1 Design features of the MVR mill
The main components of the MVR vertical roller mill are the four or six grinding rollers with a cylindrical wear part geometry, the flat grinding table, the gas-guiding housing with nozzle ring and classifier and the drive, which can optionally consist of the MultiDrive® or of a conventional drive system with planetary gear unit. All the machine components that are relevant to gas flow technology, such as the hot gas channel, nozzle ring and SLS high-efficiency classifier with the material feed are of the time-proven MPS design. Fig. 1 is a 3-D section of an MVR vertical roller mill. The designation of the mill describes the outer diameter of the grinding table, the type of material to be ground and the number of grinding rollers.
A roller module consists of the roller with cylindrical roller tyre, the roller axle, the roller arm, the bearing stand and the hydraulic force transmission unit. This special roller suspension design combined with the flat grinding table geometry ensures that the grinding gap between the rollers and the table remains parallel, ensuring uniform compression of the grinding bed and resultant smooth running (Fig. 2). Each pair of adjacent roller modules rests on a twin support that connects them to the foundation. This design provides more space between the supports, facilitating access and positively influencing the plant layout with regard to the arrangement of the hot gas ducts, the external material circulation and the accommodation of ancillary equipment.
Using the same hydraulic system that applies the grinding force during operation, the roller modules can be individually swung out of the mill. If the drive comprises a planetary gear unit, production can only be continued at reduced capacity after two opposing rollers have been lifted or swung out. The main advantage of the MultiDrive® with its several drive modules is that it enables operation to continue even if only one roller is lifted or swung out .
The trend towards higher throughputs, i.e. larger mills, is inevitably linked with the demand for larger gear units coupled with falling mill rotational speeds. The mill performance is thus determined more by the size of the torque. To avoid problems caused by using larger and larger planetary gear units, Gebr. Pfeiffer SE developed the MultiDrive® concept in co-operation with Flender/Siemens.
In the MultiDrive®, two to six identical drive modules, each with installed drive powers of up to approx. 2000 kW, drive the grinding table via a girth gear located under the grinding table. Each drive module consists of an electric motor, a coupling and a combined spur and bevel gear unit mounted on a base frame to form a transport unit. It is also equipped with a frequency converter, a transformer and an oil supply station.
The load distribution to the individual electric motors is performed by a primary control system through frequency converters associated with each drive module. This design concept enables the adjustment of the grinding table speed for the purpose of process optimization. The grinding forces from the bed of material are transmitted into the foundation via a conventional plain bearing assembly. The combined spur and bevel gear units arranged around the periphery of the girth gear are not subjected to any grinding force. If one combined spur and bevel gear unit fails, it can be taken out of meshing and the MVR mill can then be operated at reduced throughput capacity.
Benefitting from the principle of active redundancy, the MVR/MPS mill equipped with a MultiDrive® is able to maintain operation even if problems occur either with the grinding rollers or with the drive. Even though the drive power range extends from 2000 kW to 12 000 kW only five roller modules and three different drive units are used for the entire mill type series .
2 Operating experience
2.1 Test mill in the GPSE test plant facility
The test plant facility of Gebr. Pfeiffer SE is equipped with two pilot scale MPS mills as well as an MVR 400 for the performance of grinding tests. The MVR 400 was used for an extensive test series in order to establish basic mill design data for grinding cement raw materials, cement clinker and granulated blast furnace slag. The realistic pilot plant enables the determination of raw-material-related and project-relevant data such as specific power consumption, gas volume requirements and specific wear rate.
2.2 Hauri, Germany
Hauri KG, a mineral products supplier in South-West Germany, has been operating MPS mills since the late 1960s for the grinding of phonolite rock (volcanic origin) and for manufacturing various products including binding agents containing cement. To supplement the three already installed mills of type MPS 125 A (two systems) and MPS 200 BC, the company installed an MVR 1800 C-4 in 2007. The new MVR, used for manufacturing the same products as the MPS mills, achieves the same excellent operating characteristics. The ground products include, for example, an HT 35 binding agent with 60 % clinker at 5300 cm²/g acc. to Blaine and tempered phonolite with a mass-related surface of 6000 cm2/g acc. to Blaine and higher which the mill produces at a throughput rate of 22.5 t/h and a specific power requirement of 15.3 kWh/t. Fig. 3 and Fig. 4 show the mill during erection.
2.3 Lukavac, Bosnia-Herzegovina
An MVR 3750 R-3 vertical roller mill has been operating in the Lukavac cement works in Bosnia- Herzegovina since late 2008 grinding cement raw materials (Fig. 5). With an installed main drive power of 1600 kW, the throughput of this mill is 160 t/h at a screen residue of 12 % on 0.090 mm. The specific power requirement of the mill is 8.7 kWh/t and for the overall system (mill, classifier, fan) it is only 16.4 kWh/t. After an operating time meanwhile in excess of 15 000 hours, the specific wear is approx. 1.3 g/t. This MVR mill was erected in the short time of only 75 days.
