Cement Material Handling: Doubling in Size
The Rs 2,000 crore material handling equipment for cement industry is poised to double its size in coming years. Currently, there are 87 projects worth Rs 85,000 crore under various stages of implementation entailing a total production capacity of 175 million tonne. Assuming investment of 15 to 20 per cent goes into material handling equipment, an additional business worth Rs 12,750 crore or Rs 2,500 crore per annum will be generated in the next 4 to 5 years, finds
Nitin Madkaikar, FIRST Infocentre.
Materials handling account for a significant portion of the total cost of producing cement. Workers and materials have to travel long distances in the course of the manufacturing process leading to loss of time and energy while nothing is added to the value of the product. Through effective plant and equipment, much material handling operations can be mechanised and shorten process duration. The choice of material handling methods and equipment is an integral part of the plant layout design.
In many factories either the initial layout are not well throughout or, as an enterprise expands or change some of its produce or processes, extra machines, equipment or offices are added wherever space could be found. In other cases temporary arrangements are made to cope with an emergency situation, such as the sudden increase in demand for the product; but then these arrangements remain on a permanent basis even if the situation that provoked them subsequently changes. The net result is that materials handling workers often make long, round about journeys in the course of the manufacturing process leading to a loss of time and energy without anything being added to the value of the product.
The major operations carried out in a typical cement plant are as follows:
- Crushing of limestone, coal, and other materials
- Storage arid blending of raw materials
- Preparation of raw mix
- Raw mix grinding and homogenisation
- Pyroprocessing of raw materials in the kiln
- Cooling and storage of clinker
- Grinding of clinker with gypsum to cement
- Storage and dispatch of cement
Materials handling operations
Raw material preparation includes a variety of blending and sizing (grinding) operations that are designed to provide a feed with appropriate chemical and physical properties. The raw material processing operations differ somewhat for wet and dry processes.
Materials transport associated with dry raw milling systems can be accomplished by a variety of mechanisms, including screw conveyors, belt conveyors, drag conveyors, bucket elevators, air slide conveyors, and pneumatic conveying systems. The dry raw mix is pneumatically blended and stored in specially constructed silos until it is fed to the pyroprocessing system.
In the wet process, water is added to the raw mill during the grinding of the raw materials in ball or tube mills, thereby producing a pumpable slurry, or slip, of approximately 65 per cent solids. The slurry is agitated, blended, and stored in various kinds and sizes of cylindrical tanks or slurry basins until it is fed to the pyroprocessing system.
The final step in Portland cement manufacturing involves a sequence of blending and grinding operations that transforms clinker to finished portland cement. Up to 5 per cent gypsum or natural anhydrite is added to the clinker during grinding to control the cement setting time, and other specialty chemicals are added as needed to impart specific product properties. This finish milling is accomplished almost exclusively in ball or tube mills. Typically, finishing is conducted in a closed-circuit system, with product sizing by air separation.
The finished portland cement is conveyed to bulk storage silos from which it is dispensed for transportation to consuming markets. Portland cement is often loaded in bulk into hopper trucks or rail. It is also packaged in 50 kg bags made of paper or jute. The bags are loaded onto pallets for handling, warehousing, and transporting.
Process fugitive emission sources include materials handling and transfer, raw milling operations in dry process facilities, and finish milling operations.
Emissions during handling
In the raw mill, the raw material feeders, stackers, blenders and reclaimers can be a source of fugitive dust emissions. Transfer points on belt conveyor systems and bucket elevators that transport raw materials from storage to the raw mill department can also generate fugitive dust. Dry raw mills and the auxillary equipment are all designed to run under negative pressure to suppress particulate emissions.
During colder weather vents from dryers, raw mills and air separators may exhibit a steam plume that may be mistaken for particulate emissions. Fabric filters in the vent circuits for dryers, raw mills, and air separators must be insulated to prevent internal moisture condensation.
Dust in the clinker has a tendency to become airborne during handling. The free fall of clinker onto storage piles usually creates fugitive particulate emissions. Fugitive dust emissions from open storage piles are mitigated by rain and snow which causes a crust to form on the piles. Clinker in open piles is usually reclaimed with mobile equipment, such as front-end loaders. Clinker in storage halls is frequently handled with overhead bucket cranes.
