microwave calcination furnace for solid materials divided into fine particles

Abstract

Microwave calcination furnace for solid materials divided into fine particles The invention relates to a microwave calcination furnace for solid materials divided into fine particles, comprising an elongated heating chamber whose longitudinal axis is arranged vertically, an inlet tube (8) opening into the interior of the heating chamber at an upper end thereof, an outlet opening opening into the interior of the heating chamber at a lower end thereof, a mixer (28) comprising at least one mixing shaft (30) which carries a plurality of mixing blades (32) and a system (34) for driving the mixing shaft in rotation,and at least one waveguide (18) connected to a microwave generator (20) and opening inside the heating enclosure between its upper and lower ends via a protective window (22) mounted on the heating enclosure with the interposition of wave trapping seals (26). Figure for the abstract: Fig. 3.,

Description

Title of the invention: Microwave calcination furnace for solid materials divided into fine particles Technical field

[0001] The invention relates to the general field of high-temperature microwave heating to carry out a calcination treatment (possibly continuously) of solid materials divided into fine particles, and in particular powders. Prior art

[0002] A calcination heat treatment consists of heating an inert body to a high temperature (with temperatures above 150°C and up to 1900°C) in a closed enclosure in order to decompose it or obtain chemical reactions.

[0003] In the mining industry, calcination is used as a heat treatment process applied to ores such as kaolinite, clinker, bauxite, lithium and magnesite to achieve thermal decomposition, for example, removal of crystal water, removal of CO2 or phase transformation of minerals.

[0004] During this treatment process, bound metals (in ores or other solids) are converted into oxide (calcine), in the form of powder, ash or dust, or undergo a phase transition by heating. The process is usually carried out in long cylindrical furnaces or tall vertical burner furnaces.

[0005] It is known to carry out such a process by calcining materials in powder form. Thus, the calcination of powders is known using rotary heating furnaces with direct burner or by indirect heating of the furnace wall. However, these calcination processes have the disadvantages of being energy-intensive, emitting large quantities of CO2 and dust and with a low yield.

[0006] Vertical "flash" furnaces are also known in which the powder to be treated is projected into a high-temperature flame for flash calcination. However, this calcination process is very energy-intensive and a source of a significant quantity of CO2 and dust that must be treated.

[0007] There are still vertical electric furnaces with hot plates as described in publication CN 2656406. However, these furnaces have low efficiency and are unsuitable for industrial scale applications.

[0008] Publications FR 2,850,519 and FR 2,850,520 also disclose microwave radiation heating devices which treat materials in batches (or batches) in a rotating cavity. These solutions have the advantages of allowing a uniform heating, high temperature heat treatments and significant CO2 savings. However, when applied to the treatment of powders, they can pose problems of electric arcing due to the flight of fines in the furnace.

[0009] Publication FR 2,965,907 also discloses a solution for heat treatment by continuous microwave radiation per coil. However, this technology is unsuitable for temperatures exceeding 600 to 700°C.

[0010] Publication CN103822464 describes a method for heating powders of the type kaolin by microwave in a multi-stage vertical kiln. This kiln has a series of cones made of microwave-heated material over which the product to be treated flows, which makes it possible to calcine products that do not react to microwaves. On the other hand, this type of kiln has the disadvantages of creating dust and being poorly suited to industrial microwave heating (in particular due to a low bed thickness, damage and wear of the cones, and thermal runaway).

[0011] Publication CN 102305541 describes a continuous mixed microwave and traditional heating in a tower by gravity fall into a tube in which the product is preheated by microwaves and then transferred into a rotating tube to finalize cooking. This process has the disadvantages of creating a large quantity of dust, which is a source of electric arcing and which greatly limits microwave power.

[0012] Publication WO 2009 / 034418 discloses a microwave process for solving dust-related problems. This process consists of passing the product through a rectangular cross-section channel with the product being advanced by vibrations and distributing the waves over the top of this channel. Although effective, this process is completely unsuitable for use on an industrial scale and poses problems of interface at the arrival of the waves and of heating homogeneity. Statement of the invention

[0013] The present invention aims to address all of the aforementioned drawbacks by proposing a solution particularly suited to solid materials divided into fine particles such as powders and to implementation on an industrial scale.

