FR2501529A1 - Finely divided prod. mfr. from exothermic reaction - by local initiation of reaction and propagating reaction under control - Google Patents

Abstract

Method of mfg. a finely divided product using an exothermic reaction comprises locally initiating the reaction by heating, then spreading it at uniform temp. by displacing the charge of reactants w.r.t. the local heater by translatory movement, so as to one the one hand, avoid an excessive raising of the temp. and on the other hand, reduce the dwell time of the reaction products in the hot zone of the reactor. The method is used e.g. for reactions of reducing an oxide of a metal or non-metal, esp. reduction of boric oxide with Mg,. The reaction is controlled to affect the whole of the charge supplied. The reaction products are held at a high temp. only for a short time, so that contamination of the reactor is reduced and the prods. are formed as finely divided particles.

Description

 The present invention relates to a process for carrying out a highly exothermic chemical reaction and to an apparatus for carrying out this process.

 The method according to the invention is generally applicable to an exothermic chemical reaction using gods or more than two reactants.

 The exothermic reactions which can be carried out by virtue of the invention are for example those involving the reduction of a metal or non-metal oxide by one or more simple reducing bodies or even the combination of two simple bodies.

 The device used allows materials to be prepared very quickly in large quantities and with a minimum of contamination from the equipment.

 The method according to the invention is particularly suitable for the preparation of amorphous and finely divided elemental boron by reduction of boron oxide B203 by a very electro-positive metal such as magnesium (reaction proposed in principle by NOISSÂN).

 Since the MOISSON works, numerous processes have been proposed for preparing boron; they lead to products which differ greatly in terms of purity, physical nature and chemical reactivity. The industrial preparation processes for pure boron can currently be classified into two groups, depending on whether the product is obtained in bulk form or in the form of a powder.

 Solid boron is prepared either by cracking boranes or more conveniently, by reduction at high temperature (1200-14000C) of a boron halide by hydrogen, in contact with a substrate which can between consists of a filament of refractory metal or a boron rod (see for example the patents of the United BTATS OF ANSRIA NO 2 839 367, 30 53 636, 3 115 393, 3 160 476 and 3 226 248). stars brought to a very high degree of purity by the molten zone process. On the other hand, the boron thus prepared (rhombohedral variety fh) has a very low chemical reactivity due to both its massive nature and its crystallographic nature.

 All the processes for preparing pure boron in powder form are in fact based on a reduction, at moderate temperature, of an oxygenated or halogenated combination of boron (3203, BF3, BCl3, BBr3 or KBG4) by a highly electropositive metal (alkali metal or alkaline earth), which in some processes can be produced "in situ" by igneous electrolysis, as described in US Pat. Nos. 2,465,989, 2,572,248, 2,572,249, 2,686 501, 2 794 708 and 2 918 417. In all cases, the boron formed, generally amorphous or at least poorly crystallized, can only be separated from the by-products of the reaction by wet means, thanks to a complex chemical treatment .Due to the very high chemical reactivity of this type of boron, it necessarily results in oxidation of the products produced and, moreover, certain impurities cannot be completely eliminated (carbon, higher borides). The effective elemental boron titer of such products therefore hardly ever exceeds 97%, an insufficient value for certain applications but nevertheless acceptable for various industrial applications. To overcome this drawback, various purification methods have been proposed; the only one which is effective is a heat treatment carried out under vacuum, at a temperature equal to or slightly lower than the melting point of boron. This heat treatment, stil effectively makes it possible to reach a titer greater than 99%, leads to a massive boron which is not very reactive and which, because of this, must further reduced to a powder for most of the applications (operation made delicate by the very high hardness of the boron and which necessarily causes significant contamination).

Boron with a purity equal to or greater than 99% can be obtained by reducing a boron halide B (BCl3 or better
BBr3) with hydrogen in the vapor phase at a temperature which varies according to the implementation This procedure has already been used at low temperature (800-10000C) in order to obtain a pulverulent product as described by
R. XIESSLING in Acta.Chem. Scand. 2, 207, 1948, then by
R. NASLAIN, J. ETOURNEÂU and P. HAGENMULLER in "Boron", vol 3,
Proceedings of the Int.Symp. on Boron, Warsaw (1 9oe), PWN
Polish Scientific Publ. (1970) - ANVAR patent N 72 12 260, 1972 then at higher temperature (hot plasmas) by HD MIEDOCH and SM IESLEY HAMBLYN in French patent N 1 582 154.

 In the embodiments according to the last five references cited above, the boron obtained, of purity greater than 99%, is finely divided and therefore very reactive.

