- Time:Mar 22, 2023
Kiln skin, that is, the clinker layer attached to the surface of the kiln lining for firing. It protects the lining of the sintering zone, so that the lining does not directly contact the flame and high-temperature materials, weakens the chemical erosion, abrasion and high-temperature damage of the flame and materials to the lining, thereby prolonging the service life of the lining; The kiln skin also acts as a thermal insulation layer, which thickens the kiln lining in the high temperature zone, thereby reducing the heat loss on the surface of the kiln shell and improving the thermal efficiency of the kiln. Generally speaking, the kiln skin of the burning zone is the main kiln skin, and the exothermic reaction zone kiln skin after the upper transition zone is called the auxiliary kiln skin. The normal auxiliary kiln skin is only a thin layer, with a thickness of ≤70mm, and the thickest will not exceed 100mm. . The stability of the auxiliary kiln skin is very poor, and it will fluctuate with the change of kiln raw material composition, liquid phase amount, harmful components and other factors. Once the balance is broken, an abnormal kiln skin will be formed.
2500t/d single-series three-stage calcium carbide slag production line, the size of the rotary kiln is Φ4.3m×60m, the size of the calciner is Φ5800mm+Φ4300mm, and the annual average daily output is 2640t/d. After the overhaul, it is known from the display of the kiln shell scanner that the auxiliary kiln skin at the kiln tail frequently has abnormalities. ~64m barrel temperature shows the lowest 87°C. The distribution of the auxiliary kiln skin is abnormally long and thick, which leads to poor ventilation in the kiln, reduced output, and frequent kiln tail return, which seriously affects the production quality of clinker and the smooth, safe and stable operation of the kiln. We conducted a preliminary analysis of the abnormal kiln skin and A series of measures have achieved certain results.
1. Reason Analysis
The inductive reason for the change of the kiln skin in the kiln is the change of the liquid phase, that is, the rationality of the three rate values and the content of harmful components. The secondary reason is due to the mismatch in operation and the incomplete combustion of coal powder.
According to the definition of three ratios of cement clinker, we know that the aluminum ratio (IM) is too low and the liquid phase viscosity is small. Although it is beneficial to the formation of A ore, the sintering range in the kiln is narrow, and it is easy to cause large lumps in the kiln, which is harmful to calcination. Unfavorable, it is difficult to master calcination; and it is also known that when the silicon rate (SM) decreases and the liquid phase increases, it is beneficial to the flammability and operation of the clinker, but if the SM value is too low, the clinker strength is low, and the kiln is easy to form rings, Large lumps, difficult to operate.
Due to the special nature of the calcium carbide slag production line, and in order to improve clinker granulation and cement adaptability and other factors, after years of production exploration, the three rate values are KH: 0.910, SM: 2.10, and IM: 1.30. The ingredients are just in line with the innate conditions of agglomeration, egg formation, and long thick kiln skin in the kiln. Coupled with the interference of inaccurate measurement equipment, design defects of the ingredient warehouse, and poor fluidity of the material, the rate of entry into the kiln fluctuates greatly. , high and low, resulting in continuous low KH and SM entering the kiln, and frequent occurrence of high rates. When the silicon and aluminum rates are low, the decomposition furnace temperature is controlled too high, resulting in high decomposition rate of raw material entering the kiln. Too much liquid phase appears in advance. Due to the formation of liquid phase on the surface of the material, the surface tension is small and the viscosity is high. Under the action of centrifugal force, it is easy to bond with the surface of the refractory brick or the surface of the "kiln skin" to form a kiln skin, which further promotes the kiln skin. thickening.
2. The influence of harmful ingredients
In the precalcining kiln system, the circulation and enrichment of alkali, sulfur, chlorine, etc. are accompanied by two processes, one is called "inner circulation" and the other is called "outer circulation". The so-called internal circulation refers to the volatilization of alkali, sulfur and chlorine from the raw meal and fuel in the high temperature zone of the kiln, and then condenses on the lower temperature raw meal when it reaches the lower temperature area of the lowest preheater in the kiln system. As the raw meal settles and enters the kiln together, a circulation and enrichment process between the preheater and the kiln is formed. The content of alkali, chlorine and sulfur in the crust material is higher than that in the raw meal and clinker crust. The content is high. On the one hand, the reason is that the content of alkali, CI- and SO3 in the raw meal entering the kiln is much higher than that of normal enterprises due to the influence of the upstream acetylene method process conditions and the selection of surrounding raw materials. On the other hand, it is related to the reciprocating cycle of alkali, chlorine and sulfur in the calcination system.
