Patent Application
20130220077
The present invention is directed to a method and apparatus for use in the production of metals from their oxides, hydroxides, sulfides, or polyatomic forms by simple reduction at high temperature in the presence of a gaseous mixture of magnesium and carbon monoxide, where the gaseous mixture is obtained by carbothermic reduction of magnesium oxide without an intervening step or process of separating the resulting magnesium.
The reducing property of magnesium vapor for purifying metals from metal oxides, sulfides and the like is well known as represented by Hivert, et al., U.S. Pat. No. 2,881,067 which teaches the production of powder metals from oxides by reaction with magnesium vapor produced by heating elemental magnesium.
Similarly, Shekhter, et al., U.S. Pat. No. 6,171,363 and U.S. Pat. No. 6,558,447, employ elemental magnesium mixed with tantalum and niobium oxides and directly heated to about 1000° C., thereby generating gaseous magnesium in direct contact with the metal oxides to reduce the oxides to pure metal.
The problem with the prior art processes for reduction of metal oxides by magnesium vapor is in obtaining the elemental magnesium for heating to provide magnesium vapor.
Since the early part of the 1900s, Hansgirg and others have demonstrated that carbon can be used to liberate magnesium vapors from magnesium oxide at high temperatures to produce pure magnesium metal for subsequent use such as in the above noted reduction of metal oxides. Magnesium oxide is reacted with carbon, usually at or above 2200 degrees K. Carbon has an increasing affinity for oxygen at such elevated temperatures, while magnesium's bond with oxygen becomes less stable with increasing temperature. The result of this carbothermic reduction of magnesium oxide is production of a mixture of magnesium gas and carbon monoxide gas which are then separated to yield elemental magnesium for other uses.
Such separation processes are represented by Mod, et al., U.S. Pat. No. 3,560,198, in which magnesium and carbon monoxide vapors produced by typical carbothermic reduction of MgO ores are passed through a bed of metal carbides at temperatures of 1500 to about 1850° C. At those temperatures, the carbon monoxide reacts with the metal carbides to form metal oxides and carbon. However, Mod, et al., teach that the magnesium vapors do not react with either the carbides or the resulting oxides and that the elemental magnesium is separated out for recovery.
Also, Diaz, et al., U.S. Pat. No. 5,782,952, provides a continuous process for the production of elemental magnesium from magnesium oxide and a light hydrocarbon gas reacted at a temperature of about 1400° C. or greater. The resulting product stream is continuously quenched to separate elemental magnesium for use elsewhere.
However, problems with the process to yield magnesium metal occur due to magnesium's greater affinity for oxygen when the mixture cools, producing a back-reaction to magnesium oxide and carbon. Attempts to rapidly quench the mixture to minimize the opportunity for back-reaction and produce magnesium metal have a tendency to yield magnesium powder which is hazardous in large-scale environments, as noted by Diaz, et al. Other attempts used to refine the carbothermic method to produce magnesium have included rapid quenching with hydrocarbons, liquid magnesium, inert gases, and the use of supersonic nozzles to accelerate the quenching process. Filtering the carbothermic product mixture through metal carbides to separate the magnesium from the carbon monoxide has also been tried as taught by Mod, et al.
None of these prior processes teaches the direct utility of use of the gaseous magnesium and carbon monoxide mixture produced by carbothermic reduction of magnesium oxide with carbon as a reducing agent for production of metals from metal oxides, hydroxides, sulfides or polyatomic compounds, thereby avoiding the necessity of first separating the elemental magnesium and preparing it for use as a source of magnesium vapor.
The present invention takes advantage of the stability of carbon monoxide at the carbothermic reduction temperatures, such that the high-temperature mixture can be used to reduce raw materials and recover other non-magnesium substances. Carbon monoxide at approximately 2200 degrees K is stable and can be considered an inert gas in a carbothermic mixture, so that the strong reducing nature of magnesium vapors can be employed with other substances needing reduction. Examples of such substances which can be reduced using the present method include chromium oxide, polyatomic chromium compounds containing oxygen, manganese oxide, polyatomic manganese compounds containing oxygen, and zinc oxide, hydroxide, sulfide, or polyatomic zinc compounds containing oxygen or sulfur.
