Pressed Versus Moulded Refractory Bricks: What’s The Difference?
As crucial as understanding the refractory material itself is how it is formed. The modern foundryman needs to select the right refractory - both material and shape - for the right job to ensure an optimal process.
Refractory bricks are some of the most commonly used thermal insulating materials in industry today. Providing valuable protection of tooling, casing and machinery from excess heat that is required for a manufacturing process. Bricks are the logical shape, as they can be stacked and bonded to form virtually any structure, be that to line a furnace, kiln or a tundish. Broadly used choices of refractory material composing the bricks include alumina, silica and chromite. Performance of these bricks depends on myriad factors including, but not limited to, chemical composition, porosity and thermal stability. Refractory bricks are examples of ‘shaped’ refractories.(1,2,3) Refractory bricks are also known as firebricks. Whether a refractory brick is basic, neutral or acidic is a key aspect in fitting a brick to a particular application, amongst other aspects.
When considering a material or brick type or shape, the modern foundryman needs to consider many factors. These include, but are not limited to, physical strength, thermal capacity, resistance to chemical attack, uniformity, resistance to shrinkage or expansion, spalling properties and how good an insulator the refractory is. Chemical resistance and spalling properties tend to be more related to the identity of the refractory rather than whether it has been formed into a brick via pressing or moulding, with the forming process having a more meaningful effect on the other factors.
Typical Methods Of Production
Pressed Refractory Bricks
The pressing of refractory bricks is a simple idea. In basic terms, the refractory material is produced and before it has a chance to be cured (or is cured by application of heat), it is forced through a guide in a process akin to extrusion, or it is cut out in a process just like a cookie cutter. These methods may employ high pressure or hydraulic pressing to achieve the final outcome. The pressing method originated because hand forming of the insulating materials was arduous and expensive(4).
There are several characteristics that should be attained to ensure a particular brick material well suited to the pressing method, as in the table below(5):
|Can range from around 6 to 12% water by mass. Water required for pressing is added by the machinery, therefore fresh wet clay need not be used.
|Particle size going into the dry pressing equipment should be as consistent as possible. Grind size is specified exactly by the equipment, but historically clays with grind size 11 to 35 mesh were most used.
There are, broadly speaking, two primary methods of pressing: dry pressing and hot pressing.
Advantages Of The Pressing Method
The pressing method is widely credited with improvements in the production of refractory material bricks, particularly with regards to(6):
- Speed and automation
- Life of the material
More modern applications of the pressing method claim that the process is even more efficient due to less waste and systems for recovery of excess material, cycling them back into the manufacturing process(7). The pressing method does not require any external heating and the process is conducted using hydraulic pressure, the power for which is typically derived from electricity. Pressing is therefore an environmentally appropriate choice.
Compared to conventional forming methods for clay bricks, spalling is reduced by 35% and volume change upon firing is reduced by 45% and porosity is greater by almost a half(8).
Dry pressing proceeds at ambient temperature and uses hydraulic rams to force refractory materials into a mould that is centred on a table, containing cavities. The mould is fed from a charger box located above or to the side, which ensures that the material is correctly mixed and composed.
The function of the charger box is essential to the function as without the homogeneity afforded by the box’s accurate and efficient mixing, the filling of the moulds will be inefficient and therefore the overall casting process is compromised.
When pressure is applied, it is crucial that it is applied evenly. Pressures exceeding 300 to 450 bar are often used. ‘Pressure crack’ is a phenomenon in high pressure pressing moulding where high levels of plasticity in the material will cause it to split upon application of pressure - though the addition of small quantities of calcined material and/or grog can alleviate this, as can a longer (albeit slower) application of the pressure. The best refractory brick produced by this method is the one made at the highest pressure.
Hot pressing is a proprietary process developed originally by Union Carbide (now Dow Chemical Company), for refractory bricks with higher carbon contents using various organic binders. The mixture is combined as in the cold press process, but when it reaches the mould it is pressurised (as before) and subsequently has an electric current passed through it. This increase in temperature and pressure rapidly causes the volatilisation of the binders. The now gaseous organic compounds are trapped by the hydraulic press and the pores that form around them are closed off completely by the pressure. This creates an impermeable carbon-rich refractory that is some 100 times less permeable than had the same brick been made using conventional moulding and then baking regimes. A further advantage to this is that hot pressed carbon-rich refractories slightly outperform conventional ones, by way of having a 200 °C higher service temperature(9).
