experiments were carried out The dimensions of micro-bubbles generated in this study, were measured using a laser particle size analyzer. 4. CALCULATION OF METAL AND WATER RCOVERIES An example calculation of the metal and water recoveries is presented in Table 1. It is noteworthy that the weight of water within the foam section absorbing time intervals has been considered to be the same of foam weight. Therefore, the froth weights are equal to water recoveries. 5. RESULTS AND DISCUSSION 5.1. Determination of ion and non-ion species as available in the solution Because As pH has profound effect on the nature of charged ionic species in solution, it is not surprising that this parameter can have a noticeable effect on foam extraction process. According to Sebba (Sebba 1962) pH variation might be expected to have some of the following effects: (i) A change may occur in the nature of the charge on the colligend, due to hydrolysis or the formation of other complexes. (ii) Changes may occur in the degree of ionization of the collector which may

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either cause the mode of collection to alter or even cease. (iii) The colligend may be precipitated as a hydroxide thereby changing the fundamental nature of the flotation process.

Figure 2: Thermodynamic distribution of the prevalent species available in simulated waste waters used in this study as a function of solution pH. the graphs of thermodynamic distributions of Cu (II) in aqueous solutions as a function of pH. Note that the amount of salt used in preparing the simulated waste water solutions was selected such that the total metal concentration in solution was always equal to 10 mg/l. The calculations were carried out using Visual Minteq ver.2.15. The graphs in Figure 2 show that the salts dissociate to produce metal species with various degrees of hydrolysis and charging. For example, the dominant species for copper are the positively charged Cu2+, CuOH+ and at pH values less than 10 whereas they are and negatively charged Cu(OH) Cu(OH) at pH values greater than 10. At and around pH=10, precipitation of neutral Cu (OH)2 takes place. Figure

2

shows

5.2. Flotation of copper ions In this section, flotation tests were conducted as a function of frother (ethanol), collector (SDS) concentrations, the pH value and foam height in the type (a) and (b) cells. The froth developed at the surface of the flotation cells was

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removed at preset time-intervals for up to 16 min (0-2, 2-10 and 10-16 minutes). 5.2.1. Effect of collector concentration Figure 3 and Figure 4 show the effect of different concentration of SDS on ion flotation of Cu at pH=4. It can be seen that with an increase in concentration of SDS, the removal of Copper increases. The maximum recovery occurred at a SDS concentration of 200 mg/l, but, in this case, a high percent of water (41.8%) was recovered. There was no flotation at low concentrations of surfactant (50 mg/l SDS) most probably due to insufficient collector-metal ion Cu (II) pairing. At an SDS concentration of 192 mg/l, the copper and water recoveries were 58% and 38% respectively for 0.2% of ethanol concentration. As it can be seen there was optimum around 192 mg/l of SDS concentration.

Figure 3: Effect of the SDS concentration on the recoveries of Cu and water. (pH= 4.0, ethanol:0.5%v/v).

Figure 4: Effect of the SDS concentration on the floatability of Cu as a function of time (pH= 4.0, ethanol: 0.5%v/v) Usually a small excess of collector is added to guarantee maximum removal of the metallic ions in solution. Excessive collector should be avoided, not only due


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to higher cost, but also because of other negative effects, such as large foam losses, micelle formation, competition between the metal-collector complex and free collector ions for bubble surface sites and the potential toxicity of residual amounts of collector in the effluent. 52.2. Effect of frother concentration The effect of adding ethanol to the copper ions removal is shown in Figure 5 and Figure 6 at pH=4. These figures show that adding ethanol leads to increase removal rate of copper ions. Different values of frother concentration such as 0.1% , 0.2% , 0.4% and 0.5% v/v were examined into at a collector optimum value, 192ppm and that of pH=4. At SDS optimum concentration of 192 mg/l, the final copper recoveries were 63%, 45.1%, 58% and 65.3% for 0.1%, 0.2%, 0.4% and 0.5% of ethanol concentrations, respectively (Figures 5 and 6). Taking 65.3% of metal recovery and that of water at 37.1% at 0.5%v/v of ethanol frother, it was noted that the ratio of metal recovery to that of water in this concentration tops the most. Thus, the optimum density of an ethanol Frother at pH=4 is as much as or equal to 0.5%. Increasing the removal efficiency with increasing in ethanol concentration in the solution is presumably attributed to decrease of degree of coalescence of gas bubbles leading to formation of more fine bubbles. Consequently, the surface area of the gas phase increases leading to improvement in flotation efficiency.

