Pyroprocessing is a way to change a solid material physically or chemically using high temperature. It is a dry process and very energy-intensive. The three sectors that use it most are cement-making, metallurgy, and nuclear power.
Of these, cement-making uses pyroprocessing the most. First, finely ground limestone, clay and iron — the raw materials — are fed into a rotary kiln. As the temperature increases to 900° C, the limestone loses its carbon dioxide, and at around 1,450° C, the mix partly melts to form marble-sized nodules called clinker. The clinker is then ground to produce cement.
In metallurgy, pyroprocessing is used to extract metals from their ores in multiple stages. For example, sulphide ores are heated in air — or roasted — to convert them into metal oxides, like zinc sulphide. Smelting melts an ore to separate the metal from waste impurities called slag. Heating limestone to yield lime is likewise called calcining.
In the nuclear industry, pyroprocessing has a slightly different meaning. It is used to reprocess spent nuclear fuel, in techniques developed in the 1980s and 1990s. First, used nuclear fuel is broken up into pieces and placed in a salt bath — usually a mixture of lithium and potassium chlorides at 500° C or more. Then, an electric current is passed through the salt bath, causing different elements to become separated depending on their electrochemical properties. Operators then recover the elements of interest in separate streams.
This form of pyroprocessing has been the subject of study in Japan, South Korea, and the U.S. and is used as part of programmes involving advanced fast reactors.
Leave a Reply