The invention is related to additives for zinc electrowinning that inhibit hydrogen evolution and/or improve current efficiency for zinc electrodeposition, specifically cetylpyridinium-based additives.
Improving the energy efficiency of the zinc electrowinning process by inhibition of the parasitic hydrogen evolution reaction, which occurs in parallel with zinc deposition, is of major technological and commercial interest. One way of minimizing the cathodic hydrogen evolution is by the use of additives, generally organic compounds, which selectively increase the hydrogen evolution overpotential. Mackinnon et al. (Journal of Applied Electrochemistry, Volume 20, pages 728-736, 1990) and Scott et al. (Journal of Applied Electrochemistry, Volume 18, pages 120-127, 1988) describe the use of animal glue in combination with antimony to improve the current efficiency for zinc electrowinning when compared to additive-free electrolytes.
There is a need for improved additives that minimize hydrogen evolution during zinc electrowinning while providing the same or improved performance over traditional additives.
Therefore, it is an object of this invention to provide improved additives for zinc electrowinning that minimize hydrogen evolution while providing similar or improved performance over traditional additives.
Cetylpyridinium chloride (CPC), a cetylpyridinium salt, was tested as an additive in a zinc electrowinning process in two separate zinc electrowinning electrolyte compositions: 1) with antimony and 2) with both antimony and glue.
The CPC additive had the most significant influence in the presence of antimony or antimony+glue combination, where it increased the current efficiency by 23.2% and 7.6%, respectively. Moreover, the presence of 0.05 mM CPC did not increase the overall cell voltage.
Methods and Apparatus
Commercial beaker test cells containing a commercial electrowinning electrolyte (liquor) were connected to a power supply and placed in a 37° C. water bath. The anodes and cathodes were made of lead and aluminium, respectively. The laboratory supplied MSDS sheet indicated for the electrolyte the following composition: zinc sulfate 28-34% by weight, magnesium sulfate 9-15 g/l (grams/litre) and manganese 1.5-2.5 g/l.
After allowing the temperature inside the test cells to reach the desired value of 37° C., a constant current of 0.045 A, representing an electrowinning current density of 450-amperes/meter2, was applied for either 4 or 20 hours to a non-agitated electrolyte. After completion of the experiment the electrode assembly was removed from the glass beaker, rinsed with distilled water and the cathode deposit carefully scraped off and weighed with four digits precision using a digital Mettler AE 100 analytical balance. The test cells were rinsed between experiments with distilled water and acetone in order to remove traces of the organic additives. Replicates were also performed and the standard deviation was estimated.
The zinc electrodeposition current efficiency was calculated based on Faraday's law:
where
The cetylpyridinium chloride (CPC) (e.g. Sigma-Aldrich, U.S.) had the following structure:
Zinc Electrowinning Liquor with Antimony Present
Antimony (Sb) in 0.04 mg/l (milligrams/litre) concentration was added as antimony—potassium tartrate to the zinc electrowinning electrolyte. Both four and twenty hour runs were performed. The experimental results of the four-hour runs are summarized in Table 1.
Without the CPC additive present in the electrolyte, Sb had a detrimental effect on the current efficiency, i.e. between 65.1% (cell no. 15) and 74.7% (cell no. 14). On average, the current efficiency without the CPC additive present was 69.9%. Adding CPC improved the current efficiency on average by 23.2%, i.e. from 69.9% to 93.1%.
The effect of CPC in longer-term (20 hours) experiment is shown in Table 2. Without the CPC additive, the current efficiency of cell no. 15 was only 36.6%, whilst with 0.05 mM CPC present the zinc electrowinning current efficiency was 58.9%. Thus, with the CPC additive the current efficiency was higher by 22.3%.
Zinc Electrowinning Liquor with Both Antimony and Glue
Similar experiments to those in Example 1 were performed with an electrolyte containing 0.04 mg/l of Sb and 10 mg/l of glue (e.g. “pearl glue” supplied by Hudson Industries, Johnstown, N.Y.). Both 4 and 20 hour runs were carried out. The effect of the CPC additive on the zinc electrowinning current efficiency in the 4 hour runs is shown in Table 3.
The presence of glue minimizes to a certain extent the negative effect of antimony, yielding current efficiencies between 88.9% and 90%. However, adding 0.05 mM CPC brought about additional increases of current efficiency, i.e. from 89.4% on average in the absence to 97% in the presence of CPC (Table 3).
The 20 hour experiments (Table 4) showed that CPC in 0.05 mM concentration increased the current efficiency of cell no. 16 from 77.2% to 87.3%.
The cell voltage is another important figure of merit of the electrowinning process. An increase in the cell voltage represents an increase in the amount of energy required and, therefore, a less efficient electrowinning process. Table 5 shows that using 0.05 mM CPC in conjunction with Sb and glue did not induce an increase of the cell voltage.
Accordingly, while this invention has been described with reference to illustrative embodiments, this description is not intended to be construed in a limiting sense. Various modifications of the illustrative embodiments, as well as other embodiments of the invention, will be apparent to persons skilled in the art upon reference to this description. It is therefore contemplated that the appended claims will cover any such modifications or embodiments as fall within the scope of the invention.
Filing Document | Filing Date | Country | Kind |
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PCT/CA02/01250 | 8/13/2002 | WO |
Number | Date | Country | |
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60311825 | Aug 2001 | US |