But why in the precalciner kiln system, harmful components such as alkali, sulfur, chlorine, etc. are circulated and condensed in the preheater system, and they will run to the end of the kiln to circulate and enrich, resulting in abnormality of the auxiliary kiln skin at the end of the kiln and the phenomenon of returning materials at the end of the kiln. We know that the alkali, Cl- and SO3 in the system all exist in the form of salts. The alkali, chlorine and sulfur compounds brought into the system by the raw materials will gradually volatilize in the high temperature zone in the kiln and become a gas state. The normal kiln gas When alkali sulfate and alkali chloride condense, they will settle down in a molten state and form a cohesive substance together with the kiln materials and dust in the kiln, and the liquid material film formed on the raw material particles will hinder The flow of raw material particles will return material in severe cases. What we know is that the volatilization order of alkali, Cl-, SO3 is the hydroxide of alkali first, then the chloride of alkali, and finally the sulfate of alkali. The amount of circulation is related to the volatilization coefficient. The volatilization coefficient of sulfur is affected by the sulfur-alkali ratio. The higher the value of the sulfur-alkali ratio, the higher the volatilization coefficient.
Alkali chloride is more volatile than alkali sulfate. The presence of chloride ions will promote the volatilization of alkali. At 1450°C, the volatilization rate of alkali chloride is almost as high as 99.9%, so the content of chloride ions in cement clinker is very low. The volatilization rate of sulfur and alkali is related to the residence time in the high temperature zone and the physical form of the material. K2O·23CaO·12SiO2(KC23S12), Na2O·8CaO·3Al2O3(NC8A3), K2SO4, Na2SO4, K2O·8CaO·3Al2O3, etc. will remain in the clinker.
In an oxidizing atmosphere, alkali hydroxides and chlorides are more volatile, while sulfates are less volatile, namely:Cl->K2O>SO3>Na2O.
The volatility of different sulfates is also different, and the volatility at the same temperature: CaSO4>K2SO4>Na2SO4.
Under reducing atmosphere, the order of volatilization is:Cl->SO2>K2O>Na2O.
When the kiln condition is abnormal and the crusts in the smoke chamber and calciner increase, the ventilation in the kiln will gradually become poor, which will reduce the oxygen partial pressure in the kiln, increase the decomposition degree of CaSO4, and the sulfate volatiles will be mixed with incoming materials and dust. At the same time, it is easy to concentrate and sink at a certain position at the end of the kiln, adhere to the surface of the refractory brick, and form a crust. cycle.
3. The influence of pulverized coal
Because the rotor of the coal mill separator was replaced and the seal was not re-adjusted, the coal powder once ran coarse, and the maximum fineness reached 25%. In addition, due to the unique nature of Mongolian coal, the internal water is higher than 6%, which may easily cause the flame burning speed of the kiln head to be slow. , the flame is elongated, the high temperature zone moves back, the pulverized coal is not completely burned, and the volatilization coefficient of sulfur increases with the presence of local carbon (C) in the kiln. After the upper transition zone of the kiln, when the pulverized coal is incompletely burned in the kiln, the coal ash is unevenly mixed with raw materials, the flame is too long, the temperature behind the kiln is too high, the liquid phase appears early, and part of the pulverized coal falls to the surface of the material At this time, the sulfate alkali salt and CaSO4 in the material react with C to generate SO2, and the equation is as follows:
K2SO4+C→K2O+SO2+CO
CaSO4+C→CaO+SO2+CO
Coal ash settles unevenly in the material, which increases the liquid phase of the material and the thickness of the kiln skin. When the kiln condition is abnormal, some coarse coal powder will be pulled by the wind to the kiln tail for secondary combustion, and a large amount of liquid phase will be aggravated by local high temperature. The kiln skin grows and thickens.
4. Influence of operation and thermal system
(1) When the pulverized coal fineness and moisture cannot meet the production requirements and maintain a high kiln output, due to the increase in kiln output load and the mismatch of kiln speed in operation, the filling rate in the kiln increases and the ventilation gradually decreases, resulting in Incomplete combustion of fuel and deterioration of clinker quality. To ensure quality, the amount of coal used increases. Excessive coal consumption results in chemical incomplete combustion, which makes the flame reductive. Due to the large amount of CO produced in the burning zone, the material is partially oxidized. Iron is reduced to ferrous oxide, forming FeO·SiO2 low melting point compounds. The FeO·SiO2 liquid phase can promote the formation of silicon calcite [2(CaO·SiO2)CaCO3] at about 1100°C, and the silicon calcite forms a liquid phase at 1180°C~1220°C, which makes the liquid phase of the firing zone appear in advance, and will not The melted materials gather together to form a crust, plus the operation must increase the exhaust, the air velocity in the kiln increases, the flame elongates, the liquid phase appears in advance, and it is easy to form a clinker ring and a long and thick kiln skin.
