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Electroflotation of metal ions in waste water
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inmgllm illUlttnJglU; ELSEVIER
Int. J. Miner. Process. 41 (1994) 285-294
Electroflotation of metal ions in waste water L. A l e x a n d r o v a , T. N e d i a l k o v a , I. N i s h k o v CentralLaboratory of Mineral Processing, BulgarianAcademy of Sciences, Sofia 1126, P.O. Box 32, Bulgaria (Received 18 February 1992; accepted after revision 26 November i 993)
Abstract
The possibility of removal of metal ions from mining waste waters through ion flotation is investigated. Waste water of an opencast mine containing about 50 mg/l copper ions is purified. The ions undergo precipitation with xanthates forming chelate complexes with high hydrophobicity. Electroflotation is used for the obtaining of a gaseous phase with sufficient volume and high dispcrsity. Conditions are established for the realization of the precipitation and adsorbing colloid mechanism of flotation. An active potassium oleate surfactant is used for the stabilization of foam formation and for the improvement of the precipitation with the main precipitant. Removal of the metal ions present in the waste water is achieved.
1. Introduction The removal of suspended or dissolved substances is based on their surface activity, possessed or acquired as a result of their interaction with suitable reagents (Sebba, 1962). Depending On the affinity of the substances in a soluble or insoluble form to the liquid/gas surface Golman (1989) distinguishes between adsorption and adhesion mechanisms of flotation. From a kinetic point of view the adhesion mechanism is much faster and for this reason during removal of ions from solution they are precipitated with suitable complex formers before flotation. An important aspect in the complex formation from the standpoint of ion flotation is that it can change the sign of the extractable ion. Clarke and Wilson ( 1983 ) have evaluated the adhesion method of flotation and described two ways for implementing it - - the precipitate flotation and the adsorbing colloid one. The efficiency of the precipitate flotation depends on the hydrophobicity of the precipitates, which in turn depends on the nature of the precipitant and the surfactant. 0301-7516/94/$07.00 © 1994 Elsevier Science B.V. All fights reserved
SSDI0301-7516(94)00012-0
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L, Alexandrova ct ai. , Int. J. Miner. Process. 41 (1994) 285- 2~J4
The adsorbing colloid mechanism is particularly suitable for the removal of ions at very low concentrations (Rubin and Johnson, 1967; Matsuzaki and Zeitlin, 1973 ). The choice of a suitable coprecipitant with a cute sorbing activity and the usage of a suitable surfactant are of prime importance in this case. Ferrous and aluminium hydroxides are the most frequently used coprecipitants in adsorbing colloid flotation. For the flotation of ferric hydroxide, Basak and Charewich (1986) used sodium dodecyl sulphate (SDS) and potasium oleate. This brief review shows the scopes ofaplication of the precipitate flotation and of the adsorbing colloid as well as the main factor for their efficiency, i.e. the choice of suitable reagents and optimum concentrations. The combination of the precipitate mechanism and the adsorbing colloid one makes the process universal and efficient in broad concentration ranges. The present paper describes a combined scheme of precipitate - - adsorbing colloid flotation of waste water by separation of the main ions as precipitate complexes with high hydrophobicity and electrolytic generation of coprecipitants as carriers of nonprecipitated ions.
2. Experimental 2.1. Materials Waste water from an opencast copper mine, containing metal ions in mg/1 as follows: Cu - - 50.00; Zn - - 1.65; Co - - 0.75; Pb - - 0.15 etc. with pH 4.5 was subjected to precipitation and flotation. For the selection of a suitable precipitant, a model solution containing Cu, Zn, Pb and Co ions (50 mg/1 each, pH 4.5 ) was prepared by dissolution of the respective nitrates. Solutions used: 1% solutions of sodium ethyl xanthate (NaEXt), sodium propylxanthate (NaPrXt), potassium butylxanthate (KBXt) and potassium amylxanthate (KAXt) (analytical grade); 10 % solution of sodium hydroxide (reagent grade) as well as 0.1% solution of potassium oleate (KOL) (analytical grade). 2.2. Methods Precipitation was optimized in 2000 cm 3 of water, with pH preset to 8. Agitation was carried out by a magnetic stirrer while the efficiency of the precipitation of the different xanthates and potassium oleate was studied and optimized by measuring the residual concentration of metal ions. Flotation tests were carried out in a 2000 cm 3 electrocoagulation flotation cell included in a continuous regime installation - - Fig. 1. The flotation time was 5 minutes. After the waste water was treated and agitated it was fed to the flotation cell. The latter consists of an electrode and an electrode-free zone containing two chambers each. The first chamber consists of lamellar iron electrodes providing ferric ions. The electrode-free zone provides favourable hydrodynamic conditions for the flotation of the nonfloculated finely dispersed particles. Due to their
L. Alexandrova et al. ~Int. J. Miner. Process. 41 (1994) 285-294
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f
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/I
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14
F i g 1 Scheme o f t h e flotation installation; 1 - - b a s i n for waste w a t e r 2 rectifier 3 p H meter, 4 - - mixer, 5 - - peristaltic p u m p ; 6 - - electrode; 7 - - agitation vessel; 8 - - outlet o f t h e cell for clear water; 9 - - vessel for t h e f o a m product; 10 - - electrodes ( c a t h o d e a n d a n o d e ) ; 11 - - outlet for the f o a m product; 12 - - electrodes (cathode a n d a n o d e ) ; 13 - - flotation cell; 14 - - test filters;
small volume the foam product is highly watered which requires its recirculation The surface tension at the liquid/gas interface was measured by the Wilhelmy method using a glass plate. The volume of the electrolytically separated gas was measured in a 50 cm 3 cell by a platinum anode and cathode. The hydrogen ion concentration was measured by a "Radellkis" digital pH meter. The metal ion concentrations were determined on an ICP "SPECTROX=LAME" spectrometer (SPECrRO Analytical Instruments). The residual xanthate concentration was measured by a "SPECORD" spectrophotometer (Carl Zeiss, Jena). The flotation efficiency was estimated from the residual concentration of metal ions.
