Report 2 (of 6): Cementing
Alternatives - Zimbabwe
Introduction
Lime is a versatile material produced by burning
limestone and is found in two main forms: quicklime
and hydrated lime. It has been used in numerous manufacturing
and processing industries and is an essential ingredient
of many soaps, bleachers and
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IT/Kelvin Mason - Chegutu Kiln, Zimbabwe |
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fertilisers. It can also be used in building, road
construction, agriculture, water and waste treatment.
Essentially, lime is simple and cost effective to manufacture.
It can be produced to an adequate quality in sufficiently
small quantities to suit the requirements and conditions
in the rural areas of developing countries.
Quicklime Quicklime is produced by heating
any material containing calcium carbonate - usually
limestone but chalk, marble, or sea shells can be
used - for several hours, at a temperature of around
1000 degrees centigrade, in a kiln. Burning the limestone
in this way removes the carbon dioxide in the calcium
carbonate and leaves behind calcium oxide and any
impurities.
Quicklime must be treated with care because it is
a chemically unstable and slightly hazardous product.
It can cause serious burns if it comes into contact
with moisture and therefore, it must be handled carefully.
Hydrated Lime
Hydrated lime is produced by adding water to quicklime
which has been removed from the kiln. This process
is called hydration' or slaking'. If the quicklime
is pure and has been correctly burned the lumps of
lime will chemically combine with the water (providing
it is added in the right proportions) to form a dry
powder called calcium hydroxide. This hydrated lime
is a more convenient material to handle and use than
quicklime.
If more water is added the hydrate will become a
soft and smooth lime putty which can be stored indefinitely
if it is kept wet. If even more water is added the
putty will become a milky suspension known as milk
of lime' which is used in industrial processes where
the lime must flow through pipes. When the suspension
settles, a clear saturated solution is left above
the putty which is lime water.
Where the proportion of impurities, especially clay,
left in the quicklime is very high, the lumps of
lime will not readily hydrate and must be finely
ground and then can only be used for cement work
in building.
Kilns
Lime burners seek to produce the highest quality
quicklime from their limestone at the lowest possible
costs. Fuel is one of the major production costs
of lime burning and that, coupled with the increasing
scarcity of fuel wood and the environmental impact
of deforestation, has lead the efficiency of the
burning process to be assessed by how much fuel it
takes to produce a quantity of quicklime. There are
a wide variety of kilns which can be used to perform
these tasks and consideration needs to be given to
the quality of lime required and the capital, labour
and fuel available for the project before selecting
the type of kiln to be used.
Vertical Shaft Lime Kilns
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Fig.1 A Vertical Shaft Lime Kiln |
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The vertical shaft lime kiln is designed for small
scale lime producers wanting continuous production
of high grade lime with the minimum investment cost.
The principle underlying the design of the shaft
kilns is the recovery of heat by the upward passage
of air through the kiln.
In general, shaft kilns are thermally highly efficient
because they use less fuel than other kilns and still
produce high quality quicklime. A chimney is used
to provide enough natural draught to draw the air
through the kiln and to take the smoke away from
the loading area where people work.
A typical small vertical shaft kiln could be a circular
brickwork tube approximately seven metres high with
an internal diameter of around one metre. The height
of a shaft kiln should be at least six times the
diameter and these dimensions could be scaled up
or down depending on the required output of the kiln
and design considerations.
The Three Zones of a Vertical Shaft Lime Kiln
There are no physical boundries between the zones
of a vertical lime kiln (figure 1), instead they
are formed by the careful operation of the kiln.
There is a cooling zone at the bottom of the shaft
kiln, where the hot lime is cooled by incoming air
passing through the quicklime before it is extracted.
The firing zone is found about halfway up the kiln
and the air which enters it is already warm from
cooling down the quicklime. Therefore, less fuel
is needed than in traditional box kilns because the
hot air is used as a natural resource to heat the
limestone to the temperatures required to change
it to quicklime. At the top of the kiln, the limestone
is stored and warmed up using the wasted heat from
the firing zone.
The preheated limestone falls gently into the firing
zone where the fuel is burning and the limestone
begins to change to quicklime. The firing zone must
have ample poking holes for monitoring the burning
of the lime and to enable it to be readily inspected.
A portable ladder should provide sufficient access
to the poking holes. When the limestone has been
completely fired it is broken down to allow the quicklime
to fall down into the cooling zone and be cooled
before being discharged.
Operation of the Kiln
Once the shaft kiln has been lit, limestone, which
has been broken down into suitable size pieces, is
fed into the top with a measured amount of fuel.
A ramp or a hoist is needed to get the limestone
and fuel to the top of the kiln. If a hoist is used,
then the worker needs to be provided with access
to the platform by using stairs or a ladder.
Efficiency and Fuel Use
Less fuel is needed than in traditional box kilns
because the hot air is used as a natural resource
to heat the limestone to the temperatures required
to change it to quicklime.
An important feature of a vertical shaft lime kiln
is that by controlling the air flow through the kiln
and by fixing the size and monitoring the composition
of the fuel stock, it is possible to obtain a very
high quality of lime with few impurities. It is up
to the individual lime burner to decide the best
size for their limestone pieces within the specified
size range and the type of fuel to be used.
As a practical guide, good quality lime can be produced
when the limestone to coal ratio is between eight
and five to one. This means for every tonne of coal
used the kiln will burn about five tonnes of limestone
and produce between two and a half and five and a
half tonnes of quicklime. It is possible to use charcoal,
wood or other solid fuels as well as fuel oil but
coal is the fuel used in the vertical shaft kiln
at Chegutu in Zimbabwe.
The Chegutu kiln holds about two tonnes of limestone
and coal when it is full. The daily output of the
kiln is between one and a half and two tonnes of
quicklime from three to four tonnes of limestone.
Burning efficiency of this shaft kiln has reached
forty per cent compared with under fifteen per cent
for traditional box kilns.
Costing
In 1996, the total cost of building the Chegutu
Kiln was Z$47,500 ( approximately US$5009), excluding
design and supervision costs. The materials cost
Z$36,500 (US$3849) and labour costs were Z$7,000
(US$738). Infrastructure and accessories were Z$4,000(US$422).
US$1 = Z$9.4825 @ March 1996
The information provided above is a summary explanation
of one of the options available for manufacturers
interested in building a lime kiln. The decision
to build a vertical shaft lime kiln requires careful
consideration of a wide range of situation-specific
parameters, many of which are not addressed here.
For further information, please contact:
Cements and Binders Advisory Service Intermediate
Technology Myson House Railway Terrace Rugby Warwickshire
CV21 3HT Tel: +44 (0) 1788 560631 Fax: +44 (0) 1788
540270
ITDG would like to thank its Building Materials
Panel - particularly Michael Wingate - for their
help in designing the kiln and Lotte Reimer of Ove
Arup & Partners, PO Box 984, Harare, Zimbabwe
who prepared the original notes on the lime kiln
at Chegutu with the assistance of Kelvin Mason and
Peter Tawodzera.
References
Lime and Other Alternative Cements: N. Hill; S.
Holmes; D. Mather. Small-scale Lime Burning - a practical
introduction: ISBN 0 946688 01 X: M. Wingate. Small-scale
Production of Lime for Building: J. Spiropoulos
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