Granulated blast furnace slag has been used as an component of blast furnace cement for many years. The replacement of a certain clinker portion by blast furnace slag saves raw materials and thermal energy for pyroprocessing. Ingo Engeln from Polysius AG gives an overview of the Asian slag market and the use of Polysius' vertical roller mills in China and South Korea.
Granulated blast furnace slag, a by-product of pig iron production, has been used as an important component of blast furnace cement for many years, due to its latent hydraulic properties.
The replacement of a certain clinker portion by blast furnace slag does not only mean to save raw materials and thermal energy for pyroprocessing, but also provides for advantageous characteristics of the blast furnace cements:
- low heat of hydration helps to prevent cracks in the concrete due to temperature stresses,
- high resistance against sulphate attack,
- high resistance against alkali-aggregate reaction,
- high binding capacity of chlorides in blast furnace slag cement pastes,
- the low electrical conductivity of concrete made with blast furnace cement resulting in a positive effect on the corrosion protection of the concrete reinforcement.1,2
Intergrinding - separate grinding
For a long time, the applicable standards allowed cement clinker and granulated blast furnace slag to be ground together in tube mills, but especially by utilisation of tube mills the clinker mainly accumulates in the fines fraction, while the granulated slag accumulates in the coarse range (Figure 1).
Because the proportion of pulverised blast furnace slag predominates in the blast furnace cement, the slag can only inadequately participate in the hydration as a result. The consequence is loss of quality. Intergrinding in roller mills provides very similar particle size distributions for clinker and slag and thus provides a better quality of the blast furnace cement.3
The results based on studies by Schwiete and Dölbor in 1963 already show the positive influence on the strength development of cements when grinding granulated blast furnace slags to high finenesses (Figure 2).4
Therefore, it is an indisputable fact that it is necessary to grind the granulated blast furnace slags to high finenesses in order to generate good strength properties. The intergrinding of clinker and granulated blast furnace slag inevitably necessitates the energetically unfavourable grinding of the clinker to higher finenesses as necessary. The only way to avoid this is to grind clinker and slag separately.
For the design of a slag grinding plant the special properties of granulated blast furnace slag have to be considered. Granulated blast furnace slag is moist, abrasive, fine grained and difficult to grind. It also needs to be ground very finely. There are several grinding systems on the market meeting the requirements for slag grinding, but no other grinding system has gained so much importance during the last decade as the vertical roller mill.
Polysius roller mill
Fig. 3 shows the Polysius roller mill. The essential components of the Polysius roller mill are the grinding table, the two pairs of rollers, which can move vertically as well as rotationally around the guide axles, the housing, the adjustable nozzle ring, the drive unit with motor, and the separator.
Features of Polysius roller mill
The double-grooved grinding track uses a defined gap between the rollers and the grinding track to prevent the material from flowing out at the sides. This reduces the specific power consumption. The grinding rollers can move in the vertical direction and follow the wear profile, ensuring an even wear profile and a constant material bed thickness. Each grinding roller can also be twisted through 180°, i.e. the inner roller can become the outer roller, so the wear profile, and hence the throughput, remain virtually constant over the entire running period. This minimises internal recirculation in the mill and stabilises the material bed. It reduces the gas velocities in the nozzle ring and thus saves energy. It is best if used in conjunction with the external material circuit.
The Polysius roller grinding mill has used a bucket elevator from the outset for returning the recirculation material. This reduces the amount of air-borne recirculation within the mill, which lowers the pressure loss and improves the smooth running of the mill. The bucket elevator is also a great help dur-ing maintenance work.
Typical flowsheet of Polysius slag grinding plant with roller mill
The flowsheet in Figure 4 shows the typical arrangement of a Polysius slag grinding plant with a roller mill. Fresh slag and sometimes gypsum are transported from the storage to the mill feed hoppers by conveyor belts. After being discharged from the hoppers, the mill feed material is proportioned by weighbelts and then carried by conveyor belts to the mill. Protection against tramp metal is provided here by a suspended magnetic separator. A metal detector is installed in the conveying route to take away metallic objects. A heated coarse feed valve is used as an airlock device in the material feed equipment. The feed chute is very steeply inclined and is in principle integrated into the tailings cone of the high-efficiency separator. The external material circuit is a closed system, consisting of a bucket elevator, vibrating through, magnetic drum and feed chute to the tailings cone. In order to prevent material from caking onto the recirculation bucket elevator, adequate hot air is supplied by a specially installed fan. This also heats the vibrating trough and the magnetic drum. Dust removal is ensured via the mill. The hot air used for drying is generated by a hot gas generator. The finished material is transported by the circulating mill air directly to a bag filter, where it is collected and then conveyed to the pulverised slag silo. A portion of the circulating mill air is returned to the mill, which helps to save thermal energy.
