| CLASSIFICATION, CHEMICAL ANALYSIS, RAW MATERIALS AND PRODUCTION |
- Lightweight refractory bricks are shaped refractory products with a total porosity of > 45 % and an application temperature of at least 800 °C. ASTM C 155-70 and DIN EN 1094 (table 2) define the temperature at which shrinkage of the material should not exceed 2 %. The maximum bulk density is also indicated.
- If the chemical structure is considered, lightweight refractory bricks are to be classified in aluminium silicate lightweight refractory bricks, silica lightweight refractory bricks, zircon lightweight refractory bricks and corundum lightweight bricks.
- The group of aluminium silicate lightweight refractory bricks (fireclay and mullite bricks) is the most important and common group. Raw materials based on Al203, Si02 and sometimes Ca0 are used for producing these bricks.
Raw materials such as clay, kaolin, fireclay, sillimanite, andalusite, kyanite, mullite, alumina, alumina hydrate and corundum are used as alumina carriers.
- In addition to the fine grained raw materials, coarse-grained and porous raw materials are also applied. These include lightweight fireclay and hollow spheres (balls) consisting of corundum or mullite.
- The burnout process is best known and is applied most often to the production of lightweight refractory bricks. Fine saw dust, petroleum coke, lignite abrasion, styropor balls, fine waste products of the cellulose and paperboard (carton) industry are utilised as burnout materials. Burnout materials with low ash content are required in order to prevent negative effects on the hot properties.
| Table 2 : Classification of shaped heat insulating Refractory Products |
| ASTM C 155 |
|
|
|
Norm Regulation DIN - EN 1094, part 2 |
| Group |
Test Temperature*1 |
Max. Bulk Density*2 |
Group |
Test Temperature*1 |
Bulk Density* |
|
°F |
°C |
kg / m³ |
|
°C |
kg / m³ |
* 16  875°C |
1550 |
845 |
540 |
75 |
750 |
400 |
| 20 1100°C |
1950 |
1070 |
640 |
80 |
800 |
500 |
| 23 1260°C |
2250 |
1230 |
770 |
85 |
850 |
550 |
| 26 1430°C |
2550 |
1400 |
870 |
90 |
900 |
600 |
| 28 1540°C |
2750 |
1510 |
960 |
95 |
950 |
650 |
| 30 1650°C |
2950 |
1620 |
1090 |
100 |
1000 |
650 |
| 32 1760°C |
3150 |
1730 |
1520 |
105 |
1050 |
650 |
| 33 1820°C |
3250 |
1790 |
1520 |
110 |
1100 |
700 |
* abbreviated °F
(example 16 = 1600 °F ^= 875 °C) |
115 |
1150 |
700 |
| 120 |
1200 |
700 |
| *1 test temperature, at which no more than 2 % permanent linear change may occur after 24 hours |
125 |
1250 |
750 |
| 130 |
1300 |
800 |
| 135 |
1350 |
850 |
| *2 upper limit of median bulk density of products in group L. In each group of the L class the bulk density is a property used only for differentiation and is indicated with two digits after the decimal point. |
140 |
1400 |
900 |
| 150 |
1500 |
950 |
| 160 |
1600 |
1150 |
| 170 |
1700 |
1350 |
| 180 |
1800 |
1600 |
- The foam process is a further method of production. Special soaps, saponins and sulfonates are used to make stable foams. The slurry for the ceramic body is often made separately from the foam emulsion. Foam and slurry are homogenised in an intensive mixer. By the controlled mixing of foam and slurry the required bulk density is adjusted.
- In practice the gas propellant process is used less frequently. Materials which develop gas are mixed into the compound. These include the following substances:
- metal or carbide powder
- hydrogen peroxide H2O2
- dolomite and sulphuric acid
- Lightweight refractory bricks which are produced by mixing in evaporating substances (naphthalene) have distinctive differences in their properties when compared with other brick qualities. Thus it is possible to produce bricks with low density and high strength. Very fine pores guarantee low heat conduction values.
- Shaping of the lightweight refractory bricks is done by casting, centrifuging or pressing. During casting, the perforated metal moulds (forms) are lined with filter paper before being filled. Sulphite liquor, gypsum or concrete can be added in order to strengthen the mixture and to speed the setting.
- The centrifuging process is very efficient due to the continuous shaping of large blocks.
- Plastic, semi-dry and dry mixes are shaped with the corresponding presses (extrusion presses, hydraulic presses or mechanical presses).
- The bricks, unfinished cylindrical pieces or blanks are fired in chamber furnaces, bogie hearth furnaces or tunnel kilns. The firing temperature corresponds approximately to the classification temperature indicated by the producers. Due to high drying and firing shrinkage, cutting or grinding is necessary for most brick qualities in order to obtain the standard shapes.
- Bricks which are complicated in shape are produced by hand forming, vibration or moulding processes.
| PROPERTIES AND APPLICATION ADVICES |
The requirements for lightweight refractory bricks are diverse and in some cases even contradictory:
On the one hand
- high thermal insulating capability and low bulk density
on the other hand
- sufficient mechanical strength but also good workability.
Additionally
- high thermal resistance under a multitude of atmospheric conditions
- as well as resistance to temperature shocks and changes is required.
