In the process of calcining clinker in rotary kiln, the evenly and reasonably distributed kiln skin can not only protect the kiln barrel, reduce the erosion, wear and high temperature damage of flame to the kiln lining, and prolong the service life of refractory bricks in the kiln; It can also reduce the heat dissipation of kiln barrel, stabilize the thermal system and improve the comprehensive thermal efficiency. Long and thick kiln skin in the kiln is a common process problem in the industry, which often affects the ventilation in the kiln, resulting in large fluctuation of incoming materials in the kiln and difficult stability of the system thermal system, which directly affects the improvement of production quality of rotary kiln and the safe operation of equipment. Combined with the actual production process, this paper analyzes the causes of such process problems, and summarizes the treatment measures and matters needing attention in process adjustment.

1. Problem cause analysis

Generally speaking, the inducing factors affecting the change of kiln skin are complex, such as the change of chemical composition of raw materials leads to the change of liquid volume and liquid viscosity in materials; The change of pulverized coal quality (ash and water content), the position of coal pipe, flame shape and calcination temperature lead to incomplete combustion of pulverized coal; The unreasonable matching of air, coal, material and kiln speed and the abundance of harmful components will affect the structural distribution of kiln skin.

1.1 influence of raw meal chemical composition and yield value

Because the kiln skin is a process of liquid solidification to the kiln lining surface, the liquid content and liquid viscosity directly affect the formation and distribution of the kiln skin, and the change of chemical composition of raw materials directly affects the liquid content and liquid viscosity. The kiln skin can be flat, dense and firm only when the three rate value of raw materials is reasonably configured. Generally speaking, it is suggested that the raw material composition when combined with kiln skin is consistent with normal production.

According to the production practice experience, the raw material composition KH is low and the iron oxide content is high, which leads to excessive liquid phase and early formation in the calcination process. Especially in the calcination process of calcined high magnesium clinker, the fire resistance of high magnesium raw material is poor, the central control operation is adjusted frequently, the high-temperature liquid is easy to form in advance, easy to bond with the surface of refractory brick or kiln skin, long and thick kiln skin or even ring. The ventilation condition of rotary kiln deteriorates, and the thermal system of the system deteriorates, which is not easy to be stable, resulting in a series of process problems such as poor clinker granulation, many core and yellow core materials, and low clinker strength.

1.2 influence of pulverized coal quality and flame shape

Fineness of pulverized coal. Excessive moisture and mismatched kiln system air lead to the reduction atmosphere in the kiln, incomplete combustion of pulverized coal in the kiln, and the unburned coke particles continue to burn after settling, so that FeO is reduced to FeO in the reduction atmosphere, forming low melting point minerals, resulting in the problem of long and thick kiln skin. The imbalance of alkali and cl. so content ratio in raw fuel has a great influence on the time and position of liquid phase in the kiln. If the material contains too many harmful substances, the melting point will be reduced and the possibility of thick kiln skin will be increased.

Because the flame shape determines the quality of kiln skin, the stability of flame shape must be maintained in production, and the ideal flame suitable for its own kiln shape must be adjusted. In order to avoid abnormal kiln skin. In the initial stage of production, with the gradual recovery of output, in order to extend the kiln skin from the front of the kiln to the inside of the kiln, the use and adjustment of the burner, due to the improper matching of internal and external air volume and too fine flame shape, the flame high temperature area is not concentrated. Although the calcination temperature is OK, it is easy to grow thick kiln skin, uneven clinker particles and poor quality.

1.3 influence of operation and thermal system

(1) Excessive coal feeding at kiln head and system air. The proportion of coal is unbalanced, resulting in incomplete chemical combustion and flame recovery; Due to the large amount of CO produced in the combustion zone, some iron oxide in the material is reduced to ferrous oxide to form FeO · SiO compounds with low melting point. Make the liquid phase appear in the combustion zone in advance, and gather the unmelted materials together, resulting in thick kiln skin and even circles.

(2) In the process of resuming production, the kiln body is slow for a long time, resulting in excessive filling rate of materials in the kiln. The settlement and wrapping of pulverized coal in the materials is also easy to form thick kiln skin or uneven distribution of kiln skin. The kiln current is high but the amplitude is large, but the kiln skin is not firm. In the process of adjustment, it is easy to collapse due to the fluctuation of thermal system. The central control operation is adjusted frequently and is not easy to be stable.

