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During the leaching also other metallic components nobler than zinc like copper, cobalt, nickel and cadmium are leached and must be removed before electrowinning. Outotec is able to engineer the purification by either arsenic or antimony process.
The antimony process is a three-stage process consisting of coppercadmium removal and separate cobalt-nickel removal, followed by polishing. Cobalt and nickel are precipitated using antimony trioxide and zinc dust.
The chemistry of the Solution Purification Process based on arsenic was developed during the early 1970's. Outotec Solution Purification Process offers a low investment cost. The three-stage process has been tested and proven on industrial scale. The process yields pure, high-quality zinc solution for the electrowinning process. Low zinc dust consumption and increased zinc production are additional benefits. |
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Copper and chloride
The first stage of solution purifi cation is copper removal. Copper is precipitated from the zinc solution in a continuous and profi cient way. It is essential to remove copper, as it will otherwise precipitate with zinc onto the cathode and, as a result, reduce current efficiency in electrolysis. Copper is cemented using zinc dust and copper, and cuprous oxide is produced, reducing zinc dust consumption and thus operating costs.
The copper cake produced can be sold to copper smelters.
If necessary, chloride removal can be implemented at the copper removal stage. Outotec has devised an effective chloride removal method, improving the quality of zinc by keeping the chloride content under permissible levels. In order to avoid the corrosion of electrodes in electrolysis, it is advisable to remove chloride from the zinc solution.
Cobalt and nickel
Outotec is able to engineer cobalt and nickel removal, either using arsenic or a conventional antimony process. Through Outotec's continuous arsenic process, cobalt and nickel are removed in the second stage through the addition of arsenic trioxide and zinc dust. Their removal is essential since the presence of these materials lowers the effi ciency of the electrical current during electrolysis. Using this process, zinc dust consumption can be minimized and the cobalt and nickel contents in the solution are reduced in a reliable and effi cient manner.
Cadmium
Third and final step after copper and cobalt purification is cadmium removal. Cadmium is removed with zinc dust in fluidised bed reactors. This reactor type is developed and tested by Outotec. A cake containing more than 80% cadmium is produced in this step and can very economically be refined into cadmium metal.
The antimony based purification is another alternative. Outotec has built a remarkable number of antimony based purification processes all over the world under the trade name of Lurgi and is well known for its experience in this field.
Cadmium refining process
When using the arsenic based purification process to produce cake with high cadmium content, the cake can very easily be further treated into a cadmium sponge which can be briquetted.
When using the antimony based purification process the filter cake from the first and second purification steps contains all the cadmium, cobalt, nickel as well as some copper together with the excess of zinc dust. This filter cake is treated in the cadmium plant to recover a Co-Ni-Cu residue and cadmium as cadmium sponge with a high purity. The cadmium plant operates in batch process.
The cadmium cake can be further treated into briquettes. The refined metal can be cast into wire or sticks by using UPCAST® technology. The refining process yields 99.98% pure cadmium. |
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Gypsum
Gypsum must be removed from the zinc solution prior to its being sent to the electrowinning department, as it will form crystals in the tankhouse cells. Gypsum removal is performed by cooling the solution with cooling towers, and separating the precipitate from the main zinc solution using SUPAFLO® thickeners. Continuous and effi cient control of the process is essential in order to extract the maximum amount of gypsum.
For further information, please contact  zinc(at)outotec.com
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