Unslaked Lime

The process of manufacturing Unslaked lime is very simple. Boulder limestone is first crushed to small sizes (about 6" in diameter) and then mixed with coal which is also broken to about 4" in diameter in the layered format i.e., one layer of limestone, one layer of coal and so forth. This mixture is then fed to a vertical kiln. This kiln is made of bricks in a cylindrical shape with an inverted cone type discharge system. This process consists of three stages viz burning, calcination and discharge. The material which comes out of the kiln is Unslaked lime (CaO).

Chemical composition :

Uses :

Grade A – Bleaching powder, paper, textile and varnish manufacture

Grade B – Sugar and Calcium Carbide Industry

Grade C – Suitable for Soda Ash, Caustic soda by lime soda process, water treatment and tanning industry.

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Hydrated Lime

The process of producing Calcium Hydroxide from quick lime is known as dry hydration of Quick lime.

In as much as the final product has to be in dry state, the quantity of water that can be added has to be restricted and at the same time it has to be sufficiently large so that lime does not remain unhydrated. The present process is based on German technology produced by Gebt. Pfeiffer AG.

The quick lime received from the kiln is usually in the form of lumps. Even if sufficient quantity of water required for hydration is added to these lumps, the addition can only be local and no uniform mixing of the two reactants is possible. Shooting to temperature in localised position cannot be ruled out and this may  result in "Burning", It is therefore, desireable that the lime should be in such a fine form that it can be intimately mixed with water. This however, is not practical because of the inherent inhibilities cost it is neither desirable as fine grinding will prevent the elimination of the impurities by rendering them into fine stage. A compromise is there for, struck by grinding the lumps to size of about 8 to 10 mm  or so with the help of a Jaw Crusher and Hammer Mill (Disintegrator). This size of quick lime lumps will get water to penetrate through these lumps and hydration can  be achieved through whole mass. In Jaw Crusher material is fed from the Feed Hopper above which hood is fitted so that whatever dust comes out can be sucked by the blower through proper ducting and this dust is collected separately in the cyclone. The discharge of the Jaw Crusher is connected to the belt conveyor, Hammer Mill and Bucket Elevator. The Bucket Elevator  lifts the material and feed it to Quick Lime Silo in close circuit.

From Quick Lime Silo material goes to Bucket Elevator through Rotary valve in controlled feed. Elevator lifts the material to the slacker. At the top of the elevator a duct which takes air and dust to the main duct coming from the conical chute of disintegrator  is connected because due to the centrifugal force of the buckets, lots of air along with the dust is produced into the elevator. This air and dust mixture goes to bag filter and material flows into the slacker through a chute.

It is usually desirable that the hydration process should take place in the presence of liquid water rather than by water vapour. In the latter case, high temperature is attained. This not only includes the tendency of burning but may also tend to reduce the particles. High temperature destroys the colloidal nature of the hydrates and makes the product crystallized. Addition of water in small increments or spraying, appears to be preferable. Hot water if used for the process may increase firmness and or whiteness in some qualities of quick lime.

The automatic slacking machine comprises of  three slacking chambers arranged one above the other. The capacity of the chambers adopts to the increase in volume of the upper chamber (Pre-slaking Chamber) the two component, quick lime and water are intensively mixed into and hydration of  quick lime starts.

The main part of the slacking process then takes place in the second chamber, (the main slacking chamber), in the third chamber the well expended hydrator is fluidized by stirrers, and the residual moisture escapes utilizing the residual heat.

The Calcium Hydroxide (Slaked Lime) then flows out of the machine and the excess heat discharged due to the exothermic reaction is reduced by adding water. The slaking reaction takes place at approx. 100 degrees Celcius. Theoretically 0.3m3 of water is needed for the slacking reaction per ton of quick lime. For cooling purposes a further 0.3 - 0.4 m3 of water completely evaporates and leaves the plant via the wet scrubber in the form of water vapour.

Chemical compostion :

* to convert the available lime content from CaO to Ca(OH)₂ multiply the result by 1.32