The present invention takes advantage of the stability of carbon monoxide at carbothermic temperatures such that the high temperature gaseous mixture of magnesium and carbon monoxide can be used to reduce such oxides, hydroxides, sufides and polyatomic compounds to greater purity than prior methods.
For example, most chromium is currently made by the carbon reduction of chromium III oxide, or Cr2O3.
Cr2O3(c)+3C(c) at 2200° K yields Cr(l)+3CO
The Gibbs Free Energy values for this reaction are
−562 KJ/mol+0 - - - 0+−908 KJ/mol.
A negative Gibbs Free Energy means that reduction of metal oxides to the metallic state is favorable.
Unfortunately, in actual practice, small amounts of free accumulated carbon react with the chromium to produce some contaminating chromium carbide, Cr7C3(c), which is quite stable at −221 KJ/mol. This means that very-low carbon chromium is usually made by reduction with aluminum, the aluminothermic reduction, or by electrolytic methods such as electrolytic deposition from a chromium-alum electrolyte made from high carbon ferrochromium, or, in the recent FFC process, electrolyzed at 1200° K using CaCl2 liquid, producing a sponge metal. In the latter process, the metal sponge must be heated to 2200° K to melt the chromium into ingots anyway. Because of the materials required and added steps, these are more expensive processes than this proposed invention.
Similarly, manganese is currently produced carbothermically, using carbon granules interacting at high temperature with manganese II oxide, MnO, the heat decomposition product of Mn3O4. This method, similar to the chromium situation discussed above, is complicated by the formation of manganese carbide impurities. Additional reaction with MnO is usually done to try to eliminate as much of the carbon from the manganese as possible, but some impurities remain. Another method to produce manganese is by electrolysis of manganese sulfate solutions which raises environmental problems.
Zinc has been commonly produced by roasting the main ore, ZnS, in air to form ZnO and SO2. The ZnO is then carbothermically reduced to Zn, CO, and CO2 gases. However, this process presents serious problems with air pollution and the back-reaction between the CO2 and Zn. Most zinc is now made electrolytically where ZnS is oxidized to ZnSO4, either by treating roast-produced ZnO with sulfuric acid (the acid being generated by the oxidation of the ZnS), or, to avoid the pollution-plagued roasting, the ZnS is directly treated with sulfuric acid and oxygen under pressure, with deposited free sulfur used to generate the sulfuric acid. The ZnSO4 is then electrolyzed in an aqueous environment.
It is an object of the present invention to provide a method for reduction of metal oxides which takes advantage of the known reductive property of magnesium in a simple and efficient manner.
It is a further object to provide a method for reduction of metal oxides using the direct application of magnesium vapor generated by carbothermic reduction of magnesium oxide without having to first separate magnesium and carbon monoxide.
It is a still further object to provide a method whereby the magnesium oxide produced upon reduction of other metal oxides with magnesium vapor can be recycled for further use in carbothermic reduction to produce magnesium vapor.
The present invention seeks to overcome problems associated with prior carbothermic processes and to provide a method for direct reduction of metal oxides, carbides and polyatomic compounds to metals of higher purity without inclusions requiring further refining steps and without pollution plagued preliminary treatments. As such, the present invention utilizes the reducing nature of magnesium vapors obtained at high temperatures by carbothermic reduction of magnesium oxide and carbon, but without the added step of first separating and recovering magnesium for subsequent heating as in prior art methods.
In the present invention, carbothermic reduction of MgO and carbon occurs in a first chamber, producing a gaseous mixture of Mg and CO which is maintained at carbothermic temperatures as it passes through a filter to remove any sublimed carbon fragments or dust. From the filter, the gaseous mixture passes into a second chamber, maintained at carbothermic temperatures or a temperature that is at least sufficient to prevent the back reaction to magnesium oxide and carbon, in which compounds to be reduced are maintained. Preferably the compounds to be reduced by the gaseous magnesium are provided in particle or pellet form. Reduction of the target compounds produces a pure metal which is easily and readily recoverable, magnesium oxide solid which can be recycled to the first chamber, and carbon monoxide which itself can be reduced to carbon and oxygen for reuse.
