WO1992000246A1

WO1992000246A1 - Process for producing particles of magnesium spinel from waste products and the particles so-produced

Info

Publication number: WO1992000246A1
Application number: PCT/CA1991/000220
Authority: WO
Inventors: Sadashiv Kashinath Nadkarni; Luc Parent
Original assignee: Alcan International Limited
Priority date: 1990-06-28
Filing date: 1991-06-18
Publication date: 1992-01-09
Also published as: AU7976191A

Abstract

A process for producing particles of magnesium spinel (magnesium aluminate) suitable for reinforcing metal matrix composites. The process involves mixing a waste material containing alumina or a precursor, such as dross residue, ESP dust or offgrade or contaminated alumina from Bayer process plants or molten salt electrolysis factories, with magnesium oxide or a precursor to produce a mixture, calcining the mixture at a temperature suitable to produce magnesium spinel (usually between 1450 and 1650 °C), and collecting particles of the magnesium spinel having a size in the range of 5-50 νm, if necessary after comminuting larger particles to sizes in the desired range. Preferably, particle growth promoters may be added to the starting materials, examples being metal oxides and fluorides generally used in amounts of about 1-2 % by weight. The resulting inexpensive particles have a size suitable for metal matrix reinforcement and are substantially unreactive with molten aluminum and aluminum alloys.

Description

Process for producing particles of magnesium spinel from waste products and the particles so-produced

TECHNICAL FIELD

This invention relates to a process for producing particles of magnesium spinel and to the particles so- produced. BACKGROUND ART

Metal matrix composite materials are of great interest nowadays because they combine relative lightness of weight with high strength and can therefore be used for new applications in many industries. The composites comprise a metal matrix normally reinforced with ceramic material in the form of generally spherical particles, platelets or whiskers. Silicon carbide is a material that is often suggested as a suitable reinforcement, but it has the disadvantage that it reacts with molten aluminum or aluminum-containing alloys, which results in a weakening of the reinforcing effect for aluminum-based composites. Magnesium spinel (magnesium aluminate, MgAl₂0_A) is a material that is resistant to attack by molten aluminum and its alloys but it is currently manufactured by fusing commercial grade alumina and magnesia together. This makes it quite expensive and therefore unsuitable for use on a commercial scale as a reinforcing material for metal matrix composites.

While inexpensive alumina-containing materials are well known, such as clay and bauxite, they cannot be used for the formation of magnesium spinel without extensive extraction procedures because they contain too many impurities.

An object of the present invention is therefore to provide an economical process for producing particles of magnesium aluminate suitable for use as a reinforcement for metal matrix composites. Another object of the invention is to provide a process for producing particles of magnesium spinel having sizes in the range of 5-50μm.

Yet another object of the invention is to provide a process for producing magnesium spinel particles suitable for metal matrix reinforcement. DISCLOSURE OF THE INVENTION

According to one aspect of the invention, there is provided a process for producing particles comprising magnesium spinel suitable for reinforcing metal matrix composites, which process comprises: mixing particles of a waste material containing alumina or an alumina precursor in substantial amounts with particles of magnesium oxide or a magnesium oxide precursor to produce a mixture, calcining the mixture at a temperature suitable to form magnesium spinel, optionally comminuting particles of the spinel having sizes larger than 50μm to particles of smaller size, and collecting particles of said spinel having sizes in the range of 5-50μm.

The invention also relates to the magnesium spinel particles so-produced. By the term "waste material" as used in the present invention, we mean any material that is generally discarded rather than being sold for commercial use, or which is rejected for use in most commercial processes. Such materials are generally by-products of commercial processes or sub-standard products of such processes. The important requirements of such materials are that they provide an inexpensive source of alumina, contain a large proportion of alumina or alumina precursor available for the formation of magnesium spinel and contain only impurities which do not adversely affect the process of the present invention. Waste products from the aluminum industry containing at least 30% by weight of A1₂0₃ or a precursor thereof and containing less than about 0.2% by weight of other impurities that are not volatile under the reaction conditions of the process of the invention are especially preferred. BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 of the accompanying drawings is a scanning electron micrograph of a product produced according to the process of the present invention (see Example 2) . BEST MODES FOR CARRYING OUT THE INVENTION

It has now been found that certain low cost waste products of high alumina content can be used as one of the starting materials for the formation of magnesium spinel so that the cost of production of the spinel can be reduced. These materials are generally waste alumina- containing products from the aluminum industry. Most advantageously, some of the waste products contain an amount of MgO, or precursor thereof, so that less of this other starting material of the process has to be added prior to the calcination step.

