Experience in grinding raw materials for clinker production has been utilized to apply Loesche mills for the production of ground granulated blast furnace slag, ordinary portland cement, slag cement and cement with interground additives. 

The newly developed 2+2/3+3 Loesche technology combines proven elements with high reliability in a new set up of different sized rollers and a recently introduced high efficiency cage wheel type classifier, which allows fine grinding at high mass related surface areas without facing extensive vibrations. 

The drying capability of vertical roller mills allows use of moist interground additives such as granulated blast furnace slag, trass, pozzolana, limestone etc. Extensive research on product quality during recent years has shown that cements with interground additives produced in the company's mills easily meet the consumers' requirements. Water demand, strength development and the workability of cement mortar as well as of concrete made from cement produced in the Loesche mill are comparable to or even better than those from conventional ball mill systems. 
 

In 1891, the first patent had been granted world wide for a ball mill. For over 100 years, ball mills have been the standard equipment for the comminution of raw materials and products in cement production. Since 1925, vertical roller mills manufactured by Loesche GmbH have been applied for grinding raw materials, coal and minerals. In the 70s, vertical roller mills became increasingly dominant for grinding raw materials for the cement industry due to their high energy efficiency and excellent drying capability. 

In the late 70s, the cement industry became very conscious of the energy consumption in cement production and the equipment suppliers 

*Based on a paper from the Proceedings of the Association, Istanbul, Turkey, November 1997. 

consequently started to develop new technologies specially for grinding clinker, slag and other admixtures to cement. Today, a wide range of grinding technologies is available on the market. 

As one of the leaders in comminution technology, Loesche GmbH started to develop their raw mills for grinding clinker and slag more than ten years ago. The result of this research work was the development of the 2+2 technology. The first mill of this kind started industrial operation in late 1994, and since then, this grinding system has established a high position on the market based on its excellent performance, high flexibility and good operational availability. 
 

Mills of the 2+2 design (Figure 1) for grinding clinker, slag and interground additives use mainly the proven design principles of a vertical roller mill for grinding of raw materials.  

The main difference between the 2+2 mills and the conventional 4-roller mills for raw material grinding is that two differently sized pairs of rollers are used. The so called s-rollers are used to prepare the material bed on the grinding table. These s-rollers are of a light design and a smaller diameter compared to the large grinding rollers, the m-rollers. By means of hydraulic attachments, the small, light s-rollers are positioned at a certain height above the grinding table, where they slightly consolidate and deaerate the material bed. 
After this preparation, the material is properly nipped under the large and heavy grinding rollers of conventional design. The forces required for comminution underneath the m-rollers are created by means of hydraulic cylinders attached to the rocker arms of the m-rollers. All m-rollers are supported on individual stands, which are relocated on the mill foundation. Furthermore, individual hydropneumatic spring systems for each set of rollers provide a very smooth and almost vibration free operation of the mill. The 2+2 system is available for capacities of up to 170 tph OPC. In the case of higher throughput, the 3+3 system is used, which consists of three s-rollers and three m-rollers. 

As no grinding force is applied on the s-rollers, their tyres can be made of a simple steel sheet design. On the m-rollers, tyres of high, wear-resistant chromium castings are installed, and hardfacing can provide additional wear protection. 

Market demands led to the development of a new separator incorporated in the mill housing. Today, a high efficiency cage-wheel-type classifier is installed in Loesche mills. 

Many design features of the company's raw material mills have been incorporated in the mills for clinker and slag grinding. Due to the modular design of the mills, grinding rollers of different sizes can be installed in the same mill housing, thus providing a wide range of production capacity. 
 
 

The design of grinding plants with the 2+2 mills is extremely simple (Figure 2). Slag, clinker and interground additives are collected from the mill feed bins by means of weigh feeders and transported to the mill inlet. A lifting magnet and a metal detector are used to prevent tramp metal and other foreign matter being fed into the mill. 

A hydraulically operated triple gate is used as an air seal at the mill inlet. The feed material is dried and ground simultaneously in the mill. After it has passed over the grinding table and after comminution underneath the m-rollers, the material leaves the grinding table and is lifted by the air flow to the high efficiency separator. The product leaves the mill together with the air flow while the rejects are returned to the centre of the grinding table for further comminution. 

