1. Particle size
According to the particle size requirements of active lime, for the calcination process of rotary kiln, it is to ensure that the calcination process of rotary kiln can avoid under burning or over burning due to uneven particle size of limestone, excessive grade difference and uneven heating. To prevent the limestone from accumulating and staying in the container due to uneven particle size, resulting in uneven air permeability or poor gas flow.
For converter steelmaking, the requirement of particle size of active lime is to ensure slagging speed and effect in steelmaking process with time requirement. If the particle size of lime is too large, the reaction time between lime particles and molten steel will be prolonged, which will slow down the slagging speed and affect the slagging effect. On the contrary, if the lime particle is too small, it will easily cause particles or dust splashing during steelmaking, which will worsen the operating environment.
The so-called activity refers to the reaction ability of lime with water.
The activity refers to the speed of dissolution reaction between lime and water after mixing a certain amount of lime with a certain temperature and a certain amount of water. It represents the ability of lime to react with other substances (impurities) in molten steel. Because it is very difficult to measure the reaction rate of lime with molten steel during slagging.
At the same time, it can judge the quality of lime calcination and guide the production by detecting the activity. Therefore, it is necessary to test the activity of calcined lime products.
There are many methods to detect the quality or activity of active lime. Among them, hydrochloric acid titration is the main method. In the process of calcination, hydration comparison method, hydration weighing method and sampling method are used to judge the calcination quality of lime. For example:
1) Titration method
Take out a number of lime samples from the kiln, crush them, sieve them with a 1 mm pore size sieve, and then sift through a 5 mm aperture sieve. Select 50 g of lime with a particle size of 1-5 mm, dissolve it in 2000 ml water at 40 ± 1 ℃ and stir it. Add phenolphthalein as indicator in the solution, and titrate with 4N HCl (4 g equivalent hydrochloric acid) for 5-10 minutes.
At this time, the volume consumption of HCl (ML) reaching the end of titration is the activity of the tested lime sample. According to the theoretical calculation, the highest activity index of pure active Cao is 446ml.
The theoretical activity of pure active Cao is calculated as follows:
Molecular weight: Ca = 40.08 O = 16.00 H = 1.008 Cl = 35.45
Solution: it can be seen from the activity test method of active lime Cao coarse particle titration method,
CaO + H2O = Ca（OH）2 （1）
Ca（OH）2 + 2HCl = CaCl 2 + 2 H2O （2）
（1）+（2）: CaO + 2 HCl = CaCl 2 + H2O （3）
X = 50×72.92 / 56.08 = 65.01
Because: a liter of 4 N HCl solution contains 145.84 grams of HCl
Therefore, 4N HCl solution can be prepared from 65.01g HCl
65.01÷145.84×1000 = 445.79 ≈ 446 ml
2) Hydration weighing method
In the absence of chemical reagents:
a. Take some lime samples and weigh them, and the weight is g 1.
b. Dry the weighed sample in water and let it be fully digested.
c. Filter lime water, collect insoluble residue, dry and weigh it as G 2.
d. The digestion part of the reaction was calculated: G 1-g 2 = G 3.
e. The lime decomposition rate (G 3 △ G 1) × 100% can reflect the calcination quality of lime.
3) Hydration comparison method
Take out some lime clinker from kiln, cool it, put it in a container, add water to dissolve it, pour the lime solution and residue into the screen, wash away the residual lime with water, observe the size of the residue particles and compare with the amount of lime clinker taken to judge the calcination quality.
4) Sampling and tapping method
When the kiln is cooled, some lime particles are red in appearance. The surface texture of lime particles is clean and the color is white. The particle weight is light. When the lime particles are knocked with a hammer, the texture is loose and easy to be broken, with obvious nucleation and small volume.
3. SiO2 (silica)
High Cao and low SiO2 are the basic requirements and guarantee for slag forming in steelmaking process. The purpose of slagging is to remove s and P from molten steel, especially S. the basicity of slag is expressed by the ratio of Cao to SiO2. Higher SiO2 will destroy the surface structure of lime and affect the slagging speed and effect.
During the calcination of lime, the melting point of pure SiO2 can be as high as 1713 ℃. However, when the temperature is 700-800 ℃, the secondary reaction will occur between SiO2 and Cao in solid state. With the reaction going on, Cao · SiO2 (calcium metasilicate), 3CaO · 2SiO2 (wollastonite), 2CaO · SiO2 (dicalcium silicate) and 3 Cao · SiO2 can be produced successively The effect of these products on lime is to reduce the activity.
4. S (s) P (P)
In converter steelmaking, the main purpose of making high basicity slag with active lime is to remove sulfur and phosphorus from molten steel.
When the content of P and P in steel products is too high, the cold brittleness of steel will increase at room temperature (i.e. P > 0.13). That is to say, the cracking of steel is caused.
When the content of sulfur in steel products is too high, it can obviously damage the weldability of steel, reduce the impact toughness of steel, especially make the steel produce cracks during heating, rolling or casting, i.e. “hot brittleness”. The corrosion resistance (corrosion resistance) and wear resistance of the steel are obviously reduced. Therefore, the harm of sulfur to steel products has the name of “termite”.
Because lime has the characteristics of combining with sulfur, especially when lime is at high temperature, the ability of lime to absorb sulfur is very strong. Therefore, the effect of lime on the removal of sulfur from steel is very large.
However, due to the influence of sulfur content of raw materials, fuel and high-temperature calcination factors on limestone itself, the lime generated from limestone will also contain sulfur and phosphorus in different degrees. Therefore, the sulfur and phosphorus content of lime itself is required to be low. For the former, the low sulfur and phosphorus content of limestone (raw material) and fuel is required to be low.
5. Residual CO2 (CO2)
The so-called residual CO2 actually means that there is no burnt out core or sandwich in the lime particles, and there is no residual lime inner layer completely decomposed. The content of CO2 in lime is mainly controlled by calcination. It has a great influence on the quality of lime and the effect of steelmaking.
a. Small or no nucleation: the surface of lime particles is easy to sinter and produce over burning, and the characteristics of activity will be destroyed.
b. Excessive hearting: it will undoubtedly affect the effective decomposition of lime, resulting in insufficient formation of lime characteristics and reduction of activity.
In the process of C and steelmaking, if the residual CO2 is too high, the amount of scrap will be affected, the heat consumption will be increased, the utilization rate of lime will be reduced, and it is also difficult to control the slag and spatter.
Therefore, while strictly controlling the degree of lime calcination, attention should also be paid to the waterproof work during the storage and transportation of the calcined lime products, so as to reduce the pulverization rate.
d. Conversion of CO2 content:
The decomposition reaction of limestone by heating is the reaction of removing CO2. According to the decomposition equation of CaCO3, the results show that the decomposition of limestone is a reaction of removing CO2. When CaCO3 = 100, Cao = 56, CO2 = 44.
100÷44 = 2.272
When CaCO3 of 1.785 kg is needed to produce Cao per kg unit,
1.785÷2.272 = 0.79 m3
0.79÷1.97 = 0.4 Nm3 / kg
It can be concluded that when CaCO3 is 1.785 kg to produce kg of Cao, the amount of CO2 produced is 0.4 Nm3.