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Why is arsenic often recovered as a by-product of metal smelting?

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A labeled diagram of the periodic table entry for arsenic, showing its atomic number 33, symbol As, atomic weight 74.921595, electron configuration, and classification as an other nonmetal. Additional icons indicate arsenic is solid at room temperature, has a rhombohedral crystal structure, and weakly acidic oxides. — arsenic Properties of arsenic.

Arsenic is frequently recovered as a by-product of metal smelting because it is commonly found as an impurity within the ores of other valuable metals, such as copper, lead, cobalt, and gold.12

During the smelting process, these ores are heated in the presence of air. Because arsenic impurities in these ores unite readily with oxygen, they form arsenious oxide (As $_{4}$ O $_{6}$ ), also known as "white arsenic". This oxide is easily sublimed, meaning it turns into a vapor that can be collected and condensed from the furnace gases. This captured dust serves as the primary commercial source for arsenic, which is then purified or used as a starting material for other arsenic compounds.1