The quality of cement is mainly determined by the quality of the clinker. To obtain high-quality clinker, according to the process characteristics of pre-calciner production, in addition to controlling raw materials and fuel, qualified raw materials are also required, and the reasonable chemical composition, mineral composition, and rate value of the clinker must also be controlled. However, quality control during clinker calcination directly determines the quality of the clinker. This is briefly described from the following two aspects:

1. The purpose of quality control during clinker calcination

The content of free calcium oxide (f-CaO) and the clinker bulk density are important indicators for checking the clinker in the pre-calciner clinker calcination process. The increase in free calcium oxide content affects the stability and strength of the cement, and in severe cases, it causes poor stability, causing cement products to deform and crack. The measurement of clinker bulk density is to test the density of the agglomerates during clinker sintering, ensure the strength of the clinker, reflect the mineral composition of the clinker, and guide kiln system operation and batching. Therefore, the purpose of quality control during clinker calcination is to combine process production conditions and various production economic indicators, control the free calcium oxide and clinker bulk density within an appropriate range through the normal operation of the kiln system. Generally, the free calcium oxide in the pre-calciner can be controlled below 1.5%, and the clinker bulk density is greater than or equal to 1250 kg/L. However, the control range of f-CaO content and clinker bulk density varies depending on the production status of different kiln types and different cement types.

1. Influencing factors of quality control during clinker calcination

In the kiln system process operation, due to the influence of the batching rate value, raw material fineness and homogenization effect, fuel composition, calcination, cooling system, kiln atmosphere, and abnormal operation, the detection results of f-CaO content and bulk density of the sintered clinker will not be within the clinker quality control indicators, causing production process quality accidents. Therefore, in the kiln system process operation, it is very necessary to analyze the relevant process parameters, judge the influencing factors that cause the quality accidents in the production process, and make corresponding process adjustments.

2.1 Influence of batching rate value

The rate value indicates the coefficient of the relative content between various oxides in the clinker, which is used to control the mineral composition and meet the strength control of the clinker; in the kiln system operation, fluctuations in the rate value of the raw materials often lead to the destruction of the kiln thermal system. Generally, a high saturation ratio of raw materials will make the raw materials difficult to burn, resulting in high f-CaO. Operators are forced to increase coal feeding, which easily burns the kiln skin; if IM is too low, the sintering range will narrow, and large eggs are easily formed in the kiln, seriously damaging the thermal system.

2.2 Influence of raw material fineness and homogenization

Raw material fineness and homogenization are of great significance to clinker firing and clinker quality. The raw material fineness is controlled at 12-15% (0.08 mm square hole sieve residue). If a vertical mill is used, it can be relaxed to 16%. Production practice shows that there are mainly the following situations that affect the quality control of the clinker calcination process:

(1) If the raw material entering the kiln is too coarse, the decomposition rate of the raw material will decrease, and the kiln thermal load will increase. The reaction is incomplete during calcination. CaO cannot be completely absorbed, and the f-CaO content is easily too high. In process operation, the decomposition rate can be improved to reduce the kiln load and stabilize the normal calcination temperature of the kiln.

(2) Fluctuations in the raw material composition are mainly caused by low material level in the raw material homogenization bin or unstable batching. During calcination, fluctuations in the raw material composition will bring difficulties to calcination, and in severe cases, it will cause chaos in the kiln thermal system, which is not conducive to the normal operation of the kiln system. Process operation is achieved through minor adjustments to the main process parameters. For example, appropriately reducing feeding, increasing coal feeding, stabilizing the kiln system thermal system, and strengthening calcination.

