Lime production: traditional techniques in Patarra, Costa Rica
Practical Action
Hydration and sieving
Only a few producers mill the lime or use new bags as part of their production. The quicklime
(calcium oxide, CaO) discharged from the kiln is mostly fragmented to particles less than
2mm in diameter, due to the friable character of the rock and to air slaking.
Slaking is performed by building a heap of quicklime, and adding water to the surface of it.
The hydrated lime (calcium hydroxide, CaOH) forms as powder and falls to the bottom of the
pile. In some cases it is then dropped through a hand-made steel wire-mesh sieve to remove
the coarse unburnt material of sizes more than l0mm across. The sieved product will contain
a lot of partially hydrated lime including some sizeable pieces of underburnt core material
less than 10mm across. Most of this lime is sold for building use or for conditioning of
agricultural soil. Only a small proportion of the product is then milled in a hammer mill
powered by an electric engine, of the type normally used to mill corn. The milling is
undertaken in order to produce a reasonably fine and uniform product for industrial uses.
The milled hydrated lime is then bagged by locally-made mechanized packing machines.
The quality of the lime produced ranges from 40% to 45% available lime content. Available
lime content is the measure normally used to compare the quality of lime. It is the amount of
chemically active or 'free' lime and is expressed as the CaO contents in hydrated lime, the
theoretical maximum available lime con- tent being 75%.
Mode of operation
Production is carried out on a batch basis, with each cycle averaging 8 days. On average 15
batches are produced per kiln per year, although some producers may turn out 35 batches
per year in a single kiln. Each batch typically produces about 27.4 tonnes of hydrated lime
which is sold in 13 pound weight bags.
Experienced producers normally do not produce lime containing much underburnt material.
Little lime is produced the peak of the rainy season due to the difficulty in obtaining dry
fuelwood and the heavy rainstorms which adversely affect the burning process.
There are more than 35 lime producers at Patarra producing lime in the manner described.
Half of the producers are organized into a cooperative called 'Cooperativa Nacional de
Productores de Cal’. This cooperative sets selling prices and has provided financial
assistance to its associates.
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.
• The quality of calcined lime is very good.
• The characteristics of the calcium carbonate in the area make it suitable to be
calcined with biomass.
• The fuelwood from plantations and other fuel sources such as sawmill wastes could
be used in the kiln; that is the kilns are not just limited to burning a certain type of
wood.
• The wood used for fuel is low in cost and likely to remain so.
• The construction and agricultural industries, the largest markets in the country, can
tolerate a certain level of impurity in their product, the cost of the product being a
more important factor.
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 under 16%.
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