This classic series of chemical reactions is the basis for numerous applications of lime, many of which affect our lives every single day. Lime is a word used to describe a suite of products that includes four main types of materials: limestone, quicklime, hydrated lime and milk of lime. The diagram shows how each of these sits in a different stage in the sequence of processes that are collectively known as The Lime Cycle.
Industrial mineral lime is produced by heating limestones or chalks, formed of calcium carbonates, at high temperatures above 900°C in industrial kilns that operate continuously. The kilns are lined with refractory bricks that are highly resistant to the abrasion and temperature in the kilns and as efficient insulation, retain the maximum of heat within the kiln. The stone is fed into the top of the kiln and passes down through the kiln as the calcined stone is removed from the base of the kiln.
The process starts with the quarrying of limestone or chalk which is then crushed, cleaned, sorted and graded as kiln feed stone which is around fist size. The remaining smaller stone, not suited by size to be used in the kilns, can be further processed and sold to various markets.
The heating process causes a chemical reaction called calcination and this produces calcined lime also known as quicklime. The quicklimes, after cooling down, are used in various differing applications as it is removed from the kiln or further processed by crushing and grinding, and, by the controlled addition of water to form hydrated lime products, which can be made with a range of designed characteristics.
Kilns require replacement of their refractory brick lining periodically and otherwise operate almost continuously throughout their lifetime usually of many decades, maximising operational efficiency. As such, investments in lime manufacturing are planned for years in advance to ensure operational and commercial resilience are maintained. Maintenance and retrofit operations can take place at high temperatures by specialist staff and under strict health and safety regimes.
To meet the demands of lime applications across the markets where it is used, lime is manufactured using natural gas as the kiln fuel using two types of gas-fired kiln; vertical shaft kilns and parallel flow regenerating kilns (PFRK) which are widely considered to be the most energy efficient. Investment by the UK lime sector in PFRK kiln technologies over recent decades means that most of the lime production is in this type of kiln. The fuel comes directly into contact with the product within the kilns and natural gas is the preferred fuel because it introduces fewer impurities compared to other fuels and is readily available through the natural gas transmission system. Natural gas also has lower carbon emissions when compared to solid fuels.
When a calcium limestone or chalk rock, that comprises mainly of calcium carbonate (CaCO3), is heated in a kiln, it changes by a process called calcination into quicklime also known as 'burnt lime' and chemically is mainly calcium oxide (CaO), and the calcination process releases a gas from the rock which is carbon dioxide (CO2).
When water (H2O) is added very carefully to quicklime/burnt lime/calcium oxide (CaO), a further chemical process called hydration (also called slaking) takes place and the resulting material is called hydrated lime or slaked lime and is mainly calcium hydroxide [Ca(OH)2]. The hydration/slaking chemical reaction is exothermic - it releases heat - and the amount of energy is significant such that it can boil and evaporate water and will cause a thermal burn to flesh or even start a fire if in contact with combustible materials that are heated and dried by the reacting chemical. The emissions stack from a hydration vessel (also known as a bath) at a lime works is emitting steam to the atmosphere as the bath is operated at around 100°C.
When the hydrated lime is produced, the carefully controlled process of hydration enables the balance between the water and the calcium oxide to be such that the hydrated lime is produced as a dry powder when it exits the hydration bath. This is known in chemistry as the stoichiometric ratio being in balance. The hydrated lime does not need any further drying, which would require an energy source and add cost if it did.
Many applications of hydrated lime are in combination with materials in liquid phases and therefore require the lime reagent to be provided in a liquid form for easier introduction and handling in the process. Producing a liquid suspension of hydrated lime in water is readily achieved simply by adding the dry powder hydrated lime to water and stirring. The resulting liquid is known as milk of lime because it resembles cow’s milk in appearance and consistency and can be poured and pumped like water or milk. A standard dry hydrated lime can be mixed in concentrations of up to around 20% by weight that can remain as suspensions at room temperature for reasonable time periods, though they do require agitation to remain in suspension for longer periods. Special processing at the hydration stage can produce hydrated limes that are able to remain in suspension for longer without stirring.
One of the special uses of hydrated lime is in the production of Precipitated Calcium Carbonate known as PCC. This is where following hydration, the hydrated lime is immediately made into a milk of lime and then subsequently, the milk of lime [Ca(OH)2] has carbon dioxide (CO2), that has been captured from the emissions from the calcination process in the kiln, added to it. The carbon dioxide causes the reforming of calcium carbonate (CaCO3) in a process called carbonation, and the calcium carbonate then precipitates (falls) out of solution. The PCC is then dried and further processed for use in a wide range of applications from pharmaceuticals to paper coatings and from food additives to personal care products.
It could be asked ‘Why not use the original calcium carbonate?’. The answer is that the characteristics of the PCC are controllable, unlike the original natural calcium carbonate, this is what makes the difference. The PCC manufacturing process delivers highly consistent products, without natural variations.
Over the lifetime of lime products, wherever it is possible for the lime product to achieve the hydrated lime stage of the lime cycle, there is the potential for carbon dioxide to be gradually re-absorbed by hydrated lime from the air or any source of carbon dioxide, which is carbonation or can be known as recarbonation. Chemically, this begins to turn the hydrated lime back into calcium carbonate. The carbonation of lime products means that when the production of the lime from limestone can be achieved without any carbon dioxide emissions, 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.