Technology Profile No. 1: Mini-cement production*

* This technology has been developed by Regininl Research Laboratory (RRL), Jorhat, India.

A mini-cement plant is one the total installed capacity of which is not greater than 200 tons per day, including one or more kilns on one site. The case for developing mini-cement plants in India has arisen from the high cost of installing viably-sized conventional cement plants, the large number of small deposits of limestone dispersed at various parts of the country and the versatility of mini-cement plants in matching the limited availability of power, water and other inputs. By meeting the demands of local captive markets, mini-cement plants also provide for participation of local small entrepreneurs and, thus, help in building up local economies.

Major advantages of mini-cement plants are:

- Less capital-intensive;
- Short gestation period;
- Quick return of investment;
- Utilization of small deposits of limestone;
- Low transport costs, as product can be consumed locally;
- Attractive to young entrepreneurs with limited financial resources.

A mini-cement plant of capacity 20-10.0 tons per day, based on vertical-shaft-kiln (VSK) technology, has been developed by Regional Research Laboratory (RRL), Jorhat, India. The technology package is licensed through the National Research Development Corporation of India, and the plant is set up by a number of consultants, appointed for the purpose, on a turn-key basis. The product conforms to IS: 269 - 1976, the specification for ordinary Portland cement. Mini-cement plants, with technical know-how from RRL and licence from the National Research Development Corporation, New Delhi, have started commercial production successfully in several parts of the country. Complete detailed engineering reports can be offered for a 25 ton/day VSK plant. Basic design and process know-how is available from the RRL, Jorhat.

Figures 1 and 2 show outside views of two mini-cement plants in India.

Figure 1. Prag Shiva mini-cement plant in Guwahati Assam, India.

Figure 2. Bhagyanagar mini-cement plant in Nandigram, Hyderabad, India.

Production processes

Some of the processes on which mini-cement production could be based, are:

(a) VSK;
(b) Rotary kiln;
(c) Lurgi sinter bed;
(d) Belt kiln.

Processes 1 and 2 are established as commercially viable, while processes 3 and 4 are of theoretical interest. The development of the VSK process for cement production can be traced back to 1824, At that time, this process did not receive much attention, as the operations were highly labour-intensive, the clinkers produced were of non-uniform quality, and the overall economics were unfavourable. However, with the development of the pan-type nodulizer, which ensures a uniform-quality product, the situation has radically changed.

The kiln consists of a cylindrical shell with a conical sintering zone lined with refractory bricks. A rotary discharge grate at the bottom of the shell coupled with its drives and air sealing device takes care of the uniform rate of discharge of clinker. The green nodules thus move down gradually and encounter hot flue gases. In the sintering zone, nodules are calcined and oxides recombine to form the essential cement phases. The clinkers thus formed move further downward, encounter incoming air and become cooled. Finally the clinkers exit through the rotary discharge grate and reciprocating discharge gate. The RRL shaft kiln is highly efficient in calorie consumption (1030±50 kCal/kg of clinker).

The cement clinkers are then pulverized after admixing with the required amount of gypsum in a cement mill to a minimum fineness of 2250 sq cm/g.

Figure 3 shows a view of a VSK designed by RRL, and figure 4 shows another VSK designed by Cement Research Institute (CRI), India, for a 20 ton/day capacity plant.

The VSK process of cement production which is a semi-dry process consists of the following major operational steps:

(a) Primary crushing of limestone, clay and other additives, if any, to a fineness of about 12-15 mm.

(b) Pulverizing of the raw meal (stated in 1 above) and coke breeze to a fineness of 90 per cent below 170 mesh BSS.

(c) Blending of the pulverized material in suitable proportions, to ensure desired uniform-quality product.

(d) Preparation of nodules, by the addition of water to the raw meal in the nodulizer.

(e) Feeding of nodules into the VSK wherein the nodules undergo drying, calcining, sintering and cooling, resulting in the formation of cement clinkers.

(f) Grinding of clinker and blending of the ground clinker with gypsum, to obtain quality Portland cement.

Figure 5 shows a nodulizer of a VSK mini-cement plant and figure 6 shows a raw material balancing and grinding section.

Figure 3. View of a VSK

Figure 4. General arrangement of CRI type VSK for Visvakarma mini-cement plants.

(This figure is reproduced from Monograph on Appropriate Industrial Technology, Appropriate Industrial Technology for Construction and Building Materials, No. 12 (New York, United Nations Industrial Development Organization, 1980), p. 66, fig. 1.)

Figure 5. Nodulizer of VSK mini-cement plant. Pioneer Cement Plant, Ltd. Hyderabad, Andhra Pradesh, India.

The principal equipment in a VSK process for a typical plant of 25 tons per day capacity is given in table 1.

