Lime is a necessary auxiliary material in the steelmaking process. Its quality will directly affect the quantity and quality of the steel being refined. Therefore, in the metallurgical enterprises, the quality of lime is very important. China is the earliest country to produce and use lime. The Qin Great Wall and many archaeological discoveries have confirmed this indisputable fact. Although China is a big energy country, due to its backward technology, especially the old kiln type and soil-burning lime kiln are of large pollution, poor quality, high energy consumption, and low output, they cannot meet the quality requirements of steelmaking on the white ash, and the mechanization of the world Compared with automatic calcination, the gap is quite large. At present, 70% of China's ash kiln is an earthen kiln without any environmental protection measures and is protected by local protection. However, various types of serious industrial pollution problems in various regions have caused the country’s great attention, and therefore the elimination of soil Burning the lime kiln, building our own modern white ash kiln with energy saving, environmental protection and high efficiency is not only a requirement for national environmental protection but also an imperative measure for hundreds of thousands of lime production enterprises in China. The following is a simple analysis of lime raw materials, calcined fuels, calcining equipment, and processes.
一、Raw limestone
       Limestone (limestone) is a kind of rock formed by sedimentary sources. It belongs to alkaline rocks and its main component is calcium carbonate, calcium magnesium carbonate or a mixture of calcium carbonate and magnesium carbonate. According to different density ranges, formed limestone is divided into three subcategories
Low density limestone----Density range 1,760~2,160 kg/m3
Medium density limestone----Density range 2,160~2,560 kg/m3
High density limestone---- Density greater than 2,560 kg/m3 
    Limestone has good processability, polishing properties and good cementation properties. It is insoluble in water but easily soluble in saturated sulfuric acid, and can react with strong acids and form corresponding calcium salts, while releasing CO2. When limestone reaches above 900°C at atmospheric pressure, the following reaction occurs: CaCO3>900°C=CaO+CO2
1、 Limestone Impurities
As limestone (CaCO3) is a natural mineral, it contains some impurities that are not conducive to calcination. Impurities are mainly SiO2 AL2O3 Fe2O3 K2O P S, etc., which are mainly derived from three aspects
⑴、Raw limestone contains harmful substances:SiO2、Al2O3、Fe2O3、Na2O、K2O 、P、S,etc
⑵、Limestone pests in the form of soil and sand: SiO2、Al2O3、Fe2O3、Na2O、K2O 、P、S, etc
⑶、Fuel contained harmful substances:SiO2、Al2O3、 Fe2O3、Na2O、K2O 、P、S,etc
The quantity of these impurities directly affect the production and quality of calcined lime, so the choice of high-quality limestone is more critical.
2、Limestone granularity
In the limestone calcining process, the influence of the grain size of the raw limestone is very large. Since the separation of CO2 is carried out slowly on the surface of the limestone, large-diameter limestone is difficult to calcine with a smaller particle size and takes longer time. Through scientific experiments, the calcination time is proportional to the square of the limestone thickness at a certain temperature. Compared with the 40mm stone block, the 80mm block needs 4 times calcination time of the latter. It has been proved that the limestone with 40-80mm particle size is used in the shaft furnace, and the limestone with a particle size of 10-30mm is most suitable for the rotary kiln.
3、Limestone selection
  Limestone has a high density of calcium, which is not easy to burn, but calcined lime is of good quality. Conversely, limestone with low calcium content has a low density and is easy to burn, but the calcined gray quality is poor. Limestone calcium content and other material content using chemical analysis tests, wear experiments and calcination experiments to obtain accurate results. It is chosen to use after careful judgment. High-quality limestone generally has a CaO content of over 52%, a MgO content of less than 3%, and SiO2 of less than 1%.
二、fuel
There are many kinds of limestone calcined fuels. Among them are solid, liquid, gas and so on.
The solid fuel is mainly coal and its processed products. Coal is an important energy and chemical raw material, and its variety is numerous. Limestone calcined coal has the best heat content and low S content. However, the use of coal as fuel has a high cost of pollution.
Liquid fuel is petroleum and its processed products. Most of them are atomized by a spray device and diffused and burned. However, it is generally not used due to the high cost of production and pollution.
There are many types of gaseous fuels, such as coke-produced gas, producer gas, natural gas, blast furnace gas, and converter gas. Gas fuels help increase heat efficiency and save energy. Compared with coal-fired gas, gaseous fuels can increase thermal efficiency by a factor of 2 and fuel-by-fuel ratio by a factor of two; furthermore, gaseous fuels help protect the atmosphere and reduce NO x and SO 2 emissions. Therefore, it is recommended to select gas as the fuel for calcined limestone.
⑴The gas-fired lime kiln saves energy, especially the use of remaining gas in the blast furnace and residual gas in the coke oven, as well as other remaining gases in various industrial furnaces are the greatest energy saving and utilization.
