A geologically altered form of limestone known as dolomitic limestone is formed of calcium magnesium carbonates. When it is calcined, dolomitic limestone requires higher temperatures to form dolomitic quicklime or dolime than calcium limestones. Dolime is used in applications such as steel and glass manufacture and the production of kiln and incinerator refractories and refractory repair products.
The kiln feed is produced in the same ways as for lime and the stone is fed into one end of slowly rotating horizontal kilns. Horizontal kilns and solid fuels are needed to achieve the higher firing temperatures associated with dolomitic lime products that are not normally achievable in vertical kilns. The stone is moved along the kilns using the rotation motion. Preheating of the stone for maximum energy efficiency can be achieved by the passing of the hot gasses from the kiln through a preheater section before the stone enters the kiln.
The dolomitic limestone is heated to above 900°C to calcinate to a dolomitic quicklime, referred to as dolime. Horizontal kilns use a range of solid fuels including some secondary fuels such as tyres and other combustible solids as well as being capable of using biomass fuels.
The chemical reactions in making lime using the geologically altered form of limestone known as dolomitic limestone, or dolomite, are more complex than those for making lime, though they follow similar principles. Dolime is a word used to describe a suite of products that includes four main types of materials: half burnt dolime, dolime, half hydrated dolime and hydrated dolime. The diagram shows how each of these sits in a different stage in the sequence of processes that are collectively known as The Dolime Cycle.
This is the term for a limestone that has been altered by geological processes that involve the replacement of the calcium ions by magnesium ions that is called dolomitization. There are several mechanisms by which this dolomitization can take place including evaporative, seepage-reflux, mixing zone, burial and seawater dolomitization processes. The general name of the rock changes from being limestone to being dolomitic limestone also known as dolomite.
When dolomite that comprises mainly of calcium and magnesium carbonate, (CaCO3.MgCO3), is heated in a kiln, it changes by a process called calcination initially into half burnt dolomite and chemically this is a mixture of calcium carbonate and magnesium oxide (CaCO3.MgO), reflecting the earlier calcination of the magnesium carbonate in the process. The calcination process releases a gas from the rock which is carbon dioxide (CO2).
Retaining the material in the kiln at higher temperature for longer and continuing the calcination process leads to the formation of burnt dolomite or dolime, which chemically is a mixture of oxides of calcium and magnesium (CaO.MgO) as the remaining carbon dioxide is released.
When water (H2O) is added very carefully to dolime (CaO.MgO), under normal atmospheric conditions, a further chemical process called hydration (also called slaking) takes place and the resulting material is called half hydrated lime and is a mixture of calcium hydroxide and magnesium oxide [Ca(OH)2. MgO]. The hydration process chemical reaction is exothermic - it releases heat - and the amount of energy is significant though the rate of reaction of dolime is much slower than for quicklime.
When the hydration process is carried out at elevated pressure, the magnesium oxide is able to hydrate and the resulting material is called hydrated dolime and is a mixture of calcium and magnesium hydroxide [Ca(OH)2. Mg(OH)2].
As with hydrated lime, the hydrates of dolime have the potential for carbon dioxide to be gradually re-absorbed from the air or any source of carbon dioxide, to turn the calcium and magnesium hydroxides of hydrated dolime back into a forms of calcium and magnesium carbonates. The more complex chemistry of the dolomitic materials mean that the rate and completeness of carbonation will be more complex than for quicklime. Still, the carbonation potential of dolime products means that when the production of the dolime from dolomite can be achieved without any carbon dioxide emissions to atmosphere, the carbon dioxide recarbonation can remove carbon dioxide from the atmosphere and become a means to lock in the carbon dioxide to help with climate change.
MPA Lime is part of the Mineral Products Association, the trade association for the aggregates, asphalt, cement, concrete, dimension stone, lime, mortar and industrial sand industries.
MPA Lime, 1st Floor, 297 Euston Road, London NW1 3AD
T: 0203 978 3400