3. Results and discussion
3.1. Precipitation The efficiency of the process is determined by the efficiency of precipitation of metal ions and by the successful attachment of the insoluble compounds on the
288
L. Alexandrova et al. , Int. J. Mmer. Process. 41 (1994) 2 8 5 - 2 9 4
water/gas interface and their subsequent rise onto the surface. The hydrophobicity of the precipitates is of major importance for their efficient flotation. For this reason xanthates are chosen as suitable reagents, forming chelate complexes with copper ions characterized by high hydrophobicity and low solubility product.
¢.s
2 ROC..s_K
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jCu S
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The efficiency of the precipitation of xanthates which different length of the hydrocarbon radical (i.e. ethyl, propyl, butyl and amyl xanthates ) is studied. The effect of their precipitation is followed using aqueous solutions containing Cu, Zn, Pb and Co ions (50 rag/l) with optimized consumption of potassium alkyl xanthate, which is 50% of the stoichiometrically required consumption for all dissolved ions. After precipitation carried out at pH 8 for 10 rain, the residual concentrations found in the purified water are shown in Table 1. The results of the precipitation experiments do not reveal particular advantages of some of the xanthates tested. For the evaluation of their impact on the technological process of flotation, experiments are carried out with the same model solution. After flotation of the precipitated ions, analytical results are obtained. During the process increasing instability is observed, corresponding to the increase of the length of the hydrocarbon radical chain. In the case of all four xanthates the newly formed foam layer was highly sensitive to the hydrodynamic conditions. This is best expressed with the butyl and amyl metal xanthate precipitates. Except for KEXt, a spontaneous separation in the volume of aggregates from the lower part of the layer is observed with the rise of the thickness of the foam layer. Furthermore the newly formed aggregates in longchained xanthates are of finer dispersity, rendering flotation more difficult. Therefore ethyl xanthate is selected as the most suitable precipitant. In the flotation of the waste water, a potassium oleate (KOL) surfactant is used. After the establishment of the optimum consumption of reagents in flotation, an attempt is made to use KOL as a separate precipitant. The efficiency of the separate precipitation effects of KEXt and KOL is compared to their combined action in Fig. 2. Table 1 Effect of different xanthates on residual concentration of each heavy metal ion Residual concentration of:
Xanthates NaEXt
NaPrXt
KBXt
KAXt
Cu (mg/1) Zn (mg/I) Pb (mg/l) Co (rag/I)
0.043 0.051 0.043 0.038
0.038 0.046 0.041 0.046
0.039 0.047 0.039 0.045
0.043 0.052 0.042 0.048
L. Alexandrova et al. lint, J. Miner. Process. 41 (1994) 285-294
289
From Diasram B it is evident that KOL forms complexes only with the metal ions remaining in solution after precipitation with KEtX (Diagram A). However, during the optimization of the precipitant consumption in the presence of KOL, a 3-fold decrease of KEtX consumption is achieved, accompanied by an increase in the purification ratio of all ions (Diagram C). This is best expressed in the cases of copper, zinc and cadmium ions: their concentrations drop below the detectable limits of the spectrophotometric analysis. It follows that KOL participates in mixed complexes with the xanthate, leading to a considerable lowering of its consumption.
