text: The development of the Isa Process tank house technology had its beginning in the zinc industry. During the mid-1970s, MIM Holdings Limited (“MIM”) was considering building a zinc refinery in Townsville to treat the zinc concentrate produced by its Mount Isa operations. As a result, MIM staff visited the zinc smelters using the best-practice technology and found that modern electrolytic zinc smelters had adopted permanent cathode plate and mechanised stripping technology.
  MIM recognised that the performance of traditional copper refineries was constrained by the poor cathode geometry inherent in using copper starter sheets.
  MIM then developed a research program aimed at developing similar permanent cathode technology for copper refining. CRL had been operating in Townsville since 1959, using conventional starter-sheet technology and treating blister copper produced in the Mount Isa Mines Limited copper smelter at Mount Isa in Queensland. CRL incorporated the permanent cathode technology in its 1978 refinery modernisation project. The material initially selected was 316L stainless steel, stitch-welded to a 304L stainless-steel hanger bar. The hanger-bar assembly was then electroplated with copper to a thickness of 1.3 millimeters (“mm”) (later increased to 2.5 mm and then 3.0 mm to improve the corrosion resistance of the hanger bar) to approximately 15 mm down onto the blade, which provided sufficient electrical conductivity and gave the assembly some corrosion resistance.
  Electrodeposited copper adheres quite firmly to the stainless steel so that it does not detach during refining. The vertical edges of the stainless steel plates are covered with tight-fitting polymer edge strips to prevent copper depositing around the edge of the cathode plate and so make it easier to strip the cathode copper from them. The bottom of the cathode plates were masked with a thin film of wax, again to prevent the copper depositing around the bottom edge. Wax was used rather than an edge strip to avoid having a ledge that would collect falling anode slimes and contaminate the cathode copper.
  Wax was also used on the vertical edges to prolong the life of the vertical edge strip.
  The original cathode stripping machine was based on that used at the Hikoshima plant of the Mitsui Mining and Smelting Company of Japan. However, considerable development work was necessary to modify the design to handle the copper cathodes, which were heavier than those at Hikoshima, and to process the cathode plates without damaging them. The machines also had to be redesigned to allow for waxing the sides and bottoms of the cathode plates to allow the next copper cathode sheets to be removed easily.
  The stripping machines included receiving and discharge conveyors, washing, separation, cathode stacking and discharging, cathode plate separation for refurbishing, and the wax applications for the sides and bottoms of the cathode plates.
  The original CRL stripping machine had the capability of stripping 250 cathode plates per hour.
  The lower cost of the cathode plates compared to starter sheets means that shorter cathode cycle times are possible. The cycle time can range from 5 to 14 days, but a seven-day cathode cycle is common. This shorter cycle time improves current efficiency as less short circuits occur and there is less nodulation of the cathode surface.
  Initially, other refinery operators regarded the developments at CRL with scepticism. Stainless steel had been tried unsuccessfully as mother-plate material for copper starter sheets. They suffered from rapid deterioration of their strippability, resulting in “an almost daily increase in difficulty of stripping”. However, following the success of early installations in Townsville, Timmins, and many other places, the permanent stainless steel cathode technology has had widespread introduction.