Method for producing mineral source potassium fulvate through acid dissolution and alkali precipitation

Abstract

The invention discloses a method for producing mineral source potassium fulvate through acid dissolution and alkali precipitation. Comprising the following steps: crushing and levigating the coal-based solid waste to obtain a fine powdery material with uniform particle size, adding sulfuric acid into the fine powdery material, fully stirring to completely dissolve the fine powdery material into transparent clear liquid, removing insoluble impurities through vacuum filtration, collecting clear liquid for later use, slowly pouring the clear liquid into a pre-prepared potassium hydroxide solution, and stirring for 2-3 hours to obtain a potassium hydroxide solution; and continuously stirring until white floccules are generated, centrifugally separating to obtain pure potassium fulvic acid solid, and drying to obtain the mineral source potassium fulvic acid. According to the method for producing the mineral source potassium fulvic acid through acid dissolution and alkali precipitation, efficient and environment-friendly preparation of the mineral source potassium fulvic acid is achieved, the mineral source potassium fulvic acid is prepared through alkali dissolution conversion of potassium hydroxide, meanwhile, the fertilizer efficiency stability of the product under different water quality conditions is ensured, and when the same waste is applied to different areas, the fertilizer efficiency is not greatly influenced by the water quality; the requirements of agricultural production on high-quality fertilizer are met.

Description

Technical Field

[0001] The invention belongs to the technical field of mineral-source potassium humate production, and particularly relates to a method for producing mineral-source potassium humate through acid dissolution and alkali precipitation. Background Art

[0002] In the agricultural fertilizer production industry, people's awareness of soil improvement and crop nutrition needs continues to increase. Mineral potassium humate has received widespread attention as an excellent organic fertilizer additive. It can not only improve soil structure, increase soil water retention and air permeability, but also promote crop absorption of nutrients and enhance crop resistance.

[0003] The common mineral-based potassium humate preparation methods currently on the market generally pay little attention to the hard water resistance of the finished product, resulting in obvious differences in fertilizer efficiency when the same fertilizer is used in different water quality environments, which brings inconvenience to agricultural production.

[0004] The processing of mineral potassium humate mainly includes the following steps: 1. Initial screening of raw materials: Commonly used raw materials include lignite and weathered coal; 2. Raw material extraction and pretreatment: In order to improve the utilization rate and efficiency of raw materials, the humic acid content is usually increased by mechanical activation, physical separation, chemical oxidation, biodegradation and other methods; 3. Alkali extraction: Add KOH (or NaOH) to the reactor and react for a certain time under certain temperature conditions to extract free humic acid in the raw materials; 4. Solid-liquid separation: The liquid obtained by the reaction is placed in a sedimentation tank for preliminary precipitation, and then solid-liquid separation is carried out through a sedimentation centrifuge or an inclined plate. The separated liquid is dried to obtain the corresponding potassium humate or sodium humate powder; 5. Washing, evaporation, drying: Through evaporators, centrifugal separation and other links, potassium humate powder with higher purity is finally dried.

[0005] After searching, the patent number is CN217131651U, which discloses a dehydration and drying device for the production of potassium humate fertilizer. By designing a heater, the air in the box can be heated, and through the coordinated use of a motor and a rotating tube, the air outlet can be rotated to eject the jet, so that the hot air can be blown out evenly downward, so that the raw material is in uniform contact with the hot air, and the drying effect is improved. At the same time, through the action of the stirring rod, the material can be stirred and turned, so that the raw material is in complete contact with the hot air, and the drying efficiency is improved. However, when in use, when the heated air is recycled, the moisture is adsorbed by the dryer, which requires a large amount of desiccant, increases the production cost, and causes a waste of materials. When the desiccant is saturated, the water absorption ability is poor. At this time, the water vapor generated by the heating will continue to contact with the product during recycling, and the separation effect is poor, affecting the drying effect. Summary of the invention

[0006] The object of the present invention is to provide a method for producing ore-derived potassium humate by acid dissolution and alkali precipitation, so as to solve the problems raised in the above-mentioned background technology.

