Reducing variability in materials and processes is vital to making quality structural clay products. Consistency is key to raw materials in the matters of selection, mining, grinding, mixing, and extrusion. Additional processes that require consistency include drying and firing.
Variability in raw materials can cause major problems in the manufacturing of structural ceramics:
- Selection – Raw materials must be chosen carefully based on key parameters such as mineralogy, fired color, shrinkage, particle size, etc. The selection process will often include drilling for and testing many samples that will need to combined in the right ratios to achieve the desired results.
- Mining – Once the ratio of raw materials needed to meet the target parameters has been compiled, a mining plan needs to be written and then implemented. The mining plan should be written so that mining will be as efficient as possible while producing a stockpile of raw materials that will meet the needs of the plant. There are factors that can be controlled such as the type of equipment used and the number of pieces of equipment used while there are uncontrollable factors such as weather and mechanical breakdowns.
- Grinding – Preparation of raw materials is usually a multi-step process that begins at the stockpile where materials are loaded into haul trucks. The materials loaded should be representative of the pile. This requires the materials to be taken from the pile in a particular, repeatable order. Materials from the stockpile are then run through a primary crusher and a sizing system including a secondary crusher and screens. The primary crusher, secondary crusher, and screens all have wear parts that need to be maintained in order to reduce the materials’ particle size consistently.
- Mixing – Most structural clay bodies are mixes of different prepared raw materials. Whether mixing is carried out using a separate mixer or is done using the pug mill, the ratios need to be correct. The average density and moisture content of each raw material has to be considered and adjusted to in order to make repeatable, consistent mixes. In the case of using mechanical mixers, the wear parts should be maintained adequately for efficient mixing.
- Extrusion – The pug mill, vacuum chamber, and extruder contain multiple wear parts that must be properly installed, maintained, and replaced when necessary in order to temper, de-air, and form the structural clay column. The pug knives that work the raw clay mix and move the material toward the vacuum chamber have to be installed at the correct angle and replaced when worn. Pug knives that are not positioned correctly will not move the clay mix and pug knives that are worn excessively can break loose and cause catastrophic damage. The vacuum chamber starting with the sealing section contains augers and shredder knives that are subject to wear. Excessive wear in the sealing section can result in a loss of vacuum or limit the amount of mix that can be moved into the extruder. The extruder itself contains augers and liners that have to be replaced periodically as extrusion efficiency declines with wear. For optimum, consistent tempering, de-airing, and forming quality it is essential to maintain the brick machine.
Structural ceramics are usually directly set on kiln cars made out of metal frames and refractory ceramics. The kiln cars then act as a conveyance through the drying and firing processes.
The drying process starts once green brick are extruded. The bricks begin drying even before setting is complete. Brick plants that run manufacturing for 5 days per week will have inconsistent drying conditions if the holding room conditions are not monitored and adjusted as the holding room fills with green brick. The first brick made on Monday morning should see conditions that are as similar as possible to the last brick made on Friday. The dryer should not have variations from the beginning of the week to the end of the week as the dryer should always be full. Drying conditions such as temperature and air flow should be kept as steady as possible.
The firing process in a tunnel kiln is a fluid process that needs to be controlled from the beginning to the end in order to produce consistent, quality bricks. The firing process is where color, size, and strength of the final product are determined. The parameters that have to be controlled include temperature, oxygen levels, draft, air-gas ratios, and emissions. These parameters are controlled with both automatic and manual systems of burners, fans, dampers, and regulators. All of these systems are subject to harsh environmental conditions.
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