subject: Cement Grinding -- Vertical Roller Mills Versus Ball Mills [print this page] Around 110 years ago a Danish engineer, MAround 110 years ago a Danish engineer, M. Davidsen, patented a pioneering
invention in France which involved a tube mill with a charge of steel balls or flint
pebbles for fine grinding of sand or cement. F.L.Smidth acquired the rights to his
patent and started selling an improved version of this mill all over the world. For the cement industry the ball mill was really an epoch-making breakthrough as for almost 80 years it was the predominant mill for grinding of raw materials and coal, and still today is the most used mill for cement grinding.
Over the last three decades the vertical roller mill has become the preferred mill for grinding of raw materials. The grinding efficiency of the vertical roller mill combined with an ability to dry, grind and classify within a single unit gives the vertical roller mill a decided advantage over a ball mill system. However, despite these benefits, applications of the vertical roller mill for cement grinding are less prevalent.
The two-compartment ball mill operating in a closed circuit with a high efficiency
separator is thus still the most preferred arrangement for new cement grinding
installations although the vertical roller mill now has emerged as a viable alternative to the ball mill system and has increased its share of the market for cement mills over the last decade. There are a number of explanations to this situation, related to issues like cost and ease of operation, cost of installation, cost and ease of maintenance, product quality, versatility, etc.
The following presents a comparison of the two mill systems with reference to these issues.
Grinding process
The grinding process in ball mills and vertical roller mills differ fundamentally. In a ball mill the comminution takes place by impact and attrition. The comminution in the vertical roller mill takes place by exposing a bed of material to a pressure sufficiently high to cause fracture of the individual particles in the bed, although the majority of the particles in the bed are considerably smaller than the thickness of the bed.
It is thus necessary that a stable and consistent grinding bed is formed between the rollers and the table of the vertical roller mill, able to sustain such a pressure without the material being squeezed away from the pressure zone.
A stable grinding bed is usually easily obtained in raw material grinding in a vertical roller mill with a high efficiency separator. However, in cement grinding it becomes more difficult to form a stable grinding bed as
cement is ground much finer than raw meal
the feed to a cement mill is often completely dry and is significantly more difficult to grind than raw materials
requirements to the particle size distribution of the finished product are much
more strict when grinding cement than when grinding raw materials
These differences between cement grinding and grinding of raw materials made it a serious challenge to obtain a good performance of a vertical roller mill in cement grinding. Today, however, the OK mill has become an excellent example of a mill design that has overcome the difficult grinding conditions associated with finish grinding of cement clinker and related products.
The OK mill was developed by Onoda Cement Co., Onoda Engineering and
Consulting Co. and Kobe Steel in the early nineteen eighties. In 1993 F.L.SmidthFuller Engineering obtained a license to manufacture and market this mill.