Insulation and heating
Another point to consider is that the exit gas from the grinding plant has a high water vapour content. Its temperature must therefore be prevented from dropping below the dew point and plant components such as circulating bucket elevators, chutes and pneumatic trough conveyors must be carefully insulated or even heated to prevent clogging.
Slag grinding plant with roller mill in Kwangyang, Korea
The first roller mill for this application was put into operation in Korea in 1995. Polysius designed and supplied a type RMS-51/26 roller mill with an integrated high-efficiency SEPOL RMS-435 separator.
The output of the mill was 76t/h at a fineness of 4840cm2/g according to Blaine. The residue on the 44mm screen was below 2%. The specific power consumption for the mill was 33.2kWh/t, for the mill fan 8.7kWh/t and that for the separator was 1.4kWh/t, measured at the counters of their drive motors. Also remarkable is the low pressure drop of only 34mbar in the mill.
During the next years further slag roller mills of various sizes followed in France, Spain and the US. But nowhere in the world have so many roller mills for granulated blast furnace slag grinding been sold as in China dur-ing the last 6 years.
First Polysius slag roller mill in China
The first Polysius roller mill for slag grinding in China is in operation since 2002 in Anshan in the province of Liaoning. The grinding plant of the Angang Group Slag Development Company is exactly the same size as the one which has been in operation since 1995 just a few hundred kilometres away in Korea.
The grinding plant for Anshan was designed for a grinding capacity of 90tph at a ground blast furnace slag product fineness of 4000cm2/g according to Blaine. However, following the market trend, the plant now produces 83tph of ground blast furnace slag with a fineness of 4300cm2/g (acc. to Blaine). The specific power requirement for roller mill, separator, recirculating bucket elevator and system fan is 38.4kWh/t measured at the counters of the mo-ors and referred to the product fineness of 4300cm2/g (acc. to Blaine).
The hot gas needed to dry the granulated blast furnace slag, which usually has a moisture content of 8-12%, is delivered by a hot gas generator. The residual moisture content of the ground blast furnace slag is less than 0.3%.
The photo in Figure 5 shows the mill just erected complete with separator, the mill outlet duct connected to the filter and the whole mill nicely painted.
Wear protection
The grinding plant, which is carefully maintained by the owner, provided high availability and troublefree operation. None of the wear protection plates in the mill housing and in the grit cone of the separator have yet needed replacing. Only the ceramic castable in the upper housing section of the separator was replaced by wear-resistant steel plates soon after the commissioning. Once a year, after approx. 3500 operating hours and production of 300,000t of ground blast furnace slag, the surfaces of the grinding elements are refurbished by build-up welding during a planned maintenance stoppage. The specific wear of grinding track and roller tyres, taken together, is 7.3g/t.
For long service life of the grinding tools not only the appropriate wear lining is important. The iron extraction system in the external material circuit is an extremely important feature of the Polysius roller mill for slag grinding.
Iron extraction system
It is a well-known fact that iron particles in the granulated blast furnace slag are the main cause of wear in the mill. Therefore it is extremely important to remove the maximum possible amount of this iron. The external material circuit is very well suited for this purpose, because it handles a large quantity of material, some of which is already partly ground.
The material passing through the nozzle ring falls onto a discharge ring rotating with the grinding table. This material then passes via a discharge chute into the bucket elevator.
The material discharged by the bucket elevator is fed to a vibrating trough, to ensure a uniform feed rate to a magnetic drum separator. The aeration of this magnetic drum separator is so designed that practically pure iron with no granulated slag is removed from the stream of material. After the magnetic drum, the recirculated material finally returns to the grinding table. The picture in Figure 6 shows typical iron contents found in a slag grinding plant with vertical roller mill. The incoming granulated blast furnace slag has an iron content of 0.28%, corresponding to an iron quantity of 210kg/h.
The recirculating material in the bucket elevator has an iron content of 0.7%. The magnetic drum separator removes around 170kg of iron per hour in permanent operation. The iron contents found in the roller mill and in the finish product impressively illustrate the efficiency of the external material circuit for concentrating the iron and facilitating its removal. Such efficient extraction of iron in the grinding process has a very positive effect on wear in the mill.
