WO2009013177A1

WO2009013177A1 - Process for the recovery of aluminium from aluminium dross

Info

Publication number: WO2009013177A1
Application number: PCT/EP2008/059229
Authority: WO
Inventors: Placido Garcia-Juan; Andreas Becker
Original assignee: Solvay Fluor Gmbh
Priority date: 2007-07-20
Filing date: 2008-07-15
Publication date: 2009-01-29

Abstract

During melting of aluminium, e.g. for purification of aluminium, salts are added to remove impurities or to protect the molten aluminium against oxidation by air and other undesired reactions. After separation of the aluminium, aluminium dross remains in the furnaces and parts thereof, for example, on impellers. This dross contains aluminium. The process according to the present invention provides for an easy and effective process to recover aluminium contained in the dross by adding potassium fluoroaluminate and heating the resulting mixture.

Description

Process for the recovery of aluminium from aluminium dross

The invention concerns a process for recovery of aluminium from aluminium dross.

In aluminium production or refinery, protective salts are applied to protect the molten aluminium from contact with air or to form agglomerate layers with the impurities, for example, oxides.

For processing of aluminium, for example, for casting it, aluminium ingots are molten in an oven. On the walls of the oven, dross is formed which has to be removed. In aluminium refinery, aluminium is molten and stirred by an impeller, and salts are added to clean the aluminium and to bind impurities. Impellers are used to keep the liquid content of the aluminium furnace in motion to prevent separation of aluminium alloys. A dross layer forms and is separated from the aluminium by skimming or other processes. After cooling it forms solid dross. Aluminium dross is a mixture of free metal and nonmetallic substances, for example, aluminium oxide and aluminium salts. Aluminium nitride and carbide may also be present. The content of aluminium metal depends on how carefully skimming from the melt was executed. Dross with higher aluminium content often is in the form of large clotted lumps or blocks and is called white dross, while dross with a lower metal content often is in granular form and is called black dross. Often, such black dross is obtained when aluminium scrap is molten with salts in open furnaces. Often, the metal content in black dross is between about 40 and 70 % by weight. In white dross, the metal content often is in the range between 70 and 90 % by weight and even more. Traditionally, dross is disposed of in landfills. For ecological and economical reasons, recycling of the metal content in dross would be desirable.

It is an object of the present invention to provide a process for recovery of the aluminium content in aluminium dross. This object and other objects of the present invention are achieved by the process of the present invention. The process of the present invention provides for the recovery of aluminium contained in aluminium dross by adding a treating agent which contains at least 10 % by weight of a potassium fluoroaluminate with a melting point below the melting point of the aluminium to be recovered, to aluminium containing dross to obtain a reaction mixture, melting the mixture whereby liquid aluminium forms, and separating the aluminium.

The term"dros^' denotes in the present invention mixtures of metallic aluminium and impurities, for example, aluminium oxide, which is a side product in aluminium processing and refinery, for example, the residues on the wall of aluminium melting ovens, refinery processes. The term"dros^'is intended to include skimmings. Skimmings is the layer of impurities swimming on the molten aluminium which can be mechanically removed and which contains elemental aluminium along with the impurities. The term"aluminiuni' includes aluminium alloys, e.g. alloys containing magnesium. Preferably, it denotes aluminium and aluminium alloys with a melting point of at least 60CPC (pure aluminium has a melting point of about 66CPC).

The process of the invention can be applied to dross with any aluminium content. For example, it can be applied to recover the aluminium values from black dross. It as well can be applied to white dross.

The term' potassium fluoroaluminatd', in its broadest meaning, denotes all complex fluoroaluminates including the hydrates which comprise potassium cations. It is well known that fluoroaluminate anions exist with a ratio of aluminium to F of 1 :4 (the tetrafluoroaluminate anion), 1 :5 (the pentafluoroaluminate anion) and 1 :6 (the hexafluoroaluminate anion). Also, complexes are known which formally are adducts of two or more of said complexes, for example, the anion AlF 5-AlF₆ (or Al₂Fn).

The term'treating agent'refers to a composition which contains at least 10 % by weight of a potassium fluoroaluminate with a melting point lower than the melting point of the aluminium to be recovered. The remainder of the agent can be constituted by recovery salts applicable for aluminium, or by other potassium fluoroaluminates with a melting point higher than that of the aluminium to be recovered. Also, other alkali metal fluorides, e.g. the cesium fluoroaluminate, might be present. Preferably, only fluoroaluminates with the potassium ion are contained.

Preferably, the treating agent consists of potassium fluoroaluminates.

