Lime kiln designs
Practical Action
Description of the Kilns
This leaflet considers the production of quick- lime in vertical shaft kilns fired with oil.
Another leaflet in this series describes kilns which can be fired with coal or wood.
Vertical shaft kilns work on a very simple principal. The raw material is fed in at the top of the
kiln and the product is withdrawn from the bottom, causing the material to move slowly
downwards through the kiln. Heat to calcine the material is introduced roughly in the middle
of the kiln and therefore any" material above is preheated by rising hot exhaust gases, and any
below cooled by incoming air. In this way, material entering the kiln at the top is first
preheated, then calcined and finally cooled during its passage through the kiln.
For vertical shaft kilns the feed rock should be crushed to sizes between 40 and 150 mm.
Since it typically takes 1.75 kg of limestone to produce 1 kg of lime the transportation of
the raw material should be kept to a minimum. Therefore lime kilns are normally located
close to the limestone quarry.
The use of oil as fuel has both advantages and disadvantages compared with the more
traditional methods using coal or wood as a mixed feed with the limestone. The main
advantage is that there is normally no contamination of the lime with soot or ash. Better
control can also be achieved over kiln temperatures ensuring a better quality, more valuable
lime. The main disadvantages are that the kiln design needs to be more sophisticated, and
therefore is more expensive. In some locations wood or coal is both cheaper and more readily
available than oil. The latter point is significant in that use of oil in these locations may
require importation. In oil-
producing countries the use
of oil as fuel could be
practicable even when the
quality of lime is of
secondary importance.
Practically all types of fuel
oil can be used. In the case
of bunker C oil, this has to
be preheated to 100-
120°C to make it less
viscous. The use of waste
oil, such as old engine oil,
is not recommended for
very high quality lime, but
may be acceptable for lime
of lower quality. It may be
necessary to mix waste oil
with a quantity of fresh oil,
as it is rarely available in
sufficient quantities for a
continuous production run.
Figure 1: 6 ton per day kiln.
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