Background Art Small particles of alumina or alumina containing material have poor flow characteristics, creating difficulties in handling. In the Bayer process for the extraction of. alumina from alumina containing ores, precipitated alumina tri- hydrate is filtered, dried and calcined. This yields high purity alumina with a narrow range of particle sizes. However, a by-product of the calcination process comprises extremely fine particles of alumina, with an average size of less than 30 pm. This by-product is caught in the gas cleaning devices including multi- cyclones, electrostatic precipitators or bag-houses attached to the calcination device. Such particles are commonly known as ESP dust. In addition to being difficult to handle, ESP dust is difficult to reintroduce into the process stream, not being readily redigested in the highly caustic solution of the digestion phase.

Accordingly, it is desirable to be able to agglomerate small particles of alumina, such as ESP dust, to form coarser particles. Ideally, these coarser particles are of a size range suitable for use in aluminium smelting.

In the ceramics industry, particles are agglomerated by spray drying using organic polymers as binders. However, the agglomerates formed by these methods are typically weakly bonded and they are readily degraded when handled or transported.

In Australian Patent 664328, there is provided a method for agglomerating alumina particles with a binder comprising a polymer form of a hydroxy salt of aluminium. In a variant of the invention, activated alumina is used to reduce the quantities of the binding agent required.

Where the polyhydroxy aluminium salt is used alone, concentrations of at least 10% of such are required to prevent collapse of the agglomerated granules on handling. Where the polyhydroxy aluminium salt is used in conjunction with activated alumina, binder concentrations of below 10% are possible. Indeed, by the addition of activated alumina, binder levels as low as 2. 5% are able to produce sufficiently robust agglomerates.

However, the cost of the binding agent and that associated with the production of activated alumina have prevented the widespread application of this method. It is one object of the present invention to provide a method for the agglomeration of particles of alumina, or particles substantially comprising alumina, where the quantity of binder required is substantially reduced, and the need for activated alumina eliminated.

The preceding discussion of the background to the invention is intended to facilitate an understanding of the present invention. However, it should be appreciated that the discussion is not an acknowledgement or admission that any of the material referred to was part of the common general knowledge in Australia as at the priority date of the application.

Throughout the specification, unless the context requires otherwise, the word "alumina"will be understood to encompass fully dehydrated alumina, fully hydrated alumina, partially hydrated alumina or a mixture of these forms.

Throughout the specification, unless the context requires otherwise, the term "alumina particles"will be understood to include particles of an alumina containing material where the alumina content of said particles is at least about 30% by weight Ai203.

Further, throughout the specification, unless the context requires otherwise, the word"comprise"or variations such as"comprises"or"comprising", will be understood to imply the inclusion of a stated integer or group of integers but not the exclusion of any other integer or group of integers.

Disclosure of the Invention In accordance with the present invention, there is provided a method for the agglomeration of alumina particles, the method comprising the steps of : grinding the alumina particles to a D50 of less than about 1 2, um ; adding a quantity of a binding agent ; and spray drying the mixture so formed to produce agglomerated granules.

Preferably, the alumina particles are ground to a D50 of less than about 9, um. In a highly preferred from of the invention, the alumina particles are ground to a D50 of about 5pm.

In one form of the invention, a quantity of water is added to the alumina particles to form a slurry, the slurry then being subjected to grinding.

In an alternate form of the invention, the alumina particles are subjected to dry grinding before a quantity of water is added to form a slurry.

Preferably, the slurry is of as high a density as possible. Preferably still, the slurry comprises at least 50% solids. Typically, the slurry comprises between about 40 and 60% solids.

The slurry may also contain a viscosity modifier. The viscosity modifier may be one or more of acetic acid, citric acid or a polyacrylate viscosity modifier. In a preferred form of the invention, the viscosity modifier is acetic acid. In a highly preferred form of the invention, sufficient acetic acid is introduced such that the concentration of the acetic acid in the slurry is between about 0. 2 and 1. 5% by weight of the alumina particles.

In one form of the invention, the binding agent is a polymer form of a hydroxy salt of aluminium. In a specific form of the invention, the binding agent is poly aluminium hydroxy chloride. Preferably, the concentration of the poly aluminium

hydroxy chloride is less than 10% by weight relative to the alumina particles.

