RU2048664C1 - Rotary furnace for heat treatment of aluminium fluoride - Google Patents

Abstract

FIELD: rotary furnaces. SUBSTANCE: rotary furnace has housing, metal shell arranged inside housing and secured to it and heat- insulating layer in circular space formed by furnace housing and metal shell; furnace is also provided with devices for delivery and discharge of materials and discharge of flue gases. Metal shell is made in form of plain sections inter- connected by means of corrugated sections at ratio of length of plain section to length of corrugated section equal to (3-12):1; used as heat-insulating layer is powder-like material at heat conductivity factor not exceeding 0.4 W/m°C at block coefficient of circular space equal to 0.85 to 0.98. Furnace housing is provided with hatches for loading the heat-insulating material and safety valves for discharging the gas-phase components separated from heat-insulating powder. Furnace may be additionally provided with heat exchanger mounted in unloading portion of housing. EFFECT: production of aluminium fluoride of high quality; increased service life of furnace due to compensation for thermal expansion and enhanced reliability of heat insulation. 2 cl, 2 dwg

Description

 The invention relates to equipment designed for heat treatment of materials, such as aluminum fluoride, and in particular to rotary kilns.

Known rotary kiln for drying and firing of bulk and pasty materials, containing a drum and a heat exchanger placed inside it, made in the form of a flexible pipe, in which the flexible pipe is made of thin sheet steel with a thickness of about 1 mm and consists of smooth long and short corrugated sections. The diameter of the flexible pipe is equal to the diameter of the drum, the flexible pipe is attached at the ends to the wall of the drum [1]
However, the described design provides for the heat treatment of the product due to the contact of the flexible pipe with the surface of the drum, which is heated externally, which leads to rapid erosion wear of the inner pipe made of sheet steel. Also, furnaces are not acceptable in large-scale industries, for example, in the production of aluminum fluoride.

The closest to the described design in terms of technical nature and the achieved result is another known rotary kiln comprising a housing and a metal shell placed coaxially inside it attached to the furnace body, and a heat-insulating layer placed between the furnace body and the metal shell. In this design, the metal shell is made in the form of separate facing sheets installed with a gap relative to each other along their perimeter. The guiding elements are made in the form of two plates, covering on both sides the junction of the facing sheets, while the plates are mounted on supporting posts in the form of rods rigidly fixed to the furnace body [2]
The disadvantage of the design is that such a design of the metal shell can cause the lining material to enter the product (Al fluoride), contaminate it and, accordingly, make it impossible to obtain a high-quality product. In addition, the furnace is difficult to install and replace the lining.

A rotary furnace for heat treatment of aluminum fluoride is proposed, comprising a housing and a metal shell placed coaxially inside it attached to the furnace body, and a heat-insulating layer placed between the furnace body and the metal shell, a device for input and output of material and flue gases, in which the metal shell is made in the form of smooth sections interconnected by corrugated sections with a ratio of the length of the smooth section to the length of the corrugated section equal to (3-12): 1, respectively, in quality ETS insulation layer taken powdered material with thermal conductivity not greater than 0.4 W / m˙ ^C at a filling factor of the annular volume formed by the wall of the housing and the metal shell, 0.85 0.98, and the housing of the rotary kiln is provided with hatches for loading insulation material and safety valves for the release of gas-phase components released from the thermal insulation.

In order to obtain aluminum fluoride with a high content of the main substance (95 98 wt.% AlF ₃ ), the furnace is additionally equipped with a conductive heat exchanger from the material outlet side.

The invention consists in the following. The quality of aluminum fluoride has rather stringent requirements for the presence of impurities in it. Therefore, it is necessary to organize the heat treatment process in such a way that extraneous mechanical impurities, such as lining material residues, do not get into the product. In this regard, the metal shell must be airtight. On the other hand, because the process proceeds at temperatures of the material 400 550 ^о С and flue gases 1000 1200 ^о С, it is natural that the sealed metal shell undergoes thermal deformation. To compensate for thermal expansion at a length of 50 m, which is achieved at a specified temperature of at least 250 mm along the length of the furnace, it is proposed that the metal shell be made of alternating smooth and corrugated sections with a certain ratio of their lengths. This ratio of thermal deformation is determined based on, on the one hand, from the full compensation of thermal expansion, and on the other hand, from the minimum number of welds, it is l ₁ l ₂ (3-12): 1, where l _{1 is the} length of the smooth section, l ₂ corrugated length.

