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.
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
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.
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.
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.
The flowsheet of slag vertical roller mill 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.