[0007] To achieve the above object, the present invention provides the following technical solution: a method for producing ore-derived potassium humate by acid dissolution and alkali precipitation, comprising the following steps:

[0008] S1: crushing and grinding coal-based solid waste to obtain fine powder materials with uniform particle size;

[0009] S2: adding sulfuric acid to the above-mentioned fine powder material and stirring it thoroughly to completely dissolve it into a transparent clear liquid;

[0010] S3: remove insoluble impurities by vacuum filtration and collect the clear liquid for later use;

[0011] S4: Then slowly pour the clear liquid into the pre-prepared potassium hydroxide solution and continue stirring until white flocs are produced;

[0012] S5: obtaining pure potassium humate solid by centrifugation;

[0013] S6: After drying, mineral source potassium humate is obtained.

[0014] Preferably, in step S1, the particle size of the material is 200-300 mesh.

[0015] The drying device comprises a dryer, a raw material storage barrel, a first cyclone separator, a second cyclone separator and a heating box, wherein the bottom end of the raw material storage barrel is connected to a material injection pump, the discharge port of the material injection pump is connected to a material injection pipe, and the other end of the material injection pipe extends into the dryer, and one end of the material injection pipe located in the dryer is connected to an atomizing nozzle, the dryer is connected to a first discharge pipe, the bottom end of the dryer is connected to a second discharge pipe, the first discharge pipe is connected to a feed port of the first cyclone separator, the second discharge pipe is connected to a feed port of the second cyclone separator, and the bottom end of the first cyclone separator is connected to the second discharge pipe, and the heating box is provided with a heating component and a preheating component, the heating component is used to heat the outside air to dry the raw materials, and the preheating component uses the waste heat of the exhaust gas to preheat the air entering the heating box.

[0016] Preferably, the heating assembly includes an electric heater arranged at the end of the heating box, and an air pump is also fixedly installed on the top of the heating box. The inlet of the air pump is connected to an air suction pipe, and the air suction pipe is connected to the side wall of the heating box. The outlet of the air pump is connected to an air injection pipe, and the air injection pipe extends into the dryer, and the angle between the outlet of the air injection pipe and the atomizing nozzle at the bottom end of the injection pipe is 30°.

[0017] Preferably, an air inlet pipe is also connected to the side wall of the heating box, and a third filter is provided on the air inlet pipe.

[0018] Preferably, a first filter screen and a second filter screen are further provided in the heating box, and the first filter screen and the second filter screen are inserted into the heating box from the top end of the heating box.

[0019] Preferably, the top ends of the first cyclone separator and the second cyclone separator are commonly connected with a connecting pipe, and the bottom ends of the connecting pipe extend into the first cyclone separator and the second cyclone separator respectively.

[0020] Through the above scheme, the air pump can suck the outside air into the heating box from the air inlet pipe, and the high-temperature gas heated by the electric heater can enter the dryer along the air intake pipe and the air injection pipe. At this time, the high-temperature gas can immediately evaporate the water in the atomized potassium humate. At this time, the mixed gas can enter the first cyclone separator and the second cyclone separator from the first discharge pipe and the second discharge pipe respectively, and the water vapor can be separated from the potassium humate through the connecting pipe, which can ensure the drying effect of the potassium humate, and the product separated in the first cyclone separator can enter the second cyclone separator along the first discharge pipe, which can perform double separation and improve the separation effect of water vapor, and the first filter, the second filter and the third filter can filter the air sucked into the heating box, which can avoid the contamination of the potassium humate product and improve the purity of the potassium humate.

[0021] Preferably, the preheating assembly includes a first annular tube and a second annular tube arranged in the heating box, the diameter of the first annular tube is smaller than the diameter of the second annular tube, the first annular tube and the second annular tube are arranged near the air intake pipe, and the first annular tube and the second annular tube are arranged between the first filter screen and the second filter screen, and a plurality of heat exchange tubes are commonly connected between the first annular tube and the second annular tube, and the plurality of heat exchange tubes are inclined.

[0022] Preferably, a conduit is provided on the side wall of the second annular tube, and the other end of the conduit is connected to the top end of the connecting tube.

[0023] Preferably, an exhaust pipe is provided on the side wall of the first annular tube, and the exhaust pipe extends to the outside of the heating box.

