After 25 years of mainly producing slag cement, the vertical roller mill MPS 3750 C near Hannover and supplied by Gebr. Pfeiffer AG was equipped with a modern outer material recirculation circuit and a new nozzle ring. During the acceptance test in 2006 with CEM III A 42.5 containing 40% slag, the production rate was increased by 30% at a fineness of about 5500cm²/g (Blaine). The electric energy consumption of the mill, separator and fan was reduced by 10% at the same time.
Gebr. Pfeiffer AG, a medium-sized family business, has been a supplier to the cement, lime and gypsum industries for over 140 years. The company's product range comprises machines and equipment for the grinding, classification and drying of diverse materials and for slaking quicklime and calcining gypsum, which are sold worldwide.
The company, headquartered in Kaiserslautern, employs 280 staff and maintains manufacturing facilities as well as its own foundry. Gebr. Pfeiffer AG has an annual turnover of around Euro100m. With over 2000 MPS vertical roller mills in use around the world today for grinding cement raw material, coal, clinker, granulated blast furnace slag (GBFS), pozzolana, limestone, clay, gypsum and various other materials, the MPS technology represents the company's major source of revenue.
In the cement industry MPS vertical roller mills with installed powers of over 5000kW are currently in operation or on order. These mills are used to grind cement raw materials and cement or granulated blast furnace slag (Figure 1).
Structure and maintenance of Pfeiffer's MPS vertical roller mill
The structure of the MPS BC series, to which all mills producing cement or BFS powder belong, basically corresponds to that of the MPS vertical roller mills that have been used for cement raw material and coal grinding since the 1960s (Figure 2). The statically determined 3-roller-system, which is pressed against the grinding bed by a pressure frame, external pull rods and the hydraulic system, is a characteristic feature of the mill. The grinding rollers can be lifted with the help of the tensioning cylinders during start-up or for maintenance purposes.
An SLS high-efficiency classifier sits on top of the grinding zone. The supply of fresh material is integrated in such a way that the fresh material and the grits from the classifier mix well and the material drops onto the centre of the grinding table.
The MPS BC model range distinguishes itself from raw mills through its process-related design, which is characterised by the increased grinding force, an adapted, reduced speed grinding table and lower gas velocities.
The mechanical design is tailored to the higher abrasiveness of the material to be ground. The nozzle ring and other parts exposed to increased jet wear in the area of the grinding table are made of Ni-hard IV. The area of the classifier is provided with a ceramic lining, and the classifier rotor is hardfaced. The wear parts of the rollers and the grinding table are normally made of chromium alloy/cast iron and will be hardfaced at a later stage, when the exact wear profile is known. The hardfacing can be carried out inside the mill without the rollers having to be removed from the mill. The rollers are lifted up for this purpose, and, while it is being welded, the grinding plate is turned with the help of a variable-speed maintenance drive, which is integrated in the gear. Then the rollers are lowered again, driven by the grinding table and welded using several weld heads simultaneously. With an MPS 5600 BC for 170t/h slag powder, downtime for the entire hard-facing can be kept as short as approximately 110h, if the work is organised accordingly. Hardfacing at this mill is expected to be necessary every 2000h based on a production rate of 170t/h slag at 4000cm²/g Blaine. MPS mills are in operation that have been hardfaced more than ten times. In total this leads to comparatively low maintenance costs.
Operating data
The first industrial-scale vertical roller mill for slag cement, an MPS 3750 C, went into operation in Hannover 25 years ago1 and is still in operation today (Figure 3). The capacity of this mill ranges from 60t/h slag with 4100cm²/g to 70t/h CEM III A or 45t/h CEM III B with slag content between 50–80% and fineness of 3600cm²/g and 4300cm²/g. The cement quality meets all the requirements of the German market.
In 2006 this mill was equipped with a new nozzle ring and an outer material recirculation system.The principal concept (shown schematically in Figure 4) is similar to modern plants which mainly consist of:
- Equipment for metering, supplying the wet slag to the mill and for metal extraction upstream of the mill;
- The MPS BC vertical roller mill;
- The external material circulation system with a complementary metal extraction system especially aimed at the already dried and liberated pig iron;
- The process gas routing;
- Dedusting and product removal.
While producing slag cement CEM III A 42.5 with 40% slag at a fineness of 5500cm²/g, the throughput could be increased by 30% and the specific power consumption of mill, separator and fan was reduced by more than 10% after the modernisation of the plant.
Today more than 30 MPS vertical roller mills of the BC series for slag, cement and binding agents with capacities from about 20t/h up to 300t/h are used or under contract for slag powder and the production of virtually all cements of various compositions commonly used worldwide. The mills achieve finenesses ranging from 2000cm2/g to 7000cm2/g in the case of binding agents containing cement.
An MPS 5600 BC (Figure 5) with an installed power of 5300kW started production at the end of 2006 in China. It is guaranteed for 120t/h of GBFS at a fineness of 5000cm2/g or 170t/h with a fineness of 4000cm2/g. The same mill size MPS 5600 BC with 5400kW installed power was sold in 2006 to India for 145t/h slag at a fineness of 4200 Blaine. As a comparison, for OPC at a fineness of 3200cm2/g the capacity of an MPS 5600 BC can be expected to reach 275t/h.
References
1. H. Steinberg, R. Hoffmann, 'Production of slag cements in a vertical roller mill', ZKG International, 1998, 51 (3), p101–109.
2. O. Jung, 'MPS vertical roller mills for blended cements', World Cement, 1989, 20 (9).
3. S. A. Reddy, Y. Reichardt, 'Production of slag cement with an MPS vertical roller mill at the cement plant of Sagar Cements in Bayyavaram, India', ZKG International, 2001, 54 (9), p492–497.
4. Y. Reichardt, G. Link, H. Gilabert, 'Cement finish grinding with an MPS vertical roller mill at the San Rafael cement plant in Ecuador', ZKG International 2002, 55 (11), p54–57.
5. Y. Reichardt, 'The use of MPS vertical roller mils in the production of cement and blast furnace slag powder', Cement International, 2005, 2, p64–69.
