The effects of certain impurities and their interactions on zinc electrowinning
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INTRODUCTION
THE presence
of impurities in the electrolyte is a major problem for the zinc electrowinning industry. Many investigations have been conducted to determine the impurity interactions occurring at the cathode and the maximum level of each impurity that can be tolerated in the electrolyte. ~-3~ Neutral purification eliminates the bulk of the impurities, but in certain instances their concentrations may still be high enough to cause difficulties in zinc deposition. Impurity behavior is not well understood, and many questions remain regarding the acceptable limits of impurities for efficient operation of a zinc electrolysis circuit. In addition, synergism among impurities may occur which further complicates the process and causes unpredictable behavior. 32'33 Factors such as acid level, zinc concentration, time of electrolysis, agitation, current density, and temperature also influence the magnitude of the impurity effects. It is well documented that zinc deposition is very sensitive to small quantities of certain impurities. Previous studies have indicated that levels in the parts per billion range for antimony, arsenic, and germanium can greatly reduce the current efficiency) 2~ For cobalt, the parts per million range is critical. 2>3~ A complicated purification process is used to try to reach these low levels in actual practice. 1.2.34 The effects of an impurity are often directly attributable to such factors as acid concentration and the temperature of the system involved. Thus, the maximum allowable levels of the impurities must be considered in relation to the process parameters. In addition to current efficiency, deposit morphology and cathodic polarization are also influenced DONALD R. FOSNACHT is Senior Research Engineer with Inland Steel Research Laboratories. 3001 East Columbus, East Chicago. IN 46312. THOMAS J. O'KEEFE is Professor of Metallurgical Engineering and Senior Research Investigator, Graduate Center for Materials Research. University of Missouri-Rolla. Rolla. MO 65401. Manuscript submitted January 10, 1983.
ME-IALLURGICAI. TRANSACTIONS B
by the levels of impurities and addition agents in the electrolyte. Profound changes in both these features have been reported.t7 t~.~s Organic agents such as glue are often added to the electrolyte to aid in process control. Improvements in deposit appearance and current efficiency may occur when glue is added to the electrolyte, but optimum levels do exist and must not be exceeded or certain deleterious effects can r e s u l t . 17.18.19 Since impurity interactions can create severe operating problems, it would be desirable to determine the basis for many of their interactions and make the process more predictable. This study examines the effects of antimony, arsenic, cobalt, germanium, and glue on current efficiency and deposit morphology at various temperatures and acid concentrations. In order to evaluate the effects of these variables simultaneously, a factorially designed set of experiments was conducted. High and low levels of each para
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