SBM Vertical Mill

Vertical Mill Quality Control

In a cement vertical mill grinding is performed in closed circuit and with an integral high efficiency separator. This arrangement will give a good steep PSD. Experience has shown that the overall product particle size distribution is consistent with that obtained from a ball mill grinding plant with a modern high efficiency separator. During the initial vertical mill optimization period the mill is fine-tuned to match its product to the existing ball mills. This is achieved by making adjustments to operational parameters such as:

• Separator rotor speed
• Air flow rate
• Grinding pressure
• Dam ring height

Because the vertical mill has significantly higher grinding efficiency than a ball mill there is much less heat input from the grinding process. This is evident in the almost 50% less installed power, but is further taken into account with a smaller percentage of the energy being absorbed by the material. Compared to ball mills where 75% of installed power may be absorbed a good vertical mill will take only 50% of the installed motor power as heat. The end result is that the product will not be heated up as much as in a ball mill. This means that a lower degree of gypsum dehydration could occur.

A lesser degree of gypsum dehydration is not problematic considering two conditions; the inability to adequately control temperatures in ball mills creates an environment where operation is at the extreme of the gypsum dehydration. Additionally, less dehydration is not an issue if the gypsum is sufficiently reactive to control the setting reactions with a lower degree of dehydration as is normally the case.

If in special cases this is not the case different options are available to cope with the problem:

• Addition of more gypsum (within the SO3 limit)
• Increased dehydration of gypsum by adding more heat to the mill system
• Addition of a more reactive form of gypsum

Prehydration is not typically problematic in a vertical mill as it is in ball mill systems where higher temperatures and internal water-cooling systems are common. However, if cement is produced at a relatively high temperature and still has a lot of gypsum that is not dehydrated one must be aware of the potential problem of gypsum dehydration coupled with clinker prehydration that can take place during storage in the cement silos. If a problem of this kind is present it can be coped with it by one (or more) of the following options:

• Ensuring that the cement is cooled to a lower temperature before going into the silo
• Provoking a higher gypsum dehydration level in the mill
• Replacing part of the gypsum with natural anhydrite

The actual vertical mill results achieved in Phoenix are built upon the theoretical rules for quality control presented here. During the initial period of operation, extensive quality data was recorded and analyzed. The results from comparisons between the existing ball mills at the Phoenix plant and the new vertical mill are presented in an abbreviated form. The data summarized below are based on significant results to comply with all plant requirements, but more importantly to conclusively satisfy all market requirements.