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Pollution Prevention and Control Technologies for Plating Operations


Section 3 - Chemical Recovery

3.5 ELECTROWINNING

3.5.7 Operational and Maintenance Experience

The following summarizes the respondents O&M experiences and provides operating labor information.

  • The average number of annual operating hours per electrowinning unit were: 140 hrs/yr. The labor categories commonly used for operating this technology are wastewater treatment plant operator and trained technician. The following is a breakdown of the responses for skill requirements (based on data from 39 respondents):
     Environmental Engineer:................4
     Process/Chemical Engineer:.............5
     Chemist:..............................12
     Consultant:............................2
     Plumber/Pipe Fitter:..................13
     Electrician:..........................11
     Vendor:................................3
     Senior-Level Plater:..................10
     Junior Level Plater:..................14
     Wastewater Treatment Plant Operator:..20
     Trained Technician:...................17
     Common Labor:.........................10
      
  • The average percentage of downtime for this technology experienced by the respondents was 20 percent.
  • The most frequent and significant operational and maintenance problems identified with electrowinning include: labor intensive to clean (e.g., anode cleaning, electrode contact cleaning) (PS 025, PS 053,); high level of fuming or gassing (PS 036, PS 053, PS 128); sluffing off of deposit from cathode (PS 043); temperature build-up (PS 036); salting of the electrolyte (PS 036); anodes polarize at high current density and deteriorate or are attacked by chemicals (PS 213); anodes passivate (PS 239); and deterioration of fiber cathodes (PS 086).
  • Approximately 40% of the total number of electrowinning units reported in the survey forms are no longer in use.
  • Some shops reported that poor support from the manufacturer was partially the cause of their system failure. PS 008 indicated that their zinc recovery unit was removed after 4 to 5 months of operation because they could not get help with system problems ("everything went wrong...like pulling teeth to get help").
  • Two of the shops that purchased equipment from HSA Reactors Ltd. indicated that the fact the company went out of business led to the failure of their systems (PS 012, PS 276). The carbon cathodes and other equipment components used in their products were too unique to find elsewhere. Also, users cited numerous mechanical problems (e.g., pump failures-PS 086) with these units and they complained about the fragile nature, short life and high cost of electrodes (PS 039, PS 086). The labor costs for HSA systems appear to be higher than the average electrowinning system, which points to the complexity of the systems. Further, the operation of the equipment required a moderate level of expertise. As one shop (PS 276) indicated, it was "too technical for our people." Generally, it is observed that this technology was probably sound from a theoretical standpoint, but lacked good engineering design and components. A similar conclusion is presented in a report sponsored by the Canadian Branch of the AESF (ref. 351). That report concluded that such a system could be operated in a manual mode, but that more developmental work was needed before an adequate automated system could be marketed.
  • Some facilities have added sodium chloride to the electrolyte to increase the efficiency of cyanide destruction (PS 036). One shop reported unsatisfactory results, because the solution temperature increased and gassing occurred and the residual cyanide level was too high for sewering (PS 036).
  • Shops that reported difficulty with the electrowinning process cited two possible technical causes, including: (1) contamination in electrolyte (PS 036, PS 090) and (2) build-up of carbonates (PS 036).
  • Shops using the small precious metals recovery units (both commercial and in-house) reported none or very few operational and maintenance problems.

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