METHOD FOR PRODUCING THERMALLY PROCESSED MATERIAL

Abstract

Method for producing thermally processed material (50), the method comprising providing material (35) to be thermally processed, providing carbon-containing scrap material (20) from an electrolysis cell (10) for the production of primary aluminium (15), introducing the material (35) to be thermally processed into a furnace (40), processing the carbon-containing scrap material (20) to produce a scrap fuel (55), and thermally processing the material (35) to be thermally processed in the furnace (40) using energy generated by burning the scrap fuel (55) such as to produce thermally processed material (50).

Claims

1. A method for producing secondary aluminium (50) in a thermal process, the method comprising providing aluminium containing scrap material (35) to be thermally processed, providing carbon-containing scrap material (20) from an electrolysis cell (10) for the production of primary aluminium (15), introducing the aluminium containing scrap material (35) into a furnace (40) for thermal processing, processing including crushing and/or milling the carbon-containing scrap material (20) to produce a scrap fuel (55) of an average particle size between 10 μm and 300 μm, and thermally processing and melting the aluminium-containing scrap material (35) in the furnace (40) using energy generated by the scrap fuel being transported into a flame of a burner and burning the scrap fuel (55) such as to produce the secondary aluminium material (50).

2. The method according to claim 1 for producing secondary aluminium (50), the method comprising introducing the aluminium-containing scrap material (35) into a rotary furnace (40).

3. The method according to claim 1, wherein the carbon-containing scrap material (20) is obtained from an anode and/or a cathode of the electrolysis cell (10).

4. The method according to claim 1, wherein the aluminium-containing scrap material (35) comprises dross from aluminium melting and casting.

5. The method according to claim 1, wherein the aluminium-containing scrap material (35) comprises end-of-life aluminium scrap or process aluminium scrap.

6. The method according to claim 1, wherein the aluminium-containing scrap material (25) comprises fluorine-comprising bath material from the electrolysis cell (10).

7. (canceled)

8. The method according to claim 6, wherein carbon-containing scrap material (20) is mechanically processed such as to obtain panicles having an average size between 50 μm and 100 μm.

9. The method according claim 1, wherein the scrap fuel (35) is transported pneumatically into a flame of a burner (45) for generating the energy for melting the aluminium-containing scrap material (35).

10. The method according to claim 1, wherein the scrap fuel (55) is transported into a flame of a burner (45) for generating the energy for melting the aluminium-containing scrap material (35) while being dispersed in a liquid fuel, in particular oil.

11. The method according to claim 2, wherein, with respect to the caloric value, at least 30%, in particular at least 50% of the energy for melting the aluminium-containing scrap material (35) is provided by the scrap fuel (55).

12. The method according to claim 2, wherein, in addition to the aluminium-containing scrap material (35), chloride salt, in particular NaCl and/or KCl, and/or a fluoride salt, in particular CaF.sub.2, is introduced into the rotary furnace (40).

13. The method according to claim 6, wherein a fluorine content in the rotary furnace (40) is adjusted by adjusting a ratio of bath material (25) to other aluminium-comprising scrap (35).

14. The method according to claim 2, wherein an off-gas from melting the aluminium-containing scrap material (35) in the rotary furnace (40) is led into the flame of the burner (45) for post-combustion.

15. A method for producing aluminium using a substantially closed-loop mass flow, the method comprising producing primary aluminium (15) from alumina using an electrolysis cell (10), producing products (30) from the primary aluminium (15) and from secondary aluminium (50), obtaining aluminium-containing scrap material (35) from the product (35; end-of-life scrap) or the production of the product (35; process scrap), obtaining carbon-containing scrap material (20) form the electrolysis cell (10), and producing secondary aluminium (50) form the aluminium-containing scrap material (35) and the carbon-containing scrap material (20) using the method according to claim 2.

