BCR: Optimizing Firing Through TTT Analysis
TTT Analysis–The BasicsTTT 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.
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.
Putting TTT Theory into PracticeHanson 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.
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.
The Proof is in the Finished ProductAfter 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.