Zinc Refining

The Effect of Air Sparging

The effect of variations in the rate of air sparging (0 to 5000 cm³/min) on the zinc deposit is shown in Figure 7. It can be seen that with no air sparging the surface of the zinc deposit was covered with a black powder (Fig. 7(a) which suggests that the limiting current for zinc deposition was exceeded under these conditions. The substantial decrease in current efficiency (89.9%) and the higher than average decrease in HCl concentration in the spent electrolyte suggests that increased hydrogen evolution occurred. The deposit orientation was [114], [105] which seems to be a characteristic of powdery type zinc deposits.

The maximum rate of air sparging, 4750 cm³/min, yielded the smooth, compact deposit shown in Figure 7(a), which was discussed in detail in the previous section, A one-half reduction in the rate of air sparging to 2375 cm³/min had no significant effect on the current efficiency (99.7%), but as indicated in Figure 7(a), the deposit surface was rough and nodular. Although this aspect of the study was not investigated further, it appears from these results that the rate of air sparging can be reduced to <4750 cm³/min but should be >2370 cm³/min to achieve smooth 24-h zinc deposits.

The deposit obtained with no air sparging was not suitable for SEM analysis but the cross section shown in Figure 7(b) indicates it to be nodular and to contain voids. Although less nodular, the cross section of the deposit obtained with an air sparging rate of 2370 cm³/min, Figure 7(c), also reveals the presence of voids in the deposit. The morphology of this deposit, Figure 7(d), is similar to that obtained at the higher air sparging rate, Figure 5(c).

The Effect of Additives

A series of 24-h zinc deposits obtained from zinc chloride electrolytes containing various additives is shown in Figure 8. The deposit obtained from the addition-free electrolyte was rough and nodular over part of its surface and dendrite formation occurred along the edges (Fig. 8(a)). The additives, TBACl, Percol 140 and Separan NP10, were effective in producing a deposit having a smooth surface and dendrite free edges, Figures 8(b), (c) and (e). Pearl glue was less effective in producing a smooth zinc surface and in eliminating dendritic growth at the edges of the deposit, Figure 8(d).

The structural details of the zinc deposits obtained in the presence of various additives (excepting TBACl) are shown in the series of SEM and OM photomicrographs in Figure 9; the deposit structure obtained in the presence of TBACl was described earlier and is shown in Figure 5.

The deposit obtained from the addition-free electrolyte, Figure 9(a), consists of large, poorly defined hexagonal zinc platelets very similar to the 1-h deposit described in a previous paper, (cf. Fig. 1(a)). The 24-h deposit also contains pores or voids as indicated by the OM photomicrograph, Figure 9(a).

Percol 140 was effective in reducing the deposit grain size, and this resulted in a smooth, compact deposit, Figure 9(b). The deposit consists of fairly distinct hexagonal platelets, aligned at intermediate angles to the Al substrate, This deposit morphology bears a strong resemblance to the characteristic zinc deposit morphology obtained from acid sulphate electrolyte, (cf. Fig. 1(b)).

The presence of Separan NP10 in the zinc chloride electrolyte resulted in poorly defined zinc platelets which were vertically aligned to Al substrate, Figure 9(c). Although the surface of the deposit appears smooth (Fig. 8(c)), the cross section reveals it to be uneven and to contain large voids, Figure 9(c).

Unlike the 1-h deposit obtained in the presence of pearl glue, the 24-h deposit consisted of nodules having a fine grain structure which formed on the surface of the smooth initial deposit layer, Figure 9(d).

In general, TBACl was the most effective addition agent in terms of smoothing the deposit, refining the grain size and eliminating dendritic edge growth (see Fig. 5). It had the added advantage of being the least complex of the additives studied and as such would be least likely to form degradation products which would have harmful effects in other parts of a process circuit.

The current efficiency, energy requirement and orientation results obtained for zinc deposits electrowon from zinc chloride electrolytes containing various additives are summarized in Table 4. In all cases the CE was >90% and the additives generally produced deposits having a predominantly 110 orientation; i.e., the zinc platelets are aligned vertically to the aluminum substrate.


Effect of NaCl Concentration

Situations may arise when the zinc electrolyte will contain large concentrations of NaCl; e.g, from a brine leaching process to remove associated PbCl2- For this reason, the effect of varying concentrations of NaCl on the zinc deposit structure and on zinc deposition current efficiency was studied. The photographs in Figure 12 indicate that dendritic growth and powder formation increase as the NaCl concentration is increased to 3 M.

The effect of increasing NaCl concentration on the structural characteristics of the zinc deposits is shown in the SEM and OM photomicrographs of Figure 13. For NaCl concentrations <2M, the deposit morphology (Fig. 13(a), (b)) is similar to that obtained under standard conditions (Fig. 5(c)) but the deposit orientation becomes more basal; i. e., [002], [103], (Table 6) as compared to intermediate [101] for standard conditions and vertical [110] for the increased Zn concentrations. For 3 M NaCl, the deposit is nodular and contains many deep voids (Fig. 13(c)). A substantial decrease in CE, from 96.3% at 0 M NaCl to 77.3% at 3 M NaCl, occurred also, Table 6.

The effect of deposition time or total ampere- hours on the quality of the zinc deposits electrowon from an electrolyte containing 15 g/L Zn and 3 M NaCl under otherwise standard conditions is shown in Figure 14. The deterioration in the deposit quality with time can be seen from the series of photographs, Figure 14(a) to (d).

After 6-h (12-Ah) depositon time, the deposit (Fig. 14(a) is fairly smooth near the center, but powder formation is prevalent on the sides of the deposit; further, the deposit edges indicate the on-set of dendrite formation. The CE for this deposit was 96.7%.