It should be noted, however, that the proposed methods, on the one hand all use boron halides, which are expensive reagents, the implementation of which is difficult due to their high reactivity with respect to water vapor, on the other hand lead to hourly productions which remain relatively low from 1g / h to 150 g / h). The highest hourly boron production is obtained by the process cited above (patent No. 1,582,154) which employs the reduction of 13C13 at high temperature in an argon-hydrogen induction plasma, a device which still remains very expensive.

 The present invention has the object of improving these various processes by proposing a process for manufacturing a finely divided product using an exothermic reaction, characterized in that the reaction is locally initiated by heating and then propagated at temperature. uniform by moving the charge of the reagents relative to the local heating device by a translational movement in order, on the one hand, to avoid an excessive rise in temperature and, on the other hand, to reduce the residence time of the products of the reaction in the hot zone of the reactor.

 The originality of this process consists, starting from a homogeneous load of reagents, to locally initiate the reaction and to propagate it in a controlled manner to all the load so that the heat losses are balanced by energy released by the reaction . The method also has the advantage of only carrying the reaction products for a very short time at high temperature, which promotes the formation of divided products and limits their contamination by the reactor.

 Such a process is particularly satisfactory for carrying out the reaction between a reducible combination of boron such as, for example, a boron oxide and a reducing agent such as an electro-positive metal, in particular magnesium. , to obtain elemental boron.

 Indeed, according to this process, we manage to prepare amorphous elemental boron of average purity (90 to 97%) but of very large specific surface (15 to 30 m2 / g) therefore thsmically very active and on the other hand an hourly production very large (several kilograms at the level of a pilot unit).

This process is based on the following exothermic chemical reaction
3203 + 3Mg - 2B + 3Mg0
The weight ratio B203 / Mg 2 for the starting mixture leads to a product containing as impurities essentially magnesium (8%) and less than 2% oxygen, after washing with hot hydrochloric acid.

 According to another characteristic of the invention, the reaction temperature is maintained at a value less than or equal to 11000C, preferably at 12000C and the translation speed in the vicinity of 12 cm / min, so as to obtain an amorphous boron small particle size and high specific surface.

 In fact, the process, in addition to the advantage of quickly carrying out the reduction reaction, makes it possible to obtain an amorphous boron insofar as it is only brought to high temperature for an extremely short time thanks to the translational movement of the device. The temperature and the speed of cooling are very important factors since they govern the crystallization process and the particle size of the product. Boron is all the more active when it is prepared at low temperature and the faster the cooling rate.

 According to another characteristic of the imention, the boron obtained is subjected to a chemical treatment based on hydrochloric acid and fluoride, so as to reduce the magnesium and oxygen content without harming the state of division of the boron , thereby improving its purity. Thus, one can obtain a product whose purity reaches 97% of total boron.

 According to another characteristic of the invention, local heating of the reactants is provided by HP induction.

 According to another characteristic of the invention, the propagation and the control of the front of the exothermic reaction are ensured by displacing, using a mechanical translation system, the charge of the reactants inside the HF inductor .

 According to the invention, the reagents can be kept under an inert or reducing gas atmosphere or else under vacuum.

The process according to the present invention therefore makes it possible to obtain an amber, very powdery boron (average particle diameter 9 0.6 <!) And having a specific surface ranging from 15 to 30 m 2 / g. The methods previously proposed (USA patent 1,582,154 and ANVAR patent 72 12,260, 1972) do not make it possible to obtain a boron having the characteristics indicated above, but a boron whose mean particle diameter is larger (1, 5 to 2v t and the lower specific surface (4 to 8 m2 / g)
Therefore, the boron obtained by the methods of the prior art is obviously less reactive than the boron obtained according to the present invention.

 Of course, the invention is not limited to the reduction of boron oxide by magnesium, but can be applied to any exothermic reaction.

 As a result and according to another characteristic of the invention, the reactants may as well be a reducible combination of boron, carbon and a very electropositive metal making it possible to obtain boron carbide after acid washing and drying under vacuum.

 Likewise, the reagents can be a reducible combination of boron, and a reducible combination of a native and a reducer to obtain borders, or elemental boron and transition metals; this also makes it possible to prepare borides.

 For example, as borides, it is possible to prepare transition metal borides (tir2, Or32, HF32, etc.) or else alkaline earth gildings such as CaB6 or Sur36 or the thorium tetraboride ThB4.

 The present invention also relates to an apparatus used for implementing the method which has been described above.

 This device is characterized in that it consists of a metallic reactor with a double cylindrical wall containing the charge of the reactants and the diameters of which are chosen so that the quantity of reactants at the level of the propagation front of the exothermic reaction either such that the temperature does not exceed 11000C.

