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.