Preferably still, the concentration of the poly aluminium hydroxy chloride is approximately 5% by weight relative to the alumina particles.

The method of the present invention may comprise the additional step of : heating the agglomerated granules.

In one form of the invention, the agglomerated particles are dehydroxylated by heating to approximately 300°C. In an alternate form of the invention, the agglomerated granules are calcined above 500°C.

In accordance with the present invention there is further provided agglomerated alumina particles produced by any one of the above-mentioned methods.

Best Mode (s) for Carrying Out the Invention The method of the present invention will subsequently be described, by way of example only, with reference to one embodiment thereof and several examples.

In the embodiment, a quantity of water is added to particulate alumina to form a slurry of a density between about 40 and 60% solids. A viscosity modifier, in the form of acetic acid is added to the slurry such that the concentration of acetic acid in the slurry is between about 0. 2 and 1. 5% by weight of the alumina particles.

The slurry is then subjected to grinding such that the alumina particles are ground to a D50 of less than 1 2, um and preferably about 5, um.

A quantity of binding agent in the form a hydroxy salt of aluminium such as poly aluminium hydroxy chloride, is added to the slurry. The poly aluminium hydroxy chloride is less than 10% by weight relative to the alumina particles. Preferably the concentration of the poly aluminium hydroxy chloride is approximately 5% by weight relative to the alumina particles.

The mixture formed by the addition of the poly aluminium hydroxy chloride to the slurry is then spray dried to produce agglomerated granules.

The agglomerated granules may then be heated to in excess of approximately 300°C to effect dehydroxylation of such, or in excess of approximately above 500°C to effect calcination of such.

The present invention will now be described by way of several examples.

However, it must be appreciated that the following description of those examples is not to limit the generality of the above description of the invention.

Examples ESP dust was collected and milled in a Hosokawa Alpine Jet Mill. The analysis of the treated ESP dust appears in Table 1, below.

Table 1 % Na20 Dio Dso Dgo % Loss % Loss % Loss Total % to 105°C 105-300°C 300-950°C loss As Collected 0. 61 3. 7 13. 9 35. 6 As Treated 0. 27 2. 0 3. 0 5. 7 1. 17 16. 04 4. 05 21. 26

A slurry of water, poly aluminium hydroxy chloride (PAC) binder and the treated ESP dust was prepared to a solids density of approximately 50%. The slurry was then spray dried to form micro agglomerates under the conditions set out in Table 2, below.

Table 2 % PAC pH Spray Dryer Exhaust Temperature, °C 10 3. 8 138. 5 7 4. 2 130 5 5. 1 130 4 5. 5 145 3 5. 3 145 2 6. 2 145

Six runs were conducted at different PAC addition levels. Agglomeration was noted in all cases. Particle size distribution analyses were conducted on the micro agglomerates. The particle size distribution (by screening) of the product is shown in Table 3, below.

Table 3 Cumulative percent 1000 µm 500 µm 300 µm 212 µm 106 µm 45 µm passing size 10% PAC 100 95. 3 90. 2 80. 5 33. 5 2. 8 7% PAC 100 95. 3 89. 0 80. 1 31. 8 2. 4 5% PAC 100 96. 5 89. 0 72. 4 24. 6 1. 6 4% PAC 100 95. 8 84. 0 62. 3 20. 2 1. 1 3% PAC 100 89. 5 78. 4 59. 1 19. 3 0. 9 2% PAC 100 89. 3 71. 9 45. 7 5. 2 0. 2

As can be seen, the product micro agglomerates are similar to the desired smelting grade alumina sizing.

The micro agglomerates produced were tested for strength using an attrition test wherein 10 g of micro agglomerate was placed on the top screen in a stack on a RoTap, and the RoTap was activated for 5 minutes. A separate sample was placed on the RoTap for 20 minutes. The difference in particle size distribution was used as a comparative breakage test for the samples. The results of the attrition tests conducted on agglomerates formed with 7% PAC appear in Table 4, below.