(3) The system ventilation resistance increases, and the process adjustment process unilaterally emphasizes the system ventilation, resulting in too high system wind speed, high filling rate in the kiln, and too high negative pressure of kiln tail and tertiary air. The materials in the smoke chamber at the kiln tail shall be granulated in advance and the kiln tail shall be poured.

2. Treatment measures

(1) Control of raw material proportioning and three rate value; According to the production experience of clinker calcination for many years, controlling the burnability of raw materials in the kiln is of great significance to improve clinker quality and stabilize the thermal system. In the production process, considering the poor physical and chemical activity of the main silicon raw material sandstone of our plant, according to the three rate scheme designated by the laboratory, the proportion of sandstone is reduced from 3% to 2.4%, and a high saturation ratio and low silicon proportion scheme suitable for the current process conditions is gradually explored. The three rate range of clinker corresponding to the raw material proportioning scheme with high saturation ratio is KH = 0.93 ± 0.02, SM = 2.3 ± 0.1, Im = 1.3 ± 0.1, and the average 3D strength of clinker can be guaranteed to be more than 31mpa. According to the definition of three rate value of cement clinker, aluminum rate (IM) represents the proportional relationship between Ca and caf in clinker flux minerals. The higher the aluminum content, the greater the liquid viscosity. Therefore, while ensuring that the liquid phase amount of clinker is within a reasonable range, alo? o? The control index was adjusted from 5.0% to more than 4.8%. The index was adjusted to about 3.7%. In the practice of calcining clinker with high magnesium limestone, the problems of clinker lump, core material and long and thick kiln skin caused by high liquid viscosity can be effectively reduced.

Especially since the second half of 2020, the 3D strength and 28d strength of clinker have increased month by month, the clinker has uniform granulation, the qualified rate of free calcium has reached 95%, and the grindability of clinker is better. While ensuring the high yield of clinker, it provides better conditions for cement mill.

(2) Kiln air matching adjustment; In order to avoid the problem of poor ventilation in the kiln caused by long and thick kiln skin, the gate valve of the tertiary air duct can be properly closed to strengthen the ventilation in the kiln. At the same time, closing the tertiary air lock valve can also change the shape and position of the tertiary air into the calciner, so as to change the flow field distribution of the materials from the C4 spreading box in the calciner, enhance the vortex effect between the tertiary air and the fuel of the calciner, improve the problem of temperature inversion at the outlet of the calciner and C5 blanking pipe caused by insufficient tail coal combustion, and the early granulation of the smoke chamber also plays an important role.

(3) Pulverized coal quality control; Due to the coarse fineness, high ash content and large moisture content of pulverized coal, it will affect the combustion speed of pulverized coal, lengthen the flame and move the firing zone backward. Kiln skin is easy to elongate and knot. Effectively controlling the fineness and moisture of pulverized coal can improve the combustion rate of pulverized coal and reduce the settlement of pulverized coal; However, attention should be paid to the use of fuel in the kiln to avoid top burning and dead burning of mature materials, so as to form a reduction atmosphere in the kiln, promote the premature emergence of liquid phase, and cause the problem of long and thick kiln skin sticking.

(4) Adjust flame shape and coal pipe position; On the one hand, by adjusting the external air of the burner, the air coal mixing efficiency is enhanced and the combustion efficiency of pulverized coal is improved; At the same time, strengthen the rotation effect of internal air and form a circulation area for rapid ignition of fuel, which is conducive to the formation of short and stable flame. This can not only shorten the calcination range, but also strengthen clinker calcination and stabilize the thermal system. In addition, according to the change of cylinder temperature, timely adjust the position of burner in the kiln, change the high temperature point and gradually burn thick kiln skin. Attention should be paid to strictly controlling the calcination temperature to avoid large-area damage to the kiln lining caused by too high calcination temperature.

(5) When dealing with the problem of dumping at the kiln tail, first of all, it is necessary to strengthen the cleaning of the slope and shrinkage of the flue gas chamber at the kiln tail to ensure the normal ventilation at the kiln tail; Only by properly reducing the feeding amount can the problem of dumping caused by excessive material filling in the kiln be completely eliminated.

3. Conclusion

In short, a stable and reasonable thermal system is not only the fundamental task of the new dry process cement kiln, but also the basis of high quality, high output and high equipment operation rate. For the treatment of long and thick kiln skin process problems, the operation should be optimized. Combined with the monitoring data of on-site advanced instruments, scientifically and reasonably adjust the air, coal, materials and kiln speed to effectively avoid such process problems.