A Cement Plant Toolbox to Simplify Your Life
With cement demand at its highest level ever, cement plants are continually being pushed to the limit. A lot is being asked of both personnel and equipment not only to maintain production, but also to establish new records without sacrificing quality. This leads to ever greater pressure to perform. Everyone at the plant has at least some impact on:
producing a quality product;
increasing production;
reducing costs; and
preventing problems that could cause plant shutdowns.
It is therefore important that cement plant staff remain knowledgeable with the most current technologies pertaining to these issues. In other words, they must know what tools are available.
Many tests are available to provide important information beyond the scope of standard testing. Results of these tests can reveal the history of how the cement was produced, including raw material properties, pyroprocessing conditions, and finish grinding. These tests are tools, which can be used for quality control, improving and increasing production, and troubleshooting problems.
Some tools are revolutionary. For example, in thermal analysis, a new generation of testing equipment featuring lower detection limits, better signal resolution, improved sensitivity and more powerful data acquisition and processing is available today. Other tools are not new to the cement industry. However, their complete capabilities are not always fully understood or utilized. Often a single test method does not provide sufficient information by itself, but together with the correct complementary tests, the results will lead to full understanding of the problems at hand and the potential solutions.
The proper combination of tools, use of complementary techniques, and in-depth understanding of the cement manufacturing process creates an innovative toolbox that helps answer questions or solve problems.
Toolbox contentsThermal Analysis — Differential Scanning Calorimetry (DSC) and Thermogravimetric Analysis (TGA) are valuable tools when assessing cement manufacturing processes. Temperature changes affect materials and substances chemically and/or physically. The application of DSC and TGA can provide information about the changes as a function of temperature by measuring heat of reaction or transition by DSC and associated weight loss by TGA. The use of these tools can provide unique analytical information that assists in characterization of a given sample as to its nature, composition, and effect on a specific property or process.
DSC and TGA are used in assessment of cement performance and physical properties such as setting time and strength development. The results of thermal analysis can quantify cement sulfate forms, important in evaluating causes of set abnormalities, lump formation, pack set, and flowability problems. These tools can also be used in evaluation of cement hydration products, including identification, establishing the degree of pre-hydration, and influence on cement performance. Thermal analysis results help identify compounds involved in problems with kiln build-ups, finish mill operations, and cement storage.
Thermal analysis is useful in addressing the environmental concerns facing today's cement industry. The control of volatile organic compounds (VOC) emission is more significant than ever. Thermal analysis quantifies organic emissions from a specimen. Therefore, the use of thermal analysis helps in screening cement raw materials and kiln feeds as to their potentials to contribute to Total Hydrocarbon (THC) emissions from the kiln, and provides solutions to control VOC emissions. The use of alternative raw materials, new quarry development, or assessment of individual constituents used in the kiln feed and their contribution to the increase of organic materials emissions can be evaluated based on the results of thermal analysis.
Microscopy — Examining clinker under the microscope is an informative and powerful technique. The crystal microstructure is a result of the kiln feed (mineralogy and fineness) and pyroprocessing conditions. Therefore, the phase size, amount, distribution, and other characteristics reveal information about the clinker history. The kiln temperature profile also can be determined by using the microscope. Results of a microscopical examination can be used to provide clues to optimize raw feed burnability, improve clinker grindability, and troubleshoot production problems.
Analysis of the microstructure is important to monitor process changes, such as use of alternate raw materials or change in burner pipe position. In addition to clinker analysis, microscopy of the raw feed is important to optimizing production, since the feed fineness and mineralogy is critical to its ease in burning. The microscope also can be a useful tool to identify various cement and raw feed contaminants and assist in discovering the causes of buildups.
Chemical Analysis — X-ray Fluorescence (XRF) analysis for chemical composition of cements and raw materials is the most widely used technique. It is the main workhorse of the cement plant laboratory, providing important information on the chemical makeup of a given sample. It is an essential quality-control tool, used to assure the proper composition of the initial cement raw material blends and of the final product, since it furnishes data on the amounts of major cement oxides such as CaO, SiO
To complement XRF analysis, X-ray Diffraction (XRD) often is performed to establish the mineralogical composition of samples. XRF provides quantification of the elements, while XRD is used to identify actual compounds (phases). XRD identifies substances based on their crystalline atomic structure. Clinker and cement phases can, therefore, be identified and quantified. The analysis also is particularly important for identifying unknown materials, such as those in buildups and contaminants.
