The basic requirement for bricklaying in a cement rotary kiln is that the brick lining must fit closely to the kiln shell. This means that the “four corners of the cold face” of the brick must fully bear against the shell (the large end of the brick must be tightly pressed to the shell without gaps). Bricks must be tightly butted, joints straight, rings properly closed, key bricks firmly locked, and bricks correctly positioned.
Only in this way can the brick ring be reliably concentric with the shell, the lining structure remain stable, and the shell pressure and internal stresses of the lining be evenly distributed over each brick, thus avoiding stress concentration. The arrangement of refractories inside the kiln shell is shown in Figure 1.
Ficture 1
From Figure 1 it can be seen that:
The four corners of the large end (hot face) of each refractory brick must be tightly in contact with the inner surface of the shell.
The axial joints of the bricks must be strictly parallel to the kiln centreline.
The radial joints of the bricks must be strictly perpendicular to the kiln centreline.
The position of the brick lining can be determined according to the pre-marked reference lines. If the reference line is not clearly visible, the orientation can be determined by the horizontal and vertical welds on the shell.
Once any skew or deviation of the lining is found, it must be corrected in time with refractory mortar and joint-filling steel plates. When correcting, note that:
The refractory mortar joint width must be < 3 mm.
Joint-filling steel plates must be 1–1.2 mm thick, flat, without curled edges, distortion, or burrs.
Each plate must be 10 mm shorter than the brick width or height, with a width of 20–40 mm.
If the misalignment is serious, several bricks may be needed to gradually bring it back to the correct position.
During bricklaying, special attention must be paid to the following: the large and small ends of the refractory bricks must never be laid in reverse (see Figure 2). Even if only one brick has its ends reversed, the entire lining in that area must be dismantled and rebuilt. Otherwise, major equipment damage and serious personal injury accidents may occur.
Ficture 2
Large rotary kilns commonly use the arch (bracing frame) method for bricklaying. Small and medium-sized rotary kilns may also use the strut (prop bar) method.
The arch frame is a sliding pneumatic device installed on a movable platform. Through individual hydraulic cylinders, the bricks are pressed tightly against the kiln shell. The general procedure for bricklaying with an arch frame is as follows:
Lay the lower half ring of the brick lining.
Install the movable platform and mount the metal clamping device on it so that the hydraulic cylinder is in position.
Place bricks on the clamping device, and use the hydraulic cylinder to press them tightly against the kiln shell.
Before locking the bricks, use jacks to push apart the bricks on both sides to compact the already laid lining ring, and then proceed with keying (locking) the bricks.
As shown in the diagram:
First, lay the lower half ring of bricks.
Second, use cross beams and struts to build a proper support frame.
Next, rotate the kiln by 90°, lay the bricks over 1/2–3/4 of the circumference, and build the second support frame, then rotate the kiln another 90°.
Then lay the remaining bricks of the lining until the lock-brick opening is reached.
Rotate the kiln by about 45° and perform the locking operation.
Special emphasis for the strut (top bar) method:
You must pay attention to the following two points:
The height of the strut (top bar) must exceed the radius to the kiln centre, i.e. it must go beyond the kiln half-circle.
The face of the cross beam in contact with the refractory bricks must be nailed with rubber or other elastic material, or use wooden wedges to press every brick under the beam tightly.
If the strut height does not reach above the half-circle, then after the kiln is rotated 180°, the lining bricks that were previously under the strut will lose their support (now positioned above the strut) and may collapse.
Similarly, if the cross beam is not covered with rubber or tightened with wedges, and shell deformation causes some bricks not to be firmly pressed by the cross beam, those bricks may become loose or even fall out after the kiln is rotated.
Since collapse of the kiln lining may cause severe personal injury and major equipment accidents, the supports and brickwork must be carefully checked to eliminate such risks.
In addition, if wet laying is adopted:
The mortar in the brick joints must be fully filled.
The brick joint thickness must be controlled within 2 mm.
Locking (keying) bricks is the process of completing the last part of a brick ring using standard bricks, key bricks (insert bricks), steel plates, and refractory mortar, so that the brick ring is fully closed.
Insert bricks (key bricks) are lining bricks whose thickness is reduced by 10 mm or 20 mm, or increased by 10 mm, relative to the standard brick.
In general, at least two types of insert bricks should be prepared for each ring, with at least two pieces of each type.
The steel plates used for joint filling during locking should meet the following:
Thicknesses: 2 mm and 1–1.2 mm.
Length and width: slightly smaller than the standard brick.
Additional steel plates with different lengths and widths are needed for fine adjustment and deviation correction.
All steel plates must be:
Flat,
Without curled edges,
Not twisted,
Free from burrs.
The installation and tightening of the last brick during locking are as shown in the diagram.
When locking bricks, pay attention to the following points:
Do not use cut or machined bricks as lock bricks.
If multiple lock bricks are needed in one ring, they should be distributed around the ring, not concentrated in one area.
The axial joint of the lock brick must be parallel to the kiln axis.
The thickness of lock-joint steel plates must be ≤ 2 mm, and they must also be distributed, not piled in one position.
Do not drive several steel plates into a single brick joint, to avoid steel plate oxidation and expansion that could damage the refractory lining.
After locking is completed, try to drive additional thin steel plates into any suspected loose joints until no more plates can be driven in.
Do not use metal tools to directly cut or machine refractory bricks.
Special emphasis: Under no circumstances is it allowed to strike refractory bricks directly with a hammer.
Otherwise, damaged bricks may fail suddenly during operation, leading to severe personal injury and major equipment accidents.
If the kiln shell has already undergone severe deformation and cannot be replaced in time, only temporary remedial measures can be taken. The principle in handling such problems is:
Maintain production as much as possible, and
Minimize the negative effects of deformation.
To avoid stress concentration, the protrusions, depressions and other irregular areas of the deformed shell must first be leveled with refractory mortar, and then refractory bricks are laid on top.
During bricklaying, you must ensure that:
The large and small ends of the bricks remain parallel to the undeformed part of the shell, and
The extension of the large face of the brick can intersect with the kiln centreline, as shown in the figure.
In the diagram:
At least part of the large end of each brick must be in tight contact with the shell.
Any irregular gap between the large end of the brick and the shell can be filled with refractory mortar, but the maximum gap height must be < 8 mm.
The maximum thickness of mortar joints between bricks must be < 2 mm.
For the conical sections where the kiln diameter changes, it is recommended to use ISO-type or VDZ-type bricks together with refractory mortar.
By using VDZ or π/3-type bricks, only:
One brick type with a smaller curvature radius, and
One brick type with a larger curvature radius
are required to fit any kiln radius between the two curvature radii.
The main purposes of using refractory mortar in this area are to:
Fill irregular brick joints, and
Bond bricks together to form a lining with sufficient structural integrity.
At the transition between the straight cylindrical section and the conical section of the kiln (both ends of the cone), special-shaped bricks or machined bricks may be required.
Machining should be carried out on machines equipped with water-cooled diamond tools.
High-alumina bricks can be machined on site. After machining, they should be inspected and air-dried before being laid.
However, basic (alkaline) bricks are prone to hydration and therefore are not suitable for such machining.
