Clay bricks/clay pavers are a building material which has been in use in road-building in Northern Germany and The Netherlands for over 300 years. Today clay pavers are formed from loam, clay or a clayey mass with or without additives and fired to sintering. The mechanized production of clay pavers in modern factories may be divided into these production phases.
Raw material extraction
The natural clay deposits used for the production of clay pavers is mechanically extracted in an open-cast mine. Excavators, usually ladder excavators or wheel loaders, are used in the extraction process. In this way the raw material can be carefully extracted and loaded onto trolleys or trucks. The greater part of the raw material is extracted close to the clay pavers production centres resulting in very short and low-energy transport from mine to factory. Picture 3-1 shows clay extraction in a pit where the clay turns red on firing. The colour scale shows the possible colour nuances. Depending on the type of clay, the firing temperature and atmosphere different clay colours can be produced ranging from yellow through red to very dark shades.
Stages in the treatment of the raw material.
Only rarely are loam, clay and shale to be found in the composition and structure necessary for the production of clay pavers. So the mined raw material has to be processed in the production centre. This processing of the raw material consists of several stages:
• Mixing of the raw materials clay and loam as well as the addition of sand, powdered clay or other suitable stabilizing materials.
• Filtering out undesirable components
• Crushing of too large or too small components
• Adjustment of the desired plasticity through moisture control
The raw materials pass through a roll crusher, a box feeder, an edge mill and a rolling mill. The rolling mill is particularly important for the fine preparation of the raw materials. Here the coarser components of the pre-treated materials are reduced further by the circular crushers before passing on to the rolling mills where the desired grain size (0-1mm) is achieved and a fine-grained mixture is the result. Moisture is added to the mixed, crushed raw material to increase the plasticity.
The prepared raw material moves through the rolling mills to the vacuum chamber where the air is removed, is warmed by hot steam and made more malleable on to the press, usually an extrusion press. A screw pushes the raw material in a string shape out of the press. Cutters divide the string into the desired lengths. The blanks which result from this process are the same shape as the finished pavers but are larger than the finished product. This is necessary to allow for the drying and firing which reduces the size. The shrinkage varies with the raw material and is between7% and 12% . Where the blanks are for finished clay pavers with visible champfered edges these are pre-formed at the string stage.
The damp blanks are slowly dried at temperatures of up to 100° C. Water is withdrawn from the mass. The clay paver blanks are automatically stacked on special tunnel kiln trolleys so that enough distance remains between the blanks for the hot air to circulate during the firing process (setting pattern). The blanks on their trolleys are moved into the tunnel kiln.
The dry blanks are fired as the trolley passes through the tunnel kiln. At temperatures of above 1100° C the components are chemically converted to sintering level. This is how the characteristic shard of the pavers happens. During the firing process the blanks on the tunnel kiln trolleys pass first of all through the warm-up zone, then the actual firing zone and then the cooling-down zone. In this way we can guarantee a product free of stress crack. The complete firing process lasts 72 hours. During this time the blanks to be fired pass through the length of the tunnel kiln which, depending on the design, can be 60-180 metres long and pass out at the end as finished clay pavers.
The energy supply for the tunnel kiln is usually from the top, occasionally from the side. Natural gas which ensures combustion with very little residue is the predominantly used energy form. Surplus warmth is recycled via a heat recovery system for use e.g. in the drying chamber.