removal of cadmium from a liquid effluent by ion flotation

Pergamon 0892-6875(99) 00077--1

Minerals Engineering, Vol. 12, No. 8, pp. 905-917, 1999 © 1999 Elsevier Science Ltd

All rights reserved 0892-6875/99/$ - see front matter

REMOVAL OF CADMIUM FROM A LIQUID EFFLUENT BY ION FLOTATION

I.B. SCORZELLI, A.L. FRAGOMENI and M.L. TOREM*

Department of Science Materials and Metallurgy/Catholic University, Rua Marquis de S~o Vicente, 225-G~vea, 22453.900 Rio de Janeiro, Brazil. E-mail:[email protected]

* Author for correspondence (Received 19 November 1998; accepted 3 March 1999)

ABSTRACT

The removal of Cd using sodium dodecylsulfate (SDS) as collector was studied by ion flotation at laboratory scale. The effect of frothers (iso-propanol and methyl-&obutyl-carbinol (MIBC) and ionic strength (NaCI and Na2S04) were also studied, as well as characterization of the sublate by scanning electron microscopy (SEM) and the surface tension of the initial solutions. In the presence of SDS, the maximum recovery obtained at a stoichiometric metal to collector ratio of 1:3 was 99.1%, however a large volume of wet foam was produced. The best recovery (89.2%) with a dry foam was obtained at a stoichiometric ratio of 1:2. The introduction of frothers (iso-propanol and MIBC) in the system produced the highest recovery of Cd at a concentration of O.1% v/v, where the flotability was 98.8% and 97.7%for iso-propanol and MIBC respectively. An increase in magnitude of ionic strength from 4.7 x 10 -4 moles.dm -3 to 4.7 x 10 -1 moles.dm -3 significantly decreased Cd removal. Surface tension testwork indicates a decrease in flotability of Cd as surface tension drops. The SEM/EDS studies showed that the morphology of the sublate and the foam depends on the physico-chemical conditions of the system. © 1999 Elsevier Science Ltd. All rights reserved.

Keywords Flotation froths; environmental; pollution

INTRODUCTION

The growing need for development of new methods in chemical technology represents a major challenge facing mineral, metallurgical and material processing industries in the 1990's [1, 2]. The implementation of stricter waste treatment regulations constrain industries to lower the limits of allowable discharges, so that they often must treat large volumes of effluents that contain very dilute concentration of toxic metals [3].

One of the methods frequently used for cadmium separation from solutions is chemical precipitation as hydroxide, sulfide or as insoluble salts (combined with flocculation). This process presents a difficult task with respect to the removal of the generated sludges. Attempts have been made to avoid expensive filtration

Presented at Minerals Engineering "98, Edinburgh, Scotland, September 1998

905

906 I.B. Scorzelli et aL

required by this technique. Other methods such as solvent extraction, ion exchange and electrolysis have been shown to be applicable for removing metal ions from relatively concentrated waste solutions. Unfortunately, all of these techniques, including chemical precipitation, are inappropriate for large solution volumes with very low concentrations of metal ions [1, 4].

Such circumstances have revived interest in a technique known as ion flotation, which is basically one of several foam separation techniques. These methods have several advantages for treating diluted wastes: low energy requirements, low residual concentration of metals, rapid operation, small space requirements, flexibility in applying the method to a variety of metals at various levels, and production of small volumes of sludge that are highly enriched with the contaminant. All of these factors reduce the cost of processing [5].

Although foam separation techniques have a great number of advantages, the separation efficiency depends very much on ionic strength. Generally the efficiency decreases with an increase in this parameter. Because industrial wastewater is a complex mixture, this drawback is the main reason holding back their wider applications [5].

It is also important to depict that several studies on ion flotation of metals have been done with the main goal on the recovery of the metallic species in particular physico-chemical conditions [1,2,3,4,6]. There are many studies on ionic strength, nevertheless the researchers still have not clear understand what happens during the process. The aim of this work is to study the effect of collector (SDS), frothers (iso-propanol and MIBC) and ionic strength (NaCI and Na2SO4) with complementary analysis of surface tension of the initial solutions, morphology and the chemical composition of sublate and foam by SEM/EDS analysis and distribution diagrams of Cd-C1 and Cd-SO 4 systems to understand what occurs during ion flotation and bringing novel information to knowledge of basic fundamentals of Cd ion flotation.

