Production process

Abstract

A process for the production of a metal which comprises: carbothermal reduction of the corresponding metal oxide to produce a mixed gas stream comprising the metal and carbon monoxide; maintaining the mixed gas stream at a suitably elevated temperature to prevent reformation of the metal oxide; ejecting the mixed gas stream through a convergent-divergent nozzle in order to cool the mixed gas stream instantaneously to a temperature at which reformation of the metal oxide cannot take place; and separating and collecting the metal, wherein the nozzle is heated by means other than gas flow through the nozzle so that temperature of surfaces of the nozzle in contact with the mixed gas stream are maintained at a temperature sufficient to prevent deposition on the said surfaces of products from the gas stream.

Claims

1. A process for the production of a metal, comprising: performing in a reactor carbothermal reduction of a metal oxide corresponding to the metal to produce a mixed gas stream comprising the metal and carbon monoxide; maintaining the mixed gas stream at a suitably elevated temperature to prevent reformation of the metal oxide; ejecting the mixed gas stream through a convergent-divergent nozzle of the reactor in order to cool the mixed gas stream instantaneously to a temperature at which reformation of the metal oxide cannot take place; and separating and collecting the metal, wherein the nozzle is heated by gas flow through the nozzle and by an additional heat source to supply heat beyond any heat supplied to the nozzle by gas flow through the nozzle, such that surfaces of the nozzle in contact with the mixed gas stream are maintained at a temperature sufficient to prevent deposition on said surfaces of products from the gas stream.

2. The process of claim 1, wherein the additional heat source comprises (i) a heating device associated specifically with the nozzle; and/or (ii) the nozzle being positioned to derive heat from the reactor in which the carbothermal reduction reaction takes place.

3. The process of claim 1, wherein on start-up, the nozzle temperature is elevated by heating prior to any gas being allowed to flow through the nozzle.

4. The process of claim 1, wherein a requisite temperature profile for the nozzle is predetermined based on a composition of a starting material to be reduced by the carbothermal reduction and on a gaseous species that will be flowing through the nozzle at any point in time.

5. The process of claim 1, wherein the metal is magnesium.

6. A reactor for the production of a metal by a process comprising (a) performing in the reactor carbothermal reduction of a metal oxide corresponding to the metal to produce a mixed gas stream comprising the metal and carbon monoxide, (b) maintaining the mixed gas stream at a suitably elevated temperature to prevent reformation of the metal oxide, (c) ejecting the mixed gas stream through a convergent-divergent nozzle of the reactor in order to cool the mixed gas stream instantaneously to a temperature at which reformation of the metal oxide cannot take place, and (d) separating and collecting the metal, the reactor comprising: a convergent-divergent nozzle that is configured to be heated by gas flow through the nozzle and by an additional heat source to supply heat beyond any heat supplied to the nozzle by gas flow through the nozzle, such that surfaces of the nozzle in contact with the mixed gas stream are maintained at a temperature sufficient to prevent deposition on said surfaces of products from the gas stream, wherein the additional heat source comprises (i) a heating device associated specifically with the nozzle, and/or (ii) the nozzle being positioned to derive heat from the reactor in which the carbothermal reduction reaction takes place.

7. The process of claim 2, wherein: the nozzle is thermally conductive, and the heating device comprises a direct thermal coupling of the nozzle with an upstream portion of the reactor, by the use of an inductive heating system and/or by direct heat transfer.

8. The process of claim 7, wherein the inductive heating system comprises induction coils arranged around the nozzle.

9. The process of claim 2, wherein the nozzle derives heat from the reactor by being located at least partially within a heated zone of the reactor in which the carbothermal reduction takes place.

10. The reactor of claim 6, wherein: the nozzle is thermally conductive, and the heating device comprises a direct thermal coupling of the nozzle with an upstream portion of the reactor, by the use of an inductive heating system and/or by direct heat transfer.

11. The reactor of claim 10, wherein the inductive heating system comprises induction coils arranged around the nozzle.

12. The reactor of claim 6, wherein the nozzle is positioned to derive heat from the reactor by being located at least partially within a heated zone of the reactor in which the carbothermal reduction takes place.

