RU205306U1 - OVEN FOR DRYING AND HEALING OF FLUORIDE ALUMINUM TRIHYDRATE PASTE - Google Patents

Abstract

The utility model relates to a furnace for drying and calcining fluoride aluminum fluoride trihydrate (TFA) paste. The furnace contains a furnace, a cylindrical body with loading and unloading chambers installed at the ends and placed on supports with the possibility of rotation, a drive that provides rotary movement of the body, while the body is made of two cylindrical parts of different diameters and lengths - a convective part and made in the form of a tubular heat exchanger the conductive part of the furnace, rigidly connected to each other by a conical insert, while it is equipped with a receiving-screw, paddle and radial nozzle, sequentially installed in the convective part of the furnace from the side of the loading chamber, a return screw for returning dust taken from the furnace to the calcination zone, a central pipe, the diameter of which is greater than the diameter of the heat exchanger pipes, installed along the axis of the conductive part of the furnace with the formation of an annular space and made with open ends and the ability to change the flow section from the side of the unloading chamber, buckets for moving material from the convective part to the annular space in the conductive part of the furnace and annular baffles installed in the annular space of the conductive part of the furnace to divide the annular space into sections, spaced tubes installed at the discharge end of the furnace for unloading the calcined product, the height of which determines the degree of filling the shell space with material, and in the upper part of the unloading chamber a fitting is installed to remove the steam-gas generated in the shell side of the heat exchanger during the material calcination, while the furnace is located at an angle of 90 ° C to the axis of the furnace body. EFFECT: intensification of heat-mass exchange processes of removal of free and crystalline hydrate moisture from TFA paste during its drying and calcination. 1 ill.

Description

The device is intended for drying and calcining of aluminum fluoride trihydrate paste and is used in the chemical industry. The furnace can be used in other industries for drying and calcining other materials, if the temperature and other parameters of the process are close to the accepted ones, and the corrosion resistance of the furnace's structural materials corresponds to the new conditions.

Anhydrous aluminum fluoride (AlF ₃ ) is used in the production of aluminum in electrolysis baths. The process of obtaining aluminum fluoride from fluorosilicic acid can be described by the following sequence of reactions (1-4).

In the process of crystallization of aluminum fluoride in reaction (2), aluminum fluoride trihydrate (TFA), stable under standard conditions, is formed. TFA is separated from the solution by filtration, after which it is dried and dehydrated.

After precipitation of crystals of aluminum fluoride trihydrate, filtration and washing, the paste with a moisture content of 15 ... 20% (in terms of free moisture) is sent to the drying and calcination operation to remove free and crystalline hydrate moisture from the product.

Numerous gravimetric studies have established some features of the process of dehydration of aluminum fluoride trihydrate. When the material is heated to a temperature of 100 ... 105 ° C, free (surface) moisture evaporates, at a material temperature of 230 ... 250 ° C, 2.5 H ₂ O molecules of crystalline hydrate moisture are removed, the removal of 0.5H ₂ O requires significantly more heat energy and heat the material to a temperature of 450 ... 600 ° С. Moreover, the quality of the final product obtained is influenced by the conditions under which the dehydration process is carried out at the last stage. With the direct effect of flue gases on the material at the last stage of dehydration, in parallel with dehydration, the decomposition reaction of the product proceeds according to reaction (5):

In the industrial production of aluminum fluoride, various combinations of devices are used for these purposes: drying drum + tube dryer, drying drum + fluidized bed dryer, drying drum + calcining furnace [1]. The most commonly used furnaces are B3-45. Such furnaces were used in the production of aluminum fluoride at the South Ural cryolite plant (Kuvandyk, Orenburg region), at the Polevsky cryolite plant (Polevskoy, Sverdlovsk region), and are currently operated in the aluminum fluoride shop (CFA) at JSC PhosAgro (Cherepovets, Vologda region).

