JPS6159780B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention generally relates to electrical resistance heating reactors for use in reactions that monitor the presence or generation of substances that attack electrical resistors during the reaction.

The present invention relates to the implementation of a method for producing boron or boron carbide by reducing boric acid, a boron salt such as sodium borate, or boron oxide with a reducing metal in the presence or absence of a carbon source. Although the reactor of the present invention is specifically described in connection with the reaction, it is obvious that the reactor of the present invention is generally used for reactions in which the presence or generation of substances that attack electrical resistors is observed. be.

Boron, a borate such as sodium borate, or boron oxide is reduced with a reducing metal such as aluminum, calcium, or magnesium in a non-oxidizing atmosphere at 600 to 1100°C to produce boron. Alternatively, by adding a carbon source such as graphite or sucrose at the same time as a reducing metal, the material is reduced and carbonized under the same conditions to produce boron carbide.

This temperature condition can be met by an electric resistance heating furnace that uses heating wires and strips of nichrome, iron chrome, etc. as heating elements. It is extremely inexpensive compared to other types, and can be easily made larger.

However, since the reduction reaction or reduction carbonization reaction is a rapid exothermic reaction, boron oxide, reducing metals, etc. evaporate during the reaction, and both of these vapors severely deteriorate the heating element. Therefore, it is necessary to isolate the reaction chamber in which the above reaction takes place and the heating element storage chamber by some method.

On the other hand, boric acid decomposes and generates water vapor when heated, and borates and boron oxides are highly hygroscopic, so they usually contain a considerable amount of water and release water vapor when heated. This water vapor inhibits the reduction reaction or reduction carbonization reaction, resulting in residual borate, mechanical scattering of reactants and reaction products due to instantaneous generation of water vapor and thermal expansion during rapid exothermic reactions, or This causes steam oxidation of boron or carbide produced. Therefore, it is desirable to perform sufficient vacuum dehydration before starting the reaction. Furthermore, in order to create a non-oxidizing atmosphere inside the furnace, it is necessary to once place the inside of the furnace in a vacuum state and remove air.

Graphite must be used as a container for high-temperature chemical reactions, but in general, artificial graphite has fine pores and, with the exception of impregnable transparent graphite that has undergone special impregnation treatment, it is difficult for substances to diffuse and for fluids to flow. To allow penetration,
The walls of a graphite reaction vessel cannot completely prevent the passage of gas or metal vapor or the diffusion of substances. Therefore, it is impossible to create a high degree of vacuum only inside the graphite container, and if there is a pressure difference between the inside and outside of the graphite reaction container, or if the graphite reaction container is used for a long period of time, the reactants leaching out of the graphite container is unavoidable.

The two requirements mentioned above, that is, the isolation of the reaction chamber and the heating element storage chamber, and the evacuation of the reaction chamber, can be met at the same time by: (a) a reactor core that serves as both a separation wall and a vacuum-resistant container; (b) Both the reaction chamber and heating element storage chamber are installed inside a vacuum-resistant container, and there is a Two types of separation walls can be considered. Type (a) seems to be better in terms of downsizing the vacuum exhaust system and simplifying the role of the reactor core tube, but it has the following serious problems. . That is,
In addition to the fact that the maximum temperature during an exothermic reaction is 1200 to 1400℃, there is no single material that can withstand long-term use in terms of temperature and atmosphere, and because the core tube has a multilayer structure, In order to avoid this problem, the effective volume of the reaction chamber in which the reaction is carried out is drastically reduced due to the increase in the wall thickness of the reactor core tube, and the increase in the heat capacity of the reactor core tube itself is transmitted through the reactor core tube to the outside of the reactor. As a heating furnace, this has a fatal flaw in that the thermal efficiency is drastically reduced due to the increase in heat escaping to the furnace.

Therefore, boron or iodine carbides produced by the conventional method (a) or by a method using an expensive graphite heater Tammann furnace have low productivity and are expensive.

In the type (b), the inventors have found that by adopting a structure in which the graphite partition wall and the heating element storage chamber outside thereof can be separately evacuated, the above temperature and atmospheric problems can be solved, and at the same time thermal efficiency can be improved. Significant improvement〓〓〓〓
The present invention was completed based on this discovery.

According to the invention, an exhaust port with a valve (first exhaust port)
and a vacuum-resistant container equipped with a gas inlet (first gas inlet) with a valve; a reaction container made of graphite provided within the vacuum-resistant container; One having an exhaust port (second exhaust port) and a gas inlet with a valve (second gas inlet);
A chemical reaction furnace is provided which includes an electric resistance heating furnace surrounding the reaction vessel and disposed in the vacuum-resistant vessel.