The particle size distribution of the product can be influenced by adjustments in the speed of the separator rotor, of the air flow and of the grinding pressure. A jet pulse filter is installed downstream of the Loesche mill where the product is collected.  

The system fan of the circuit is installed behind the jet pulse filter. The VVVF drive of the system fan allows an adjustment of the air flow according to the requirement of the circuit. Depending on the mode of operation, the air can be recirculated to the mill or discarded through the stack. A hot gas generator can be connected to the recirculation duct and can be used as a hot gas source in case of drying of moist feed materials such as granulated blast furnace slag, pozzolana or other moist interground additives. If other sources of hot gases like cooler waste gas or preheater waste gas are available, they might be used as a hot gas source as well. An intake for ambient air can be used for controlling the product temperature. 

Granulated blast furnace slag additionally contains certain amounts of metallic iron in the range of 1 % and 3%, which accumulates on the grinding table during grinding. These 'iron particles are discharged from the grinding table and fall down into the hot gas channel in the lower part of the mill. From there, they are reclaimed by scrapers and discharged from the mill housing through one opening. Together with the metallic iron, small amounts of feed material are extracted as well. This reject material is lifted by means of a bucket elevator to a hopper, from which it is extracted in a controlled flow. In slag grinding plants, the reject passes over a magnetic drum separator, which separates the magnetic iron from the rest of the reject material. The iron is collected separately in a container, while the rest of the reject is fed back into the mill together with the feed material. This arrangement reduces the wear on the tyres of the grinding rollers and the lining of the grinding table considerably. 

The entire grinding plant can be arranged in a layout as a very compact design (Figure 3). The mill itself is located on ground level as well as the system air fan. Only the jet pulse filter requires a steel structure for its support. The whole grinding circuit is completely enclosed. The material 

transport inside the grinding circuit itself takes place in closed ducts and no conveying equipment is required, which might need an attendance during operation or might need maintenance. The grinding plant does not need any building structure and can be set up in the open air. Compared to other grinding systems, the noise level of the grinding plants with Loesche mills is extremely low. 
 
 

The excellent drying capacity of these mills makes it a useful tool for grinding granulated blast furnace slag. Moisture contents of 15% or even more are acceptable.  

In isolated grinding plants, hot gas generators are generally used as a heat source. If the grinding plant is installed close to a kiln cooler, waste gas might be used as hot gas source as well. In the application of a hot gas, generator fuel with a low sulfur content should be used as high sulfur contents may cause corrosion in the jet pulse filter when temperatures below the dew point of sulfuric acid are observed during start-up or shutdown of the plant. 

The comminution efficiency of grinding of granulated blast furnace slag of the company's mills is good. An installation with a production capacity of 60 tph ground granulated blast furnace slag with a mass related surface of 4.300 cm2/g according to Blaine requires a specific power consumption of approximately 36 kWh/t for the Loesche mill, the separator and the mill system fan (Table 1). Compared to a conventional close circuit ball mill system, 54% of the power can be saved in this particular case. The pressure drop over the Loesche mills in the case of slag grinding is extremely low compared to grinding of raw materials. Pressure drops between 320 daPa and 380 daPa have been monitored over long production periods. 
 
 

The European Standard EN 197 allows the production of a wide range of cements with interground additives (Table 2). Cement plants which can utilize the whole range of allowable interground additives according to EN 197 can lower their specific power consumption per t of sold product significantly and can simultaneously increase the yearly production rate. 

In ball mill systems, which are not designed for drying operation, the production capacity drops significantly in case the moisture of the mill feed increases above 1%. Numerous ball mills world-wide suffer from 20% or even 30% decrease of their production capacity due to the fact that moist pozzolanas, trass or slag is added. Under these conditions, these mills are operated at a very high temperature level resulting in the disadvantage of high product temperatures. Provided that sufficient hot gases are available, the Loesche mills are not sensitive to variations in the mill feed moisture. Therefore, it is ideal for the production of cement with interground additives with a high moisture content such as granulated blast furnace slag, pozzolana, trass or even moist fly ash. 