3.2. Flotation
The effectiveness of the precipitate flotation depends on different factors: dispersity and volume of the gaseous phase, type and concentration of surfactants, hydrodynamic conditions in the ceil, pH and ion content in the media. Effective precipitation and adsorbing colloid flotation require the presence of a gaseous phase with sufficiently large area and maximal dispersity due to the sensitivity of precipitated and coprecipitated particles to hydrodynamic condi-
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Fig. 2. Purificationratios of metalscontainedin the studiedwater. (A) Qr~ = 125 mg/1,QKOL=0, (B) QKOL----12.5rag/l, QKe~=0;(C) Qr~= 37.5 rag/l, Q-----KOLI2.5mg/L
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L. Alexandrova et al. /Int. J. Miner. Process. 41 (1994) 285-294
tions. Electroflotation proves appropriate in this case since it combines the electrocoagulation effect and electrolytic gas separation. The size of the bubbles formed depends on the electrode potential, the surfactant concentration and the electrode shape (Frumkin, 1987). It is determined by the difference in the attachment force Fy = n-a'7-sin0 and the buoyant force Fa = V . g . p , where a is the diameter of the circle of bubble attachment to the electrode surface, ~, is the surface tension of the liquid/gas interface, and 0 is the contact angle. The value of the wetting angle depends on the surface tensions and is defined by Neumann's equation: COS0= 7s/g -- 7s/[ }'W g
It follows that the dispersity of the gaseous phase could be increased by the rise of the difference (Ta/g-7s/l).This could be achieved either by increasing of the electrode polarity or by diminishing the surface tension at the liquid/gas interface. The values of 7wg depend on the rate of adsorption of KOL ions on the xanthate precipitates Curve l in Fig. 3 shows the dependence of the changes of the equilibrium values ofyw8 of waste water with precipitated metal ions on the KOL concentration; it is seen that the decrease of ~'l/g starts at KOL concentration of 6 × 10-5 tool/1 i.e. at the completion of the adsorption of oleate ions on the colloid particles. At this concentration saturation with oleate of the monolayer at the water/particle interface is observed while its adsorption on the water/air in-
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Fig. 3. Changeof)~/sdependingon the concentrationof KOL in the wastewater ( 1) and in distilled alkalizodwater (2).
L. Alexandrova et al. ~Int. J. Miner. Process. 41 (1994) 285-294
291
tefface continues, leading to the decrease of ~,. In order to elucidate the mechanisire of the above effects, the change of the surface tension as a function of KOL concentration for distilled alkalised water is studied (see Fig. 2, Curve 2 ). Consequently, by electroflotation of the waste water the size of the gas bubbles can be influenced at KOL concentrations higher than 6 × 10- 5 tool/1. Microscopic studies show that at KOL concentrations below 6 × 10-5 tool/1 the size of the electrolytically separated bubbles remains stable at permanent electrode density of the electric current. The investigations of the effect of KOL on the volume of electrolytically separated gas show that in this case much lower KOL concentrations influence this separation. Numerous studies report the effect of surfactants on electrode processes, (e.g. Srinivasan and De Levie, 1986). At a given concentration of the surfactant, dense condensed monolayers are created on the electrode surface, inhibiting the electrode reactions. As it can be seen in Fig. 4, this concentration is 4 × 10-5 tool/1 in the case of water and above this value the volume of the separated gas decreases abruptly. In the process of flotation KOL acts as a surfactant stabilizing the foam layer, as a collector of the generated by electrocoagulation ferric hydroxides and as a coprecipitant of the nonprecipitated cations in solution. The influence of KOL on the flotation is demonstrated in Fig. 5. Curve 1 shows the percentage of recovery of copper depending on KOL concentration corresponding to the mollar concentrations, and Curve 2 shows the percentage of the yield of foam product as a function of the same KOL consumption. The KEXt consumption is 15% of the stoichiometrically required one for the copper ions present (37.5 rag/l). As seen in Curve 1, the recovery of copper increases with the increase of KOL consump-
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L
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KOL CONCENTRATION[m0(xL"]
Fig. 4. Change of the volume of the released electrolytic gas depending on the concentration of KOL.
292
L. Alexandrova et ai. / Int. J. Miner. Process. 4 l (1994) 285-294
--
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Fig. 5. Influence of KOL on the copper recovery ( 1 ) and the yield of foam product (2).
tion up to 12.5 mg/1, corresponding to 4)< 10 -5 mol/l. The favourable effect of KOL is due to the fact that it acts as a coprecipitant and as foam stabilizing reagent and it is namely this stabilizing action which leads to the increase of the recovery by structuring of the foam layer making it less sensitive to hydrodynamic influences. This improvement of the foam stability is not related to the action of KOL as a surfactant, which is adsorbed on the gas/liquid interface, KOL enhances the adhesion between the xanthate precipitates; this can be deduced from Curve 2 in Fig. 5 showing that up to concentrations of 4)< 10 -5 tool/ I the yield of foam product decreases. This is due to the push out of the hydrate water by the colloid particles during the adsorption of oleate anions on their surface. At concentrations higher than 4)< 10-5 tool/1 the recovery of xanthate precipitates decreases drastically regardless of the continuing adsorption of oleate ions on the surface of the colloid particles up to 6 )< 10-5 mol/1 as it is evident from Curve 1 in Fig. 3. According to Sbeiman (1967) this could be due to two reasons: 1. Competition between the ions of the surfactant and the hydrophobic precipitate for a position on the liquid/gas interface. 2. Manifestation of a solubilizing effect, especially in the case of surfactant tending to micellization. In the present case a competition between KOL ions and the hydrophobic partides is not possible since at concentrations of up to 6)< 10-5 tool/1 KOL is adsorbed on the surface of the colloid particles. Evidently, at concentrations higher than 4)< 10-5 adsorption of a new layer of oleate ions begins, their heads being directed towards the solution. This phenomenon has al ready been mentioned by Bernasconi et al. (1987) for the barium ion flotation with sodium laurylsulfate, by Bernasconi et al. (1988) for the ion flotation of zircconium by sodium alkylsulfates. They showed that the