[0024] Through the above scheme, when in use, after the water vapor separated by the first cyclone separator and the second cyclone separator is discharged through the connecting pipe, the water vapor can enter the second annular tube along the conduit, and further enter the first annular tube along the multiple heat exchange tubes. Since the diameter of the first annular tube is smaller than the diameter of the second annular tube, the multiple heat exchange tubes gradually tilt upward from the first annular tube to the second annular tube, which can increase the contact between the air sucked from the intake pipe and the heat exchange tube, improve the heat exchange effect of the heat exchange tube, further ensure the preheating effect, save the energy consumed by the electric heater to heat the air to a specified temperature, and utilize the thermal energy of the exhaust gas, which is green and environmentally friendly, and saves energy and reduces emissions.

[0025] The technical effects and advantages of the present invention are as follows: 1. The method for producing mineral potassium humate by acid dissolution and alkali precipitation realizes efficient and environmentally friendly preparation of mineral potassium humate. The mineral potassium humate prepared by alkali dissolution and conversion using potassium hydroxide ensures the stability of the fertilizer effect of the product under different water quality conditions, so that the fertilizer effect of the same waste material is not greatly affected by water quality when applied to different areas, thereby meeting the demand for high-quality fertilizers in agricultural production.

[0026] 2. The present invention provides a heating component, and the air pump can suck the outside air into the heating box from the air inlet pipe. The high-temperature gas heated by the electric heater can enter the dryer along the air intake pipe and the air injection pipe. At this time, the high-temperature gas can immediately evaporate the water in the atomized potassium humate. At this time, the mixed gas can enter the first cyclone separator and the second cyclone separator from the first discharge pipe and the second discharge pipe respectively. The water vapor can be separated from the potassium humate through the connecting pipe, which can ensure the drying effect of the potassium humate. The product separated in the first cyclone separator can enter the second cyclone separator along the first discharge pipe, and double separation can be performed, which can improve the separation effect of water vapor. The first filter, the second filter and the third filter can filter the air sucked into the heating box, which can avoid the contamination of the potassium humate product and improve the purity of the potassium humate.

[0027] 3. The present invention sets a preheating component. After the water vapor separated by the first cyclone separator and the second cyclone separator is discharged through the connecting pipe, the water vapor can enter the second annular tube along the conduit, and further enter the first annular tube along the multiple heat exchange tubes. Since the diameter of the first annular tube is smaller than the diameter of the second annular tube, the multiple heat exchange tubes gradually tilt upward from the first annular tube to the second annular tube, which can increase the contact between the air sucked from the intake pipe and the heat exchange tubes, improve the heat exchange effect of the heat exchange tubes, further ensure the preheating effect, save the energy consumed by the electric heater to heat the air to a specified temperature, and utilize the thermal energy of the exhaust gas, which is green and environmentally friendly, and saves energy and reduces emissions. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Figure 1 This is a schematic diagram of the overall structure of one side of a drying device for producing potassium humate from mineral sources in an embodiment of the present invention;

[0029] Figure 2 This is a schematic diagram of the overall structure of the other side of a drying device for producing mineral-based potassium humate in an embodiment of the present invention.

[0030] Figure 3 It is a front cross-sectional structural schematic diagram of a dryer in an embodiment of the present invention;

[0031] Figure 4 It is a schematic diagram of the side cross-sectional structure of the drying device and the heating box in the embodiment of the present invention;

[0032] Figure 5 Schematic diagram of the structure of the first annular tube and the second annular tube in an embodiment of the present invention;

[0033] Figure 6 It is a schematic diagram of the cross-sectional structure of the first cyclone separator and the second cyclone separator in an embodiment of the present invention.

[0034] In the figure: 1, dryer; 102, first discharge pipe; 103, second discharge pipe; 2, raw material storage barrel; 201, injection pump; 202, injection pipe; 3, first cyclone separator; 4, second cyclone separator; 5, connecting pipe; 501, conduit; 6, heating box; 601, first filter; 602, second filter; 603, air pump; 6031, air injection pipe; 6032, suction pipe; 604, air inlet pipe; 605, third filter; 606, electric heater; 7, exhaust pipe; 701, first annular pipe; 702, second annular pipe; 703, heat exchange pipe. DETAILED DESCRIPTION

[0035] The following will be described clearly and completely in conjunction with the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0036] Example 1

[0037] A method for producing ore-source potassium humate by acid dissolution and alkali precipitation comprises the following steps:

