Patent Application
20130338421
The invention relates to a method for processing used carbon-containing cathode material, in particular used cathode troughs from aluminum production, in which the cathode material is placed in a shaft furnace and, in order to gasify carbon, is subjected to a thermal treatment in the shaft furnace at a temperature above the ignition temperature of the carbon and above the evaporation temperature of toxic substances contained in the used cathode material.
In aluminum production according to the Hall-Heroult process, aluminum oxide is reduced during molten mass electrolysis. The electrolysis cell most often is comprised of a steel trough clad with carbon material (graphite/anthracite) and a refractory material, such as fireclay. The carbon cladding serves as the cathode during electrolysis, and as such will subsequently be referred to as a cathode trough. Used cathode troughs, also called spent potliners, accumulate in large quantities during aluminum production according to the Hall-Heroult process, and have always posed problems relating to disposal due to their high content of toxic substances. The toxic substances include cyanide, which forms out of the carbon in the cathode troughs and nitrogen in the air, and various metal fluorides, such as sodium and aluminum fluoride, which are formed out of the metal species contained in the bauxite in conjunction with the fluoride of the cryolite used in the Hall-Heroult process. When changing out the used cathode troughs, the latter are removed from the electrolysis cell together with a refractory encircling wall, so that the spent potliners cleaned in this way predominantly still contain aluminum, silicon, calcium and magnesium oxide from the refractory material.
For example, a method of the kind mentioned at the outset became known from Canadian patent application CA 2 308 545 A1, corresponding to U.S. Pat. No. 6,074,623. In this method, used cathode troughs from aluminum production are gassed in a shaft furnace, and the formed reaction gasses are removed. However, the disadvantage to the process according to this prior art is that the alkali metals also contained in the used cathode troughs tend to form circuits in such shaft furnaces, thereby becoming concentrated in the shaft furnace, and leading to accretions and slopes in conjunction with elevated refractory wear and dust emission, and to a diminished quality for potentially formed slag from the starting materials.
Therefore, the object of the present invention is to indicate an improved method with which the mentioned toxic substances can be completely separated and obtained from the used cathode material, while at the same time preventing the formation of alkali circuits.
In order to achieve the object, the method mentioned at the outset is developed further according to the invention so that the reaction gases are conducted co-currently with the carbon in a first longitudinal section of the shaft furnace, and counter-currently to the carbon in a second longitudinal section of the shaft furnace. The reaction gases are drawn from a region of the shaft furnace having an enlarged cross section, in particular an enlarged diameter, between the specified longitudinal sections, and preferably subjected to an after-treatment. Because the reaction gases are drawn in a central area along the longitudinal axis of the shaft furnace (central gas vent), alkali circuits are interrupted, and the reaction gases containing the mentioned toxic substances in the form of sodium fluoride (NaF), aluminum fluoride (AlF3), hydrofluoric acid (HF), nitrogen (N2) and potentially additional alkaline and alkaline earth fluorides in addition to synthesis gas (CO+H2) can be relayed to further reprocessing.
The process itself takes place autothermally due to the high carbon content once the carbon has reached the ignition temperature and enough oxygen is available for gassing purposes. However, in order to initiate the process or provide support during the process, another embodiment envisages that the furnace be heated to heat the carbon of the used cathode material to the reaction temperature. This heating can basically be conducted in any of the ways known to the expert. In one embodiment, for example, the energy for thermal treatment can be introduced into the carbon to be reprocessed via electric induction. In this case, induction coils can be arranged around the periphery of the shaft furnace in the area of the dispensing end of the shaft furnace, and used to build up an induction field to which the introduced carbon of the used cathode material becomes coupled, and thereby heated. According to an alternative embodiment to the above, heating can also be accomplished through fossil firing, for example by positioning a combustion chamber in the shaft furnace, preferably in the first longitudinal section of the shaft furnace, and in particular in the upper region of the first longitudinal section, by which the shaft furnace has introduced into it a fuel, such as natural gas, petroleum, coal dust, dust generated by used cathode troughs or the like, optionally mixed in with an oxidation gas, such as oxygen or air. In another variant of this embodiment, a supply line that empties into the first longitudinal section of the shaft furnace, preferably into the upper region of the first longitudinal section of the shaft furnace, can be provided, through which the shaft furnace is supplied with fuel, optionally mixed in with oxidation gas. In another variant of this embodiment, the first longitudinal section of the shaft furnace, preferably the upper region of the first longitudinal section of the shaft furnace, can incorporate one or more lances that preferably extend in the direction of the longitudinal extension of the shaft furnace, through which the shaft furnace is supplied with fuel and oxidation gas, either separately or mixed together. As an alternative to heating via induction or fossil firing, the carbon of the used carbon-containing cathode material can also be brought to the reaction temperature by placing a partial quantity of already incandescent coke or graphite on the used cathode material introduced into the shaft furnace, or by adding a partial quantity of already incandescent coke or graphite to the used cathode material during or before its introduction into the shaft furnace.
