Nurhan Gürel
CemenTürk Editor-in-Chief
Summary
The production of high-quality cement in the cement industry is largely based on comminution and classification processes. Comminution, which involves the reduction of raw materials’ size, and classification, which separates particles into different size ranges, are critical in cement production. This paper examines the historical evolution of these technologies frequently used and the fundamental advancements that have significantly impacted the industry. The study covers modern comminution technologies such as ball mills, vertical mills, and highpressure grinding rolls (HPGR), as well as air classifiers.
Introduction
The cement industry plays a vital role in infrastructure development, and the efficiency of its production processes directly influences economic growth and sustainability. Among the various stages of cement production, comminution and classification are of great importance and constitute the bulk of energy consumption. Over the years, advancements in comminution and classification technologies have resulted in significant improvements in energy efficiency, production capacity, and product quality. As the industry faces increasing pressure to reduce its environmental footprint, these advancements have become more critical than ever. This article aims to provide a comprehensive overview of the technological developments in comminution and classification within the cement industry since the invention of modern cement, emphasizing both historical progress and recent innovations. It also addresses the future direction of the technology.
At the beginning of the 19th century, the cement industry was still in its infancy, and the methods of grinding and crushing raw materials were quite primitive. The comminution process was mostly carried out manually in the early stages. Workers would break down limestone and other raw materials using hammers and other simple tools. The Industrial Revolution, which spanned from the late 18th to the early 19th century (Table 1), marked a significant turning point for the cement industry. With the invention of electricity, mechanization began in grinding technology.
Table 1: Stages of the Industrial Revolution
19th Century: The Historical Development and Impact of Ball Mills
In the early 20th century, the demand for finely ground coal and Portland cement for power plants and construction work was rapidly increasing. There was a need for equipment capable of finely grinding cement clinker, which led to the development of the ball mill in 1885 by H. Gruson in Magdeburg. Known as the “Krupp-Grusonwerk mill,” this was the first ball mill used in the industry, and its design laid the foundation for future innovations. However, early designs had significant drawbacks. The drum had large perforations that would clog, resulting in coarse product. Furthermore, wear and tear were major issues. Eventually, a variation was developed where the product was discharged through a shaft at the mill’s end, significantly improving performance. By 1900, mills with diameters of 1.2 meters and lengths of 6 meters were producing 3 tons of cement per hour. (1)
Figure 2: Krupp-Grusonwerk mill of 1885.
When operated in open circuit, the mills would produce an abundance of large particles, leading to non-standard cement. To solve this issue, the mills began to be operated in closed circuits with air classifiers, where fine particles were immediately removed, preventing coarse particles from exiting the circuit. (2)
This approach led to the continuous closed-circuit grinding concept, facilitated by the invention of the Askham air separator in 1885. The separator fed the product from the mill outlet, and its function was to separate coarse particles and send them back to the mill for regrinding. The success of the closed-circuit system sparked a trend toward closed-circuit operation in industrial grinding processes.
Short tube mills are typically used in closed circuit systems with air separators, while long tube mills are used in open circuit systems for final grinding operations. For example, a typical grinding line operates at 110 horsepower, producing at a capacity of 4.5 tons per hour with a 90-micron residue at 16%. These types of systems are designed to provide high efficiency and quality, making them suitable for various industrial applications.
As ball mills became widely used in the cement industry, their designs and efficiency were continuously improved. These mills, which used rotating drums filled with steel balls, provided the fine grinding needed to produce high-quality cement essential for modern construction (100 microns). Ball mills proved to be versatile machines, used for various purposes. However, they were not a solution for fine grinding challenges that emerged as finer grinding standards were required. Specifically, they were inefficient in grinding particles smaller than 100 microns, as such small particles could not easily be captured between the balls or the liners. This led the industry to explore new grinding technologies.
Figure 3: A ball mill used for the initial grinding of clinker and a long tube mill used for final grinding. (3)
Mid-20th Century: Developments in Vertical Roller Mill Technology
The mid-20th century saw another significant change in comminution technology with the introduction of vertical roller mills (VRMs). VRMs, based on the “treadmill” principle used since prehistoric times, grind materials between a rotating table and grinding rollers. These mills offered several advantages over traditional ball mills, including greater energy efficiency and the ability to grind a wider range of materials, such as slag, pozzolan, and other cement additives. The increased efficiency of VRMs led to their widespread adoption in cement plants worldwide, especially in regions where energy costs were high.