The B3-45 furnace with an outer diameter of 3.0 m and a length of 45 m is made in the form of a cylindrical body with a constant diameter along the entire length, equipped with loading and unloading chambers, support bands with support stations, a drive and a furnace located, as a rule, along the axis ovens. The inner surface of the furnace is partly or along its entire length lined with heat-resistant and corrosion-resistant stainless steel.

Structurally, the furnace is made in accordance with the requirements imposed by the standards for the designs of furnaces for chemical plants with general-purpose rotating drums [2].

The disadvantages of this furnace design should be pointed out.

1. Since the inner surface of the furnace is not equipped with any weighing nozzle and the material predominantly moves in the lower sector of the furnace, in the so-called “heap”, the contact surface of the material-flue gases phases is poorly developed. As a result, the length of the furnace sector increases, where free and 2.5H ₂ O molecules of crystalline hydrate moisture are removed.

2. Low intensity of interaction of phases material - flue gases leads to incomplete utilization of the heat of flue gases and they leave the furnace with a temperature of 250 ... 300 ° C. To reduce the temperature of the exhaust gases from the furnace, it is necessary to additionally install recuperative heat exchangers.

3. Direct contact of the material with flue gases at the final stage of high-temperature calcination of the material leads to partial decomposition of the product and a decrease in its quality.

4. At the unloading end of the furnace, the body temperature is increased due to its direct contact with flue gases with an initial high temperature of about 800 ... 900 ° C, which affects the reliability of the support located in this zone.

5. There is an increased loss of heat through the walls of the furnace into the surrounding space.

6. For the operation of the furnace, three support stations are required, which significantly increases the complexity of the furnace maintenance.

The low intensity of interaction of the material with the flue gases requires an increase in the residence time of the material in the furnace and an increase in its length, and poor use of the heat of the flue gases leads to the need to increase its temperature at the inlet and, therefore, at the outlet.

The closest to the proposed one is a heat-mass transfer apparatus containing a rotating drum with zigzag tubular elements located along the drum in diametrical planes and connected at the ends to the inlet and outlet distribution chambers, and a system of coolant inlet and outlet pipes [3]. The disadvantage of this design is that due to temperature stresses and corrosive effects, the pipes collapsed along the welded seams and the calcined material spilled into the annular space. An attempt to isolate the material to be calcined from flue gases by passing it through zigzag pipes was technologically justified, but for technical reasons it did not find application in industrial production when calcining TPA.

Tasks that are solved in the claimed furnace:

- increasing the reliability of the furnace;

- increasing the intensity of heat and mass transfer processes;

- reduction of the mass-dimensional dimensions of the furnace;

- reduction of natural gas consumption for the process of drying-calcining of TFA paste.

The set tasks are solved by the fact that a furnace for drying and calcining aluminum fluoride trihydrate paste is declared, in which the body is made of two cylindrical parts of different diameters and lengths - convective and conductive, rigidly connected to each other by a conical insert, and in the convective part they are installed in series, starting from the charging end of the furnace, the receiving screw L ₁ , the blade L ₂ and the radial L ₃ nozzles, the return screw for returning to the calcination zone of the dust carried away from the furnace, and the conductive part is made in the form of a tubular heat exchanger; buckets are installed to move material from the convective part to the conductive part, for unloading the calcined product, distance tubes are installed at the discharge end of the furnace, the height of which "H" determines the degree of filling the intertube space with the material, and in the upper part of the discharge chamber there is a fitting for steam-gas removal formed in the shell space of the heat exchanger during material calcination.

The design of the inventive furnace in the conductive part provides for the installation of a central pipe of a larger diameter, in comparison with the heat exchanger pipes, with open ends, with the possibility of changing the flow section from the side of the unloading chamber.

The annular space of the conductive part is divided into sections by installing several annular baffles.

In the proposed device, the firebox is located at an angle of 90 ° to the axis of the furnace body.

