WO2003051773A1

WO2003051773A1 - Process for the production of elemental boron by solid state reaction

Info

Publication number: WO2003051773A1
Application number: PCT/CA2002/001939
Authority: WO
Inventors: Sabin Boily; Houshang Darvishi Alamdari; René DUBUC; Julie Gaudet
Original assignee: Groupe Minutia Inc.
Priority date: 2001-12-19
Filing date: 2002-12-19
Publication date: 2003-06-26
Also published as: CA2365541A1; AU2002350330A1

Abstract

The invention relates to a process for the production of elemental boron. The process of the invention comprises subjecting a mixture of a reducible boron compound and a reducing agent to mechanical activation, whereby the boron compound is reduced to elemental boron by the reducing agent. When the reducible boron compound contains sodium, the reducing agent is used in combination with a salt thereof in order to enable the desired solid state reaction to occur. Such a process enables one to produce elemental boron at low cost.

Description

PROCESS FOR THE PRODUCTION OF ELEMENTAL BORON BY SOLID

STATE REACTION TECHNICAL FIELD

The present invention relates to a process for the production of elemental boron by solid state reaction. BACKGROUND ART

Boron, the fifth element in the periodic table, is a hard and brittle element which is classified among nonmetals such as carbon, arsenic, germanium, etc... Non-metals differ markedly from metals in electronic structure, physical and chemical properties such as electronegativity, thermal and electrical conductivity.

Boron was discovered as an element by the English chemist Davy and by the French chemists Gay-Lussac and Thenard in 1808. It took almost 100 years before this element was obtained in 80% purity. Now, boron may be prepared from its compounds by different methods such as chemical reduction, nonaqueous electric reduction or thermal decomposition.

¹ Reduction with hydrogen at high temperatures, especially hot filament reaction with boron halides, is the conventional method for obtaining high purity boron (>99% pure). Boron can also be prepared by electrolysis of melts containing borates or fluoroborates. Another way to obtain elemental boron is direct decomposition of boron from compounds such as halides and hydrides, to high purity boron at high temperatures (800-1100 °C). All these methods need high temperature operations and are costly and difficult to scale-up.

The most common method for producing large amounts of elemental boron is the exothermic reduction of boron trioxide with magnesium: B₂O₃ + 3Mg →3MgO + 2B

In this method, B₂O₃ and Mg powders are mixed and heated. The exothermic reaction occurs and causes an increase in temperature in excess of 800°C.

The resulting material is a mixture of boron and MgO which is removed by acid washing. Boron obtained by this method is amorphous and impure (usually <90%). hi order to obtain crystalline or pure boron, further processes are required.

Boron can be purified by zone refining or other thermal techniques. Crystalline boron is obtained by dissolving amorphous boron in liquid aluminum and cooling it down slowly. During cooling, crystalline boron is precipitated in the Al matrix. The matrix is removed by chemical reactions and the crystalline boron is recovered. Amorphous boron can be converted to β-rhombohedral above 1000 °C. As amorphous boron is highly reactive and spontaneously flammable at high temperatures, handling or processing of it at high temperatures requires special and costly precautions.

Ball milling, specially high energy ball milling, is an alternative technique to induce solid-state reactions. US Patent No. 5,328,501 teaches that certain metal oxides can be reduced by milling the oxide and a reducing agent. For example, metal oxides such as CuO, CdO, Fe₂O₃, V₂O₅, ZnO, can be reduced to elemental Cu, Cd, Fe, V, Zn, ... by high energy ball milling. However, the reduction of nonmetal compounds to nonmetal elements by this technique is not evident. For example, milling of SiO₂ in the presence of active metals results in the formation of intermetalic compounds such as silicates instead of elemental silicon. It is therefore an object of the present invention to overcome the above drawbacks and to provide a process for the production of elemental boron by solid state reaction. DISCLOSURE OF THE INVENTION

According to the invention, there is thus provided a process for the production of elemental boron, comprising subjecting a mixture of a reducible boron compound and a reducing agent to mechanical activation, whereby the boron compound is reduced to elemental boron by the reducing agent. When the reducible boron compound contains sodium, the reducing agent must be used in combination with a salt thereof in order to enable the desired solid state reaction to occur. Applicant has found quite unexpectedly that elemental boron, a nonmetal element, can be produced by subjecting a mixture of a reducible boron compound and a reducing agent to mechanical activation, with the proviso that when the reducible boron compound contains sodium, the reducing agent is used in combination with a salt thereof. MODES FOR CARRYING OUT THE INVENTION

Examples of suitable boron compounds include boron trioxide (B₂O₃), orthoboric acid (H₃BO ), methaboric acid (HBO₂), sodium borate (NaBO₃-4H₂O) and anhydrous sodium tetraborate (Na₂B₄O₇). Boron trioxide is preferred. Examples of suitable reducing agents include aluminum, magnesium and calcium. Magnesium is preferred.

