Bioleaching

Bioleaching is a way of extracting metals, mainly copper, from low-grade ores using bacteria. The bacteria break down the metal compounds in the rock and produce a solution called a leachate that contains metal ions. The copper is then recovered from the leachate, either by displacement with scrap iron or by electrolysis.

Why bioleaching is used

Copper is usually extracted from rich ores by smelting: heating the ore strongly. But the Earth's metal ores are limited, and high-grade copper ores are becoming scarce. What is left are low-grade ores, which contain only a small amount of copper. In an ore that is 0.4% copper, a tonne of rock holds just 4 kg of copper, so digging up and smelting 250 tonnes of rock would give one tonne of copper.

Biological methods, bioleaching and phytomining, can extract copper from these ores and from mine waste without the traditional mining work of digging, moving and disposing of huge amounts of rock.

How bioleaching works

1. The low-grade ore is piled into heaps or left in place, and kept wet with an acidic solution.
2. Bacteria grow on the ore. Species of Acidithiobacillus, such as A. ferrooxidans, get their energy by oxidising iron and sulfur in metal sulfide minerals.
3. The metal goes into solution. As the sulfide minerals break down, copper ions are released into the liquid trickling through the heap. This copper-rich liquid is the leachate.
4. The leachate is collected from the bottom of the heap.
5. Copper is extracted from the leachate by displacement or electrolysis.

Bioleaching is slow. In one study of low-grade copper sulfide ore, the bacteria had recovered nearly 45% of the copper after 75 days.

Getting the copper out of the leachate

Displacement with scrap iron

Iron is more reactive than copper, so it displaces copper from solutions of copper compounds. Scrap iron is cheap, which makes this a good way to use it:

As an ionic equation, Fe + Cu²⁺ → Fe²⁺ + Cu. The iron is oxidised (it loses electrons) and the copper ions are reduced (they gain electrons). See the reactivity series guide for more on displacement.

Electrolysis

When a current is passed through the leachate, the positive copper ions are attracted to the cathode. Each one picks up two electrons there and is deposited as copper metal:

Electrolysis gives very pure copper but uses more electricity. The electrolysis guide explains how it works.

Example: copper from scrap iron

How much copper could 2.8 kg of scrap iron displace from a leachate, if all the iron reacts? Use Ar: Fe = 56, Cu = 63.5.

Moles of iron = 2800 ÷ 56 = 50 mol. The equation shows 1 mol of Fe gives 1 mol of Cu, so 50 mol of copper forms. Mass of copper = 50 × 63.5 = 3175 g, about 3.2 kg.

Check your understanding.

Advantages and disadvantages of bioleaching

In the exam you may be given data about an extraction method and asked to evaluate it. Weigh up the points on both sides, use any numbers you are given, and finish with a judgement.

Bioleaching vs phytomining

Common mistakes

• Saying bacteria produce copper metal. They produce a solution of copper compounds; another step gets the metal out.
• Mixing up bioleaching and phytomining. Bacteria and leachate means bioleaching; plants and ash means phytomining.
• Saying bioleaching is fast. It is slow, which is one of its main disadvantages.
• Writing the displacement the wrong way round. Iron displaces copper because iron is more reactive.
• Forgetting why it is needed. High-grade copper ores are running out.

Practice questions

1. What is a leachate?
2. Give one reason why bioleaching is used to extract copper.
3. Write a word equation for extracting copper from copper sulfate solution using iron.
4. Give one environmental disadvantage of bioleaching.

Answers: 1) A solution containing metal compounds, produced by bacteria. 2) Any one: high-grade copper ores are scarce; it can use low-grade ores; it avoids digging and moving large amounts of rock; it uses less energy than smelting. 3) Iron + copper sulfate → iron sulfate + copper. 4) The acidic, toxic leachate can pollute soil and water if it escapes.

Practise with a short timed paper

Extracting metals and using the Earth's resources come up in GCSE Chemistry. The daily GCSE Chemistry Higher paper is a short timed paper of 8 questions, with a new one every day.

Read next

Chemistry · Reactions and electrolysisPhytomining and bioleaching: how they extract copperWhat phytomining is, how plants extract copper from low-grade ores, how bioleaching works, getting copper from solution, and the pros and cons, for GCSE.