L. Alexandrova et al. / Int. J. Miner. Process. 41 (1994) 285-294
293
surface properties of the precipitated phase are controlled by adsorption of the collector on its surface: the solid particles are flocculated and float rapidly only when the amount of adsorbed collector is close to the monolayer. As mentioned above in the preliminary study of the effect of xanthates with different length of the hydrocarbon radical, the flotation of the precipitates is carried out most efficiently with ethyl xanthate. The xanthate consumption is optimized in the flotation of the waste water in the presence of KOL. The results in Fig. 6 suggest that the recovery of copper is substantial at a 5% consumption of KEXt (of the stoichiometrically required one for copper ions in the solution) The recovery of Cu increases to about 98% at KEtX consumptions from 10 to 15%. The further increase of the KEXt consumption does not affect the recovery. Following the determination of the optimal concentrations of the basic reagents, experiments are carried out on a laboratory equipment with continous operation. The process parameters relative productivity and alkalinity of the medium are optimized (Miloshev and Nishkov, 1991 ). As a result of our studies an the flotation of waste water a foam product yield is obtained of approximately 5% by volume. Its high structural resilience makes it suitable for foam fractionation, decreasing the yield to 1-2% by volume. The dry residue from the foam product is about 0.06%, its copper content being 8%. In the established technological regime the contents of manganese, zink, cobalt
100
90
80 >n~ (D
i~:
7o
y
60
50
I
I
I
I
I
I
i
5
10
15
20
~5
30
35
KEtX CONSUMPTION [%]
Fig. 6. Influence of the consumption of KEXt on the copper recovery.
294
L. Alexandrova et aL ~Int. J. Miner. Process. 41 (1994) 285-294
and iron in the waste water are reduced to 0.14 mg/l; 0.09 mg/l; 0.06 mg/l and 0.24 mg/l, respectively.
4. Conclusions 1. By combining the precipitate and the adsorbing colloid mechanisms of ion flotation, an effective removal of the ions present in waste water is achieved. 2. The use of precipitants such as xanthates, forming chelate complexes with the metal ions with low solubility and high hydrophobicity, contributes not only to the efficient precipitation of the metal ions but also to their flotation. 3. The electrolytic process is used both for the high saturation of the solution with finely dispersed bubbles and for the electrolytic provision with ferric ions, their hydroxides possessing strong adsorptive ability. 4. A cation-active surfactant - - potassium oleate is used for the improvement of precipitation, for enlarging the number of precipitated ions, for removing of colloid carriers and for enhancing the foam formation. Acknowledgements Financial support from the National Fondation "Scientific Investigations" is gratefully acknowledged.
References Basak, S. and Charewich, W., i 986. Flotation of metal hydroxide precipitates, l. Chem. Technol. Biotechnol., 36(2): 74-78. Bernasconi, P., Poirier, J.E., Bouzat, G., Blazy, P., Bessiere, J. and Durant, R.R., 1987. Barium ion flotation with sodium laurylsulfate, I. Mechanisms controlling the extraction process. Int. J. Miner. Process., 21: 25-44. Bernasconi, P., Poirier, J.E., Bouzat, G., Blazy, P., Bessiere, J. and Durant, R.R., 1988. Zirconium ion flotation with long-chain alkylsulfates from nitric and uranyl nitrate solutions. Int. J. Miner. Process., 23: 293-310. Clarke, A.N. and Wilson, D.I., 1983. Foam Flotation. Theory and Applications. Marcel Dekker, New York. Frumkin, A.N., 1987. Electrode Processes. Nauka, Moscow, 171 pp. Golman, A.M., 1989. Ion Flotation. Nedra, Moscow, 135 pp. Matsuzaki, C. and Zeitlin, H. 1973. Separation of collectors used as coprecipitants of trace elements in sea water by adsorption colloid flotation. Separ. Sci., 8:185-192. Miloshev, S. and Nishkov, I. 1991. Flotation removal of heavy metal ions from mining waters. Enviromental controls in metallurgical industries and scrap metal recycling. In: M.E. Chalkley and A.J. Oliver (Editors), 30th Annual Conference of Metallurgists. CIM, p. 243-251. Rubin, A.I. and Johnson, I.D., 1967. Effect of pH on ion and precipitate flotation systems. Anal. Chem., 39: 298-302. Sebba, F., 1962. Ion Flotation. Elsevier, New York, 172 pp. Sheiman, I., 1967. Precipitate flotation. Ph.D. Thesis, Univ. of the Witwatersrand, Johannesburg. Srinivasan, R. and De Levie, R., 1986. Condensed thymine films at the mercury/water interface. Part II. Effects on electrode kinetics. J. Electroanal. Chem., 201: 145-152.
inmgllm illUlttnJglU; ELSEVIER
Int. J. Miner. Process. 41 (1994) 285-294
Electroflotation of metal ions in waste water L. A l e x a n d r o v a , T. N e d i a l k o v a , I. N i s h k o v CentralLaboratory of Mineral Processing, BulgarianAcademy of Sciences, Sofia 1126, P.O. Box 32, Bulgaria (Received 18 February 1992; accepted after revision 26 November i 993)
Abstract
The possibility of removal of metal ions from mining waste waters through ion flotation is investigated. Waste water of an opencast mine containing about 50 mg/l copper ions is purified. The ions undergo precipitation with xanthates forming chelate complexes with high hydrophobicity. Electroflotation is used for the obtaining of a gaseous phase with sufficient volume and high dispcrsity. Conditions are established for the realization of the precipitation and adsorbing colloid mechanism of flotation. An active potassium oleate surfactant is used for the stabilization of foam formation and for the improvement of the precipitation with the main precipitant. Removal of the metal ions present in the waste water is achieved.