Mechanical activation is advantageously performed by mechanical milling in a ball mill, attrition mill, shaker mill, rod mill or any other suitable milling device. High energy ball milling using a SPEX (trademark) vibratory ball mill operated 8-25 Hz is preferred. It is also possible to use a rotary ball mill operated at 300-1500 rpm or a rotary attritor operated at 50-300 rpm. A ball-to-powder ratio of 1:1 to 20:1 is generally used. The mechanical impact on the powder mixture results in a solid-state chemical reaction between the boron compound, the reducing agent and, optionally, the salt of the reducing agent.

The mechanical activation is preferably carried out in an inert gas atmosphere such as argon, to prevent oxidation of the elemental boron as well as undesirable oxidation of the reducing agent. Since the product obtained comprises a powder mixture of elemental boron and an undesired compound, usually an oxide of the reducing agent, pure boron can be obtained by removing the undesired compound by leaching or other purification techniques. When the reducing agent and a salt thereof are mixed with a reducible boron compound that contains sodium, a sodium salt is also produced during milling. This undesired sodium salt can be removed by solubilization in water during the leaching process. Preferred leaching media include:

HC1 when the reducing agent is Mg;

KOH or NaOH when the reducing agent is Al; and

H₂SO₄ or HC1 when the reducing agent is Ca. When the reducing agent is Mg and use is made of HC1, the leaching can be carried out at a temperature in the range of 10°C to boiling point.

A preferred leaching method consists in adding the aforesaid powder mixture into an acid solution bath, after the reduction reaction has occurred. To help the dissolution of the undesired compound, the solution can be milled in the ball mill. Any agglomerates are broken during milling and the leaching medium is in contact with fresh surfaces so that the rate of dissolution increases. However, such a method requires a special acid resistant coating on the inner surface of the crucible. Crystalline boron can be directly obtained by the process according to the invention. Milling times of generally less than 40 hours and ball-to-powder ratios generally ranging from 1:1 to 10:1 result in the formation of an ultrafϊne crystalline boron powder. The following non-limiting examples illustrate the invention.

EXAMPLE 1

3.6 g pure Mg powder (99.8%) were mixed with 3.4 g pure B₂O₃ powder (99.98%) and milled in a hardened steel crucible with a ball-to-powder ratio of 3.3:1 using a SPEX 8000 vibratory ball mill operated at a frequency of about 17 Hz. The operation was performed under a controlled argon atmosphere to prevent oxidation. After 20 hours of milling, the reaction was substantially completed. The resulting powder was then leached in an acid solution bath containing 12.5 ml of water and 50 ml of concentrated HC1 (35% w/w) to remove undesirable magnesium oxide. The solution was heated under agitation during 5 hours at a temperature between 70°C and 80°C. The temperature of the solution was then increased until the boiling point of the solution is reached and the ebullition was maintained for one additional hour. The solution was then filtered, washed with water and dried at a temperature between 90°C and 140°C overnight. The resulting powder was subjected to further washing and drying steps. The powder obtained was crystalline boron with 94% purity, or better; C, S, N, Cl, Al, Cr, Fe, Mg, Mn, Mo, Si and V were determined by ICP or calorimetric analysis.

EXAMPLE 2

3.06 g pure Al powder (99.8%) were mixed with 3.94 g pure B₂O₃ powder (99.98%) and 2% wt. of stearic acid. The mixture was milled in a hardened steel crucible with a ball-to-powder ratio of 3.1:1 using a SPEX 8000 vibratory ball mill operated at a frequency of about 17 Hz. After 20 hours of milling, the reaction was substantially completed.

EXAMPLE 3

3.8 g pure Mg powder (99.8%) were mixed with 3.2 g pure H₃BO₃ powder (99.0%) and milled in a hardened steel crucible with a ball-to-powder ratio of

3.3:1 using a SPEX 8000 vibratory ball mill operated at a frequency of about 17 Hz.

The operation was performed under a controlled argon atmosphere to prevent oxidation. After 3 hours of milling, the reaction was substantially completed. The resulting powder was then leached in an acid solution bath containing 7.7 ml of water and 50 ml of concentrated HC1 (35%) w/w) to remove undesirable magnesium oxide. The solution was heated under agitation during 5 hours at a temperature between 70°C and 80°C. The temperature of the solution was then increased until the boiling point of the solution is reached and the ebullition was maintained for one additional hour. The solution was then filtered, washed and dried at a temperature between 90°C and 140°C overnight. The resulting powder was subjected to further washing and drying steps. The powder obtained was crystalline boron with 91%) purity, or better; C, S, N, O, Cl, Al, Cr, Fe, Mg, Mn, Mo, Si, V and Sc were determined by ICP or calorimetric analysis.

