Lime production: improved techniques in Malawi
Marble is quarried by means of first drilling and blasting with explosives, and then by manual
breaking using hammers to a feed size of between 75 and 125 mm. An experienced blaster
is hired every 3 to 6 months to extract sufficient rock for the period. The rock is then taken to
the kiln site at the nearby town of Balaka by hired truck.
The fuel used is charcoal produced from renewable fast growing plantation reserves planted
by the Government for industrial fuel supply. Alternatively coal or wood could also be used.
The kiln design and firing procedures
The kiln design is of the vertical shaft type, with an effective shaft height of 6m and an
internal diameter of 1.1m, tapering 1m from the top to 800 mm diameter.
The kiln is lined internally with locally produced refractory bricks followed by a 230 mm
thick layer of insulating bricks and a 120 mm skin of ordinary stock bricks. The brickwork is
encased in a 6 mm mild steel casing with a diameter of 2.3m (see figures 1 & 3).
Four discharge points are located at the bottom of the kiln, at 90° to each other and each
fitted with steel fire doors.
The kiln is free standing with a cat ladder providing access to 4 inspection ports and to a
steel platform at the top of the kiln, which facilitates charging.
With this particular kiln a fan forces air through the kiln and thereby helps to control kiln
temperature. The fan is powered by a 5.5 kW motor and air is fed into the kiln through
a cast iron manifold at the centre of the lower section of the kiln (see figure 3).
Marble and charcoal are manually hoisted to the top of the kiln and into a steel charging
hopper on a 24 hour per day continuous basis. In total 5.7 tonnes of marble and 0.8 tonnes
of charcoal are fed into the kiln during every 24 hours.
The kiln temperature is maintained at between 1000 and 1100°C.
The quicklime (predominantly calcium oxide, CaO) is discharged from the kiln once every
hour and-a-half from the four discharge openings by shovel directly into wheelbarrows.
In practice it is the rate of discharge which determines the rate of flow through the kiln and
the quantity of feed required. The rate of discharge is determined by the required period of
calcination and to some extent by the position of the firing zone.
Hydration, sieving and classification
Slaking is carried out in a mechanical
hydrator. This comprises a horizontal U-
shaped trough 2 m long and 800 mm wide,
covered with a lid hinged on one side and a
gate along the bottom for discharge. Agitation
is provided by a rotating internal shaft with a
series of angle sections welded to it
alternately set off at 90°.
Quicklime is charged into the top of the
hydrator in batches of 220 kg. Once the
rotating agitator is turning 60 litres of water
is added through holes in pipes running along
the top of the machine, at a rate of 10 litres
per minute. The time taken to hydrate one
batch is approximately 12 minutes, after
which the hydrated lime is discharged.
Figure 2: Prototype mechanical batch