BASIC PRINCIPLES

The technique of ion flotation provides a simple physical method for removing and concentrating the ions present in very dilute solutions. The method is classified as a foam separation process, which relies on the direct interaction between an ionic surfactant and an oppositely charged metal ion (either simple or complex) [7]. If the ions to be removed are not surface active, they can be made so through union with or adsorption of surface-active agents [8]. With the aid of suitable surfactants the metal ions are rendered hydrophobic giving origin to metal-collector complexes known as "sublate". When rising bubbles are introduced into the system, the sublate adsorbs on the bubble-water interface and are carried upwards producing a foam layer at the top of the bulk solution. This layer can then be physically separated from the bulk solution [6, 8].

Precipitate and adsorbing colloid flotation are processes that, together with ion flotation, possess some distinct advantages for treating large volumes of dilute waste solutions. These three processes are collectively termed foam extraction. Precipitation flotation requires precipitation of the metal-bearing species in preparation for subsequent flotation. Adsorbing colloid flotation involves removal of metal ions by adsorption onto, or co-precipitation with, a carrier floc (the most commonly used are Fe(OH) 3 and AI(OH)3). The precipitate formed is then floated [1,9]. Ion flotation is generally the most selective among the three, while the other two are characterized by significantly increased kinetics.

Many physical and chemical factors influence flotation such as: type and concentration of collector and frother, ionic strength, pH, bubble size, processing of the froth (or foam) layer, hydration of surfaces, etc. Since these factors are interrelated, flotation is quite difficult to model.

Effect of collector type and concentration

The key to flotability (or nonflotability) of chemical species is hydrophobicity. Substances are rendered hydrophobic by addition of the appropriate collector, in which the polar groups are eliminated by reaction

Removal of cadmium from a liquid effluent by ion flotation 907

(or adsorption) leaving non-polar groups exposed to solution. These groups can now be adsorbed on the gas/liquid interface [ 10].

An important factor relates to the ratio of collector to metal ion. The quantity of collector used in ion flotation should be at least stoichiometric. 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 to higher cost, but also because of other negative effects, such as large foam losses, micelle or hemi-micelle formation, competition between the metal-collector complex and free collector ions for bubble surface sites and the potential toxicity of residuals amounts of collector in the effluent.

Effect of frother type and concentration

The frother aids flotation by adsorbing onto the gas-liquid interface, thereby reducing its surface tension which results in stable bubbles [I 1, 12]. The creation of a finer bubble size distribution through changes in dynamic surface tension is also a very important result of frother addition. (11) However, similar to collector chemicals, higher concentrations of alcohol-based frothers are deleterious.

Effect of ionic strength

The presence of neutral salts in solution reduces the efficiency of ion flotation, because colligend ions must have to compete for collector with other ions in solution [11].

Ions with opposite charge to the collector have a stronger effect than do ones with the same electrical charge. Moreover, the higher the charge of these ions, the greater the contribution to overall ionic strength of the solution. The order of interference is: trivalent ions > divalent ions > monovalent ions [11].

MATERIALS AND REAGENTS

Experimental apparatus

The ion flotation tests were carried out in a 95 cm high acrylic column cell with internal diameter of 5.7 cm (Figure 1). Bubbles were generated by sparging air through a sintered glass frit (10-15 [am, porosity 4) at a controlled flowrate of 2.02 ml.s -t. Each experiment used an initial solution volume of 1 liter, filling approximately half the column.

smlcm

ouMlt

Fig. 1 Column flotation.

908 I.B. Scorzelli et al.

Reagents

The following reagents were used: cadmium sulfate (CdSO4.8/3H20) from Riedel de Harn, sodium dodecylsulfate (SDS: ClzH25NaO4 S) from Riedel de Harn as collector; iso-propanol (C3H80) from Vetec and methyl-isobutyl-carbinol (MIBC - (CH3)2CHCHzCHOHCH3) from Carlo Erba as frothers; sodium chloride (NaCI) and sodium sulfate (NazSO 4 ) from Vetec to alter the ionic strength; caustic soda (NaOH) and sulfuric acid to adjust solution pH.

