As an important excipient, active lime with high activity and high added value is widely used, and its demand and quality requirements are constantly increasing. What are the quality requirements for active lime? As a professional manufacturer of lime calcination plant, we collect some parameters that are related to the quality requirements of active lime, Let’s find more useful things together.
Calcination equipment: lime kiln
Calcination is a key process in the active lime production line. Active lime calcination equipment mainly refers to the lime kiln, which is used for the calcination of lime ore. The choice of calcination equipment has a great influence on the quality of active lime. General mine products (0–30 mm) fine-grade limestone account for about 30–40% of the total production; if not fully utilized, this will seriously waste limestone resources.
At present, the active lime calcining equipment is different, and the limestone grain grade used is also different. vertical lime kiln and rotary lime kiln are two types of lime kiln commonly used in active lime production line.
A rotary kiln can directly calcinate 10–50 mm of fine-grade limestone so that most of the limestone below 40mm can be used, saving a lot of mining costs every year and extending the mining life of the mine. A lime rotary kiln is mainly composed of a kiln head, motor, kiln tail, shell, support device, rotary cylinder, nozzle device, feeding part, control system, energy-saving components, noise suppression circuit, and protection circuit. From the configuration, it can be seen that the lime rotary kiln has the advantages of low energy consumption and strong environmental protection performance. It has a good advantage and a good development prospect in making full use of limestone resources.
Granularity
In view of the particle size requirements of active lime, the calcination process of a rotary kiln is to ensure that in a stable temperature environment, it avoids underburning or overburning due to the uneven size of limestone particles, excessive grade difference, and uneven heat. Prevent the limestone accumulation in the container from staying in the process of uneven particle size, resulting in uneven air permeability or leading to poor airflow.
For converter steelmaking, the particle size requirement of active lime is to ensure the slag formation speed and effect in the steelmaking process with time requirements. If the particle size of lime is too large, the reaction time between lime particles and molten steel will be prolonged, the slag formation speed will be slowed down and the slag formation effect will be affected. On the contrary, if the particle size of the lime is too small, it is easy to cause particles or dust to splash during steelmaking and worsen the operating environment.
Activity
Activity refers to the ability of lime to react with water.
Activity degree refers to the rate of dissolution reaction between lime and water after mixing a certain amount and a certain particle size range 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 directly measure the reaction rate between lime and molten steel in the slagging process.
At the same time, it can determine the calcination quality of lime and guide the production by detecting the activity level. Therefore, it is necessary to test the activity of calcined lime products.
There are many ways to test the quality or activity of active lime. Among them, hydrochloric titration is the main method. In the process of calcination, it is quick and practical to analyze the calcined quality of lime by hydration comparison, hydration weighing and sampling.
SiO2 (silicon dioxide)
High CaO and low SiO2 are the basic requirements and guarantees for completing slag making in the steelmaking process. The purpose of slag making is to remove S and P from molten steel, especially S, and the alkalinity of slag is expressed by the ratio of CaO to SiO2. Higher SiO2 will damage the surface structure of lime, affecting the slag making speed and effect.
During the calcination process of lime, the melting point of pure SiO2 can reach as high as 1713 ℃. However, at 700-800 ℃, SiO2 undergoes a secondary reaction with CaO in a solid state. As the reaction progresses, CaO · SiO2 (calcium metasilicate), 3CaO · 2 SiO2 (calcium silicate), 2CaO · SiO2 (dicalcium silicate), and 3CaO · SiO2 (calcium trisilicate) can be generated in sequence. The impact of these products on lime leads to a decrease in activity.
S (sulfur) P (phosphorus)
The main purpose of using activated lime to produce high alkalinity slag during converter steelmaking is to remove sulfur and phosphorus from the molten steel.
When there is a high content of P phosphorus in steel products, it will increase the cold brittleness of the steel at room temperature (i.e. when P>0.13). That is to say, it causes cracking of the steel.
When the sulfur content in steel products is too high, it can significantly damage the welding performance of the steel, reduce the impact toughness of the steel, especially cause cracks in the steel during heating, rolling or casting, which is called “hot brittleness”. And it can significantly reduce the corrosion resistance (rust) and wear resistance of steel. So, the harm of sulfur to steel products is known as “termite”.
Due to its characteristic of combining with sulfur, especially at high temperatures, lime has a particularly strong ability to absorb sulfur. So, the role of lime in removing sulfur from steel is very significant.
However, due to the influence of raw materials, fuel sulfur content, and high-temperature calcination factors on the inherent existence of limestone, the lime generated from limestone itself will also contain varying degrees of sulfur, phosphorus, and other components. Therefore, there are low value requirements for the sulfur and phosphorus content of the lime itself. And there are also low value requirements for the low sulfur and phosphorus content of its former limestone (raw material) and fuel.
Residual CO2 (carbon dioxide)
The so-called residual CO2 actually refers to the unburned core or sandwich in the lime particles, and the residual inner layer of lime that has not been completely decomposed. The content of CO2 in lime is mainly controlled through calcination. It has a significant impact on the quality of lime and the effectiveness of steelmaking.