Moulded Refractory Bricks
The moulded method is a logical progression from hand moulded bricks, akin to traditional mud brick production. Moulding is the other popular contemporary method for forming refractory materials. Also described as ‘refractory castables’, these materials are formed when the pre-cured refractory material is poured into some kind of mould bearing the desired shape and size of the final product, where the refractory material is allowed to cure in situ and is removed at a later stage(10). Depending on the identity of the mould, it may be either reused or discarded after use. Naturally, reuse is preferred from both economic and environmental standpoints. The moulded method is key in the production of monolithic refractories, i.e. those that are of a single piece continuous construction, particularly desired for the highest performance demand applications.
Briefly, the process proceeds via placing the uniformly mixed refractory material into a mould that is typically made of steel. The material is left to cure in the mould for lengthy periods of time, with a minimum 24 hours being common(11). Upon release from the mould, the material is fired as if it were any other type of refractory material. Interestingly, the temperature and atmosphere can be controlled around the mould, meaning that properties like humidity can be modulated to afford certain end results(12).
Advantages And Disadvantages Of The Moulded Method
The moulded method is widely tolerant of the widest range of materials, porosities and moisture contents. Because of the lack of pressure requirement, the moulded method is simpler in terms of construction and operation - and therefore comes at a lower cost. Additionally, moulds can be constructed in virtually any shape and size and can be reused(14). However, because of the lack of external forces, the moulded method takes longer than the pressed method.
Which Is More Suitable?
Both pressed and moulded refractories have their advantages and so, naturally, the specific use case needs to be considered, alongside any demands placed on production by available resources, the need for automation, etc. The common advantage to both of the methods is relative speed compared to making bricks by hand and consistent shape, size and finish(13).
It should be noted that shape formation - by pressing or by moulding - is only part of the conversation around selecting and designing a refractory. How the material is cured is as important, i.e. at what temperature and for how long. Furthermore, pressing and moulding is primarily concerned with the production of bricks, including monoliths - and only part of the story for refractory pastes and other compound products.
- Refractory bricks are blocks of highly thermally resistant ceramic or ceramic-like material, comprised mostly of refractory materials such as silica or chromite
- These bricks find wide use in industry, from tundish linings in continuous casting to fire bricks for kilns
- Bricks are primarily made via either a moulded or pressed method, depending on the desired refractory brick outcome
- Both production methods have significant advantages over forming by hand, especially in terms of speed and consistency, with the pressing process being fastest
- Pressing used hydraulic pressure to compress the uncured brick material into a mould, which is subsequently fired. Advantages include speed, uniformity and better porosity properties. Hot pressing is a subset, which is largely proprietary in nature and use
- The moulding process is effective and reliable, albeit slower than the pressing method. It is widely tolerant of many materials
1 W. E. Lee and W. M. Rainforth, Ceramic Microstructures - Property Control by Processing, Chapman and Hall, London, 1994
2 K. Matusmoto, Chem. Abst., 1963, 59, 3626
3 T. R. Lyman and W. J. Rees, Trans. Ceram. Soc., 1937, 36, 110
4 F. C. Steimke, Bachelor's Thesis, University of Missouri School of Mines, 1941
5 US Patent US1911152, 1929
6 E. Hagar, J. Am. Ceram. Soc., 1925, 8. 122
7 China Patent CN2312120Y, 1996
8 J. H. Kruson and C. A. Smith, J. Am. Ceram. Soc., 1925, 8, 829
9 C. Schacht (ed), Refractories Handbook, CRC Press, Boca Raton, United States , 2004
10 A. M. Garbers-Craig, J. S. Afr. Inst. Mining Metallurg., 2008, 108, 491
11 W. N. Dos Santos, J. Eur. Ceram. Soc., 2003, 23, 745
12 A. V. Gropyanov, Refract. and Indust. Ceram., 2009, 50, 198
14 J. McMeekan, J. Inst. Engineers, 1920, 1, 380
13 J. D. Gilchrist (ed), Fuels Furnaces and Refractories, Pergamon, Oxford, 1977