The production of lime in kilns is a more than 2000 year old technology, believed to have been developed by the Romans around 300 B.C. The process of burning limestone at temperatures above 900° C to produce quicklime, which is subsequently slaked with water to produce hydrated lime, has since become traditional practice in most countries, as lime is one of the most versatile materials known, being used for numerous industrial and agricultural processes, environmental protection and building construction.
Lime is also obtained as a by-product in the form of lime sludge (which contains calcium carbonate and various impurities) from sugar manufacture, and from acetylene and paper industries.
The chemical reactions in lime burning are:
Reaction 1: (900° C, depending on type of limestone)
CaCO3 + heat à CaO +CO2
Reaction 2: (at around 750° C):
CaMg(CO3)2 +heat àCaCO3 +MgO+CO2
then Reaction 1 (at around 1100° C)
• The chemical process of lime burning shows that the main constituent in the raw material (limestone) is necessarily calcium carbonate (CaCO3). Limestone can have CaCO3 contents exceeding 98 % (as in chalk and various types of shells and coral) or as low as 54 % (in pure mineral dolomite).
• Each type of limestone yields a different quality of lime, depending on the type and quantity of impurities. The purest forms of lime are needed for chemical and industrial use, while impurities can be desirable in limes used for building and road construction. Limestones, called "kankar" in India, that contain 5 to 25 % of clay can produce a hydraulic lime, which hardens in the presence of water, like a cement.
• By-product lime sludge is moulded into bricks or briquettes before firing in kilns.
• The presence of impurities in the limestones influences its behaviour during burning, so that the kiln design and choice of fuel are largely dependent on the raw material and the kind of end product required. Expert advice is therefore essential at a very early stage, in order to achieve satisfactory results, both for the lime producer and user.
• Preparing the raw material is extremely important as only one size of stone (about the size of a man's fist) should be used, in order to facilitate an even gas flow and uniform burning of the lumps. Small-scale firing trials are important to study the behaviour of the raw material and the quality of quicklime it yields, and also to make sure that the lumps do not break apart until they leave the kiln.
• Wood and coal are the most common, traditional fuels. Wood firing produces some of the best quality limes, as it burns with long, even flames generating steam (from the moisture content of the wood), which helps to lower the temperature needed for dissociation (separation of CO: from the carbonates), thus reducing the danger of overburning.
• The wood must be seasoned (dried) and cut into relatively small pieces. The wood supply should be close to the kiln in order to avoid heavy transport costs. About 2 m3 bulk of wood is needed for each tonne of hydrated lime produced. This is a problem, in view of the rapid depletion of timber resources, but a possible solution is to establish fuelwood plantations for continuous replacement of the harvested wood.
• Charcoal gives a higher fuel efficiency, but the lime produced is not as good as that burnt with wood.
• Coal with a high carbon content produces a good lime and can show good fuel economy even in small kilns. Coke is preferable because of its low volatile content (hydrocarbons which can be driven off as vapour), but is hard to ignite, and is, therefore, often mixed with coal.
• Liquid and gaseous fuels, though more expensive, are easier to handle than solid fuels, and burn without producing ash which contaminates the lime.
• The main types are heavy fuel oils, often mixed with used motor oil. The fuel is vaporized, mixed with air and ignited in chambers located around the kiln, producing a fully developed flame before it comes into contact with the limestone.
• Liquified petroleum gases, mainly propane (C3H8) and butane (C4H10), are other useful liquid fuels. Natural gas, such as methane (CH4), and producer gas, which is made from wood, plant material or coal, are used in the same way.
• Whether oil or gases are used, the kilns will necessarily be more sophisticated than those needed for solid fuels.
• Possible alternative fuels are peats and oil shales, and biomass energy, derived from plant material including agricultural and forestry wastes. There are several ways in which they can be used.
• Solar and wind energy are unlikely to be used in the near future.
Kiln design and operation
• A lime kiln is a built structure, in which limestone is heated to a temperature at which CO2 is released, converting the stone into quicklime. The heat is provided by burning suitable fuels, which are either placed in layers between the limestone or mixed with it. Liquid or gaseous fuels are either injected from the sides of the kiln or burnt in adjacent chambers, from which hot gases are passed through the kiln.
