WO1991009978A1

WO1991009978A1 - Process for converting dross residues to useful products

Info

Publication number: WO1991009978A1
Application number: PCT/CA1990/000454
Authority: WO
Inventors: Gaetan Chauvette; Frank M. Kimmerle; Roger Roussel
Original assignee: Alcan International Ltd Canada
Current assignee: Rio Tinto Alcan International Ltd
Priority date: 1989-12-27
Filing date: 1990-12-24
Publication date: 1991-07-11
Anticipated expiration: 1992-06-27
Also published as: AU6970391A

Abstract

A process for converting dross residues having an aluminum nitride content of at least 5 wt % and a metallic aluminum content of no more than 10 wt % ( particularly plasma dross residues) to useful products. The process comprises treating the dross residues with a solution having a pH of 7 or more, the reaction at alkaline pH values being AlN + 4H2O ← Al(OH)3 + NH4+ + OH- followed by: Al(OH)¿3 + OH?- ← Al¿2?O?- + 2H¿2O and NH4+ + OH- ← NH¿3? $(1, 3)$ + H2O. The initial treatment of the dross residues may be under mild conditions to generate ammonia from the nitride. The ammonia can be collected as a useful product or converted to other useful products such as ammonium salts. Under harsher conditions, e.g. elevated temperature and pressure with caustic alkalis, a substantial portion of the dross residue dissolves to form an alkali metal aluminate solution which is itself a useful product (e.g. it can be used in the Bayer process) or can be converted into other useful products such as aluminum trihydrate. A portion of the dross residue normally remains undissolved and this can also be a useful product, e.g. a refractory precursor, or may be disposed of by dumping since it is non-polluting.

Description

PROCESS FOR CONVERTING DROSS RESIDUES TO USEFUL PRODUCTS

TECHNICAL FIELD

This invention relates to a process for converting dross residues to useful products. BACKGROUND ART Dross is a material which forms on the surface of molten aluminum or aluminum alloys during remelting and metal holding and handling operations when the molten metal is in contact with a reactive atmosphere. Dross normally consists of metal oxides and nitrides and a considerable quantity of molten free (unreacted) metal. For economic reasons, it is normally desirable to extract the free metal for re-use, but afterwards the remaining dross (dross residue) has in the past been dumped in land fill sites since it has no economic value. The dumping of dross in this manner has recently come to be regarded as environmentally unacceptable because harmful chemicals may leach out of the dross into ground and surface water and thus cause widespread pollution.

Part of the problem caused by dross results from the presence of soluble salts in the dross as a consequence of the traditional process for removing the free metal. This process involves heating and tumbling the dross in the presence of a molten salt bath in order to remelt the metallic fraction and to cause the resulting small molten metal droplets to coalesce and form an easily separable molten metal pool. While the process is quite efficient in extracting the metal, the residual salt cake forms a large proportion of the dross residue.

We have previously devised a process for extracting the free metal component from dross without the use of molten salt baths. This process is disclosed in our Canadian patent number 1,255,914 issued on June 20, 1989 (the disclosure of which is incorporated herein by reference) and involves the treatment of dross in a fur- nace heated by means of a plasma torch. This heating pro¬ cedure can be carried out on the dross without any prior treatments and results in the coalescence of molten metal droplets in the dross in the absence of molten salts. While our improved process of treating dross results in an efficient extraction of metallic aluminum and avoids the environmental problems caused by the salt content of dross treated in the traditional way, the resulting "plasma dross" residue still contains appreciable amounts of aluminum nitride and may therefore still be unsuitable for disposal in land-fill sites since this chemical may also be a pollutant.

There is therefore a need for a process of dealing with aluminum nitride-containing dross which does not result in. environmental problems and yet can be operated economically.

Proposals have been made in the past to convert drosses of various kinds into useful products,, such as refractories, with the intention not only of avoiding disposal problems but also of bringing in an economic return, but these proposals have not been put into wide- scale use because various problems have been encountered. US patent 4,252,776 to Huckabay et. al. discloses a process in which dross residues from the salt process are washed to remove water-soluble salts and are then reacted with water at elevated temperatures. The reaction is performed in the liquid phase and under sufficient agitation to ensure particle-to-particle attrition to break the protective aluminum hydroxide film which forms about the aluminum metal contaminant particles, thereby achieving substantially complete oxidation of the aluminum metal contaminant to aluminum oxide trihydrate. Thus the reaction is aimed at dross residues which still contain a relatively large amount of metallic aluminum and the reaction is concerned primarily with the reaction of this metallic component.

