Brick making shares many similarities with baking a cake. By following the recipe, mixing all the right ingredients in the right order, and baking at the right temperature for the right length of time, a perfect finished product should result. Baking at the wrong temperature or for the wrong amount of time will lead to the cake being either under- or over-cooked. The same thing happens with brick making, but understanding what is happening during the process will allow you to write the recipe correctly in the first place, rather than relying on trial and error. Time, temperature and transformation (TTT) analysis is a methodology that can help you optimize your firing process.
Figure 1. Diagram of the mineral transformations that take place through the firing curve.
TTT Analysis–The Basics
TTT analysis was developed to understand the mineralogical changes that take place during the firing of a ceramic body. The effects of time and temperature on the mineral transformations of a known raw material composition can be related to the desired physical properties of the brick, and this data can be used to identify the optimum soak time and temperature for that brick type.
Figure 1 shows a diagram of the mineral transformations that take place during firing for a typical UK brick making clay.
Figure 2. Laboratory generated TTT plot showing mineral transformations in time and temperature space. The position of the mineral name and the corresponding line indicates whether a mineral is being destroyed or formed, e.g., those below the line are being destroyed and those above are being formed.
By plotting the mineral transformations in time and temperature space, the TTT plot (Figure 2) shows where and when key mineral reactions produce the ceramic transformations that give brick their physical characteristics.
While Figures 1 and 2 will mean a lot to mineralogists, they are not of much use to the majority of brick makers. However, such plots do form the fundamental foundations for making consistently high-quality products. By plotting the routinely measured physical properties of the bricks rather than the mineral transformations, the plots become more relevant to the majority of brick makers. This information can then be used to determine the minimum quality requirements, as well as the optimum soak time and temperature needed to achieve these requirements.
Figure 3. Laboratory generated TTT plots for water absorption, compressive strength and frost resistance (durability).
Putting TTT Theory into Practice
Hanson Brick’s Kirton Brickworks is located in Nottinghamshire, UK. The works consumes approximately 6000 metric tonnes of raw material per week to produce 2 million red and buff bodied brick in that same timeframe using an extrusion process. Following an increase in production, concern was expressed as to the impact upon durability (frost resistance) of the red bodied products. TTT analysis was used to identify the optimum firing conditions and justify kiln modifications to address the problem.
Initial laboratory trials consisted of forming a TTT plot (Figure 3) for test pieces of the red body mix design. Test pieces were fired at different times and temperatures to achieve the plot.
Figure 4. Underload firing curve for the Kirton red body. Densification resulting from mineral transformations became evident at roughly 950¿C.
In addition, underload firings (Figure 4) of the test pieces were used to identify the point at which the body began to densify, as this indicated when the majority of mineral transformations were having a volumetric effect on the brick body.
The initial 90 data points were modified to 18 in the lab firings, which created a TTT plot upon which the critical physical properties could be plotted. From routine testing of the works-made products, it was established that the minimum requirements for the desired physical properties to ensure durability were <15% water absorption and >45 Nmm2 compressive strength. This was verified by the results of the laboratory firings.
Figure 5. Thermocouple positions on the kiln car.
While the lab results were very promising, the theory needed to be verified in the production environment. This was done by using a Datapaq Kiln Tracker system (2x Tpaq 100 data loggers in a TB6200 hot box and type K thermocouples). Nine thermocouples were placed at specific points throughout the kiln car. Each thermocouple was sandwiched between two brick to accurately measure the brick body temperature rather than the passing air temperature (see Figure 5).
Figure 6. Works TTT plots indicating the maximum thermocouple temperatures and soak time above 950¿C in minutes. Representative brick sampled from the corresponding thermocouple positions were tested, and the results of water absorption and compressive strength were plotted.
Collection of the data and the subsequent routine testing of the brick immediately adjacent to the thermocouples allowed production TTT plots to be created (see Figure 6). By transferring the results of the TTT plots back onto the kiln car, the zones of fired and under-fired brick were easily identified.
Figure 7. Quality data plotted back onto the kiln car indicating where kiln adjustments need to be made.
Using this information, the plant operators were able to rebalance the kiln to bring the under-fired zones back into line (see Figure 7).
The Proof is in the Finished Product
After evaluating the TTT plots, the plant operators at Kirton agreed that the best method to gain better quality was to improve the heat distribution at the front end of the kiln by slowing the push rate and lowering the kiln draft. Improvements in the product quality were almost instantaneous.
TTT analysis can be used as both an indicative and predictive tool for establishing optimized firing curves for each body type. It identifies the minimum time and temperature required to achieve a specified quality, ensuring that a consistent product quality is maintained. It also reduces energy consumption, minimizes emissions into the environment, and uses equipment and data that are already available in the majority of brick plants.
For More Information
For more information about TTT analysis, contact Dr. Andrew Smith, Hanson Brick Ltd., Unicorn House, Wellington St., Ripley, Derby, England DE5 3DZ; (44) 8705-258258; fax (44) 1773-514044; e-mail firstname.lastname@example.org
; or visit http://www.hanson-brickseurope.com