• Careful control is needed to maintain the correct temperature long enough to burn the stone completely. Underburnt limestone will not hydrate, while overburnt material is too hard and dense for slaking, or hydrates very slowly.
• As the variety of kiln types is extremely wide, they can only be described here in general terms. The more sophisticated types (eg rotary and fluidized bed kilns) are not dealt with, although in certain situations their use may indeed be worth consideration.
• Batch or intermittent kilns are generally used in remote places, where continuous supplies are not needed (eg small building projects or road construction). They are loaded with limestone and fired until all the stone has been burnt. After cooling, the quicklime is extracted, the limestone reloaded and the kiln fired again. The fuel efficiency is naturally very low, as the kiln walls have to be reheated each time a new batch is fired. On the other hand, it requires little attention during firing. The fuel is burnt below the limestone (in updraught or flare kilns) or within the entire batch (in mixed feed batch kilns).
• Vertical shaft kilns are designed mainly for continuous production: the stone, fed in from the top, gradually drops into the burning zone, then into the cooling zone, and is finally extracted from below, making room for the next load, and so on. The top layer is preheated by the exhaust gases and the air intake below is preheated by the cooling quicklime, thus achieving maximum use of the available heat.
The main design features and operational considerations with regard to vertical shaft mixed feed kilns are:
• Foundations and kiln base: built on a firm ground and dimensioned to carry the shaft and kiln contents; an engineer's advice is needed.
• Shaft dimensions and shape: the cross-sectioned area is related to the desired output (rule of thumb: 1 m2 produces about 2.5 tonnes per day): a circular plan provides better heat distribution; the ratio of height to diameter should be at least 6: 1 for optimum gas flow; the height must be related to the type of limestone, as soft stones tend to get crushed under the pressure, thus restricting the gas flow (kilns for soft chalk should not exceed 5 m height); shafts that taper towards the top (angle about 3°) minimize "hanging" (stone sticking to the sides and forming arches).
• Structural walls: must support the lateral pressure of the limestone (by provision of greater wall thickness at the base, or buttresses, or by means of steel tension bands at intervals of 80 cm, as developed by the Khadi and Village Industries Commission, Bombay); must resist cracking due to heat expansion (by using small bricks rather than big blocks, and lime-sand mortar in narrow joints); wall thicknesses of at least 50 cm for good thermal performance; weather resistant material (natural stone or well-burnt bricks) at least for the top wall courses.
• Linings: at least 22 cm thick, in the upper part of the kiln, resistant to abrasion (eg hard stone or blue engineering bricks); in the firing zone and below, resistant to heat and chemical action (hard, fine-textured refractory bricks laid with very fine joints of fireclay mortar).
• Insulation: usually 5 to 10 cm thick, between wall and lining to retain the heat in the kiln, especially around the calcining zone; different insulations are possible (eg air-gap, rice husk ash or other pozzolana, light-weight aggregate, rockwool).
• Openings: at the top for charging, preferably with lid, if a chimney extends beyond the opening, at the bottom for air to flow in and to remove the cooled quicklime, whereby with a single opening in the centre (inflow type) draught control is easier than with two or more openings (outflow type); around the kiln at different levels as pokeholes and inspection holes, usually the size of a brick (which is used for closing), to regularly loosen stuck limestone lumps and to monitor the temperature within the kiln.
• Chimney: between 2.5 and 6 m high, to improve the draught and thus provide sufficient oxygen for combustion, to cool the quicklime, and to draw the exhaust gases away from operators loading the kiln.
• The type of lime that is used for building and numerous other processes is hydrated or slaked lime. This is obtained by adding hot water or steam to quicklime. Pure quicklimes react vigorously evolving considerable heat, while impure limes hydrate slowly, or only after the lumps are ground.
Three forms of hydrated lime are commonly produced:
a. dry hydrate, a dry, fine powder, formed by adding just enough water to slake the lime, which is dried by the heat evolved;
b. milk of lime, made by slaking quicklime with a large excess of water and agitating well, forming a milky suspension;
c. lime putty, a viscous mass, formed by the settling of the solids in the milk of lime.
