WO2008053460A1

WO2008053460A1 - Elemental magnesium production by carbothermic reduction for use in the regeneration of hydrogen storage compounds

Info

Publication number: WO2008053460A1
Application number: PCT/IL2007/001248
Authority: WO
Inventors: Jonathan Goldstein; Menachem Givon
Original assignee: HyoGen Ltd
Current assignee: HyoGen Ltd
Priority date: 2006-11-02
Filing date: 2007-10-18
Publication date: 2008-05-08
Anticipated expiration: 2009-05-02
Also published as: IL179005A0

Abstract

The invention provides a process for the production of elemental' magnesium from feed materials containing magnesium oxide and carbon, said process comprising: exposing excess magnesium oxide and carbon to a high temperature reaction zone, in the presence of a catalyst, which is optionally supported, for disproportionation of carbon monoxide back to carbon and carbon dioxide, wherein said magnesium oxide and carbon react at a temperature of at least 1100. OC in said reaction zone to produce gaseous magnesium and carbon monoxide, and.removing said gaseous magnesium and carbon dioxide from said reaction zone. The obtained magnesium powder is used in a process for synthesizing or regenerating hydrogen storage compounds such as metal borohydrides.

Description

ELEMENTAL MAGNESIUM PRODUCTION BY CARBOTHERMIC REDUCTION FOR USE IN THE REGENERATION OF HYDROGEN STORAGE COMPOUNDS

The present invention relates to a process for improving the regeneration of hydrogen storage compounds based on chemical hydrides such as sodium borohydride and related borohydrides.

Sodium borohydride in aqueous solution can be catalytically decomposed to give hydrogen and sodium metaborate according to reaction 1 :

NaBH₄ + 2H₂O => NaBO₂ + 4H₂ (Reaction 1 ).

Goldstein's US application (US 20060042162) describes preparing sodium borohydride in a slurry form having beneficially high borohydride content, and treating the metaborate product so it is free of waters of crystallization.

It is of vital importance to be able to regenerate the metaborate back to the borohydride as cheaply as possible. Suda, in patent application

US 20040249215 has proposed to react anhydrous metaborate with magnesium powder and hydrogen at 400 to 600 ⁰C according to reaction 2:

2Mg + 2H₂ + NaBO₂ => 2MgO + NaBH₄ (Reaction 2).

The product borohydride can then be separated from the magnesium oxide by solvent extraction or other methods. Zhou in US patent application 20050207959 suggests a similar process. Both methods have the same drawback: converting expensive magnesium powder to cheap magnesium oxide. As a result, said process is costly and creates a large volume of solid magnesium oxide effluent.

The magnesium oxide product from Reaction 2 is most cheaply and directly converted back to magnesium metal by carbothermic reduction at high temperatures (>1500 degrees C), providing magnesium and carbon monoxide vapors according to reaction 3:

MgO + C => Mg + CO (Reaction 3).

Hydrocarbons such as methane can be used in place of carbon in Reaction 3 but are more costly. In a typical process the carbon monoxide product will be burnt with air according to reaction 3a:

2CO + O₂ => 2CO₂ (Reaction 3A) to reclaim its thermal energy and the carbon dioxide combustion product sequestered in-plant. It is difficult to attain complete conversion in Reaction 3 since the gaseous products tend to recombine. The prior art e.g., US patents 5782952 and 4290804, describes strategies for quenching the gaseous magnesium product while letting the carbon monoxide pass on. For example, supersonic cooling, solvation of magnesium vapor, inert gas injection or latent heat cooling of the products using a spray of molten magnesium have been proposed. These methods all have an energy penalty.

According to a first embodiment of the present invention it has now been found that in Reaction 3 the presence of a catalyst or supported catalyst for disproportionation of carbon monoxide can greatly aid the kinetics. The disproportionation reaction (Boudouard Equilibrium) for carbon monoxide is according to the following reaction 4:

2CO o C + CO₂ (Reaction 4).

