No 1. Traditional Batch Techniques at Chenkumbi, Malawi
Practical Action
The quality of the lime produced ranges between 32 and 45% available lime content.
(Available lime content is the measure normally used to compare the quality of limes. It
means the amount of chemically active or “free” lime and is expressed as the CaO content. In
a hydrated lime the theoretical maximum available lime content is 75%)
Mode of Operation
Production is carried out on a batch basis with each cycle averaging 60 days. On average 3
batches are produced per kiln per year, although some producers may produce 5 batches per
year in a single kiln.
Each batch typically produces about 50 tonnes of lime hydrate which is sold in units of 25
kg. Approximately 15 tonnes of waste unburnt material is also produced which is normally
used as feed in the next kiln batch.
Little lime is produced in the wet season due to the difficulty in obtaining fuelwood and to
heavy rainstorms adversely effecting the burning process.
In 1991 there were more than 40 lime producers at the Chenkumbi Hills producing lime in
the manner described. The producers are organized into a Lime Makers Association which
sets agreed selling prices and has provided an avenue through which technical and financial
assistance can be channelled.
Comments
Traditional lime-burning techniques have the following advantages:
the capital costs involved in production are minimal;
the operation of the kiln is simple and well understood by the producers. In addition
the batch method of production is flexible and well suited to fluctuating demands for
lime;
the labour-intensive methods of production provide both permanent and casual
employment opportunities.
However, there are also a considerable number of problems and disadvantages:
the quarrying methods employed are both laborious and inefficient;
the kiln design and the batch method of production are very energy inefficient. Fuel
efficiency is estimated at below 15%;
the kilns' demand for slow burning hardwoods has contributed to deforestation in the
area surrounding Chenkumbi. This has resulted in considerable environmental
damage and led to difficulties in the lime producers obtaining adequate fuel
supplies, particularly as Malawi Government policy is against the felling of
indigenous trees for industrial purposes. In the longer term the technology is neither
environmentally nor economically sustainable;
a combination of dense, coarse crystalline rock (which is difficult to calcine fully),
and inefficient kiln design and firing procedures result in very low calcining
efficiencies, estimated at around 60%. In other words only 60% of the marble in the
kiln is converted into quicklime, the other 40% remaining as unburnt waste
material;
the tendency to leave the quicklime exposed in the kiln before hydration results in
undesirable air slaking,
due to inefficient slaking techniques the quicklime is rarely fully hydrated;
the sieving to separate the hydrated lime from unbumt material is ineffective with a
large proportion of unbumt material being passed on for milling;
the mills used are not designed for lime and are therefore not particularly efficient at
this particular task. The presence of hard unbumt material produces wear on the mill
hammers and, as there is no separation of powdered material prior to milling, -
energy is wasted in attempting to mill already fine material.
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