Activity is an important indicator of lime for slagging. The activity of light burnt lime is related to the chemical composition of lime, the facies structure of lime and the calcination process conditions. The chemical composition and facies structure are the intrinsic factors affecting the activity of calcined lime. Conditions, and calcination process conditions are extrinsic conditions that affect the activity of calcined lime. Due to the difference of origin and mineralization conditions, the chemical composition of lime is different, especially the composition and content of the associated impurity minerals have certain influence on the activity and subsequent use of calcined lime. The ore-forming conditions affect the composition, distribution and grain size of limestone facies, thus affecting the activity of light burnt lime. These are all intrinsic conditions affecting calcined lime.
The calcination conditions are the process conditions used in the lime calcination process, including: calcination temperature, holding time, heating rate, and particle size of calcined limestone. These conditions can be studied to grasp the law of influence, so that the calcination process conditions can be adjusted according to requirements to obtain the products we need. This paper mainly studies the influence of external conditions on the activity of calcined lime. In order to explain the difference in calcination activity of limestone in different producing areas under the same conditions, the microstructure of the limestone ore and the microstructure of the product are explained.
1 experiment
Three kinds of limes from different producing areas were selected for the study, which were represented by the origins A, B and C respectively. The light burning experiment was carried out in a box type electric resistance furnace, and the heating rate was 5 ° C·min -1 , and the temperature was raised to the corresponding temperature for heat preservation, and the natural temperature was lowered after the holding time.
The effect of calcination temperature on the activity of light burnt lime was studied at a holding time of 90 min. The calcination temperatures were 950 ° C, 1000 ° C, 1050 ° C, 1100 ° C, 1150 ° C and 1200 ° C, respectively. The effects of holding time on the activity of light burnt lime were studied at 1050 ° C and 1100 ° C. The holding time was 30 min, 60 min, 90 min and 120 min, respectively.
The lime after light burning is rapidly cooled, and a sample with a particle size of less than 10 mm is prepared, and 50 g each time is taken into 1000 mL of distilled water, and phenolphthalein is used as an indicator, and titrated with a concentration of 4 N hydrochloric acid under stirring of an electric mixer. The red color disappears within 10 min, and the total volume of hydrochloric acid consumed (mL) is the activity of lime. The microstructure of the sample before and after calcination was observed using a scanning electron microscope of the type VEGATS 5136XM.
2 Results analysis and discussion
2.1 Effect of calcination temperature on the activity of light burnt lime
Table 1 shows the effect of calcination temperature on the activity of lime after light burning of limestone in three different producing areas, and the holding time is 90 min. It can be seen from Table 1 that the activity of lime after three kinds of limestone light burning first increased first, and after reaching the highest value, the activity decreased.
The lime of the place A is lightly burned at 1000 °C, and the activity reaches the highest value, while the limestone B and C both reach the highest value at 1050 °C. The two lime activities have slightly different trends with the light burning temperature.
Limestone B is lightly burned from 950 ° C to 1050 ° C, and the activity is kept at a high value. When the temperature exceeds 1050 ° C, the activity decreases rapidly.
The C activity of limestone changes slowly with the calcination temperature, and maintains a high activity value at 1000~1100 °C.
For this reason, it can be considered that limestone C has a wide calcination temperature range, and temperature fluctuation has little effect on the activity of lime after light burning, so that the calcination process is easier to control. The difference in activity after light calcination of different limes under the same calcination regime depends on the intrinsic conditions of the lime, ie its composition and structure, which will be explained later from the microstructure. 
