Lime: an introduction
Practical Action
Almost any fuel can be used in lime burning. Traditionally, wood was most commonly used
but increasing cost, environmental concerns and deforestation have restricted its use. Coal is
probably the most common fuel used in recent years. Others fuels include oil, gas, some
agricultural wastes and even, in a few cases, peat.
Burning
The techniques employed in burning lime can
vary considerably. The simplest (and least
efficient) method consists merely of a circular
pile of logs with limestone heaped on top of it.
This is then ignited and is likely to produce
less than 500kg of lime per burn.
At the other end of the scale, sophisticated
and energy-efficient rotary kilns operating
continuously can produce over 500 tonnes of
lime per day.
Probably the most common type of lime kiln is
the vertical shaft, of which there are many
variants. Basically they consist of a tall,
chimney-like, cylindrical shaft. Limestone is
loaded at the top and while being burned falls
slowly under gravity as lime is withdrawn from
the bottom.
The limestone is first pre-heated, burned and
then cooled as it passes slowly through the
kiln. Vertical shaft kilns will normally produce
between 2 and 250 tonnes of lime per day
depending upon kiln size.
Figure 2: A vertical shaft kiln for small-
scale production of lime. Chegutu,
Zimbabwe. Photo: Practical Action / Kelvin
Mason.
Other leaflets in this series describe a number of kiln types and give case studies of lime
production at varying levels of output and mechanisation.
Hydration
Hydration, in small quantities, can be undertaken manually by sprinkling water onto a pile of
quicklime which is then turned and mixed with a rake as more water is added. In large
quantities, hydration is normally automated and done in large hydrators where measured
quantities of water and quicklime are fed in and mechanically agitated.
During hydration, the quicklime lumps will disintegrate to a fine powder. For high quality
limes some form of screening and/ or classification, during which the lime is sorted by
particle size and density, will be required to grade the lime. Hydrated lime is normally
supplied and sold bagged, as a dry powder.
If quicklime is hydrated with an excess of water and well agitated, it forms a milky
suspension known as milk of lime. Allowing the solids to settle and drawing off the excess
water forms a paste-like residue known as lime putty. Lime in this form is considered, by
many, to be preferable for use in building as it ensures complete hydration, produces
excellent mortars and, if kept in a saturated condition, will not deteriorate over time.
Dolomitic and hydraulic limes
In some locations, raw material with high calcium carbonate contents will not be available.
This may not be a constraint for lime used in the construction industry since lime containing
impurities can be tolerated and may even have advantages.
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