In a preferred embodiment, in a first inert chamber magnesium oxide and carbon are heated to produce a gaseous mixture of magnesium and carbon monoxide in accordance with the following formula:
3MgO(c)+3C(c) at 2200 degrees K yields 3Mg(g)+3CO(g)
This gaseous mixture is fed through a high temperature MgO zone to filter out any carbon other than that present as CO gas. The purified mixture is directed into a chamber wherein oxides, polyatomic oxides, or sulfides of metals are kept at the same temperature as the first chamber or temperatures at least high enough to prevent the back-reaction of the Mg/CO mixture. The magnesium gas combines with the oxygen or sulfur atoms, leaving the reduced desired metal. The desired reduced metal is then collected, the CO gas is separated and the resulting magnesium oxide or sulfide can be recycled.
Thus, the present invention provides a method for reduction of metal oxides, hydroxides, sulfides or polyatomic compounds thereof by direct usage of the products of carbothermic reduction of magnesium oxide, comprising conducting carbothermic reduction of magnesium oxide in the presence of carbon at an elevated temperature of from 1800 to 2600 degrees Kelvin, producing a mixture of magnesium vapor and carbon monoxide gas and contacting metal oxides, hydroxides, sulfides or polyatomic compounds thereof with that mixture of vapor and gas mixture at the elevated temperature, whereby the mixture of magnesium vapor and carbon monoxide gas is prevented from undergoing back-reaction to magnesium oxide and carbon, and the magnesium vapor effects reduction of said metal oxides, hydroxides, sulfides or polyatomic compounds thereof to metallic forms with the production of magnesium oxide. Furthermore, the magnesium oxide is recovered and recycled back to the carbothermic reduction step.
The present invention particularly provides a method for production of metals from metal oxides using a carbothermically produced mixture of magnesium vapor and carbon monoxide gas to reduce the metal oxides and comprising maintaining the metal oxides and the mixture of magnesium vapor and carbon monoxide gas at carbothermic temperatures during reduction of the metal oxides, producing pure metal, magnesium oxide and carbon monoxide, separating the metal and the magnesium oxide and recycling the magnesium oxide to carbothermic production of magnesium vapor and carbon monoxide gas. By maintaining the temperature, a back-reaction of magnesium vapor and carbon monoxide gas to magnesium oxide and carbon before reduction of the metal oxides is prevented.
The present invention still further provides a method for production of metals from metal oxides comprising, providing a reactor body comprising first and second reaction chambers, a heating means capable of heating the reactor to carbothermic temperatures and maintaining those temperatures and a filter means separating the first and second chambers. The first chamber is charged with a mixture of solid magnesium oxide and solid carbon while the second chamber is charged with a solid metal oxide in powder, particulate or pellet form. The reactor is heated to carbothermic temperature carbothermically reducing the magnesium oxide to magnesium vapor and producing a mixture of magnesium vapor and carbon monoxide gas which is passed through the filter means into the second chamber where it contacts the metal oxide, reducing the oxide to metallic form and oxidizing the magnesium vapor to magnesium oxide.
Magnesium oxide, MgO, can be made in an assortment of ways well known to those skilled in the art. These methods include decomposing MgCO3 with heat, decomposing Mg(OH)2 with heat, treating dolomite(CaCO3a.MgCO3b) with heat to form calcined dolomite(CaOa.MgOb) which is then treated with water to separate the magnesium oxide, treating MgCl2 with steam, etc.
