Hydrometallurgy, 28 (1992) 423-433
Elsevier Science Publishers B.V., Amsterdam
Effect of gas sparging on the removal of heavy metal ions from industrial wastewater by a cementation technique Mahmoud A. Zarraa Chemical Engineering Department, Faculty of Engineering, Alexandria University, Alexandria, Egypt (Received January 8, 1991; revised version accepted August 20, 1991 )
ABSTRACT Zarraa, M.A., 1992. Effect of gas sparging on the removal of heavy metal ions from industrial wastewater by a cementation technique. Hydrometallurgy, 28: 423-433. The effect of nitrogen sparging on the rate of diffusion controlled cementation of copper on a single vertical zinc rod from a simulated waste solution of copper sulphate was studied. Variables investigated were nitrogen superficial velocity, diameter and height of the zinc rod, and the physical properties of the solution (adding glycerol). These variables were studied for their effect on the mass transfer coefficient of copper cementation. The mass transfer coefficient was found to increase with increasing superficial gas velocity. Increasing both diameter and height of the zinc rod were found to decrease the mass transfer coefficient. Mass transfer data were correlated by the equation: j=0.48 (Fr.Re) -°'l~.(dr/d) "°'s3.(L/d) -°'6' for the conditions: 1430
Cementation is an electrochemical process by which the ion of a more noble metal is displaced from solution by a less noble metal placed in contact with it. The metal displaced from solution is deposited on to the surface of the less noble metal which is simultaneously dissolved [ 1 ]. Cementation is one of the most effective and economic techniques used for recovering toxic and valuable metals from industrial waste streams [ 2 ]. Cementation is also considered an important reaction in hydrometallurgical processing and in metal winning [ 2,3 ]. Recently, gas sparging in bubble columns has received a great deal of attention as a means of enhancing the rate of mass transfer [4Correspondence to: M.A. Zarraa, Chemical Engineering Department, Faculty of Engineering, Alexandria University, Alexandria, Egypt.
0304-386X/92/$05.00 © 1992 Elsevier Science Publishers B.V. All rights reserved.
12 ] in chemical, biochemical and electrochemical reactors. It has been claimed that gas stirring is more effective and more economic than mechanical stirring [ 6-8,10 ]. Although much work has been done on the rate of cementation reactions under mechanical stirring conditions, little has been done on the effect of gas stirring on the rate of solid-liquid mass transfer during cementation. The present work was carried out to explore the possibility of using gas stirring as a means for enhancing the rate of mass transfer during the removal of heavy metal ions (e.g., cupric ions ) from a simulated waste solution using a cementation technique in a batch reactor with no solution flow. Dilute copper-containing solutions are encountered in chemical industries as wastes from pickling and electroplating industries. The rate of cementation reaction of cupric ions in solutions with zinc metal has been studied on a number of previous occasions [ l, 13-15 ]. These authors found that the rate of copper cementation with zinc is a diffusion-controlled process, with the reaction rate being highly dependent on the quantity and nature of the deposit remaining attached to the zinc surface. Centnerswer and HeUer [ 13 ] found that the reaction was diffusion controlled at low stirring speeds but chemically controlled at high rotation speeds. On the other hand, King and Burger [ 14 ], who used a more efficient stirring system, found that the reaction was diffusion controlled at all stirring speeds investigated. It has been reported [ 16,17 ] that the reaction rate increases once a porous deposit is formed. This was attributed to the increase in surface area and to the generation of small eddies in the laminar boundary layer by the deposit protrusions. When a coherent deposit was formed, the rate was found to decrease with increasing deposit mass and tended to fall to zero [ 18 ]. Lee et al. [ 16 ] and Strickland and Lawson [l 5] reported a slow initial rate followed by an enhanced rate period using a rotating zinc disc for the cementation of copper from solution. Nadkarni and Wadsworth [ 18 ] and EI-Tawil [ 19,20 ] reported a decrease in the rate of cementation of copper by either iron or zinc powder from copper sulphate solution and attributed this decrease in rate to the formation of a solid layer on the surface. Ghabashy and Fawzy [21 ] studied the effect of a magnetic field on the rate of cementation of copper from copper sulphate solution on zinc. They found that the rate of copper cementation increases significantly with increasing intensity of the magnetic field. EXPERIMENTAL
The apparatus (Fig. 1 ) consisted of a 100 mm diameter vertical cylindrical plexiglass column 300 mm in height. The column was fitted at its bottom with a perforated plexiglass distributor which contained 1800 holes, each hole of l mm diameter; the holes were separated from each other by a distance of 1.5 mm. Smooth zinc rods of various diameter and height were brazed to a 2 mm diameter stainless steel wire which acted as a holder. The stainless steel rod
EFFECT OF GAS SPARGING ON CEMENTATION RATE
(I) Plexiglass cylindrical column (2) Gas distributor (3) Gas humidifier (4.) Nitrogen cylinder (5) Rotameter (6) Electrolyte level, (7) Zinc rod (8) Rod holder (9) GLass valve (10) Walt (S)~
(!o) (8) ----"
T (9) i
roll am am
Fig. 1. Experimental apparatus.
holder was isolated with an inert epoxy resin. Rod diameter ranged from 7 to 30 mm, and rod height ranged from 20 to 100 mm. Usually, the surface of the zinc rod was prepared by abrasion with 400 grade carborundum paper followed by 600 grade paper. It was then washed with water and acetone and dried. Before each run 1000 ml ofcopper sulphate solution were placed in the column. Nitrogen gas was allowed to pass through the distributor at the required rate, the zinc rod was positioned vertically in the middle of the column, 50 mm from the distributor, and the rod holder was held firmly in position by two clamps to a wall. Before entering the reactor, nitrogen was humidified in copper sulphate solution identical in concentration with that in the reactor. Nitrogen flow rate was measured by a calibrated rotameter. Samples of I ml of the solution were taken every 2 min for analyzing cupric ions in the solution by iodometry [22 ]. All the solutions, which were 0.05 M in copper sulphate, were prepared from Analar grade chemicals and distilled water. Each run was carried out twice using a fresh solution. Physical properties of the solution were altered by adding glycerol, ranging in concentration from 5 to 20 wt.%. Solution density and viscosity were determined using a density bottle and an Ostwald viscometer, respectively [ 23 ]. The diffusivity of cupric ions in the solution was obtained from the literature [24 ] and was corrected for the change in temperature and viscosity. During each run, the temperature was fixed at a value of 22 _+1 °C. The superficial velocity o~ nitrogen ranged from 7.56 to 23.34 mm/s.
RESULTS A N D DISCUSSION
For the batch reactor used in the present work, the rate of diffusioncontrolled copper cementation with zinc can be expressed in terms of the disappearance of cupric ions from the solution by the equation [ 25 ]"
- Vs ( d C / d t ) = K A C
which upon integration yields:
ln C o / C = K A t / Vs
Figure 2 shows a typical In Co/C versus t relation; the mass transfer coefficient, K, was obtained from the slope of the In Co/C versus t line. This curve is characterized by two distinct rates; a slow initial rate and a final faster one. It is clear that the present finding agrees with the results [ 15,16 ] for the cementation of copper with a rotating zinc disc. In the present work, the increase in the rate of mass transfer in the first stage of copper cementation on Initial concentration ot copper sulphate -0.05 M
Rod diameter-_ 12 mm Rod heighl = 40ram Superficial gas velocity, V(mmls) V
,.Y C _J
't, mi~q Fig. 2. Ln Co/C versus t for a single ve~ical zinc rod.