The present invention is basically not limited with respect to either the absolute or relative dimensions of the individual longitudinal sections. However, especially good results are obtained when the length of the first longitudinal section measures 40 to 80%, preferably 50 to 70%, and especially preferably 60 to 70% of the overall length of the shaft furnace.
A further development of the inventive idea proposes that the length of the second longitudinal section preferably measure 20 to 60%, especially preferably 30 to 50%, and eminently preferably 30 to 40% of the overall length of the shaft furnace.
The shaft furnace or its longitudinal sections can here exhibit a circular cross section. However, a rectangular cross section is preferred for inductively heated shaft furnaces. This is preferred in particular in light of the skin effect, which ends up limiting the penetration depth of the electrodynamic field.
In order to maintain the carbonization of the carbon, a preferred embodiment of the present invention provides that oxygen be blown into the first and/or second longitudinal section of the shaft furnace. In regions where the oxygen hits the carbon heated at least to ignition temperature, the carbon is oxidized to CO2, but the latter is again reduced to CO in the carbon fill in the shaft furnace, thereby satisfying Boudouard's equilibrium. As a result, the carbon is not completely burned, but merely gassed, so that carbon monoxide with a calorific value can be obtained. Under the prevailing reaction conditions, the cyanide (CN− compound) contained in the used cathode material is also converted into carbon monoxide and nitrogen, and thereby completely destroyed.
In order to separate out alkali metals, their compounds and fluoride, the method according to the invention is preferably further developed in such a way that water or steam is blown into the first and/or second longitudinal section of the shaft furnace. Fluorides and alkali compounds, for example those present as sodium fluoride, are volatilized in the presence of water or steam according to the following reaction scheme, and thereby converted into the gaseous phase:
2NaF+H2O→Na2O+2HF
Na2O+C→2Na+CO
Na2O+CO→2Na+CO2
Elementary sodium is gaseous, and is removed via the central gas vent, before condensation on colder filling material wandering downward through the furnace causes circuits to form in the shaft furnace. In the gaseous phase, which also contains hydrofluoric acid (HF), sodium fluoride is again formed, and can be separated out during the ensuing waste gas treatment.
Another option for removing fluorides is to react them with silicon dioxide as follows:
SiO2+4NaF→SiF4+2Na2O.
In a preferred embodiment, the method according to the invention is implemented in such a way as to blow oxygen and water or steam into the shaft furnace by lances. This measure deals with a problem where, when oxygen or water/steam are only blown in through nozzles that empty at the wall of the shaft furnace, just the border areas of the shaft furnace or the carbon fill in the shaft furnace are supplied, while the interior region of the carbon fill cross section remains undersupplied, as a result of which the reaction rate is very low in these areas, or it might be impossible to achieve an autothermal process at all. By introducing oxygen and water/steam with the help of lances, the mentioned substances can be introduced precisely to those locations lying in the area of inductive heating, thereby generating reaction spaces that are hot enough for the process to subsequently run its course autothermally along the entire shaft furnace. At this juncture, it is also possible to blow in powdery or dusty used carbon-containing cathode material, in particular cathode troughs.
If the carbon of the used cathode material is not completely gassed in the shaft furnace, the method can preferably be developed further by dissolving the non-gassed carbon in an iron bath. The iron bath can here be inductively heated, for example. The iron bath ensures outstanding dissolution kinetics for carbon, and is easy to regenerate by blowing in oxygen, so-called refining. Since this refining is an exothermal reaction, the thermal balance of the method is improved in this way. The oxygen can also be blown in continuously, so as to keep the iron bath constantly C-undersaturated.
In a further development of the inventive idea, it is proposed within the framework of an especially preferred embodiment of the present invention that an additive be incorporated into the used carbon-containing cathode material during or before its introduction into the shaft furnace in order to slag refractory material, i.e., as described above, in particular aluminum, silicon and magnesium oxide, which are present in the used carbon-containing cathode material as residues of the refractory lining. Possible additives include elementary calcium and all calcium-containing compounds, which are also referred to below as Ca carriers, elementary magnesium as well as all magnesium-containing compounds, which are also referred to below as Mg carriers. Examples of suitable Ca carriers are CaO and CaCO3, e.g., which are available in the form of steel mill slag, limestone or anhydrous lime, and preferably introduced into the process in coarse pieces to ensure that the carbon fill or column can always be thoroughly gassed. Examples of suitable Mg carriers are MgO and MgCO3, which are also preferably introduced into the process in coarse pieces to ensure that the carbon fill or column can always be thoroughly gassed. In this embodiment, the reaction between the additive, i.e., preferably the Ca carrier or Mg carrier, and the aluminum, silicon and magnesium oxides of the refractory material can yield high quality slag, specifically, when adding Ca carriers, high quality calcium aluminate slag, which exhibit outstanding hydraulic properties, and can thus be advantageously used in the cement industry for manufacturing hydraulic binders. The addition of Mg carriers produces high quality magnesium aluminate slag or spinel slag, for example which can be used as refractory concrete.