[0014] According to the invention, this aim is achieved by means of a microwave calcination furnace for solid materials divided into fine particles, comprising: - an elongated heating enclosure with a longitudinal axis arranged vertically; - at least one inlet tube for materials to be treated opening inside from the heating enclosure to an upper end thereof; - at least one discharge orifice for the treated materials opening inside the heating enclosure at a lower end thereof; - a mixer mounted inside the heating enclosure and comprising at least one mixing shaft which is arranged parallel to the longitudinal axis of the heating enclosure and which carries a plurality of mixing blades, and a system for driving the mixing shaft in rotation around its axis; and - at least one waveguide which is connected to a microwave generator and which opens inside the heating enclosure between its upper and lower ends via a protective window mounted on the heating enclosure with the interposition of wave trapping seals.

[0015] The oven according to the invention is remarkable in particular in that it allows the materials to be treated to be placed directly in contact with the microwave field, in their mass. The oven according to the invention allows the materials to be treated to circulate at a controlled speed just in front of the arrival of the microwaves to heat them homogeneously, at a high power density while avoiding the creation of dust and optimizing the yield. In addition, the area subjected to the microwaves is very limited because the waves are quickly damped in the product.

[0016] According to one embodiment, the heating enclosure has a straight section of oblong shape with two half-cylinders.

[0017] In this embodiment, the mixer advantageously comprises two parallel mixing shafts, each centered on one of the axes of the two half-cylinders and each carrying a plurality of mixing blades, the mixing blades carried by one of the mixing shafts interlocking with those carried by the other mixing shaft, which makes it possible to ensure optimal mixing.

[0018] Also preferably, the system for rotating the mixing shafts is configured to drive the two mixing shafts in opposite directions and at synchronized speeds, always with the aim of optimizing mixing.

[0019] More preferably, the protective window has a curved trapezoidal cross-section so as to fit the space not subjected to the mixing blades carried by the two mixing shafts.

[0020] According to an advantageous arrangement, the waveguide opens at a lower part of the heating enclosure so as to create a cold upper zone not subjected to microwaves and a hot lower zone subjected to microwaves.

[0021] In this case, the heating enclosure is preferably provided at the level of the cold zone with a thermal insulation coating.

[0022] In addition, the furnace may further comprise induction heating means for located above the waveguide so as to create an intermediate preheating zone located between the cold zone and the hot zone.

[0023] The furnace may further comprise means for heating by Joule effect the mixing blades located at least at the level of the preheating zone, which makes it possible to ensure additional heating of the materials to be treated.

[0024] According to an advantageous arrangement, the discharge orifice for the treated materials opens inside the heating enclosure via a hopper and its flow rate is regulated by a motorized adjustable opening valve.

[0025] According to another advantageous arrangement, the oven further comprises a plurality of temperature probes opening into the interior of the heating enclosure at different heights.

[0026] The heating enclosure may be provided with an external thermal insulation coating.

[0027] Preferably, each wave trapping seal comprises a plurality of grooves which extend around the entire periphery of the protective window and which have a depth equal to a quarter of the wavelength of the emitted microwaves.

[0028] According to an advantageous arrangement, the oven may comprise a plurality of waveguides positioned one above the other and connected to microwave generators delivering microwaves at different frequencies and powers so as to define several heating stages distributed over the height of the heating enclosure. Brief description of the drawings

[0029] [Fig-1] [Fig.l] is a schematic and perspective view of a calcination furnace by microwave according to one embodiment of the invention.

[0030] [Fig.2] [Fig.2] is a front view of the furnace of [Fig.l].

[0031] [Fig.3] [Fig.3] is a side sectional view of the furnace of [Fig.l].

[0032] [Fig.4] [Fig.4] is a cross-sectional view of the furnace of [Fig.l] with a view of above.

[0033] [Fig.5] [Fig.5] is a cross-sectional view showing a protective window according to one embodiment of the oven according to the invention.

[0034] [Fig.6] [Fig.6] is a front view of an oven according to an alternative embodiment of the invention. Description of the embodiments

[0035] The invention relates to a calcination heat treatment, which can be carried out continuously, of solid materials divided into fine particles, and in particular powders. In the context of the present invention, the calcination heat treatment is carried out with heating temperatures between 150°C and 1900°C.

