Impurities remaining in partially refined copper manufactured by typical metallurgical processes make it unacceptable for certain uses. Electrorefining, the purification of metals by electrolysis, has been used to remove remaining impurities such as bismuth, tellurium, arsenic and antimony and to recover any gold or silver sometimes associated with the copper.
In electrorefining, the metal sought to be refined is used as the anode, or positive electrode of the electrochemical process. Placed in an appropriate electrolyte solution and subjected to a weak electric current, the copper anode is gradually consumed. The copper ions go into solution, migrate through the electrolyte, and collect at the negative electrode or cathode where they are deposited as essentially pure copper, called electrolytic cathode copper.
One problem with the electrorefining process is that impurities associated with the partially refined copper anode may become dissolved or suspended in the electrolyte and collect at the cathode to contaminate the desired pure copper. If the copper used to form the anode is too impure, the electrorefined copper will still contain too many contaminants. Even very small amounts of impurities on the order of several ppm or less make the electrorefined cathode copper unacceptable for certain applications. The problem caused by the impurities carried along with the copper cathode may necessitate the costly and time consuming recycling through the anode casting and electrolysis process in order to achieve a copper product of acceptable purity.
Copper anodes used in electrorefining are ordinarily rapidly cooled after being cast. Fire refined copper, or a similar partially refined copper, is cast into anode molds and immediately flooded with water after solidification. The anode is then immersed in water for final quenching. U.S. Pat. No. 3,227,644 and U.S. Pat. No. 4,050,961 disclose rapid cooling methods for preparing metal anodes. Anodes prepared by the usual rapid cooling processes have a rather homogeneous structure, i.e. they contain impurities as discrete particles highly dispersed in the matrix of the dominant metal or as a solid solution in the dominant metal. It is thought that rapid cooling from the liquid to solid state does not permit time for the impurities to migrate within the molten mass and segregate from the copper. Further, rapid cooling in the solid state prevents precipitation and coalescing of impurities in solid solution. When such an anode is used in the electrorefining process, the finely divided impurities contained within it are more easily dissolved and suspended in the electrolyte solution increasing the likelihood of their deposition and entrainment at the cathode to contaminate the copper being refined.
It is thus one object of the instant invention to prepare a copper anode from which a smaller amount of the impurities dissolve and/or become suspended in the electrolyte to cause contamination of the cathode.
Another object of the invention is to formulate an anode which can utilize copper containing more than the usual allowable amount of impurities and yet yield an acceptable electrorefined copper product.