Crushing and grinding are collectively referred to as pulverization. The industry is used to calling the process of processing large pieces of material into small pieces of material crushing. The process of turning coarse particles into fine powder is called grinding. The cement grinding process is divided into two parts: raw material preparation process and cement production process, referred to as raw material grinding and cement grinding.
Limestone, clay, iron powder, etc. are ground together and called raw materials. Clinker, gypsum, and mixed materials are ground together and called cement.
Purpose of Material Crushing in Cement Production
1) After the material is crushed, the surface area (specific surface) per unit mass of the material increases. Therefore, the effect of physical action and the speed of chemical reaction can be improved.
(2) Several different materials can be easily mixed evenly in powder form.
(3) Powdered materials are also convenient for drying, transportation and storage. They also create conditions for calcining clinker and making cement to ensure the qualified rate of cement leaving the factory.
Reasonable Control of Raw Material Fineness
When grinding fineness is below 10% on the 0.08 mm sieve, power consumption increases sharply. As residue decreases, grinding output drops. This reduces overall efficiency. To balance energy use and output, fineness is usually kept around 10% on the 0.08 mm sieve.
The 0.20 mm sieve residue is controlled below 1.0%.
When using large ball mills for production, the product particle size is more uniform and there are fewer coarse particles. Under the premise of easy burnability, the 0.08mm square hole sieve residue can be relaxed to 12-16%, but the 0.20mm square hole sieve residue should be controlled to be less than 1.5%.
Grinding Balls and Their Gradation
During the cement grinding process, materials need impact on one hand and grinding on the other. The use of grinding balls of different specifications can also reduce the void ratio between each other, so that they have more contact opportunities with the materials, which is conducive to improving energy utilization. Under the condition of a certain grinding body loading, the total surface area of small steel balls is larger than that of large steel balls. To break up large pieces of material, the steel balls must have greater energy. Therefore, the size of the steel balls (segments) should be larger. If the material needs to be ground finer, smaller steel balls (segments) should be selected. Therefore, in the grinding operation, the grinding body must be correctly selected and reasonable grading must be carried out.
Grinding System
The grinding system in which the material passes through the ball mill once and becomes the product is called an open circuit system . After the material is ball-milled, it must be sorted by a classifying device, and qualified fine powder is used as the finished product. The grinding system in which unqualified coarse powder is returned to the ball mill for re-grinding is called a closed circuit system .
The classifier is the grading equipment of the closed circuit grinding system. It sorts the milled material in time, and qualified fine powder is used as the finished product, and unqualified coarse powder is returned to the mill for re-grinding. It can adjust the particle composition of the finished product to meet the process requirements and ensure the quality of the grinding product. The performance of the classifier is one of the main factors affecting the production and quality of the closed circuit grinding system.
Factors Affecting Ball Mill Production and Quality
- Particle size of the material entering the mill.
- Grindability of the material.
- Moisture content of the material entering the mill.
- Temperature of the material entering the mill.
- Fineness requirements of the material leaving the mill.
- Grinding process
- Adding grinding aids.
Effect of Cement Particle Size on Cement Properties
The size of cement particles directly affects its hydration rate and strength development. Fine particles (such as cement with a larger specific surface area) have a larger contact area with water and a faster early hydration reaction. It helps to improve early strength, but may increase water demand, reduce fluidity, and may cause shrinkage cracks. Coarse particles hydrate more slowly, and the later strength growth is stable, but the early strength is low. The ideal cement particle distribution needs to take into account the reasonable combination of fine particles (3~30μm) and coarse particles to optimize the setting time, mechanical properties and durability. In addition, too fine particles (<3μm) are easy to agglomerate, which is not conducive to strength development. Therefore, modern cement production controls the particle grading by adjusting the grinding process to balance workability, strength and shrinkage performance.
Epic Powder
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