Modification of the Partition Diaphragm for the Slag Tube Mill

When grinding slag powder with a tube mill, the exceptionally good fluidity of the slag powder causes it to flow rapidly through the mill, which tends to shorten its residence time inside the mill and results in a very low material bed level. This not only leaves a large amount of grinding media without material to grind, but also raises the internal temperature of the mill, significantly reducing the grinding efficiency of the media. As a consequence, the slag cannot be ground finely enough, the specific surface area of the finished slag powder is low, and the mill output decreases. In addition, to improve the grinding efficiency of slag, the last two compartments of the slag tube mill typically use grinding media of smaller diameters.

Therefore, when grinding slag with a tube mill, in order to control the flow velocity of the slag inside the mill and also to prevent the grinding media from migrating between compartments, the slot openings of the partition diaphragms and the discharge grate plate at the mill end should both be reduced to about 5 mm.

In the author’s production practice of grinding slag with a tube mill, various modifications have been attempted. Ultimately, the most successful solution was found to be the diaphragm made by welding wear‑resistant round steel bars side by side.

The following describes the modification technique for the partition diaphragm of a slag tube mill, using a Φ4.2 m × 13.5 m slag mill as an example.

An open‑circuit tube mill is preferable for slag grinding. The mill is usually divided into three compartments, and the partition diaphragms are welded using Φ60 mm round steel bars, with the material being 40Cr alloy steel. Each diaphragm panel requires approximately 4.78 tonnes of round steel bars, with a total length of about 214 m. During fabrication, first draw a circle of Φ4190 mm on flat ground, then cut the round bars to the corresponding lengths. Use steel plates measuring 66 mm (length) × 60 mm (width) × 5 mm (thickness) as spacers between the round bars for welding. The inner diameter of the tube mill is 4200 mm, and the diaphragm diameter is made to 4190 mm to prevent bending deformation caused by thermal expansion of the diaphragm. The slots of the diaphragm should be staggered relative to each other; each slot is 400 mm long and 5 mm wide. The welded joints between bars measure 66 mm long, 60 mm wide, and 5 mm thick. Due to the limited width of the manhole, during welding the diaphragm is first welded into several smaller sections, and then final assembly is completed by welding these sections together inside the tube mill to form the full diaphragm, as shown in Figures 2.21 and 2.22.

Special attention should be paid to the minimum opening distance (i.e., the distance from the first slot to the mill shell) when fabricating the partition diaphragms and grate plates.
Due to the high fluidity and rapid flow of slag inside the mill, if the minimum opening distance of the diaphragms and grate plates is too small, the slag bed level inside the mill will inevitably become too low, reducing grinding efficiency and thus lowering mill output. Therefore, the minimum opening distance of the partition diaphragms and the discharge grate plate should be appropriately increased to control the material flow velocity, raise the material bed level, and increase mill production. However, if the minimum opening distance is increased too much, it will adversely affect mill ventilation. As a general rule, the minimum opening distance is set at 12% to 24% of the diaphragm radius (i.e., 250–500 mm).

Since the temperature of slag gradually rises during the grinding process, the temperatures in the various compartments of the slag tube mill differ considerably—typically the first compartment is cooler, while the third compartment becomes very hot. Moreover, as moisture continuously evaporates during grinding, the moisture content of the slag powder often varies greatly between compartments, resulting in significantly different flow velocities through each compartment. Generally, the flow velocity is slower in the first compartment, making it prone to overfilling, while the third compartment tends to have a faster flow and is more likely to run empty. Therefore, the minimum opening distances for the diaphragms in each compartment and for the discharge grate plate should be different: smaller for the front compartments and larger for the rear compartments. In addition, the minimum opening distance of each diaphragm should also take into account factors such as the specific surface area requirement of the finished slag powder, the moisture content of the feed slag, the grinding media filling rate, and the mill ventilation rate. In short, to reduce the speed at which slag passes through the diaphragm, the minimum opening distance should be increased; conversely, to increase the speed, the distance should be decreased.