The gist of the present invention resides in that a graphite reaction vessel and a vacuum-resistant vessel enclosing an electric resistance heating furnace are constructed so as to be able to be separately evacuated and ventilated, and the exhaust port and gas inlet may be provided in various ways. formats are possible. It is possible to provide an exhaust port and a gas inlet directly to the graphite reaction vessel.

Furthermore, the chemical reaction furnace of the present invention is not limited to the manner in which the graphite reaction vessel and the electrical resistor surrounding it are arranged in the vacuum-resistant vessel space, and includes so-called vertical furnaces and horizontal furnaces. Horizontal furnaces in which the walls of the graphite reaction vessel laterally partition the vacuum-resistant vessel space generally have the advantage that reactants or products can be taken in and out easily, but have the disadvantage that the gas flow within the furnace is complicated. On the other hand, in a vertical furnace, by causing the gas flow to undergo natural convection, it is possible to reduce turbulence in the gas flow and effectively purge the reducing metal vapor.

Next, one embodiment of the present invention will be described with reference to the drawings.

As described above, the chemical reaction furnace of the present invention consists of the vacuum-resistant container 5, the electric resistance heating furnace 6, and the graphite reaction container 8 as the main parts.

The vacuum-resistant container 5 has a cylindrical side wall 51 and a ceiling plate 5.
2. It consists of a bottom plate 53, and the side walls 51 are provided with water cooling jackets at key points (not shown). A cylindrical graphite reaction vessel 8 is arranged in the center of the vacuum-resistant vessel, and an electric resistance heating furnace 6 is arranged so as to surround it. Electric resistance heating furnaces are known, in which a heating element 7 made of, for example, Kanthal alloy is wrapped around the inner wall and filled with a heat-resistant material such as asbestos. The conductor tap 4 is pulled out from the place, and this is pulled out airtightly to the outside of the vacuum-proof container as seen on the right side of the drawing, and the thermocouple 2 is inserted as seen on the left side of the figure, and its conductor is similarly pulled out. It is drawn out airtight to the outside of the vacuum-resistant container. Advantageously, the outlet is removable. The furnace body of an electric resistance heating furnace does not need to be airtight.

A graphite reaction vessel 8 placed in the center of the vacuum-resistant container has an opening at its upper end closed and fixed by a ceiling plate of the vacuum-resistant container. "Closed" here
This term refers to the ceiling board that surrounds and fixes the cylindrical graphite reaction vessel, and although the surroundings do not necessarily have to be strict, It is the meaning.

The vacuum-resistant container 5 has a first exhaust port 3 that has a valve and communicates with the vacuum system, a first sub-exhaust port 3' that has a valve and a check valve and communicates with the atmosphere, and also has a valve. A first gas introduction pipe 10 is provided. The first exhaust port 3 communicates with a normal vacuum system, and the gas introduction pipe 10 normally communicates with a pressure accumulation source (cylinder) of an inert gas such as nitrogen or argon. In a preferred embodiment, the first exhaust port and the first sub-exhaust port are configured into one exhaust pipe (first exhaust pipe) equipped with a three-way switching valve.

In order to provide the first gas introduction pipe 10 on the bottom plate 53, it is preferable that the electric resistance heating furnace 6 is placed on a pillow 12 made of a heat insulating material, rather than being placed directly on the bottom plate.

A preliminary chamber 13 is provided at the top of the ceiling plate 52 of the vacuum-resistant container, surrounding the upper opening of the graphite reaction container, and this has a second exhaust port 1 having a valve and communicating with the vacuum system, and a second exhaust port 1 that communicates with the atmosphere. a second sub-exhaust port 1';
An airtight hatch 14 that can be opened and closed is provided. In a preferred embodiment, the second exhaust port and the second sub-exhaust port are one exhaust pipe (second exhaust pipe) equipped with a three-way switching valve.
It is composed of The communication ports of the first exhaust pipe and the second exhaust pipe to the vacuum system can be integrated and connected to one common vacuum system.

Rather than simply opening into the vacuum-resistant container, the first gas introduction tube 10 extends into the vacuum-resistant container as shown in the figure, and is brought close to the outer periphery of the bottom of the graphite reaction container. formed into a tube;
By providing a large number of gas release holes inside or on the upper side of the annular tube, the gas released into the vacuum-resistant container is carried by natural convection generated in the vacuum-resistant container space, and is connected to the graphite reaction container 8 and electrical resistance. Above the gap between body 7〓〓〓〓
It is preferable to raise the temperature.

