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The amazing chemistry of cement kilns: What happens at 1450°C?
In the cement production process, cement kiln plays an indispensable role. Cement kilns are the core part of the production of more than one billion tons of cement each year. Not only are these kilns a major source of energy consumption and greenhouse gas emissions, improving kiln efficiency has long been a focus of cement manufacturing technology. The amazing chemical reaction that takes place at temperatures of 1450°C makes the production of cement possible and efficient.
Cement clinker manufacturing process
The manufacturing process of cement clinker can be divided into three stages. First, limestone is mixed with clay or shale and ground into a fine powder. This “raw material mixture” is then fed into a cement kiln where it is fired at temperatures as high as 1450°C. Finally, the resulting clinker is ground into cement.
At temperatures of 1450°C, a chemical reaction combines the calcium carbonate in the limestone with silica-containing minerals to form a mixed calcium silicate, which is what gives cement its properties.
The process of key chemical reactions
When the raw material mixture enters the cement kiln, various chemical reactions occur successively as the temperature gradually rises:
- 70 to 110°C: Free water evaporates.
- 400 to 600°C: Clay minerals decompose into their component oxides.
- 650 to 900°C: Calcium carbonate reacts with SiO2 to form belite (C2S).
- 900 to 1050°C: The remaining calcium carbonate decomposes into calcium oxide and carbon dioxide.
- 1300 to 1450°C: Partially melts, belite reacts with calcium oxide to form alite (C3S), which is also the main component of Portland cement.
Ultimately, the molten state at up to 1450°C causes the materials to combine into masses of about 1-10 mm in diameter, called clinker.
History of cement kiln development
The history of cement dates back to 1825 when Portland cement clinker was first manufactured in static lime kilns. As technology developed, continuous kiln designs began to appear from 1885, eventually developing into the rotary kiln used today, which now accounts for more than 95% of the world's cement production.
The wonderful structure of the rotary kiln
The rotary kiln consists of a steel cylinder lined with refractory material. The cylinder has a slight slope and will rotate slowly at a speed of 30 to 250 times per hour. The raw materials enter at the high end of the cylinder and gradually descend to the other end of the kiln as it rotates. When the fuel burns, the flame formed heats the raw materials to a high temperature, and finally pushes the clinker into the cooler for cooling.
The design of the rotary kiln revolutionizes cement production efficiency, allowing it to run 24 hours a day, 365 days a year, and only stop for necessary maintenance.
Modern technology and environmental protection challenges
While modern cement kilns have become technologically very efficient, they still have an impact on the environment. Cement plants account for around 2.5% of global non-natural carbon emissions. As a result, many manufacturers are looking for ways to reduce emissions, such as by utilizing waste materials or developing new fuel systems. Additionally, many companies are focusing on improving the calcium hydroxide process, trying to find more environmentally friendly alternatives.
Future prospects
As the world’s need to reduce carbon emissions increases, the cement industry may move towards a more sustainable future. The application of advanced technologies, such as preheaters and pre-burning furnaces, will further improve efficiency and help reduce CO2 emissions.
So, as environmental awareness continues to grow, what innovative measures will the cement industry take in the future to solve current challenges and achieve sustainable development?