FIG. 1 shows the external structural elements of the furnace. Here: I - convective part of the furnace, II - conductive part of the furnace, III - conical insert; 1 - support at the loading end of the furnace, 2 - support at the discharge end of the furnace, 3 - furnace drive, 4 - unloading chamber, 5 - loading chamber, 6 - chute, 7 - return pipe of the captured dust, 8 - flue gas outlet, 9 - steam gas outlet, 10 - firebox, 11 - flue gas inlet to the unloading chamber, 12 - manhole.

FIG. 2 shows the internal structural elements of the furnace. Here: 13 - receiving screw nozzle, 14 - lifting blade nozzle, 15 - radial nozzle, 16 - return auger, 17 - buckets, 18 - tube sheet, 19 - annular partition, 20 - central pipe, 21 - pipes for passage flue gases, 22 - remote discharge tubes, 23-annular zone formed by two annular discs.

The device works as follows. The operation of the furnace is preceded by the loading of the paste into the supply hopper and the ignition of the furnace.

The wet paste of aluminum fluoride trihydrate through the dosing units and the feeder is fed to the tray 6 and enters the furnace space, where the receiving screw nozzle 13 is located along the length L _{1 of the} furnace. The receiving screw nozzle, when the furnace rotates, the material moves along the furnace and, as it is freed from free (surface) moisture, it enters the zone where the lift-blade nozzle 14 is located, located along the length of the furnace L ₂ . Along the length L _2, it is possible to install successively various known designs of blade nozzles, but the main purpose of using this type of nozzles is to lift and weigh the material across the flow of flue gases, to intensify the process of heating it and removing free and partially crystalline hydrate moisture. The material in this zone becomes free-flowing, becomes mobile, in connection with this, the entrainment of fines with flue gases increases. Further, the material moves to the zone where the radial type of packing 15 is located along the length L _{3 of the} furnace. The radial nozzle agitates the material in the rubble, raises it to a small height and, spilling it, contributes to the renewal of the contact surface of the material with flue gases, which contributes to its further heating and the removal of 2.5H ₂ O molecules of crystalline hydrate moisture. The convective part of the furnace I from the radial nozzle to the conical insert III is smooth and the material moves only in the heap.

The sequential installation of the above nozzles takes into account the properties of the processed material, intensifies heat and mass transfer processes at the stage of removing free and partially crystalline hydrate moisture, the processes are completed at a shorter furnace length with a deeper use of the heat of flue gases.

As the analyzes of the fine fraction of the material carried away from the furnace have shown, the bulk of the dust is carried away from the weighing and tedding zone of the material, and has a residual crystalline hydrate moisture of 1.0 ... 1.5H ₂ O water molecules. Therefore, the dust caught in the dry gas cleaning system from the bunkers of the apparatus by gravity, spills out into the annular zone 23 through the return pipe of the captured dust 7 and returns to the calcining zone by the return screw 16, namely, to the convective part of the furnace, behind the radial nozzle 15.

From the conical insert III, the material is moved by ladles 17 into the annular space of the conductive part of the furnace II, where the process of dehydration and removal of, mainly, 0.5H ₂ O water molecules takes place by heating the material through pipes for the passage of flue gases 21.

Calcining the material without contact with flue gases improves the quality of the calcined product, while reducing the diameter of the conductive part of the furnace and organizing the movement of flue gases in the pipes reduces the temperature of the furnace body, and increases the reliability of the support structure in this area.

To eliminate material slip from the inlet to the outlet due to the formation of a through flow, in order to ensure a given residence time in the calcination zone, the annular space of the conductive part of the furnace is sectioned by installing several annular baffles 19. By changing the inner diameter of the annular baffles 19, the degree of filling the annular space with material is regulated sections. The filling capacity of the last section with material, before unloading the product, is determined by the height of the unloading distance tubes 22. The number of unloading tubes 22, their diameter and height "H" are determined by design calculations.