2.4 Holcim France
Since 2009, an MPS 4750 BC mill equipped with a MultiDrive® has been in service at a Holcim grinding plant in France, producing CEM I and ground granulated blast-furnace slag. The MultiDrive® comprises three modules with a power of 1450 kW each. The grinding plant produces 120 t/h of CEM I 52.5 or 130 t/h of granulated blast furnace slag with a mass-related surface of 5000 cm²/g acc. to Blaine.
2.5 Balaji, India
The Jaypee Group (Jaiprakash Associates Ltd) is the third largest cement manufacturer in India. The company‘s various plants are already equipped with six raw mills of types MPS 4000 B, MPS 4750 B and MPS 5000 B, as well as six coal mills of type MPS 3070 BK and two MPS mills for cement grinding (MPS 4750 BC). For cement grinding at its Balaji plant, the client decided for the new MVR mill solution with MultiDrive®. At this plant there are two MPS 5000 B mills for raw material grinding and two mills of size MPS 3070 BK for coal grinding.
The ordered MVR 5600 C-4 vertical roller mill has a grinding table diameter of 5.6 m and roller diameters of 2.83 m. It is driven via a MultiDrive® with four modules with drive powers of 1650 kW each. The guaranteed output rate for Portland fly ash cement PPC with 30 % fly ash content (corresponding to CEM II/B-V acc. to EN 197-1) is 320 t/h at a fineness of 3500 cm²/g acc. to Blaine. The guaranteed specific power requirement of the mill is 18.7 kWh/t. A second product manufactured with the MVR is Portland Cement OPC (CEM I acc. to EN 197-1) at 3000 cm²/g acc. to Blaine.
Fig. 6 and Fig. 7 show the erection of the mill housing between the preassembled supports. The swung-in rollers are shown in the top view in Fig. 8. In the subsequent erection phases, the lower and upper parts of the classifier are mounted on the lower section of the mill (Fig. 9).
In May 2012, the MVR was put into service (Fig. 10). The mill has meanwhile been in operation for more than 3000 hours, achieving an output rate of up to 360 t/h for PPC at a mass-specific surface of 3600 cm²/g (Blaine). The specific power consumption at the main drive of the mill is better than the guaranteed value. For OPC cement the mill has achieved an output rate of 300 t/h at 3000 cm²/g (Blaine) and a specific power consumption at the main drive of 15.2 kWh/t.
Table 1 shows the strength development of the CEM I produced in the MVR compared to other plants that also manufacture CEM I at low Blaine values.
3 Projects in hand
Holcim S.A. (Brazil) has awarded the Spanish company Cemengal a turnkey project for the construction of a grinding plant at their Barroso works. For this plant, Gebr. Pfeiffer SE is supplying an MVR 6700 C-6 with output rates of up to 450 t/h. This will be the largest cement mill in the world. Holcim decided to contract Cemengal/GPSE because the MVR 6700 C-6 represents a single mill solution that can achieve the lowest specific purchase costs per tonne of cement. Installation of a single mill avoids the expenditure for a second feed bin and two sets of peripheral systems. Holcim expects the operational reliability of the MultiDrive® concept to be higher than that of a single large planetary gear unit. The MultiDrive® will be equipped with six modules of 1920 kW each.
The mill is to be used for manufacturing five cement types with different portions of granulated blast-furnace slag at output rates of up to 450 t/h for a CEM II/B-S acc. to EN 197 at a mass-related surface of 4300 cm²/g acc. to Blaine. The hot gases required for drying purposes will be taken from the clinker cooler of the kiln line, if necessary backed-up by a hot gas generator. The installation schedule stipulates a period of 21 months from the contract signing to production of the first cement. The various component groups have been delivered, with the core parts being manufactured in European workshops. The first items were already shipped at the end of June 2012.
In Port Kembla, Australia, Cement Australia (a JV between Holcim and HeidelbergCement) is currently building a grinding terminal for the production of ground granulated blast-furnace slag cement and CEM I. Fig. 11 presents a visualization of the planned facility. The planned annual production is 1.1 million tonnes. To achieve this, Cement Australia has ordered an MVR 6000 C-6 to be driven by a MultiDrive® equipped with three modules of 1840 kW each. In this case the decision for the MVR with MultiDrive® was taken on the basis of the expected shorter stoppage times due to the redundancy concept, and the lower risk of gear unit failure. A further reason was the cost saving spare parts inventory for the gear unit modules in contrast to a conventional planetary gear unit. Like the plant in Barroso/Brazil, this project is a partnership undertaking of Cemengal and GPSE. In order to minimize work on the construction site, some plant sections, e.g. the process filter and the finished product silos are already being preassembled in Spain. Each of the silos will be delivered to the plant site in only three sections. Plant commissioning is scheduled for the 1st quarter of 2014.
4 Summary and prospects
The MVR mill and the MultiDrive® concept enable the achievement of throughput rates of up to 12 000 t per day by a single mill solution. The active redundancy of the roller modules and of the MultiDrive® modules will assure outstanding availability of the mill. The described MVR vertical roller mill and MultiDrive® systems that have already achieved total operating times in excess of 35 000 hours show excellent operating behaviour and have confirmed all the design assumptions and maintenance concepts.