EFFECT OF GAS SPARGING ON CEMENTATION RATE
zinc as a result of gas stirring was considered for quantitative treatment. The enhanced second stage of the cementation reaction, which has been ascribed to deposit roughening [ 15,16 ], was not treated quantitatively. Figure 3 shows the effect of superficial gas velocity on the mass transfer coefficient at a single vertical zinc rod. The mass transfer coefficient can be related to the superficial gas velocity by the equation: Koc V °'77
The rate of cementation of copper with zinc may be enhanced by gas stirring through the following effects: ( 1 ) The rising bubble swarm induces an upward flow of the solution past the stationary zinc rod, where a hydrodynamic boundary layer and a diffusion boundary layer are formed around the zinc rod. The higher the superficial gas velocity, the higher the solution flow and the thinner the diffusion layer, that is the higher the rate of transfer of copper ions to the zinc surface and, at the same time, through the porous deposit layer formed on the zinc rod which, in turn, enhances the rate of cementation. (2) According to Kast [ 26 ] a fluid element in front of the rising bubble receives a radial momentum in addition to the axial momentum. This radial mass transport forced by the axial bubble motion weakens and breaks up the boundary layer at the surface of zinc rod. Consequently, the occurrence of fast initJat concentration of copper s u t p h a t e =0.05 M
Rod d i a m e t e r
= 12 mm
= / . 0 mm
Sc = I~30
SC = 2103
S c = 24Ba
Sc - 1805
t J l,I
Fig. 3. The effect of superficial gas velocity on the mass transfer coefficient at a single vertical zinc rod.
radial flow rates can be regarded as lateral eddy diffusivity, that is, a radial mass dispersion which contributes to sustaining the cementation reaction. (3) The rising bubbles induce axial solution flow, which moves radially when it reaches the top of the solution and then reflects downwards at the wall of the column and finally moves radially again in the bulk of the solution, as shown in Fig. 4. This internal circulation of the solution which moves countercurrent to the uprising gas bubbles could affect the rate of copper deposition on the zinc surface. The data shown in Fig. 3 also indicate that within the present range of conditions the physical properties of the solution have a marked effect on the mass transfer coefficient. The mass transfer coefficient tends to increase with decreasing solution viscosity, probably because of the increase in the cupric ion diffusivity owing to the decrease in the diffusion layer thickness according to the relation:
Figure 5 shows that the mass transfer coefficient decreases with increasing rod height according to the equation: K~L-0'58
Figure 6 shows that the mass transfer coefficient decreases with increasing rod diameter according to the equation: (6)
Koc d r °'~4
In bubble columns with no net liquid circulation flow, studies  have
L..,.--) tt ttCJ
Fig. 4. Approximate pattern of flow of gas-liquid dispersion.
EFFECT OF GAS SPARGING ON CEMENTATION RATE Initial concentration of copper sulphate = 0.05M Rod d i a m e t e r =12 rnrn Sc = 1430
velocity, V (rnmls)
I m J~ll 0.05
Fig. 5. Effect of rod height on the mass transfer coefficient.
revealed the presence of two opposing streams, an upward stream in the co~e zone and a downward stream at the column wall (Fig. 4). As the rod diameter increases, the gap between the rod and the column wall decreases. Accordingly, the two opposing streams interact strongly, with a consequent decrease in the rise velocity of the upward stream and the mass transfer coefficient at the zinc rod. Previous studies in the area of heat transfer in bubble columns [ 27-31 ] differ on the effect of the length of the transfer surface on the heat transfer coefficient. Investigators who used relatively long bubble columns found that the heat transfer coefficient is independent of the column height. Lewis et al. [ 29 ], who studied heat transfer in bubble columns using vertical and horizontal heaters placed inside the column, found that the heat transfer coefficient decreases with increasing the height of vertical heater and the diameter of horizontal heater. In the area of solid-liquid mass transfer in bubble systems similar results were obtained; Ibl et al. [6 ] and Ettel et al. [7 ], who studied the effect of gas sparging on the rate of mass transfer at a vertical plate, found that plate height has little effect o~ the mass transfer coefficient.