[0038] S1: crushing and grinding coal-based solid waste to obtain fine powder materials with uniform particle size;

[0039] S2: Add sulfuric acid to the above-mentioned fine powder material and stir it thoroughly to completely dissolve it into a transparent clear liquid. The amount added is in a mass ratio of 1:1;

[0040] S3: remove insoluble impurities by vacuum filtration and collect the clear liquid for later use;

[0041] S4: Then slowly pour the clear liquid into the pre-prepared potassium hydroxide solution, adding the fine powder material and potassium hydroxide in a mass ratio of 1:0.8, and continue stirring until white flocs are produced;

[0042] S5: obtaining pure potassium humate solid by centrifugation;

[0043] S6: After drying, mineral source potassium humate is obtained.

[0044] Preferably, in step S1, the particle size of the material is 200-300 mesh.

[0045] Example 2

[0046] See also Figures 1 to 6 A drying device for producing potassium humate from mineral source, comprising a dryer 1, a raw material storage barrel 2, a first cyclone separator 3, a second cyclone separator 4 and a heating box 6, wherein the bottom end of the raw material storage barrel 2 is connected to a material injection pump 201, the material outlet of the material injection pump 201 is connected to a material injection pipe 202, and the other end of the material injection pipe 202 extends into the dryer 1, and one end of the material injection pipe 202 located in the dryer 1 is connected to an atomizing nozzle, and the dryer 1 is connected to a first discharge pipe 102 The bottom end of the dryer 1 is connected to a second discharge pipe 103, the first discharge pipe 102 is connected to the feed port of the first cyclone separator 3, the second discharge pipe 103 is connected to the feed port of the second cyclone separator 4, and the bottom end of the first cyclone separator 3 is connected to the second discharge pipe 103. A heating component and a preheating component are provided in the heating box 6. The heating component is used to heat the outside air to dry the raw materials, and the preheating component uses the waste heat of the exhaust gas to preheat the air entering the heating box 6.

[0047] The heating assembly includes an electric heater 606 arranged at the end of the heating box 6. An air pump 603 is also fixedly installed on the top of the heating box 6. The inlet of the air pump 603 is connected to an air suction pipe 6032, and the air suction pipe 6032 is connected to the side wall of the heating box 6. The outlet of the air pump 603 is connected to an air injection pipe 6031, and the air injection pipe 6031 extends into the dryer 1. The angle between the outlet of the air injection pipe 6031 and the atomizing nozzle at the bottom end of the injection pipe 202 is 30°.

[0048] An air inlet pipe 604 is also connected to the side wall of the heating box 6 , and a third filter screen 605 is provided on the air inlet pipe 604 .

[0049] A first filter screen 601 and a second filter screen 602 are also provided in the heating box 6 . The first filter screen 601 and the second filter screen 602 are inserted into the heating box 6 from the top end of the heating box 6 .

[0050] The top ends of the first cyclone separator 3 and the second cyclone separator 4 are connected to each other by a connecting pipe 5 , and the bottom ends of the connecting pipe 5 extend into the first cyclone separator 3 and the second cyclone separator 4 , respectively.

[0051] In this embodiment, when in use, the air pump 603 can suck the outside air into the heating box 6 from the air inlet pipe 604, and the high-temperature gas heated by the electric heater 606 can enter the dryer 1 along the air intake pipe 6032 and the air injection pipe 6031. At this time, the high-temperature gas can immediately evaporate the water in the atomized potassium humate. At this time, the mixed gas can enter the first cyclone separator 3 and the second cyclone separator 4 from the first discharge pipe 102 and the second discharge pipe 103 respectively, and the water vapor can be separated from the potassium humate through the connecting pipe 5, which can ensure the drying effect of the potassium humate, and the product separated in the first cyclone separator 3 can enter the second cyclone separator 4 along the first discharge pipe 102, and double separation can be performed, which can improve the separation effect of water vapor, and the first filter 601, the second filter 602 and the third filter 605 can filter the air sucked into the heating box 6, which can avoid the contamination of the potassium humate product and improve the purity of the potassium humate.

