During the electrowinning steps in the SPE, electrochemical reduction, corrosion, competing side reactions, and metal displacement reactions occur at the cathode particle surfaces [18
]. The metal displacement reaction between Cu2+
and reduced nickel metal was found to be important during the co-electrodeposition of copper and nickel from solution [20
In a similar fashion, in Cu/Ni/Cd mixtures, additional metal displacement reactions occur that play an important role in the overall removal of the three metals; viz.
) has been investigated during electroplating [21
]. It has a significant effect on the resultant alloy composition, forming compositionally modulated alloys during pulsed plating [23
]. The metal displacement reactions between copper and cadmium [25
], and nickel and cadmium [26
] have also been investigated.
The P/R process steps are used to increase the metals concentrations to levels sufficient for efficient electrowinning, and as the final cleanup step for the discharged water. The increase in metal ion concentration with each P/R cycle was linear, as expected. For a cycle time of 8 min., and initial metal concentrations of 20 ppm each, the increase per P/R cycle was about 16 ppm, which is accounted for as follows: The inlet and outlet tubing of the P/R tank was located at about 1/5 of the distance from the bottom of the tank. Thus, for each P/R cycle, about 1/5 of the solution remains in the tank after discharge of the supernatant water. The accumulated concentration in a P/R/redissolution cycle is then (1-1/5) × 20 ppm = 16 ppm. For the current hardware, the precipitation process worked well for total metal ion concentrations less than about 200 ppm. For higher total concentrations, the amount of accumulated hydroxide precipitate sludge would exceed the level of the tank outlet tubing, and some precipitate would be carried over and lost upon draining the tank. Of course, the operating capacity can be increased by using a larger tank and/or relocating the tank outlet.
Metal ion concentrations in the filtered effluent water were also measured for the same runs as 0.23, 0.37, and 1.50 ppm for Cu2+
, and Cd2+
, respectively, averaged over the 7 cycles. The solubility products of copper, nickel, and cadmium hydroxide are 4.8×10−20
, and 7.20×10−15
, respectively, at 25°C [27
]. Therefore, at equilibrium at pH 11, the Cu2+
ion concentration in solution should be 2.3×10−7
(0.015 ppm), and Ni2+
should be 5.2×10−6
(0.3 ppm), and 1.2×10−5
(1.3 ppm), respectively. These calculated concentrations are slightly less than the experimental values for nickel and cadmium, and much less for copper. These discrepancies are explained by the filter performance. A 20 μm inline filter (ISC, model No SJC-40-20) was used to retain the metal precipitate particles in the P/R tank. Three different filters were tested: 5, 20, and 50 μm. The 5 μm filter was easily plugged, and the 50 μm filter allowed through much more precipitate than the 20 μm filter. Comparison of the calculated concentrations with the data suggests that the 20 μm filter retained 93–99% of the metal hydroxide precipitate particles. The mean size of copper hydroxide precipitate particles has been reported to be on the order of 0.1 – 5 μm [28
], which is much less than the 20 – 50 μm for nickel hydroxide [29
], and 400 μm for cadmium hydroxide [30
]. The filter size used is less than that of the average nickel hydroxide and cadmium hydroxide precipitate particles, but larger than that of copper hydroxide particles. The reasonably good agreement between the experimental and calculated nickel and cadmium ion concentrations indicates that the filter retained almost all the nickel hydroxide and cadmium hydroxide precipitate particles. However, the larger difference between the experimental and calculated copper ion concentrations indicates that some copper hydroxide precipitate passed through the filter.
USEPA MCLG (maximum contaminant level goal) values are 1.3 mg L−1
(TT, treatment technique) for copper; 0.1 mg L−1
(remanded 1995; currently under reconsideration) for nickel; and 0.005 mg L−1
for cadmium [31
]. The experimental results indicate that the precipitation process reduces the copper ion concentration below its MCLG. However, additional treatment may be necessary to reduce the nickel and cadmium concentrations to below their respective MCLG values to drinking water standards, if so required.
