The OK™ raw mill rolls into production

August 28, 2017 John Terembula

The latest chapter in the OK™ mill legacy opens: a solution for raw grinding 

The world’s largest raw mill in operation at Indarung VI in Indonesia.

The OK mill has long served the cement industry as an energy-efficient and reliable solution for cement grinding. Since the 1980s, the robust design has evolved to include the best-available technology and incorporate the knowledge gained from over 150 installations. This has helped it continue to exceed the expectations of the cement industry: lowest power consumption, highest availability, top product quality, and flexibility to grind the most diverse mix of materials. Now, the same proven technology and modular design approach of the standard OK cement mill is available for raw grinding applications. In early 2017, an OK 54-6 raw mill, with 8,700 kilowatts of installed power, successfully began operation.  

A common platform
Given the OK mill’s inherent stability and proven ability to grind wet components, FLSmidth is well-positioned to offer one common mill design for both cement and raw grinding. This has been achieved in the OK raw mill recently released by FLSmidth. 

One of the key advantages is parts commonality, which is achieved by a modular design that uses the same roller and swing lever assemblies, grinding tables and liners, hydraulic system, and drive systems. This allows OK mill owners to benefit from a high degree of spare part flexibility and lower volumes of inventories. Similarly, because most operation and maintenance procedures and tools are identical, OPEX costs as well as staffing and training resources are reduced. Furthermore, common parts are not only shared between applications, but also across different mill sizes, providing much greater flexibility in the use of standard component combinations. 

​FLSmidth’s modular design uses the same common OK mill parts for raw and cement applications and across different mill sizes.

Ever-increasing capacity
The market trend for large-capacity plants demands larger and larger mills. Carsten Schoessow, Vice President of Global Product & Upgrades Sales at FLSmidth, explains some of the reasons behind this trend:

“Acknowledging regional differences, there are a number of contributing factors to the demand for larger grinding mills. First, there is greater general acceptance of the economic benefits of larger single mill installations. Second, VRM development over several decades has shown that VRMs are mechanically reliable. And third, there are now many VRM references being used within all cement types, such that the global industry knows they meet or exceed quality requirements.” - Carsten Schoessow, Vice President - Global Products & Upgrades, Sales at FLSmidth

New advances in manufacturing combined with decades of design experience make it possible to produce reliable mills of ever-larger size and capacity. Increasing the size of the mills not only takes advantage of economies of scale, but also reduces the CAPEX investment or total installed costs, while also increasing total plant productivity. FLSmidth had this firmly in mind in the development of the new OK raw mill, and currently, the largest VRM for raw grinding in the world is the FLSmidth OK 54-6 raw mill at Semen Padang, Indarung VI in Indonesia.

Operational excellence
Attention to detail in designing the critical grinding components for the specific application improves operating stability and reliability, contributing to 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. This minimises the risk of lost production due to unplanned stoppages. 

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.” 

Since its introduction, the OK mill has proven to be the most efficient cement vertical roller mill (VRM) available, with the highest reliability and ease of operation. For example, many installations have achieved guaranteed performance two weeks or less after the start of commissioning. 

The OK™ mill can be operated with a number of rollers out of service, minimising the risk of lost production.

Recent innovations
The latest developments regarding the mechanical design include the use of segmented roller tires.  When the roller grinding surfaces are worn, the tire segments can either be replaced or they can be re-welded, without removing the rollers from the mill. The swing-out feature allows rollers to easily be removed from the mill, making it possible to perform maintenance work externally.

A recent innovation to replace the full conventional steel stand with a concrete stand results in cost savings and easier installation. With less steel, the material costs are reduced and unnecessary transportation of heavy parts is eliminated. Plant-specific requirements can be accommodated through the inherent flexibility of having concrete stands. Furthermore, concrete mill stands allow greater standardisation of the mechanical parts of the mill and a more standardised erection procedure.  

A bright future 
With an established reputation as the preferred mill for grinding of both raw materials and cement, VRMs have been designed and supplied to the global cement industry by FLSmidth for several decades. The FLSmidth OK raw mill continues that tradition. Cement plant owners gain new opportunities to profit from one common mill, benefitting from parts commonality and consistent maintenance practices. Furthermore, a complete size range, including the largest VRM available for raw and cement grinding, enables plant owners to meet all possible capacity requirements in a single mill – even for the hardest materials. The VRMs’ high availability of scheduled operating time implies that large mills can be installed with confidence, minimising resources and delivering greater productivity.

*This article was first published in International Cement Review, July 2017

John Terembula

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