[0036] Figures 1 to 5 show, in different views, an oven 2 according to the invention. intended for the implementation of such calcination heat treatment.

[0037] According to the invention, the oven 2 comprises in particular a heating enclosure 4 of elongated shape and positioned so that its longitudinal axis XX is substantially vertical.

[0038] In cross section (as shown in [Fig.4]), the heating enclosure 4 of the furnace according to the invention has an oblong shape, that is to say an elongated shape which is longer than it is wide and which ends in two half-cylinders each centered on a vertical axis Yl-Yl, Y2-Y2.

[0039] The heating enclosure 4 is typically made of an electrically conductive material resistant to high temperatures, for example refractory steel. Its thickness is generally between 1 and 10 mm and its dimensions include a length between 200 mm and 5 m and a width between 200 mm and 2 m.

[0040] The heating enclosure 4 is also provided with an external thermal insulation coating (for example a layer of aluminized rock wool having a thickness of the order of 5 mm - not shown in the figures).

[0041] At its upper end, the heating enclosure 4 is closed by an upper cover 6. At least one inlet tube 8 for materials to be treated passing through this upper cover 6 opens into the interior of the heating enclosure 4 at an upper end thereof.

[0042] Similarly, at its lower end, the heating enclosure 4 is closed by a drain bottom flange 10 in which an evacuation orifice 12 for the treated materials is made.

[0043] Advantageously, this discharge orifice 12 opens inside the heating enclosure via a hopper 14 and its flow rate is regulated by a motorized adjustable opening valve 16.

[0044] Furthermore, the oven 2 according to the invention comprises at least one waveguide 18 which is connected to a microwave generator 20 and which opens into the interior of the heating enclosure 4 between its upper and lower ends.

[0045] Typically, the microwave generator 20 comprises a magnetron or klystron (having a unit power which can vary between 1kW and 10MW) coupled to a frequency generator (which can vary from 200MHz to 5000MHz).

[0046] In the example shown in Figures 1 to 6, two waveguides 18 are provided which are positioned vertically one above the other and which are each connected to a different microwave generator. Of course, the number of waveguides could be greater and distributed differently depending on the dimensions of the heating enclosure.

[0047] In the case of several waveguides positioned one above the other, the The frequencies and powers of each microwave generator can be different in order to optimize the heating of the materials to be treated. In particular, it is particularly advantageous to have microwave generators with defined frequencies per heating stage for targeted materials. Similarly, it can be advantageous to be able to vary the power of each stage of microwave generators to optimize the heating efficiency of the materials.

[0048] Indeed, the response of a material to microwaves depends on its dielectric permittivity which varies according to the frequency of the electromagnetic field and the temperature of the product in the form of a function which is variable according to the chemistry of the materials.

[0049] Preferably, the waveguide(s) 18 open(s) at a lower part of the heating enclosure 4 so as to create a cold upper zone ZF not subjected to microwaves and a hot lower zone ZC subjected to microwaves.

[0050] In this case, the heating enclosure 4 is advantageously provided at the level of the cold zone ZF with a thermal insulation coating 21 ([Fig.3]).

[0051] Each waveguide 18 opens into the interior of the heating enclosure 4 via a protective window 22 which is mounted in an opening of the heating enclosure by a mounting flange 24 with the interposition of wave trapping seals 26.

[0052] The protective window 22 is made of a material transparent to the microwaves generated by the wave generator and resistant to high temperatures. It is for example made of quartz or ceramic and makes it possible to isolate the waveguide from dust.

[0053] The interface between each protective window 22 and the heating enclosure 4 necessarily leaves retention zones. The disadvantage of these retention zones is that they are immobile, so that heating them by microwaves causes thermal runaways which can go as far as vitrification of these zones and their damage.

[0054] Also, in order to cancel the effect of the microwaves at these retention zones, it is provided to position wave trapping seals 26 all around each protective window. The effect of these wave trapping seals 26 is to cancel the effect of the microwaves on the periphery of the protective window over a thickness of approximately 5 mm corresponding to the retention zone between the protective window 22 and the heating enclosure 4.

[0055] The principle of the wave trapping joints 26 is to make a part of the incident wave travel an additional half-wavelength so that the other part of the wave cancels itself out when it encounters it.