Table 1. List of equipment for a 25 ton/day VSK mini-cement plant

Equipment	Quality	Capacity
Limestone crusher	1	6 ton/h
Limestone conveyor	1	6 ton/h
Hammer mill	1	6 ton/h
Belt conveyor	1	6 ton/h
Limestone elevator	1	3 ton/h
Coke-breeze, clay and additive elevator	1	2 ton/h
Table feeders (1 each)	4	(a) 2 ton/h
		(b) 120 kg/h
		(c) 300 kg/h
		(d)100 kg/h
Belt conveyor for raw material	1	4 ton/h
Raw material elevator	1	4 ton/h
Raw material grinding mill	1	3 ton/h
Screw conveyor	1	3 ton/h
Raw meal elevator	1	4 ton/h
Homogenizer	2	10 ton/h
Raw meal feeder	1	3 ton/h
Nodulizer	1	3 ton/h
Nodule screen	1	3 ton/h
Nodule elevator	1	3 ton/h
Vertical shaft kiln	1	25 ton/day
Clinker elevator	1	4 ton/day
Clinker and gypsum elevator	1	4 ton/day
Gypsum feeder	1	50 to 150 kg/h
Clinker feeder	1	1-4 ton/h
Cement mill	1	2 ton/h
Cement elevator	1	4 ton/h
Screw feeder	1	4 ton/h
Weighing machine	1	0-100 kg

A process flow diagram of a VSK mini-cement plant is shown in figure 7 and a typical layout of 50/100 ton/day-capacity VSK cement plant is shown in figure 8.

Raw materials, including fuel

Limestone:
	CaO	45 per cent minimum
	SiO2	12 per cent maximum
	Al2O3	4 per cent maximum
	Fe2O3	2-4 per cent maximum
	MgO	2.0 per cent maximum
Clay/fly-ash/shale:
	SiO2	60-66 per cent
	Al2O3	12-18 per cent
	Fe2O3	5-9 per cent
	Plasticity	Medium plastic
Coke breeze (fuel):
	Calorific value	6000 kCal/kg minimum
	Ash	30 per cent maximum
	Volatile matter	8 per cent maximum
Gypsum:
	CaSO4 2H2O	80 per cent minimum

The typical consumption pattern of these raw materials per ton of product is as follows:

(a) Limestone	1.36 T
(b) Clay	0.17 T
(c) Coke-breeze	0.25 T
(d) Gypsum	0.04 T

Utilities

Consumption of utilities per ton of product is as follows:

(a) Power	About 135 kWh
(b) Water	About one cubic metre

Product specifications

Physical
Specific surface:		2250 cm2/g minimum
Setting time:
	(a) Minimum	30 minutes
	(b) Maximum	600 minutes
Soundness:
	(a) Le Chatelier	10 mm maximum
	(b) Autoclave	0.8 per cent maximum
Compressive strength (kg/cm2):
	(a) 3 days	160 minimum
	(b) 7 days	220 minimum
	(c) 28 days	330 minimum
Chemical
	Loss-of-ignition	5.00 per cent maximum
	Insoluble residue	4.00 per cent maximum
	SO3 content	2.75 max., when C3A <7 per cent 3.00, when C3A >7 per cent
	MgO content	6.00 maximum
	Alumina ratio	0.66 minimum
	Lime saturation factor	Between 0.66 and 1.02
Special requirements:
	(a) C3A	less than 7 per cent
	(b) SO3	2.75 per cent maximum

Characteristics of cement IS: 269-1976 limits

Specific surface area:
	3100-3300 cm2/g	Minimum 2250 cm2/g
Setting time (min.):
	Initial 106-130 minutes	Not less than 30 min.
	Final 220-260 minutes	Not more than 600 min.
Compressive strength:
	3 days 190-220 kg/cm2	Minimum 160 kg/cm2
	7 days 280-310 kg/cm2	Minimum 220 kg/cm2
	28 days 405-450 kg/cm2	Minimum 330 kg/cm2
Le Chatelier expansion:
	1-2 mm	10 mm maximum
Autoclave test:
	0.05-0.2 per cent	0.8 per cent maximum

Figure 6. Raw material balancing and grinding section. Bhagyanagar Cement Plant Ltd., Nandigram, Hyderabad, Andhra Pradesh, India.

	Labour
	50 ton/day	100 ton/day
Skilled	45	50
Unskilled	75	80
Designation
Managing director	1	1
Works manager	1	1
Shift-in-charge	4	4
Plant operator	8	8
Nodulizer operator	8	8
Raw mill operator	4	4
Cement mill operator	4	4
Senior chemist	1	1
Junior chemist	3	3
Storekeeper-cum-accountant	1	1
Electrician	2	2
Mechanic fitter	2	2
Peon	2	2
Driver	-	2
Guards	4	4
Helper	-	3
Unskilled labour	35	35
Unskilled contract labour	40	45
Total	120	130

Figure 7. Process flow diagram of a VSK mini-cement plant.