Methods

All solutions contained 20 mg/L Cd 2÷. Each test solution was prepared by combining the required amount of metal salt stock solution, SDS stock solution, and the necessary frother or sodium salt stock solution with distilled water to make up 1 liter of solution. All tests were carried out at a pH range between 4.0-5.0, where the main species in solution is Cd 2÷.

The pH of the solution was adjusted and the solution was stirred for approximately 15 minutes to ensure consistent mixing of all reagents. The solution was then introduced into the column with the air flowing through the sparger. Each test was run for two hours with foam and solution samples taken at 0, 5, 15, 30, 60, 90 and 120 minutes. The metal content of the samples taken from the bulk solution was determined by atomic absorption spectroscopy (AAS). The metal removal at each time interval was calculated based upon the metal content of the initial solution. The initial solution was also taken for surface tension measurements using the du Notiy ring method. After two hours, the sublate contained in the foam was collected with a glass stick and placed on an aluminum plate in a dryer following which it was metalised with gold for SEM/EDS characterization.

Typical ion flotation test

Bubble generation began just after the solution was introduced into the column, filling it with bubbles. The movement of the bubbles upward was extremely turbulent. Immediately, a foam layer formed at the top of the solution, creating a distinct interface. The height of this interface tended to move regularly during the experiment, which can be attributed to cycles of foam generation and liquid drainage. During the first 30 minutes of the experiment, the upper part of the column was filled with a wet foam. As the test progressed, the foam began to dry out, through drainage and evaporation, especially at the very top. As the foam broke down, a solid phase precipitated.

RESULTS AND DISCUSSION

In this study, the experiments were carried out on a long flotation time basis to reach the maximum recovery of cadmium; for industrial application, this time can be reduced modifying the size of the equipment and the conditions of the process like air flow rate and sparger porosity.

Figure 2 shows the effect of the concentration of SDS on ion flotation of cadmium. It can be seen that with an increase in concentration of SDS, the removal of cadmium increases. The maximum recovery occurred at metal:collector ratio of 1:3 (99.1%), but, in this case, a large volume (250 ml) of wet foam was produced. A recovery of 89.2% was obtained for a metal:collector ratio of 1:2 in which a dry foam (2 ml of liquid when the foam collapsed) was produced indicating that the metal was concentrated in the foam.


100.

~ 8 0 - ~., e> e0-

20-

O: - A - j ;3

I I I I

30 . 60 90 120

time

Fig.2 Effect of the SDS concentration on the flotability of Cd. Air flow rate: 2 ml.s-~; pH= 4.0-5,0; metal:collector ratio of 1:2.

The results obtained in Figure 2 are in accord with the literature [11, 13], in that the maximum recovery of a particular metal ion is related to the amount of collector added and the amount of foam produced. It can be verified that a small excess of collector (metal:collector ratio 1:2) increases the flotability of cadmium and produces a dry foam.

Figure 3 presents the morphology of the foam in the system Cd-SDS. In this case, the foam collapsed just after its removal from the column cell, leaving only the sublate and a SDS film in an aluminum plate. The sublate presented an acicular form containing Cd and S which indicates the formation of cadmium dodecylsulfate.

Fig:3 Photomicrography of the sublate in the system Cd/SDS (metal:collector ratio of 1:2).

Figures 4 and 5 show the effect of using iso-propanol and MIBC, respectively, as the frother at a metal:collector ratio of 1:2. An increase in frother concentration significantly affects the removal of cadmium. The highest removal, 98.8% for iso-propanol and 97.7% for MIBC, was obtained using a concentration of 0 .1% v/v. However an increase in the concentration of frother from 0.1% v/v to 1.0% v/v reduced the recovery of cadmium from 89.2% without frother to 67.0% for iso-propanol and 76.1%


for MIBC. However, note the significant improvement in the rate of flotation with iso-propanol as compared to MIBC (91% versus 52% at a concentration of 0.1% v/v after only 30 minutes).