1. Introduction The removal of suspended or dissolved substances is based on their surface activity, possessed or acquired as a result of their interaction with suitable reagents (Sebba, 1962). Depending On the affinity of the substances in a soluble or insoluble form to the liquid/gas surface Golman (1989) distinguishes between adsorption and adhesion mechanisms of flotation. From a kinetic point of view the adhesion mechanism is much faster and for this reason during removal of ions from solution they are precipitated with suitable complex formers before flotation. An important aspect in the complex formation from the standpoint of ion flotation is that it can change the sign of the extractable ion. Clarke and Wilson ( 1983 ) have evaluated the adhesion method of flotation and described two ways for implementing it - - the precipitate flotation and the adsorbing colloid one. The efficiency of the precipitate flotation depends on the hydrophobicity of the precipitates, which in turn depends on the nature of the precipitant and the surfactant. 0301-7516/94/$07.00 © 1994 Elsevier Science B.V. All fights reserved
SSDI0301-7516(94)00012-0
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L, Alexandrova ct ai. , Int. J. Miner. Process. 41 (1994) 285- 2~J4
The adsorbing colloid mechanism is particularly suitable for the removal of ions at very low concentrations (Rubin and Johnson, 1967; Matsuzaki and Zeitlin, 1973 ). The choice of a suitable coprecipitant with a cute sorbing activity and the usage of a suitable surfactant are of prime importance in this case. Ferrous and aluminium hydroxides are the most frequently used coprecipitants in adsorbing colloid flotation. For the flotation of ferric hydroxide, Basak and Charewich (1986) used sodium dodecyl sulphate (SDS) and potasium oleate. This brief review shows the scopes ofaplication of the precipitate flotation and of the adsorbing colloid as well as the main factor for their efficiency, i.e. the choice of suitable reagents and optimum concentrations. The combination of the precipitate mechanism and the adsorbing colloid one makes the process universal and efficient in broad concentration ranges. The present paper describes a combined scheme of precipitate - - adsorbing colloid flotation of waste water by separation of the main ions as precipitate complexes with high hydrophobicity and electrolytic generation of coprecipitants as carriers of nonprecipitated ions.
2. Experimental 2.1. Materials Waste water from an opencast copper mine, containing metal ions in mg/1 as follows: Cu - - 50.00; Zn - - 1.65; Co - - 0.75; Pb - - 0.15 etc. with pH 4.5 was subjected to precipitation and flotation. For the selection of a suitable precipitant, a model solution containing Cu, Zn, Pb and Co ions (50 mg/1 each, pH 4.5 ) was prepared by dissolution of the respective nitrates. Solutions used: 1% solutions of sodium ethyl xanthate (NaEXt), sodium propylxanthate (NaPrXt), potassium butylxanthate (KBXt) and potassium amylxanthate (KAXt) (analytical grade); 10 % solution of sodium hydroxide (reagent grade) as well as 0.1% solution of potassium oleate (KOL) (analytical grade). 2.2. Methods Precipitation was optimized in 2000 cm 3 of water, with pH preset to 8. Agitation was carried out by a magnetic stirrer while the efficiency of the precipitation of the different xanthates and potassium oleate was studied and optimized by measuring the residual concentration of metal ions. Flotation tests were carried out in a 2000 cm 3 electrocoagulation flotation cell included in a continuous regime installation - - Fig. 1. The flotation time was 5 minutes. After the waste water was treated and agitated it was fed to the flotation cell. The latter consists of an electrode and an electrode-free zone containing two chambers each. The first chamber consists of lamellar iron electrodes providing ferric ions. The electrode-free zone provides favourable hydrodynamic conditions for the flotation of the nonfloculated finely dispersed particles. Due to their
L. Alexandrova et al. ~Int. J. Miner. Process. 41 (1994) 285-294
2
/ ~
ao [ ]
4
287
~ f
f
1
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F i g 1 Scheme o f t h e flotation installation; 1 - - b a s i n for waste w a t e r 2 rectifier 3 p H meter, 4 - - mixer, 5 - - peristaltic p u m p ; 6 - - electrode; 7 - - agitation vessel; 8 - - outlet o f t h e cell for clear water; 9 - - vessel for t h e f o a m product; 10 - - electrodes ( c a t h o d e a n d a n o d e ) ; 11 - - outlet for the f o a m product; 12 - - electrodes (cathode a n d a n o d e ) ; 13 - - flotation cell; 14 - - test filters;
small volume the foam product is highly watered which requires its recirculation The surface tension at the liquid/gas interface was measured by the Wilhelmy method using a glass plate. The volume of the electrolytically separated gas was measured in a 50 cm 3 cell by a platinum anode and cathode. The hydrogen ion concentration was measured by a "Radellkis" digital pH meter. The metal ion concentrations were determined on an ICP "SPECTROX=LAME" spectrometer (SPECrRO Analytical Instruments). The residual xanthate concentration was measured by a "SPECORD" spectrophotometer (Carl Zeiss, Jena). The flotation efficiency was estimated from the residual concentration of metal ions.