One measure of CEP system performance is the net metal removal rate over the programmed number of P/R steps per electrodeposition step. The removal of a single metal can be expressed as [18
is the metal ion concentration during electrowinning, te
, is the electrowinning time, k
is the electrochemical metal reduction rate constant, kL
is the interphase mass transfer coefficient, a
is the specific interfacial surface area, and kc
is the metal corrosion rate. This expression cannot be solved analytically since k
varies with overpotential, and thus with time. Consequently, the experimental data were fit to polynomials for the sake of convenience. Some typical Ce vs te
data are presented for copper removal in the CEP SPE as a function of applied current in .
Figure 2 (a) Copper electrowinning in the SPE as a function of applied current. The initial Cu2+ concentration in each case was 99.5 ppm, which was prepared from the 20 ppm stock solution with five P/R cycles. The symbols are the experimental data, and the curves (more ...)
The net rate
at which a metal is removed in the CEP system, R
(e.g., in g min−1
), is given by the expression:
is the initial metal concentration prepared for the electrodeposition cycle; Ce
is the metal concentration at the end of the electrodeposition cycle, te
is the time for P/R process associated with accumulating the metal for electrodeposition; and V
is the solution volume in the SPE. The removal rate attains a maximum at dR/dte = 0.
Thus, the optimal value of te
occurs at the root of the transcendental equation:
For the data presented in for copper removal, C0 = 99.5 ppm, which was accumulated from a 20 ppm solution over five P/R cycles. Each P/R cycle was 8 min, so for the five P/R cycles shown, tp = 5 × 8 min = 40 min. The net CEP removal rate, R, for these data as a function of total CEP process time, te + tp, was calculated as a function of applied current from the polynomial curve fits of the data presented in , and the results are presented in . As shown, the net CEP removal rate exhibits a maximum. The reason for this behavior is as follows. For a fixed value of tp (i.e., 40 min. in this case) at low CEP process times, or at low values of te, there is little metal removal by electrowinning, so (C0 – Ce) is low. As te increases, however, more metal is removed by electrowinning, such that (C0 – Ce) increases, and the effect of fixed tp decreases in comparison to te. This behavior causes R to initially increase with CEP process time. However, since Ce decreases exponentially with te, the metal ion deposition rate decreases with te. That is, the rate of increase of (C0 – Ce) decreases with te. This causes the net CEP removal rate to exhibit a maximum, which represents the optimum net CEP removal rate achieved at this maximum.
From it is also noted that the optimal value of R increases and occurs at earlier times with increasing applied current. The observed net removal rate is the competitive result of metal removal by electrochemical reduction and metal redissolution via corrosion. The electrochemical reduction rate increases with current, while the corrosion rate remains approximately constant for fixed electrowinning conditions, such as temperature and pH. Thus, at a particular time, the removal rate increases with applied current. The higher electrowinning rate requires less time to process the results of P/R cycles at the maximum. Consequently, the optimal time decreases with applied current, while the optimal rate increases.
Using the resultant polynomial curve fits from , the corresponding optima for the copper data are 73, 90, and 100 min., corresponding to maxima in the copper removal rates of 0.014, 0.010, and 0.006 g min−1, for 20, 15, and 10A, respectively. It is noted that the initial electrowinning concentration obtained with multiple P/R cycles was 99.5 ppm, while in the corresponding values are 107, 109, and 108 ppm for 10, 15, and 20A, respectively. The reason for this “discrepancy” is that after the solution is pumped to the SPE, a few minutes are required to heat the solution to the desired temperature (50°C in this case). During that time, some metal corrosion occurs which increases the initial concentration slightly. This also explains the initial “negative” removal rates in .
Knowledge of the optimal times is useful for setting the electrowinning time for multiple CEP cycles. From , at 15A, the net copper removal rate maximum occurs at 90 min. The results for copper removal at 15A over multiple CEP cycles are presented in . Overall, the feed water copper ion concentration was reduced from 20.1 ppm to 0.23 ppm in the effluent water. The difference between these values is the total amount of copper deposited on the cathodic particles. The copper lost in the process was determined to be on the order of ±1.5%, such that the copper mass balance closes reasonably well.