An object of the present invention is thus to provide a process for treating aluminum nitride-containing dross which contains only a small or no metallic component in order to convert the dross into commercially desirable products and/or to avoid environmental pollution.

SUBSTITUTE SHEET DISCLOSURE OF INVENTION

According to one aspect of the present invention, there is provided a process for converting aluminum nitride-containing dross residue into ammonia-containing and alumina-containing products, which comprises partially digesting said aluminum nitride-containing dross residue having a content of AIN of at least 5 wt % and no more than about 10 wt % of metallic aluminum with a solution having a pH of 7 or more to solubilize about 10-50 wt % of said residue and to produce an ammonia-containing reaction product, a residual solution and a solid residue; and separating said ammonia-containing reaction product from said solid residue and residual solution.

The ammonia-containing reaction product and the solid residue can be collected and optionally further treated or reacted to produce commercially valuable end products.

The invention thus provides a way of converting aluminum nitride-containing dross residues, and especially plasma dross residues, into useful products, thus avoiding the need for disposal and producing an economic return. BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawing is a block diagram showing the steps of a basic form of one aspect of the process of the invention and also two optional additional treatment processes.

BEST MODES FOR CARRYING OUT THE INVENTION

The basis of the present invention is the partial digestion of the aluminum nitride-containing dross residue with a solution having a pH of 7 or more to produce at least an ammonia-containing product and a solid residue. For effective operation of the invention, the dross residue must contain at least 5 wt % of aluminum nitride and no more than 10 wt % of metallic aluminum. Drosses with larger amounts of metallic aluminum generate large quantities of unwanted hydrogen in violent reactions when digested and drosses with smaller amounts of aluminum nitride generate insufficient quantities of the desired ammonia. More preferably, the dross residue should contain at least about 10 wt % AIN and most preferably at least about 25 wt % AIN and preferably only 2 wt% of Al or less. Dross treated by the plasma process indicated above has suitable contents of aluminum nitride and metallic aluminum and is the preferred type of dross residue and has the advantage (in terms of cost, convenience and wear and tear on machinery) that it can usually be used in the process of the invention without intermediate treatment steps, such as grinding to reduce the particle size; however, dross residues from other sources having the required composition may be employed, if desired. Dross which has been cooled rapidly to give particles having a size of 100% -20 Tyler mesh is particularly preferred. Dross which has been cooled slowly may produce sintered lumps which are not very suitable.

The digestion should be carried out in such conditions that about 10 to 50 wt % of the residue is solubilized.

During the digestion step, the aluminum nitride in the dross is converted to ammonia (or an ammonium- containing product) and either a soluble product (if the pH is greater than 7) or an aluminum hydroxide precipitate (if a neutral solution is used) . This is illustrated by the reaction scheme below, which shows the effect of digestion in alkaline solution:

AIN OH^" followed by:

and

The above reactions take place quite quickly in relatively mild conditions e.g. at atmospheric pressure in mildly alkaline or neutral solutions and at temperature below 100^βC although, as described below, harsher conditions may be used, if desired. In neutral solutions, the first reaction may be the only one to take place.

This digestion step accomplishes two things; firstly

SUBSTITUTE SHEET ammonia is produced and may be collected and liquefied or reacted with an acid to produce a soluble or insoluble salt. Ammonia and ammonium salts are valuable products and can be used as fertilizers and in industry. Secondly, either a useful aluminum trihydrate precipitate is formed (digestion in neutral solutions) or an aluminate solution is obtained (digestion in alkaline solutions) which can be used for the purposes outlined below.