In the finish mill, various material-handling system vents are the source of particulate matter emissions. In the bagging and loading operations, particulate emissions are generated from silo openings, cement handling equipment, and the various bulk and package loading operations. The dust generated during the loading of trucks and railcars is controlled by venting the transport vessel to a fabric filter.
Modernisation of MHE
Until recently, pneumatic material conveying (air lift and screw pump) are being used in most cement plants for pulverised material conveying (e.g. raw meal, pulverised coal and cement). This system uses compressed air and is therefore very energy intensive compared to mechanical conveying systems. The main advantage of pneumatic conveying is its low maintenance requirement and operating flexibility.
However, modern bucket elevators require less maintenance and offer considerable energy savings for transporting pulverised materials like raw meal and cement. These are now increasingly being used in the cement industries.
Another development has been the dense phase conveying system. Unlike the pneumatic screw pump, this system moves material with low velocity, which significantly reduces power consumption.
For handling dusty materials and also to avoid spillage and environmental pollution, pipe conveyor material transport systems are used. The advantages of this system are:
- Dust free transport of materials
- Zero spillage
- Ability to transport at steep angles
- Return belt can also be used for material transport
Air lift and screw pump systems when used for kiln feed, introduce a considerable amount of air into the top stage pre-heater cyclones which results in a larger quantity of kiln exit gases to be handled by the fan and requires a dust collecting system with a larger capacity. The effective temperature of raw meal from the top cyclone is also reduced, adversely affecting fuel consumption. Thus, in modern plants invariably bucket elevators are used for kiln feed, which saves both heat and electrical energy.
Material Handling and Environment
The emission during materials handling in cement industry has been a cause of concern until technology stepped in and cement manufacturers abided to environment pollution norms. Explains
P B Ravikumar, Chief Executive Officer, GMV Projects and Systems
"Gone are the days, when heavily polluting equipments were installed with name sake dust collectors for the sake of pollution control boards. Now even the operators of the machines are well informed and demand better operating conditions. Companies are repeatedly selecting environmental-friendly methods like water spray, etc." While on increased cost of equipments he stated that "There is no question of any premium. Environment-friendly feature will be a basic necessity. Best performance feedback from actual users will be the premium for all suppliers.
On domestic availability of equipment Ravikumar believes that Indigenous designers mostly bench mark their products and does not have a research back up. Imported qualities are far better and have a researched development background. GMV has developed a complete range of alternate fuel handling system for cement plants to use waste fuels like plastic, municipal waste, moulded parts, coconut kernels, rice husk, paint sludge etc
Technical layout of MHE in cement plants
Conventionally F K pumps and airlifts were used for raw meal transport to blending silo, for kiln feed and for cement transport to cement silo. F K pumps and airlifts use compressed air thus consuming major share of power. Replacing F K pumps and air lifts with bucket elevator for blending silo/cement silo, kiln feed can make substantial saving in power. This can be made possible by locating the blending silo and cement silos near the raw mill and cement mill building respectively.
As far as possible, a streamlined layout should lead to minimum number of transfer towers in order to reduce the number of belt conveyors. Use of conveyor belt in diagonal direction can reduce the number of belts. Thus saving on an additional drive of belt conveyor, transfer tower, bag filter, lighting arrangement for transfer tower can save lot of energy.
Crusher and screen station can be located in the quarry itself so as to convey the screened material through cross-country belt. The movement of dumpers can be avoided by this and also can avoid the reject handling, thus substantial saving on the fuel can be achieved through this.
Prospects for MHE in cement industry
The Rs 2,000 crore material handling equipment for cement industry is poised for the tremendous growth in coming years. As of June 2011, there were 87 cement projects under various stages of implementation entailing a total production capacity of 175 million tonne. The total investment envisaged on these projects is Rs 85,000 crore and are schedule for completion in next 4-5 years. Assuming a typical one million tonne cement plant invests about 15 to 20 per cent of the total project cost in material handling equipments. This implies a total additional business of Rs 12,750 crore to Rs 17,000 crore or Rs 2,500 crore per annum on an average. Further another 290 million tonne worth of projects are being planned or have been announced. This would provide further impetus to the material handling equipment industry. The need of the hour is technology upgradation as demand for world class equipments will be the basic necessity of this fresh demand.