(2) When the kiln skin at the kiln tail is abnormal, it is usually adopted to reduce the temperature of the calciner and the tail temperature to cause the kiln skin to shrink, and the low-temperature materials are poured in and washed to further reduce the temperature of the auxiliary kiln skin at the kiln tail to make it shrink and fall off , the basic oxides that were originally volatilized in the preheater system were brought into the kiln. When the kiln condition was abnormal and the free calcium was unqualified, the habitual operation of reducing materials and kiln speed was not found out. At a high temperature of 1450°C, almost all the chlorine salts are volatilized, and at the same time, the sulfur originally solidified in the clinker is volatilized. In addition, the prolongation of the residence time of the material in the high temperature zone increases the volatilization rate of sulfur and alkali, and aggravates the harmful components of sulfur and chlorine in the kiln. The amount of circulation will lead to further deterioration of the auxiliary kiln skin at the kiln tail.
(3) When the ventilation in the kiln is poor, the flying sand at the kiln head is large, and there is positive pressure, some operators will open the fan valve of the grate cooler in order to pursue the output and increase the high-temperature fan and the kiln head exhaust fan to no avail. The temperature decreases, resulting in poor cooling effect, lower secondary air temperature, incomplete combustion of coal powder, and uneven mixing of coal ash into clinker, resulting in an increase in the local liquid phase in the kiln, increasing the probability of a long and thick kiln skin .
5. Handling measures
Due to the limitations of its own technology, equipment, materials and other conditions, it is impossible to fundamentally remove harmful components in the raw materials entering the kiln, and can only improve, alleviate and reduce the production of crusts from other aspects.
(1) To eliminate or slow down the impact of sulfur on the calcination of the rotary kiln, due to the influence of the upstream and raw coal resources, it is not easy to reduce the sulfur content in the raw fuel, so it is considered from the aspect of reducing the ratio of sulfur to alkali; on the other hand, reducing sulfate in the kiln The amount of volatilization is to reduce the volatilization coefficient of sulfur, specifically, the firing temperature should not be too high, and the residence time of clinker at high temperature should be shortened.
(2) Ensure that the air volume in the kiln increases the oxygen content in the kiln gas, and the volatilization coefficient of sulfur decreases with the increase of the oxygen content in the kiln gas. The volatilization of sulfur in the kiln mainly comes from CaSO4, and its thermal decomposition equation is as follows:
CaSO4<=>SO2+CaO+1/2O2
In the above reaction equation, when the temperature is constant, when the O2 content increases, the CaO and SO2 content in the material decreases, the CaSO4 content increases, and the volatilization coefficient of sulfur decreases. When the O2 content decreases, the CaO and SO2 content in the material increases, the CaSO4 decreases, and the volatilization coefficient increases. When the sulfur content is high, the oxygen content of the kiln exhaust gas should be controlled at about 2.5%.
(3) Determine an economical and reasonable output and quality target based on the actual situation, and within a reasonable range, ensure a fast turnover rate of the kiln, which is to increase the kiln speed and adopt the method of fast burning of thin materials. Quickly unload the material, shorten the clinker residence time, and reduce the amount of sulfur and chlorine circulating in the kiln.
(4) Adjust the batching plan reasonably, increase the silicon rate and reduce the aluminum rate. On the premise of ensuring the quality of the clinker and the flammability of the material, it should be considered that the amount of liquid phase should not be too much, and the viscosity of the liquid phase should not be too large. The amount of liquid phase is within a reasonable range of 24% to 26%.
(5) Once the abnormality of the kiln skin at the end of the kiln affects the normal operation, ensure that the oxidizing atmosphere in the kiln, the complete combustion of the head coal and the temperature in the kiln cannot be too high. The position of the burner in the kiln changes the high temperature point so that the kiln skin at the kiln tail shrinks due to the temperature difference. If it does not improve for a long time, the skin at the kiln tail should be considered for cleaning.
(6) When the kiln tail is long and thick, in order to avoid a large amount of material pouring into the kiln when cleaning the kiln tail smoke chamber and shrinking the mouth, resulting in excessive filling at the kiln tail and material leakage, the cleaning should be adjusted according to the amount of crust on site The number of times, focus on cleaning the kiln tail shrinkage and the feeding slope to ensure the ventilation of the system.
6. Conclusion
As long as the existing resources are reasonably matched, adjusted, and used, summed up experience, and systematically standardized operations, the impact of abnormal kiln skin on the kiln system will be reduced and eliminated, and finally the high-quality and high-yield kiln system will be realized through the stability of the thermal system in the kiln.
ZHENJIN REFRACTORIES is a manufacturing enterprise integrating R&D, production, sales, international trade and technical services. It is dedicated to the supply, construction and installation of refractory products in cement, lime, non-ferrous smelting, steel, glass and chemical industries. services. Long-term focus on the research and development and production of high-quality refractory materials, which are used in 31 provincial-level administrative regions across the country and exported to Greece, South Korea, Japan, the United States and other international markets. The products have been recognized for their stable performance, excellent quality and exquisite craftsmanship. The trust and praise of users.
The perclaes-hercynite refractory bricks produced by ZHENJIN REFRACTORIES have been safely operated for 410 days in the first cycle of the 10,000-ton production line of Huaxin Cement Huangshi Company, and 414 days in the second cycle. The third cycle will start on January 28, 2023. expected to be better.
