IE960714A1 - Carbothermic magnesium manufacturing process using a¹hydrocarbon & metal alkyl quench & seperation method - Google Patents

Abstract

This invention concerns an improved process of magnesium manufacture by the carbothermic process. The carbothermic process involves producing magnesium vapour and carbon monoxide by reducing magnesium oxide with carbon or metal carbides. The problem with this process is that it involves a rapid quench step to prevent the reverse reaction which proceeds below 1780 degrees Centigrade at 1 atmosphere and the quench steps hitherto used are inefficient. In this improved process a reactor operating at above 1 atmosphere and 1780 to 2000 degrees Centigrade, is fed with a mixture of 2.7 tonnes of carbon, 3 tonnes of oxygen & 1.7 tonnes of magnesium oxide, producing 6.4 tonnes of carbon monoxide and one tonne of magnesium. The high temperature mixture of carbon monoxide and magnesium vapour from the carbothermic reaction is quenched by a high speed spray of fine droplets of medium boiling point hydrocarbons followed by a high speed spray of fine droplets of metal alkyls. The mixture of carbon monoxide, vapours and magnesium metal dust is cooled to produce a liquid, the liquid density adjusted to 1.74 to 1.80 and the magnesium seperated from the other chemicals. Gases such as carbon monoxide are burned to produce power. Medium boiling point hydrocarbons and alkyls are recycled. Byproducts produced are power, carbon dioxide, nitrogen and argon. Some C3 and C4 hydrocarbons are produced by thermal cracking of the hydrocarbon used in the quench step. Principal use is in magnesium manufacture.

Description

CARBOTHERMIC MAGNESIUM MANUFACTURING PROCESS USING A HYDROCARBON & METAL ALKYL QUENCH & SEPERATION METHOD PATENT APPLICATION BY PADRAIG MC ALISTER, AN IRISH CITIZEN OF THORMANBY LAWNS, HOWTH^tUNTY P1 ,DI ,M REPUBLIC OF IRELAND. // 17 //// 1/ / OPEN TO PUBLIC INSPECTION UNDER SECTION 28 AND RULE 23 JNL. No.. V & Si·? OF This invention concerns an improved process of magnesium manufacture using a carbothermic process. In a carbothermic process magnesium vapour mixed with carbon monoxide is produced using reduction of magnesium oxide either by carbon or metal carbides.

The state of the art in magnesium manufacturing is described in engineering literature describing the Norsk Hydro process, the Permanente process, the silicothermic process, conference publications such as the 1986 proceedings on magnesium and patents such as US patents 2880151, 2888389, 3389062, 3396094, 3907651, 4495037, 4409083, Japanese patent 36-9055 & European applications 84201741.0, & 84100122.2.

In the case of production by the carbothermic route, the Permanente process involved reducing magnesium oxide by carbon at about 1 atmosphere and above 1780 degrees Centigrade. The co-produced carbon monoxide and magnesium vapour were quenched in about 20 microseconds using a high speed stream of natural gas. In the case of the metal carbide process cited in Japanese patent 810506, operation is between 5 and 200 torr and at least 1600 degrees Centigrade and quenching is by isentropic expansion of magnesium vapour.

While the carbothermic processes can use cheap feedstocks, the processes have not found widespread use hitherto because of the poor efficiency of the quench step.

It has been found by the inventor’s research that the efficiency of the quench step can be improved by choosing, instead of the natural gas quenching or isentropic expansion cooling methods used hitherto, quenching using a high molecular weight liquid quenching agent with a high specific gravity, which evapourates at moderate temperatures & is stable at high temperatures, so that it has the highest heat capacity per unit volume of quench spray. This reduces the quench time below the 20 microseconds achievable with the state of the art methods such as the Permanente process. It has been found by the inventors research that stable metal alkyls saturated with methyl or ethyl groups are particularly suitable for this use.

It has also been found by the inventor's research that by using medium boiling point hydrocarbons for a part of the quench, that the latent and sensible heat of the magnesium and carbon monoxide vapour eo 7 1 4 may be used to good effect, as these hydrocarbons may undergo thermal cracking to produce lower molecular weight hydrocarbons which can be economically seperated and sold.

It has also been found by the inventor's research that by using a mixture of metal alkyls, or metal alkyls with hydrocarbons, with a specific gravity above 1.74 - the specific gravity of magnesium metal, and below that of any co-produced carbon -1.8 - 2.1 or of magnesium oxide - 3.2 to 3.6, typically using a mixture of metal alkyls & hydrocarbons with a specific gravity of 1.77 to 1.8, that a sharp seperation of the magnesium metal produced can be achieved in conventional centrifuges, or other state-of-the-art solid seperation processes, using relatively small amounts of energy. The metal alkyls used together with any gases present can be easily seperated from magnesium metal or from any impurities present and recycled for reuse in the process.