Wear measurement on grinding tools
Special gauges for rollers and grinding table enable the operator of the grinding plant to measure the wear on the grinding tools at regular intervalls (Fig. 7).Transferred to a wear sheet (Fig. 8) the measured values provide an accurate picture of the wear profiles for rollers and grinding track. In black colour the worn of material is indicated. This wear sheet shows typical wear profiles for slag grinding and it can be clearly seen that the wear on the inner rollers is lower than on the outer rollers. In this case the roller units can be turned by means of the roller pair dismantling device. The former inner roller now becomes the outer roller. Thus all four rollers get equal wear profiles and maximum utilisation of the roller liners is achieved. For once rewelded, hardfaced liners the turning of the roller units usually happens after 3000-4000 operating hours. A further important aspect when using a roller mill are the conditions and possible ways of carrying out maintenance work. Simple access and rapid exchangeability of the grinding elements are the prime requirements here.
Polysius roller mills therefore offer the option of replacing complete roller pair units. Special installation equipment, which facilitates rapid replacement through the large maintenance doors in the housing, is provided for this purpose.
Rewelding of grinding tool liners
In China most customers decide for refurbishment of the grinding tool liners by build-up weldings, a work which is usually carried out by local compa-nies. The photo in Figure 9 shows the arrangement for build-up welding of the grinding track in a Polysius slag roller mill in China.
The welding material used is usually much harder than the material of the original liners and thus provides the advantage of longer service lives. Reportedly the intervalls between the reweldings of the grinding tool liners can be 6000-7000 operating hours.
Design optimisation
During the last few years the design of the Polysius roller mill has even been improved (Fig. 10). On the existing mill of the size RMS 60/29 in Camden, New Jersey, several modifications have been carried out to improve the performance. The mill housing now is smaller and has a round cross section to avoid dead zones, the separator housing is smaller, the recirculating material is passed into the grit cone and the wear protection for guide pins and brackets has been improved.
After optimisation the capacity jumped from 86.3t/h to 99.0t/h based on the same compressive strength values to achieve the grade 120 quality of the finish product. The desired product quality was now achieved with 5700cm2/g according to Blaine and thus approx. 300cm2/g less than before. The modifications also led to a smoother mill operation and to reduced fuel consumption.
The optimised mill housing design leads to continously increasing gas velocity over the height of the roller mill, the conveying capacity of the gas stream is increased and in consequence undesired internal circulations which cost energy are avoided. Thus it is ensured that particles of finish product fineness are exposed to the separator and can leave the grinding circuit as early as possible. This again leads to coarser material on the grinding table, more stable mill operation and better grinding efficiency.
The first 2 Polysius roller mills for slag grinding with optimised housing in China belong to ACC Ya-Dong Cement Corporation. The slag grinding plant in Nanchang is in operation since November 2005. The roller mill is a type RMS-51/26 with an SEPOL RMS-435 separator.
For a feed material consisting of 95% slag, 4% gypsum and 1% limestone a capacity of 80t/h finish product with a fineness of 4400cm2/g according to Blaine was warranted. The residue on the 45mm screen must not exceed 5%. The specific power consumption warranty for mill, separator and fan was 35.5kWh/t, measured at the counters of the drive motors.
During commissioning in November 2005 for a mixture of 97% slag and 3% gypsum a capacity of 94t/h at a fineness of 4580cm2/g according to Blaine was achieved. The residue on the 45mm screen was 2.4%. The specific power consumption for mill, separator and fan was 35.48 kWh/t referring to the fineness of 4580cm2/g according to Blaine, measured at the counters of the drive motors. For the mill drive only a bonus compared to a tube mill of 2.95 was determined. This means that a tube mill would require almost 3 times as much energy for the mill drive as the Polysius roller mill.
In the last few years further slag meal producers decided for the Polysius roller mill and made it one of the most popular slag grinding mills in the world and especially in China. Eight Polysius roller mills for granulated blast furnace slag are meanwhile in operation in China and 5 further units have been sold in recent months.
In China and South Korea, many granulated blast furnace slag grinding plants are already in operation, but the number of granulated blast furnace slag grinding plants in South-East Asia is still low. Customers from South-East Asia who are now interested in slag grinding are invited to send their inquiries to the Polysius subsidiary in Singapore
References
1. Geiseler, J., Kollo, H., and Lang, E.: "Influence of Blast Furnace Ce-ments on Durability of Concrete Structures" ACI Materials Journal, 92 ( 1995 ) 3, ( May-June ), S. 252/257
2. Lang, E.: "Blast Furnace Cements" in "Structure and Performance of Cements, 2nd Edition" by J. Bensted and P. Barnes
3. Lohnherr, L., Menzel, K.: "Operating experience with the production of slag cements in a roller mill" in "Polysius teilt mit – 209"
4. Schwiete, H.-E., Dölbor, E.: "Einfluss der Abkühlungsbedingungen und der chemischen Zusammensetzung auf die hydraulischen Eigenschaften von Hämatitschlacken" Westdeutscher Verlag, Köln und Opladen 1963.