Fig.4

100"

"-" 8 0 -

V

>., 6 0 - 0 pH = 4,0 - 5,0

air flow: 2 ml/s I Z Cd/SOS - 1:2

Iso-propanoh - - = - 0 , 0 % v/v - - e - - 0,1% v/v _ A _ 0,5% v N

, , - - v - 1 0% vN

/o ' 0 30 90 120

time (min)

Effect of iso-propanol concentration on the flotability of Cd. Air flow rate: 2 ml.s-'; pH= 4.0-5.0; metal:collector ratio--1:2.

2o4O O T - -

> 0 (3 G)

100 ̧

80 ¸

60,

40.

20, --&-- 0,5% v/v - - v - - 1,0% v/v

0 T f / I 1

0 30 60 90 120

CeSDS-12 MIBC: - - " - - 0,0 % v/v - -e - - 0,1% vN

t i m e ( m i n )

Fig.5 Effect of MIBC concentration on the flotability of Cd. Air flow rate: 2 ml.s-'; pH= 4.0-5.0, metahcollector ratio of 1:2.

Figure 6 shows the surface tension of the initial solutions in the system Cd/SDS/frother. It can be observed that the surface tension of the solution decreases with an increase in frother concentration. This can be attributed to the adsorption of frothers species at the liquid-gas interface. On can observe that the chain structure of the frother plays an important role in surface tension. As isopropanol is an aliphatic alcohol it promotes a steeper decrease in surface tension for lower concentrations. Nevertheless, this effect is counterbalanced during the increasing of MIBC concentration.


E v

6 0 .

5 0 ¸

4 0

3 o I I I I I

0,0 0,2 0,4 0,6 0,8 1,0

conc.frothers (% v/v)

Fig.6 Surface tension of the initial solution in the system Cd/SDS/frothers.

Duyvesteyn [11] has shown that frother type and concentration have a complex effect on the rate of metal removal and the maximum metal removal. There is a combination of effects that the alcohol has on the formation of metal-collector complexes, the solubility of these complexes, the competition between alcohol molecules and metal-collector complexes for sites on the bubble surfaces, and the coalescence of the bubbles as they are produced at the frit.

In addition, even at low concentrations of alcohol, the metal-collector complex may be stabilized by alcohol molecules through "hydrophobic interaction" with the hydrocarbons chains of the frother. The detrimental action of higher concentrations of alcohol (also observed by other researchers [14]) may be due to the fact that the metal-collector complexes are stabilized so well in the aqueous solution that they are less likely to adsorb at the air-interface. Also at the higher alcohol concentrations, the number of sites on the interface liquid-gas available for metal-collector complex adsorption will be reduced due to adsorption of alcohol molecules, which may lower metal removal [ 11].

Figure 7 presents a photomicrograph of the sublate in the system Cd/SDS/MIBC at 0.1% v/v concentration of MIBC. It is clear that the sublate has the same morphology observed in the system Cd/SDS shown in Figure 3. An EDS analysis indicated the presence of Cd and S.

Fig.7 Photomicrography of the sublate in the system Cd/SDS/MIBC (metal:collector ratio of 1:2 and 0.1% v/v of MIBC).


The effect of ionic strength by addition of NaC1 or Na2SO 4 can be seen in Figures 8 and 9, respectively. An increase in ionic strength from 4.7 x 10 -4 to 4.7 x 10 -l moles.dm -3 decreased the

Fig.8

E" >= 8

100-

80-

60-

40-

20-

0;

pH = 4,0- 5,0 air flow: 2 ml/s Cd/SDS - 1:2 -~ Z- ionic $trenght (NaCI) (rnolelu~m ~ _ T _ --a-- 4,7x 104 ~ ~ . _ ~ - - ~ ' ~ _ T _ --e--4,7x 10 3 ~ - - ' - - 4,7x 10 "2 J ~ J - -v - - 4,7x 10 ~

. - ~

30 60 90 120

t i m e (ra in)

Effect of ionic strength with NaC1 on the flotability of Cd. Air flow rate: 2 ml.s -1 pH= 4.0-5.0, metal:collector ra t io - - t :2.

