A case study in lime production ~ Sri Lanka
Practical Action
quicklime is recovered from the bottom with stone and fuelwood continuously being
replenished at the top.
Although lime producers in the Akurala and Matale areas claim that the percentage of
unburnt material is around 5 per cent, observations reveal that it could be as much as 10 per
cent or more. The amount of fuelwood used also varies considerably and observations made
reveal that the quantities used are much more than is claimed.
Hydration of quicklime
Most of the quicklime taken from the kiln is of a reasonable particle size for hydration. In
any case there is a fair amount of air slaking before the actual slaking operation is carried
out, which is done by pouring water over a fairly large pile of quicklime which is then mixed
manually with a shovel. The hydrated lime is then sieved to remove the larger lumps of lime
which have not hydrated properly before the product is bagged.
No milling is undertaken because the building industry and other users of lime are satisfied
with the product in this state, and to keep the processing costs low. The average calcium
oxide content by mass in lime samples from the Akurala area is 58 per cent and in dolomitic
lime samples 43 per cent. The magnesium oxide content by mass in the Matale dolomitic
samples of lime is around 26 per cent.
Mode of operation
Production in the Akurala area is on a batch basis with the recovery of about a ton of lime
each day. Kilns are fired depending on the availability of coral, which is getting more scarce
each day. Lime is normally sold by the ton, although units of 50kg and 25kg are also
available. In the Akurala area there are over 40 families producing lime. In recent years, due
to non-availability of coral the number of lime producers has been decreasing fast.
With production of dolomitic lime it should be noted that in dolomite the magnesium
carbonate (MgCO3) components dissociates to magnesium oxide (MgO) at around 775°C. The
dolomite when heated to 900°C, the lowest temperature normally used to break down the
calcium carbonate (CaCO3) component to calcium oxide (CaO), will produce MgO which is
over- burnt and not readily converted to the hydroxide, Mg(OH)2. If this material is used
without allowing the hydration to be completed, blistering of plasters can occur if used for
plastering walls. If the dolomite lime is to be used for mortar it need only be kept for about
three days after slaking. Masons and plasterers claim that dolomitic lime should be added to
water and not water to dolomitic lime for satisfactory results and kept for as long a period as
possible.
Comments
Traditional small-scale artisanal techniques of lime production have the following advantages.
Small-scale lime industries of the traditional type form a significant segment of the
economy. Collectively they represent the entrepreneurial ability of the local people.
The capital costs involved in production are minimal; they are labour intensive and
geared to supply local requirements.
The batch method as practised in the coastal areas is flexible and suited to
availability of raw materials and to the fluctuating market demand.
The disadvantages may be listed as follows:
Due to coastal erosion the State has banned the mining of coral close to the coasts
for lime burning and so this is difficult to obtain. Removal of inland coral deposits,
which is still being done, has resulted in large areas becoming flooded with water.
These ponds are now a serious environmental hazard.
The batch method of production in the Akurala area in traditional type kilns is
not energy efficient. This practice of lime burning dates back over 80 years.
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