Reaction 4 is catalyzed by many transition metals, their alloys and their refractory compounds (Co, Ni, Fe, platinum group metals, W in particular and their oxides, carbides, nitrides) but their usual effectiveness lies between 400 ⁰C and 600

When one or more of these catalysts are mixed with the magnesium oxide and carbon reactant mixture of Reaction 3, however, magnesium and carbon monoxide will form, but some of the carbon monoxide will disproportionate on the catalyst to give carbon and carbon dioxide according to Reaction 4. This disproportionation reaction on its own (that is with no other components present) is normally slight at elevated temperatures above 1000 ⁰C even on the catalyst , but according to the present invention the carbon will be continuously removed by reaction with excess magnesium oxide in the reaction bed vicinity as per Reaction 3. By Le Chatelier's Principle, since one of the products of the reaction (the carbon) is being continuously removed,

Reaction 4 will be driven to the right and, effectively, magnesium and carbon dioxide will leave the reaction bed. Not only the kinetics and yield are improved and the bed operating temperature somewhat reduced, but the magnesium vapor may be condensed to powder with no further product losses or energy losses. This process will be hereafter referred to as the improved carbothermic reduction of magnesium oxide. Thus according to the present invention, there is now provided a process for the production of elemental magnesium from feed materials containing magnesium oxide and carbon, the process comprising: exposing excess magnesium oxide and carbon to a high temperature reaction zone, in the presence of a catalyst, which is optionally supported, for disproportionation of carbon monoxide back to carbon and carbon dioxide, wherein the magnesium oxide and carbon react at a temperature of at least 1100.⁰C in the reaction zone to produce gaseous magnesium and carbon monoxide, and removing the gaseous magnesium and carbon dioxide from the reaction zone.

In a second preferred embodiment of the present invention the improved carbothermic reduction process of magnesium oxide is integrated with the metaborate reduction process (Reaction 2, of US patent applications 20040249215 and 20050207959, the relevant teachings of which are incorporated herein by reference) as shown in Figure 1. Thus in the combined process the effluent stream of magnesium oxide powder from the metaborate reduction process (9, Figure 1) is cheaply and efficiently reduced back to magnesium powder, to be reused by the metaborate reduction process (15, Figure 1).

In a third preferred embodiment of the present invention, it has been found that producing tetrahydroborates per Reaction 2 (Suda, US application 20040249215) can be greatly enhanced by the addition of hydride forming materials or supported materials selected from the transition metals and their alloys, such as Pt, Pd Al, Fe, Ni, Co, Mg, Zn, V, Zr, Ti, La, Y, Ce, Ca, Nb, Nd, Pr or any metal or alloys that can form oxides with oxygen.

In a fourth preferred embodiment of the present invention, it has been found that the catalysts mentioned in the first and third embodiment can be added to magnesium powder as it is formed from the gaseous phase in the Mg powder condensation (Figure 1).

In a fifth preferred embodiment of the present invention it has been found that utilizing the heat generated in the improved carbothermic reduction section for preheating of the raw material of the metaborate reduction section (Figure 1) can reduce the energy costs of the combined process. In preferred embodiments of the present invention MgO, C and the catalyst are all in powder form and preferably MgO, C and the catalyst powders are pelletized together.

Preferably, said catalyst or the supported catalyst is selected from the group consisting of transition metals, their alloys and their refractory compounds.

In preferred embodiments of the present invention, said catalyst is selected from the group consisting of compounds of Co, Ni, Fe, platinum group metals, and W.

Preferably said compounds are selected from the group consisting of oxides, carbides, nitrides.

In some preferred embodiments of the present the gaseous magnesium is condensed directly to magnesium powder.

In other preferred embodiments of the present the gaseous magnesium is condensed to liquid magnesium.

Preferably the source of the magnesium oxide powder is a second process, said second process requiring magnesium powder as a raw material.