Table 4 Not Calcined 600°C 900°C Time 5mins 20mins delta 5mins 20mins delta 5mins 20mins delta Size (mic)-% Retained- +300 6. 47 4. 44-2. 03 4. 44 2. 11-2. 33 3. 91 1. 58-2. 33 -300/+212 9. 88 10. 55 0. 67 9. 11 9. 03-0. 08 11. 04 10. 02-1. 02 -212/+150 23. 42 23. 92 0. 49 23. 36 23. 44 0. 07 26. 15 26. 50 0. 34 -150/+106 26. 75 27. 01 0. 27 29. 00 28. 30-0. 70 29. 27 30. 15 0. 88 -106/+75 16. 64 16. 79 0. 15 17. 39 18. 29 0. 90 16. 41 17. 26 0. 84 -75/+53 9. 29 8. 94-0. 36 9. 13 9. 52 0. 39 7. 81 7. 94 0. 13 -53 7. 54 8. 35 0. 81 7. 57 9. 31 1. 74 5. 40 6. 56 1. 16

The results of the attrition tests conducted on agglomerates formed with 5% PAC appear in Table 5, below.

Table 5 Not Calcined 6000C 9000C Time 5mins 20mins delta 5mins 20mins delta 5mins 20mins delta Size (mic) % Retained +3007. 674. 99-2. 692. 020. 19-1. 831. 650. 18-1. 47 -300/+212 15. 74 16. 71 0. 97 14. 56 10. 32-4. 24 15. 13 4. 50-10. 63 -212/+150 25. 92 26. 65 0. 73 26. 35 29. 14 2. 80 26. 92 33. 93 7. 01 -150/+106 24. 78 25. 14 0. 36 27. 03 28. 33 1. 30 26. 86 29. 13 2. 27 -106/+75 13. 96 13. 93-0. 03 15. 55 16. 02 0. 47 15. 54 16. 24 0. 70 -75/+53 7. 18 7. 08-0. 10 8. 44 8. 48 0. 04 8. 03 8. 42 0. 39 -53 4. 74 5. 50 0. 77 6. 05 7. 51 1. 47 5. 89 7. 62 1. 73

The results of the attrition tests conducted on agglomerates formed with 3% PAC appear in Table 6, below.

Table 6 Not Calcined Time 5mins 20mins delta Size (mic) % Retained +300 19. 34 8. 87-10. 47 -300/+212 19. 77 21. 66 1. 90 -212/+150 22. 16 27. 33 5. 17 -150/+106 21. 43 23. 57 2. 14 -106/+75 9. 46 9. 60 0. 14 -75/+53 5. 46 5. 61 0. 14 -53 2. 38 3. 36 0. 98

The results of the attrition tests conducted on agglomerates formed with 10% PAC appear in Table 7, below.

Table 7 Not Calcined 600°C 900°C Time 5mins 20mins delta 5mins 20mins delta 5mins 20mins delta Size (mic) % Retained +300 6.05 4.82 -1.23 4.34 2.37 -2.00 3.38 1. 36-2. 02 -300/+212 11. 40 8. 78-2. 62 7. 81 7. 53-0. 29 7. 65 7. 24-0. 41 -212/+150 19.92 20.86 0.94 19.14 19.39 0.25 18.62 18. 98 0. 36 -150/+106 26. 23 27. 20 0. 97 28. 06 28. 11 0. 04 27. 24 27. 79 0. 56 -106/+75 17. 39 18. 05 0. 66 19. 31 19. 49 0. 18 19. 25 19. 69 0. 43 -75/+53 10. 30 10. 21-0. 09 10. 88 11. 11 0. 23 11. 84 11. 69-0. 15 -53 8. 71 10. 07 1. 36 10. 46 12. 01 1. 55 12. 02 13. 25 1. 23 The results of the attrition tests conducted on agglomerates formed with 4% PAC appear in Table 8, below.