Other main constituents of dross residues, i.e. oxides of aluminum and other metals (e.g. Mg present in drosses from Al/Mg alloys) are generally not digested in mild conditions and remain as a solid residue. However, this solid residue can be used for commercial purposes as will be indicated later. A preferred additional aspect of the invention is based on the surprising discovery that digestion of dross residue in caustic alkali under harsh conditions results in the dissolution of a large component of the dross previously thought to be insoluble. In such conditions, it appears that a proportion of the aluminum and other metal oxides in the dross dissolve to form soluble aluminates. Furthermore, aluminum metal, finely dispersed throughout the dross residue, hydrolyses in the following manner: Al + 3H₂0 -^*• A1(0H)₃ + ³/_≥H₂i and any aluminum carbide present yields methane:

A1₄C₃ + 12H₂0 ^*• 4A1(0H)₃ + 3CH^t

It is also found that dross residues containing magnesium spinel are partially digested to a more nearly stoichiometric form yielding additional hydrate:

(MgO)₁._χ(Al₂0₃)_1+χ + 6xH₂0 → (1-X) [MgO.Al₂0₃] + 4x[Al(OH)₃]

The result of these reactions is that a considerable proportion of the dross is digested and the remaining insoluble fraction normally consists of more nearly stoichiometric magnesium spinel and insoluble α-alumina,

ET although the exact nature of the insoluble fraction depends on the composition of the dross residue itself.

The digestion under caustic conditions can be carried out separately from the AIN hydrolysis step mentioned above but the two steps are usually carried out simultaneously since the AIN hydrolysis takes place under the harsh and caustic conditions as well as under the mild conditions.

The conditions required for the digestion of otherwise insoluble fractions of the dross residue are usually as follows. The dross residue is normally heated in an autoclave or similar pressure reactor to a temperature up to about 325'C, preferably 50 - 300°C and usually 100 - 225°C, in the presence of a solution containing a caustic alkali (preferable NaOH for economy, although KOH or LiOH could also be used) . A particularly preferred solution is one containing 50 - 200 g/1 of NaOH (expressed as Na₂C0₃) . The solutions are normally aqueous but could conceivably be organic (e.g. in alcohol or acetone) . Autogenous pressure is preferably generated since the reactor is closed and this pressure varies according to the reaction temperature employed. Generally, the autoclave is provided with a safety valve set at a pressure slightly higher than the vapour pressure of water at the intended digestion temperature. For example, when the digestion is carried out at 170^βC, the vapour pressure of water is 114 psi and the safety valve is set at about 150 psi. If the digestion is at 200'C, the vapour pressure is 225 psi and the valve is set at 275 psi. Other suitable pressures are apparent from vapour pressure tables. If desired, the reaction mixture may be subjected to agitation to accelerate the solubilization.

The time required for the digestion step varies according to the size of the charge of the dross residue and its composition, but the digestion is normally complete within a few hours. The optimum ratio of caustic solution to dross residue charge also varies, but in general it can be said that the greater the amount of the

S caustic solution, the greater the proportion of the dross residue that dissolves. Higher temperatures and more concentrated caustic solutions also generally result in a larger fraction of the dross residue dissolving. In a basic form of the invention, after the digestion reaction is complete, the aluminate solution is normally separated from the insoluble fraction e.g. by filtration with a filter press or the like or by means of any other solid/liquid separator. The aluminate solution and the insoluble fraction (together with any ammonia or ammonium reaction product that may have been collected) are then used, sold, converted to other products or (in the case of the insoluble fraction) even disposed of by dumping since the product is non-polluting. In the case of the caustic aluminate solution, several treatments are possible. Firstly, the aluminate solution may be used in the conventional Bayer process as "Bayer liquor" for the production of alumina.

Secondly, so-called aluminum trihydrate (aluminum trihydroxide) can be precipitated either by cooling the caustic solution, by diluting the caustic solution with water or by a combination of the two. Seed crystals of Al(OH)₃ are generally added to the liquid to promote the precipitation of the solid. The trihydrate produced in this way is very white and is thus a valuable product. It appears that traces of iron and organics responsible for red and yellow hydrate products are either not present in the original dross residue (although this depends on its origin) or, if present, are not solubilized in the caustic digestion process. Furthermore, the aluminate solutions produced by the invention generally do not contain ions, such as ions of Ca, Fe, P and Mg, that slow down the precipitation of the hydrate, nor "poisons" such as sugars, gum arabic and other organics that often contaminate other hydrate products. Preferably, the solution containing the aluminate should contain less than about 0.1 wt % of dissolved organic carbon and substantially no suspended colloidal iron in order to produce a very white trihydrate product. The solution remaining after the precipitation of the trihydrate can be recycled to the caustic digestion step, if desired.

Thirdly, the aluminate in the solution resulting from the caustic digestion step can be converted to other useful chemicals, such as aluminum silicate by the addition of sodium silicate or to zeolites by the addition of Si0₂, etc.