[0056] As shown in particular in [Fig.5], each wave trapping joint 26 thus comprises a plurality of grooves 26a which extend over the entire periphery of the protective window 22 and which have a depth P equal to a quarter of the wavelength of the emitted microwaves, such that a round trip corresponds to half a wavelength.

[0057] The geometry of these wave trapping joints 26 depends on the frequency of the emitted microwaves. For different magnetron frequencies, the dimensions of the wave trapping joints will therefore be different.

[0058] The oven 2 according to the invention also comprises a mixer 28 which is mounted inside the heating enclosure 4.

[0059] This mixer 28 comprises at least one mixing shaft 30 which is arranged parallel to the longitudinal axis XX of the heating enclosure and which carries a plurality of mixing blades 32, as well as a system 34 for driving the mixing shaft in rotation around its axis.

[0060] The mixing shaft 30 extends from the upper end of the heating enclosure 4 towards its lower end and the mixing blades 32 are regularly distributed over the entire height of the heating enclosure. At the upper end thereof, the mixing shaft 30 passes through the upper cover 6 and is connected to the rotation drive system 34.

[0061] In the embodiment of Figures 1 to 6, the mixer 28 comprises two mixing shafts 30a, 30b which are each centered on one of the axes Yl-Yl, Y2-Y2 of the two half-cylinders and which each carry a plurality of mixing blades 32a, 32b

[0062] In this configuration, the mixing blades carried by one of the two mixing shafts interlock with those carried by the other mixing shaft so as to ensure optimal mixing of the materials to be treated (see [Fig.4] in particular).

[0063] Similarly, still in this configuration, the rotation drive system 34 of the mixing shafts 30a, 30b is advantageously configured to drive the two mixing shafts in opposite directions and at speeds synchronized with each other, still with the aim of improving the mixing of the materials to be treated.

[0064] For this purpose, as shown in particular in [Fig. 2], the rotational drive system 34 of the mixing shafts 30a, 30b consists for example of a geared motor assembly 34a which is connected to the mixing shafts by belt or gear systems 34b ([Fig. 2]). In addition, the mixing shafts are guided by bearings or rollers 36 ([Fig. 3]).

[0065] Furthermore, the mixing blades 32a, 32b are dimensioned so that the mixing of the materials to be treated is carried out over the entire height of the heating enclosure and to give the material fluid behavior.

[0066] Preferably, the shafts 30a, 30b and mixing blades 32a, 32b are coated with an insulating ceramic allowing operation at high temperature (up to around 1900°C) and to avoid excessively rapid wear.

[0067] Also preferably, as shown in [Fig.5], each protective window 22 has a curved trapezoidal cross-section so as to fit the space not subjected to the mixing blades 32a, 32b carried by the two mixing shafts 30a, 30b.

[0068] The operation of the oven 2 according to the invention is as follows.

[0069] The materials to be treated which are introduced into the heating enclosure 4 by the inlet tube(s) 8 are brought in a continuous and fluid layer by the rotation of the mixing blades 32a, 32b in front of the outlets of the waveguides 18 to be heated uniformly in their volume.

[0070] It will be noted that the ratio between the diameter of the intake tube(s) 8 and their length makes it possible, for a given frequency of the microwave generator 20, to cancel out wave leaks (for example 60mm in diameter and 100mm in length for a microwave generator frequency of 2450MHz).

[0071] The materials to be treated are immediately mixed by the mixing blades 32a, 32b and penetrate regularly into a cold zone ZF not subjected to microwaves, adding to the mass of product already present in the heating enclosure.

[0072] When the product reaches the heating zone ZC, the microwaves emitted by the waveguides 18 penetrate a layer of 1 to 10 cm or more of product depending on the frequency of the microwave generator 20 (approximately 2 to 5 cm for a frequency of 2450 MHz and 5 to 20 cm for a frequency of 915 MHz) and the type of material to be treated. The rotation of the mixing blades 32a, 32b ensures that all the particles pass in front of the protective windows 22.

[0073] The product descends as it is calcined to the lowest point of the heating enclosure where it is recovered by the hopper 14 and then collected by the discharge orifice 12.

[0074] It will be noted that the heating temperatures are recorded by temperature probes 38 opening inside the heating enclosure 4 at different heights thereof.