 According to another characteristic of the invention, the metal tube is parcounipar by HP currents which locally and homogeneously heat the reaction mixture.

 According to another characteristic of the invention, the heating is provided by HF induction of an independent metal tube and outside of an enclosure containing the reagents.

 The apparatus which is the subject of the invention will be described in more detail with reference to the accompanying drawing which is a diagram of this apparatus.

 According to this figure, the device which is the subject of the invention consists of a metal reactor 1 with a double wall 10 and 11, cylindrical, containing the charge of the reactants 2. the diameters of the metal tubes 10 and 11 are chosen so that the amount of reagent at the 1.1 ′ propagation front of the reaction does not exceed 11000C.

 A protective tube 3, for example made of silica glass, constitutes the external enclosure of the reactor I and makes it possible to operate under vacuum or under an inert gas, such as, for example argon; the entire reactor is connected to this source of vacuum or neutral gas by means of a pipe 4.

 A high frequency inductor of suitable geometry provides local heating of the metal tube 1 (800-9000C) to initiate the reaction. If the priming is carried out at the upper end of the reactor, as shown in the figure, the reaction tends to propagate from top to bottom along arrow A, but, due to heat losses, the propagation front I, I '' moves only a few centimeters. To compensate for these heat losses, a mechanical device represented diagrammatically by the arrow B allows the reactor 1 to move from bottom to top inside the high frequency inductor 5 at the same speed as the propagation front X, X ' of the reaction.

 In the present description, the local heating is provided by high frequency induction, but it is clear that this heating could be achieved using other methods known to those skilled in the art without calling into question the originality of the process. It is therefore in controlling the exothermic propagation and its effects that the originality of the invention resides.

 the process and the apparatus which are the subject of the invention will be illustrated in the examples below.

Of material constituting the cylindrical reactor, which is preferably stainless steel in the case of the preparation of boron, is chosen according to its compatibility at high temperature with the products prepared.

gX3PLE 1: a powder mixture consisting of 1000 g of
B203 and 500 g of magnesium are placed under an argon atmosphere inside a stainless steel tube 90 mm in diameter and 700 mm in length. The reaction is initiated by locally heating the upper part of the tube to 8500 C using an HF inductor. After initiation of the reaction, the temperature of the reaction medium rises to approximately 1000 ° C.

At this instant, the reactor is driven in a translational movement allowing the propagation front of the reaction to move at 12 cm / mm, so as to maintain the temperature of the reaction medium at the value indicated above. Not to exceed 11000C is desirable for two essential reasons: 1) to avoid the crystallization of boron 2) to avoid the reaction between boron and stainless steel which
form a eutexia above 11000C.

 After complete reaction, the product obtained, although compact, remains very brittle. It consists essentially of boron, magnesia, magnesium borate and boric anhydride. After grinding, the product is treated with a hot hydrochloric acid solution (7000), then washed with methyl alcohol. After drying under vacuum at 1100C, a product titrating 90% of total boron is obtained. The main impurities are magnesium (8, '), metals such as iron, manganese, copper, titanium, calcium and potassium (0.2%) and oxygen (1.8%). The hourly output at the pilot installation level is around 2 kg / hour, a value much higher than that indicated in the cited patents.

 X-ray diffraction analysis indicates that the boron obtained is amorphous. The median diameter of the boron particles is 0.629w (Joyce Loebl distribution) and the specific surface of the material, determined by a BT type method, is 30 g / m2.

EXAMPLE 2 The procedure indicated in Example 1 was
say again. The product obtained, titrating 90% of total boron, was treated at 9000C under argon with a mixture of fluorides, for one hour. After washing with hot water using a hydrochloric acid solution, a product titrating 97% of total boron was obtained. The main impurities are still oxygen (0.8%;), magnesium (0.3%) and other metals (Pe, Mn, Cu, Ti, Ca, K).

 The boron obtained remains amorphous and finally divided (specific surface: 14 m2 / g). The fluoride treatment therefore improves the product's total boron titer and clearly reduces the magnesium and oxygen contents. The specific surface of the material, although lower than that indicated in example 1, remains however much higher than those reported in the patents we have cited (4 to 8 m2 / g).

EXAMPLE 3: the procedure indicated in example 1 was
repeated in a molybdenum reactor.

 The reaction starts violently but the temperature reached is much higher (; - 12000C) because, locally at the initiation level, the amount of reactants is twice as large as that used in Example 1.

 The product obtained, as in Example 1, has 90% total boron. The higher reaction temperature (s 12000C) than in Example 1, leads to a partially crystallized boron (rhombohedral variety) and with a lower specific surface (7 m2 / g).

 The main factor which influences the crystallinity and the specific surface of the boron prepared is mainly the temperature of the reaction medium which depends on the quantity of reactants present at the initiation of the reaction.