Figure 6: Effect of the frother concentration on the floatability of Cu as a function of time (pH= 4.0, SDS: 192 mg/L). 5.2.3. Effect of pH The effect of solution pH was also tested. The reason was to observe the response of the various copper species versus the collectors used. The experiments with the anionic SDS (192 mg/l) were carried out at pH values of 4, 6, 8 and 10 since the copper species prevalent in solution were mainly positively charged below pH = 10 (see Figure 2). The results of these experiments were presented in Figures 7 and 8. In the experiments with the SDS, the figure shows that copper is selectively floated at pH 4 and 6 where doubly charged Cu(II) species are prevalent. At pH=4, the recoveries of metal and water were 65.3% and 37.1% respectively.

Figure 7: Effect of pH value on the recovery of Cu and water (SDS: 192 mg/L, ethanol: 0.5%v/v).

Figure 5: Effect of the frother concentration on the recoveries of Cu and water. (pH= 4.0, SDS:192 mg/L).

The ratio of metal recovery to water recovery at pH=4 has the highest value. When pH increased to 8, however, selectivity decreased probably due to the appearance of mono-charged copper

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species which show smaller affinity towards the SDS most probably due to a smaller charge density.

Figure 9: Effect of froth height on the recovery of Cu and water (SDS: 192 mg/L, ethanol: 0.5%v/v, pH=4). Figure 8: Effect of pH value on the floatability of Cu as a function of time. If pH increased to 10, the selectivity of copper increased once again. This seemed somewhat surprising since the amount of positively charged copper species should be lower at pH =10 where the dominant copper form is the neutral Cu(OH)2 precipitate. The high recovery at pH =10 (actually the highest recovery within this set of tests, Cu recovery >70%) implies that the precipitated copper must be floating. Actually, a visual inspection at this pH clearly showed precipitated copper in the froth products. The removal of copper ions at pH=10 where copper hydroxide; precipitation took place more efficiently, rapid and virtually complete within a relatively short time compared with the removal of copper ions at pH=4. 5.2.4. Effect of foam height The effect of foam height on water recovery in copper ion flotation is shown in Figure 9. Increasing the foam height from about 30% of cell height in type (a) cell to 50% of cell height in type (b) cell resulted in decreasing the water recovery considerably. It reduced the water recovery by 66.85% when the copper recovery was 65% in both cells.

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Owing to low water recovery in this experiment, flotation time continued up to 40 minutes (Figure 9) and so that, after 40 minutes, copper and water recoveries reached to 85.6% and 15.5% respectively, which are highly favorable. From the results it was clear that increasing the froth depth to allow drainage occurrence was beneficial in reducing water recovery. Wet foams were made up of excess volumes of entrained water containing unwanted spectator ions which reduced the specificity for the desired metal ion. Upgrade ratios also suffer a reduction in magnitude. A top section is necessary in a foam column to promote drainage of liquid from the foam product. 5.2.5. Effect of coalescent phenomena (Flotation with enhanced bubbles size distribution) At this stage (phase 3), ion Flotation was followed with the coalescence of micro bubbles with bubbles of normal size belonging to mechanical Flotation cell. Figure 10 shows the results of water and metal recovery that has been undertaken with a normal Flotation (general flotation) method and that of bubble coalescence in a five liter cell.