13. A reactor for the production of metal, the reactor comprising: a convergent-divergent nozzle configured to eject a mixed gas stream; and a heat source configured to heat the convergent-divergent nozzle beyond any heat supplied to the nozzle by gas flow through the nozzle, such that surfaces of the nozzle in contact with the mixed gas stream are maintained at a temperature sufficient to prevent deposition on said surfaces of products from the gas stream, wherein the additional heat source comprises (i) a heating device associated specifically with the nozzle; and/or (ii) the nozzle being positioned to derive heat from the reactor in which the carbothermal reduction reaction takes place.

14. The reactor of claim 13, wherein: the nozzle is thermally conductive, and the heating device comprises direct thermal coupling of a suitably conductive nozzle with an upstream portion of the reactor by use of an inductive heating system and/or direct heat transfer.

15. The reactor of claim 14, wherein the inductive heating system comprises induction coils arranged around the nozzle.

16. The reactor of claim 13, wherein the nozzle is positioned to derive heat by being located at least partially within a heated zone of the reactor in which the carbothermal reduction takes place.

Description

BRIEF DISCUSSION OF FIGURES

(1) Embodiments of the present invention are illustrated in the accompanying non-limiting drawings in which:

(2) FIG. 1 is a backscattered SEM (Scanning Electron Microscope) image of a blockage cross-section, the scale of which applies also to FIGS. 2-6;

(3) FIG. 2 is a Calcium (Ca) element map in nozzle blockage (in this and images 3-6, lightness indicates concentration of element mapped);

(4) FIG. 3 is an Iron (Fe) element map in nozzle blockage;

(5) FIG. 4 is a Silicon (Si) element map in nozzle blockage;

(6) FIG. 5 is a Magnesium (Mg) element map in nozzle blockage; and

(7) FIG. 6 is an Oxygen (O) element map in nozzle blockage.

(8) FIG. 7 is a backscattered SEM of nozzle cross-section, showing growth of blockage and irreversible choking of flow.

(9) FIG. 8 is a backscattered SEM of nozzle cross-section after operation with additional heating, showing negligible blockage.

(10) FIG. 9 is a schematic showing a reaction chamber and nozzle assembly, illustrating arc-furnace reaction chamber and specific induction heating apparatus for maintaining nozzle temperature.

(11) FIG. 10 is a schematic showing a reactor assembly and nozzle, illustrating induction heating of furnace and positioning of nozzle to achieve separate controlled induction heating of the nozzle surface.

(12) FIG. 11 is a schematic showing a reactor assembly and nozzle, illustrating position of the nozzle mostly within the reaction chamber in order to maintain surface temperature.

(13) FIG. 12 shows gas flow data for experiments TMG-84, 85, and 88-89. The absence of additional nozzle heating resulted in catastrophic and irreversible nozzle blockage, and the experiments were forced to terminate early.

(14) FIG. 13 shows gas flow data for experiments TMG-87 and 91-95. Additional direct nozzle heating was provided, and the chart shows that reliable operation can be achieved by maintaining the throat temperature above 1600° C.

DETAILED DISCUSSION OF INVENTION

(15) There are believed to be a number of mechanisms by which deposits are formed on the relevant surfaces of the nozzle. The first is relevant to start-up of the process and the corresponding conditions that exist at start-up. The remainder relate to steady-state operation of the carbothermal process. In an embodiment of the invention heating of the nozzle may be required during start-up as well as during steady-state operation. On start-up hot inert gas may be flushed through the nozzle and additional heat supplied as described herein. In an alternative embodiment on start-up the nozzle temperature is elevated by heating prior to any gas being allowed to flow through the nozzle. This is done to prevent deposition prior to establishment of steady-state operating conditions (temperature).

(16) FIG. 1 is a back-scattered SEM (Scanning Electron Microscope) image of a blockage cross-section, with the graphite nozzle wall visible at the top. Adjacent to the wall may be observed a bright deposit, which is primarily calcium, iron, and silicon (see FIGS. 2-4). These species deposit during the start-up of the process.