2.3 Influence of pulverized coal

The influence of pulverized coal on the quality control of the clinker calcination process mainly refers to the ash content, volatile matter, calorific value, fineness, and moisture content of the pulverized coal. Most pre-calciners in domestic cement enterprises use bituminous coal as fuel, and pulverized coal preparation uses wind-swept coal mills or vertical mills. The fineness of pulverized coal is controlled within 12% (0.08 mm square hole sieve residue), and the moisture content is controlled within 1.0%. According to the statistics of practical production data, due to the addition of coal ash, the lime saturation ratio of clinker will be reduced by 0.04-0.16, the silica ratio will be reduced by 0.05-0.2, and the alumina ratio will be increased by 0.05-0.3. Therefore, in normal production, the ash content and volatile matter of pulverized coal should be adapted to the batching rate value. If the lime saturation coefficient KH is too high and the ash content of pulverized coal is too low, the f-CaO content of clinker is difficult to control. In the batching, the saturation ratio should be reduced or the calcination temperature should be increased. Therefore, in the process operation, combined with the properties of pulverized coal and the rate value of raw materials entering the kiln, corresponding adjustments should be made in operation to determine the clinker calcination temperature, while stabilizing the thermal system.

2.4 Influence of calcination temperature

Normal clinker firing has a temperature range (sintering range). Due to fluctuations in the raw material composition, the calcination temperature required for raw materials is generally 1350-1450℃. If the kiln temperature is too low, underfired clinker is likely to occur. CaO is not fully absorbed to generate C3S, but exists in a free state, causing the clinker strength to decrease. If the calcination temperature is too high, the free calcium oxide is very low, the clinker agglomerate size increases, it becomes dense and difficult to grind, and it is easy to damage the kiln skin, which is very unfavorable for the protection of refractory materials. The practice of cement kiln burner projects by Zhengzhou Zhongwei Environmental Protection Equipment Co., Ltd. shows that controlling the calcination temperature is crucial for controlling the quality of clinker. The method of process adjustment is: to judge the high and low of the calcination temperature through parameters such as kiln current, NOx, and kiln head flame temperature, and adjust the kiln coal feeding amount, raw material feeding amount, kiln ventilation amount, secondary and tertiary air temperature, and kiln speed to control the calcination temperature within an appropriate range, avoiding underfiring or overfiring of materials.

2.5 Influence of kiln atmosphere:

Normal clinker is fired under excess air in the kiln, that is, there is enough oxygen to ensure complete combustion of the fuel. If the oxygen content in the kiln is insufficient, incomplete combustion of the fuel will easily produce CO, forming a reducing atmosphere. Incomplete combustion of the fuel results in heat loss, resulting in low thermal intensity in the kiln. High-valence transition elements are reduced to low-valence ones, producing clinker with abnormal colors, such as yellow heart material. At the same time, due to the early appearance of the liquid phase in the kiln skin, the viscosity decreases, causing the formation of rings at the back of the rotary kiln, which also causes poor ventilation in the kiln, aggravating the reducing atmosphere. This forms a vicious cycle, destroying the normal calcination temperature in the kiln, making it difficult to control the quality of the clinker.

During the process operation, the reducing atmosphere in the kiln can be judged by the appearance characteristics of the clinker and relevant process parameters, such as gas component analysis, such as the reduction of CO2 and NOx content, the increase of CO, and the high temperature of the kiln tail flue. This can be improved by improving the ventilation in the kiln and reducing the generation of reducing atmosphere. If the reducing atmosphere is generated due to the distortion of coal feeding measurement and incomplete combustion of coal powder, the coal feeding amount should be adjusted in time during operation.

2.6 The impact of cooling

In pre-calcination kiln production, clinker cooling mainly refers to the rapid quenching process of clinker exiting the kiln in the cooler. Since the clinker needs to undergo two processes of liquid phase condensation and mineral phase intersection during the cooling process, cooling has a significant impact on the quality of the clinker. Under the condition that the clinker calcination is normal, the key in the process operation is to strengthen the rapid cooling of the clinker, because rapid cooling has the following obvious advantages: (1) Prevent the transformation of C2S in the clinker, which causes the clinker to pulverize. (2) Rapid cooling causes internal stress in the clinker balls, improving the grindability of the clinker. (3) Rapid cooling can reduce C3A crystallization, improve the sulfate resistance and stability of cement. Therefore, in the operation, the operation of the cooler is often optimized, the clinker cooling system is improved, and the cooling of the clinker is strengthened.

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