Figure 10: Effect of Coalescence phenomena on the recovery of Cu and water (SDS: 192 mg/L, ethanol: 0.5%v/v, pH=4). This recovery obtained the best result, 85.6% and 15.5% under the best conditions (such as collector concentration of SDS=192mg/l, ethanol frother of 0.5%v/v and pH=4) and using a mechanical flotation method. Under the optimum conditions, recovery of both metal and water were 92.8% and 13.1% respectively if a coalescence Flotation was used. Figure 10 shows the effect of micro-bubbles on the recovery- flotation time curves in which the Cu and water recoveries are plotted against the flotation time. The curves indicate that the Cu recovery in the presence of microbubbles was significantly higher than in the absence of micro-bubbles. When the cumulative flotation time is 15 min, the Cu recovery in the presence of microbubbles was about 13% higher than in the absence of them. The difference of the Cu recovery increased as the cumulative flotation time increased from 15 minutes to 25 minutes. The fact that the curve with micro-bubbles is above another curve, indicates that the presence of micro-bubbles improved the flotation kinetics. The presence of micro-bubbles on Cu ions surfaces facilitates attachment to conventional-sized bubbles. The microbubbles do not have sufficient buoyancy to float the collector-metal ion Cu (II) pairing by them. But the surface of Cu coated with micro-bubbles is more

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hydrophobic than without micro-bubbles. The wetting film separating the colliding convention-sized bubbles and microbubbles is more unstable than the film between the convention-sized bubbles and Cu ions. The significant difference in diameter between the convention-sized bubble and micro-bubble leads to a huge differential capillary pressure, which facilitates the rupture of water films separating convention-sized bubbles and micro-bubbles. Thus the presence of micro-bubbles accelerates the water film rupture and bubble-metal ion collision. Bubbles did not have any direct role in ion flotation. Reducing the water recovery to 16% is just due to the decrease of water height as a result of existence some solution in tubes of the pumps. 6. CONCLUSION The Copper removal was carried out in synthetic wastewater by ion flotation technique in a laboratory flotation cell. Various parameters such as surfactant and frother concentrations, pH, foam height and dimensional distribution of bubbles were tested to determine the optimum flotation conditions. Sodium dodecyl sulfate (SDS) was used as collector. Ethanol was used as frother. The results obtained from different tests indicate how a lower height of foam affects negatively on copper removal by ion flotation. At this stage, the more a metal is recovered, the more the water will be recovered. Excessive water is available in foam and lack of insufficient time to lose it accounts for this problem. It was observed that with the coalescence of micro bubbles with bubbles of normal size belonging to mechanical flotation cell, improved the flotation kinetics. The presence of micro-bubbles on Cu ions surfaces, and facilitates attachment to conventional-sized bubbles. Under the optimum conditions (such as collector concentration of SDS=192mg/l,

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ethanol frother: 0.5%v/v and pH=4, froth height: 125 mm) the best removal obtained for the system Cu/SDS with a dry foam (water recovery: 15.5%) was 85.6%. Coalescence of micro bubbles with bubbles of normal size belonging to mechanical flotation cell improved the removal of Cu to a maximum floatability of 92.8% and reduced the water recovery to 13.1%. REFERENCES Hudson, J.B. C., Daniel, G. N., Reiner, bubble size distribution measurements Minerals Engineering, 22, pp. 330 335. In Proceedings of 5th IMPC,London, pp. 309-322. Liu, Zh. thermodynamic approach to ion flotation. I. Kinetics of cupric ion Colloids and Surfaces A: Physicochemical and Engineering Aspects, Volume 178, Issues 1-3, pp. 79-92. Liu, Zh. and Doyle F.M. thermodynamic approach to ion flotation. II. Metal ion selectivity in the SDS Cu Ca and SDS Cu Pb Colloids and Surfaces A: Physicochemical and Engineering Aspects, Vol. 178, Issues 1-3, pp. 93103. inetics. NATO ASI, Series E: Applied Science, Vol. 208, Kluwer Academic Publishers, Dordrecht, pp. 183 210. Elsevier monographs. Tomlinson, H.S. andSebba, F., 1962, urfactant ions by AnalyticaChimicaActa, Vol. 27, pp. 596-597.

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