The development history of industrial kilns
12,000-7,000 years ago, human beings entered the age of pottery. The early pottery was made of clay rich in silicate and aluminate, which was formed by adding water and fired under a flame. The firing temperature is generally related to the characteristics of the clay, and the sintering temperature of a good clay is 700-800 °C. But this temperature is slightly difficult for humans, mainly because the flame will exchange heat with the surrounding air, making it difficult to increase the temperature. In order to burn pottery, human beings had to start thinking about the problem of combustion and heat preservation, and the earliest kiln was born.
It can be said that the earth kiln was born very early, at least in the Neolithic Age. Judging from the current situation, at least 5,000 years ago, humans have already made a relatively complex kiln, which is what we call a horizontal cave kiln. The temperature of this kiln can be said to be more than enough for firing pottery. At the same time, improvements were also made to the fuel. Due to the low calorific value of wood, the subsequent kilns mainly used charcoal as fuel.
In the 2000 years after the appearance of the horizontal cave kiln, humans have further improved the kiln. This improvement is mainly in terms of ventilation. The early kilns were all naturally ventilated. When the air in the kiln is insufficient, the charcoal combustion is not sufficient. It is difficult to reach the predetermined temperature. In order to supplement the air in the kiln, the method of forced ventilation was adopted, so the bellows was invented, and the corresponding humans began to consciously process and manufacture the tuyere, and the temperature in the kiln was further increased. In the Shang and Zhou dynasties, human beings had reached a high temperature of more than 1000 ℃ in the kiln, which made it possible to smelt copper, iron and celadon.
Now let's talk about the difference between the development routes of eastern and western kilns. In fact, in terms of kilns, my country has been ahead of the times for a long time. As mentioned above, due to the difference in mineral resources, the use of inorganic materials in China and the West has moved towards two routes. Due to the abundance of kaolin and other resources, China has moved towards the skill of ceramics. In the West, due to the large number of volcanoes and crystals, it finally moved to the skill point of cement and glass. This has also indirectly led to two different directions of Eastern and Western kiln technology.
For a long time, the Western requirements for kilns were actually not high. First of all, the Western countries basically did not produce their own porcelain, and generally used pottery, iron and even used lead to prepare containers. Another important point is that Westerners accidentally discovered glass, and the smelting temperature of early glass was only 800 ℃, so for a long time, there has been little improvement in Western kilns.
China is different. During the Qin and Han Dynasties, the firing temperature of Chinese kilns could reach 1200-1300 °C. At the same time, the atmosphere in the kilns could be controlled during the firing process. The kiln shapes in this period were mainly dragon kiln and steamed bread kiln. This is generally related to the terrain. It is said that there are many plains in the north, so steamed bread kilns are generally the main ones, while there are many mountains in the south, and dragon kilns can be made directly by relying on the mountains.
After the development of the Long Kiln in the Song Dynasty, it gradually evolved into a stepped kiln, which is the so-called Dehua kiln. The Dehua kiln has 4-6 kiln chambers connected in series. It can also be understood as adding a partition inside the dragon kiln, dividing the dragon kiln into several sections. Compared with the Dragon kiln, the structure of the Dehua kiln has been improved, the kiln body can be made larger, and larger porcelain can be fired, and the thermal efficiency is also significantly improved. In the Southern Song Dynasty, Zhen kiln appeared in Jingdezhen, and the biggest feature of Zhen kiln was the appearance of thermal insulation layer. A layer of gap is made between the inner wall and the outer wall of the kiln, which is filled with air. The whole kiln body has a stronger thermal insulation capacity and less heat loss. During this period, the temperature in the kiln body can reach 1400 ℃.
Chinese kilns have successively experienced the evolution of the dragon kiln, the step kiln and the town kiln. It can be said that Chinese kilns are far ahead of their contemporary Western counterparts in terms of thermal insulation performance, atmosphere control, kiln structure and volume. But it is a pity. As mentioned earlier, kilns in my country are generally used to burn finished products, while Western kilns are generally used to burn powders. This ultimately leads to the development of kilns in my country for a long time. It is impossible to point out a key point. The skill point is continuous production. In continuous production, at least the kiln realizes the continuous inflow and outflow of materials, which is easier for powder materials to achieve. At the same time, it is easy to lead to other derivative results, such as fluidization technology. The continuous operation of the kiln to realize the forming material waited until the tunnel kiln appeared in the 20th century.
It is a pity that because China's basic science is very weak, although it was in the lead in production technology for a period of time, it soon encountered a bottleneck in development, and the development of Chinese kilns has not progressed since the Ming Dynasty. Not long after, an industrial revolution took place in Europe, which soon achieved corner overtaking in kiln technology.
First of all, from the middle of the 18th century, the West began to have demand for cement. At this time, cement needs to be fired. The earliest cement fired was a shaft kiln, which is used for burning lime. In 1877, the first rotary kiln appeared, and the cement industry was basically formed.
The emergence of the rotary kiln is not an overnight process. If you still have an impression, we will mention the equipment used in Lubran’s soda production when we talk about the soda production. This should be called a rotary calciner in a strict sense. There are still differences between rotary kilns. This is actually one of the prototypes of rotary kilns. In this system, the rotation of the kiln body and the continuous in and out of materials are realized. However, the temperature of the sodium bicarbonate decomposition process for soda production is only about 200 °C, so the structure of the kiln body is very simple, and there are no structures such as burners and refractory bricks. .
At the beginning, when the rotary kiln appeared, it was not directly used in the cement industry for two reasons. First of all, the cement industry probably began to develop at the end of the 18th century. When the rotary kiln first appeared, human beings only had a little understanding of the formula of cement. In addition, the cement firing temperature is high, and the technology of refractory brick lining is not yet developed, and the high temperature of 1400 ° C is not acceptable for metals at that time. In addition, the corresponding pulverized coal combustion technology also needs a certain accumulation. Therefore, the rotary kiln was not used in the cement industry until 100 years after its appearance.