No 1. Traditional Batch Techniques at Chenkumbi, Malawi
Practical Action
Arched vaults are built over the trenches with large marble boulders. The kiln is charged with
5 alternate layers of fuelwood and marble starting with kindling and small logs at the base
and ending in a heaped layer of marble feed at the top. Approximately 75 tonnes of marble
and 55 tonnes of wood are required for each batch.
The kiln is ignited and stoked for about 48 hours, after which the firing holes are sealed and
the kiln is left to bum out, which takes approximately 2 to 3 days. The kiln is then allowed to
cool for a further 6 to 8 days before discharging.
Figure1: Illustration of the Malawian traditional lime kiln.
Hydration and sieving
The kiln is discharged from the kiln top. Discharging may be done in two or three batches due
to lack of storage space for hydrated lime and to the producers preferring not to undertake
slaking, milling and bagging until a sale has been made. This unfortunately tends to result in
air slaking.
The quicklime (calcium oxide, CaO) discharged from the kiln is mostly fragmented to less
than 5mm diameter, due to both the friable character of the rock and to air slaking.
Slaking is performed by pouring water on to a reasonably large pile of quicklime, which is
then turned manually using a shovel, much in the same way as concrete is mixed by hand.
The hydrated lime (calcium hydroxide, Ca(OH)2) is then sieved through a hand punched metal
sheet sieve to remove the coarse unburnt material. In practice, material of less than 10 mm
will pass through the sieve and therefore the product will contain quite large quantities of
smaller unburnt cores.
The product from sieving is then milled in a hammer mill, powered by a diesel engine, of the
type normally used for milling maize. The milling is undertaken in order to produce a
reasonably fine and uniform product.
The milled hydrated lime is then bagged by hand.
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