The operation of the industrial furnace is a decisive criterion for the behaviour of lightweight refractory bricks in service. For furnaces operated on a continuous basis the mass of the constructed refractory lining plays a less important role in energy efficiency. The degree of thermal insulation is significant for efficient operation, so that bricks with low ?-values are preferred.
The basic rule is: 
The lower the bulk density, the lower the thermal conductivity. There is demand for a bulk density adapted to the service temperature. This demand is based on the existence of a minimum thermal conductivity and the shift to higher bulk density at higher temperatures.
Special knowledge is required with regard to
maximum service limit temperatures. The classification temperature as producer information is usually determined in the lab on standard half size bricks. Shrinkage should not exceed 2 % after being subjected to heat on all sides for 24 hours.
In practice the start of shrinkage and softening, due to the long-term effect of high temperatures, static pressure, vibrations, reduced atmosphere or fluxed vapours, is approximately 100 to 200 K below the classification temperature determined in the lab. (Graph 2)
If there is
chemical attack by gas or dustloaded constituents as part of the furnace atmosphere, the following has to be considered:
- Reducing furnace atmospheres require bricks with low iron content (carbon bursting).
- Furthermore it has been determined that higher shrinkage occurs in reducing atmospheres.
- Alkaline vapours and condensates result in modification of the structure and melt phases (alkali bursting).
- Ca0-bonded bricks are sensitive to overheating due to short sintering ranges.
- Sulphur contents in the brick can attack heating elements in electric furnaces.
The thermal conductivity values indicated in brochures are valid for normal atmospheres. The values change e.g. under inert gas. In H2 atmospheres the actual thermal conductivity can be up to 7 times higher. Furthermore the heat flow changes as a result of increased furnace pressure. In a vacuum the convective heat transfer decreases.
Measurements of thermomechanical behaviour such as resistance to permanent compression and fire resistance under load are much more informative than the classification temperature or shrinkage.
Graph 3 shows the compressive creep of lightweight mullite bricks (CT 1540 to 1650 °C ) at various temperatures and loads. The curve shows that compressive creep does not stop until below 1150 °C, even if the mechanical load is minimal. For the construction of the furnace – especially for arches – this means that the hot face of the brick in the brickwork may continue to be distorted until a layer temperature of 1150 °C is reached. Thus the result is that the thermal load on the brick must be regulated via the heat transfer value in such a way that there is still a sufficient supporting „cold“ part of the brick left.
The deformation curves measured in the high temperature zone in a large test furnace are generally similar to those shown in
graph 4. For this reason it is advisable to install bricks with a higher classification temperature in the arch than at the walls in order to obtain higher safety for the construction.
The
cold compressive strength is less important than the hot properties. In general the mechanical loads are not that high, so that the strength of lightweight refractory bricks is fully sufficient. Approximately 0.5 N/mm² is required as a minimum strength. This ensures enough strength for transport and handling during installation work. Often a compromise between strength, bulk density and thermal conductivity has to be found. In the case of parts of the construction with higher mechanical load, bricks with higher cold crushing strength are required. It is important that a higher cold compressive strength is not the result of a higher fluxing agent content.
Thermal shock resistance is an important property for furnaces operated periodically.
Depending on the heating-up and coolingdown speeds for the furnace, it is important to know the stress limits of the material. For lightweight refractory bricks, there are permissible temperature differences in the range between 130 to 250 K and these affect the material and result in cracking. Such temperature differences are often exceeded repeatedly in wall linings. Each temperature change results in an increasing loosening of the structure.
In technical literature 5-10 K/min. is mentioned as the critical heating-up speed for lightweight fireclay bricks – depending on the shape.
The individual grades of lightweight refractory bricks show different resistance behaviour to thermal shock. High cristobalite contents > 10 % have a negative effect on thermal shock resistance. Below 10 % other criteria are more essential.
Microcracks in the structure are
advantageous because any stress that occurs can be absorbed without further crack development.
When
testing the lightweight refractory bricks conclusive differences are evident in the qualitative results of air quenching according to DIN 51068.
On pages 90 - 91 of the catalogue properties of the
PROMATON® lightweight refractory bricks are shown.
The producers` brochures do not often include information on fire resistance under load, compressive creep and thermal shock resistance.
Suspended blocks and roll ducts are made by gluing standard bricks or boards with high temperature bonding agents.
Experience has shown that the
weak points in the brickwork bond are mortar joints and adhesive joints. Due to high porosity and the resulting fast removal of water, there are two important factors for the bricklaying of lightweight refractory bricks:
- The bonding agent must be „plastic“ and must have a high capability to bond water. If this is not a given property, the bricks cannot be laid precisely according to measurements and the bond of the individual bricks is lost when corrected later.
- The bonding agent should have a low alkali content. The lightweight refractory bricks suck the binder out of the bonding agent during bricklaying, which means that the bricks are infiltrated on the outside by the binder liquid. If the alkali content is higher (water glass bonding agent), it is possible that outer areas are formed which are enriched with alkali. These areas sinter at higher temperatures and have a vitreous form. Thus the brick has a different structure on the outside than on the inside. During temperature changes this results in cracks and spallings.
Consequently bonding agents without water glass should be preferred for high temperature applications.
ALSIFLEX® bonding agent.