[0075] It will also be noted that the control of the heating, the flow rate of products and the power emission of the different microwave generators can be carried out by an automaton and a human-machine interface.

[0076] In connection with [Fig.6], an alternative embodiment of the oven according to the invention will now be described.

[0077] In this variant embodiment, the furnace 2' further comprises induction heating means 40 which are positioned above the waveguide(s) 18 so as to create an intermediate preheating zone ZPC located between the cold zone ZF and the hot zone ZC.

[0078] This variant makes it possible to heat the heating enclosure 4 in the preheating zone ZPC in order to preheat the product to be treated.

[0079] According to another variant of the furnace not shown in the figures, the mixing shafts and the mixing blades which they carry can be heated by Joule effect over a certain height, preferably corresponding to the preheating zone ZPC described previously.

[0080] This variant is particularly suitable up to temperatures of the order of 600 / 700°C by allowing additional heating of the product to be treated.

Claims

Claims

1. Microwave calcination furnace (2; 2') for solid materials divided into fine particles, comprising: - a heating chamber (4) of elongate shape, a longitudinal axis (XX) of which is arranged vertically; - at least one inlet tube (8) for materials to be treated opening into the interior of the heating chamber at an upper end thereof; - at least one discharge orifice (12) for the treated materials opening into the interior of the heating chamber at a lower end thereof; - a mixer (28) mounted inside the heating chamber and comprising at least one mixing shaft (30) which is arranged parallel to the longitudinal axis of the heating chamber and which carries a plurality of mixing blades (32), and a system (34) for driving the mixing shaft in rotation about its axis;and - at least one waveguide (18) which is connected to a microwave generator (20) and which opens into the interior of the heating enclosure between its upper and lower ends via a protective window (22) mounted on the heating enclosure with the interposition of wave trapping seals (26), in which: - the heating enclosure (4) has an oblong cross-section with two half-cylinders, and - the mixer comprises two parallel mixing shafts (30a, 30b), each centered on one of the axes (Yl-Yl, Y2-Y2) of the two half-cylinders and each carrying a plurality of mixing blades (32a, 32b), the mixing blades carried by one of the mixing shafts interlocking with those carried by the other mixing shaft mixing.;

2. Oven according to claim 1, wherein the drive system (34a, 34b) for rotation of the mixing shafts (30a, 30b) is configured to drive the two mixing shafts in opposite directions and at synchronized speeds.

3. Oven according to one of claims 1 and 2, in which the protective window (22) has a curved trapezoidal cross-section so as to fit the space not subjected to the mixing blades carried (32a, 32b) by the two mixing shafts.

4. Oven according to any one of claims 1 to 3, in which the waveguide (18) opens at a lower part of the heating enclosure so as to create a cold upper zone (ZF) not subjected to microwaves and a hot lower zone (ZC) subjected to microwaves.

5. Oven according to claim 4, in which the heating enclosure (') is provided at the level of the cold zone (ZF) with a thermal insulation coating (21).

6. Oven according to one of claims 4 and 5, further comprising induction heating means (40) positioned above the waveguide (18) so as to create an intermediate preheating zone (ZPC) located between the cold zone (ZF) and the hot zone (ZC).

7. Oven according to claim 6, further comprising means for heating by Joule effect the mixing blades located at least at the level of the preheating zone (ZPC).

8. Oven according to any one of claims 1 to 7, in which the discharge orifice (12) for the treated materials opens inside the heating enclosure (4) via a hopper (14) and its flow rate is regulated by a motorized adjustable opening valve (16).

9. Oven according to any one of claims 1 to 8, further comprising a plurality of temperature probes (38) opening into the interior of the heating enclosure at different heights.

10. Oven according to any one of claims 1 to 9, in which the heating enclosure (4) is provided with an external thermal insulation coating.

11. An oven according to any one of claims 1 to 10, wherein each wave trapping seal (26) comprises a plurality of grooves (26a) which extend over the entire periphery of the protective window (22) and which have a depth (P) equal to a quarter of the wavelength of the emitted microwaves.

12. An oven according to any one of claims 1 to 11, comprising a plurality of waveguides (18) positioned one above the other and connected to microwave generators (20) delivering microwaves at different frequencies and powers so as to define several heating stages distributed over the height of the heating enclosure (4).

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