AQA AS Chemistry Unit 2: Extraction Of Metals

Chemistry Unit 2 The Extraction Of Metals

• Metals are usually found in ores containing minerals, in which the metals are combined with oxygen or sulphur. In order to obtain the metal, these other elements have to be removed using a reduction reaction.

• The chemist must choose a suitable reducing agent. Ideally this should be cheap and readily available. Any by-products should be non-polluting and easy to dispose of.

• The ores contain unwanted minerals such as clay and rock that have to be removed, these are called gangue. An ore must contain enough metal for it to be economic to mine and extract the metal. If the metal is valuable, the ore could be worth mining even if it contains relatively little of the metal.

Converting sulphide ores to oxides

• Before reduction, sulphide ores are usually converted to oxides by heating them in air, a process usually called roasting.

e.g. ZnS _(s) + 1½ O_{2 (g)} → ZnO _(s) + SO_{2 (g)} Zinc Sulphide + Oxygen → Zinc Oxide + Sulphur Dioxide

• The problem is that the by-product, sulphur dioxide, is an acidic gas which, if it is allowed to escape, is converted into sulphuric acid by reaction with water and oxygen in the atmosphere. This is a contributor to acid rain.

• This sulphur can be collected and converted into sulphuric acid in controlled conditions. It can then be sold for a variety of purposes which makes the industrial process more economical.

SO_{2 (g)} + H₂O _(g) → ½O_{2 (g)} + H₂SO_{4 (l)}

Choosing a reducing agent

• Coke is often chosen as a reducing agent. It is cheaply attained by heating coal in the absence of air. For some metals the temperature required is so high that the process is uneconomic.

• Hydrogen is a possible reducing agent (made from methane and water), this method is used to extract tungsten.

• Metals higher up the reactivity series may be reduced by electrolysis.

• Another possibility is to use a more reactive metal as a reducing…