High-grade ores are running out. Low-grade ores contain only a little metal, and extracting metal from them by smelting or electrolysis uses a lot of energy. Two biological methods can be used instead.
In bacterial leaching, bacteria feed on the minerals in a low-grade copper ore and release copper ions into a solution called the leachate. The copper is then recovered from the leachate, for example by displacement using scrap iron or by electrolysis. In phytoextraction, plants are grown on soil that contains metal compounds. The roots take up the metal ions and store them in the plant. The plants are harvested and burned, and the metal is obtained from the ash.
To evaluate them: both methods use less energy than smelting, need no huge quarries, and can use low-grade ores or waste. But both are slow, taking months or years. Phytoextraction needs a large area of land and gives only a small amount of metal, and the leachate from bacterial leaching can be acidic or toxic and can pollute water if it escapes.