[0052] As an embodiment of the present invention, refer to Figure 4-Figure 5 The preheating component includes a first annular tube 701 and a second annular tube 702 arranged in the heating box 6. The diameter of the first annular tube 701 is smaller than the diameter of the second annular tube 702. The first annular tube 701 and the second annular tube 702 are arranged near the air inlet pipe 604, and the first annular tube 701 and the second annular tube 702 are arranged between the first filter 601 and the second filter 602. A plurality of heat exchange tubes 703 are commonly connected between the first annular tube 701 and the second annular tube 702, and the plurality of heat exchange tubes 703 are inclined.

[0053] A guide tube 501 is disposed on the side wall of the second annular tube 702 , and the other end of the guide tube 501 is connected to the top end of the connecting tube 5 .

[0054] An exhaust pipe 7 is disposed on the side wall of the first annular pipe 701 , and the exhaust pipe 7 extends to the outside of the heating box 6 .

[0055] In this embodiment, after the water vapor separated by the first cyclone separator 3 and the second cyclone separator 4 is discharged through the connecting pipe 5, the water vapor can enter the second annular tube 702 along the conduit 501, and further enter the first annular tube 701 along the multiple heat exchange tubes 703. Since the diameter of the first annular tube 701 is smaller than the diameter of the second annular tube 702, the multiple heat exchange tubes 703 gradually tilt upward from the first annular tube 701 to the second annular tube 702, which can increase the contact between the air sucked in from the intake pipe 604 and the heat exchange tubes 703, improve the heat exchange effect of the heat exchange tubes 703, further ensure the preheating effect, save the energy consumed by the electric heater 606 to heat the air to the specified temperature, and utilize the thermal energy of the exhaust gas, which is green and environmentally friendly, and saves energy and reduces emissions.

[0056] Working principle: First, the potassium humate solution in the raw material storage barrel 2 is sprayed into mist through the atomizing nozzle at the end of the injection pipe 202 by the injection pump 201, and then the air pump 603 is started. The air pump 603 can suck the outside air into the heating box 6 from the air inlet pipe 604. The high-temperature gas heated by the electric heater 606 can enter the dryer 1 along the air intake pipe 6032 and the injection pipe 6031. At this time, the high-temperature gas can immediately evaporate the water in the atomized potassium humate. At this time, the mixed gas can be separated from the first discharge pipe 102 and the second discharge pipe 103. The water vapor can be separated from the potassium humate through the connecting pipe 5, thereby ensuring the drying effect of the potassium humate. The product separated in the first cyclone separator 3 can enter the second cyclone separator 4 along the first discharge pipe 102, thereby performing double separation, thereby improving the separation effect of the water vapor. The first filter screen 601, the second filter screen 602 and the third filter screen 605 can filter the air sucked into the heating box 6, thereby preventing the potassium humate product from being contaminated and improving the purity of the potassium humate.

[0057] After the water vapor separated by the first cyclone separator 3 and the second cyclone separator 4 is discharged through the connecting pipe 5, the water vapor can enter the second annular tube 702 along the conduit 501, and further enter the first annular tube 701 along the multiple heat exchange tubes 703. Since the diameter of the first annular tube 701 is smaller than the diameter of the second annular tube 702, the multiple heat exchange tubes 703 gradually tilt upward from the first annular tube 701 to the second annular tube 702, which can increase the contact between the air sucked from the intake pipe 604 and the heat exchange tubes 703, improve the heat exchange effect of the heat exchange tubes 703, further ensure the preheating effect, save the energy consumed by the electric heater 606 to heat the air to the specified temperature, and utilize the thermal energy of the exhaust gas, which is green and environmentally friendly, saves energy and reduces emissions. The water vapor after heat exchange can be discharged from the heating box 6 through the exhaust pipe 7.

[0058] Finally, it should be noted that the above is only a preferred embodiment of the present invention and is not intended to limit the present invention. Although the present invention has been described in detail with reference to the aforementioned embodiments, it is still possible for those skilled in the art to modify the technical solutions described in the aforementioned embodiments or to make equivalent substitutions for some of the technical features therein. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention should be included in the protection scope of the present invention.