The bottom of the graphite reaction vessel 8, like the upper opening, is fixed to the vacuum-resistant vessel in a closed manner. A hole 81 is bored in the bottom of the graphite reactor to allow a graphite support rod 17 for supporting the crucible 9 to enter. Although the contact between the support rod 17 and the hole 81 is not airtight, it is desirable that it be as closed as possible.

A second support rod 17 is provided at the bottom of the vacuum-resistant container 5 to airtightly and slidably support the graphite support rod 17.
A spare room 15 is provided. A hole is provided in the center of the bottom of the preparatory chamber to allow the support rod to pass through.For example, an O-
The graphite support rod is configured to be able to hold the O-ring 16, while the portion of the graphite support rod that makes sliding contact with this O-ring is covered with a stainless steel coating.
The lower part may be a bare stainless steel rod.

This preliminary chamber has a gas introduction pipe 1 having a second valve.
1 is provided. Like the first gas conduit, this also communicates with an accumulator source of inert gas.
The inert gas introduced from here is introduced into the latter through the gap between the support rod 17 and the hole in the bottom of the graphite reaction vessel, and exits from the vessel through the first sub-exhaust port.

This second auxiliary chamber is not essential in the present invention, and is unnecessary if a crucible support rod is not provided.

The graphite reaction vessel may be provided with a lid. The purpose of this lid is to keep the scattering of substances in the reaction vessel during sputtering of the reactant, but it has a hole through which gas can pass, and can be easily removed from the vessel.

With this structure, the upper hatch 14 is opened, the support rod is raised to the outside of the hatch by a drive mechanism (not shown), the necessary reaction material is charged into the crucible, the lid is applied, and the support rod is raised to the outside of the hatch. The rod is lowered to the desired position within the reaction vessel, the graphite reaction vessel is also covered if necessary, the hatch is closed, and heating, degassing, and ventilation operations are performed as necessary.

There is no need to describe the design or components in detail, as such a device can be easily fabricated based on this drawing with knowledge known to those skilled in the art. For reference, the vacuum-resistant container made by the inventors is made of stainless steel with a thickness of 6 mm.
Its outer diameter 730mm, height 790mm; outer diameter of graphite reaction vessel
220 mm, graphite wall thickness 15 mm, and a water cooling jacket around the outer periphery.

Next, an example of an operation in which boron carbide was synthesized using the above-mentioned reactor will be described.

The hatch 14 is opened, the support rods are raised to the outside of the hatch by a drive mechanism (not shown), a mixture of boric acid, magnesium, and carbon is charged into the crucible, the lid is closed, and the support rods 17 are moved into the reaction vessel 8. Lower it to the center, cover the graphite reaction container, and add hatch 1.
4 closed. Next, the valves of the first gas introduction pipe 10, the second gas introduction pipe 11, the first sub-exhaust port 3', and the second sub-exhaust port 1' are closed, and the valves of the second exhaust port 1 and the first exhaust port 3 are closed. The inside of the vacuum resistant container 5 and the graphite reaction container 8 are evacuated to a pressure of 1 torr or less using the vacuum pump provided at the end of the crucible, and the reaction mixture in the crucible is heated to 480°C by energizing the electric resistor 7. in
After heating for 90 minutes and vacuum dehydration, the drive mechanism
The crucible was raised to the preliminary room. Next, the valves of the first exhaust port 3 and the second exhaust port 1 are closed, and the first gas introduction pipe 1 is closed.
0 and the valves of the second gas introduction pipe 11 are opened, nitrogen is introduced from the first gas introduction pipe 10, and the second gas introduction pipe 1 is supplied with nitrogen.
After introducing argon from 1, the first sub-exhaust port 3' and the second sub-exhaust port 1' equipped with check valves are opened, and electricity is applied to the electric resistor 7 to control the temperature of the graphite reaction vessel 8. of
The crucible was then lowered to the center of the reaction vessel 8 by a drive mechanism, and the reaction mixture in the crucible was brought to 1020°C and held for 90 minutes. Nitrogen gas passed through the first gas introduction pipe 10 and was introduced from a number of small holes provided inside the annular pipe around the graphite reaction vessel 8 at the lower part of the electric resistance heating furnace 6 . The nitrogen flow is carried by the generated natural convection and raises the gap between the graphite reaction vessel and the electrical resistor, causing reducing metal vapor and the electrical resistor to leak out of the reaction vessel 8 through the pores of the graphite partition wall. Contact with them was prevented as much as possible.
The nitrogen flow rate was set to 1/100 to 1/10 of the void volume in the vacuum-resistant container 5 per minute. A portion of the nitrogen is exhausted from the first sub-exhaust port 1' while refluxing inside the vacuum-resistant container. A trace amount of magnesium metal vapor that leaked into the electric resistance heating furnace through pores in the graphite reacts with the nitrogen gas sent from the first gas introduction pipe 10 and becomes inert nitride, which is removed, causing deterioration of the electric resistor. was almost completely prevented. Also, the graphite reaction vessel was not attacked by reducing metal vapor or boric acid vapor. Next, raise the support rod,
Store the reaction product in a preliminary chamber and leave it to cool sufficiently〓〓〓〓
After that, the hatch 14 was opened and the reaction product was taken out. The obtained boron carbide intermediate is a pumice-like solid mainly composed of boron carbide and magnesium oxide, with small amounts of magnesium boride, carbon, and magnesium borate, and after immersing it in hydrochloric acid to remove the magnesium oxide. After heat treatment at 1800℃, highly pure boron carbide powder could be obtained.