Carbon, from any of such sources as solid petroleum coke, solid purified coal, solid charcoal, or gaseous hydrocarbon compounds which decompose at temperatures lower than the carbothermic temperatures are mixed with the MgO in a first reaction zone. The first reaction zone chamber is preferably made of any refractory material constituted by materials inert to the reactants involved in this described invention, for example graphite. Other materials meeting these refractory and inert qualities include 1) refractory noble metals, 2) any structurally strong, refractory material lined with an inert refractory substance such as graphite, lime, passive metal, or other inert fused oxide, nitride, or carbide. Although graphite may be used to construct the first reaction zone chamber, it is noted that non-carbon refractory materials should be used in all other reaction zones to prevent contamination of product. Graphite or carbide materials can be used in these other reaction zones only if they are thoroughly lined with non-carbon, inert, refractory materials to prevent exposed carbon from contaminating the product.
The MgO/C ratio in the starting mixture is preferably that which meets the stoichiometric needs of the reaction.
The MgO/C mixture is heated in a carbothermic reaction zone to between 1800 and 2600 degrees Kelvin, preferably 2000 to 2400 degrees Kelvin, but most preferably to about 2200 degrees Kelvin to enable the carbothermic reduction of MgO to proceed and prevent back-reaction of the resulting magnesium and carbon monoxide. Heat may be provided by any means commonly known to be used in high temperature furnaces including electrical resistance, electrical induction, combustion of fuels and the like. The carbothermic reaction yields a mixture of Mg gas and CO gas. If hydrocarbon gases are used as the carbon source, the hydrocarbons are first heat-decomposed into C atoms and H2 gas before being mixed with heated MgO dust.
The gaseous Mg/CO mixture from the first reaction chamber is directed through a filtering supply of hot MgO to ensure no carbon species other than CO gas is being swept along. The gaseous Mg/CO mixture is then directed into a second chamber to intermingle with the oxide or sulfide compounds of the desired metal product. The second chamber and its contents are maintained at carbothermic temperature or at a temperature at least high enough to prevent back-reaction of the Mg/CO mixture to MgO/C. In addition, filter chamber, second chamber and any subsequent chambers, conduits, etc., are preferably made from or thoroughly lined with any non-carbon materials which are inert to the reactants involved. Because the filter through which the Mg/CO mixture passes removes any errant forms of carbon other than CO gas, exposed carbon materials should be avoided downstream of the filter to prevent contamination of the metals being produced. Suitable materials would include 1) refractory noble metals, 2) any structurally strong refractory material which is thoroughly lined with an inert refractory substance such as magnesia, lime or other inert metal or fused oxide or nitride. Carbon materials, such as graphite, may be used only if thoroughly lined with an inert refractory to prevent exposure of the carbon material to the reactants.
The magnesium component of the gaseous mixture reduces the compound of the desired metal which is then collected from the chamber. If the desired metal is a liquid or solid, the gases can be vented and the desired metal separated from the magnesium oxide or sulfide. If the desired metal is gaseous, it can be distilled in a distillation chamber from the CO and any other gas. The magnesium compound solids can be recovered and recycled to the MgO feed reactant.
The boundary between the first chamber holding the MgO and C and the second chamber holding the oxide or sulfide pellets to be reduced includes a filter holding MgO at the same carbothermic temperature and through which the gaseous Mg/CO mixture passes to filter out any carbon not bound as CO gas. That filter may comprise all or part of the boundary.
In those embodiments in which the resulting metal is in gaseous form and must be condensed for recovery, for example, manganese and zinc and, to a lesser extent, chromium, the gases including the metal are directed to a third chamber for such condensation.
Reactor 1 is divided into first and second chambers 2, 3. Reactor 1 is provided with heat from electrical heaters 4, although other heat sources suitable for providing the required carbothermic temperatures may be used. Chambers 2 and 3 are separated by boundary wall 5 in which is disposed filter 6. Filter 6 provides access between first chamber 2 and second chamber 3 for gaseous Mg/CO mixture generated in first chamber 2. Alternatively, filter 6 may comprise all or most of the extent of boundary wall 5. Filter 6 comprises MgO in a form to permit passage of the gaseous Mg/CO mixture and is maintained at or about carbothermic temperatures.