Initially developed for grinding softer materials like coal and limestone, vertical mills emerged as improved versions of existing machines. The first of these was the Huntington mill, invented in 1883, which is considered the ancestor of today’s vertical roller mills. Huntington’s mill used centrifugal force instead of gravity to press the roller against a fixed ring. Various vertical mills using balls or rollers pressed against a ring were subsequently developed and commercially produced. (4)
By the early 20th century, machines such as the Maxecon ring roller mill, Fuller ring ball mill, and Raymond centrifugal ring roller mill were in use. The Maxecon mill had a vertical grinding ring rotating around a horizontal axis, with three convex rollers pressing against the ring independently. The Fuller mill featured a U-shaped grinding ring with large steel balls rotating within it, while the Raymond mill had rollers mounted on vertical shafts attached to a horizontal beam, with centrifugal force pressing the rollers against the grinding ring. (5)
Figure 4: Maxecond mill, Fuller mill and Raymond centrifugal mill
By the 1920s, vertical roller mills evolved rapidly. The experiences gained from the Maxecon and Raymond mills led to the development of new designs by Loesche. Loesche introduced a design with stationary rollers and a rotating table, which allowed for the application of higher grinding forces. The first Loesche mill was built in 1928, and this design became the foundation for the mills that bear its name today. Loesche mills, particularly successful in grinding hard minerals, were widely used in power plants and cement factories. (6)
Throughout the 1930s, improvements in mill design were made, but capacities did not increase significantly due to the Great Depression and World War II. By the 1960s, with rising demand in cement plants and the development of larger kiln technologies, mill sizes and capacities grew rapidly. By the 2000s, mills capable of producing 900 tons per hour were developed using larger rollers and tables.
The main advantage of vertical roller mills over ball mills was their ability to achieve the same grinding results with less energy consumption per unit. Therefore, vertical roller mills became the preferred choice for fine grinding cement clinker. Over recent years, the technologies used in these mills have made significant progress, especially in terms of capacity and energy savings, making them the preferred option for grinding cement, coal, limestone, and other raw materials.
Late 20th Century and Early 21st Century: Technological Improvements and High-Pressure Grinding Rolls (HPGR)
From the 19th century onwards, roller crushers, which consist of two steel rollers rotating towards each other, have been widely used. High pressure was applied to materials passing between the rollers. While one roller rotates in a fixed bed, the other is mounted on a movable bed with high-pressure springs. Materials were drawn into the rollers by rotation, reaching the impact zone where they were crushed by gravitational and frictional forces until they passed through the narrowest gap between the rollers. By 1850, double-roller machines could rotate 50 times per hour, were powered by 20-horsepower motors, and could grind 60 tons per day. More powerful, compact, and high-speed machines were developed. By 1925, roller diameters reached 1.8 meters, with widths of 0.5 meters, driven by 150-horsepower motors, capable of grinding to a fineness of -3 mm. (7)
Figure 5: The first Loesche Mill (6)
In the early 20th century, due to the materials and spring designs used, double-roller crushers could only apply a maximum of 70 tons of pressure per roller. With the development of hydraulic pistons, much higher pressures became possible. However, demand for double-roller crushers decreased during that time.
In the 1920s, the Allis Chalmers company began producing double-roller machines designed to granulate and press fine solid materials into pellets. By using hydraulic pistons, they created a compact roller that used the basic design of granulators and pellet mills. The compact roller consisted of two rollers rotating in opposite directions. Fine material fed from the top was drawn into the gap between the rollers, where it was compressed under high pressure and agglomerated into a sheet product. The void content of the product could approach zero, and the sheet thickness could vary between 0.25 to 0.020 inches. (8)
As materials passed through the gap between the rollers, they were redirected to reduce void content, with large particles breaking when they reached minimum voids, followed by plastic deformation. This crushing principle under high pressure laid the foundation for a mill developed by Dr. Klaus Schoenert in Germany 30 years later. In 1982, Dr. Schoenert patented a highpressure roller crusher for the fine and ultra-fine grinding of brittle solids. The use of high-pressure rollers to prepare cement clinker for tube mill feed began in 1985. HPGRs were introduced to cement plants as a means to reduce energy consumption by up to 50% compared to traditional ball mills. (9)
As the 20th century progressed, the focus in comminution technology shifted to improving energy efficiency and reducing operating costs. The development of high-pressure grinding rolls (HPGR) in the 1980s represented a significant technological advance. HPGRs operate by crushing materials under high pressure between two rotating rollers. The high pressure creates micro-cracks in the material, making subsequent grinding stages easier. This technology has been proven to be more energy efficient than traditional mills and has been adopted by cement plants seeking to reduce energy consumption and increase productivity. Initially, HPGRs were used as pre-crushers, but today, they are also used in final cement production.
The Historical Development of Air Classifier Technology
Air classifiers play a critical role in industrial grinding and material separation processes, helping to determine production quality.
The development of air classifiers emerged in response to the growing production needs of the cement industry. The first air classifier was patented in 1885, specifically designed to meet the demand for large volumes of ground cement clinker. As efficiency and quality requirements in cement production increased, significant design changes and improvements were made to air classifiers. However, due to the high cost of redesigning air classifiers, major innovations have occurred only twice: the development of static and dynamic air separators. (3) There are three main generations in the development of dynamic air separator technology:
1. First-Generation Dynamic Classifiers
The development of first-generation dynamic classifiers began with the Askham air separator, developed by Mumford and Moodie in 1885. This device used a fan system designed to separate fine and coarse particles. The basic principle involved using an air current created by a fan to lift fine particles upwards while heavier coarse particles fell down. Although first generation classifiers were low-cost and provided reasonable classification success, they had several shortcomings, such as the inability to fully separate fine particles from recirculated air and overheating of circulating air in clinker circuits, which resulted in excessive fine material circulation in the system and contamination of the final product.
