Classification and inventory of common industrial kilns in the powder industry

Industrial kilns refer to thermal equipment that utilizes heat generated by fuel combustion or conversion of electrical energy in industrial production to smelt, melt, bake (calcin), heat, retort, gasify, etc., materials or workpieces. There are many types of industrial kilns with diverse uses. In metallurgy, chemical industry, non-ferrous metals, building materials, machinery, light industry, advanced ceramics, and other industries, different industrial furnaces are used for different industrial purposes. Usually, a specific kiln has more than one characteristic. Therefore, the same kiln has different classification points and multiple classification methods and naming methods. For example, a continuous sintering furnace is both a sintering furnace and a continuous furnace.

In industrial kilns, common heat sources include solid fuels, liquid fuels, gaseous fuels, and electrical energy conversion. These heat sources generally fall into two categories during their formation: those existing in flame form and those existing in non-flame form. Therefore, based on the heat source, kilns can be classified into two types: flame furnaces and electric kilns.

1. Flame Furnaces

A furnace that uses the flame of fuel combustion as its heat source is called a flame furnace. The flame inside the furnace can directly contact the material for heating, with temperatures generally below 2000℃. During combustion, the furnace wall radiates heat and partially reflects projected heat, playing a crucial role in heat exchange. However, sometimes, to prevent oxidation of the material (workpiece), the flame needs to be separated from the material, with the flame's heat indirectly transferred to the material through a partition wall. Flame furnaces can be used to both heat and melt materials.

A high-temperature downdraft kiln is a type of flame furnace, consisting of three parts: the kiln body (round or rectangular), combustion equipment, and ventilation equipment. In operation, fuel is added to the grate of the combustion chamber, air is introduced into the ash pit, passes through the grate, and combusts with the coal bed. The flame and combustion products are ejected from the nozzles formed by the fire baffle and kiln walls to the top of the kiln, then reflected downwards to the bottom, and discharged into the flue through the suction holes, branch flues, and main flue. As the flame flows over the products, its heat is transferred to the products through convection and radiation.

Flame furnaces are characterized by rapid melting and high output, but they also suffer from low thermal efficiency and environmental pollution from exhaust gases.

Therefore, the use of flame furnaces should ensure:

(1) complete combustion of fuel under specified heat load conditions;

(2) stable combustion process, capable of continuously supplying heat to the furnace;

Currently, flame furnaces are commonly used in traditional ceramics, industrial ceramics, glass products, refractory materials, new material sintering, and metal heat treatment.

2. Electric Furnaces

Electric kilns generally convert electrical energy into heat energy through electric heating elements. Based on the method of current generation, they can be further divided into five categories: resistance furnaces, induction furnaces, electric arc furnaces, electron beam furnaces, and ion furnaces. Since there is no fuel combustion, there is no need for ventilation equipment such as air supply and exhaust systems. Their structure is simple, they occupy a small area, and the products inside the kiln are not affected by flue gas or ash. Electric kilns can also achieve temperatures exceeding 2000℃, which is difficult to achieve with flame kilns. Temperature and atmosphere are also easier to precisely control and monitor, resulting in high-quality fired products. They are often used for the production and processing of high-quality, special materials, such as artificial graphite anode materials for lithium-ion batteries.

However, to ensure uniform temperature distribution across the kiln cross-section, the cross-sectional size is usually limited. Therefore, electric kilns are not suitable for firing large, thick-walled products, and their output is relatively low. Furthermore, the auxiliary electrical equipment of electric kilns is more complex, and the consumption of electric heating elements is higher, leading to higher equipment and operating costs.

Industrial kilns are essential thermal equipment in industries such as metallurgy, chemicals, building materials, ceramics, glass, electronics, and food. Classified by industry process, they can be categorized into sintering furnaces, debinding furnaces, calcining furnaces, drying furnaces, graphitization furnaces, and vulcanization furnaces. In the advanced ceramics industry, debinding furnaces and sintering furnaces are commonly used thermal equipment in the production process.