EXAMPLE 4

2.3 g pure Mg powder (99.8%) were mixed with 3.2 g pure Na₂B₄O₇ powder (99.95%>) and 1.5 g pure MgCl₂ powder (99%). The mixture was milled in a hardened steel crucible with a ball-to-powder ratio of 3.2:1 using a SPEX 8000 vibratory ball mill operated at a frequency of about 17 Hz. The operation was performed under a controlled argon atmosphere to prevent oxidation. After 20 hours of milling, the reaction was substantially completed. The resulting powder was then leached in an acid solution bath containing 43 ml of water and 43 ml of concentrated HC1 (35% w/w) to remove undesirable magnesium oxide and sodium chloride. The solution was heated under agitation during 5 hours at a temperature between 70°C and 80°C. The temperature of the solution was then increased until the boiling point of the solution is reached and the ebullition was maintained for one additional hour. The solution was then filtered, washed and dried at a temperature between 90°C and 140°C overnight. The resulting powder was crystalline boron. EXAMPLE 5

3.8 g pure Mg powder (99.8%) were mixed with 3.2 g pure H₃BO₃ powder (99.0%) and milled in a hardened steel crucible with a ball-to-powder ratio of 3.1:1 using a SPEX 8000 vibratory ball mill operated at a frequency of about 17 Hz. The operation was performed under a controlled argon atmosphere to prevent oxidation. Nine additional millings were performed under the same conditions. After 3 hours of milling, the reaction was substantially completed. All resulting powders were mixed and then leached in an acid solution bath containing 125 ml of water and The solution was agitated at ambient temperature during 1 hour. The solution was then filtered, washed and dried at a temperature between 90°C and 140°C overnight. 2.6g of the resulting lixiviated powder was charged in a Si₃N₄ ceramic crucible together with 10 ml of concentrated HCl and 5 ml of water. The solution was milled for 1 hour. The solution was then filtered, washed and dried at a temperature between 90°C and 140°C overnight. The resulting powder was crystalline boron.

EXAMPLE 6

4.6g pure Ca (99.5%) were mixed with 2.4g pure H₃BO₃ (99.0%) and

2% wt. of stearic acid. The mixture was milled in a hardened steel crucible with a ball-to-powder ratio of 3.1:1 using a SPEX 8000 vibratory ball mill operated at a frequency of about 17Hz. After 10 hours of milling, the reaction was substantially completed.

Claims

CLAIMS:

1. A process for the production of elemental boron, comprising subj ecting a mixture of a reducible boron compound and a reducing agent to mechanical activation, with the proviso that when the reducible boron compound contains sodium, the reducing agent is used in combination with a salt thereof.

2. A process according to claim 1, wherein the boron compounds is selected from the group consisting of boron trioxide, orthoboric acid, methaboric acid, sodium borate and anhydrous sodium tetraborate.

3. A process according to claim 2, wherein the boron compound is boron trioxide.

4. A process according to claim 1, 2 or 3, wherein the reducing agent is selected from the group consisting of aluminum, magnesium and calcium.

5. A process according to claim 4, wherein the reducing agent is magnesium.

6. A process according to claim 4, wherein the elemental boron produced is in admixture with an undesired compound, and wherein the undesired compound is removed by leaching.

7. A process according to claim 6, wherein the reducing agent is magnesium and wherein the mixture of elemental boron and undesired compound is leached in hydrochloric acid at a temperature of 10°C to boiling point.

8. A process according to any one of claims 1 to 5, wherein the mechanical activation is performed by mechanical milling in a ball mill, attrition mill, shaker mill or rod mill.

9. A process according to claim 8, wherein the mechanical activation is performed by high energy ball milling using a vibratory ball mill operated 8-25 Hz.

10. A process according to claim 9, wherein the vibratory ball mill is operated at about 17 Hz.

11. A process according to claim 8, wherein the mechanical activation is performed in a rotary ball mill operated at 300-1500 rpm.

12. A process according to claim 11, wherein the rotary ball mill is operated at about 1000 rpm.

13. A process according to claim 8, wherein the mechanical activation is performed in a rotary attritor operated at 50-300 rpm.

14. A process according to claim 13, wherein the rotary attritor is operated at 250 rpm.

15. A process according to any one of claims 8 to 14, wherein the mechanical activation is carried out in an inert gas atmosphere.

16. A process according to any one of claims 8 to 15, wherein the mechanical activation is carried out for a period of time of less than 40 hours at a ball- to-powder ratio ranging from 1:1 to 10:1, whereby elemental boron having a crystalline structure is obtained.