First chamber 2 is charged with magnesium oxide and carbon in stoichiometric amounts relative to the overall reaction and is heated to carbothermic temperatures producing gaseous Mg/CO which passes through filter 6 whereby any unreacted carbon is removed from the gaseous stream.
Second chamber 3 is charged with oxides or sulfides of metals to be purified by the method, the oxides or sulfides being in particulate or pellet form. Within second chamber 3, gaseous magnesium reacts with the metal oxide or sulfide at the elevated temperatures maintained in second chamber 3 reducing the metal oxide or sulfide to the pure metal and forming magnesium oxide or magnesium sulfide. In the example shown, Cr2O3 is reduced to produce liquid chromium metal which is recovered from the bottom of second chamber 3 at conduit 7. The resulting solid magnesium oxide may also be recovered from second chamber 3 at conduit 8.
Still gaseous carbon monoxide together with any residual metal vapor is drawn off at 9 from the upper end of second chamber 3 and passes to condenser 10 where such residual metal is separated as liquid or solid 11 and gaseous carbon monoxide is drawn off for reduction and reuse 12.
The method and apparatus may operate in batch form, as shown in
The following examples are representative of the present invention.
In the method schematically illustrated in
3MgO(c)+3C at 2200° K yields 3Mg(g)+3CO(g).
This gaseous product, without any free C, is passed over Cr2O3 in second chamber 3, for reduction of Cr2O3 in accordance with:
3Mg(g)+3CO(g)+Cr2O3(c) at 2200° K yields 3MgO(c)+2Cr(l)+3CO(g)
The Gibbs Free Energy values for this reaction are
0+−908 KJ/mol+−562 KJ/mol - - - yields −839 KJ/mol+0+−908 KJ/mol
Or, by viewing the gaseous CO as an inert gas at 2200° K,
−562 KJ/mol - - - yields −839 KJ/mol (Gibbs Free Energies).
The overall negative Gibbs Free Energy favors reduction of Cr2O3 to chromium so that the chromium liquid is tapped out from below. The magnesium oxide crystals can be periodically cleaned of any adherent chromium by vacuum removal of the chromium because its vapor pressure is substantially higher than the magnesium oxide crystals. The magnesium oxide can then be recycled. An alternate embodiment of this process may use a flux to better separate adherent chromium from the magnesium oxide. The CO is vented and used for any purpose including heat recovery for the process through a pre-heater or use in another synthesis.
The usual source of the Cr203 starts with purified chromite, FeCrO4. The present invention can also be used to reduce FeCrO4 by the purified Mg/CO gas in second chamber 3, where the reduced product is very-low carbon ferrochromium, a useful material employed in the production of numerous alloys such as stainless steel. The present invention can further be used to reduce numerous chromium compounds, producing mixed metals or alloys, of any starting material where chromium atoms and oxygen or sulfide atoms exist.
In addition, sodium dichromate Na2Cr2O7 can be used as the chromium compound in second chamber 3. Sodium dichromate is made by treating chromite with molten NaOH and O2 to convert the Cr III to CrO4(−2). The melt is then dissolved in water and sodium dichromate is precipitated. In the present invention, the Na2Cr2O7 is reduced by the gaseous Mg/CO mixture, producing very-low carbon chromium, with vented gases being CO and Na gas which can be collected and separated. The MgO is recycled.
The present invention can be applied to alternate sources of chromium obtained from the ores listed in Table 1.
Chromite, technically (Mg,Fe+2)(Cr,Al,Fe+3)2O4, has served historically as the main source of chromium, and is capable of being directly reduced using the method of the present invention for chromium or chromium alloy production. Similarly, other chromium ores listed in the above Table, or modifications of them to change their cation content, can be reduced by the present invention because they each contain oxygen or sulfur atoms. Each can be used in this invention to produce very-low carbon chromium, ferrochromium, or many low-carbon chromium alloys.