In the aforementioned embodiment, as much Ca carrier and especially preferably calcium oxide is added to the used carbon-containing cathode material during or before its introduction into the shaft furnace to ensure that a calcium aluminate slag with an Al2O3 content of 50 to 70%, preferably 55 to 65% w/w, and especially preferably about 60% w/w, is generated in the shaft furnace. A melt phase here arises at under 1,600° C. As an alternative thereto, it is equally preferred that as much Ca carrier and especially preferably calcium oxide be added to the used carbon-containing cathode material during or before its introduction into the shaft furnace to ensure that a calcium aluminate slag with an Al2O3 content of more than 70 to 90% w/w, preferably 75 to 85% w/w, and especially preferably about 80% w/w, is generated in the shaft furnace. The high melting point of the slag here gives rise to a sintering phase. In light of their composition and quality, these calcium aluminates are suited in particular for use in generating quick cements, increasing the early strength of composite elements, or in the form of sulfate-induced slag cements or gypsum-slag cements. Therefore, the use of these calcium aluminates in cements is especially advantageous, since it enables a significant reduction in the clinker factor during cement production, i.e., in the percentage of Portland cement clinkers per ton of cement. This is advantageous in terms of climate policy, because roughly 1 ton of carbon dioxide is produced during the manufacture of 1 ton of Portland cement clinkers. Furthermore, the calcium aluminates fabricated in this way can be used for manufacturing refractory materials and manufacturing ⅓ slag, i.e., a mixture comprised of CaF2, Al2O3 and CaO, which in turn is employed for obtaining heavy-duty steel components. Aside from the above, the calcium aluminates fabricated in this way can also be used in the area of pig iron desulphurization or secondary metallurgy, which is also advantageous among other things since it avoids the problematic use of calcium fluoride.
The method according to the invention preferably employs used carbon-containing cathode material with an aluminum oxide content of 10 to 45%, especially preferably of 15 to 30%.
The aforementioned embodiment, in which an additive is incorporated into the used carbon-containing cathode material during or before its introduction into the shaft furnace, can also be combined with the embodiment in which the carbon of the used carbon-containing cathode material is not completely gassed, and the carbon that was not gassed is dissolved in an iron bath.
However, as an alternative to executing the process with an iron bath, the method can also be implemented in such a way according to another preferred embodiment as to completely gas the carbon in the cathode material and remove the remaining calcinate. In this case as well, an additive, especially preferably a Ca carrier and/or Mg carrier, can, but does not have to, be incorporated in the used carbon-containing cathode material during or before its introduction into the shaft furnace. If no additive is incorporated, a dry, practically carbon-free calcinate made up of aluminum and silicon oxide is discharged, which can also find application in the cement industry.
It was observed that incomplete calcinate can disintegrate into carbon-containing powder. However, in light of the powder form, the latter is then no longer permeable to gas, and the corresponding shaft furnace section can no longer be thoroughly gassed. The following after-treatment options are conceivable. After-treatment can take place in the iron bath as described above. Alternatively, the powder can be treated with steam for decarbonization purposes, as a result of which a simultaneous cooling is achieved due to the endothermic, heterogeneous water-gas reaction.
As already indicated further above, elementary sodium along with sodium oxide (Na2O) recombines in the gaseous phase in the presence of hydrofluoric acid (HF) to sodium fluoride (NaF). For sodium oxide, the reaction equation is to be written out as follows:
Na2O+2HF→2NaF+H2O.
For the gaseous sodium, the reaction equation must here be written out as follows:
Na+HF→NaF+½H2
The following secondary reaction can be observed:
2Na+H2O→Na2O+H2
If the objective is now to obtain problematic, but economically interesting hydrofluoric acid in the gaseous phase, the method is preferably implemented using a strongly acidic slag. Given the presence of a strongly acidic slag, wherein the acid component can be formed by silicon oxide and/or aluminum oxide, for example, hydrofluoric acid is regenerated according to the following reaction equation:
2NaF+SiO2+H2O→Na2SiO3+2HF
or
NaF+Al2O3+H2O→NaAl2O4+HF.
