Improving Processes: Metals, LCAs and Recycling
Atom Economy and Percentage Yield
In industrial chemistry, two key measures tell us how efficiently a reaction uses its raw materials. Atom economy tells you what fraction of the mass of the reactants ends up as the wanted product rather than waste. A high atom economy means less waste is generated, lowering environmental impact and production costs.
The formula is:
$$\text{atom economy} = \frac{M_r \text{ of desired product}}{\text{sum of } M_r \text{ of all products}} \times 100$$
Percentage yield compares the actual amount of product made to the theoretical maximum:
$$\text{percentage yield} = \frac{\text{actual yield}}{\text{theoretical yield}} \times 100$$
Percentage yield is often below 100% because of incomplete reactions, side reactions, reversible reactions that do not go to completion, or product lost during purification. The limiting reactant determines the theoretical yield — once it runs out, the reaction stops regardless of how much of the other reactant remains.
Atom economy measures what proportion of reactant atoms become the desired product (from the balanced equation). Percentage yield measures how much product was actually made compared to the theoretical maximum. Both should be as high as possible for economic and environmental reasons.
Life Cycle Assessments
A life cycle assessment (LCA) evaluates the environmental impact of a product at every stage of its life. The four main stages are: (1) extracting and processing raw materials, (2) manufacturing, (3) using the product (including transport and packaging), and (4) disposing of the product at the end of its useful life.
LCAs consider: energy consumption at each stage, raw material use, water use, emissions to air and water (including greenhouse gases and toxic substances), waste produced, and the potential for recycling or reuse at the end of life. However, some factors — such as visual impact, noise, or local ecosystem disruption — are difficult to quantify precisely. This means LCAs involve some subjective judgement, and different analysts can reach different conclusions if they weight factors differently. Companies sometimes use selective LCA data to support marketing claims, so evaluating LCA evidence critically is important.
Recycling
Recycling reduces the need to extract new raw materials, saves energy, and reduces the volume of waste going to landfill. Factors that affect decisions about recycling include: the cost of collection and sorting, the energy needed to process recycled material compared to making new material, the market value of the recycled product, and public willingness to separate and recycle materials.
Metals are particularly worth recycling because mining and refining metal ores is very energy-intensive. Recycling aluminium, for example, uses only a small fraction of the energy required to extract aluminium from its ore (bauxite) by electrolysis. Glass and paper can also be recycled effectively, though the economic viability depends on collection costs, sorting complexity and market conditions.
Recycling conserves finite resources, reduces energy use, and cuts landfill waste. Metals such as aluminium are especially valuable to recycle because extracting them from ore by electrolysis is very energy-intensive. The overall benefit must weigh the energy and cost of collection and processing.
Extracting Metals from Low-Grade Ores
Traditional smelting requires high concentrations of metal ore and generates significant environmental impact. Where ore is too dilute (low-grade) to make smelting economically viable, alternative biological methods can be used. Two techniques are particularly important: phytomining and bioleaching.
Phytomining uses plants that can absorb and accumulate metal ions from the soil through their root systems. These plants, sometimes called hyperaccumulators, concentrate metal ions in their leaves and stems. The plants are then harvested and burned; the resulting ash contains a much higher concentration of the metal compound than the original soil. The metal is then extracted from the ash, often by smelting or chemical treatment. The process is slow (growing the plants takes time) but has a lower environmental impact than traditional opencast mining.
Bioleaching uses bacteria that produce acidic solutions (leachate) by reacting with metal sulfide ores. The leachate dissolves the metal compounds out of the ore. The resulting metal-rich solution (often called the ‘pregnant leach solution’) can then have the metal extracted from it either by displacement (adding a more reactive metal) or by electrolysis. Bioleaching is also slower than conventional extraction but requires less energy input and can process waste heaps and low-grade ores that would otherwise be uneconomical to use.
Phytomining uses plants to absorb metal ions; the plants are burned and the metal extracted from the ash. Bioleaching uses bacteria to produce acidic solutions that dissolve metals from low-grade ores; the metal is then removed by displacement or electrolysis. Both are slower but more sustainable alternatives to conventional smelting for low-grade ores.
Traditional reduction with carbon is used for metals below carbon in the reactivity series (such as iron, zinc and copper). Carbon displaces the metal from its oxide when heated at high temperatures. Electrolysis is required for metals above carbon in the reactivity series (such as aluminium) because they cannot be displaced by carbon. Electrolysis uses large amounts of electrical energy, making it expensive. By contrast, phytomining and bioleaching require very little energy but are slow, making them best suited to low-grade or dilute ore bodies where faster methods are not viable.
Sustainability in Chemistry
The principles of sustainable chemistry combine all the ideas above: maximising atom economy to reduce waste, conducting LCAs to guide product design, recycling where possible to conserve finite resources, and using biological or lower-energy extraction methods where appropriate. Chemistry plays a central role in meeting the needs of the present without depleting resources that future generations will need.