Fourthly, the aluminate can be obtained as a solid product simply by evaporating the water from the solution. Other possible uses of the aluminate solution no doubt exist.

The insoluble fraction remaining after the digestion is usually washed to remove traces of alkali and other soluble compounds and is then dried. If the solid contains hydroxide, it may be desirable to calcine the product to produce the corresponding oxide. The product can then be used as such as a refractory precursor or can be treated (e.g. by melting) to form a refractory product such as a rock wool. If desired, the alumina component of the solid product can be physically separated from any spinel component (present when the dross residue contains MgO) , e.g. by making use of their considerably different densities (alumina = 2.3; spinel = 3.6). If the spinel component remains non-stoichiometrical, MgO or Al₂0₃ can be added to make the proportions correspond with the stoichiometrical amounts.

The compositions of dross residues vary widely even when the residues come from the same type of dross treatment process, and so it is difficult to give indications of the proportions of the solid that can be solubilized. However, it is not unusual to be able to dissolve about 27% by weight of plasma dross residue by the mild hydrolysis and about 48% by weight in total following the caustic digestion step.

The process of the invention can be operated continuously or batchwise. If preliminary hydrolysation is employed, this step is quite easily made continuous.

UBSTITUTE SHEET The caustic digestion step can be made continuous by the use of continuous autoclaves.

The accompanying drawing represents typical applications of the present invention as will be explained 5 in the following.

Plasma dross residue 10 and caustic sodium hydroxide solution 11 are introduced into a digester 12 to bring about partial digestion of the residue 10. During this step, ammonia 13 is evolved and removed from the digester 10 12 along with traces of hydrogen and methane generally formed as by-products. The contents of the digester 12, i.e. and aluminate solution and a solid residue, are passed to a solid/liquid separator 14 where the solid 15 is separated from the liquid 16. The resulting solid 15 product 15 generally contains alumina, magnesium oxide and magnesium spinel. The liquid product 16 contains sodium hydroxide and aluminate.

If desired, the aluminate solution can be converted to solid A1(0H)₃ or to a zeolite. To form solid Al(OH)₃, 0 the liquid is transferred to a digester 17 where seed crystals 18 of Al(OH)₃ are added and the liquid is cooled. The resulting suspension is then transferred to a solid/liquid separator 19 for removal of the A1(0H)₃ solid 20. The remaining solution 21 of sodium hydroxide and 5 unprecipitated aluminate is then recycled to the feed to digester 12.

To produce a zeolite, the liquid product 16 is transferred to a digester 22 to which solid Si0₂23 is also added, preferably a source of soluble silica such as 0 silica fumes or kaolinite. A solid zeolite precipitate is formed and the resulting suspension is transferred to a solid/liquid separator 24. A solid zeolite product 25 is obtained and the remaining liquid 26 can be recirculated to the digester 12 for use in the initial digestion. 5 The invention is described in more detail with reference to the Examples given below. EXAMPLE 1

Plasma dross residues were submitted to tests for the determination of available and extractable alumina (available alumina means the portion of dross which can be solubilized under mild alkaline conditions and extractable. alumina means the total amount of dross that can be solubilized under both mild conditions and caustic digestion at high temperature and pressure) . The tests involved placing the dross in a 70 ml closed bomb with a concentrated caustic alkali solution and heating the contents under pressure. The results are shown in Table 1 below.

Table 1 Available and Extractable Alumina

These results show that a considerable proportion of the dross is solubilized.

EXAMPLE 2

Similar tests were carried out on plasma dross residues and it was found that a large proportion of the dross solubilizes. The proportion not solubilized was analyzed by x-ray diffraction and magnesium spinel and α alumina were the major components. The influence of the process parameters an the conversion rate of the plasma dross residues were evaluated and the results are presented in Table 2 below. TABLE 2

Influence of Process Parameters on Conversion Rate of Plasma Dross Residue

The results show that the effects of the temperature on the conversion rate is more important than the caustic concentration and the residence time. As aluminum nitride is totally converted into alumina hydrate below 150"C using a 100 g/1 in caustic, it appears that another component of the initial dross contributes to increasing the amount of alumina in solution.

INDUSTRIAL APPLICABILITY

The invention can be applied to the utilization of dross residues which are currently discarded as waste in order to avoid environmental pollution and to create an economic return and valuable products.