M.A.ZARRAA Initial concentration of copper sulphate = 005 M Rod height = 40 mm Sc = 1430
Superficial gas v e t o c i t y j V (mm Is) +
, O 0 0.005
0 1 0,01
d r ~rn
Fig. 6. Effect of rod diameter on the mass transfer coefficienl.
Ibl and co-workers used plates of height ranging from 0.2 to 1 m, while Ettel and co-workers used plates of height 1.1 m. Cavatorta and Bohm [ 32 ] studied the effect of gas sparging on the rate of mass transfer from the wall of a vertical tube of a working section ranging in height from 5 to 17.9 mm. These authors found that the mass transfer coefficient decreases with increasing tube height and then becomes constant with further increase in tube height. The firs' decrease in the mass transfer coefficient with increasing tube height was ascribed by the authors to the fact that the flow of the liquid-gas dispersion at short tubes is developing; when the flow becomes fully developed the mass transfer coefficient becomes independent of height. In a recent study, Zarraa et al. [ 33 ] ~tudied the effect of gas sparging on the rate of mass transfer at a single sphere of different diameters, ranging from 4.8 to 35 mm. These authors found that the mass transfer coefficient decreases with increasing sphere diameter. They reported that this decrease in the mass transfer coefficient may be attributed to the build-up of a hydrodynamic boundary layer and a
EFFECT OF GAS SPARGING ON CEMENTATION RATE + Sc
0 Sc =1612
6 Sc = 1805
o Sc = 2103 q Sc =2488
10 9 B 7
+ + o ~ °v-
4 6 8 10 12 lZ,, ( Ft. IRe %o.,, ( dr/d )_0.8, ( L / d )_o.6..
Fig. 7. Overall mass transfer correlation at a single vertical zinc rod.
diffusion layer which increases in thickness with sphere diameter in a manner similar to single phase flow past spheres. An overall mass transfer correlation was envisaged using the dimensionless groups j, Fr and Re, usually used in correlating heat and mass transfer in gasstirred vessels. In view of the present finding that both rod diameter and rod height affect the mass transfer coefficient significantly, and since the flow pattern of the recycled solution depends on the column diameter (as shown in Fig. 4); two extra dimensionless terms were used to account for these effects, namely dr/d and Lid. Figure 7 shows that, for the conditions: 1430
(Fr.Re) -°'13. (dr/d) -°'83. ( L / d ) -°'61
with an average arithmetic deviation of +_ 11%. CONCLUSIONS
The following conclusions can be drawn from the present study: ( 1 ) The mass transfer coefficient increases with increasing superficial gas velocity.
(2) The mass transfer coefficient decreases with an increase in either diameter or height of the zinc rod. (3) The ratios (dr/d and L/d) have marked effects on mass transfer coefficients. (4) Solid-liquid mass transfer coefficients are well correlated by eq. ( 7 ) in terms of the mass transferj factor, the Froude number, Fr, the Reynolds number, Re, and the ratios dr/d and L/d. (5) The reactor can be operated batchwise or continuously at low solution flow rates. Such a continuous reactor would have a high degree of conversion per pass, owing to the high residence time of the solution (resulting from the use of low flow rates) and the high mass transfer coefficient resulting from gas sparging. NOMENCLATURE A C Co D dr d
Fr g .] K L
St t 1/ lA
rod surface area, m 2 concentration of copper sulphate at time t, mol/m 3 initial concentration of copper sulphate, mol/m 3 diffusion coefficient, m2/s rod diameter, m column diameter, m Froude number, V2/(drg) acceleration dueto gravity, m/s 2 mass transfer j factor (Sc°'66S0 mass transfer coefficient, m/s rod height, m Reynolds number, (pVdr) /iz Schmidt number, It~ (pD) Stanton number, K/V time, s superficial gas velocity, m/s volume of the solution in the reactor, m a
Greek Symbols p
solution density, kg/m 3 solution viscosity, kg/m.s diffusion layer thickness, m
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EFFECT OF GAS SPARGING ON CEMENTATION RATE
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