How to build a small vertical shaft lime kiln
Practical Action
people work. The fuel used at Chegutu is coal, but it is also possible to use charcoal and
other solid fuels.
In general, shaft kilns use less fuel than batch kilns because less heat is wasted. In a shaft
kiln there should be three ‘zones’. At the bottom there is a cooling zone where incoming
air passes through the quicklime. The air which is drawn in cools the quicklime so that
less heat is wasted when the kiln is discharged. Also, the air which arrives in the firing
zone is already warm which saves fuel. The firing zone is held about halfway up the kiln.
In this zone the fuel is burning and the limestone is changing to quicklime. Above the
firing zone the limestone is stored and warmed up. Using the waste heat from the firing
zone to warm the limestone also saves fuel.
Construction
Materials
Square brick skin
Common bricks laid in mortar, 1:1:6 (Portland cement: hydrated lime: pit sand). Brick
force for each fourth brick course. Brick force is standard brick reinforcing, consisting of
two 2.5mm diameter longitudinal wires, connected by 2.5mm diameter cross wires at
300mm centres. The brick skin should be plastered with a lime plaster for weather
protection, 1:5 (hydrated lime: pit sand).
Circular shaft
Refractory bricks laid in mortar of approximately 1:1 (refractory clay: refractory brick finely
crushed).
Concrete ring beam, slabs, slope at shaft floor, draw box and foundation
1:1:5:5 (Portland cement: hydrated lime: river sand: 19mm limestone aggregate).
Reinforcing as shown on drawing. The reinforcing is square twist steel with a yield strength
of 410MPa. Ribbed steel of similar strength and same cross sectional area can be used
instead depending on availability.
Vermiculite concrete infill between the two brick skins
Make a vermiculite mix, using 2 parts vermiculite and 1 part grog (finely crushed burned
bricks). Use 1:4 (hydrated lime: vermiculite mix) to make up vermiculite concrete.
Lime concrete infill below the floor of the kiln
1:3:6 (hydrated lime: river sand: 75–100mm limestone aggregate).
Tension bands
50mm x 5mm flat straps on two sides, 12mm diameter mild steel rods with threaded ends
on two sides.
Inspection holes
50mm diameter steel steam pipe, or similar, with 50mm flanges.
Discharge door at draw box
6mm mild steel shutter door set in steel frame as per drawing.
Chimney
2mm mild steel plate. 10mm diameter mild steel holding down bolts.
Design assumptions
Wind load is assumed for central Africa. In other parts of the world with different wind
conditions, the chimney fixing should be checked.
Soil conditions are assumed to be non-expansive soil, with good load bearing pressure,
as would be the case in most limestone deposit areas. Difficult soil conditions or
construction on sloping ground would require a special foundation design and further
advice should be sought.
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