In portland cement manufacturing process, the quality of cement clinker directly affects the quality of the finished product cement.
The following are several important factors affecting the cement clinker quality. In the process of clinker production, strict control of these factors can ensure the production of high-quality, high-yield clinker.
1. Lime Saturation Factor
The lime saturation factor (LSF) is a ratio of CaO to other oxides, it is used to control the proportion of cement raw meal. In the operation of the cement kiln, the thermal system of the kiln can be affected by the fluctuation of LSF. The high saturation ratio will make the raw meal difficult to burn to clinker.
2. Fineness & Homogenization of Raw Meal
The fineness and homogenization of raw meal have an important impact on the clinker quality.
Generally speaking, the fineness of the raw meal needs to be contained between 12% and 16% to ensure the quality of the product clinker. If the raw meal fed to the kiln is too coarse, it will increase the heat load of the kiln, decrease the decomposition rate of the raw meal during the calcination process, and the solid-state reaction in the kiln will not be completely done, which will reduce the quality of finished clinker.
The low homogenization degree of the raw meal will cause the composition fluctuation of raw meal, which will bring difficulties to the calcination and even cause disorder of the kiln thermal system, which is not conducive to the normal operation of the kiln system.
3. Coal Fines
Many cement plants use bituminous coal as the fuel of rotary kiln, so the ash content, volatile matter, calorific value, fineness, and moisture of coal fines will affect the clinker calcination process in the kiln.
Coal fines are usually prepared by air swept coal mill or vertical mill. In order to produce high-quality clinker, the fineness of coal fines should be controlled at about 12%, and its moisture content should be controlled within 1.0%. The machines that are usually used to dry the coal fines are rotary dryers.
According to the data statistics obtained in the actual production, the coal ash infiltration in the process of calcination will cause the lime saturation factor in the clinker to be reduced by 0.04 ~ 0.16, the silicon ratio to be reduced by 0.05 ~ 0.20, and the aluminum ratio to be increased by 0.05 ~ 0.30. Therefore, in normal production, it is necessary to make the ash and volatile matter of coal fines adapt to the proportioning of raw meal.
For example, a high lime saturation factor of raw meal and a low ash content of coal fines will make the f-CaO content of the clinker hard to control. Under this situation, we should lower the saturation during raw meal proportioning or raise the calcination temperature to maintain the clinker quality.
Therefore, in the operation of rotary kiln, we need to make corresponding adjustment according to the properties of coal fines and raw meal proportioning.
4. Calcination temperature
During normal production process, the clinker is always calcined in a fixed temperature range. According to the fluctuation of raw meal composition, the calcination temperature of clinker is generally between 1350℃ and 1450℃.
If the temperature in the kiln is too low, it is easy to produce underfired clinker. Under the low-temperature condition, CaO in the raw meal is not fully absorbed to form C3S but exists in free state, which makes the strength of calcined clinker decrease.
If the temperature in the kiln is too high, the size of the clinker will increase, which makes the clinker dense and hard to grind. The large-sized clinker is also easy to damage the kiln skin, which is not conducive to the protection of the refractory lining of the rotary kiln.
Therefore, in the operation of rotary kiln, we need to judge the calcination temperature through the parameters of current NOx and flame temperature at the kiln head and adjust the coal feeding rate, raw material feeding rate, airflow rate, secondary and tertiary air temperature, and kiln speed to control the calcination temperature in an appropriate range.
5. Atmosphere inside the Kiln
Under normal conditions, the clinker is calcined in the oxidation atmosphere, that is, there is sufficient oxygen in the rotary kiln to make the fuel completely burn. If the oxygen content in the kiln is insufficient, the fuel will not be able to be completely combusted and will generate CO to form a reduction atmosphere inside the kiln.
In the kiln, the high value of the fuel and the heat intensity of the clinker is not reduced completely. In the meantime, because the liquid phase in the kiln skin is led out ahead of time and the viscosity is reduced, the ventilation in the kiln is not good, and the reduction atmosphere is heavier, forming a vicious cycle.
During the calcination process in the reduction atmosphere, incomplete fuel combustion causes heat loss and thermal strength reduction in the kiln, and the transition elements with high valence are reduced to that with low valence, which makes the clinker color abnormal. Moreover, the kiln skin will appear liquid phase in advance, which reduces its viscosity and deteriorates the kiln’s ventilation, and eventually forms an even heavier reduction atmosphere.
In the production process, we can tell if there is a reduction atmosphere by observing the clinker color and the gas composition parameters in the kiln. If the reduction atmosphere appears, it can be eliminated by improving the ventilation condition of the kiln and adjusting the coal feeding rate.
6. Cooling Process
In the clinker production of dry process, the calcined clinker is quenched in a grated cooler or a rotary cooler after being discharged from the kiln. Because there are two processes of liquid phase condensation and mineral intersection in the clinker cooling process, the cooling process will have a certain degree of impact on the clinker quality.
High quenching speed has the following advantages: 1) prevents the clinker from pulverizing due to C2S transformation; 2) makes the clinker ball produce internal stress and improves its grindability; 3) reduces the crystallization of C3A in clinker and improves the sulphuric acid resistance of finished cement.