Why is cadmium usually obtained as a by-product of zinc processing?

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Periodic table — zinc group element Modern version of the periodic table of the elements (printable).

Cadmium is considered a rare element in the Earth's crust, occurring primarily in small quantities within other ores rather than in independent mineral deposits. It is most commonly found associated with sphalerite, the chief ore of zinc, which typically contains between 0.1 and 0.3 percent cadmium.1

Because cadmium is naturally integrated into these zinc-bearing minerals, it is recovered as a by-product during the standard industrial processing of zinc. During the initial stages of zinc production, the ore is roasted to convert zinc sulfide into zinc oxide. This process causes the cadmium to become concentrated in the resulting fumes or flue dusts, which are then collected and treated through various chemical steps to isolate and refine the cadmium. Consequently, the production of cadmium is inextricably linked to the extraction and smelting of zinc.1