(a) Cu2+ concentration in the process water over multiple CEP cycles at 15A. Four P/R steps were used to accumulate the initial Cu2+ concentration for electrowinning. After that, each CEP cycle consisted of one SPE and three P/R steps.
Nickel and cadmium electrowinning are more sensitive to corrosion rate than for copper. Consequently, nitrogen sparging plays a more important role in reducing dissolved oxygen in the electrolyte solution to reduce the corrosion rate, as discussed in References [18
]. The optimal time for nickel electrowinning was determined in the same manner as for copper. The optimal times and maximum net CEP removal rates for nickel were 112 min, 0.006 g min−1
, and 120 min, 0.004 g min−1
for 20A and 15A, respectively, and 173 min. for cadmium at 20A.
Slightly different than for copper, the nickel and cadmium electrowinning times for multiple CEP cycles are determined by both the optimal time and the final concentration at the end of each electrowinning step. Since an integral number of P/R cycles occur for each CEP cycle, the nickel and cadmium ion concentrations are not sufficiently depleted when shorter time intervals than the optimal time are used. Consequently, the initial ion concentration for the second CEP cycle will be greater than that for the first CEP cycle, etc., and the metal ion concentration would accumulate with each successive CEP cycle. Therefore, a time longer than the optimal time was used as the electrowinning time to maintain the initial metal ion concentration at the same level for the subsequent electrowinning step.
Nickel removal data at 20A over multiple CEP cycles are presented . Each CEP cycle consisted of one electrodeposition step of 180 min. (i.e., greater than the optimal time). With this program, no nickel ion accumulation occurs, while the electrodeposition rate remains close to the maximum rate at the optimal time), and three P/R cycles of 8 min. each for another 24 min. This program was effective for nickel removal. It is noted that the optimal time for nickel removal is greater than that for copper, and also that the maximum nickel removal rate is less than that for copper. The lower nickel electrodeposition rate means that less nickel is deposited over the same time interval, or a lower maximum removal rate for nickel. A lower removal rate requires a greater optimal time for the same initial metal concentration of 100 ppm.
The rate of cadmium removal is less than that of nickel. Cadmium electrowinning rates were found to increase with applied current, pH, and temperature. At 20A, pH 4.0, and 50°C, a 100 ppm cadmium solution was accumulated (from a 20 ppm solution) with five P/R cycles. The resultant optimal removal time and maximum rate were 340 min., 0.003 g min−1, respectively. These values represent a greater time and lower maximum rate than were found for copper and nickel under similar conditions.
CEP data for co-removal of binary mixtures of the metals over multiple CEP cycles are presented in under similar conditions: (a) Cu2+/Ni2+ co-removal, te = 180 min.; (b) Cu2+/Cd2+ co-removal, te = 240 min.; and (c) Ni2+/Cd2+ co-removal with te = 300 min.
Figure 4 Binary metal ion concentrations in the process water over multiple CEP cycles. In each case, three P/R steps were employed to accumulate the initial ion concentrations for electrowinning. After that, each CEP cycle consisted of one SPE step for the t (more ...)
CEP data for co-removal of a ternary Cu2+
mixture over multiple CEP cycles are presented in . As shown, all the metal mixture co-removal data over multiple cycles exhibit a characteristic flattened curve at the inception of each electrodeposition step for the last metal to be removed. This characteristic is attributable to the metal displacement reactions [20
Figure 5 Cu2+, Ni2+, and Cd2+ concentrations in a ternary mixture over multiple CEP cycles. Three P/R steps were employed to accumulate the initial ion concentrations for electrowinning. After that, each CEP cycle consisted of one 300 min SPE step and one P/R (more ...)