In order to control the products obtained in the gaseous phase as respectively desired, the method is thus preferably further developed in such a way that the basicity in the shaft furnace can be adjusted by adding basic or acidic ingredients, in particular CaO or SiO2.
An especially preferred process provides for the alternating addition of basic and acidic ingredients. In this way, sodium fluoride and hydrofluoric acid are alternately obtained. The hydrofluoric acid can be reacted with alumina (Al2O3) to form aluminum fluoride (AlF3) and water, wherein the aluminum fluoride can be further processed with the sodium fluoride (NaF) obtained during the basic process to form cryrolite (Na3AlF6), which in turn can be used in aluminum production according to the Hall-Heroult process.
12NaF+4AlF3→4Na3AlF6
If the load of toxic substances, such as cyanides, alkali metals and their fluorides, is too high, it may happen that the carbon of the used cathode material will not be coupled through electric induction when exposed to heating as described above. In this case, it is preferred that the method according to the invention be further developed in such a way as to also introduce unused graphite or coke to the carbon of the used cathode material. At a suitable frequency, the unused carbon is coupled to the induction field and heats up, wherein the actual carbon to be reprocessed is also brought to temperature through contact with the incandescent graphite or coke.
The method is preferably implemented in such a way that the reaction gases are drawn at a temperature of 800° C. to 1200° C., in particular 900° C. to 1100° C., especially 1000° C., and relayed to after-treatment, thereby preventing a condensation of reaction gases or toxic substances in the line systems, and ensuring a controlled waste gas treatment.
The maximum temperature in the furnace hearth here preferably measures 1,200 to 1,700° C., especially preferably 1,400 to 1,700° C., and eminently preferably 1,500 to 1,600° C. This maximum temperature is diminished up to the central gas vent by endothermic reactions and heat losses to a maximum of 1,200° C.
In a further development of the inventive idea, it is proposed that the after-treatment of the reaction gas drawn from the shaft furnace take the form of a waste gas treatment free of wastewater. For example, this can take place by routing reaction gas drawn from the shaft furnace, which aside from carbon monoxide, carbon dioxide, hydrogen, possibly water, and dust discharged from the reactor, also contains volatilized compounds, including in particular hydrofluoric acid, sodium fluoride, aluminum fluoride and others, into a cooled and preferably isothermally operated fluidized bed reactor, which contains particles comprised of aluminum oxide, sodium fluoride, aluminum fluoride, calcium carbonate, aluminum hydroxide or the like. The temperature of the fluidized bed can here be easily set to a suitable value, for example by a water-powered heat exchanger or a water evaporator, wherein the strong turbulence of the fluidized bed ensures a uniform temperature in the shaft furnace, for example of about 1,000° C., about 1,100° C. or about 1,200° C. The volatilized compounds of the waste gas are quickly cooled, condensed and possibly reacted on the fluidized aluminum oxide particles, as a result of which cryolite or similar compounds form on the aluminum oxide particles. Waste gas cleaned in this way is drawn from the fluidized bed reactor, wherein any remaining aluminum oxide particles loaded with reaction products are separated out of the waste gas in a cyclone situated downstream from the reactor, or in another dry absorption process on alumina or aluminum oxide in a counter-current at a low temperature. Silicon fluoride (SiF4) can optionally also be separated out of the waste gas obtained in this way. Aside from being free of waste water, this after-treatment has the advantage of requiring only comparatively small devices. Another advantage to this after-treatment is that heat is generated in the process, which can be used to heat up the process steam required for the shaft furnace. In addition, the product comprised of the particles and waste gas can be used in aluminum production.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a method for processing used carbon-containing a cathode material, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.
Referring now to the figures of the drawings in detail and first, particularly to
While the upper portion of the second longitudinal section 9 exhibiting the nozzles 14 represents a counter-current heating section, a lower region 17 must be understood as a cooling section, since nozzles 15 are intended for introducing primarily water in liquid form, and possibly as steam or wet steam.
In the third alternative configuration of the shaft furnace according to
As may be gleaned from
As evident from
As further depicted based on the example of
| Number | Date | Country | Kind |
|---|---|---|---|
| A 245/2011 | Feb 2011 | AT | national |
This is a continuation application, under 35 U.S.C. §120, of copending international application No. PCT/EP2012/053006, filed Feb. 22, 2012, which designated the United States; this application also claims the priority, under 35 U.S.C. §119, of Austrian patent application No. A 245/2011, filed Feb. 23, 2011; the prior applications are herewith incorporated by reference in their entireties.
| Number | Date | Country | |
|---|---|---|---|
| Parent | PCT/EP2012/053006 | Feb 2012 | US |
| Child | 13974302 | US |
