Kilns are used for high-temperature processing of industrial and building materials. Such equipment can have different designs, sizes and their operational features. The drum or rotary kiln occupies a separate place in the segment, providing opportunities for efficient drying of bulk raw materials.
Unit design
Industrial models of drum kilns are predominantly formed by steel pipes with brick refractory lining. A prerequisite for the layout is to ensure the possibility of rotation of the cylinder around its axis at a speed of 30-250 rpm. Accordingly, the larger the diameter of the drum, the lower the rotation speed. The movement is provided by a shaft fixed on a bearing rack with rollers made of heat-resistant metal. Thermal impact is provided in the process of combustion of fuel materials (gas, oil, gasoline or solid raw materials), which is placed in a separate chamber. In some versions, the rotary kiln contains heat exchange devices that implementauxiliary processes of roasting and drying.
Oven operation principle
A cylindrical container in the form of a drum has a slight inclination relative to the horizontal - this is the initial position from which the movement begins. But before switching on, the cavity of the structure is filled with working material. The workpiece is fed through the upper nozzle of the drum. Next, the operator closes the structure and turns on the electric motor. During operation, the rotary kiln cyclically lowers the stirred substance down, blowing hot gases over the mass. The admission of thermal flows can be made through an external firebox, but in classic models, gas generation is carried out inside the drum. In the second case, a Bunsen burner can be activated, which forms flames through the pipes of the furnace nozzle. Such tasks require an additional source of fuel in the form of oil, gas, crushed coal or wood chips.
Heat treatment zones
Throughout the entire working cycle, the material being served can meet with furnace gases several times under different temperature conditions, which determine one or another state of the processed mass. Depending on the characteristics of heat treatment in the furnace, the following zones are distinguished:
- Drying zone. The space of this part is about 25-35% of the total capacity of the drum. Gases at a temperature of about 930 ° C provide the processes of moisture evaporation.
- Heating zone. In this part, processing is carried out with flows with a temperatureup to 1100 °С. Heating is carried out against the background of heat transfer from the combustion product with the possible support of third-party chemical reactions.
- Temperature softening zone. The temperature treatment mode in this zone can be 1150 °C. The main task of this part of the rotary kiln is to ensure complete combustion of excess air in the open structure of the material.
- Cooling zone. At this stage, the target material is exposed to cold streams and hardens. Some of the metal granules of the billet may also be oxidized here with a brownish-red hue.
Technical and operational features of the equipment
In itself, the rotation of the unit with the movement of the contents of the material increases its efficiency and the quality of firing. Especially advantageous is the use of long tubular structures, thanks to which the consumption of thermal energy is minimized. The longer the drum, the denser the granules interact with the furnace gases during their movement inside the tank. Accordingly, unproductive heat losses are also minimized. It is worth noting the uniformity of firing, which also affects the quality of heat treatment of bulk materials. For example, a rotary kiln for cement raw materials in the form of crushed gypsum and clinker allows the mass to be sintered so that a homogeneous structure is obtained. Sometimes several raw material groups are combined with the addition of calcium silicates, limestone and clay. The drum in the process of rotation forms an almost singleproduct consistency.
Calculation of the heat output of the furnace
For uniform firing of the material, it is necessary to ensure its movement along the entire length of the furnace with an optimal speed regime. The pace of movement, on the one hand, must create the conditions for performing the necessary reactions, and on the other hand, it must not hold the mass in a state of crystallization, otherwise the already acquired technological properties will be lost. Optimum power balance can be achieved by choosing the right motor.
At a basic level, the calculation of a rotary kiln is based on the residence time of the material in the heat treatment vessel - with a dry method, the intervals are on average 1.5-2 hours, and with a wet method, 3-3.5 hours. Also should be taken into account the time to complete the firing process, which in the case of dry processing will be about 1 hour, and for wet firing - 1.5 hours. As for power, an electric motor is provided for performing standard tasks, the power potential of which varies from 40 to 1000 kW in case of with industrial units. Specific indicators are also determined taking into account the connection of auxiliary communications, the nature of the strapping and the inclusion of modifying components in the main fired composition.
Rotary kiln lining
In addition to the selection of optimal operating parameters, maintenance will also affect the quality of firing. One of the key activities aimed at maintaining high technicalperformance indicators of the furnace, will be its lining. In essence, this is the isolation of the metal surface of the drum using a heat-resistant material. The thermal insulation function is effectively performed by cast refractory concrete and bricks. But even after lining, the rotary kiln must be coated with protective coatings that protect the structure of the same concrete from the propagation of small cracks. The lining itself is made with a thickness of 8 to 30 cm, depending on the size of the furnace structure. Refractory should be calculated for temperatures of the order of 1000-1200 ° C.
Conclusion
Kilning units today are widely used in the manufacture of building mixtures, tile materials and all kinds of consumables that require drying. The advantages of rotary kilns include high productivity and quality of thermal effects, but operation is not complete without drawbacks. This equipment is characterized by large dimensions, massive working bodies and a low level of automation. To this it is worth adding the requirements for power supply. In full-cycle production, drum ovens are connected to 380 V networks, as well as to ventilation and cooling systems.