(17) The remaining blockage is primarily magnesium (FIG. 5), present as the oxide (FIG. 6), which has deposited progressively during steady-state operation of the nozzle.

(18) Initially, and at relatively low temperatures, certain impurities in the starting materials (metal oxide), such as Ca, Fe, and Si in the case of reducing magnesium oxide, will be capable of being carbothermally reduced. Reduction of such oxides, of which the above list is by means of example, takes place at a temperature in the range 500 to 1000° C., well below the temperature at which the magnesium oxide can be reduced. At this time the nozzle is coming up to its intended operating temperature and is thus undercooled. Metallic vapours of the impurities will condense in the nozzle, leading to the commencement of the blockage process (FIGS. 2-4).

(19) In accordance with an embodiment of the present invention the nozzle is heated so that at this critical time condensation of metallic vapours is avoided. In this embodiment the nozzle temperature may be elevated as necessary prior to any gases being ejected through it from the carbothermal reactor provided upstream of the nozzle so that when gases do flow through the nozzle it is already above the temperature at which condensation of species can occur. The critical temperature in this regard will depend upon the composition of the starting material and this can be determined based on such composition. When the nozzle temperature is elevated as required, gases from the upstream reactor can be allowed to pass through the nozzle without fear of deposition of solids in the nozzle. Typically, the temperature of the (relevant surfaces of the) nozzle are maintained above 1100° C., for example above 1300° C.

(20) In this embodiment heating of the nozzle may be accomplished by any suitable means, including resistance heating, induction heating, direct external convection heating, or any other means appropriate to the materials and construction of the nozzle.

(21) At temperatures above 1700° C. magnesium and CO vapour are produced, along with impurities such as Al, Mn, and S that may be produced as a result of reduction of corresponding oxides at such high temperatures. If the temperature of the relevant surfaces of the nozzle is below the condensation temperature of any of these species, condensation will occur and deposits will form in the nozzle (FIGS. 5 and 6).

(22) Furthermore, the reversion oxide products, such as CaO, SiO.sub.2, MgO, and C, are stable at high temperatures and cannot be dislodged once formed in the nozzle. Thus, the temperature of the nozzle must be maintained above the critical reversion temperatures for these species, and any others that are likely to deposit under such temperature conditions.

(23) The formation of deposits in the nozzle is very important since even small amounts of deposits can interrupt gas flow through the nozzle, expanding the boundary layer and causing turbulence and increased reversion, the solid products of which contribute to further deposition and possibly blockage. FIG. 7 illustrates the progressive and catastrophic blockage resulting from this process.

(24) In accordance with the present invention it has been found that maintaining the nozzle surface temperature above the critical condensation temperatures for any, and all, gaseous species flowing through the nozzle is essential to reliable operation of the convergent-divergent nozzle (FIG. 8). Thus, the minimum temperature of surfaces of the nozzle that come into contact with gases flowing through the nozzle should always be sufficient to avoid condensation of species present in the gas at any point in time.

(25) The present invention may be implemented using the same basic methodology and componentry/reactor as disclosed in Hori acknowledged above. However, a fundamental distinction over such conventional approaches is that in accordance with the present invention specific steps are taken to heat the nozzle to, and maintain the nozzle at, a suitable high temperature. In this context the invention relies on heating of the nozzle over-and-above any heating effect due to gas flowing through the nozzle. Such an approach would not be required if the nozzle operated adiabatically when hot gas flows through it. However, the present inventors have found this not to be the case so that “passive” heating of the nozzle by gas flow alone will not avoid the formation of deposits in the nozzle.

(26) In accordance with the present invention the temperature of the nozzle may be controlled as required by a number of different approaches. In one embodiment, and as noted above, the nozzle may be heated by suitable heating apparatus associated with the nozzle and specifically provided with that function in mind. For example, the nozzle may be heated by induction coils that are arranged around the nozzle.

