British Lime Association



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	The British Lime Association
Information for Education What is lime Uses of Lime Information for Education A to Z of Lime Case Studies News and Events Publications Useful Links Jobs Home British Lime Association Gillingham House 38 - 44 Gillingham Street London Tel. 0207963 8000 Fax. 0207 9638001 email info@qpa.org			Calcium Calcium is the 5th most abundant element in the Earth’s crust and limestone / chalk forms 20% of the World’s sedimentary rock. In the UK, calcium carbonate is found as limestone or chalk. The Peak District limestone was formed during the Carboniferous geological period, some 340 million years ago. At this time, Britain was part of a large continental landmass close to the equator. In these tropical conditions rivers flowed into shallow warm seas teeming with primitive fish, molluscs, and coral reefs. Their calcium shells combined with silt to form layer upon layer of calcium carbonate rich sediments several hundred metres thick. The fossil remains of these ancient plants and animals are easily recognisable in limestone. In the chalk deposits, found mainly on the east of the UK, the rock is formed from the skeletons of billions of microscopic marine algae called coccolithophorids, which used sunlight to synthesise food. They died and settled on to the sea bed in the Upper Cretaceous period (between 65 – 100 million years ago). These algae can still be found in warm waters. The word calcium is derived from the Latin "calx", meaning lime. It was discovered early in the history of civilised man and there are references to lime in both Egyptian and Roman times, in fact as far back as the first century. Lime is a generic term which can cover limestone, chalk, quicklime and hydrated lime. There are a number of synonyms for each material, some of which are given below in the Lime Cycle: Limestone/Chalk Chemical Name - Calcium Carbonate Chemical Formula CaCO3 Synonyms calcium carbonate, limestone, chalk Molecular Mass 100.09 Decomposes when calcined at 900°C to form calcium oxide Calcining (or burning) - This involves heating the limestone/chalk to approx. 1100°C in a kiln CaCO3 = CaO + CO2 Quicklime Chemical Name - Calcium Oxide Chemical Formula CaO Synonyms calcium oxide, quicklime, burnt lime Molecular Mass 56.08 Melts at 2614 °C, boils at 2850 °C Hydration (or slaking) - This involves adding water, which produces an exothermic reaction (gives out heat). CaO + H2O = Ca(OH)2 Hydrated Lime Chemical Name - Calcium Hydroxide Chemical Formula Ca(OH)2 Synonyms calcium hydroxide, hydrated lime, slaked lime, Molecular Mass 74.09 Decomposes at 580°C to form calcium oxide Re - Carbonation - This involves reaction with carbon dioxide to return to chalk Ca(OH)2 + CO2 = CaCO3+ H2O (Back to calcium carbonate, this is sometimes known as precipitated calcium carbonate or PCC) Limestone/chalk, quicklime and hydrated lime are all alkalis of varying strengths. Which means that they will neutralise acids, in fact this is one of their major industrial uses. Treatment (scrubbing) of acid flue gases from power stations and incinerators uses either Limestone/chalk or hydrated lime and prevents “acid rain” by removing sulphuric and hydrochloric acid. Liquid acid waste streams can also be treated using limestone/chalk, quicklime or hydrated lime. Calcium salts of chemicals tend to be insoluble. This is another important property of lime products and can be used to remove soluble contaminants from a liquid waste stream by precipitating them out as a sludge which can be removed as a solid.