A particularly preferred waste product for use as a source of alumina in the present invention is aluminum dross residue. 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, such as air. Dross normally consists of aluminum oxide and aluminum nitride, as well as oxides and nitrides of other metals, e.g. magnesium, which may have been present in the metal. Dross from magnesium-containing aluminum alloys may therefore already contain a part of the magnesium oxide required for the process of the invention, or even an amount of the desired final product, magnesium spinel. Dross normally also contains a certain amount of free metal, most of which is removed for re-use to leave a dross residue, which has in the past been viewed as a waste product.

Dross residues differ not only according to the composition of the metal on which the dross was formed but also according to the process employed for the removal of the free metal. A traditional 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 for extracting the metal, the residual salt cake forms a large proportion of the dross residue. When dross residue produced in this manner is to be used in the present invention, it should be treated to remove the salt fraction prior to use in the process. This can be done for example by washing the dross residue with water to dissolve the salt, separating the undissolved solid from the salt solution and then drying the remaining dross residue.

We have previously devised an alternative method for extracting the free metal component from dross without the use of a molten salt bath. 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 furnace heated by means of a plasma torch. This procedure can be carried out on the dross without any prior treatments and results in the coalescence of molten metal droplets without the need for molten salts. The resulting dross residue (so-called plasma dross residue) typically contains about 30% by weight of A1N, 30% by weight of A1₂0₃ and 30% by weight of MgAl₂0₄. If desired, the large proportion of A1N can be converted to aluminum oxide (alumina) prior to use in the present invention by contacting the dross residue with water or steam at a temperature of up to about 300^βC. This converts the nitride to the hydroxide and ammonia and subsequently leads to the dehydration of the hydroxide to form the oxide. However, this prior conversion of AlN to A1₂0₃ is not generally necessary prior to the process of the present invention because it has been found that the nitride reacts directly during the calcination step with available oxygen to produce the desired magnesium spinel and thus acts as an alumina precursor. The use of the dross without conversion of the nitride component is, in fact, preferred because the nitride reacts exothermically, and thus reduces the heat input required for the reaction step. Dross residues resulting from yet other metal removal procedures can also be used in the present invention, e.g. dross residue resulting from the electric arc treatment of dross or the procedure described by Montagna in U.S. Patent No. 3,999,980 issued on December 28, 1976 (the disclosure of which is incorporated herein by reference) . Even dross which has not been treated to remove most of the free metal content may be used as a starting material for the present invention provided the metal content is not too high. If the aluminum content in the dross is low, it is possible to grind the dross to the desired particle size. However, if the aluminum content is high, the dross is usually in the form of an agglomerate of large pieces and it is not easy to grind to a smaller particle size, so the excess aluminum must be removed, prior to use, to form a dross residue.

Other preferred starting materials for use in the present invention include electrostatic precipitator (ESP) dust and offgrade or contaminated alumina which is unsuitable for use in other manufacturing processes, e.g. the Bayer process for producing alumina or molten salt electrolysis for the product of metallic aluminum. ESP dust is very fine undercalcined alumina separated from the exhaust gases of alumina calcinerε. Typically more than 90% by weight of the material is less than 44 microns in size. It consists of a mixture of calcined, partially calcined and uncalcined particles, consequently its loss on ignition can vary between 1 and 35% by weight. It is further characterized by containing small amounts of oxides of other metals, such as sodium oxide (typically 0.3 to 0.7% by weight or higher). The material has very limited commercial or industrial value, and is normally disposed of as waste material or transformed by physical or chemical treatments into more useful products.

The magnesium oxide used as the other starting material in the process of the invention may itself be a waste product, but is normally a commercial product available in fine powder form. If desired, precursors such as magnesium carbonate may be employed instead. The magnesium spinel particles produced by the process of the invention will only be suitable for use as metal matrix reinforcements if the particles have sizes in the range of 5-50μm, more preferably 10-30μm, and most preferably 10-15μm. In those cases where the reaction does not produce particles in this size range without intervention, steps must be taken to modify the process. This can be done in a number of ways as outlined in the following.

