Since the 1980s, the OK mill has served the cement industry as an energy efficient and reliable solution for cement grinding.  The mill’s long-standing success, with over 150 installations worldwide, can be attributed to a core robust design concept which has continuously evolved to include the best available technology and incorporate the knowledge gained through experience. As a result of this continued investment, the mill has retained its leading position to meet the current needs of the market; low power consumption, high availability, product quality, and flexibility to grind diverse materials.

The first OK™ raw mill

The first OK™ raw mill recently began operation in Indonesia. The OK 54-6 raw mill with 8,700 kW installed power, follows FLSmidth’s modular mill design where rollers from the OK 39-4 mill are applied to a larger diameter table.  

Lower power consumption

Compared with other cement grinding VRMs, the OK mill consistently operates with lower airflow and the lowest power consumption. As a result of the patented cement grinding profile and an integral separator with industry-leading high efficiency, the mill consistently uses 15 to 20 percent (3 to 5 kWh/MT) less power than other cement VRMs. 

Design advantages

The design of the FLSmidth OK mill presents several clear advantages. It allows the combination of standard VRM features such as drying, grinding and separation process in one unit, thus simplifying the plant layout. It also incorporates unique design elements in the roller and table profile that improve operating stability and reliability, giving a typical availability of 90 to 95 percent of scheduled operating time.

Due to its inherent flexibility, the OK mill can be operated with half of its rollers out of service and still achieve 60 to 70 percent of the nominal output, minimising risk of lost production due to unplanned stoppages and guaranteeing long-term availability.No text in field]
Ok raw mill

Improved control over particle size distribution

Product quality is a function of cement particle size distribution (PSD) and the dehydration of the gypsum within the cement. In the OK mill, parameters, such as mill air flow, separator speed and grinding pressure, can be easily adjusted during operation to control or alter the PSD curve to a profile (steeper or flatter) that achieves the desired quality standards. When needed the PSD curve can match that from an existing ball mill.



Handle multiple materials with ease

As the operating parameters can be adjusted instantly and the retention time of the system is very short, the switching between different types of products can be performed with almost immediate effect. As illustrated on the graph, most OK mills switch between products such as Ordinary Portland cement to slag blended cement or from a lower to a higher Blaine. Furthermore, the temperature profile of the system can be controlled to adjust the level of dehydration of gypsum.


Particle size distribution adjustment on OK cement mill

The OK mill has proven to be effective for grinding blended cements with one or more wet components, not only because of its highly effective drying performance, but also due to its ability to maintain a stable grinding material bed. As an example, the OK mill has been used to grind a wide range of materials from 100 percent slag with feed containing more than 20 percent moisture to limestone, pozzolan and fly ash. Because the mill can utilise much higher quantities of hot gases for drying than both a ball mill or a roll press with ball mill circuits, it means that the percent substitutions of slag, and other wet materials is not limited by the system’s drying capacity.  


OK mill feed table
The composition of cement products produced using the OK™ cement mill.

Grinding products above 5500 Blaine has proven its versatility as well as its capability of consistent, stable operation throughout a range of cement, blended cement, and slag cement products.


The legacy continues

Today’s market forces of increased imports and large capacity plants demand larger and larger mills, and FLSmidth has again met the need. As reported in the article regarding KAR Cement, the first six roller OK mill has reached its production guarantees within four weeks of commissioning. The OK 54-6 cement mill’s time as the “world’s largest OK mill” will be fleeting as an even larger mill, an OK 62-6 cement mill, has been sold to Shah Cement.


The installation of the first OK mill for raw grinding is the next step in the evolution of the FLSmidth OK™ mill. The mill’s inherent stability and proven ability to grind wet components serves as strong foundation for a raw grinding application. The common design platform uses the same parts including the roller assembly and swing lever system, grinding tables and liners, hydraulic system as well as the drive system. Consequently, owners can benefit from both parts commonality as well as consistent maintenance procedures. 


Continued development of the FLSmidth OK™ mill 

Over almost 40 years on the market, FLSmidth has continuously updated and improved the OK™ mill design since it was first introduced to the market in the early 1990’s. In addition to featuring six rollers for significant increases in capacity, several other design improvements have been implemented.

Søren Holm, Vice President in FLSmidth Global Cement Technology, comments: “In updating the OK mill, we have combined our broad experience in optimising mechanical design with customer input, our own observations and data gained from operating a wide range of mills for cement and raw material grinding.”

The new mill comprises an adapted mechanical design to reflect actual operating and maintenance practices and recent developments within metallurgy. The simple layout and fewer machines in the mill circuit ensure high run-factor, reduced civil construction costs and low maintenance costs.

The separator design has been significantly improved, allowing producers to achieve better product quality. Additionally, internal components have been adapted to improve airflow and reduce wear from jet abrasion, and features have been added to help keep the liners in place to provide greater long-term reliability in table liner segments.


John Terembula, Product Line Manager, Grinding



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