(9

8 G)

IZ

100-

80-

60"

40-

20-

0 0

pH = 4,0 - 5,0 air flow: 2 ml/s Cd/SDS - 1 : 2 ~- ~- io:c strenght N0~S0 4 (moles/dm~)) ~ - - - Z_

--4, xlo

• j/ J

T 1" T 3" 30 60 90 120

t ime (rain)

Fig.9 Effect of ionic strength with Na2SO 4 on the flotability of Cd. Air flow rate: 2 ml.s-~; pH= 4.0-5.0, metal:collector ratio--1:2.

maximum cadmium removal to 4.5% for the system Cd/SDS/NaC1 and to 1.0% for the system Cd/SDS/NaaSO 4. Through the distributions diagrams of Cd-CI (Figure 10) and Cd-SO 4 (Figure 11), it can be observed that the species, in an ionic strength of 4.7 x 10 -] moles.dm -3 (that correspond to [NaC1] = 4.21 x 10 -t moles.dm -3 and [Na2SO4]= 1.05 x 10 -l moles.dm -3) are CdCI-, CdC12 and CdC13- and Cd 2÷, CdSO 4 and Cd(SO4)22-, respectively. The drastic reduction on the flotability of Cd can be explained due to the formation of neutral and anionic complexes that can not interact with the anionic collector and the excess of sodium ions that overcome Cd 2÷ concentration. So the Cd 2÷ ions are not associating in the bulk solution with dodecyisulfate ions and the sodium ions must be competing for collector ions.


0,5 M 0,05 M 0,006 M 0,0005 M

O,g

l- 0,8

o,7 J ~ N C d C I ÷ \ +,+X \

\ \ \ o. CdCl+"

-1 0 1

Cd 2÷

2 3 pC,

Fig. 10 Distribution diagrams of cadmium chloride complexes.

1

OJ

0.1 M 0,01 M 0,001 M 0.0001 M

0 ,8

0,7

0,6

X

0,5

o a 0,4 o

~ 0,3

0,2

C d ( S 0 4 ) , 6"

C d 2+

0,1

O+ -1 0 1 2 3 4

p S O ,

Fig. 11 Distribution diagrams of cadmium sulfate complexes.

The presence of complexes compounds can be confirmed in the analysis of sublate and the foam produced in the systems Cd/SDS/NaC1 and Cd/SDS/Na2SO 4 (ionic strength: 4.7 x 10 -2 moles.dm -3) by SEM/EDS as shown in Figures 12 and 13. For the system Cd/SDS/NaC1 , two phenomena are observed: an acicular sublate (Figure 12 (a)) containing Cd, S and C1 which indicates the association of Cd and CI to form a complex (CdCln) 2-" and an almost perfect bubble structure (Figure 12 (b)) containing only Na and S. This analysis suggests that just SDS is present. For the system Cd/SDS/Na2SO 4, the sublate is concentrated at the border of the collapsed bubbles (Figure 13 (a)) and can be seen in globular shape (Figure 13(b)). The EDS analysis showed the presence of Cd and S indicating formation of cadmium dodecylsulfate. For ionic


strength of 4.7 x 10-~moles.dm -3, no precipitate of Cd was formed and the foam produced, analyzed, by EDS showed only Na and S indicating the presence of SDS.

Fig. 12

(a) (b)

Photomicrography of the sublate in the system Cd/SDS/NaC1 (metal:collector ratio of 1:2 and ionic strength: 4.68 x 10 -2 moles.din-3). (a) acicular sublate (1000x) and (b) bubble structure (200x).

(a) (b)

Fig. 13 Photomicrography of the sublate in the system Cd/SDS/Na2SO 4 (metal:collector ratio of 1:2 and ionic strength: 4.68 x 10 -2 moles.dm-3). (a) sublate concentrated at the border of the foam collapsed (200x) and (b) globular sublate (1000x).

