The article covers the benefit of pre-cast technology, comparison with in situ castings, gunning and shotcreting technology and few specific case studies in cement plant with recommendation of selection of pre-cast technology for cement plant refractory repairs.
Shaped refractories are one of the oldest forms of refractories. Pre-cast pre-dried block development is inspired by civil engineering. After the development in concrete formulation technology, it was observed that higher speed of construction and improved durability of concrete can be achieved through precast shapes. Similarly, after the development of low cement based monolithic refractory formulations, pre-cast pre-dried refractory shapes are considered for the speed and performance in terms of longer refractory life. Pressed firebricks could not be produced in all the different shapes in the needed quality. Pre-cast pre-dried shape solutions give the flexibility of material selection and variability of shapes and sizes as required.
Innovation inpre-cast pre-dried solution occurred after the development of high-end monolithic solutions and manufacturing technology. More sophisticated low cement (LC) and ultra-low cement (ULC) constables were developed with higher densities, lower porosity and significantly improved properties upon exposure to service temperature. Lower cement contents requireaccurate control of water addition, the use of high-intensity mixers together with adequate vibration of the material during placement. Adopting these operational controls in field installations is challenging and so for some high performance demanding areas, the pre-cast technology offers advantageous properties in comparison with in situ cast installation.
Why pre-cast pre-dried solution?
Over the decades, Indian cement industry has changed. The use of monolithic refractory increased over bricks due to many reasons like:
At present situation, the challenges are different. Refractory work in India is a manpower intensive job and demands a special skill set for the quality work. There is an acute shortage of skilled manpower working in this field. Along with the above reasons, the most important thing is to improve upon the performance and precision of installation. This is creating the demand for pre-cast pre-dried shapes solutions. The pre-cast solution is a precision solution due to the modular lining which combines the advantages of both bricks and castables. Pre-cast pre-dried blocks may have the insulation and the anchoring system pre-attached.
Pre-cast refractory shapes have many advantages, including simplicity and durability.The precast shapes are cast and air-cured in well controlled atmosphere (temperature plus humidity) in a special manufacturing plant that offers more consistent wet mixing and optimum level of vibration (frequency, amplitude and time-controlled cycles) in achieving excellent particle packaging with minimum water demand. Physical properties of the material can further be optimised and improved by firing the precast shapes in a PLC controlled oven or furnace.
The precision of firing is developed by doing a TGA (thermo-gravimetric analysis) study of the target formulation being used for casting and then designing a suitable firing cycle to drive out the moisture through the entire thickness of the shape, not just the surface. Since the initial firing is precisely controlled, there is also a reduction in micro-cracking within the shape. Pre-cast shapes treated to such cycles result in a truly homogeneous, incredibly reliable lining. In service, pre-cast shape refractory linings see less stress and cracking without the fear of steam spalls or damage during initial heating. These risks are taken away from cement manufacturing process. In throughput driven cement operations, maintenance planning is done meticulously. Unscheduled outage due to spalling is not appreciated.
Therefore, pre-cast pre-dried shapes can offer significant benefit through faster installation, higher refractory performance and lowering down the overall maintenance cost.
Study on end properties of pre-cast & gunning/shotcreting panel
An experimental study of various (gun, shotcrete, and castable) 70 per cent alumina low cement composition has shown that the pre-cast clearly comes out with better installed and dried physical properties over similar composition of its gun and shotcrete variety and even when it is cast in-situ.
The reason being controlled water demand, mixing and vibration and finally the quality care that it goes through in a manufacturing plant. The controlled firing of the pre-cast shapes adds to its robustness as explained earlier.
Precast solution study for cement plant: Cooler Bullnose
Cooler Bullnose is one of the critical areas of the cement plant. This area is prone to heavy abrasion due to rising clinker dust. It is also affected by severe thermal spalling and chemical attack. For a cement plant this area is one of the bottlenecks for the performance. By using precast solutions this performance is improved.
Below are a couple of case studies of pre-cast pre-dried shapes solution for cement plant refractory application.
CASE STUDY - 1
Precast solution case study details
Plant Size 2.2 MTPA
Equipment IKN cooler
Primary Fuel Coal through the main burner. Calciner Feed with alternative fuel
(AF) and waste-derived fuel.
Performance 36 months
Acknowledgement The solution repeated for the second line
Block Size 600 X 600 mm, and some locking shapes
Blocks were 600 mm x 600 mm, with four stainless steel anchors, with threaded ends. These were inserted in back-to-back steel section channels (crossover sections between the main structural beams) and suspended in position with nuts. Following completion lightweight insulating castable was poured over the top of the blocks. (So, the roof was open, with no steel casing). The blocks were lifted into place using a "block and tackle" arrangement, suspended from structural beams.
CASE STUDY - 2
Precast solution case study details
Plant Size 3.3 MT/annual
Primary Fuel Coal
Performance 25 months and still running
Time required for application The product has been pre-assembled and marked in plant hence has taken a day to complete installation at site.
Block Size Approximately 600 x 800 mm, and some locking shapes
The pre-cast pre-dried shapes were pre-assembled and installed at the site, under operation for 25 months and still running.
Pre-cast pre-dried designs are increasingly seen to offer effective solution in Cooler Bullnose, tip casting, preheater system, tertiary air duct (TAD) modular system and other areas like (Planetary) cooler bend, damper blade, firing pipe, etc. The benefit it offers with respect to significant improvement in performance is primarily due to the reason that these shapes are manufactured under controlled conditions which eliminate site variability in inputs, water, temperature, mixing energy and vibration energy.
These pre-cast pre-dried shapes being dried out at the manufacturing stage, provide faster and lower risk of return to high temperature service. Finally, faster installation, engineered anchor solution, built-in expansion allowance and excellent engineering supporting system makes it a robust and preferable industry solutions to enhance the refractory performance in critical areas of cement plant.
ABOUT THE AUTHOR:
The article is authored by Purushottam Bedare, Parthasarathi Mukhopadhyay and Stephen Woodcock of Vesuvius India.
The article was presented during 16th NCB seminar in New Delhi.