In some preferred embodiments of the present invention said second process is a process for synthesizing metal borohydrides as described in US patent application 20050207959.

In other preferred embodiments of the present invention said second process is a method for producing tetrahydroborates as described in US patent application 20040249215.

Preferably said method for producing tetrahydroborates is enhanced by the addition of hydride forming materials.

In other preferred embodiments of the present invention said method for producing tetrahydroborates is enhanced by the addition of supported materials.

In preferred embodiments of the present invention said hydride forming materials or supported materials are selected from the group consisting of transition metals and their alloys.

Preferably, said hydride forming materials or supported materials are selected from the group consisting of Pt, Pd, Al, Fe, Ni, Co, Mg, Zn, V, Zr, Ti, La, Y, Ce, Ca, Nb, Nd, Pr. Alternatively, said hydride forming materials or supported materials are selected from any metal or alloy that can form oxides with oxygen.

In preferred embodiments of the present invention heat produced in the reaction zone is used to preheat the feed material for said second process.

In some preferred embodiments of the present invention the catalysts are embedded in the magnesium oxide powder produced in said second process.

In other preferred embodiments of the present invention said hydride forming materials are embedded in the magnesium powder fed from the process defined herein.

While the invention will now be described in connection with certain preferred embodiments in the following examples and with reference to the attached figures, so that aspects thereof may be more fully understood and appreciated, it is not intended to limit the invention to these particular embodiments. On the contrary, it is intended to cover all alternatives, modifications and equivalents as may be included within the scope of the invention as defined by the appended claims. Thus, the following examples which include preferred embodiments will serve to illustrate the practice of this invention, it being understood that the particulars shown are by way of example and for purposes of illustrative discussion of preferred embodiments of the present invention only and are presented in the cause of providing what is believed to be the most useful and readily understood description of formulation procedures as well as of the principles and conceptual aspects of the invention. Brief description of drawings

Figure 1 is a block diagram showing the integration of a metaborate reduction process (A) with improved carbothermic reduction of magnesium oxide (B).

Figure 2 is a schematic presentation of components for carrying out the improved process of the present invention as described in the example below. Example

Referring to Figure 2, magnesium oxide is reacted at temperature with carbon in the presence of a tungsten catalyst for disproportionation of carbon monoxide, in order to generate magnesium powder and carbon dioxide with high yield. Magnesium oxide powder (40 gm) was intimately mixed with carbon powder (5 gm) and tungsten powder (1 gm) as catalyst, using powders of below 300 mesh particle size, and the mixture was placed in a stainless steel crucible (1) that was fitted with a stainless steel tube section (2). The magnesium oxide reactant is in stoichiometric excess to the carbon. The crucible was placed in a furnace (3) with arc heating capability so that the tube section projected from the furnace hot zone out of the furnace directly into an oil cooled stainless steel condenser (4). A further length of tubing (5) exiting from the condenser proceeded into a bath of aqueous 10% sodium hydroxide solution (6) where gas, emerging from the tube (5), could displace alkali from an inverted cylinder (7). The system was fitted with argon purge line (8).

The system was first flushed with argon, the furnace was heated to a temperature close to the magnesium melting point (900 ⁰C) and when argon traces stopped bubbling from the tube the alkali filled inverted cylinder (7) was placed over the tube (5). To initiate the reaction the furnace temperature was gradually raised above 1100 ⁰C, which was the boiling point of magnesium. Gas was seen to evolve again and initially displace alkali from the cylinder. When no further gas was observed, the condenser (4) was disconnected from the furnace (3) area and the furnace was shut off. The condenser when opened was found to contain 17 gm of magnesium powder, together with small amounts of magnesium oxide and carbon. The alkali in the inverted cylinder (7) absorbed nearly all the gas generated, confirming it was mainly carbon dioxide (and not carbon monoxide which is not dissolved by alkali). The crucible (1) when opened was found to contain unreacted excess of magnesium oxide together with the tungsten powder catalyst which was unchanged. The yield of magnesium in this experiment was in excess of 70%.

