Extracting metals

Methods of extracting metals The Earth's crust contains metals and metal compounds such as gold, iron oxide and aluminium oxide, but when found in the Earth these are often mixed with other substances. To become useful, the metals have to be extracted from whatever they are mixed with. A metal ore is a rock containing a metal, or a metal compound, in high enough concentration to make it economic to extract the metal. Ores are mined. They may need to be concentrated before the metal is extracted and purified. The economics of using a particular ore may change over time. For example, as a metal becomes rarer, an ore may be used when it was previously considered too expensive to mine.

Reactivity and extraction method Metals are produced when metal oxides are reduced (have their oxygen removed). The reduction method depends on the reactivity of the metal. For example, aluminium and other reactive metals are extracted by electrolysis, while iron and other less reactive metals may be extracted by reaction with carbon or carbon monoxide.

Metals (in decreasing order of reactivity) Method of extraction potassium sodium calcium magnesium aluminium extract by electrolysis carbon zinc iron tin lead extract by reaction with carbon or carbon monoxide hydrogen copper silver gold platinum extracted in various ways Reactivity and extraction method

Extraction by reaction with carbon Metals such as zinc, iron and copper are present in ores as their oxides. Each of these oxides is heated with carbon to obtain the metal. The metal oxide loses oxygen, and is therefore reduced. The carbon gains oxygen, and is therefore oxidised. Using iron as an example: iron oxide + carbon → iron + carbon dioxide 2Fe 2 O 3 (s) + 3C(s) → 4Fe(l) + 3CO 2 (g) Note that the iron is liquid when it is formed, due to the very high temperature at which the reaction takes place. Some metals, such as aluminium, are so reactive that their oxides cannot be reduced by carbon.

Electrolysis Ionic compounds contain charged particles called ions. For example, copper(II) chloride contains positively charged copper ions and negatively charged chloride ions. Ionic substances can be broken down into the elements they are made from by electricity, in a process called electrolysis. For electrolysis to work, the ions must be free to move. When an ionic compound is dissolved in water, or melts, the ions break free from the ionic lattice. These ions are then free to move.

For example, if electricity is passed through copper(II) chloride solution, the copper(II) chloride is broken down to form copper metal and chlorine gas

There is a similar result if electricity is passed through molten copper(II) chloride. The solution or molten ionic compound is called an electrolyte. The negative electrode is called the cathode, while the positive electrode is called the anode. This is what happens during electrolysis: Positively charged ions move to the negative electrode. Metal ions are positively charged, so metals are produced at the negative electrode (cathode). Negatively charged ions move to the positive electrode. Non-metal ions, such as oxide ions and chloride ions, are negatively charged, so gases such as oxygen or chlorine are produced at the positive electrode (anode).