As the heat-insulating material is a powdery material with a low coefficient of thermal conductivity (no more than 0.4 W / m˙ ^C at the maximum temperature of the inner wall). The choice of material with a certain coefficient of thermal conductivity is determined by the need to obtain the outer body surface temperature not higher than 40 90 ^° C as such materials can be used, for example, expanded perlite, vermiculite, etc. At a degree of filling of the annular space defined by the metal shell and the housing wall, 0.85 0.98. With a smaller filling of the annular space due to the rather active movement of the material, conditions are created for convective heat transfer from the heated shell to the furnace body, which, in turn, leads to an increase in the temperature of the external wall of the body. An increase in fill factor above 0.95 will require the use of fine powder, which is practically not feasible. The coefficient of thermal conductivity of the powder, the degree of filling of the annular space with it are determined in each case, based on the properties of the powder and the fineness of its grinding, within the limits stated in the application materials.

 The body of the rotary kiln, which provides for the use of powdered insulating material, is equipped with hatches for filling it. Filling is done by feeding the material through the hatches and periodically turning the furnace, distributing it evenly throughout the volume. Since the powdery lining layer can emit gaseous components during operation of the furnace, it is provided that the furnace body is provided with safety valves through which these components are removed from the thermal insulation.

 The use of powders as the lining material makes the design reliable and easy to use, as it allows you to quickly replace or backfill the insulation without dismantling the inner shell. Moreover, such thermal insulation allows you to constantly keep the required temperature of the outer wall of the housing.

For aluminum fluoride with a high content of the basic substance necessary to avoid contact of the combustible flue gases with the heated material to a temperature of 450 ⁵⁵⁰ C. In this case, from the material output can be set any conductive heat exchanger of known construction.

 In FIG. 1 is a longitudinal section through a rotary kiln for heat treating aluminum fluoride. The furnace consists of a housing 1, inside of which there is a metal shell, consisting of smooth 2 and corrugated 3 sections. The annular volume 4 formed by the wall of the housing and the metal shell is filled with a powdery heat-insulating material. Smooth sections of the metal shell are attached to the housing by elements 5. The loading end of the furnace is equipped with an opening 6 for introducing the wet material and exhaust gases. The wet material enters through estrus 7, and the exhaust gases enter the junction box 8. The discharge end of the furnace has an unloading box 9 and a junction box 10 for flue gases. The furnace body is equipped with hatches for filling the insulating material 11 and valves 12 release of gas-phase components released from the thermal insulation.

 In FIG. 2 shows the discharge end of a furnace equipped with a conductive heat exchanger 13.

 Preparation of the furnace for operation is as follows.

First, open the hatches 11 to fill the insulating material, fill it up, close the hatches and turn the furnace. The furnace is stopped, the material is again poured into the hatches and turned again. The operation is repeated until the annular volume 4, formed by the housing 1 and the metal shell, is filled until its fill factor is 0.85 0.98. After that, flue gases are fed through the junction box 10 into the internal volume of the furnace, and wet aluminum fluoride is fed into the furnace through the heat supply 7. Due to the countercurrent movement of the flue gases and the wet material, the product is dried and calcined. During the heat treatment the metal shell is heated to a temperature of 200 ^{500 °} C depending on the heat treatment zone. In this case, axial thermal expansion of the shell occurs. The elongation of smooth sections of the metal shell 2 is compensated by the compression of the corrugated sections 3, as a result of which the shell rigidly welded to the ends of the furnace and in the places where the shell is attached to the cylindrical surface of the housing by the elements 5 is not destroyed. The used dry thermal insulation provides the possibility of unhindered radial thermal expansion of the shell. The heat-treated material moves through the furnace volume and is discharged through the unloading box 9. The exhaust gases through the opening 6 enter the junction box 8 and are sent to the absorption system by a fan. If the rotary kiln on the discharge end side is equipped with a conductive heat exchanger 13, the flue gases are fed into the heat exchanger, and the calcined material moves in the gap between the shell and the outer wall of the heat exchanger. When the gas phase accumulates in the insulation material, it is thrown out through the safety valves 12.

 Using the proposed design allows to obtain high-quality fluoride aluminum, significantly increase the service life of the furnace by compensating for thermal expansion and the reliability of thermal insulation. The furnace is easier to operate due to the ease of replacement and updating of the insulating layer. The operation of the furnace ensures sanitary standards, as the temperature of the outer wall of the furnace body is easily adjustable.

Claims (2)

1. ROTATING FURNACE FOR THERMAL TREATMENT OF FLUOROUS ALUMINUM, comprising a housing mounted inside it coaxially with a gap, a metal shell attached to the housing, and a thermal insulation layer in the annular gap, as well as a device for introducing and discharging the processed material and heating gases, characterized in that the shell is made of alternating smooth and corrugated sections with a ratio of the length of the smooth section to the length of the corrugated (3 12) 1, the thermal insulation is made of powder material with a coefficient of t thermal conductivity of not more than 0.4 W / m · ^o C, with an annular gap filling factor of 0.85 0.98, while the casing is made with hatches for loading thermal insulation material and safety valves for releasing gas components released from the thermal insulation powder.  2. The furnace according to claim 1, characterized in that it is equipped with a heat exchanger installed in the discharge part of the housing.

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