The 12-h (24 Ah) deposit (Fig. 14(b) is also smooth at the center but large dendrites have formed at the deposit edges and at the bottom of the deposit. The CE has decreased to 94.9%. After 18-h (36-Ah), the deposit (Fig. 14(c)) is rougher and more dendrites have formed on the surface of the deposit. The CE for this deposit was 89.9%.

Finally, after 24-h (48-Ah), the deposit (Fig. 14(d)) consists of a rough, dark-grey, powdery surface with large dendrites at the deposit edges. The CE was only 76.0%.

The fact that the Zn deposits deteriorate with increasing Cl concentration may be attributed to an increase in the formation of Zinc-chloro complex ions (e.g. ZnCl4=) at the expense of Zn2+ cations. An increase in total Cl concentration would favor the formation of zinc-chloro complex ions which could result in a corresponding decrease in Zn2+ concentration and hence give rise to a limiting current condition for Zn deposition which favors powder formation and dendritic growth.

The effect of increasing deposition time on the structural characteristics of the zinc deposits obtained from an electrolyte containing 3 M NaCl is shown in the SEM and OM photomicrographs, Figure 15. The deposit obtained after 12 Ah is even and compact, Figure 15(a), and its morphology is similar to that obtained after 48 Ah under standard conditions; i.e., in the absence of NaCl (cf. Fig. 5(c)).

Increasing the total Ah to 24 and 36 results in thicker deposits which remain even and compact; however, as revealed by the SEM photomicrographs (Fig. 15(b) and (c)), the tendency toward powder formation increases with increasing deposition time.

After 48 Ah (Fig. 15(d)), a high degree of powder formation occurs; the cross section is thinner than that after 36 Ah (cf. fig. 15(c) and (d)) because much of the growth has occurred on the deposit edges as large dendrites (see Fig. 14(d).

Effect of Current Density

The effect of varying the current density on zinc deposition from chloride electrolyte was studied over the range 161 A/m² (15 ASF) to 646 A/m² (60 ASF). The deposits obtained at the various current densities for a total deposition tine of 48-Ah are shown in Figure 16. The deposits were all very similar; the 646 A/m² deposit (Fig. 16(a)) showed more edge growth than was observed at the lower current densities. Variations in the current density had no significant effect on the deposit morphology (Fig. 16(b) which remained similar to that obtained under standard electrolysis conditions (Fig. 5(c). The cross section (Fig. 16(c)) for the deposit obtained at 484 A/m² was smooth and compact.

The current efficiency was >96% for all current density values, except 161 A/m² (15 ASF) where it decreased to 82% (Table 7). As expected, the energy requirement increased with increasing current density (Table 7) because of the higher voltages required. The deposit orientation was not affected by these variations in the current density.

Effect of Impurities

The effect of the impurities: Co, Cu, Cd, Ni, Fe(II), Fe(III), Sb and Pb, both individually and in various combinations, on the zinc deposit quality and on the current efficiency of zinc deposition was also studied. Zinc deposits obtained from chloride electrolyte containing various concentrations of Co, Cu, Fe and Sb under standard electrolysis conditions are shown in Figure 17.

The addition of 0.08 mg/L Sb to the electrolyte had no significant effect on the physical appearance of the zinc deposit (Fig. 17(a)) but the CE was reduced to 90% (Table 8). At an Sb concentration of 0.2 mg/L (Fig. 17(b)), the deposit edges became rougher but the CE increased to 94.12. At 0.5 mg/L Sb, the deposit edges consisted of a mixture of dendrites, powder and large modules; the CE was 88.6%.

The addition of 5 and 10 mg/L Cu to the electrolyte, Figures 17(c) and (d), respectively, resulted in zinc deposits characterized by black powder formation along the top and bottom edges. The deposit surface was noticeably rougher at 5 mg/L Cu; the CE decreased to 91% in both cases (Table 8).

The deposit obtained from an electrolyte containing 30 mg/L Co (Fig. 17(e)) was similar to that obtained from an “impurity-free” electrolyte; the CE was 94%. The presence of cadmium (10 mg/L) in the electrolyte resulted in a rough nodular zinc deposit; the CE was 91%.

The deposit obtained from an electrolyte containing nickel (10 or 30 mg/L) was rough and nodular. Black powder formed along the deposit edges, particularly at the higher Ni concentration. Deposit re-solution occurred and the CE decreased to 816% for 10 mg/L Ni and to 51.7% for 30 mg/L Ni.

The addition of 200 mg/L Fe(II) or Fe(III) to the electrolyte had no significant effect on the physical appearance of the zinc deposit but the CE was reduced to 89%.

Although reasonably high levels of certain metallic impurities (e.g. Co, Fe) could be tolerated, combinations of two or more of the impurities were detrimental to zinc electrowinning from chloride electrolyte. 30 mg/L Co and 0.2 mg/L Sb, Figure 18(a)), resulted in a rough, nodular deposit; some re—solution occurred and the CE was 88%.

A similar deposit (Fig. 18(b)) was obtained when the electrolyte contained 5 mg/L Cu and 0.2 mg/L Sb; black powder formation occurred along the deposit edges and the CE was 86.2%. The entire surface of the deposit obtained from an electrolyte containing 30 mg/L Co, 1 mg/L Cu and 0.2 mg/L Sb was covered with a black powder, Figure 18(c); the CE was 84.4%.

The deposit obtained from an electrolyte containing 1 mg/L Cu and 10 mg/L Pb was relatively smooth but showed some signs of re-solution near the bottom (Fig. 18(d)); the CE was 89.9%. The SEM photomicrograph (Fig. 18(e)) reveals a poorly crystalline deposit, typical of that obtained from sulphate electrolytes containing similar levels of Pb.