It has also been found by the inventors research that by using the partial oxidation of carbon to carbon monoxide at the source of heat for the endothermic magnesium producing reaction using magnesium oxide & carbon, a carbon monoxide gas phase is produced which can be easily seperated from metal alkyls or hydrocarbons & burned to produce power.

The process can thus recover practically all of the energy not used in the magnesium oxide reduction reaction itself. This results in an improved production step & entirely novel quench & seperation steps.

In this improved carbothermic process using a novel quench & seperation process, a carbothermic reactor operating at above 1 atmosphere and 1780 to 2000 degrees Centigrade, is fed with a mixture of 2.7 tonnes of carbon, 3 tonnes of oxygen & 1.7 tonnes of magnesium oxide, per tonne of magnesium required. This is the stoichiometric mixture required to reduce the magnesium oxide and supply the heat required, by partial oxidation of carbon to carbon monoxide. This reaction produces 6.3 tonnes of carbon monoxide and 1 tonne of magnesium vapour.

The carbothermic reactor feed is purged of air by a purge of bone dry carbon dioxide or argon, prior to entering the reactor to eliminate nitrogen & minimise water vapour in the reactor feed.

The high temperature mixture of carbon monoxide & magnesium vapour from the carbothermic reaction are passed to a high temperature cyclone seperator, operating at the same temperature as the reactor, to remove dust or grits and then to the quench condenser.

The above magnesium production step can usefully be carried out in a single three compartment reactor, with compartments (a) to introduce feed after purging of air present and preheat feed by exchange of heat with some outgoing gases, (b) to carry out the magnesium producing reaction and (c) to enclose the high temperature seperator for dust and grits, thereby reducing heat and other losses, reducing costs and improving safety.

In the quench condenser a high speed spray of fine droplets of medium boiling point hydrocarbons followed by a high speed spray of fine droplets of metal alkyls, is rapidly mixed with the magnesium vapour & carbon monoxide to reduce the temperature to 185 to 250 degrees Centigrade in a high speed two stage liquid injection quench system.

A spray of kerosene & recycled C5+ hydrocarbons optionally entrained in high pressure argon gas jets, may usefully be used for the first step & a spray of the metal alkyls tetraethyl or tetramethyl lead, optionally entrained in high pressure argon gas jets, may usefully be used for the second step, either on their own or mixed with other metal alkyls. Typically a spray of 100 tonnes of total liquid per tonne of magnesium vapour will be used. Kerosene and metal alkyls, in addition to producing a very rapid quench effect, also provide a reducing atmosphere and inhibit the reverse carbothermic reaction, by forming metal carbonyls with traces of any free radical CO ions formed at high temperatures.

The mixture of carbon monoxide, vapours and magnesium metal dust from the quench condenser, is then passed to a cooler to reduce temperature to below the condensation point of the principal hydrocarbons & metal alkyls present. Typically this will involve cooling to below 50 degrees Centigrade.

This produces a liquid component consisting of C5+ hydrocarbons, metal alkyls & magnesium metal with small quantities of carbon and magnesium oxide and a gas component consisting of carbon monoxide plus C5- hydrocarbons & metal alkyl vapours with argon or carbon dioxide & traces of metal carbonyls. This liquid is seperated from the vapours and gases for magnesium recovery. 060714 The vapours & gases are sent to a vapour recovery plant to seperate and recover the hydrocarbons, metal alky Ils & carbonyls from the principal stream of carbon monoxide & other gases. C5+ hydrocarbons & alkyls are recycled to the process. C4- hydrocarbons & other products, except carbon monoxide & argon are sold or disposed of.

The carbon monoxide & argon from the vapour recovery plant is sent to a power plant, to burn the carbon monoxide & any undesirable impurities using stoichiometric oxygen, to produce power & carbon dioxide mixed with argon & water traces. This step will produce power of about 6 megawatt hours per tonne of magnesium metal produced. This power is partly used in the process & partly exported to the air seperation plant used to provide oxygen & to seperate carbon dioxide from argon.

Downstream of this power plant the carbon dioxide mixed with argon is sent to a seperation plant to seperate & purify the argon and carbon dioxide for use in the process or for sale. Gaseous impurities are disposed of by discharge to stack. Typically this part of the process will be integrated with the air seperation plant used to produce the oxygen required for the process. The air seperation plant used to produce the oxygen needed can be any of the state of the art methods.

