Alternative methods of extracting metals (HT only) — AQA GCSE Chemistry

Metal ores are naturally occurring rocks that contain enough of a metal compound to make extraction worthwhile. They formed over millions of years by geological processes, and the rate at which humans mine and use them is far faster than the rate at which they form. This means the Earth’s resources of metal ores are finite and will eventually run out if extraction continues at current rates. As high-grade ores become scarce, extracting metals from lower-grade ores or from waste becomes more important, but these alternative methods often need more energy or more complex chemical processes. Recycling metals also helps conserve ore resources and reduces the environmental impact of mining.

Copper is a finite resource obtained from ores that contain copper compounds. Rich, high-grade ores are being used up, so copper ores are becoming scarce; this raises cost and makes traditional extraction from low-grade rock uneconomic. Low-grade ores contain only small percentages of copper compounds, so large masses of rock must be processed to obtain useful copper. Two biological alternatives help here: phytomining, which uses plants, and bioleaching, which uses bacteria. Both release copper compounds into a form that can be recovered, for example by displacement or electrolysis. They suit low-grade ores because they concentrate copper without digging, moving and disposing of enormous volumes of rock.

Traditional copper extraction from low-grade ore involves digging out huge quantities of rock, transporting it and disposing of waste after processing. These steps use large amounts of energy, damage habitats and create spoil heaps. Phytomining and bioleaching avoid digging, moving and disposing of large amounts of rock because the plants or bacteria act on copper compounds in the ground or in heaps of low-grade ore. The copper is concentrated into plant ash or into a leachate solution, so only a much smaller mass needs further processing. This reduces landscape disturbance, waste rock and transport, although the methods are slower and may still need chemical processing to obtain copper metal.

Phytomining uses plants that grow in soil containing low-grade copper ore. The plants absorb copper compounds through their roots and store them in their tissues. After a period of growth, the plants are harvested. The harvested plant material is burned, and the ash that remains contains concentrated copper compounds. Because the copper has been concentrated from a large volume of soil into a small mass of ash, the ash can be processed further, for example by displacement using scrap iron or by electrolysis, to obtain copper metal. The method is slow and depends on plant growth, but it can make low-grade or contaminated land economically useful.

Bioleaching uses bacteria that feed on low-grade copper ore. As the bacteria break down the ore, copper compounds dissolve into a solution called leachate. The leachate contains copper compounds, which can be recovered by displacement using scrap iron or by electrolysis. Bioleaching avoids digging, moving and disposing of large amounts of rock because the bacteria act on the ore in situ or in heaps, and the copper is carried in a relatively small volume of solution. The process is slower than traditional mining and depends on conditions that suit the bacteria, but it can make low-grade ores economically useful and reduces landscape disturbance.

Alternative extraction methods like phytomining and bioleaching produce metal compounds, not the pure metals themselves. Phytomining yields ash containing metal compounds, while bioleaching produces a leachate solution containing dissolved metal ions. To obtain the useful metal, these compounds must be further processed. For example, copper compounds obtained from these alternative methods can be processed to extract pure copper. This is typically achieved by displacement using scrap iron, which is more reactive than copper, or by electrolysis of the leachate solution. Processing is essential because the metal remains chemically combined in the compound. These methods allow for the economical extraction of metals from low-grade ores while reducing environmental impacts compared to traditional mining.

Copper is less reactive than iron, so iron can displace copper from a solution of a copper compound. If scrap iron is placed in copper sulfate solution, the iron atoms lose electrons and dissolve as iron ions, while copper ions gain electrons and form copper metal. The word equation is iron + copper sulfate → iron sulfate + copper, and the ionic equation is Fe + Cu²⁺ → Fe²⁺ + Cu. Scrap iron is cheap and often waste material, which makes this an economical method. Alternatively, electrolysis of a copper compound solution uses a direct current to reduce copper ions at the negative electrode, producing copper metal. Electrolysis can produce very pure copper and can be used when displacement is unsuitable, but it needs electrical energy. Both methods obtain copper from a solution of a copper compound rather than from copper ore directly.

Biological methods use living organisms to extract metals from low-grade ores or waste. In bioleaching, bacteria obtain energy by breaking down metal sulfide ores, producing a solution containing metal ions that can be processed to obtain the metal. In phytomining, plants absorb metal ions from the soil through their roots; the plants are harvested and burned, and the ash contains the metal compound, which is then processed. To evaluate these methods you must weigh advantages against disadvantages using the information given. Advantages can include working on low-grade ores, reduced need for high temperatures, and less landscape disruption than open-cast mining. Disadvantages can include slow rates, the need for large areas of land, and the energy or chemical processing still required afterwards. A judgement must be based on the evidence provided, not on a general preference.