The association of the metal ion, M 2+, and the dodecylsulfate, RSO4-, can be represented by the following reaction and equilibrium constant, Kassn [3]:

{M(RS0 4) 2} M2+ + 2RSO 4- ) M(RSO4)2 kass n = (1)

{M 2+}{RSO;} 2


AG log Kassn = - a s s n ( 2 )

2.30RT

Reaction 1 shows the equilibrium constant in terms of thermodynamics activities of the product and reactant species. Duyvesteyn and Doyle [3] comments that at lower ionic strengths, the ionic activities will be very close to the analytical concentrations (i.e., the activity coefficients are close to unity). At higher ionic strength, the activity coefficients of both the M 2÷ and of dodecylsulfate ions are decreased, either through electrical interactions, as approximated by the Debye-Htickel extend theory, or possibly through formation of M2+-inorganic anion complexes. The activity coefficient of the neutral M(RSO4) 2 complex would not be expected to decrease as dramatically, consequently its analytical concentration at equilibrium would be lower than that at lower ionic strengths. This would leave free dodecylsulfate ions that could adsorb at liquid-gas interfaces, with concurrent removal of sodium ions.

Figure 14 show the surface tension of CD/SDS solutions as a function of ionic strength. The surface tension decreased significantly with an increase in ionic strength, although its nominal value was almost the same for NaC1 and Na2SO 4. The decrease in surface tension can be explained due to adsorption of other ions, such as Na ÷ and dodecylsulfate ions, at the liquid-gas interface. Generally, an increase in inorganic salt concentrations should increase surface tension, however in these systems (metallic ions-collector-inorganic salts) the contrary happens. Jurkiewcz [15] comments that the cations of higher valency and smaller hydrated ion size, i.e., of higher ionic potential, undergo the preferential adsorption. Thus, in the presence of metal--collector of 1:2 (without addition of inorganic salt) the number of Me2÷-dodecylsulfate ionic pairs is higher than that of Na ÷- dodecylsulfate in the adsorption at the interface liquid-gas. However, with an increase of inorganic salts of sodium, three factors can occur [15]: (1) Mass cation effects; this result is consistent with the added Na ÷ competition with separated Me 2÷ for RSO 4- in a cation exchange mechanism; (2) A higher ionic strength leads to relaxation of the repulsives forces among the adsorbed dodecylsulfate anions; (3) The salting-out action of the dodecylsulfate by the electrolyte. All these factors affect the increased surface excess (expressed as the inverse of surface tension) of Na ÷ and RSO 4- ion pairs and is consistent with the experimental data on surface tension in Figure 14 and the lower recovery of Cd ions in high ionic strength.

ii : . c . n o . , ]_ -" NaCI

55"~_ ----o-- Na2S04

3o I I ! ! I

0,0 0,1 0,2 0,3 0,4 0,5

Ionic Strenght (moles.dm ~)

Fig.14 Surface tension of the initial solution in the system Cd/SDS/frothers.

916 I.B. Scorzelli et aL

CONCLUSIONS

The best removal obtained for the system Cd/SDS with a dry foam was 89.2% for a the metal collector ratio of 1:2. The presence of frothers at a concentration of 0.1% v/v, improved the removal of Cd to a maximum flotability of 98.8% and 97.7% for iso-propanol and MIBC, respectively.

The ion flotation process is very sensitive to the increase in ionic strength reducing drastically the recovery of Cd. The distribution diagrams showed that the species presented in the systems are neutral or anionic complexes that are not able to interact with the anionic surfactant (SDS). The surface tension data and SEM/EDS also showed that the reduce in flotability is related to the adsorption of Na +- dodecylsulfate at the interface liquid-gas. The Cd species remain in solution as of chloride and sulfate complexes. The flotability of Cd could be improve using a cationic collector as cetyl trimethyl ammonium bromide (CTAB) or a complex agent that have a specific interactions with cadmium ions.

The characterization of the morphology of the sublate using SEM/EDS showed that the sublate and foam structure depend on the physico-chemical conditions of the system. The SEM/EDS studies proved to be a powerful tool in understanding the colloid chemistry behavior of ion flotation.

A C K N O W L E D G E M E N T S

The authors would like to thank Professor John Meech (University of British Columbia) for his assistance, Maria de F~tima S. Lopes and Marcos Bella-Cruz Silva (Catholic University of Rio de Janeiro) for the assistance in the SEM/EDS analysis and the AAS analysis, respectively. Financial assistance from CNPq and FAPERJ are gratefully acknowledged.

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removal of cadmium from a liquid effluent by ion flotation

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