Figure 8. Typical layout of a 50/100 ton/day VSK cement plant.

Technology Profile No. 2: Production of lime*

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

Production of lime is a simple process in which limestone is calcined at elevated temperatures. Theoretically, 900°C is a sufficiently high temperature to carry out the process. However, in practice, it has been found necessary to maintain the temperature at a much higher level than this to complete the chemical reaction. In the absence of adequate temperature over sufficient time, the lime produced will be of inferior quality: it might be underburnt or overburnt. The success of the process, therefore, lies in maintaining proper conditions for calcination.

Kiln design

Lime kilns of various designs have been used. However, vertical-shaft types are thermally the most efficient. Consequently, their use results in savings in fuel. In India, different types of kiln have been employed through the ages, but investigations carried out at the Central Building Research Institute (CBRI) have shown that most of the traditional designs produce an inferior quality of product with a higher consumption of fuel. CBRI, in recent years, has developed lime kilns of several types, which are being offered for exploitation by the industry. The smallest kiln has about a 5 ton/day capacity below which hardly any efficiency can be expected. Figure 1 shows a lime kiln developed by CBRI.

Some salient features of the kilns

(a) The kilns are of brick or stone masonry;

(b) The kiln designs ensure smooth running and periodic withdrawal of lime;

(c) The kilns lend themselves to a fair degree of instrumentation, if required;

(d) They work on natural draft and have an arrangement for their control;

(e) They work continuously but can be adapted for day working only;

(f) They are thermally efficient, and heat losses are minimal;

(g) They produce a uniform quality lime, by avoiding overburning or underburning;

(h) Under standard working conditions, these kilns produce very little core or unburnt material;

(i) They can be operated by trained unskilled labour;

(j) Contamination of lime with fuel is minimal.

Raw material and chemical composition

The impurities in limestone are primarily SiO₂, Al₂O₃ and Fe₂O₃. They are non-volatile in nature and remain as contaminants in the lime produced. Limestone generally contains some MgCO₃ also. Calcite stone usually contains CaCO₃ exceeding 95 per cent, and dolomitic stone has an MgCO₃ content of 35-40 per cent. In the burning operation, the carbonates are converted to their corresponding oxides. Dolomitic lime is used largely in refractories where a high MgO content is essential.

The principal reactions involved in the calcination of calcitic and dolomitic limestones are:

CaCO₃ ® CaO + CO₂
(Calcite)

CaCO₃ MgCO₃ ® CaO.MgO. + 2CO₂
(Dolomite)

The average dissociation temperatures for the above two types of limestone at atmospheric pressure are 900° and 725°C respectively. Certain materials, such as sulphur dioxide, present in the stone or fuel tend to react with lime and oxygen to form CaSO₄ which is unstable at high temperatures. Al₂O₃ and SiO₂ combine with CaO and MgO to form various silicates and aluminates at very high temperatures. These compounds are water-insoluble and are undesirable, as they decrease oxide values and also coat the lime particles and so reduce its reactivity.

The reaction of quicklime is also influenced by high operating temperatures and retention times. With an increase in temperature, the reaction rate increases, and, consequently, the reaction time decreases. However, the maintenance of high temperatures beyond an optimum limit causes overburning of the lime.

Production process

Limestone is broken to a size of about 75 to 125 mm, and coal to half this size. These are mixed in mixers near the kiln. To initiate fire, a layer of firewood is first laid. Above this, some steam coal is spread. Thereafter, the kiln is filled with previously mixed coal and limestone. Generally, the coal requirement is 12-15 per cent that of limestone, but this varies, depending upon the type of limestone and the quality of coal. Fire is introduced from the bottom and rises. Charging and discharging are so adjusted that the firing zone is maintained in the middle of the kiln.

Figure 1. Lime kiln.

Figure 2. Production process of quicklime.

Scheme for the production of quicklime

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

(b) Production scale

(i) Rate of production

10 tons per day of three shifts,
3000 tons per year of 300 working days

(ii) Inputs

Land	3000 sq m
Building	20 sq m
Shed	200 sq m
Machinery and	1 lime kiln
equipment	1 feeding device
Electric power	50,000 kWh per year
Water	1000 kl per year
Coal (steam)	900 tons per year
Limestone	6000 tons per year
Labour	1 manager
	3 operators
	4 skilled labourers
	2700 work days labour
	2 guards

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.