Table 1 Effect of calcination temperature on the activity of light burnt lime
2.2 Effect of holding time on the activity of light burned lime
Table 2 shows the effect of calcination time on the calcination product activity of the three limestone samples at two calcination temperatures. It can be seen from Table 2 that calcination at 1050 ° C for 90 min has a higher activity of light calcined limestone; while calcination at 1100 ° C, the calcination activity of the same limestone is also higher in a shorter time. Calcination temperature and holding time have a great influence on the activity of lime. In the limestone calcination process, CaCO3 first decomposes, but at the same time is accompanied by recrystallization of CaO and crystal growth. Heat transfer affects both processes. When the calcination temperature is low, the temperature gradient is small and the heat transfer rate to the center is slow. In this case, if the holding time is short, the decomposition temperature of CaCO3 is not easily reached at the center, and the decomposition of CaCO3 is incomplete, and the phenomenon of unburning occurs, and the activity of lime is low. When the temperature is high, the temperature gradient is large, the center is easy to reach a higher temperature, and CaCO3 decomposes rapidly. At the same time, the recrystallization of CaO and the growth rate of crystals are also faster. In this case, if the holding time is too long, the CaO crystal develops more completely, and it is easy to form CaO of larger crystal grains, which converts the active calcium oxide into inactive calcium oxide, resulting in a decrease in the activity of lime. It can be seen from Table 2 that limestone C has little change in calcination time with calcination time at both calcination temperatures of 1050 ° C and 1100 ° C, and maintains high activity, which is not sensitive to holding time, while limestone B As the holding time changes, its activity changes rapidly and is sensitive to the holding time. As can be seen from Table 2 and the above discussion, for the same limestone calcination, in order to obtain higher activity, a lower calcination temperature and a longer holding time can be selected; or a higher calcination temperature and Shorter holding time. Due to the different origin of raw materials, the activity of limestone calcined products is different from the holding time. Therefore, in the process of tissue limestone calcination, the light burning process system should be adjusted according to the origin.
                                    Table 2 Effect of holding time on the activity of light burnt lime
2.3 Effect of raw material particle size on the activity of light burnt lime
Table 3 shows the effect of the particle size of the three limestone raw materials on the activity of the calcined product at 1050 ° C for 90 min. It can be seen from Table 3 that the activity of calcined products of limestone large particles in the three producing areas is higher than that of the small particle calcined products. The decrease in the activity of the small particle calcined product under the calcination condition may be due to the excessive burning of the small particle calcined product, so that the product grains are increased and the porosity is lowered, thereby affecting the activity of the lime. Different results may occur with changes in calcination temperature and calcination time.
Table 3 Effect of limestone particle size on the activity of light burnt lime
2.4 Influence of the microstructure of limestone ore on the activity of light burnt lime
Figure 1 shows the microstructure of three limestone ore. It can be seen from Fig. 1 that the limestone A and B grains are finer and the limestone B grains are more uniform; while the limestone A grains are finer but not uniform, and there are larger size grains. The limestone C grains are coarser, and smaller grains are embedded in the middle of the large grains. The grain of limestone C is much larger than limestone A and B. It can be seen from the fracture morphology that the limestone grain size is C>B>A.
Figure 1 Microstructure of three limestone ore
Figure 2 shows the microstructure of lime after light burning of three limestones at 1050 ° C for 90 min. It can be seen from Fig. 2 that the sintering of the limestone A calcined product grains is severe, so that the porosity of the sample is lowered, the specific surface area is decreased, and the activity is lowered. The calcined product of limestone B has fine crystal grains, lighter sintering between crystal grains, large specific surface area and high activity. The limestone C light-sintered product has fine crystal grains, but the crystal grains have a certain adhesion due to sintering, and there are certain pores between the adhered crystal grains. According to the activity data of lime after light burning at 1050 °C for 90 min, it can be seen that the sintering between the crystal grains is better, and the light-burning lime with more adhesion is generated, and the activity is lower; while the crystal grains are fine, the sintering between the crystal grains is better. Light, its activity is higher.
Figure 2 Microstructure of three limestones after light burning (1050 ° C, 90 min)
It can be seen from the above experimental and analytical results that the grain size of limestone ore has a certain influence on the activity of light burnt lime, but there is no simple correspondence between the grain size and the activity of the product after light burning. Therefore, it cannot be considered that the ore crystal The particle size is large, and the activity of the light-burning product is low; the grain size of the ore is small, and the activity of the light-burning product is high. Because the activity of light burnt lime is related to the grain size, it is also related to the degree of sintering. The degree of sintering is also related to factors such as grain size, impurity concentration and impurity distribution.
3 Conclusion
From the above experimental research and analysis, the following conclusions can be drawn.
1) Under a certain calcination time, as the calcination temperature increases, the activity of limestone calcined product increases first with the increase of calcination temperature, and then decreases. When calcined at a certain temperature, the activity reaches a maximum.
2) The holding time at a certain calcination temperature has a great influence on the activity of the calcined product.
3) The particle size of the limestone has an effect on the activity of the calcined product.
4) The microstructure of the limestone ore also has a great influence on the activity of the calcined product.
Zhang Bin, Yin Hongfeng
School of Materials and Mineral Resources, Xi'an University of Architecture and Technology
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