Claims

1. A method for producing ore-derived potassium humate by acid dissolution and alkali precipitation, characterized in that: The following steps are involved: S1: crushing and grinding coal-based solid waste to obtain fine powder materials with uniform particle size; S2: adding a weak acid to the above-mentioned fine powder material and stirring it thoroughly to completely dissolve it into a transparent clear liquid; S3: remove insoluble impurities by vacuum filtration and collect the clear liquid for later use; S4: Then slowly pour the clear liquid into the pre-prepared potassium hydroxide solution and continue stirring until white flocs are produced; S5: obtaining pure potassium humate solid by centrifugation; S6: drying the mineral source potassium humate through a drying device.

2. The method for producing ore-derived potassium humate by acid dissolution and alkali precipitation according to claim 1, characterized in that: In step S1, the particle size of the material is 200-300 mesh.

3. The method for producing ore-derived potassium humate by acid dissolution and alkali precipitation according to claim 1, characterized in that: In step S6, the drying device comprises a dryer (1), a raw material storage barrel (2), a first cyclone separator (3), a second cyclone separator (4) and a heating box (6), characterized in that: the bottom end of the raw material storage barrel (2) is connected to a material injection pump (201), the discharge port of the material injection pump (201) is connected to a material injection pipe (202), and the other end of the material injection pipe (202) extends into the dryer (1), and one end of the material injection pipe (202) located in the dryer (1) is connected to an atomizing nozzle, and the dryer (1) is connected to a first discharge pipe (102), a second discharge pipe (103) is connected to the bottom end of the dryer (1), the first discharge pipe (102) is connected to the feed port of the first cyclone separator (3), the second discharge pipe (103) is connected to the feed port of the second cyclone separator (4), and the bottom end of the first cyclone separator (3) is connected to the second discharge pipe 103, a heating component and a preheating component are arranged in the heating box (6), the heating component is used to heat the outside air to dry the raw materials, and the preheating component uses the waste heat of the exhaust gas to preheat the air entering the heating box (6).

4. A drying device for producing mineral-source potassium humate according to claim 3, characterized in that: The heating assembly comprises an electric heater (606) arranged at the end of the heating box (6); an air pump (603) is also fixedly installed at the top of the heating box (6); an inlet of the air pump (603) is connected to an air suction pipe (6032); the air suction pipe (6032) is connected to the side wall of the heating box (6); an outlet of the air pump (603) is connected to an air injection pipe (6031); the air injection pipe (6031) extends into the dryer (1); and the angle between the outlet of the air injection pipe (6031) and the atomizing nozzle at the bottom end of the injection pipe (202) is 30°.

5. A drying device for producing mineral-source potassium humate according to claim 4, characterized in that: An air inlet pipe (604) is also provided on the side wall of the heating box (6), and a third filter screen (605) is provided on the air inlet pipe (604).

6. A drying device for producing mineral-source potassium humate according to claim 5, characterized in that: A first filter screen (601) and a second filter screen (602) are also provided in the heating box (6); the first filter screen (601) and the second filter screen (602) are inserted into the heating box (6) from the top end of the heating box (6).

7. A drying device for producing mineral-source potassium humate according to claim 3, characterized in that: The top ends of the first cyclone separator (3) and the second cyclone separator (4) are connected to each other by a connecting pipe (5), and the bottom ends of the connecting pipe (5) extend into the first cyclone separator (3) and the second cyclone separator (4), respectively.

8. A drying device for producing mineral-source potassium humate according to claim 3, characterized in that: The preheating component comprises a first annular tube (701) and a second annular tube (702) arranged in a heating box (6); the diameter of the first annular tube (701) is smaller than the diameter of the second annular tube (702); the first annular tube (701) and the second annular tube (702) are arranged near an air intake pipe (604); the first annular tube (701) and the second annular tube (702) are arranged between a first filter net (601) and a second filter net (602); a plurality of heat exchange tubes (703) are arranged in communication between the first annular tube (701) and the second annular tube (702); and the plurality of heat exchange tubes (703) are arranged obliquely.

9. A drying device for producing mineral-source potassium humate according to claim 8, characterized in that: A conduit (501) is provided on the side wall of the second annular tube (702), and the other end of the conduit (501) is connected to the top end of the connecting tube (5).

10. A drying device for producing mineral-source potassium humate according to claim 9, characterized in that: An exhaust pipe (7) is arranged on the side wall of the first annular tube (701), and the exhaust pipe (7) extends to the outside of the heating box (6).

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