Rotary kiln refractory castable shrinkage joint and refractory brick masonry expansion joint

Rotary Kiln Refractory Brick Masonry Expansion Joint Reservation Scheme

Due to the stresses caused by thermal expansion of the refractory bricks and the lateral elliptical deformation (ellipticity) of the rotary kiln, expansion joints must be preserved in the kiln lining design. The size of the expansion joint can be determined by the properties of the refractory brick and the temperature of the masonry. Usually, 50%-100% of the theoretical expansion of the refractory brick at high temperature is taken as the value of the expansion joint. There are three types of expansion joints in rotary kilns: radial joints, circumferential joints and longitudinal joints.

Before masonry, radial joints are usually covered with a layer of refractory mortar. For alkaline bricks, a radial seam of about 1mm, but no more than 2mm, is usually reserved to prevent loosening of the kiln lining. For non-alkaline bricks, it can be directly dry laid. Longitudinal joints can only retain small joints. If wet masonry is used, 2-3mm thick refractory clay joints should be reserved. When dry laying, a 1mm thick steel plate should be sandwiched in the middle. Considering the big and small heads of the refractory bricks, the big head joint should be smaller and the small head joint should be larger. Because the small head is the working surface of the refractory brick, it swells some people. When setting circumferential joints, in the conveyor belt with alkaline bricks as kiln lining, 3-4mm clay joints can be left for wet laying, and about 2mm cardboard joints can be left for dry laying. In the lining belt of non-alkaline brick kilns, circular joints cannot be set, and the zero-joint masonry scheme is adopted. Expansion joint refers to the gap left by the refractory castable in the construction process according to the construction regulations. Its function is to make the refractory castable expand and contract freely when it encounters high temperature.

How to reserve expansion joints for refractory castables?

1. Shrinkage and expansion of refractory castables

During the secondary heating process, the refractory castable will shrink twice during dehydration at 50°C to 200°C and sintering at 900°C to 1000°C. In other temperature ranges, heating will cause the castable to expand. After the first heating, the castable generally no longer shrinks. In order to prevent the stress caused by the volume change from damaging the castable, the lining must be divided into small areas with a diagonal size of no more than 1.5m, and poured in sections, with expansion and contraction gaps left at the boundaries of each pouring area.

2. The scope of influence of expansion joints
The size of the gap should ensure that the castables of each unit can expand freely, and the expansion joints in appropriate positions can also be used as control joints. In the high temperature section (>600℃), the width of the expansion joint should be controlled at 3-4mm according to the distance between the expansion joints. Set expansion joints and control joints on the plane 200mm away from the lobes, and set up non-woven halls at the lobes and sharp cones. The castables on both sides of expansion joints and control joints are easy to crack, and the nail density can be appropriately increased.

3. Expansion joint width control
The width of the expansion joint is related to the operating temperature and the length of the control line. The higher the temperature, the longer the control length, and the expansion joint can be appropriately widened. In areas with lower temperatures (<400°C), castables can be used directly on the equipment housing without thermal insulation. In this case, inserting a layer of 2mm thick cardboard or plastic film every 1.5m can meet the expansion space requirements.

4. The best location for expansion joints
When determining the expansion position, the reasonable arrangement of the vibration process should be comprehensively considered, and the vibration of the castable on the side of the gap should not affect the castable that has been poured, vibrated and started to harden. The position of the expansion joint should avoid the stress parts, the furnace frame and the holes of the furnace lining. In areas with complex shapes, such as corners, protrusions, etc., expansion joints that are compatible with the surface should be provided on convex corners and curved surfaces with small curvature radii. When two casting surfaces meet to form a reentrant corner, it shall be at the reentrant corner. Set "<" type expansion joints. The lining drawing shall show the location and dimensions of the major component expansion joints. When the working temperature of the castable exceeds 1200°C, an expansion joint with a width greater than 3mm should be set and filled with fiber felt.

5. Control seam settings
Since the castable will crack due to shrinkage, a control seam is set at the convex corner, at about 200mm, so that the shrinkage crack appears at a predetermined safe position. Control seams can be set by filling predetermined seams with paper, paraffin or veneer, pouring flammable or lost materials at high temperatures. When these combustibles burn or are lost, control seams are left behind. The control seam must be placed between adjacent nails, approximately the same distance from the nails on both sides. Should be installed on continuous curved surfaces, tapered surfaces and pipes, and should be installed on flat surfaces and curved surfaces where the risk of shrinkage cannot be controlled. Avoid setting expansion joints in areas where expansion joints cannot be set, such as setting expansion joints at small pouring surfaces and raised corners.

Influence of lock seam brick and retaining ring on the lining of rotary kiln

lock seam brick

In order to lock the kiln lining, it is necessary to calculate the theoretical quantities of locking seam bricks and locking seam steel plates. Calculated as follows. The perimeter of the cold end of the lining body (main brick + locking brick + steel plate) = the perimeter of the inner surface of the kiln (12-4) The perimeter of the hot end of the lining body (main brick + locking brick + steel plate) = the reserved perimeter of the kiln body +(-20-0mm)

According to formula (12-4), the perimeter of the cold end of the kiln lining must be equal to the perimeter of the kiln body to ensure "big end extrusion". When the "big end is squeezed", the circumference of the small end is between the perimeter of the lining reserved for the kiln body and the perimeter of the lining reserved for the kiln body. Ideally, you can achieve "small head relaxation".

First, the number of lock bricks should meet the requirements of formula (12-4).
Second, there should be more than one surplus for each refractory brick ring and each type of lock brick. When building a kiln, the type and quantity of refractory bricks are determined. If the lining brick cannot be locked, it can only be solved by using steel plate and refractory. In short, "squeeze the big head and relax the small head" is not only a theory, but also has certain guiding significance for practice. If the small end of the brick cracks when the temperature rises, try increasing the width of the small end joint. For example, use refractory clay or bricks with a small diameter of curvature. In case of draw and drop bricks, you must
Check whether the big end is compressed, whether the firing shrinkage rate of the refractory brick is too large, and whether the bonding strength of the refractory soil is sufficient.

Fixed ring
The inclination of the cement rotary kiln is about 3%. As the kiln body rotates, the kiln lining tends to slide to the kiln head. In order to prevent the kiln lining from sliding and causing pressure on the kiln mouth, there should be at least one retaining ring near the kiln head. To withstand sliding thrust and prevent the brick lining from moving to the kiln head. Multiple retaining rings can be provided if necessary. However, the hot end of the firing zone and transition zone cannot be provided with brick rings. In the disintegration area, an additional retaining ring can be provided at least 4 m from the tire. The height of the brick retaining ring shall not exceed 50mm and the section dimensions shall be 50mm x 50mm. If the height is too low, the area of ​​the brick fixing ring is small, and the refractory material in contact with the fixing ring is easily crushed. If the height is too high, as the kiln lining becomes thinner, the brick retaining ring will oxidize, causing expansion and damage to nearby refractories. In any case, refractory bricks with grooves should be built on the kiln head side and ungrooved refractory bricks on the kiln tail side of the kiln head to prevent the formation of weak grooves. The thrust of the kiln lining is directly damaged by sliding.