CASE STUDIES: Problem solving in the cement industry I. Preheater BuildupProblem: A preheater kiln without alkali bypass system was regularly experiencing buildups in the preheater tower.
Investigation: Samples of the buildup were initially analyzed by XRF. The sulfur level (8.15%), and to a lesser extent the alkali level (0.78% Na
These results show several facts:
The presence of spurrite, a needle-forming crystalline material often responsible for flow problems.
The calcite peaks are diffuse, suggesting that there may be variable crystallinity of the material.
There is very little free lime or silica, suggesting almost complete reaction of the silica and lime to belite (or retention as calcite).
One of the compounds contains chloride as a constituent. To quantify the chloride content, potentiometric titration with silver nitrate was performed. The sample was shown to contain 0.97% chloride; this is a significant amount.
The buildup sample was then investigated by thermogravimetric analysis (TGA) to determine the cause of the diffuse calcite peaks (Figure 1, page 19). The results demonstrate three high-temperature weight losses, occurring at 578° to 760°C, 760° to 868°C, and 868° to 998°C. The higher two weight losses are consistent with those expected from normal calcite and from spurrite. However, the low temperature loss is probably attributable to CaCO
To further confirm the buildup mechanism, the sample was examined microscopically. The calcium carbonate was found in two crystalline modifications: one that appeared not to be fully decomposed calcite, and the other that had morphological features consistent with lime but optical properties consistent with calcite. This result confirms that recarbonated free lime was a component of the buildup. The build up sample was also examined in powder mount by microscopy. The analysis verified the occurrence of spurrite (Photo 1, page 20) and re-carbonation of free lime (Photo 2, page 20).
Solution: Chloride is known to be a catalyst for the formation of spurrite. This is especially true if the kiln feed contains illite-type clays. These conditions were all fulfilled at the plant. The presence of belite and recarbonated free lime provides further evidence that the silica and lime have essentially all combined in the buildup — evidence that a reactive form of silica such as illite led to low-temperature formation of belite and spurrite and furthered this buildup mechanism. The mechanism itself is now seen to consist of at least two parts: the consolidation in place of loose material through recarbonation, and the inhibition of flowability through spurrite formation. The possible solutions to the problem were to:
identify and remove or reduce the source of chloride in the system;
replace the illite-containing clay with an alternate raw material without illite; or
install a bypass system on the preheater to remove the chloride.
| Original Raw Mix | After Process Changes | |
|---|---|---|
| Lime Saturation Factor (LSF) | 99 | 98 |
| Silica Ratio (SR) | 2.9 | 2.5 |
| Coarse calcite grains % | 7.2 | 5.5 |
| Coarse quartz grains % | 1.2 | 0.8 |
| % Free Lime (“virtual burnability”) | 5.8 | 3.7 |
The most desirable and cost effective alternative was to locate the source of the chloride. Careful study revealed that the coal was the principal source of chloride; it contained 0.3% chloride. Since the kiln uses 0.1 lb of coal per lb of clinker, this is equivalent to a clinker chloride of 0.03%, or a kiln feed chloride equivalent of about 0.02%. This is higher than the threshold limit of 0.015% chloride traditionally cited by preheater kiln manufacturers as the highest value consistent with successful operation of a preheater kiln without bypass. A recommendation was made that the plant change coal sources to reduce chloride input.
II. Sources of High Organic Emissions/Raw Material EvaluationProblem: A cement plant was experiencing unacceptably high emissions of volatile organic compounds (VOC) and carbon monoxide. The principal question was whether the emissions stemmed from raw materials, or from combustion problems. As raw materials, the plant had available chalk, cement rock, clay, silica sand, and iron ore and was targeting the kiln feed for Type II composition. The plant used the chalk, clay, silica sand, and iron ore for the kiln feed because the chalk was soft, and the plant operators believed that they could achieve better raw mill production with the chalk as calcium source, rather than the cement rock. This decision was made even though use of the chalk also required silica sand to be used, and this quartz was hard to grind.