(27) In the embodiment illustrated in FIG. 9, pelletised reactants are fed from the hopper (1) via a feed tube (2) into the main reactor. The arc furnace is encased in a steel shell (3), lined with appropriate refractory (4) and hearth material (5). Electrodes (6) provide heating to the furnace. Induction coils (7), controlled independently of the furnace temperature, and encased in additional refractory (8), provide heating to the convergent-divergent nozzle (9) to prevent deposition and blocking. The level of reactants (10) is maintained at an appropriate level to optimise the reaction.

(28) In another embodiment however the nozzle may derive heat by being closely associated with the reactor in which the carbothermal reduction reaction takes place. In this case the nozzle derives heat by being located at least partially within the heated zone of the reactor. For example, the nozzle may derive heat from the primary induction coil of an inductively-heated reactor. In this embodiment heating of the nozzle may take place by one or more mechanisms: convective heating at medium temperatures and low gas flow rates (below 1000° C.); radiative heating (likely to be more prevalent at temperatures above 1000° C.; and heating due to coupling of the nozzle (usually graphite) with the induction field of the coil used to effect heating of the reactor, or with additional induction heating (see FIG. 10). In accordance with the present invention the position of the nozzle may be varied in order to derive the most beneficial heating effect given the intended outcome of the present invention. It may also be appropriate to insulate the nozzle in order to minimise heat loss. In the embodiment illustrated in FIG. 10, pelletised reactants are fed from the hopper (1) into the main reactor. The induction furnace is encased in a steel shell (2), lined with appropriate refractory (3), additional insulation (4) for the induction coil (5), and an appropriate conductive material (6), wherein the reactants (7) are maintained at an appropriate level. Additional induction coils (8) provide heating to the convergent-divergent nozzle (9).

(29) In practice one or more of the approaches described for heating of the nozzle may be applied to achieve the most effective and economic result in the context of the present invention. The requisite temperature profile for the nozzle may be predetermined based on the composition of the starting material(s) to be reduced and on the gaseous species that will be flowing through the nozzle at any point in time. The input temperature of gas flowing through the nozzle may contribute to heating of the nozzle but, as has been noted, the gas temperature will not be determinative of the nozzle temperature since gas flow of the nozzle can cause cooling thereof

(30) In accordance with the invention the metal to be produced may be selected from the group consisting of Mg, Mn, Ca, Si, Be, Al, Ba, Sr, Fe, Li, Na, K, Zn, Rb and Cs.

(31) The present invention may be particularly useful for the production of magnesium, and here it should be noted that the thermochemistry of metals can vary considerably. This point can be illustrated by considering aluminium and magnesium. Importantly, the reaction products from the carbothermal reduction of alumina have relatively high boiling points (aluminium boils at about 2500° C. and Al.sub.2O (aluminium sub-oxide) has an appreciable vapour pressure above about 1800° C.) when compared with the reaction products from the carbothermal reduction of magnesia (magnesium boils at about 1050° C. and no sub-oxide species exist). Accordingly, using conventional nozzle methodology, in which the nozzle is heated by gas flow, alumina-derived reaction products require higher nozzle temperatures in order to avoid deposition problems. Under the prevailing nozzle temperature attributable to gas flow (about 1100° C. on average assuming no losses), alumina-derived reaction products tend to readily condense and form deposits on the nozzle. In contrast during the carbothermal reduction of magnesia conventional thinking would lead to the expectation that the nozzle temperature would be suitably high so that the deposition problem would be unlikely to occur. However, the present inventors have found the opposite and this is somewhat unexpected given conventional wisdom in the art.

(32) The reductant used in the present invention may be derived from a variety of conventional carbon sources including graphite, petroleum and coke (such as metallurgical coke).

(33) The present invention also provides a reactor suitable for implementation of the process of the invention as described herein. The reactor design and construction is essentially the same as described in Hori acknowledged herein. However, the reactor of the present invention is adapted to achieve active heating of the nozzle (i.e. other than by gas flow) in order to avoid deposition problems. As described, the nozzle may be heated by heating means associated specifically with the nozzle (FIG. 10) and/or the nozzle may be positioned to derive heat from the reactor in which the carbothermal reduction reaction will take place (see FIG. 11).