1. Debinding Furnace

New ceramics often incorporate a significant amount of organic binders and plasticizers during forming, such as paraffin wax in hot-press casting and polyvinyl alcohol in roll forming and tape casting, resulting in high internal porosity. Direct sintering causes a large amount of organic matter in the ceramic body to melt, decompose, and volatilize, leading to deformation, cracking, and adhesion of the body, affecting the yield and thermal conductivity of the substrate. Furthermore, the high carbon content of these organic materials, when insufficient oxygen is present to create a reducing atmosphere, negatively impacts sintering quality. Therefore, before firing, the ceramic body is typically placed in a debinding furnace and baked at a temperature lower than that used for sintering to remove the organic matter, ensuring the product meets shape, size, and quality requirements.

2. Sintering Furnace

The debinding ceramic green body can be sintered in a sintering furnace. In the sintering furnace, due to the high temperature, the solid particles of the ceramic green body bond together. Grains grow, porosity and grain boundaries gradually decrease, and through mass transfer, the overall volume shrinks, density increases, and finally, a dense polycrystalline sintered body with excellent mechanical properties is formed.

During the sintering process, sintering temperature and sintering time are the most important factors affecting ceramic sintering. They largely determine the size and distribution of grains in the ceramic. The sintering atmosphere and sintering pressure in the furnace can affect the number of pores inside the ceramic, thus affecting the degree of densification. Therefore, these parameters in the sintering furnace should be precisely controlled and monitored during sintering to ensure that the sintered product does not crack or deform.

In addition to kilns that separately perform debinding and sintering processes, integrated furnaces can often be used with pre-set temperature profiles to meet the needs of integrated production of multiple processes.

In the product manufacturing process, industrial kilns mainly operate in two modes: continuous and intermittent (or periodic).

1. Intermittent Kilns
The characteristic of intermittent kilns is that the furnace chamber is not divided into temperature zones. Materials are manually loaded into the furnace in batches for heating or melting. After the heating or melting process is completed inside the furnace, the materials are manually unloaded in batches. The furnace charge does not move within the furnace, and the furnace temperature changes over time. Common types include pit furnaces, box furnaces, bell furnaces, and shuttle furnaces.

① Bell Furnace
A bell furnace generally consists of a bell jar, a base, and a lifting structure. The heating element is installed on the inner wall of the furnace jar. Its characteristic is that when the blank is fired and cooled to a certain temperature, the bell jar can be quickly transferred to another base to begin firing blanks in another furnace. Therefore, it is very suitable for sintering components that are easily damaged during movement. Generally speaking, it has good sealing performance, stable and uniform temperature and atmosphere inside the furnace, low heat loss, and high thermal efficiency.

② Box Furnace

As the name suggests, a box furnace resembles a box in shape, with heating elements distributed along the inner wall of the furnace chamber. During production, the blanks are placed on a support plate and then fired together in the center of the furnace chamber. After one batch of blanks is fired, they are removed from the furnace for the next batch to be loaded and fired. Box furnaces are highly versatile and suitable for the heat treatment of single pieces and small batches of various shapes. However, due to the poor sealing of the furnace opening and door, heat loss is significant, resulting in uneven temperature distribution. The temperature near the furnace door is lower than the temperature at the center of the furnace chamber, so it usually cannot operate at full capacity, leading to low production efficiency. This can be addressed by adding heating elements to the furnace door or installing a fan.

③ Pit Furnace

Most pit furnaces have a circular structure, and workpieces are vertically loaded into the furnace for heating using specialized cranes or other tools. The furnace body is constructed with high-temperature refractory fiber, which reduces heat loss and saves energy. Its thermal efficiency and sealing performance are higher than box furnaces, but it is not as convenient to operate and maintain.