EXAMPLE 4: the procedure indicated in Example 1 was
applied to prepare boron carbide B4C.

Boron carbide can be prepared by reducing B203 with carbon, but the reaction is endothermic and the proposed method cannot therefore be used any other. To overcome this drawback 7a reduction of 3203 is achieved by a mixture of carbon and magnesium following the exothermic reaction
2 B2O3 + 6 Mg + C
BC + 6 MgO
An intimate, powdery mixture of 1000 g of
B203, 1050 g of magnesium and 86 g of carbon was produced.

As for the preparation of boron, the exothermic reaction begins around 850-9000C. The product obtained after reaction is brittle.

After grinding the product is washed with a hydrochloric acid solution, then dried under vacuum at 1100C. The X-ray diffraction spectrum of the product is that of crystallized boron carbide 34C. The impurities present are essentially those contained in the boric acid and the starting magnesium, ie:
0.5 to 1% (Be + Mn + Cu; Ti, Ca, K
The boron carbide prepared is very pulvenllent (average particle diameter 1-2 <). The product obtained is in all respects comparable to that prepared by the process described in European patent 0 011 524.

EXAMPLE 5 The TiB2, ZrB2, HPB2 and Th34 borides were prepared
from stoichiometric mixtures of boron and powdered metal. The reactions are violently exothermic and start around 800-900 C. In order to avoid any contamination of the borides by the reactor, the reactions are carried out under argon in a cooled copper basket. The device is slightly different from that used in the previous examples. The boron + metal mixture is introduced in a thin layer (2 to 3 mm) in a nacelle which moves horizontally, inside an HP inductor

 The borides obtained are crystallized and their purity is greater than 99%. A few minutes are enough to prepare several tens of grams of borides.

Claims (15)

 10) Process for manufacturing a finely divided product using an exothermic reaction, characterized in that the reaction is initiated locally by heating, then it is propagated, at uniform temperature, by shifting the charge of the reactants relative to the local heating device by a translational movement in order, on the one hand, to avoid an excessive rise in temperature and on the other hand to reduce the residence time of the reaction products in the hot zone of the reactor.  20) Process according to claim 1, characterized in that the reagents used are a reducible combination of boron and a reducing agent making it possible to obtain elemental boron.  30) Process according to claim 2, characterized in that the reagents used are a boron oxide and an electropositive metal, in particular magnesium.  40) Method according to claim 3, characterized in that lton maintains the reaction temperature at a value less than or equal to 11000C preferably at 1200 C, and the translation speed in the vicinity of 12 cm / min, so as to obtain a amorphous boron of small particle size and high specific surface.  50) Process according to claim 4, characterized in that the boron obtained is subjected to a chemical treatment based on hydrochloric acid and fluorides so as to reduce the magnesium and oxygen content without harming the state of division of boron, thereby improving its purity.  60) Method according to any one of claims 1 to 5, characterized in that one ensures the local heating of the reagents by induction R.P.  70) Method according to any one of claims 1 to 6, characterized in that the propagation and the control of the front of the exothermic reaction are ensured by moving, using a mechanical translation system, the charge of the reagents inside the HP inductor  80) Process according to any one of claims 1 to 7, characterized in that the reactants are kept under an inert or reducing gas atmosphere.  90) Process according to any one of claims 1 to 7, characterized in that the reagents are placed under vacuum.  100) Method according to claim 1, characterized in that the reagents are a reducible combination of boron, carbon and a very electropositive metal making it possible to obtain boron carbide after acid washing and drying under vacuum.  110) Process according to claim 1, characterized in that the reactants are a reducible combination of boron, a reducible combination of a metal and a reducing agent, making it possible to obtain borides.  12) Process according to claim 1, characterized in that the reactants are elemental boron and transition metals thus making it possible to prepare borides.  130) Process according to any one of claims 11 and 12, characterized in that the borides prepared are the diborides of the transition metals (tir2, ZrB2, HPB2 ...> the alkaline earth borides (CaB6, Sur36) and the ThB4 thorium tetrabride.  14) Apparatus for implementing the method according to any one of claims 1 to 13, characterized in that it consists of a metal reactor with a cylindrical double wall containing the charge of the reagents and whose diameters are chosen so that the quantity of reactants, at the propagation front of the exothermic reaction, is such that the temperature does not exceed 11000C.  150) Apparatus according to claim 14, characterized in that the metal tube is traversed by HP currents, which locally and homogeneously heat the reaction mixture.  160) Apparatus according to claim 14, characterized in that the heating is provided by HP induction of an independent metal tube and outside the enclosure containing the reagents.