2. Second-Generation Dynamic Classifiers
In the 1960s, second-generation dynamic classifiers brought significant design improvements. These classifiers moved the internal fan outside the system, allowing for better control of the airflow. They also included external fans to optimize air circulation and cyclones to collect fine material, with independent control over classifier speed. These innovations resulted in sharper separations, reduced bypass, and continuous control of product fineness. However, these classifiers were quite large, making them difficult to use in confined spaces.
3. Third-Generation Dynamic Classifiers
Developed in the 1980s, third-generation dynamic classifiers introduced several important design changes. One of the most notable differences was the horizontal entry of air into the classifier, with the feed distributed as a fully dispersed particle curtain. Additionally, fine materials were required to pass through a rotating cage before being separated from the air, which facilitated the removal of coarse particles. This new design resulted in sharper classification. As a result of this innovation, the efficiency of cement production circuits increased by 20%.
Third-generation air classifiers are not only used in the cement industry but are also commonly used in industries producing ultra-fine particles. Devices such as the Alpine Turboplex, for instance, have been successful in producing particles smaller than 10 microns. These classifiers use small rotating cages and counterflow air to separate extremely fine particles.
Static Classifiers
Static air classifiers, which have no moving parts, are less commonly used due to their lower efficiency. These classifiers rely on changes in airspeed and direction to separate fine and coarse particles. However, their classification accuracy is quite low, which is why they are more commonly referred to as “sand separators.”
As a result, air classifiers have played a critical role in cement production and the separation of fine materials in many industries. This technology, which has shown significant advancements over three generations, has provided important innovations, especially in increasing production efficiency and improving product quality.
Recent Developments and the Future of Comminution in the Cement Industry
In recent years, the focus of comminution technology has shifted towards energy efficiency, environmental impact, and sustainability. Advances in grinding technologies, such as the increased use of vertical mills and high-pressure grinding rolls (HPGR), have further optimized the comminution process.
Alongside these developments, the creation of more efficient classifiers and separators has also enhanced the overall process. As the cement industry continues to develop, classification technologies will play an increasingly important role in meeting rapidly changing market demands. Future trends in classification technologies are likely to focus on increasing efficiency, improving automation, and reducing the environmental impact of cement production.
• Smart Classification Systems: The development of smart classification systems using machine learning and artificial intelligence is a promising research area. These systems can analyze the large amounts of big data obtained from the production process to make real-time adjustments, thereby increasing efficiency and product quality.
• Sustainable Classification Technologies: As the industry places greater importance on sustainability, new classification technologies that minimize energy consumption and reduce emissions will be critically important. Innovations such as lowenergy classifiers and systems capable of effectively processing alternative raw materials will be vital for the future of cement production.
As the demand for specific performance characteristics of cement products increases, ultra-fine grinding technologies are becoming increasingly important. These technologies enable the production of cement with very fine particle sizes, which can enhance the durability and strength of concrete.
• Stirred Mills: Stirred mills are commonly used in super fine grinding technologies, typically for grinding cement additives such as slag and fly ash. These mills utilize a combination of grinding media and rotating disks to achieve very fine particle sizes.
• Jet Mills: Jet mills use high-speed air flows to grind materials into super fine particles. This technology is especially suitable for producing cement with very fine particle sizes, thereby enhancing the reactivity and performance of the final product. The integration of automation and advanced process control systems into grinding technology has improved cement production. Modern cement plants are equipped with sophisticated control systems that monitor and adjust the grinding process in real time. These systems optimize energy use, reduce waste, and ensure consistent product quality.
• Real-Time Monitoring: Sensors and data analytics provide real-time feedback on the performance of grinding equipment, allowing for immediate adjustments to optimize efficiency and product quality.
• Predictive Maintenance: Automation systems can anticipate maintenance needs, minimizing downtime by reducing the likelihood of unexpected equipment failures.
• Optimal Energy Usage: Advanced control systems help cement plants optimize their energy consumption by adjusting grinding parameters according to the characteristics of the feed material and desired product specifications.
• Integration with Digital Twins: The use of digital twins—virtual models of physical systems—can revolutionize the cement industry. By creating digital replicas of processes, cement plants can simulate and optimize changes before implementing them, leading to more efficient and sustainable production processes.
Conclusion
Looking to the future, the cement industry is exploring new methods and technologies to further increase grinding efficiency. Research into alternative grinding technologies such as ultrasonic grinding and the use of nano materials promises the next generation of cement production. Additionally, the industry is increasingly focusing on reducing the carbon footprint of cement production, with grinding processes playing a significant role in achieving this goal. As the cement industry continues to evolve, the grinding process will remain at the forefront of technological innovations, guiding improvements in efficiency, sustainability, and product quality.
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