Description

SHORT DESCRIPTION OF THE FIGURE

[0035] The FIGURE shows a schematic view of the method according to an embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

[0036] The method according to the invention produces secondary aluminium, that is, aluminium that is not directly produced from alumina, from aluminium-containing scrap material. With reference to FIG. 1, an electrolysis cell 10 produces primary aluminium 15 from alumina (not shown). Generally, this is achieved via the Hall-Heroult process involving the flow of an electric current from the cathode placed on the bottom side of the electrolysis cell through a reaction chamber of the electrolysis cell 10 to the anode placed on the upper side of the electrolysis cell 10. The reaction chamber is filled with alumina and cryolite (Na.sub.3AlF.sub.6) and optionally other additives such as lithium fluoride. The anode and the cathode both comprise carbon. While the anode is consumed at a higher rate than the cathode via the oxidation of carbon that takes place during the reduction of alumina, the cathode also deteriorates over time and must eventually be replaced. As both anode and cathode come into contact with fluorine or fluorine-comprising compounds in the reaction chamber, used (also called “spent”) anodes and cathodes may comprise fluorine. According to the present invention, spent/used anodes and/or cathodes from the electrolysis cell 10 are referred to as carbon-comprising scrap material 20. A further byproduct of the production of primary aluminium in the electrolysis cell 10 is bath material 25, a mixture of impurities from the alumina, the cryolite/additives, impurities from the anode/cathode and the cell itself that accumulates on the upper side of the reaction chamber of the electrolysis cell 10 and is removed from time to time. The bath material 10 may comprise significant amounts of fluorine (F).

[0037] The primary aluminium 15 is used to produce products 30 such as cast products, sheet metal, extruded products and the like. Aluminium-containing scrap material 35 is obtained from process waste that is generated during production of the products 30 (process scrap) as well as from the products 30 themselves at the end of their lifetime (end-of-life scrap). Aluminium-containing scrap material 35 may also be obtained from dross generated while casting aluminium or from casting process scrap.

[0038] The aluminium-containing scrap material 35 is according to the invention introduced into a rotary furnace 40. The rotary furnace 40 is heated by a burner 45 such as to melt the aluminium-containing scrap material 35. The melting may result in a purification of the material in the rotary furnace with respect to the specific aluminium content, as organic compounds in the aluminium-containing scrap 35 such as lacquers and coatings are burned off and other impurities and oxides may accumulate in dross that forms on top of the melt in the rotary furnace 40 and can be separated from the melt. The melted and solidified material that is produced in the rotary furnace 40 is referred to as secondary aluminium 50, as it is-in contrast to primary aluminium-not directly produced from alumina.

[0039] The secondary aluminium 50 may be used like primary aluminium 15 to produce products 30 resulting in a substantially closed cycle with respect to the mass flow of aluminium. The cycle:

[0040] product 30->aluminium-containing scrap material 35->secondary aluminium 50-product 30-> . . .

[0041] may be repeated indefinitely in principle, however in practice some aluminium is lost during recycling, there are issues with the purity of secondary aluminum 50 in some applications, and the global demand in aluminium is rising so that on a global scale secondary aluminium 50 has to be supplemented by primary aluminium 15 to meet demand.

[0042] According to the invention, the carbon-comprising scrap material 20 is processed such as to obtain scrap fuel 55. The processing may include crushing and/or milling the carbon-comprising scrap material 20 such as to reduce the particle size. Depending on the design of a burner 45, an average particle size determined using the largest circle method (e.g. using a light optical microscope) of the scrap fuel 55 may be between 10 μm and 300 μm, in particular between 50 μm and 100 μm. While smaller average particle sizes have a better ratio of surface to volume than larger average particle sizes and therefore burn better in a flame of the burner 45, processing the carbon-comprising scrap material 20 is a resource-intensive and the stated average particle size of between 10 μm an 300 μm, in particular between 50 μm and 100 μm, is thought to be a good compromise between quality of the scrap fuel 55 and production efficiency of the scrap fuel 55. According to one embodiment that is thought to be particularly efficient in terms of processing effort while still producing particles having a favorable surface to volume ratio, the carbon-comprising scrap material 20 is milled such that an average particle size above or equal to 150 μm is obtained. Said milled material is then fed through a sieving means, e.g. a sieve, that allows particles having a diameter of 150 μm or smaller to pass through the sieving means to obtain a sieved fraction. The sieved fraction is then used as the scrap fuel 55. In case the particles are too large, the combustion speed is too low and the burner may get clogged resulting in a less effective use of combustion energy and more downtime. On the other hand, when the particles are too small, the cumulative energy balance deteriorates as too much energy was spent on mechanical processing the carbon-comprising scrap material 20.