Firstly, if the starting materials themselves have a particle size significantly larger than 20μ, the starting materials are preferably first comminuted to a size of about 20μ or smaller. This can be achieved by crushing or grinding the starting materials, e.g. in a ball mill or the like. This preliminary size reduction step not only helps to ensure that the product of the process will be of the desired particle size, but also makes it possible to thoroughly mix the starting materials together as a dry powder mixture prior to calcination so that the starting materials come into intimate contact suitable for complete reaction. Moreover, mixtures of fine powders take less time to react during the calcination step and thus make the process more economic. When dross residue is used as a source of alumina, it is usually in the form of large particles which are preferably ground to a size of -325 Tyler mesh prior to use. ESP dust is already of submicron size and commercial MgO is also generally already of -325 Tyler mesh size. Typically, the starting materials of the desired small particle size are mixed in a V-blender or other mixing apparatus for a period of at least one hour to achieve the desired intimate mixture. Secondly, if the particles produced by the process are larger than the stated range, e.g. if the product is in the form of large fused lumps, the product can be comminuted and classified to obtain a suitable product. On the other hand, if the product includes particles having a range of sizes, including sizes in the desired range, then screening to obtain the desired fraction can be carried out with subsequent milling and screening of any larger size fractions. Although particles larger than the desired range can be reduced in size by comminution as indicated above, particles smaller than the desired range generally have to be discarded. Unfortunately, in many cases, the particles produced without intervention do tend to be too small, often about 2-5μm, or contain a substantial fraction within this size range. This is usually the case when lower calcination temperatures and shorter calcination times are employed. In such cases, materials which act as particle size modifiers can be added to the reactants to promote the formation of larger particles. Suitable particle size modifiers include oxides of other metals, e.g. Si0₂, B₂0₃, etc. or mineralizers (grain growth additives) such as fluorides (e.g. MgF₂, CaF₂, Na₃AlF₆, NaF, A1F₃, Na₂SiF₆ and H₂SiF₆) , in quantities sufficient to promote crystal growth.

The particle size modifiers are generally used in amounts in the range of 1-2% by weight of the reaction mixture and are generally added to the starting materials in fine powder form (preferably less than 20μm) prior to the thorough mixing of the starting materials.

The reason why such materials promote the growth of larger particles in the process of the invention is not precisely known, but possibly relates to the formation of a liquid phase during the reaction causing observable glassy phases to form at the boundaries of the particles in the final product. The existence of a liquid phase may permit the rapid mass transfer of the material necessary to make particles of the desired large size.

The calcination step of the process of the invention usually, but not necessarily, requires a temperature of at least 1450^βC, a reaction time in the range of 1 to 3 hours 5 and is normally carried out under an atmosphere of air, but other non-reactive atmospheres (such as oxygen or argon) could be employed, if desired. Larger particles tend to be formed at higher calcination temperatures, and temperatures in the range of 1650°C + 50"C often produce 0 the most desirable particles.

The starting materials are generally used in relative quantities which provide approximately the stoichiometrical amounts of alumina and MgO required for the production of alumina spinel during the calcination 5 step, although this is not absolutely essential since some contamination of the product with the starting materials can be tolerated. Naturally, the relative amounts of the starting materials vary with the alumina content of the waste material used as the alumina source and the 0 magnesium content (if any) of the alumina-containing waste material. In the case of ESP dust, the ratio of the dust to MgO is generally about 2.5:1.0 by weight. In the case of dross, a ratio of dross residue to MgO of 3.5:1.0 by weight has proven satisfactory, although this ratio would

25 be increased for dross residues of high MgO content.

The invention is illustrated in more detail below with reference to the following Examples. EXAMPLE 1

Plasma dross residue, MgO and B₂0₃ in the ratio of

30 100:29:2 by weight were mixed together for one hour in a V-blender and then calcined in air at 1650"C for 2 hours.

X-ray diffraction analysis of the resulting particles showed that the product was essentially composed of MgAl₂0₄, i.e. magnesium spinel.

35 Scanning electron microscopy showed that most of the particles were between 5 and 30 μm in size.

Similar results were also obtained when Si0₂ was used in place of the B₂0₃ and the same calcination conditions were employed. EXAMPLE 2

Four different compositions were prepared, each containing 100 g of plasma dross residue and 29 g of MgO plus a different particle growth promoter, namely the following:

COMPOSITION GROWTH PROMOTER AMOUNT(g)

1 Na₃AlF₆ 2 2 B₂0₃ 2

3 Siθ₂ 2

4 cryolite 1

+ CaF₂ 1

The mixtures were heated in air at 1450°C for two hours for a first series of tests and identical mixtures were then heated in air at 1650"C for the same period of time for a second series of tests.

Particles of suitable size were formed in all cases and larger particles were obtained at the higher temperature. The best growth agents were, in increasing order of effectiveness, B₂0₃, Si0₂, Na₃AlF₆ and the mixture of CaF₂ with Na₃AlF₆.

Accompanying Figure 1 is a scanning electron micrograph of the product of the treatment at 1650°C containing B₂0₃. Most of the particles are between 5 and

20 μ in size, which is an effective size for the reinforcement of aluminum and aluminum alloys.

COMPARATIVE EXAMPLE 1

Dross residue resulting from plasma treatment of aluminum dross and MgO were mixed together in the ratio of

100:31 by weight and calcined in air at a temperature of

1300°C for 1-s hours. The product was analyzed by X-ray diffraction which showed that it was essentially MgAl₂0₄.

The A1N present in the dross residue had been oxidized completely. Scanning electron microscopy showed the product to be in the form of particles having a size of 2-

5 microns.