Since the present invention is a furnace having the above-mentioned configuration, the atmosphere inside the furnace can be well controlled, and at the same time, an extremely durable graphite reaction vessel is used, and an electric resistor is created by combining this with nitrogen gas purge. In addition, it is possible to prevent deterioration of the heating capacity, and also to make effective use of heating volume and improve thermal efficiency.

[Brief explanation of the drawing]

The figure is a sectional view showing the concept of the device of the present invention, mainly showing the parts related to the present invention. 1: second exhaust port, 1': second sub-exhaust port, 2: thermocouple, 3: first exhaust port, 3': first sub-exhaust port,
4: Conductor tap, 5: Vacuum resistant container, 6: Electrical resistance heating furnace, 7: Kanthal alloy heating element (electrical resistor), 8: Graphite reaction vessel, 9: Graphite reaction crucible, 10: First gas Inlet, 11: Second gas inlet, 12: Heat insulating material, 13: Preliminary chamber, 14: Airtight hatch, 15: Second preliminary chamber, 16: O-ring, 1
7: Support rod, 51: Side wall, 52: Ceiling board, 53:
Bottom plate, 81: hole. 〓〓〓〓

Claims (1)

[Claims] 1. A vacuum-resistant container equipped with an exhaust port with a valve (first exhaust port) and a gas inlet with a valve (first gas inlet port); A reaction vessel, which has an exhaust port with a separate valve (second exhaust port) and a gas inlet with a valve (second gas inlet) independently of the vacuum-resistant container; A chemical reaction furnace comprising an electric resistance heating furnace placed in the vacuum-resistant container. 2. The chemical reactor according to claim 1, wherein the vacuum-resistant container is provided with a sub-exhaust port (first sub-exhaust port) having a valve and a check valve, and the graphite reaction container is also provided with a sub-exhaust port having a valve and a check valve. It is characterized by having a sub-exhaust port (second sub-exhaust port) having a stop valve. 3. The chemical reaction furnace according to claim 2, in which the vacuum-resistant container, the graphite reaction container, and the electric resistance heating furnace are all vertically cylindrical; Closely fixed to the ceiling plate of the vacuum vessel; a preliminary chamber surrounding the opening of the reaction vessel is provided on the ceiling plate, and the second exhaust port, the second sub-exhaust port, and the second exhaust port are connected to the reaction vessel. The bottom of the reaction vessel is closed-closely fixed to the bottom of the vacuum-resistant vessel, and the bottom of the reaction vessel has ventilation means; An apparatus characterized in that it has a second preliminary chamber, which communicates with the ventilation means at the bottom of the reaction vessel, and in which a second gas inlet is provided. 3. The chemical reactor according to item 3, wherein the reactor has a support rod that penetrates the bottom plate of the reaction vessel and enters the reaction vessel from below, the penetrating portion of the support rod constitutes a ventilation means, and the support rod A chemical reactor characterized in that the rod is slidably and airtightly supported at the bottom of the second preparatory chamber and can support a crucible at the upper end.5 Claims 2 to 4 In the chemical reactor according to any one of the above, the first gas introduction pipe is provided on the bottom plate of the vacuum-resistant container, and the introduction pipe penetrates and extends into the inside to form an annular pipe surrounding the bottom of the graphite reaction container. , a device characterized in that the annular portion thereof has a large number of small holes, and the introduced gas is released toward the gap between the graphite reaction vessel and the electric resistor. 6. Claims 2 to 5. The chemical reactor according to any one of the above, characterized in that the first exhaust port and the first sub-exhaust port, and the second exhaust port and the second sub-exhaust port are each integrated by a three-way valve. 7. A chemical reactor according to any one of claims 2 to 6, characterized in that the first exhaust port and the second exhaust port communicate with a common vacuum system. 8. A chemical reactor according to any one of claims 1 to 7, which cools a main part of a vacuum-resistant container.
It is also characterized by being able to