3. Results and discussion
3.1. Precipitation The efficiency of the process is determined by the efficiency of precipitation of metal ions and by the successful attachment of the insoluble compounds on the
288
L. Alexandrova et al. , Int. J. Mmer. Process. 41 (1994) 2 8 5 - 2 9 4
water/gas interface and their subsequent rise onto the surface. The hydrophobicity of the precipitates is of major importance for their efficient flotation. For this reason xanthates are chosen as suitable reagents, forming chelate complexes with copper ions characterized by high hydrophobicity and low solubility product.
¢.s
2 ROC..s_K
+ Cu 2+ ~
ROC
jCu S
COR
(1)
"S
The efficiency of the precipitation of xanthates which different length of the hydrocarbon radical (i.e. ethyl, propyl, butyl and amyl xanthates ) is studied. The effect of their precipitation is followed using aqueous solutions containing Cu, Zn, Pb and Co ions (50 rag/l) with optimized consumption of potassium alkyl xanthate, which is 50% of the stoichiometrically required consumption for all dissolved ions. After precipitation carried out at pH 8 for 10 rain, the residual concentrations found in the purified water are shown in Table 1. The results of the precipitation experiments do not reveal particular advantages of some of the xanthates tested. For the evaluation of their impact on the technological process of flotation, experiments are carried out with the same model solution. After flotation of the precipitated ions, analytical results are obtained. During the process increasing instability is observed, corresponding to the increase of the length of the hydrocarbon radical chain. In the case of all four xanthates the newly formed foam layer was highly sensitive to the hydrodynamic conditions. This is best expressed with the butyl and amyl metal xanthate precipitates. Except for KEXt, a spontaneous separation in the volume of aggregates from the lower part of the layer is observed with the rise of the thickness of the foam layer. Furthermore the newly formed aggregates in longchained xanthates are of finer dispersity, rendering flotation more difficult. Therefore ethyl xanthate is selected as the most suitable precipitant. In the flotation of the waste water, a potassium oleate (KOL) surfactant is used. After the establishment of the optimum consumption of reagents in flotation, an attempt is made to use KOL as a separate precipitant. The efficiency of the separate precipitation effects of KEXt and KOL is compared to their combined action in Fig. 2. Table 1 Effect of different xanthates on residual concentration of each heavy metal ion Residual concentration of:
Xanthates NaEXt
NaPrXt
KBXt
KAXt
Cu (mg/1) Zn (mg/I) Pb (mg/l) Co (rag/I)
0.043 0.051 0.043 0.038
0.038 0.046 0.041 0.046
0.039 0.047 0.039 0.045
0.043 0.052 0.042 0.048
L. Alexandrova et al. lint, J. Miner. Process. 41 (1994) 285-294
289
From Diasram B it is evident that KOL forms complexes only with the metal ions remaining in solution after precipitation with KEtX (Diagram A). However, during the optimization of the precipitant consumption in the presence of KOL, a 3-fold decrease of KEtX consumption is achieved, accompanied by an increase in the purification ratio of all ions (Diagram C). This is best expressed in the cases of copper, zinc and cadmium ions: their concentrations drop below the detectable limits of the spectrophotometric analysis. It follows that KOL participates in mixed complexes with the xanthate, leading to a considerable lowering of its consumption.
