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The core secret of cement manufacturing: How does the cement kiln transform raw materials into precious cement?
Cement is one of the indispensable materials in modern construction, with an annual production volume of more than one billion tons. Behind this glory, the cement kiln is the core of its production process and plays an irreplaceable role. The cement manufacturing process is complex and energy-consuming, and cement kilns are large energy consumers in the process and one of the main sources of carbon dioxide emissions. With global expectations for sustainable development, improving the efficiency of cement kilns has become an important research direction in cement manufacturing technology.
The core function of a cement kiln is to convert raw materials into cement clinker. The effectiveness of this process is directly related to the quality and cost of the final cement product.
Cement clinker manufacturing process
The cement manufacturing process can be divided into three main stages. First, limestone and other clay or shale are ground to produce a fine "raw material mixture." The mixture is then heated in a cement kiln to temperatures of up to 1450°C and finally ground into cement. During this process, as the temperature of the raw materials gradually rises, a variety of chemical reactions occur one after another.
At 70 to 110°C, water evaporates; at 400 to 600°C, the minerals in the clay decompose into their oxides; at 650 to 900°C, calcium carbonate reacts with silicon dioxide to form belite, It is then regenerated into alite, the main component of Portland cement.
This series of reactions ultimately converts the raw material into clinker, forming particles with a diameter between 1 and 10 mm. This hot clinker is then dropped into a cooler, which successfully recovers most of the heat energy and cools it down to 100°C for further storage and processing.
The evolution history of cement kilns
As early as 1825, the manufacture of Portland cement clinker had begun, and most of the equipment at that time used modified static lime kilns. This design has not changed significantly in the past half century. The capacity is limited by the weight of raw materials. Usually, each batch can only produce up to 30 tons of clinker, and the output is only 1,500 tons per year.
In 1885, the design of the continuous kiln appeared, and this concept finally achieved a breakthrough in 1890. The successful development of the rotary kiln greatly improved the efficiency of cement production.
Operation principle of rotary kiln
Today, the operation of rotary kilns has significantly improved cement production efficiency. The kiln consists of a steel cylinder lined with refractory bricks, and the entire system is slightly tilted and rotates continuously. The raw material is fed into the upper end of the kiln and rotation causes it to gradually travel downwards along the kiln. The lower end of the kiln is equipped with a fuel feed port, where the fuel will burn to form a circular flame that generates a large amount of heat.
Early successful rotary kiln designs originated mainly in Pennsylvania, and with the evolution of technology, the production capacity of modern rotary kilns has been significantly increased, reaching a daily clinker output of 10,000 tons.
Comparison between wet and dry methods
In the manufacture of cement, there are two different processing methods for raw material preparation: wet method and dry method. The wet process traditionally consumes more fuel to evaporate the water in the feedstock, while the dry process directly mixes the gas with the solid feedstock.
Modern preheating system
With the advancement of science and technology, the introduction of preheating systems has greatly improved the thermal efficiency of cement kilns. Among them, the chain preheater preliminarily dries the raw materials with higher efficiency, and then enters the kiln for further calcination and clinker generation.
The airflow suspension preheater uses a cyclone separator to perform efficient heat exchange on the feed material, thereby further improving the performance of the kiln.
Impact on the environment and future challenges
However, despite the continuous advancement of cement production technology, a large amount of carbon dioxide emissions are still generated in the process, accounting for approximately 2.5% of global unnatural carbon emissions. As the world attaches great importance to climate change, the pursuit of low-carbon cement production processes has become the focus of the industry's future development.
The cement industry needs to more actively explore alternative fuels and materials to meet today's society's environmental impact requirements and make changes to future burdens.
The development of cement kilns not only reflects the progress of industrial technology, but also involves all aspects of environmental protection and sustainable management. While pursuing more efficient and environmentally friendly production methods, how will the cement industry meet this challenge in the future?
