For high-temperature processing of industrial and building materials, kilns are used. Such equipment can have different designs, sizes and its operational features. A drum or rotary kiln occupies a separate place in the segment, providing the possibility of efficient drying of bulk materials.
Unit design
Industrial models of drum furnaces are mainly formed by steel pipes with brick refractory lining. A prerequisite for the layout is the ability to rotate 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 means of a shaft fixed on a support rack with rollers made of heat-resistant metal. The thermal effect is provided during the combustion of fuel materials (gas, oil, gasoline or solid-state raw materials), which is placed in a separate chamber. In some implementations, the rotary kiln comprises heat exchange devices that implement auxiliary processes of firing and drying.
The principle of operation of the furnace
The cylindrical capacity 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. In the process, the rotary kiln cyclically lowers down the mixed substance, pouring the mass of hot gases. Thermal fluxes can be tolerated through an external firebox, but in classical models gas is generated inside the drum. In the second case, a Bunsen burner can be used, which forms flames through the pipes of the furnace nozzle. Such tasks require an additional fuel source in the form of oil, gas, crushed coal or wood chips.
Heat treatment zones
Throughout the entire working cycle, the serviced material can several times meet with furnace gases under different temperature conditions that determine one or another state of the treated mass. Depending on the characteristics of the heat treatment in the furnace, the following zones are distinguished:
- Drying area. The space of this part is about 25-35% of the total drum capacity. Gases at a temperature of about 930 ° C provide moisture evaporation processes.
- Heating zone. In this part, processing occurs in streams with temperatures up to 1100 ° C. Heating is carried out against the background of heat transfer from the combustion product with the possible support of third-party chemical reactions.
- The zone of temperature softening. The temperature treatment 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 flows and solidifies. Some of the metal granules of the workpiece can also undergo oxidation operations with the acquisition of a brownish-red hue.
Technical and operational features of the equipment
By itself, rotating the unit with moving the contents of the material increases its efficiency and firing quality. Especially beneficial is the use of long tubular structures, due to the device of 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 the heat treatment of bulk materials. For example, a rotary kiln for cement raw materials in the form of crushed gypsum and clinker allows sintering of the mass so that a homogeneous structure is obtained. Sometimes several raw material groups are combined with the addition of silicates of calcium, limestone and clay. The drum during the rotation process forms an almost uniform consistency of the product.
Calculation of the thermal power 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 mode. The pace of movement, on the one hand, should create the conditions for the necessary reactions to be performed, and on the other hand, not to delay the mass in the crystallization state, otherwise the already acquired technological properties will be lost. An optimal power balance can be achieved with the help of the correct selection of an electric motor.
At a basic level, the calculation of a rotary kiln is based on the residence time of the material in the heat treatment tank - with the dry method, the intervals are on average 1.5-2 hours, and with the wet method 3-3.5 hours. Also take into account the time to complete the firing process , which in the case of dry processing will be about 1 hour, and with wet firing - 1.5 hours. With regard to power, an electric motor is provided for standard tasks, the power potential of which varies from 40 to 1000 kW in the case of 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 calcined composition.
Rotary kiln lining
In addition to selecting optimal performance indicators, maintenance will also affect the quality of firing. One of the key works aimed at maintaining high technical and operational performance of the furnace will be its lining. In essence, this is the insulation of the metal surface of the drum with a heat-resistant material. The heat-insulating function is effectively performed by cast refractory concrete and brick. 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 at temperatures of the order of 1000-1200 ° .
Conclusion
Firing units today are widely used in the manufacture of building mixtures, tile materials and all kinds of raw materials that require drying. The advantages of rotary kilns include high productivity and quality of heat exposure, but operation is not without its drawbacks. This equipment is characterized by large size, massive working bodies and low level of automation. To this, it is worth adding the requirements for power support. In full-cycle plants, drum kilns are connected to 380 V networks, as well as to ventilation and cooling systems.