3.2. Flotation
The effectiveness of the precipitate flotation depends on different factors: dispersity and volume of the gaseous phase, type and concentration of surfactants, hydrodynamic conditions in the ceil, pH and ion content in the media. Effective precipitation and adsorbing colloid flotation require the presence of a gaseous phase with sufficiently large area and maximal dispersity due to the sensitivity of precipitated and coprecipitated particles to hydrodynamic condi-
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Fig. 2. Purificationratios of metalscontainedin the studiedwater. (A) Qr~ = 125 mg/1,QKOL=0, (B) QKOL----12.5rag/l, QKe~=0;(C) Qr~= 37.5 rag/l, Q-----KOLI2.5mg/L
290
L. Alexandrova et al. /Int. J. Miner. Process. 41 (1994) 285-294
tions. Electroflotation proves appropriate in this case since it combines the electrocoagulation effect and electrolytic gas separation. The size of the bubbles formed depends on the electrode potential, the surfactant concentration and the electrode shape (Frumkin, 1987). It is determined by the difference in the attachment force Fy = n-a'7-sin0 and the buoyant force Fa = V . g . p , where a is the diameter of the circle of bubble attachment to the electrode surface, ~, is the surface tension of the liquid/gas interface, and 0 is the contact angle. The value of the wetting angle depends on the surface tensions and is defined by Neumann's equation: COS0= 7s/g -- 7s/[ }'W g
It follows that the dispersity of the gaseous phase could be increased by the rise of the difference (Ta/g-7s/l).This could be achieved either by increasing of the electrode polarity or by diminishing the surface tension at the liquid/gas interface. The values of 7wg depend on the rate of adsorption of KOL ions on the xanthate precipitates Curve l in Fig. 3 shows the dependence of the changes of the equilibrium values ofyw8 of waste water with precipitated metal ions on the KOL concentration; it is seen that the decrease of ~'l/g starts at KOL concentration of 6 × 10-5 tool/1 i.e. at the completion of the adsorption of oleate ions on the colloid particles. At this concentration saturation with oleate of the monolayer at the water/particle interface is observed while its adsorption on the water/air in-
J
Bo ~-
T
E x
70 ~ 60
g ~ so i.--
un
30
) i0-~ KOL
CONCENTRATION
I0-~ [molxl-' I
Fig. 3. Changeof)~/sdependingon the concentrationof KOL in the wastewater ( 1) and in distilled alkalizodwater (2).
L. Alexandrova et al. ~Int. J. Miner. Process. 41 (1994) 285-294
291
tefface continues, leading to the decrease of ~,. In order to elucidate the mechanisire of the above effects, the change of the surface tension as a function of KOL concentration for distilled alkalised water is studied (see Fig. 2, Curve 2 ). Consequently, by electroflotation of the waste water the size of the gas bubbles can be influenced at KOL concentrations higher than 6 × 10- 5 tool/1. Microscopic studies show that at KOL concentrations below 6 × 10-5 tool/1 the size of the electrolytically separated bubbles remains stable at permanent electrode density of the electric current. The investigations of the effect of KOL on the volume of electrolytically separated gas show that in this case much lower KOL concentrations influence this separation. Numerous studies report the effect of surfactants on electrode processes, (e.g. Srinivasan and De Levie, 1986). At a given concentration of the surfactant, dense condensed monolayers are created on the electrode surface, inhibiting the electrode reactions. As it can be seen in Fig. 4, this concentration is 4 × 10-5 tool/1 in the case of water and above this value the volume of the separated gas decreases abruptly. In the process of flotation KOL acts as a surfactant stabilizing the foam layer, as a collector of the generated by electrocoagulation ferric hydroxides and as a coprecipitant of the nonprecipitated cations in solution. The influence of KOL on the flotation is demonstrated in Fig. 5. Curve 1 shows the percentage of recovery of copper depending on KOL concentration corresponding to the mollar concentrations, and Curve 2 shows the percentage of the yield of foam product as a function of the same KOL consumption. The KEXt consumption is 15% of the stoichiometrically required one for the copper ions present (37.5 rag/l). As seen in Curve 1, the recovery of copper increases with the increase of KOL consump-
~,,0
>3.5
uJ -J i~J 3.0 u.i
O >
10-s
I
L
10-.~
10-~
KOL CONCENTRATION[m0(xL"]
Fig. 4. Change of the volume of the released electrolytic gas depending on the concentration of KOL.
292
L. Alexandrova et ai. / Int. J. Miner. Process. 4 l (1994) 285-294
--
99°t ......................
I~so
97.o)
T I'° ~
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~- ~o t
\/
~
i
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94o +
so •
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3.0
92.0 ~
1,0
T 0
I
I
1~10 4 KOL
2x10'
CONCENTRATION
I 3×10 ~j
I 4x10 -f
5xt0 "~I
[molxL"]
Fig. 5. Influence of KOL on the copper recovery ( 1 ) and the yield of foam product (2).
