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a saturated calomel electrode (SCE) as a reference electrode. Experiments were conducted in 500 ml of test electrolyte solution. The cathodic potentials were cycled between -0.5 and -1.4 volt vs SCE at a potential sweep rate of 10 mVs" using a potentiostat, Princeton Applied Research, model: 273A.

Table 2 a saturated calomel electrode (SCE) as a reference electrode. Experiments were conducted in 500 ml of test electrolyte solution. The cathodic potentials were cycled between -0.5 and -1.4 volt vs SCE at a potential sweep rate of 10 mVs" using a potentiostat, Princeton Applied Research, model: 273A.

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1: Electrolyte circulation pump; 2: DC power source; 3: Electrowinning cell; 4: Water bath had been added at the plant. To degrade the added suppressants (Dowfroth 250 C mainly), the supplied electrolyte was heat treated at 100C for 1 hr with the expectation of destroying the previously added organic surfactants (5, 19). To ensure that most of the organic additives had been destroyed, TOC (total organic carbon) tests were performed on the spent electrolyte before and after heat treatment using a Total Organic Carbon Analyzer, Model: TOC-Vcpuicpn, Shimadzu Corporation. The TOCs before and after heat treatment of spent electrolyte were 2.045 and 0.491 mg/l respectively, i.e. 80.72% of the organic received from the plant was degraded. For comparison, TOC in deionized water was found to be 0.120 mg/l. After cooling down, the required amount of additives (or surfactants) were added to make a stock solution before performing the electrowinning tests.
1: Electrolyte circulation pump; 2: DC power source; 3: Electrowinning cell; 4: Water bath had been added at the plant. To degrade the added suppressants (Dowfroth 250 C mainly), the supplied electrolyte was heat treated at 100C for 1 hr with the expectation of destroying the previously added organic surfactants (5, 19). To ensure that most of the organic additives had been destroyed, TOC (total organic carbon) tests were performed on the spent electrolyte before and after heat treatment using a Total Organic Carbon Analyzer, Model: TOC-Vcpuicpn, Shimadzu Corporation. The TOCs before and after heat treatment of spent electrolyte were 2.045 and 0.491 mg/l respectively, i.e. 80.72% of the organic received from the plant was degraded. For comparison, TOC in deionized water was found to be 0.120 mg/l. After cooling down, the required amount of additives (or surfactants) were added to make a stock solution before performing the electrowinning tests.
TABLE I. Zinc electrowinning experimental conditions. Characterizations
TABLE I. Zinc electrowinning experimental conditions. Characterizations
Figure 2. Effect of different additive concentration on acid mist suppression efficiency. TABLE IIL. Effects of additives on acid mist concentration, acid mist suppression efficiency, current efficiency and energy consumption in zinc electrowinning. surfactant as has been observed in the case of orthophenylene diamine and sodium lignin The registered current efficiencies (CE) for the different acid mist suppressants are shown in Table HI and Fig. 3. The current efficiencies range from a minimum of 84 % (Dowfroth 250 obtained in the than those obta led to higher C] at 15 ppm) to a maximum of 90.8% (blank, 0 ppm). Thus the CEs solutions containing the acid mist suppressants are all only slightly lower ined from the blank solution. Licorice and Saponin containing solutions The CEs for Te E's than for the other suppressants like Dowfroth 250 and Tennafroth 250. nnafroth 250 and Dowfroth 250 were reasonably good at 5 ppm compared to the blank solution as shown in Fig. 3.
Figure 2. Effect of different additive concentration on acid mist suppression efficiency. TABLE IIL. Effects of additives on acid mist concentration, acid mist suppression efficiency, current efficiency and energy consumption in zinc electrowinning. surfactant as has been observed in the case of orthophenylene diamine and sodium lignin The registered current efficiencies (CE) for the different acid mist suppressants are shown in Table HI and Fig. 3. The current efficiencies range from a minimum of 84 % (Dowfroth 250 obtained in the than those obta led to higher C] at 15 ppm) to a maximum of 90.8% (blank, 0 ppm). Thus the CEs solutions containing the acid mist suppressants are all only slightly lower ined from the blank solution. Licorice and Saponin containing solutions The CEs for Te E's than for the other suppressants like Dowfroth 250 and Tennafroth 250. nnafroth 250 and Dowfroth 250 were reasonably good at 5 ppm compared to the blank solution as shown in Fig. 3.
Spent electrolyte preheated at 90 °C for 1 hr and electrolysis done at 38 °C
Spent electrolyte preheated at 90 °C for 1 hr and electrolysis done at 38 °C
the 10 ppm level which is clearly reflected in the energy consumption curve shown in Fig 4. For all the cases, the energy consumption increases with increasing concentration of additives. The same has been reflected in the average cell voltage depicted in Table II, which showed an increasing cell voltage with increasing additive concentration. This could be caused by the influence of the additives on electrolyte characteristics such as electrical conductivity and surface potentials between the electrodes and the electrolyte (21). Our experimental cell voltage varies from 2.80-2.95 volts whilst industrial potentials are between 3.3 — 3.8 V [22]; the increased imposed voltage required in industry is likely due to stray currents and resistance of plant hardware e.g., bus bars etc.
the 10 ppm level which is clearly reflected in the energy consumption curve shown in Fig 4. For all the cases, the energy consumption increases with increasing concentration of additives. The same has been reflected in the average cell voltage depicted in Table II, which showed an increasing cell voltage with increasing additive concentration. This could be caused by the influence of the additives on electrolyte characteristics such as electrical conductivity and surface potentials between the electrodes and the electrolyte (21). Our experimental cell voltage varies from 2.80-2.95 volts whilst industrial potentials are between 3.3 — 3.8 V [22]; the increased imposed voltage required in industry is likely due to stray currents and resistance of plant hardware e.g., bus bars etc.