⑵Benefiting environmental protection, because the gas-fired kilns do not emit a large amount of harmful gases after combustion of solid fuels, and the gas originally discharged into the atmosphere also reduces the emission of pollutants into the atmosphere after being fueled, so it is very beneficial to environmental protection.
⑶The furnace temperature is even and the calcined lime is of good quality. Gaseous fuel can be burned in all the voids of limestone. There is no dead angle, and the use of coal as a fuel will cause the temperature to be high or low. Because the gas-burning flame is even and at the same time it emits heat, rapid combustion and rapid cooling can be achieved, so the lime activity is good.
⑷Easy to detect, good operation. Because the gas kiln's temperature, gas, air flow and pressure can all be detected by the instrument, the furnace operator can know the furnace condition and adjust the furnace condition according to the test data. Unlike the solid fuel, it is not easy to control. Even if the furnace is stopped, the gas continues to burn. The gas-fired kiln has a short regulation cycle and the solid fuel kiln has a long regulation cycle.
The most ideal gas fuels are blast furnace and coke oven gas, but the coke oven gas has a high calorific value and must be detarred, so it is not easy to control the use.
The high pressure air blower (Roots blower) is blown into the blast furnace after being heated by a hot blast stove. This hot blast and coke help to generate carbon dioxide and carbon monoxide. Carbon dioxide and hot coke produce carbon monoxide, and carbon monoxide rises. The iron element in iron ore is reduced to become pig iron. This is the chemical process of iron making. Molten iron is temporarily stored at the bottom of the furnace and is periodically discharged for direct steelmaking or ingot casting. At this time, there is a large surplus of carbon monoxide in the blast furnace gas. This mixture gas is "blast furnace gas." This kind of gas containing flammable carbon monoxide is a kind of low calorific value gas fuel, its calorific value is generally only 3140-3560KJ/M3, can be used for self-use gas of metallurgical enterprises, such as heating hot-rolled steel ingot, preheated steel ladle, etc. Wait. Blast furnace gas is a by-product of the ironmaking process. The main components are: CO, C02, N2, H2, CH4, etc. The content of combustible CO is about 25%. The composition and calorific value of blast furnace gas are related to the fuel used in the blast furnace, the type of pig iron produced, and the smelting process. The gas from the blast furnace contains a lot of dust, which can clog gas pipelines and equipment during transportation and use. Therefore, it must be dusted before it can be transported and used. Most steel mills use gravity dust collectors and bag dust removal. The ignition point of blast furnace gas is not high, it seems that there is no fire obstacle, but in the actual combustion process, due to various factors, the temperature of the mixed gas must be much greater than the ignition point in order to ensure the stability of combustion.
三、Ash Calcination Equipment - White Ash Kiln
There are many kinds of old furnace type lime kiln. Such as soil kiln, ordinary shaft furnace type, rotary kiln and so on. The conventional shaft furnace type kiln, in which the earth kiln and the use of coal as the calcined fuel, is a phase-out equipment. Most of the furnaces used today are gas-fired shaft furnaces and rotary kilns. Ordinary gas-fired shaft furnace type lime kiln is divided into 25, 50, 140, 180, and 300M3 according to furnace capacity. The smaller the easier it is to grasp. Now take the 140M3 ordinary gas-fired shaft furnace ash kiln for analysis:
The outer diameter of the ring-shaped base wall is 3.95 meters, wall thickness is 400mm, upper standard is 4.5m, the outer diameter of the steel kiln shell is 4.29m, and the highest point of the top elevation is 26.55m, that is, the height of the metal kiln shell is 22.05m, and the top of the inclined bridge is 31.25m (level ) Kiln internal diameter 3m, material column height 20m, effective volume 140m3. Six thermocouples were installed, with a position of 5.6m, two 10.8m, two 14.8m, and one 23.5m. Burner nozzles were located at an elevation of 13.3m and nozzles (burners) were located at 16 positions. Height 10.8m; Combustion air surrounding pipe 16-sided, 325mm diameter, opposite to the center of the burner, cooling air surrounding the pipe 300*250; explosion-proof hole ф 250,250 to disperse the ground operation; cooling fan, combustion fan 6.3A, 30Kw. Hoist 3 Ton 18-22Kw.
Design requirements: raw material particle size 40-80mm, gas calorific value 860*4190KJ/m3, flow rate 7000m3/hour, design utilization factor> 0.6.
The ratio of gas to air is between 1:2 and 1:2.5. The cycle time of the furnace is calculated and the ash discharge time is adjusted according to the cycle. The ash discharge interval of 130 cubic lime shaft kiln is generally at, such as the gas pressure, the air pressure is At 10 kilopascals, put about 7 tons of ash on a one-time ash in 60 minutes. Keep the material surface stable and the positive and negative values do not exceed 10 cm.
Excerpt from "Metallurgical Information Equipment Network"