The method of the present invention can also be used in the production of very-low carbon manganese using the batch process of
MgO(c)+C(c) at 2200° K yields Mg(g)+CO(g)
The resulting gaseous Mg/CO mixture is filtered to remove free C, is passed through a MgO fed filter 14 into second reaction chamber 3 to react with MnO (the resulting oxide from heating Mn3O4) supplied by feed mechanism 15 into second chamber 3 at 2200° K
Mg(g)+CO(g)+MnO(c or l) at 2200° K yields MgO(c)+Mn(l or g)+CO(g)
The Gibbs Free Energy values are
0+−303 KJ/mol+−213 KJ/mol - - - yields −280 KJ/mol+0+−303 KJ/mol
Or, by viewing the gaseous CO as an inert gas at 2200° K
−213 KJ/mol - - - yields −280 KJ/mol
The MnO at 2200° K is just below the boiling point of Mn. Mn(g) at 2200° K has a Gibbs Free Energy of +13 KJ/mol. In the batch process some of the Mn as liquid can be tapped out at the bottom of second chamber 3 and existing Mn vapors mixed with CO gas can be drawn off and cooled to about 1800° K, where the Mn is distilled away from the CO gas. The CO gas can then be vented. At 1800° K, the Gibbs Free Energies of the carbon monoxide gas and manganese oxide are; CO(g) is −269 KJ/mol, MnO(s) is −248 KJ/mol. So the CO can be vented away from the liquid Mn at this temperature without back-reaction to MnO and C. The MgO would be recycled. In the continuous process of
In addition, the method of the present invention can be used to reduce manganese and iron mixed oxides yielding very-low carbon ferromanganese, a useful alloy, often used in the steel industry.
The method of the present invention can be used with any of the raw manganese compound sources to produce very-low carbon manganese or alloys if they have oxygen or sulfur atoms. The following table shows several of the ores with manganese atoms.
These ores can be used directly or after modification to adjust the other cations present to produce the desired atom ratio in the alloy product.
The method of the present invention can further be used to produce zinc and zinc alloys. In the process of this invention, the carbothermic magnesium products around 2200° K are again used.
MgO(c)+CO(c) at 2200° K yields Mg(g)+CO(g)
The gaseous Mg/CO mixture is passed over ZnS so that
Mg(g)+CO(g)+ZnS(l) at 2200° K yields MgS(c)+CO+Zn(g)
The Zn is condensed from the CO quickly to prevent a back-reaction between the Zn and CO, or, as employed in the Imperial Smelting Furnace method, condensed into a spray of molten lead and distilled out later.
The MgS can be roasted or treated with an oxygen compound such as steam to convert the Mg entity back to MgO. If steam is used,
MgS(c)+H2O(g) at 1800° K yields MgO+H2S(g) with Gibbs Free Energy of:
−194 KJ/mol+−147 KJ/mol - - - yields −361 KJ/mol+−1 KJ/mol
The H2S can then be subjected to the Claus Process, currently the main process used to generate elemental sulfur from H2S found in natural gas, 2 H2S(g)+O2(g) to yield 2 S2+2 H2O. The MgO is again recycled.
The batch process of
The present process can also be applied to the production of zinc from zinc sulfide in a single chamber reaction apparatus 21 as illustrated in
The sources of zinc listed in Table 3, can also be reduced using the method of the present invention producing zinc either in purified form or as an alloy when mixed with other cation compounds.
aZinc blend, wurtzite.
bCalamine.
For example if ZnO is used in second chamber 3, the Mg/CO gas mixture will reduce the ZnO to yield MgO solid, Zn gas, and CO gas. The MgO can be recycled and the Zn and CO gases separated.
A desired alloy of zinc can be made by reducing an ore including the cation or by mixing the alloying metal oxide or sulfide with the zinc starting compound and co-reducing them in second chamber.
While the invention has been described with respect to certain specific embodiments, it will be appreciated that many modifications and changes may be made by those skilled in the art without departing from the spirit of the invention. It is intended, therefore, that all such modifications and changes are within the true spirit and scope of the invention as recited in the following claims.
