BRICK & CLAY RECORD: Controlling Quality with light
For centuries, the ceramic and brick industries have been striving to conquer the battle of maintaining a consistent final moisture content in the extrusion or forming stage. A body formulation with consistent moisture levels at the forming stage will also have a predictable plasticity-and therefore quality. However, achieving tight moisture control of the final formed product requires a control/detection mechanism that can maintain a real-time evaluation of the raw material moisture content of the mix prior to forming. The equipment must also be easy to maintain, and must be durable enough to withstand the abrasive nature of the brick raw materials.
For an increasing number of ceramic and brick manufacturers, infrared absorption spectroscopy instruments meet these requirements and provide the best way to analyze moisture content. The instruments perform the analysis without physically touching the material. As a result, they can be used on-line during the forming process. They are placed high enough above the process to allow easy access for maintenance, and they can be used with virtually any material found in the ceramic industry-including highly abrasive clays. With infrared absorption technology, analyzing and adjusting moisture content can be a relatively simple process.
The Power of LightThe concept of infrared absorption can be understood by considering the effect that the sound of a singer's voice can have on a normal drinking glass. When a singer scales the octaves (frequency) at a high enough degree of loudness (sound pressure), a drinking glass can shatter. The point at which the shattering occurs is called the resonant frequency of that drinking glass.
Another classic example is filling several drinking glasses to different levels with water and then rubbing a wet finger along the rim of the glass. This action produces sound waves from each glass at tone levels represented by the distance from the water level to the top of the glass. The glass-water combination will resonate at exact frequencies proportional to the height of the liquid in the glass.
Instead of using sound, infrared absorption uses infrared light to excite molecules at particular wavelengths. Figure 1 illustrates the electromagnetic spectrum from the ultraviolet to the far infrared region. The visual spectrum (the range of wavelengths that stimulate the rods and cones of the retina in the human eye) and the ultraviolet region (invisible to the human eye) are to the left of the chart. These wavelengths have higher levels of energy than the infrared region, which is on the right side of the chart. Infrared light can be further broken down into near infrared (NIR), far infrared (FIR) and sometimes into a third category, the mid-infrared (MIR) region. The near- and mid-infrared regions are the most useful regions in infrared absorption studies.
When molecules of water are excited with infrared energy, the molecules will resonate at specific wavelengths, much like the shattering or "singing" glass mentioned earlier. The increased energy causes the water molecules to stretch, vibrate and bend.
When a substance is analyzed using infrared absorption spectroscopy, the infrared light typically passes through a very thin film of the substance to a receiving detector on the other side. However, when used to analyze the moisture content of clay, the instrument passes the infrared light through the water that is coating the surface of the microscopic particles of the raw material. The "reflectivity" of the raw material acts as a mirror, bouncing the infrared light back through the surface water toward the detector. A double pass through the surface water helps optimize the signal-to-noise ratio and produces a curve that is proportional to the amount of water in the clay. The instrument then compares this curve against the base curve to determine whether the moisture content of the mix is adequate to achieve the desired plasticity in the forming stage. Some infrared absorption spectroscopy instruments* are so sophisticated that they will automatically add the appropriate amount of water if the moisture content is below the desired level.
*The MC-C Series moisture controller, suppied by Journey Electronics Corp., Monroe, OH, is one example of this type of system.
Understanding the TechnologyAs with any technology, infrared absorption can only be truly optimized if the operator understands the potential drawbacks and how to avoid them. As noted previously, a double pass through the surface water can help increase the signal-to-noise ratio and ensure that an accurate curve is generated. However, it is important to note that virtually all clays differ in reflectivity and will produce different base curves. As a result, the change in returned intensity at certain wavelengths can be nonlinear when compared to different types of clay. For this reason, each raw material used in a process should be calibrated so that the readings can be associated with a standard curve for that material. Once the base curves for individual raw materials have been determined, mixes of these materials will produce predictable curves that are proportional to the amount of water in the raw clay.
When a raw material is irradiated with infrared energy, the water molecules become agitated by absorbing energy at specific wavelengths in the infrared region of the spectrum. Other molecules that possess hydrogen-to-oxygen bonding can also absorb energy at these wavelengths and, at high enough levels, can affect the base curve. Some examples of substances that might cause changes in base curves for different raw materials are large concentrations of decaying organic impurities, or the addition of amine-like groups (N-H bonds), some peroxides, and certain carbonyl-hydroxyl groups (O-H bonds), such as aldehydes, which give absorption at nearby wavelengths. However, in most brick clays, the normal percentage weight of these compounds compared to the total weight of the raw materials is usually low and is therefore insignificant to the base curves.
Another potential change in base curves can occur due to the hydrated water content of the raw materials. Hydrated water is normally formed as complex ions bonded to cations (positive ions such as Mg++ and Al+++) as part of a stable molecular structure, and dehydrates in the firing process between 300 to 1800ºF, depending on how strongly it is bonded to the clay. During infrared analysis of the raw material, the hydrated water will absorb infrared energy and will be recorded as part of the moisture content. If calibration curves have been determined for the materials used in the body formulation, the hydrated water content will have been recorded as part of these curves, and the values will be negated. The final moisture determinations will not be affected unless the percentage of hydrated water changes drastically in the raw material. Since the dry water weight of a raw material is generally 5% to 12% of the total weight, minor variances in the hydrated water weight of a raw material will be negligible.
Although water molecules can be coordinated in and around the crystalline lattice structures of salts in several other ways, these occurrences are more rare, and associated concentrations are usually low enough to ignore.
Improving PlasticitySince the plasticity of a material can better be exploited through tight control of moisture content, infrared spectral detection of raw material moisture provides a definite means to achieve improved "green" stage quality. With accurate, efficient, on-line moisture analysis, inconsistent plasticity can be avoided, and A-grade products can be easier to manufacture.
For more information about infrared absorption spectroscopy, contact Journey Electronics Corp., 902 Garver Rd., Monroe, Ohio 45050; (513) 539-9836; or e-mail firstname.lastname@example.org .