Intergrinding of clinker with additives of different grindability in ball mills leads to overgrinding of softer particles. However, the more difficult grindable components can be found in the coarser fraction of the particle size distribution of the cement. This is the result of the similar retention type of softer and harder particles in the grinding bed of a ball mill. In this respect, conditions are different in a Loesche Mill. Once the individual particles have passed over the grinding table and under a grinding roller, they are transported immediately to the high efficiency separator at the top of the mill. Particles of the size defined as products leave the mill immediately. No overgrinding happens in Loesche mills. However, this does not influence the product quality. Numerous tests of setting behaviour and strength development of cements with interground additives used in a Loesche mill and the comparison with the same type of cement produced in a ball mill have shown that in both systems the same quality level at the same mass related fineness could be achieved easily. Some typical examples are shown below. 

Table 3 shows the comparison of CEM 1 according to EN 197. No interground additives have been used. In this case, a high mass related surface area was required which could be managed easily. 
 
 

The use of interground limestone is very common in many countries for cements which will be used for nonstructural buildings or for the production of binders of all kinds. Table 4 shows the comparison of strength development of such a cement produced in a ball mill and a vertical roller mill. 

 The addition of pozzolana or trass is a very common practice in many countries. Table 5 gives the results of a comparative study with a pozzolana cement. Such cements are usually ground to higher surface areas than OPCs which can be done quite well in Loesche mills. As pozzolanas are usually fairly moist materials, the drying capability of a Loesche mill is a further advantage to the producer. 
 

The comparison in Table 6 shows a fly ash cement. Fly ash is currently available from coal fired power plants. Production of fly ash cement can be considered an environmentally friendly step towards intelligent recycling of this by-product. 

Although the retention time of the material in a Loesche mill is between three and four minutes, a dehydration of gypsum takes place. Compared to a ball mill, the degree of dehydration of gypsum in cements with low mass related surface areas is less in cements used in a Loesche mill, but on higher mass related finenesses (e.g. 4.000 cm2/g and higher), the same degree of dehydration in ball mill cements and cements produced in vertical roller mills has been observed. However, it is necessary to adjust the percentage of the sulfate carrier in accordance with the desired setting behaviour in case of the application of a vertical roller mill individually. 

When introducing the 2+2 technology to the market three years ago, most customers requested that the cements produced in the mill should have approximately the same particle size distribution as the cements produced in conventional close circuit ball mills. Contrary to other technologies brought into the market in recent years, this target could be achieved easily. By adjustment of the process parameters, it was possible to achieve an even less inclined slope of the particle size distribution than obtained in ball mill systems, if required. In the meantime, it was recognized that in many countries low mass related surface areas are requested but steep particle size distributions are desired. Due to the installation of the high efficiency separator, this target can be easily achieved. 

Comprehensive tests with cements with interground additives from various sources all over the world proved that the workability of the cements produced in a Loesche mill is better than the workability of cements produced from the same raw materials in other grinding systems. This statement is valid for the workability of concrete as well, which seems to be a great advantage for cement manufacturers who supply ready mix plants or plants which manufacture prefabricated concrete structures. 
 

After close circuit roller press systems for finish grinding had been introduced into the market, the installation of mixing plants for the production of blended cements became a common technology. In comparison to the operation of a ball mill, the power saving by application of roller press systems for slag grinding only is significantly high. 

 Therefore, separate grinding of the individual components reduces the energy demand for cement production. 

However, switching over from grinding pure CEM 1 to grinding granulated blast furnace slag or trass is easy. 

Grinding of the components to different mass related surface areas and a later mixing of the components increases the flexibility of the plant and allows fast response to the market demand. 

The decision of whether to mix or to intergrind must be based mainly on the sales strategy of the cement manufacturer. 
 

The 2+2/3+3 technology developed by Loesche GmbH for the grinding of slag and cement with interground additives offers high flexibility in the production of a wide range of different cements in one single production unit. Furthermore, low energy consumption, minimized costs for civil constructions as well as a high reliability and high operational availability make this system an ideal grinding tool.