These particles were too small to be useful as reinforcements of aluminum matrices. The formation of such particles shows that low calcination temperatures tend to produce smaller particles in the absence of particle growth promoters. INDUSTRIAL APPLICABILITY

The magnesium spinel particles of size 5-50 μ produced according to the present invention can be used as reinforcements for metal matrix composites, particularly when the metal phase is aluminum or an aluminum alloy.

Claims

WHAT WE CLAIM IS:

1. A process for producing particles comprising magnesium spinel suitable for reinforcing metal matrix composites, wherein alumina or an alumina precursor is mixed with particles of magnesium oxide or a magnesium oxide precursor to produce a mixture, the mixture is calcined at a temperature suitable to form magnesium spinel, and particles of said spinel having sizes in the range of 5-50μm are collected, if necessary after comminuting particles of said spinel larger than 50μm to particles within the range of 5-50μm, characterized in that a waste material is used as a source of said alumina or alumina precursor.

2. A process according to Claim 1 characterized in that said waste material contains substantial amounts of said alumina or alumina precursor.

3. A process according to Claim 1 characterized in that said waste material contains at least 30% by weight of said alumina or said precursor.

4. A process according to Claim 1 characterized in that said waste material contains less than 0.2% by weight of impurities that are not volatile under the reaction conditions.

5. A process according to Claim 1 characterized in that said waste material is an aluminum dross residue.

6. A process according to Claim 5 characterized in that said aluminum dross residue is substantially aluminum metal-free and salt-free.

7. A process according to Claim 5 or Claim 6 characterized in that said dross residue contains magnesium oxide.

8. A process according to Claim 5 or Claim 6 characterized in that said dross residue is a residue resulting from the treatment of dross by a plasma process to remove free metal therefrom.

9. A process according to Claim 1, Claim 2, Claim 3 or Claim 4 characterized in that said waste material is ESP dust.

10. A process according to Claim 1, Claim 2, Claim 3 or Claim 4 characterized in that said waste material is offgrade or contaminated alumina derived from an operation selected from a Bayer process for the production of alumina and molten salt electrolysis for the production of aluminum.

11. A process according to Claim 1, Claim 2, Claim 3, Claim 4, Claim 5 or Claim 6, characterized in that particles of size 10-3Oμm are collected.

12. A process according to Claim 1, Claim 2, Claim 3, Claim 4, Claim 5 or Claim 6, characterized in that particles of size 10-2Oμm are collected.

13. A process according to Claim 1, Claim 2, Claim 3, Claim 4, Claim 5 or Claim 6, characterized in that a particle size modifier is incorporated into said mixture to ensure that said calcination produces particles having a size of at least 5μm.

14. A process according to Claim 13 wherein said particle size modifier is an oxide of a metal other than Mg and Al.

15. A process according to Claim 14 characterized in that said oxide of a metal other than Mg and Al is Si0₂ or B₂0₃.

16. A process according to Claim 15 characterized in that said oxide is used in an amount of about 1-2% by wt. of said mixture.

17. A process according to Claim 13 characterized in that said particle size modifier is a mineralizer.

18. A process according to Claim 17 characterized in that said mineralizer is a fluoride.

19. A process according to Claim 18 characterized in that said fluoride is MgF₂, CaF₂, Na₃AlF₆, NaF, A1F₃, Na₂SiF₆ or H₂SiF₆.

20. A process according to Claim 19 wherein said fluoride is used in an amount of about 1 to 2% by wt of said mixture. 5

21. A process according to Claim 1, Claim 2, Claim 3, Claim 4, Claim 5 or Claim 6, characterized in that said calcination is carried out at a temperature of at least about 1450°C.

22. A process according to Claim 1, Claim 2, Claim 3, 10 Claim 4, Claim 5 or Claim 6, characterized in that said calcination is carried out at a temperature of 1650^βC ± 50^βC.

23. A process according to Claim 1, Claim 2, Claim 3, Claim 4, Claim 5 or Claim 6, characterized in that said

15 calcination is carried out under an atmosphere selected from the group consisting of air, oxygen and argon.

24. A process according to Claim 1, Claim 2, Claim 3, Claim 4, Claim 5 or Claim 6, characterized in that said particles of said waste material and magnesium oxide or

20 precursor have a size of less than 2Oμm.

25. Particles of magnesium spinel having a size in the range of about 5-50μm, characterized in that said particles are produced by a process according to Claim 1, Claim 2, Claim 3, Claim 4, Claim 5 or Claim 6.

25 26. A metal matrix composite comprising a metal matrix and a reinforcement characterized in that said reinforcement comprises particles according to Claim 25.

27. A composite according to Claim 26 characterized in that said metal is an aluminum-containing metal.