Investigation: Microscopy of the chalk revealed small pockets of bituminous material (Photo 3, page 20). A DSC study was performed on the chalk-containing kiln feed. The results (Figure 2, page 21) indicate a significant amount of organic material. It is clear that the source of the organic emissions from the kiln feed was the bituminous material in the chalk. In addition, almost half of the organic emissions (more than 115 J/g, or about 50 Btu/lb) was released at temperatures below 350°C; lower temperature release indicates a greater tendency to be emitted.
Solution: To reduce emissions, the chalk needed to be replaced by another calcium source. Fortunately, cement rock was available. DSC on the cement rock showed much lower organic contamination than in the chalk.
DSC Results: Energy released from combustion in two calcium sources
| Total Energy J/g |
<350°C J/g |
|
|---|---|---|
| Chalk | 252.69 | 115.05 |
| Cement Rock | 80.12 | 11.60 |
The cement rock showed only about one-third of the total organic contamination of the chalk. Furthermore, the energy released below 350°C was only about 10% that of the chalk. It was suggested that the plant consider the use of the cement rock as a substitute for chalk and sand, and raw mixes were calculated.
With the new mix, the sand use was reduced to about half, and the organic input was reduced more than 65%. The emission problem was successfully addressed as the emissions dropped below the regulatory threshold and the kiln feed burnability improved due to the reduction of coarse quartz resulting from the reduction in the amount of sand. The homogeneity of the kiln feed permitted a reduction in its fineness, compensating for the harder grindability of the cement rock compared to the chalk.
III. Hard-Burning Raw FeedProblem: A cement plant experienced problems with burnability of the kiln feed, resulting in decreased production and increased fuel consumption. The raw materials available were limestone, clay, and iron ore.
Investigation: A tool called the “virtual burnability test” was used to analyze how changes in raw mix composition could improve the burnability. This test uses an equation to evaluate the effects of chemical composition, mineralogy, and fineness: %CaO = a*LSF+b*Ms+c + d*(calcite > 125μmm) + e*(coarse grains > 45 μm) + f*(coarse grains of other silicates)
where, %CaO is the burnability index; a*LSF + b*Ms + c is the contribution from the chemical composition of the kiln feed; + d*(calcite larger than 125μm), + e*(coarse grains larger than 45 μm), and + f*(coarse grains of other silicates) are the contributions from the fineness and mineralogy.
* “a” through “f” are constants based on time and temperature for the laboratory test
The results of chemical analysis of the kiln feed (corrected for coal ash absorption) showed the lime saturation factor (LSF) and silica ratio (SR) to be high. Microscopic examination demonstrated high contents of coarse calcite and quartz in the feed (Photo 4, page 21). The resulting microstructure of clinker produced from coarse kiln feed is shown in Photo 5 (page 21).
The raw mix parameters used in calculation of the “virtual burnability” are provided in Table 1 (page 22). The largest contribution was from the content of the coarse grains of calcite and quartz; finer grinding would be required to reduce this effect. In order to reduce the contribution from the chemical composition, the LSF and silica ratio should be decreased.
Solution: Based on mineralogical and chemical observations, recommendations were made to grind the raw mix finer and to use the alternate clay source with less SiO
A variety of tools are available to the cement manufacturing industry, including thermal analysis, microscopy, and chemical analysis. An understanding of the capabilities of these tools individually and how they compliment each other, together with the knowledge of their relationship to cement manufacturing process provides a complete toolbox to solve cement plant operation problems and increase production.
Linda M. Hills, Ella Shkolnik, F. MacGregor Miller, Vagn Johansen all work at Construction Technology Laboratories, Inc., 5420 Old Orchard Road, Skokie, IL 60077; (847) 965-7500; www.ctlgroup.com.
This article is summarized from “An Innovative Cement Plant Toolbox to Simplify Your Life,” first published and presented at Cement Americas' International Cement 2000 conference in Charleston, S.C. December 2000).
AcknowledgementsThe authors wish to thank Alex Mishulovich, Ronald Sturm, Fulvio Tang, Howard Kanare, and Laura Powers for their contributions in providing analysis and expertise.