Generally, as mentioned above, the content of lower melting fluoroaluminate or fluoroaluminates is at least 10 % by weight of the treating agent. More preferably, the content of potassium fluoroaluminate with a melting point lower than that of the aluminium to be recovered is equal to or greater than 30 % by weight of the treating agent, still more preferably, it is equal to or greater than 50 % by weight, especially preferably, it is equal to or greater than 80 % by weight.

It is well known that potassium tetrafluoroaluminate and potassium pentafluoroaluminate and their hydrates have melting points well below 60CPC, while tripotassium cryolite melts above lOOCPC. Thus, as potassium fluoroaluminates with a melting point which is below the melting point of the aluminium to be recovered, the terra- and pentafluoroaluminates are especially suitable. Thus, the presence of potassium tetrafluoroaluminate and/or dipotassium pentafluoroaluminate and/or their hydrates in a very preferred embodiment is mandatory. Nevertheless, tripotassium hexafluoroaluminate may additionally be present.

Highly preferably, the content in tripotassium fluoroaluminate ("potassium cryolite") is at most 10 % by weight of the treating agent, and preferably is even lower, down to 0 % by weight of the treatment agent, see also below.

Preferably, a treatment agent is used which consists essentially of potassium tetrafluoroaluminate ; or consists of mixtures of potassium tetrafluoroaluminate with dipotassium pentafluoroaluminate including the hydrates thereof ; or consists of mixtures of potassium tetrafluoroaluminate with dipotassium pentafluoroaluminate including the hydrates thereof and tripotassium hexafluoroaluminate. Preferably, the content of tripotassium hexafluoride is equal to or lower than 10 % by weight, more preferably, equal to or lower than 5 % by weight, still more preferably, equal to or lower than 3 % by weight, in the fluoroaluminate composition (the treatment agent). Often, the treatment agent contains only undesired amounts of the hexafluoroaluminate due to the production process. Especially preferably, the content in the hexafluoroaluminate is lower than 0,5 % by weight ; in one especially preferred embodiment, it is 0 % by weight.

If mixtures of two or more potassium fluoroaluminate compounds are used, the content of potassium tetrafluoroaluminate preferably is equal to or higher than 20 % by weight of the mixture. More preferably, the content of potassium tetrafluoroaluminate is equal to or higher than 50 % by weight. Still more preferably, it is equal to or higher than 70 % by weight. Most preferably, the content is equal to or higher than 75 % by weight. Also here, the content of tripotassium fluoroaluminate, as mentioned above, is very preferably is equal to or lower than 5 % by weight or still lower, as indicated above, and can be less than 0,5 % by weight and can even be 0 % by weight. The content of dipotassium pentafluoroaluminate is preferably equal to or higher than 5 % by weight of the total weight of the fluoroaluminate composition. Preferably, it is equal to or lower than 80 % by weight of the total weight of the fluoroaluminate composition (which is also denoted as the treatment agent). Preferred mixtures comprise 75 to 85 % by weight of potassium tetrafluoroaluminate, 15 to 25 % by weight of dipotassium pentafluoroaluminate and 0 to 5 % by weight of tripotassium hexafluoroaluminate. The terms 'potassium tetrafluoroaluminatd' and "dipotassium pentafluoroaluminate" are meant to include that hydrates can be contained.

The term'to comprise" includes, in the context of the present invention, the meaning of to consist of.

If desired, the potassium fluoroaluminate consists essentially of potassium tetrafluoroaluminate or its hydrates. The term"essentiall^' denotes in this context that preferably, at most 5 % by weight of the potassium fluoroaluminate are potassium fluoroaluminates (for example, dipotassium pentafluoroaluminate or its hydrates or tripotassium hexafluoroaluminate) other than potassium tetrafluoroaluminate (KAlF₄). The content in tripotassium fluoroaluminate again is as low as given above, is preferably equal to or lower than 5 % by weight, and is often 0 % by weight.

A very suitable composition is a flux composition consisting essentially of potassium tetrafluoroaluminate and dipotassium pentafluoroaluminate and optionally their hydrates. For example, a mixture of potassium tetrafluoroaluminate and dipotassium pentafluoroaluminate which is available under the trademark Nocolok^® is highly suitable.

An effective amount of potassium fluoroaluminate is added to the dross. It may vary depending on the metal content. Often, equal to or more than 1 g, preferably equal to or more than 2 g of potassium fluoroaluminate are added per 100 g of the dross. Very preferably, equal to or more than 3 g of potassium fluoroaluminate are added per 100 g of the dross, still more preferably equal to or more than 5 g per 100 g of the dross. Often, equal to or less than 50 g of potassium fluoroaluminate are added per 100 g of the dross.