Water vapors and fluorine gases released during the calcination of the material at the last stage, possible suction gases, are discharged through the steam-gas outlet 9 and enter the gas path for flue gas removal from the furnace to the gas cleaning system (not shown in Fig. 1).

Flue gases obtained by burning natural gas in the furnace 10 with an initial temperature of 750 ... 650 ° C (coolant), through the flue gas inlet 11 enter the space of the unloading chamber 4 and then into the pipes for the passage of flue gases 21, where they give off heat through the wall to the calcined material. Further, the flue gases are combined into a common flow and provide the process of drying and dehydration of the material in the convective part of the furnace. The furnace is located at an angle of 90 ° to the axis of the furnace, which makes it possible to make the unloading chamber 4 sliding and open access to the tube sheet and pipes for the passage of flue gases for inspection, repair or replacement of pipes.

With the central pipe 20 installed, during commissioning, it is possible to diaphragm the opening of the central pipe from the side of the discharge chamber of the furnace and partial bypass of hot flue gases into the space of the convective part of the furnace to maintain an optimal temperature regime in this zone.

A hatchway 12 at the front wall of the loading chamber 5 provides access to the interior of the furnace, up to ladles 17 and tubesheet 18.

Thanks to the adopted design changes in the 02.9 / 3.5-27 furnace, a reduction in overall dimensions and weight has been achieved. The length is reduced by 18 meters, due to the use of a lift-blade and radial weighing attachment, and the weight is reduced by 50%. The productivity of the furnace was increased by 1.5 times due to the intensification of all stages of drying and calcining of TPA. Due to the intensification of the process in the convective part of the furnace, the heat of the flue gases is more fully used, the temperature of the exhaust gases after the furnace has decreased from 250 ... 300 ° С to 100 ... 115 ° С, the temperature of the flue gases leaving the furnace has been reduced from 850 ... 900 ° С to 650 … 750 ° C. Due to a decrease in the contact area of the furnace body with flue gases and thermal insulation of the entire furnace body, losses through fences into the surrounding space have decreased, in general, the consumption of natural gas per ton of produced product has decreased from 200 to 165 m ³ / t.

The working conditions of the support of the furnace 2 at the discharge end have improved due to a decrease in the temperature of the furnace body in this zone.

Used sources.

1 Pilkauskas K.K. Improvement of aluminum fluoride technology at the Keidar chemical plant // Proceedings of NIUIF. No. 254. - BONTI - M, 1988 - S. 148-151.

2 GOST 27120-86 Furnaces for chemical production with rotating drums for general purposes.

3 USSR author's certificate for application No. 2969472, cl. F26D 11/04, 1980.

Claims (1)

A furnace for drying and calcining the paste of aluminum fluoride trihydrate, containing a furnace, a cylindrical body with loading and unloading chambers installed at the ends, placed on supports with the possibility of rotation, a drive providing a rotational movement of the body, characterized in that the body is made of two cylindrical parts of different diameters and length - of the convective part and made in the form of a tubular heat exchanger of the conductive part of the furnace, rigidly connected to each other by a conical insert, while it is equipped with a receiving-screw, blade and radial nozzle, sequentially installed in the convective part of the furnace from the side of the loading chamber, a return screw for returning into the calcination zone of the dust carried away from the furnace, by a central pipe, the diameter of which is greater than the diameter of the heat exchanger pipes, installed along the axis of the conductive part of the furnace with the formation of an annular space and made with open ends and the possibility of changing the flow section from the side of the unloading chamber ers, ladles for transferring material from the convective part to the annular space in the conductive part of the furnace and annular baffles installed in the annular space of the conductive part of the furnace to divide the annular space into sections, spacer tubes installed at the discharge end of the furnace for unloading the calcined product, the height of which determines the degree of filling the annular space with material, and in the upper part of the unloading chamber a fitting is installed to remove the vapor-gas generated in the annular space of the heat exchanger during material calcination, while the furnace is located at an angle of 90 ° to the axis of the furnace body.

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