(34) In the embodiment illustrated in FIG. 11, pelletised reactants are fed from the hopper (1) via a feed tube (2) into the main reactor. The arc furnace is encased in a steel shell (3), lined with appropriate refractory (4) and hearth material (5). Electrodes (6) provide heating to the furnace. In this case radiative and convective heating maintains an appropriate temperature of the convergent-divergent nozzle (7). The level of reactants (8) is maintained at an appropriate level to optimise the reaction.

(35) In accordance with the present invention the temperature of the nozzle may be determined as the production process proceeds with the nozzle temperature being controlled as required to avoid deposition problems. The nozzle temperature may be measured using conventional methodology and apparatus. Alternatively, the temperature characteristics of the nozzle may be determined experimentally based on gas flow through the nozzle at varying temperatures with the nozzle temperature being adjusted in practice based on such determination, and supported by additional modelling. The latter approach would avoid the need to actively measure the nozzle temperature during the course of the production process.

(36) The present invention is illustrated with reference to the following non-limiting example.

EXAMPLE

(37) The two main series of experiments conducted were from TMG-84 to TMG-90 and TMG-91 to TMG-95. The first series had no additional heating of the nozzle surface, except for TMG-87, which is included here within the second series. Experiments TMG-91 to TMG-95, and TMG-87, included additional heating of the nozzle. The following summarises the results obtained. a. TMG-84. No additional heating of the nozzle. Blockage occurred early and irreversibly; reaction terminated. b. TMG-85. No additional heating of the nozzle. Blockage early and irreversible; no significant data obtained. c. TMG-86. No additional heating. Nozzle blocked at a throat surface temperature around 1200° C. d. TMG-87. Additional heating provided by nozzle position. Run proceeded to completion (300 g). e. TMG-88. No additional heating provided. Experiment failed early. f. TMG-89. No additional heating provided. Experiment failed early. g. TMG-90. No additional heating provided, but reactor heated more slowly to allow some equilibration with nozzle. Insufficient temperature increase resulted in blockage. h. TMG-91. Additional heating provided by nozzle location. Nozzle un-blocked above approximately 1650° C. 300 g charge consumed. i. TMG-92. Repeat of TMG-91, with similar result. j. TMG-93. Slightly faster heating resulted in a lower nozzle temperature and greater blockage during heat-up, but the blockage was similarly reversible above 1650° C. 400 g charge consumed. k. TMG-94. Further additional heating provided by means of removal of internal insulation. (Heating now caused by induction coupling and radiation within reaction chamber.) Nozzle heated faster, resulting in a much flatter blockage profile (less blockage). 400 g charge consumed. l. TMG-95. Repeat of TMG-94, with 500 g charge consumed.

(38) As shown in FIG. 12, the experiments conducted with no additional heating of the nozzle surface resulted in irrecoverable blockage, demonstrated by the drop in gas flow rate through the nozzle. The gas flow rate is directly proportional to the available cross-sectional area of the throat; as the flow is constricted sonic flow cannot be maintained.

(39) FIG. 13 illustrates the improvement achieved by heating the nozzle surface further. While some early constriction is evident in the earlier tests, additional heating results in the maintenance of the integrity of the gas flow path and continued safe operation of the nozzle. The critical surface temperature of the nozzle throat is around 1600 to 1700° C.

(40) In another embodiment of the present invention the momentum of the gas stream exiting the nozzle may be used for energy regeneration. Such energy may be recovered as electrical or thermal energy. In the latter case thermal energy may be re-used directly in the process of the invention, for pre-heating the reactants or providing additional control over the nozzle temperature.

(41) Throughout this specification and the claims which follow, unless the context requires otherwise, the word “comprise”, and variations such as “comprises” and “comprising”, will be understood to imply the inclusion of a stated integer or step or group of integers or steps but not the exclusion of any other integer or step or group of integers or steps.

(42) The reference in this specification to any prior publication (or information derived from it), or to any matter which is known, is not, and should not be taken as an acknowledgment or admission or any form of suggestion that that prior publication (or information derived from it) or known matter forms part of the common general knowledge in the field of endeavour to which this specification relates.