In a further experiment magnesium powder from the first part of the example is reacted at temperature with sodium metaborate and hydrogen in the presence of a nickel-lanthanum alloy hydriding catalyst to generate sodium borohydride with high yield.

Some of the magnesium powder (12 gm) from the process above was mixed with anhydrous sodium metaborate (8.7 gm) and nickel-lanthanum alloy (LaNi₅) powder hydriding catalyst (1 gm). This mixture was preheated in a tube furnace in a hydrogen atmosphere at over 5 bar at 400 degrees C, and then heated at an heating rate exceeding 10 degrees C per minute to 600 degrees C, maintaining this temperature for an hour and allowed to cool in hydrogen. The resulting powder was treated with 10% aqueous sodium hydroxide to extract the sodium borohydride, and analysis showed that 8 gm of sodium borohydride had been produced, corresponding to a yield exceeding 80%. The remaining powder comprised unreacted magnesium oxide, sodium metaborate and unchanged catalyst.

Claims

WHAT IS CLAIMED IS:

1. A process for the production of elemental magnesium from feed materials containing magnesium oxide and carbon, the process comprising: exposing excess magnesium oxide and carbon to a high temperature reaction zone, in the presence of a catalyst, which is optionally supported, for disproportionation of carbon monoxide back to carbon and carbon dioxide, wherein the magnesium oxide and carbon react at a temperature of at least 1100.⁰C in the reaction zone to produce gaseous magnesium and carbon monoxide, and removing the gaseous magnesium and carbon dioxide from the reaction zone.

2. The process of claim 1 wherein MgO, C and the catalyst are all in powder form.

3. The process of claim 2 wherein MgO, C and the catalyst powders are pelletized together.

4. The process of claim 1 wherein said catalyst or the supported catalyst is selected from the group consisting of transition metals, their alloys and their refractory compounds.

5. The process according to claim 4 wherein said catalyst is selected from the group consisting of compounds of Co, Ni, Fe, platinum group metals, and W.

6. The process according to claim 5, wherein said compounds are selected from the group consisting of oxides, carbides, nitrides.

7. The process of claim 1 wherein the gaseous magnesium is condensed directly to magnesium powder.

8. The process of claim 1 wherein the gaseous magnesium is condensed to liquid magnesium.

9. A process as in claim 1, wherein the source of the magnesium oxide powder is a second process, said second process requiring magnesium powder as a raw material.

10. A process as in claim 9, wherein said second process is a process for synthesizing metal borohydrides as described in US patent application 20050207959.

11. A process as in claim 9, wherein said second process is a method for producing tetrahydroborates as described in US patent application 20040249215.

12. A process as in claim 11, wherein said method for producing tetrahydroborates is enhanced by the addition of hydride forming materials.

13. A process as in claim 11, wherein said method for producing tetrahydroborates is enhanced by the addition of supported materials.

14. A process according to claims 12 and 13, wherein said hydride forming materials or supported materials are selected from the group consisting of transition metals and their alloys.

15. A process according to claim 14, wherein said hydride forming materials or supported materials are selected from the group consisting of Pt, Pd, Al₁ Fe, Ni, Co, Mg, Zn, V, Zr, Ti, La, Y, Ce, Ca, Nb, Nd, Pr.

16. A process according to claim 14, wherein said hydride forming materials or supported materials are selected from any metal or alloy that can form oxides with oxygen.

17. A process as in claim 9, wherein heat produced in the reaction zone mentioned in claim 1 is used to preheat the feed material for said second process.

18. A process as in claim 9, wherein the catalysts mentioned in claim 4 are embedded in the magnesium oxide powder produced in said second process.

19. A process as in claim 13, wherein said hydride forming materials are embedded in the magnesium powder fed from the process according to claims 1 and 7.