Table 8 Not Calcined 600°C 900°C Time 5mins 20mins delta 5mins 20mins delta 5mins 20mins delta Size (mic) % Retained +300 13. 02 10. 72-2. 30 5. 25 0. 20-5. 05 2. 938 0. 24-2. 70 -300/+212 22. 13 23. 06 0. 93 24. 28 14. 42-9. 86 24. 37 11. 24-13. 13 -212/+150 23. 47 23. 92 0. 45 24. 92 33. 95 9. 02 26. 18 36. 16 9. 98 -150/+106 21. 91 22. 18 0. 27 23. 00 25. 98 2. 98 23. 92 26. 85 2. 93 -106/+75 10. 53 10. 25-0. 28, 11. 70 11. 10-0. 60 11. 34 11. 06-0. 28 -75/+53 5. 73 5. 89 0. 15 5. 69 6. 16 0. 47 5. 91 5. 80-0. 12 -53 3. 20 3. 98 0. 78 5. 16 8. 19 3. 03 5. 33 8. 66 3. 33

The results of the attrition tests conducted on agglomerates formed with 2% PAC appear in Table 9, below.

Table 9 Not Calcined Time 5mins 20mins delta Size (microns) % Retained +300 19. 65 11. 52-8. 13 -300/+212 30. 03 18. 13-11. 90 -212/+150 24. 81 29. 85 5. 04 -150/+106 14. 64 23. 52 8. 87 -106/+75 4. 81 4. 80-0. 01 -75/+53 3. 72 7. 01 3. 29 -53 2. 32 5. 17 2. 85 The results were compared to micro agglomerates formed using unground ESP and 10% PAC. The results of the attrition tests conducted on agglomerates formed with 10% PAC and unground ESP dust appear in Table 10, below.

Table 10 Not Calcined 600°C 900°C Time 5mins 20mins delta 5mins 20mins delta 5mins 20mins delta Size (mic) % Retained +300 4.12 3.26 -0.86 0.82 0.49 -0.33 0.79 0.29 -0.50 -300/+212 6. 17 6. 01-0. 16 6. 16 5. 07-1. 09 5. 84 4. 43-1. 41 -212/+150 15.64 15.84 0.19 16.11 15.83 -0.27 15.84 15. 62-0. 23 -150/+106 25. 57 25. 45-0. 12 28. 62 28. 90 0. 28 29. 15 29. 64 0. 49 -106/+75 20. 44 20. 51 0. 068 24. 27 24. 19-0. 08 23. 99 24. 27 0. 28 -75/+53 13. 61 13. 13-0. 48 15. 91 15. 34-0. 57 15. 90 15. 22-0. 68 -53 14. 43 15. 8 1. 37 8. 10 10. 17 2. 07 8. 49 10. 54 2. 05

The results were compared to the attrition behaviour of a typical smelting grade alumina. Results of the attrition tests conducted on a typical smelting grade alumina appear in Table 11, below.

Table 11 Attrition Time, (min) 5mins 20mins delta Size Range, microns % retained +300 0. 15 0. 02-0. 13 -300/+212 0. 13 0. 00-0. 13 -212/+150 4. 48 3. 94-0. 54 -150/+106 25. 15 24. 67-0. 48 -106/+75 35. 33 34. 92-0. 41 -75/+53 20. 96 20. 97 0. 00 -53 13. 80 15. 48 1. 68 100.00 100.00

The results demonstrate that before calcination, the products of mixes containing greater than 4% PAC binder have comparable resistance to the product of the unground ESP with 10% PAC binder. After heating to 600°C, there is more breakage of coarse particles at the 5% PAC level, and considerably more at the 4% PAC level. Most of this material goes into the 106 to 212 pm range and there

is only a small increase in the smallest fraction measured. After heating to 900°C, there is much more breakage from the coarse fractions of the 4% PAC and 5% PAC samples than the product of the unground ESP. Again, most of this material goes into the 106 to 212 urn range and there is only a small increase in the smallest fraction measured.

From these results, it can be seen that the method of the present invention is highly effective at agglomerating alumina particles into micro agglomerates of appropriate size and resistance to attrition to be readily incorporated into smelting grade alumina. This is achieved using levels of binder much lower than that utilised in the prior art, affording cost savings and industrial hygiene benefits.

It is envisaged that a catalyst metal may be added to the slurry to produce a high surface area catalyst product on an alumina binder.

Modifications and variations such as would be apparent to the skilled addressee are considered to fall within the scope of the present invention.