Preferably, less than 2O g of the potassium fluoroaluminate are added per 100 g of the dross, still more preferably, equal to or less than 1O g per 100 g of the dross. In one embodiment, the figures mentioned above denote the total amount of potassium fluoroaluminates added to the dross (including the amount, if contained, of tripotassium hexafluoroaluminate). Preferably, the figures denote the amount only of potassium tetrafluoroaluminate, potassium pentafluoroaluminate, or, if a mixture of them is used, the amount of both, which is added to the dross. If hydrates of the tetra- or pentafluoroaluminate are contained, they are also included into the calculation.

In one embodiment, dross is removed from the apparatus (e.g., the furnace or melting apparatus) and mixed with the potassium fluoroaluminate for Al recovery. In another embodiment, the potassium fluoroaluminate can be applied directly to the dross in the furnace, e.g. it can be applied directly to the dross on the furnace walls without the need to scratch the solids off the wall.

The mixture of dross and potassium fluoroaluminate is heated to a temperature which is sufficiently high so that the dross releases aluminium which forms a separate liquid. It is at least as high as the melting point of aluminium, namely 661°C. Preferably, the temperature is equal to or higher than 68CPC. Generally, it is equal to or lower than 850PC, preferably equal to or lower than 75 CPC.

The potassium fluoroaluminate can, as was mentioned above, be applied together with other salts, for example, with alkali metal chlorides or bases. In a preferred embodiment, the potassium fluoroaluminate is the only salt added to the dross.

The advantage of the process is that it allows to recover the aluminium content of dross which otherwise must be disposed in land fills. The process can be applied to dross from various sources wherein aluminium is smelted and dross is produced, for example, from processes intended to purify aluminium. For example, dross recovered from furnaces, e.g. used for melting aluminium, or impeller-driven furnaces, e.g. those used for refinery, can be treated. The following examples are intended to explain the invention in further detail without intention to limit it. Examples

Example 1 : Aluminium recovery from impeller dross

1.1. Dross from a furnace in which aluminium was molten, stirred by an impeller and purified under addition of protective salts and protective gas, comprised about 70 % by weight of aluminium. 200 g of this dross and 10 g of a mixture of potassium tetrafluoroaluminate and dipotassium pentafluoroaluminate, available from Solvay Fluor GmbH, Germany, under the tradename Nocolok^®, were heated to a temperature of 700⁰C. Liquid aluminium separated ; 78 g were recovered. 1.2. Example 1.1 was repeated, but potassium tetrafluoroaluminate available from KBM Master Alloys under the tradename PAF-grey^® was applied. A yield comparable to that of example 1.1 was achieved. Example 2 : Aluminium recovery from furnace dross

Dross adhering to the walls of an oven for aluminium smelting was removed, a mixture of potassium tetrafluoroaluminate and dipotassium pentafluoroaluminate (available under the tradename Nocolok from Solvay Fluor GmbH) was added, and the formed solid was heated. Liquid aluminium separated, was removed and thus could be recovered.

Claims

C L A I M S

1. Process for the recovery of aluminium contained in aluminium dross by adding a treating agent which contains at least 10 % by weight of a potassium fluoroaluminate which potassium fluoroaluminate has a melting point below the melting point of the aluminium to be recovered, to aluminium containing dross to obtain a reaction mixture, melting the mixture whereby liquid aluminium forms, and separating the aluminium.

2. Process according to claim 1 wherein the treatment agent comprises potassium fluoroaluminate based on potassium tetrafluoroaluminate, dipotassium pentafluoroaluminate, their hydrates, or mixtures of tetrafluoro- and pentafluoroaluminate and/or their hydrates.

3. Process according to claim 1 wherein the treatment agent consists of potassium fluoroaluminates, preferably consists of potassium tetrafluoroaluminate, dipotassium pentafluoroaluminate, and/or their hydrates.

4. Process according to claim 1 wherein equal to or more than 1 g of potassium fluoroaluminate are added per 100 g of dross.

5. Process according to claim 1 wherein equal to or less than 20 g potassium fluoroaluminate are added per 100 g dross.

6. Process according to claim 1 wherein the reaction mixture is heated to a temperature which is equal to or higher than the melting point of aluminium.

7. Process according to claim 6 wherein the reaction mixture is heated to a temperature which is equal to or higher than 68CPC.

8. Process according to claim 6 wherein the reaction mixture is heated to a temperature equal to or lower than 85CPC.

9. Process according to claim 1 wherein the content of tripotassium hexafluoroaluminate in the treatment agent is equal to or lower than 10 % by weight of the treatment agent, preferably equal to or lower than 5 % by weight, more preferably equal to or lower than 2 % by weight, and especially preferably about 0 % by weight.

10. Process according to claim 1 wherein the aluminium dross is obtained during aluminium production, aluminium processing or aluminium refinery.