Figure 5. Cyclic voltammograms of spent electrolyte in the presence of (a) Saponin, (b Tennafroth, (c) Licorice and (d) Dowfroth.
Figure 5. Cyclic voltammograms of spent electrolyte in the presence of (a) Saponin, (b Tennafroth, (c) Licorice and (d) Dowfroth.
TABLE IV. Nucleation over potentials and surface tension values for the different test conditions.
TABLE IV. Nucleation over potentials and surface tension values for the different test conditions.
Figure 6. Variation of surface tension of spent electrolyte containing additives at their difference concentrations. of the suppressants except for Saponin, where a maximum surface tension of 68.9 mN/m was observed at 15 and 10 ppm addition level. At the same concentration, e.g. at 5 ppm, the surface tensions were consistently highest for Saponin while Licorice, Dowfroth and Tennafroth followed with increasingly smaller surface tensions (e.g. 68.9, 68.8, 68.7, and 68.5 mN/m, respectively). This indicates that the natural additives (Saponin and Liorice) possess higher surface tension than the synthetic additives (Tennafroth and Dowfroth). It is noteworthy that the acid mist suppressant efficiencies for Tennafroth and Dowfroth were observed to be higher than those of Licorice and Saponin, as shown in Fig. 2. Again, higher suppression efficiency is an indication of better bubble or foam formation on the surface of the electrolyte during electrowinning. The highest suppression efficiencies were observed at minimum surface tension values for all the additives at 15 ppm addition evel as listed in Table IV.
Figure 6. Variation of surface tension of spent electrolyte containing additives at their difference concentrations. of the suppressants except for Saponin, where a maximum surface tension of 68.9 mN/m was observed at 15 and 10 ppm addition level. At the same concentration, e.g. at 5 ppm, the surface tensions were consistently highest for Saponin while Licorice, Dowfroth and Tennafroth followed with increasingly smaller surface tensions (e.g. 68.9, 68.8, 68.7, and 68.5 mN/m, respectively). This indicates that the natural additives (Saponin and Liorice) possess higher surface tension than the synthetic additives (Tennafroth and Dowfroth). It is noteworthy that the acid mist suppressant efficiencies for Tennafroth and Dowfroth were observed to be higher than those of Licorice and Saponin, as shown in Fig. 2. Again, higher suppression efficiency is an indication of better bubble or foam formation on the surface of the electrolyte during electrowinning. The highest suppression efficiencies were observed at minimum surface tension values for all the additives at 15 ppm addition evel as listed in Table IV.
Figure 7. SEM picture of deposited Zn (a) and XRD pattern (b) of SE/Blank.
Figure 7. SEM picture of deposited Zn (a) and XRD pattern (b) of SE/Blank.
TABLE V. Preferred orientation of deposited Zn during different electrowinning conditions.
TABLE V. Preferred orientation of deposited Zn during different electrowinning conditions.
ppm level of addition as shown in Fig. 10 (b). Higher addition (10 and 15 ppm) levels of suppressants produce less compact and less crystalline structures as shown in Figs. 8 (b) & 8 (c), 9 (b), 9 (c), 11 (b), and 11 (c) respectively for Tennafroth 250 at 10, and 15 ppm evel, Saponin 10 and 15 ppm level, and Dowfroth 250 at 10 and 15 ppm level respectively. XRD analysis revealed that the SE/Tennafroth 250 at 5 ppm [Fig. 8 (d)] favored a lattice orientation of [101], [102] [103] and [112] (Table V).
ppm level of addition as shown in Fig. 10 (b). Higher addition (10 and 15 ppm) levels of suppressants produce less compact and less crystalline structures as shown in Figs. 8 (b) & 8 (c), 9 (b), 9 (c), 11 (b), and 11 (c) respectively for Tennafroth 250 at 10, and 15 ppm evel, Saponin 10 and 15 ppm level, and Dowfroth 250 at 10 and 15 ppm level respectively. XRD analysis revealed that the SE/Tennafroth 250 at 5 ppm [Fig. 8 (d)] favored a lattice orientation of [101], [102] [103] and [112] (Table V).
Figure 11. SEM pictures of deposited Zn of SE/Dowfroth 250 5(a), 10 (b), 15(c) ppm. and XRD patterns (d) of all the deposited Zn.
Figure 11. SEM pictures of deposited Zn of SE/Dowfroth 250 5(a), 10 (b), 15(c) ppm. and XRD patterns (d) of all the deposited Zn.
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