tion up to 12.5 mg/1, corresponding to 4)< 10 -5 mol/l. The favourable effect of KOL is due to the fact that it acts as a coprecipitant and as foam stabilizing reagent and it is namely this stabilizing action which leads to the increase of the recovery by structuring of the foam layer making it less sensitive to hydrodynamic influences. This improvement of the foam stability is not related to the action of KOL as a surfactant, which is adsorbed on the gas/liquid interface, KOL enhances the adhesion between the xanthate precipitates; this can be deduced from Curve 2 in Fig. 5 showing that up to concentrations of 4)< 10 -5 tool/ I the yield of foam product decreases. This is due to the push out of the hydrate water by the colloid particles during the adsorption of oleate anions on their surface. At concentrations higher than 4)< 10-5 tool/1 the recovery of xanthate precipitates decreases drastically regardless of the continuing adsorption of oleate ions on the surface of the colloid particles up to 6 )< 10-5 mol/1 as it is evident from Curve 1 in Fig. 3. According to Sbeiman (1967) this could be due to two reasons: 1. Competition between the ions of the surfactant and the hydrophobic precipitate for a position on the liquid/gas interface. 2. Manifestation of a solubilizing effect, especially in the case of surfactant tending to micellization. In the present case a competition between KOL ions and the hydrophobic partides is not possible since at concentrations of up to 6)< 10-5 tool/1 KOL is adsorbed on the surface of the colloid particles. Evidently, at concentrations higher than 4)< 10-5 adsorption of a new layer of oleate ions begins, their heads being directed towards the solution. This phenomenon has al ready been mentioned by Bernasconi et al. (1987) for the barium ion flotation with sodium laurylsulfate, by Bernasconi et al. (1988) for the ion flotation of zircconium by sodium alkylsulfates. They showed that the
L. Alexandrova et al. / Int. J. Miner. Process. 41 (1994) 285-294
293
surface properties of the precipitated phase are controlled by adsorption of the collector on its surface: the solid particles are flocculated and float rapidly only when the amount of adsorbed collector is close to the monolayer. As mentioned above in the preliminary study of the effect of xanthates with different length of the hydrocarbon radical, the flotation of the precipitates is carried out most efficiently with ethyl xanthate. The xanthate consumption is optimized in the flotation of the waste water in the presence of KOL. The results in Fig. 6 suggest that the recovery of copper is substantial at a 5% consumption of KEXt (of the stoichiometrically required one for copper ions in the solution) The recovery of Cu increases to about 98% at KEtX consumptions from 10 to 15%. The further increase of the KEXt consumption does not affect the recovery. Following the determination of the optimal concentrations of the basic reagents, experiments are carried out on a laboratory equipment with continous operation. The process parameters relative productivity and alkalinity of the medium are optimized (Miloshev and Nishkov, 1991 ). As a result of our studies an the flotation of waste water a foam product yield is obtained of approximately 5% by volume. Its high structural resilience makes it suitable for foam fractionation, decreasing the yield to 1-2% by volume. The dry residue from the foam product is about 0.06%, its copper content being 8%. In the established technological regime the contents of manganese, zink, cobalt
100
90
80 >n~ (D
i~:
7o
y
60
50
I
I
I
I
I
I
i
5
10
15
20
~5
30
35
KEtX CONSUMPTION [%]
Fig. 6. Influence of the consumption of KEXt on the copper recovery.
294
L. Alexandrova et aL ~Int. J. Miner. Process. 41 (1994) 285-294
and iron in the waste water are reduced to 0.14 mg/l; 0.09 mg/l; 0.06 mg/l and 0.24 mg/l, respectively.
4. Conclusions 1. By combining the precipitate and the adsorbing colloid mechanisms of ion flotation, an effective removal of the ions present in waste water is achieved. 2. The use of precipitants such as xanthates, forming chelate complexes with the metal ions with low solubility and high hydrophobicity, contributes not only to the efficient precipitation of the metal ions but also to their flotation. 3. The electrolytic process is used both for the high saturation of the solution with finely dispersed bubbles and for the electrolytic provision with ferric ions, their hydroxides possessing strong adsorptive ability. 4. A cation-active surfactant - - potassium oleate is used for the improvement of precipitation, for enlarging the number of precipitated ions, for removing of colloid carriers and for enhancing the foam formation. Acknowledgements Financial support from the National Fondation "Scientific Investigations" is gratefully acknowledged.
References Basak, S. and Charewich, W., i 986. Flotation of metal hydroxide precipitates, l. Chem. Technol. Biotechnol., 36(2): 74-78. Bernasconi, P., Poirier, J.E., Bouzat, G., Blazy, P., Bessiere, J. and Durant, R.R., 1987. Barium ion flotation with sodium laurylsulfate, I. Mechanisms controlling the extraction process. Int. J. Miner. Process., 21: 25-44. Bernasconi, P., Poirier, J.E., Bouzat, G., Blazy, P., Bessiere, J. and Durant, R.R., 1988. Zirconium ion flotation with long-chain alkylsulfates from nitric and uranyl nitrate solutions. Int. J. Miner. Process., 23: 293-310. Clarke, A.N. and Wilson, D.I., 1983. Foam Flotation. Theory and Applications. Marcel Dekker, New York. Frumkin, A.N., 1987. Electrode Processes. Nauka, Moscow, 171 pp. Golman, A.M., 1989. Ion Flotation. Nedra, Moscow, 135 pp. Matsuzaki, C. and Zeitlin, H. 1973. Separation of collectors used as coprecipitants of trace elements in sea water by adsorption colloid flotation. Separ. Sci., 8:185-192. Miloshev, S. and Nishkov, I. 1991. Flotation removal of heavy metal ions from mining waters. Enviromental controls in metallurgical industries and scrap metal recycling. In: M.E. Chalkley and A.J. Oliver (Editors), 30th Annual Conference of Metallurgists. CIM, p. 243-251. Rubin, A.I. and Johnson, I.D., 1967. Effect of pH on ion and precipitate flotation systems. Anal. Chem., 39: 298-302. Sebba, F., 1962. Ion Flotation. Elsevier, New York, 172 pp. Sheiman, I., 1967. Precipitate flotation. Ph.D. Thesis, Univ. of the Witwatersrand, Johannesburg. Srinivasan, R. and De Levie, R., 1986. Condensed thymine films at the mercury/water interface. Part II. Effects on electrode kinetics. J. Electroanal. Chem., 201: 145-152.