The liquid plus entrained solids from the quench condenser/cooler is sent to a density adjusting mixer where the specific gravity of the liquid is adjusted to the range 1.74+ to 1.80. For this step kerosene or the metal alkyls tetraethyl or tetramethyl lead may usefully be used.

The liquid plus entrained solids from the mixer is sent to a centrifuge or other state-of-the-art solids seperation system to seperate the stream into a magnesium stream with a solids specific gravity of about 1.74, a hydrocarbon plus metal alkyl stream with a specific gravity of 1.74+ to 1.80, a carbon stream with a solids specific gravity of 1.8 to 2.1 and a magnesium oxide stream with a solids specific gravity of 3.2 to 3.6.

The magnesium stream is treated to remove any traces of hydrocarbon or metal alkyl and then sent to a packing plant, pelleting plant or to an ingot casting plant using any of the state of the art methods.

The carbon and magnesium oxide residues are also treated to remove any traces of hydrocarbon or metal alkyl and then either recycled to the carbothermic reactor or further seperated into pure »60714 carbon and pure magnesium oxide powders for sale. Solid impurities are disposed of by any usual state-of-the-art methods.

The liquid hydrocarbon plus metal alkyl stream is seperated into a principally hydrocarbon and a principally metal alkyl stream and recycled to the quench condenser.

As the metal alkyls are thermally stable & argon is inert, and both are recycled with only small losses due to decomposition or other losses, the overall process material balance consists of using 2.7 tonnes of carbon, 6.6 tonnes of oxygen & 1.7 tonnes of magnesium oxide to produce 1 tonne of magnesium, 11 tonnes of carbon dioxide, 6 megawatt hours, with some thermal cracking of medium boiling point hydrocarbons to produce more valuable C3 and C4 hydrocarbons & byeproducts of over 25 tonnes of nitrogen and about .5 tonnes of argon per tonne of magnesium produced.

The quench & seperation steps in this process are entirely novel steps. The carbothermic process involved is also an improvement to the state of the art carbothermic magnesium production methods. The overall process is a major improvement in the state of the art in magnesium production technology.

Claims (13)

1. A carbothermic magnesium production process in which the heat required for the endothermic reduction step is provided by partial oxidation of carbon to carbon monoxide at a temperature above 1780 degrees Centigrade.

2. A process in which the carbon monoxide and magnesium vapour produced by a carbothermic process, are rapidly quenched using a liquid hydrocarbon spray or a metal alkyl spray or both in combination.

3. A method according to claim 2 in which the liquid hydrocarbon is a hydrocarbon capable of being thermally cracked to produce bye products.

4. A method according to claim 2 in which the metal alkyl is tetraethyl or tetramethyl lead.

5. A method of seperating magnesium from carbon or magnesium oxide by seperating them in a liquid medium of specific gravity in the range 1.74+ to 1.80. «60 7 14

6. A method according to claim 5 in which the liquid specific gravity is adjusted to 1.74+ to 1.80 by mixing a light hydrocarbon, tetraethyl or tetramethyl lead with each other or with a pre-existing mixture of these.

7. A method according to claim 1 in which, after seperation of the materials cited in claims 2 to 6, the carbon monoxide produced according to claim 1 is burned to produce power.

8. A method according to claim 1 in which any carbon monoxide leaving the reactor is cooled by heat exchange with feed magnesium oxide and carbon or by external cooling to below the temperature at which the reverse carbothermic reaction takes place to convert any magnesium present to magnesium oxide and recycle it to the reactor with the fresh feed.

9. A process according to any preceding claim in which the steps of (a) feed to the reactor and heat exchange between fresh feed and exiting carbon monoxide mixed with argon & some magnesium, (b) a carbothermic magnesium production process and (c) seperation of dust/grits are carried out in a three compartment kiln in which the three seperate compartments involve steps (a), (b), or (c).

10. A method according to any preceding claim, in which the state of the art technology as described in the description of the process, together with the novel elements, is used to produce magnesium, argon, carbon, magnesium oxide, light hydrocarbons, carbon monoxide, or carbon dioxide substantially as hereinbefore described.

11. A process according to any preceding claim for the preperation of magnesium, argon, carbon, magnesium oxide, light hydrocarbons, carbon monoxide, or carbon dioxide substantially as hereinbefore described.

12. Magnesium, argon, carbon, magnesium oxide, light hydrocarbons, carbon monoxide, or carbon dioxide whenever prepared by a process claimed in a preceding claim.

13. Use of any process substantially as hereinbefore described.