How does a rotary kiln work?
Robust design with high-tech finesse
In the chemical industry, the rotary kiln's robust design is contrasted by highly complex chemical and material processes taking place inside the drum. The result is a large set of factors that must be taken into account for process and plant design. Consequently, there are many rotary kiln modifications and countless optional or necessary applications that complement the process. In the following, the product path through the rotary kiln will be outlined in general terms to generate an initial understanding of potential challenges.
Prior to the actual heat treatment, the educt is fed into the furnace. Complexity and the number of necessary conveyors and sensor technology, as well as additional equipment, depend strongly on the material properties, the furnace's operating mode, and the conditions on-site. The filling can take place at discrete times (batch operation) or continuously (continuous operation). In addition, material properties such as transportability or reactivity determine the suitability of different applications. A feed screw, a dosing system, and chambers for calibrating the process atmosphere frequently realize this function.
The furnace head functions as a system boundary to the process chamber and has ports for various devices for process monitoring and control. These include, for example, thermocouples, optics, and process gas lines. Seals are used to prevent a potentially safety-critical exchange of atmosphere between the process or heating chamber and the environment. The complexity of the sealing system depends strongly on the application. For processes with high requirements regarding atmospheric separation, ONEJOON uses a specially developed and patented dust- and gas-tight multi-chamber system, which reliably operates at over- and negative pressure. This allows the use of protective gas atmospheres such as N2, H2, or O2 as well as the heat treatment of input materials that are harmful to health.
Heat treatment process in the rotary kiln
The actual heat treatment process takes place in the drum (process chamber). Here the material is heat-treated for a defined residence time and according to characteristic thermal profiles that need to be determined. In batch processes, the rotary kiln is filled at discrete intervals before the thermal profile is being followed at a certain rotational speed of the drum. For continuous operations, the mass throughput is defined by the length and diameter of the drum, rotational speed, angle of inclination, and internal fittings geometry.
Both process types can be operated at temperatures up to 1200 °C, depending on the drum material and heating type. In special cases, temperatures up to 2000 °C are also possible. Depending on the process, the material and gas flow are either equally-oriented (co-current flow) or reciprocated (counter-current flow). For high-quality materials, the furnace is heated indirectly by either electric heaters, gas burners, or an upstream combustion chamber. Insulation, which is located behind the heating elements, enables an ideal heat transfer to the pipe and minimizes heat emissions to the environment.
An electric motor with a chain drive realizes the characteristic rotational movement of the rotary kiln. The drum is supported by a running ring, which is mounted on the kiln's circumference and runs on rollers. The rollers are fixed to the furnace frame via corresponding bearings. The frame consists of a stable steel framework, which gives the system the necessary mechanical qualities.
After the heat-treatment, the product must potentially be chilled down. The cooling zone consists of either a cooling drum or alternative indirectly cooled conveyor technology, such as a water-cooled vibration channel. Suppose a drum is used, then an open system with a medium flowing around the tube is typically employed. In industrial practice, water or air functions as a cooling medium. The dimensioning of the cooling zone depends primarily on the material's target temperature, the mass throughput, and the product's specific heat capacity.
On the discharge side, a furnace head and a sealing system are again used as system boundaries to the environment. The furnace head has a product discharge shaft through which the heat-treated product leaves the furnace. Depending on the process requirements, the product ejection shaft can be supplemented with additional equipment for atmosphere separation and further processing.
As illustrated, the supposedly simple design of the rotary kiln is contrasted by complex process handling, which significantly influences the product quality and the possible mass throughput. Therefore, the equipment complementing the rotary kiln is critical for high product quality and comfortable operation. This is why ONEJOON systematically develops solutions to optimize the design of rotary kilns. At some customer plants, we installed mobile furnace heads and sealing systems saving 25-30% of the time required for maintenance work. Other applications include temperature measurement of the product or a self-cleaning pipe.
Covers and protectors
If particularly strict limits (e.g., high OEB levels) for contamination or material emissions regarding process or heating chamber have to be complied with, a closed extraction system outside the furnace can be used to complement the sealing system. This ensures the safety of the plant even in the event of a potential incident.