④ Shuttle Kiln

The shuttle kiln's structure resembles a matchbox. During operation, the blanks to be fired are pushed into the kiln by kiln cars. After firing, they are pulled out in the opposite direction to unload the fired blanks. Because the kiln cars move back and forth within the kiln like a shuttle, it is named a shuttle kiln. The unique structure of the shuttle kiln makes it well-suited for small-batch, multi-variety production, offering great flexibility in production methods and scheduling. Furthermore, loading, unloading, and partial cooling of the finished products can be carried out outside the kiln, improving working conditions and shortening kiln turnaround time. However, due to the kiln cars constantly moving in and out of the kiln, heat storage and dissipation losses are significant, resulting in high flue gas temperatures and high heat consumption. This drawback is usually mitigated by adding a waste heat recovery device.

2. Continuous Kilns

In a continuous kiln, the temperature at various points within the furnace remains relatively constant over time. The green body passes sequentially through the preheating, high-temperature, and cooling sections of the furnace at a controlled speed, completing the sintering process. This type of sintering furnace allows for uninterrupted continuous sintering, is very simple to operate, and has high thermal efficiency, resulting in large production volumes and uniform product quality. However, it requires significant investment and its thermal regime is not easily adjusted, so it is mostly used for products with high output and a relatively limited product variety. Common continuous kilns in the powder industry include tunnel kilns and roller kilns.

① Tunnel Kiln

The structure of a tunnel kiln resembles a long, straight tunnel, with trolleys running on tracks laid at the bottom of the tunnel. The combustion equipment is located in the middle of the tunnel kiln, forming the high-temperature firing zone. An induced draft fan or chimney is installed at the front of the kiln to guide the high-temperature flue gas from the firing zone to the kiln head, preheating the green body to be fired, forming the preheating zone. At the kiln tail, cold air is typically blown in to cool the fired products, forming the cooling zone. Since most of the waste heat from the high-temperature flue gas in the firing zone can be utilized, it is more energy-efficient than intermittent kilns. However, some heat is still wasted because cooling is required at the kiln tail.

② Roller Kiln

A roller kiln places the product directly or indirectly on closely spaced rollers. Each roller is driven by a chain via a sprocket at its end, causing the rollers to rotate continuously. This rotation of the rollers transfers the product from the kiln head to the kiln tail. Because the temperature difference during preheating, firing, and cooling is extremely small, production time is very short. Furthermore, roller kilns have much better airtightness than tunnel kilns, resulting in better energy efficiency. However, it also has a drawback: firing high-temperature reduction products requires high-quality rollers. Silicon carbide rollers are generally used, which effectively handle the firing of high-temperature ceramic products up to 1350℃.

③ Pusher Kiln

The bottom of a pusher kiln is paved with precisely laid tracks made of sturdy refractory bricks. A pushing mechanism pushes refractory plates containing the ceramic blanks into the kiln, successively passing through the preheating zone, firing zone, and cooling zone. Sometimes, to reduce friction, ceramic balls can be placed under the refractory plates. Because the principles and advantages of pusher kilns are similar to those of roller kilns, pusher kilns are limited by the thrust-bearing capacity of the refractory plates, and are generally not very long, ranging from a few meters to twenty meters in length. They are generally used for firing high-temperature powder or special ceramics, and their daily output is not large.

With technological advancements and the continuous implementation of environmental protection policies, industrial kilns are evolving from low-end to high-end models. They have progressed from being characterized by low output and quality, high fuel consumption, high labor intensity, low firing temperatures, and inability to control the atmosphere, to higher output and quality, lower fuel consumption, higher firing temperatures, controllable atmosphere, and mechanization and automation. Besides the classifications mentioned above, kilns can also be categorized by temperature (high-temperature, medium-temperature, and low-temperature) and by working system (radial, convection, and layered working systems). Production enterprises can choose appropriate industrial kilns based on their product's specific requirements regarding processing technology, atmosphere, and output to improve production efficiency and product quality.