Abstract:
A cetylpyridinium salt, cetylpyridinium chloride (CPC) is used as a hydrogen evolution inhibitor (a current efficiency improver) in a commercial zinc electrowinning process. Zinc electrowinning compositions containing a) antimony and b) antimony and glue were tested. Adding CPC at a 0.05 mM concentration to the electrowinning liquor resulted in increased current efficiency for both electrolytes.

Description:
FIELD  
       [0001]     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.  
       BACKGROUND  
       [0002]     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.  
         [0003]     There is a need for improved additives that minimize hydrogen evolution during zinc electrowinning while providing the same or improved performance over traditional additives.  
         [0004]     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.  
       SUMMARY  
       [0005]     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.  
         [0006]     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. 
     
    
     DETAILED DESCRIPTION  
       [heading-0007]     Methods and Apparatus  
         [0008]     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.  
         [0009]     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/meter 2 , 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.  
         [0010]     The zinc electrodeposition current efficiency was calculated based on Faraday&#39;s law:  
               CE   ⁢           ⁢     (   %   )       =         z   ·   F   ·     m   d         I   ·   t   ·     A   Zn         ·   100             (   1   )             
 
 where 
    CE—current efficiency for Zn electrodeposition (%)     z—no. of electrons exchanged [=2]    F—Faraday&#39;s number [=96485.3 C mol −1]      m d —amount of zinc deposit (g)     I—applied current [=0.045 A]    t—time (s)     A Zn —atomic weight of zinc [=65.39].    
 
         [0019]     The cetylpyridinium chloride (CPC) (e.g. Sigma-Aldrich, U.S.) had the following structure: 
    C 21 H 38 N + Cl −  
                         
   
 
       EXAMPLE 1  
       [heading-0021]     Zinc Electrowinning Liquor with Antimony Present  
         [0022]     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.  
         [0023]     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%.  
                                                                 TABLE 1                           Effect of CPC on the current efficiency of       zinc electrowinning in the electrolyte containing       0.04 mg/l Sb (as Sb-tartrate).            Conditions:   Current efficiency (%)           Temperature: 37° C.   per Cell No.   Average            Time: 4 hours   14   15   CE (%)                    No additive   74.7   65.1 ± 1.0   69.9 ± 6.8       CPC 0.05 mM (mM = millimolar)   93.7   92.7 ± 2.9   93.1 ± 0.9                  
 
         [0024]     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%.  
                                           TABLE 2                           Effect of CPC on the zinc electrowinning       current efficiency in 20 hour experiment with 0.04 mg/l       Sb in the electrolyte.                Conditions:   Current efficiency (%)           Temperature: 37° C.   per Cell No.           Time: 20 hours   15                            No additive   36.6           CPC 0.05 mM   58.9                      
 
       EXAMPLE 2  
       [heading-0025]     Zinc Electrowinning Liquor with Both Antimony and Glue  
         [0026]     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.  
         [0027]     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).  
                                                                                 TABLE 3                           Effect of CPC on the current efficiency of       zinc electrowinning in the electrolyte containing       0.04 mg/l Sb (as Sb-tartrate) + 10 mg/l glue.                Conditions:       Current efficiency               Temperature: 37° C.       (%) per Cell No.   Average                Time: 4 hours   13   16   CE (%)                            No additive   88.9   90.0 ± 0.9   89.4 ± 0.8           CPC 0.05 mM   98.3   95.8   97.0 ± 1.8                      
 
         [0028]     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%.  
                                           TABLE 4                           Effect of CPC on the zinc electrowinning       current efficiency in 20 hour experiment with 0.04 mg/l       Sb and 10 mg/l glue in the electrolyte.                Conditions:   Current efficiency (%) per           Temperature: 37° C.   Cell No.           Time: 20 hours   16                            No additive   77.2           CPC 0.05 mM   87.3                      
 
         [0029]     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.  
                                           TABLE 5                           Effect of CPC on the average cell voltage       in 4 hour experiments with 0.04 mg/l Sb and 10 mg/l       glue in the electrolyte.                Conditions:               Temperature: 37° C.           Time: 4 hours   Average cell voltage (V)                            No additive   2.83           CPC 0.05 mM   2.83                      
 
         [0030]     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.