[0043] The scrap fuel 55 is burned by the burner 45 to produce secondary aluminium 50 in the rotary furnace 40 by heating the carbon-containing scrap material 35. The heat produced by the burner 45 that heats the rotary furnace 40 is schematically shown by arrow 46 in FIG. 1. According to embodiments of the invention, the burner 45 is located outside of the furnace 40, e.g. rotary furnace 40. In other words, the burner 45 may be located outside of a furnace chamber of the furnace 40. According to embodiments of the invention, the scrap fuel 55 does not come into contact with the aluminium-containing scrap material 35 or the secondary aluminium 50. This also means that no spent potlinings come into contact with the material that is located inside the furnace 40 (e.g. inside the furnace chamber).

[0044] To improve the performance of the burner 45, the scrap fuel 55 may be supplemented by natural gas or by oil (petroleum) or hydrocarbons derived from gas/petroleum, such as diesel fuel, gasoline or otherwise processed petroleum.

[0045] The scrap fuel 55 may be blown into a flame of the burner 45 or the scrap fuel 55 may be mixed with the supplemented fuel. In case the supplemented fuel is a gas, a mixture of said gas and the particulate scrap fuel 55 may be incinerated to generate heat. In case the supplemented fuel is a liquid such as oil, the scrap fuel 55 may be dispersed in the liquid and the resulting dispersion may be incinerated to generate heat.

[0046] According to embodiments of the present invention, a rotary furnace 40 is used as the flame generated by the burner 45 has a high volume and generates a large amount of (heat) radiation caused by the solid scrap fuel 55. It has been found that a rotary furnace 40 has an optimal transfer area for transfer of the energy of the flame that is generated by the burner 45 burning the scrap fuel 55. Further, a rotary furnace 40 allows to utilize off-gas cleaning of the gases generated by the burner 45 and/or generated by melting the aluminium-comprising scrap material 35.

[0047] An off-gas (not shown in FIG. 1) that is produced by heating the aluminium-containing scrap material 35 in the rotary furnace 40 may be led to the burner 45 to be burned by the burner 45. This results in a cleaning of the off-gas, as harmful compounds in the off-gas are burnt. In case the off-gas comprises compounds that can be burned in an exothermic reaction (for example stemming from organic coatings or lacquers or plastics in the aluminium-containing scrap material 35), said compounds generate additional heat and improve the energy efficiency.

[0048] According to the invention, chlorine comprising salts, such as NaCl or KCl may be introduced into the rotary furnace 40 together with the aluminium-containing scrap material 35. It has been found that such an addition of chlorine comprising salts can increase the quality of the secondary aluminium 50 produced by the rotary furnace 40. It is thought that the mechanism involves an increased accumulation of high-melting intermetallics in the dross forming in the rotary furnace 40 caused by the chlorine salt additions.

[0049] According to the invention, fluorine-comprising bath material 25 may optionally also be introduced into the rotary furnace 40 together with aluminium-containing scrap material 35. It is thought that the fluorine from the bath material 25 reduces the surface tension of Al-droplets formed during melting in the rotary furnace 40 which results in the formation of droplets having a higher volume. The larger the droplets are, the less oxidation takes place on the surface of the droplets due to the geometric relationship of surface to volume. Further, fluorine may help in keeping the walls of the rotary furnace 40 clean. It is noted that the fluorine-comprising bath material 25 according to the present invention is not a spent potlining but is taken from the interior of an electrolysis cell.

[0050] In addition to or instead of bath material 25, optionally fluorine comprising salts such as CaF.sub.2 and/or Na.sub.3AlF.sub.6 or other salts may be introduced into the rotary furnace 40 together with the aluminium-containing scrap material 35.

[0051] Accordingly, the present invention provides methods that enable an efficient production of secondary aluminium 50 by using waste material from the production of primary aluminium 15 in an electrolysis cell 10 that reduce energy consumption and reduce the amount of waste that is disposed in landfill sites.