In are presented Cu2+/Ni2+/Cd2+ co-removal data at 20A, pH 4.0, 50°C. For these data, the initial solution concentration of about 100 ppm of each metal was prepared directly from the reagents, and not via a series of P/R steps, as previously. This was done simply for convenience due to the limited capacity of the P/R tank. As shown in , the copper electrowinning rate is the greatest, while that of cadmium is the lowest, and that for nickel is intermediate between the two. In are presented the corresponding removal rates of Cu2+, Ni2+, and Cd2+ determined from the data in . The resultant corresponding optimal times and maximum removal rates are 65, 91, and 280 min., and 0.017, 0.006, and 0.002 g min−1, respectively.
Figure 6 (a) Cu2+, Ni2+, and Cd2+ electrowinning in the CEP SPE at 50°C, pH 4, with 3.5 SLM nitrogen sparging. The initial ion concentrations prior to electrowinning in each case were 100 ppm from a prepared solution. The symbols are the experimental data, (more ...)
Although not specifically shown here, it is noted that for the ternary mixture, the copper and nickel removal rates are greater, and the cadmium removal rates are less in comparison to their single metal behavior. This is also reflected in the optimal times that decrease for copper and nickel, and increases for cadmium. This behavior is due to the displacement reactions between copper ion and nickel and cadmium metal on the cathodic particles in a similar fashion as was found for Cu2+
]. In the ternary mixture, Cu2+
can be spontaneously reduced by both nickel and cadmium metal previously deposited on the surface of the cathodic particles, due to the positive standard potentials for the displacement reactions (Eqns. (1
). This enhances the net copper reduction rate, and decreases the optimal time. In a similar fashion, the nickel removal rate is enhanced by the cadmium displacement reaction, Eq. (3)
. However, since the standard potential difference between nickel ion and metal cadmium is less than that between copper ion and nickel, and copper ion and cadmium, its removal rate is enhanced somewhat less than that for copper. It is also noted that the displacement reactions all act in the same sense to increase the effective cadmium “corrosion” rate, such that its net removal rate decreases and its optimal time increases, in comparison to its single metal behavior.
The effects of the metal displacement reactions are also evident in modifying the initial electrowinning concentrations of the three metal ions. Even though the initial concentration of each metal ion was about 100 ppm, it is noted in that by the time the SPE was heated to the 50°C operating temperature and electrowinning was initiated, the metal ion concentrations had changed to 99, 110, 117 ppm for Cu2+, Ni2+, and Cd2+, respectively. This is due to the action of the metal displacement and corrosion reactions over this period. That is, a small amount of copper was net removed, while both the nickel and cadmium ion concentrations increased due to corrosion.
The time interval selected for multiple co-deposition cycles for the Cu2+/Ni2+/Cd2+ mixtures was greater than the optimal times of each of the single metals. Similar to the behavior of multiple CEP cycles with the binary mixtures, both the criteria of avoiding metal ion accumulation with subsequent cycles and maintenance of high electrowinning rates, were used to determine the optimal electrowinning time interval. That is, if a time shorter than the optimum for Cd2+ (but longer than those for Cu2+ and Ni2+) is used, the concentration of Cd2+ (the last metal ion removed in the mixture), will increase with each subsequent electrowinning step, which is counter to the general overall desired effect. Consequently, the time interval for the electrowinning cycle was increased to sufficiently deplete cadmium to maintain decreasing cadmium concentrations with each successive electrowinning cycle. Of course, a number of other program strategies could also be employed as well. For example, the electrowinning time could be programmed so that it starts out as near optimal for Cu2+, which allows Ni2+ and Cd2+ to accumulate with each cycle, while preferentially removing copper, and then operate on the resultant Ni2+ and Cd2+ - concentrated solution with an electrowinning time between the optimal values for Ni2+ and Cd2+ to preferentially remove Ni2+ while accumulating Cd2+, and then finally run at an optimal time for Cd2+ removal on the residual, etc., as well as other permutations thereof. The metal displacement reactions serve to “spread out” the removal of the different metals in time. This characteristic, coupled with the types of programs described above, could be used to allow the preferential accumulation of each metal on a particular set of particles which, since they are entrained, can be removed and introduced hydrodynamically at selected times in the program, so that metal separation and recovery can also be achieved in a continuous fashion.