• The most common form is dry hydrate, which is very suitable for storage in silos or airtight bags, and easy to transport. Lime putty, which is an excellent building material, can be stored indefinitely under moist conditions. Milk of lime is generally produced in conjunction with other process industries.
• In small limeworks, slaking is usually done by hand, either on platforms to produce a dry hydrate or in shallow tanks to make lime putty.
• Although the hydration of quicklime is a simple process, it must be carried out with special care, for instance, to see that all the quicklime is completely slaked. Pieces that hydrate too slowly and as a result are overlooked, can cause serious problems later on.
• If water is added too slowly, the temperature of the lime may rise too fast, forming an inactive white gritty compound ("water burnt" lime). If water is added too quickly, a skin of hydroxide may develop, preventing further hydration ("drowned" lime).
The Central Building Research Institute in India has developed a small hydration plant, which requires very little space and eliminates most of the problems of hydration, producing uniform qualities of dry hydrate in a relatively short time.
The location and layout of a lime-works are vital factors that influence the economy and quality of lime production. The illustration (from Bibl. 06.08) shows an appropriate site organization in which distances between successive operations are relatively short.
• Lime is used as a stabilizer in soil constructions with clayey soils, because the lime reacts with clay to form a binder.
• Lime is mixed with a pozzolana (rice husk ash, fly ash, blast furnace slag, etc.) to produce a hydraulic binder, which can partially or completely substitute cement, depending on the required performance.
• Hydraulic lime (made from clay-rich limestone) can be used without a pozzolana.
• Non-hydraulic lime (pure calcium hydroxide) is also used as a binder in renders. It hardens on reaction with the carbon dioxide in the air to change back to limestone (calcium carbonate). This process can take up to 3 years, depending on the climatic conditions.
• Lime is used in cement mortars and plasters to make it more workable.
• Limewash (diluted milk of lime) is used as an external and internal wall coating.
• Lime is produced with less energy input than cement, making it cheaper and environmentally more acceptable.
• In mortars and plasterwork, lime is far superior to portland cement, providing gentle surfaces which can deform rather than crack and help to control moisture movement and condensation.
• Since the strengths produced by portland cement are not always required (and sometimes can even be harmful), lime-pozzolanas provide cheaper and structurally more suitable substitutes, thus conserving the cement for more important uses.
• Limewashes are not only cheap paints, but also act as a mild germicide.
• Soil stabilization with lime requires more than twice the curing time needed for soils stabilized with cement.
• If quicklime is stored in moist conditions (even humid air), it will hydrate.
• Hydrated lime, stored for long periods, gradually reacts with the carbon dioxide in the air and becomes useless.
• Lime bursting (hydration of remaining quick lime nodules) can take place long after the component has dried, causing blisters, cracks and unsightly surfaces.
• Plain limewashes take a long time to harden, and are easily rubbed off.
• Traditional lime burning in intermittent kilns waste a great deal of fuel (usually firewood) and often produce non-uniform, low quality limes (overburnt or underburnt).
• The value of lime is greatly underestimated, especially since portland cement has become a kind of "miracle" binder almost everywhere.
• The curing time of lime stabilized soils can be shortened by using hydraulic limes or adding a pozzolana to non-hydraulic limes.
• Quicklime has to be hydrated before use in construction work, therefore this should be done soon after it is unloaded from the kiln, as hydrated lime is much easier to store and transport.
• To prevent rapid deterioration of dry hydrated lime, it should be stored in air-tight bags.
• It is advantageous to store the lime in the form of lime putty, This can be done indefinitely, as the quality of the lime putty improves the longer it is stored. By this method, even the slowest hydrating quicklime particles are slaked, thus avoiding lime bursting at a later stage.
• A great deal has to be done to disseminate information and assist local lime producers in constructing more efficient lime kilns (in terms of fuel consumption and lime output).
• Similar efforts are needed to rehabilitate lime as one of the most important building materials.
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