Traditional extraction needs ores that contain a lot of metal, and it uses large amounts of energy. High-grade ores are running out, so there is a growing pile of low-grade ore that contains too little metal to be worth smelting. Two biological methods can use it.
Bacterial leaching uses bacteria that feed on the ore and break it down. The metal ions dissolve in water to make a solution called the leachate. The metal is then recovered from the leachate, for example by electrolysis or by displacement with scrap iron. Phytoextraction grows plants on soil that contains metal compounds. The plants absorb the metal ions through their roots and store them in their tissues. The plants are harvested and burned, and the metal is then extracted from the ash.
To evaluate them, compare the advantages and disadvantages. Both methods work on low-grade ores, use less energy than smelting and avoid the pollution from high temperature furnaces. Phytoextraction can also clean up contaminated land. However, both methods are slow and can take a long time, often months or years. The leachate can be acidic and toxic, and it can pollute rivers if it escapes. Phytoextraction needs a large area of land, and burning the plants releases carbon dioxide. Overall, the methods are best when the ore is low-grade and time is not critical.