Vertical Roller Mills for Finish Grinding

With the continual increasing demand for portland cement and constant pressure for reduced energy consumption, producers are exploring a wide variety of cost-saving manufacturing options. One option is vertical roller mill technology for finish grinding.

Traditionally, plants used ball mills to grind clinker and gypsum into cement. The result: the majority (60%) of finish grinding in the world is still performed using the ubiquitous ball mill. Ball mills are cylindrical steel shells with steel liners. These rotating drums contain grinding media that tumble inside the cylinder. The grinding balls cascade and tumble onto the clinker and gypsum to produce cement. Almost all ball mills use a form of closed circuit grinding that returns material that is too coarse back to the ball mill inlet while material fine enough to meet product requirements is collected. The separator or classifier determines which particles will be returned and which particles are sufficiently fine. With an effort to increase production, ball mill physical size has increased almost to the physical limitation dictated by the gas velocities and accompanying pressures necessary for the process. Ball mills may not be the most efficient means of size reduction but their reputation for product consistency and their simplicity of operation have made them an historic plant favorite.

Since the 1980’s, cement plants are increasingly looking to vertical roller mill technology for their finish grinding needs. Vertical roller mills present a compact and efficient grinding method. Clinker and gypsum is ground on a rotating table that passes under large rollers. Material is forced off the table by centrifugal force, where it is then swept up into an airstream to a classifier immediately above. Just as with a ball mill, material that is too coarse is returned to the table for additional grinding while material that is fine enough is collected as product. The compact design of a vertical roller mill allows it to dry, grind, and classify, all within one piece of equipment and all in a relatively compact space. Vertical roller mill technology allows: (1) power consumption savings; (2) consistent product quality and (3) process simplification.

1. Cost Savings

The single largest energy consumption point in the manufacture of portland cement is in the finish milling process. A reduction in power consumption here makes a dramatic impact on the plant’s overall power consumption. VRM technology makes significant power reductions possible by more efficient grinding.

2. Quality

Vertical roller mills create a particle size distribution with a sharper cut in comparison to cement milled in a traditional ball mill. That is, the particle size distribution is in a narrower band. This sharper cut means potentially less variability in product consistency and, hence, more predictable results in product performance.

3. Process Simplification

By changing various vertical roller mill operating parameters, significant adjustments in the particle size distribution, retention time, and fineness of the finished cement can be achieved. This can help with plant operations as production is switched between different cement types.

In light of these issues, vertical roller mills are becoming a popular choice for both existing ball mill conversion and new mill construction. From 2000-2002, 56% of the mill orders (comprising both raw mills and finish mills) came from vertical mills – compared to 32% for ball mills. Clearly vertical roller mills are an increasingly attractive option for improving production, lowering energy costs, and maintaining product quality.

References

Bhatty, J.I., Miller F.M., and Kosmatka, S.H., Innovations in Portland Cement Manufacturing, SP400, Portland Cement Association, Skokie, IL, 2004, 1370 pages.

Harder, J., “Advanced Grinding in the Cement Industry”, ZKG International, BertelsmannSpringer, Güterslosh, Germany, Vol. 92, No. 3, 2003, pp31-37.

Oesch, Chris and Jurko, Blaz, “Finish Grinding with Vertical Roller Mills – Operating Data”, IEEE-IAS/PCA 2002 Cement Industry Technical Conference Record, IEEE-02-15, Portland Cement Association, Skokie, IL, 2002, pp 187-192.

Roy, Gary, “Increasing Cement Grinding Capacity with Vertical Roller Mill Technology”, IEEE-IAS/PCA 2002 Cement Industry Technical Conference Record, IEEE-02-17, Portland Cement Association, Skokie, IL, 2002, pp 205-211.