How many arrangement patterns are there for the slots of the partition diaphragm?

Broadly speaking, there are five types: concentric circular, radial, diagonal (oblique linear), polygonal, and herringbone (or chevron).

• With the concentric circular arrangement, the material moves tangentially along the arcs (i.e., along the direction of the slots), so it can pass through the slots either with or against the material flow direction. The slots are not easily blocked, which is beneficial for increasing throughput, but there is a higher risk of coarse particles passing through (i.e., “run‑through” or over‑grinding).
• With the radial (also called radial straight‑line) arrangement, the relative movement of the material being ground is perpendicular to the slot direction. The material passage capacity is lower, which favours finer product fineness; because the material is less likely to flow back, the slots tend to become blocked more easily.
• With the diagonal (also called oblique straight‑line) arrangement, the slots are neither easily blocked nor prone to coarse‑particle run‑through, but the material flow velocity is somewhat lower than with the concentric circular type.
• With the polygonal arrangement, it has a certain effect in reducing material back‑flow (compared with the concentric circular type).
• With the herringbone (also called inclined herringbone) arrangement, it has the advantages of the diagonal type, but the effective open area is smaller than that of the diagonal type, and the construction is more complex.

What should be the total slot area and the slot width of the partition diaphragm?

The slots of the partition diaphragm are designed for material passage. The slot width on the outlet face of the diaphragm is approximately 1.5 to 2.0 times that on the inlet face. The total slot area, also referred to as the effective area of the diaphragm, is expressed as a percentage of the diaphragm area. A larger effective area allows greater material throughput, but if it is too large, the strength of the diaphragm decreases. The effective area on the inlet face is typically 5% to 15%; for dry‑process closed‑circuit raw mills, it can reach 20%, while for wet‑process mills, a smaller value is preferable.

The slot width is related not only to the material throughput but also to the maximum particle size that can pass through, so it must be carefully controlled. In multi‑compartment tube mills used in the cement industry, the effective slot width (i.e., the inlet‑side width) of the first diaphragm is generally 6 to 14 mm, and for some raw mills it may be up to 16 mm. The effective slot width of the second diaphragm is usually 6 to 10 mm, and that of the third diaphragm is typically 5 to 8 mm. Based on practical experience in both domestic and overseas cement industries, the effective slot width of the first diaphragm should not be too large: for open‑circuit dry mills, 6 to 8 mm is recommended; for closed‑circuit dry mills, 8 to 12 mm; and for wet‑process raw mills, 3 to 7 mm.

The slot length is calculated according to the required open area and the effective slot width, and is generally 300 to 400 mm, with the longest abroad reaching 700 mm.

For the diaphragm of a two‑compartment coal mill, the effective slot width is generally 5 to 8 mm. The effective slot width of the discharge grate plate should be about 2 mm wider than that of the diaphragm, to facilitate material discharge and the removal of broken iron chips.