Technology Profile No. 3: Hydrated lime*

* This technology has been developed by the Central Building Research Institute (CBRI), Roorkee, India.

Lime produced by the calcination of limestone in a kiln is known as quicklime. Before using it in construction, it needs to be hydrated. Chemically the process is:

CaO + H₂O ® Ca(OH)₂

In this process, if any magnesia is present, it may also be hydrated partially or fully as:

MgO + H₂O ® Mg(OH)₂

Although the conversion of quicklime into hydrated lime appears to be a simple process, the reaction is governed by numerous factors which affect the properties of the final product. It is, therefore, desirable that the manufacture of hydrated lime is carried out in a factory under controlled conditions, rather than in the field where hardly any control can be effected.

Process of hydration

Lime samples hydrated in the machine during trial runs were evaluated for their physical and chemical properties.

The chemical properties of hydrated lime are shown in table 1.

Table 1. Chemical properties of hydrated lime

Chemical constituents	Percentage composition
SiO2	2.5 - 4.6
Al2O3	0.7 - 5.5
CaO	82.0 - 96.3
MgO	0.56 - 5.86
CO2	1.65 - 1.89
Loss-on-ignition (LOI)	23.45 - 25.75

Some of the physical properties of hydrated lime are shown in table 2.

Table 2. Physical properties of hydrated lime

Constituents	Properties
Residue on 2.36 mm sieve	0.0
Residue on 850 micron sieve	1.2 - 1.46 per cent
Residue on 300 micron sieve	1.6 - 3.97 per cent
Residue on 212 micron sieve	3.64 - 3.80 per cent
Soundness (Le Chatelier)	0.5 - 1.0 mm
Workability	40 - 44 per cent

The above results show that the hydrator can be used for producing class B and C limes.

Advantages of the use of hydrated lime

The use of hydrated lime has the following advantages.

(a) Properly manufactured and carefully packed hydrated lime possesses definite and uniform properties;

(b) There is hardly any deterioration even after long storage, if the material is properly packed;

(c) It is easy to handle, store and transport and can be used without any further processing at the site;

(d) It can be incorporated in mortars in exact proportions;

(e) The plasticity of lime putty can be improved, if so desired, by soaking it in water.

Figure 1. Lime hydrator.

Hydrated lime, possessing definite advantages, is finding increasing demand in construction and various other industries, such as paper, sugar, leather tannery and agriculture, among others. That is why, there is always a considerable demand for a suitable indigenous machine for hydrating quicklime.

Lime hydrating machine

Based on extensive research work carried out at the Central Building Research Institute (CBRI), Roorkee, a lime hydrating machine has been developed, which is commercially produced in two different sites. Special features of the CBRI lime hydrating machine are:

(a) The machine has three tiers with consequent saving of space;

(b) Each of the three tiers of the machine has a well-defined function:

(i) The first tier acts as mixer;

(ii) The main process of hydration takes place in the second tier;

(iii) In the third one the hydration process is completed and the final product is dried.

(c) The design of the machine has been kept flexible so that movement of materials and, consequently, the contact period for the reaction between lime and water can be adjusted to achieve complete hydration;

(d) Steam generated during hydration is used for pre-heating the water used for hydration and thereby speed of the reaction is accelerated;

(e) The smaller model is transportable as one unit and, hence, it is possible to carry it to the site of use;

(f) Lime obtained is in an almost dry state;

(g) The machine is suitable for high-calcium and soft-burnt dolomite lime;

(h) Machines capable of hydrating about three, five and ten tons of quicklime per shift of eight hours have been designed, fabricated and tested in the laboratory. They have also been commissioned in the field.

Scheme for the production of hydrated lime

The manufacturing process is shown in the flow chart (see figure 2).

The production scale is as follows:

Rate of production

22 tons per day of 2 shifts
6600 tons per year of 300 working days

Land and building
		sq m
	Land	2000
	Building	20
	Shed	250
Machinery and equipment
	Crusher for quick lime	1
	Lime hydrator	1
	Bucket elevator	1
	Vibrating screen	1
	Storage Bins	2
	Belt conveyor for quick lime	1
Raw materials
	Quicklime	6000 tons per year
Utilities
	Electric power	100,000 kWh per year
	Water	10,000 kl per year
Workforce requirement
	Plant supervisor-cum-manager	1
	Chemist-cum-analyst	1
	Mechanic-cum-operator	6
	Electrician-cum-mechanic	2
	Storekeeper	1
	Clerk-cum-typist	1
	Skilled labour	12

Energy consumption for a day's production*

Machinery/equipment	Energy
	Electrical	Thermal
Crusher, lime hydrator, bucket elevator, vibrating Screen, belt conveyor
	334 kWh

* Requirement for 22 tons of hydrated lime

Figure 2. Production process of hydrated lime.