SUBSTITUTE SHEET

Claims

CLAIMS:

1. A process for converting aluminum nitride-containing dross residue into ammonia-containing and alumina- containing products, characterized in that said process comprises: partially digesting said aluminum nitride-containing dross residue having a content of AIN of at least 5 wt % and no more than about 10 wt % of metallic aluminum with a solution having a pH of 7 or more to solubilize about 10-50 wt % of said residue and to produce an ammonia-containing reaction product, a residual solution and a solid residue; and separating said ammonia-containing reaction product from said solid residue and residual solution.

2. A process according to claim 1 characterized in that said dross residue is a plasma dross residue.

3. A process according to claim 2 characterized in that said plasma dross residue is used in a form resulting from a plasma dross treatment process without an intermediate grinding step.^""

4. A process according to claim 1, claim 2 or claim 3, characterized in that said partial digestion is carried out at a temperature of less than about 100"C at about atmospheric pressure.

5. A process according to claim 1, claim 2 or claim 3, characterized in that said partial digestion step is carried out at a temperature of about 100-300°C in an autoclave.

6.- A process according to claim 1, claim 2 or claim 3, characterized in that said dross residue contains at least about 10 wt % of AIN.

7. A process according to claim 1, claim 2 or claim 3, characterized in that said dross residue contains at least about 25 wt % of AIN.

8. A process according to claim 1, claim 2 or claim 3, characterized in that said solution has a pH of more than 7 and said residual solution contains aluminate, and characterized in that said aluminate of said residual solution, following separation from said solid residue, is reacted with a reagent to convert said aluminate to a different product.

9. A process according to claim 8 characterized in that said residual solution contains less than about 0.1 wt % of dissolved organic carbon and substantially no suspended colloidal iron.

10. A process according to claim 8 characterized in that said aluminate is reacted to produce white aluminum trihydrate as said different product.

11. A process according to claim 8 characterized in that said aluminate is reacted to produce zeolite as said different product.

12. A process according to claim 1, claim 2 or claim 3, characterized in that said dross residue contains A1₂0₃ and

MgO and wherein said partial digestion is carried out in a solution containing a caustic alkali at a temperature and under a pressure suitable to partially dissolve said A1₂0₃ so that said solid residue contains a mixture of alumina spinel and α-alumina.

13. A process according to Claim 1, claim 2 or claim 3, characterized in that ammonia is generated during said partial digestion as said ammonium-containing compound and said ammonia is collected.

14. A process according to Claim 13 characterized in that said solution has a pH of more than 7 and a temperature below about 100^βC during said contacting step.

15. A process according to claim 1, claim 2 or claim 3, characterized in that said solid residue is removed from said residual solution, dried and calcined.

16. A process for producing ammonia from dross residue containing at least 5 wt % of aluminum nitride and no more than 10 wt % of metallic aluminum, characterized in that said process comprises contacting said residue with a solution having a pH of 7 or more and collecting the ammonia thereby generated.

17. A process according to Claim 16 characterized in that said ammonia is reacted with an acid to produce an ammonium salt.

18. A process for producing an alkali metal aluminate solution, characterized in that said process comprises at least partially digesting aluminum dross residue containing at least 5 wt % of aluminum nitride and no more than 10 wt % of metallic aluminum in a solution containing an alkali metal hydroxide at a temperature and under a pressure suitable to produce said at least partial digestion.

19. A process for producing a mixture of spinel and α- alumina from aluminum dross residue containing MgO, characterized in that said process comprises partially digesting said aluminum dross residue in a solution containing a caustic alkali at a temperature and under a pressure suitable to produce said at least partial digestion, and separating an undissolved portion comprising a mixture of spinel and α-alumina from said solution.

20. An ammonia-and/or alumina-containing product characterized in that said product is obtained by treating a dross residue having an aluminum nitride content of at least 5 wt % and a metallic aluminum content of less than 10 wt % with a solution having a pH of 7 or more and extracting said product from the resulting mixture.

21. A product according to claim 20 characterized in that said product is ammonia or an ammonium compound.

22. A product according to claim 20 characterized in that said product comprises magnesium spinel.

23. A product according to claim 20 characterized in that said product comprises aluminum hydroxide.

24. A product according to claim 20 characterized in that said product comprises a zeolite.

SUBSTITUTE SHEET