The first LM63.3+3
by Dr Daniel Strohmeyer, Loesche GmbH, Germany
In this article, that was presented at Cemtech Middle East 2010 in Dubia, Dr Strohmeyer outlines the basic design features of Vertical Roller Mills(VRM) and their role in the cement plant, before highlighting Loesche’s largest mill the LM63.3+3 as well as a recent project to install the first mill of this type in Nallalingayapalli, India.
There are three potential positions where a VRM can be utilised within a cement plant: raw material grinding, finish cement milling and – in the case of coal fired kilns – coal grinding. The general working principles of VRMs begin with the mill taking material in through the rotary feeder, moving down the feed shoot, hitting the revolving grinding table in the middle and, due to the acting centrifugal forces, the material moves outwards underneath the rollers where grinding takes place. The material then moves over the dam ring and is moved upwards with the gas flow to the classifier. It is at the classifier where material is deemed either to have reached the required product fineness or not. If it is rejected, it is sent back through the grid funnel to the grinding table where it undergoes another grinding cycle.
The VRM raw mill is slightly different from the finish VRM mill design, because on the raw mill grinding table rollers always work in pairs. The master roller carries out the main grinding, while support is provided by the slave roller. The slave roller is needed for precompaction of the grinding bed. In the development, when grinding cement with a VRM, engineers found that it was difficult to keep the mill stable. The reason was due to the higher fineness of material being worked in the finish mill compared to the coarser material being ground in the raw mill.
The cement bed is more fine grained so there was a higher tendency for air to be trapped in the finish mill inside the grinding vent and this could affect the stability of the material bed. To reduce this effect Loesche designed the system to have a pre-running slave roller which lies on the bed with its own weight to carry out a pre-compaction and deaeration of the grinding bed (see Figure 2). Internal friction is required to grind material as without the friction the material acts more like a fluid and moves away from the master grinding roll. With this type of grinding system which Loesche introduced to the market in 1994, the company is able to grind OPC up to a fineness of 5000Blaine. The main advantage of grinding cement with a VRM is the specific energy consumption. This specific energy consumption is dependent on the Blaine being achieved (See Figure 3). The higher the Blaine, the more pronounced is the effect of saving energy with a VRM compared to utilising a ball mill for the same purpose.
The smaller Loesche VRMs begin with the 35.2+2 that grind at 50tph and go up to the LM63.3+3 which has a range of 300-350tph depending on the grindability of the material being ground and the Blaine (see Figure 4). A typical grinding system layout is shown in Figure 5, with the mill at the centre, feeding system, reject system and bucket elevator and a bag filter, where the product is finally recovered from. Such a grinding system may also include a hot gas generator (HGG), but if we are talking about OPC with hot clinker this is not usually necessary. A HGG is more often required for a plant producing PPC with pozzolan and high moisture content, especially if hot gases cannot be supplied off of the cooler. There are also certain advantages in having a HGG if the plant owners wants to use the mill independently of the kiln.
The Nallalingayapalli LM63.3+3 mill project
with regards to the grinding requirements were 370-375tph raw mill, 45tph coal grinding.
Initially, Raghuram selected two cement mills each with 175tph OPC. Loesche then looked at the different materials to carry out a proper sizing of the machine. This is essential as all materials have different physical properties and the grindability would be unknown. Consequently, a grinding test was performed with Loeche’s laboratory mill essentially to determine the grindability factor, the specific energy consumption as well as the wear factor which gives an indication of the life time of the wear parts inside the mill, such as the tyres and grinding tread on the table. The specific energy consumption is a general parameter that stands for the grindability of a material and is used as an indicator of the grindability factor worldwide. The ‘MFLF’ is a Loesche standard. The company has a standard material with its standard grindability that is kept in the company’s laboratory for comparisons against the grindability of all other raw materials that are tested in the lab. This helps Loesche to find the correct size of mill for the client.
As the plant did not have any clinker available, Loesche had to use clinker from a neighbouring plant to estimate the clinker grindability factor. The assumption for the grindability for the clinker was approximat ely 15kWh/t making this material of comparitively ‘good’ grindability. Based on the assumptions and grinding tests, Loesche selected the mills and asked Raghuram Cement to try finish grinding with just one large
cement mill as the capacity could be achieved with a single mill. This had the advantage of less investment and reuced maintenance later on.
On the raw material side, Loesche chose its 56.4 VRM and for the coal grinding its 26.3D mill.
The nominal capacity of the mill is 280tph OPC based on a certain Blaine, while the actual capacity that can be achieved is much higher. The gear box is a Flender unit with installed power of 6700kW and the dynamic classifier has a drive of 710kW. Finally, the fan motor power is 3500kW and the table diameter is 6.3m.
Raghuram Cement required four products to be produced with the mill. The VRM has to be flexible to cope withthis product variation. The products included:
• OPC 3000Blaine at 360tph (25.40kWh/t) – 95 per cent clinker, five per cent gypsum
• PPC 3400Blaine at 355tph (26.95 kWh/t) – 65 per cent clinker, five per cent gypsum and 30 per cent dry fly ash
• Slag cement 3600Blaine at 330tph (33.20kWh/t) – clinker 50 per cent, 45 per cent slag and five per cent gypsum
• Slag grinding 4000Blaine at 300tph (38.39=0kWh/t) – 100 per cent slag.
Loesche was responsible for the entire engineering of the plant and its scope of supply included the mill itself, the fan and other items like metal detector/separators on the conveyors, while the bag filter was provided by another supplier.
After completing the flow sheet, engineering and general layout of the plant, Loesche started building the mill section with the stand, pedestals and the bridges in between. At this stage the mill is already 6m off the ground (see Figure 6). The gear box base plate is glued to the foundations with shotfast to give the best connection between the gear box base plate and the ground. The grinding table weighs about 100t and after it is fitted pre-assembled mill housing was installed (see Figure 7). At this stage the mill housing is about 10.5m off the ground.
Next followed the rollers and the classifier and classifier rotor. The hot gas ducts were then fitted to the mill and the elevation on top at this stage is about 23m high (see Figure 8). The master rollers have a diameter of about 2.7m.
The feeding system is special in that there are two rotary valves and an additional two feeding systems for dry fly ash which is fed separately from the clinker (see Figure 9). The fly ash is fed in the region of the classifier as it has a fineness of 2500-2800Blaine. Consequently, some of the fly ash already has the product fineness so it makes sense to feed it close to the classifier where it can leave the mill easily without entering the grinding cycle.
The Nallalingayapalli project was started by Loesche in January 2007. The first delivery was in October 2008 and erection began in February 2009. Commissioning was completed in August 2009 and the first feed was in September 2009. Loesche also reached the 360tph OPC target in September 2009. Up until February 2010, two different products have been ground in the mill. The plant could not mill the slag cement or the slag itself as the slag has not been available. The specific power consumption was obtained almost exactly (see Table 1).
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Published by Loesche GmbH on May, 2010