Process for producing enhanced alumina

Alumina powders are widely used in many fields of industry and life, such as catalysts, adsorbents, additive carriers, ceramics, fillers and the like.

There is a need for an alumina carrier that has a high BET surface area and pore volume, with adequate bulk density, stability, and the ability to easily accommodate additives. One of them is, for example, an alumina carrier such as enhanced alumina (EA). A simple method for producing alumina powder is also required.

Dosonite, sodium aluminum dihydroxide carbonate (NaAlOH)₂CO₃) And their alkali metal and ammonium analogs are useful compounds, which are either naturally occurring or synthesized by a variety of processes that have been performed in aqueous media in the prior art. In US4,356,157, dosonite was produced by a combination of aluminum hydroxide with ammonium hydrogencarbonate or alkali metal at high pressures and temperatures from 150 ° C to 250 ° C.

The present invention provides a essentially dry method for the preparation of reinforced alumina powders. This involves three important steps. The first step is an aluminum compound, usually hydroxide, and typically Al (OH) produced by conventional methods in the Bayer process.₃(Givezite), including rapid (flash) firing. This step is carried out industrially to produce activated alumina fine particles and special powders capable of rehydration. The alumina powder produced in this first step is mixed with solid ammonium carbonate in a blender with the addition of a small amount of water. Hydrogen carbonate (bicarbonate) ammonium NH₄HCO₃Is a preferred carbonate, but other ammonium carbonates such as (NH₄)₂CO₃, Ammonium sesquicarbonate or ammonium carbamate, etc. can also be used. The mixture heats itself without external heat, but moderate heating and curing may be applied to accelerate the reaction, resulting in hydroxyammonium aluminum. hydroxycarbonate) (dawsonite type) NH₄AlCO₃(OH)₂Is prepared. The third step involves the decomposition of hydroxyammonium aluminum carbonate to obtain reinforced alumina (EA) with specific morphology and nano-sized material characteristics. In addition, carbon dioxide (CO₂) And ammonia (NH₃) And water are gaseous decomposition products of hydroxycarbonate.

Carbon dioxide and ammonia may be recovered and reused in the preparation of ammonium bicarbonate for use as a raw material. The present invention differs from known methods for the production of alumina powders, such as hydrolysis of aluminum alkoxides and various precipitation pathways. It is also a method using rehydration of flash calcined alumina (FCA), which is usually different from the method of performing autoclave treatment and heat treatment after that. No significant liquid or emission is involved. The carbonate additive may be used in a dry form, and the NH generated in the decomposition stage₃And CO₂Can be easily recovered for reuse by supplementing.

FCA alumina powder industrially produced by flash firing of givezite, known as UOP LLC's A-300 product, is a reagent grade ammonium bicarbonate powder in a blender with the addition of a small amount of water. Mix with. The mixture flows freely, but the flow pattern and resistance to mixing with the addition of water vary somewhat. In addition, the temperature rises and exceeds 45 ° C to 50 ° C. After 30-45 minutes, the resulting powder is transferred to a plastic container and further cured in an oven at 55 ° C-60 ° C to complete the reaction. The final step of the method is to produce reinforced alumina with high BET surface area and pore volume, NH.₄-Pyrolysis of dosonite.

If desired, the raw material alumina may be treated with an acidic solution, followed by washing and drying. The purpose of this treatment is to reduce residual sodium when the raw material alumina is produced by the Bayer process of alumina production. Such alumina materials usually contain 0.2-0.5% by weight sodium, represented as an oxide. When used as a catalyst carrier, low soda alumina is desirable. The treatment with the acidic solution may be carried out directly on the strengthened alumina produced after the decomposition of the hydroxycarbonate product, not on the raw material alumina.

The method of producing fortified alumina via the hydroxy carbonate, (dersonite-type) intermediate described may be combined with the use of additives to further enhance the properties of the final product. Additives in the form of solids and additives in the form of liquids can be used. When the water in the hydroxycarbonate forming step is replaced with a colloidal silica solution, alumina with increased thermal stability is produced.

The present invention provides a essentially dry method for the preparation of reinforced alumina powders. This involves three important steps. The first step is an aluminum compound, usually hydroxide, and typically Al (OH) produced by conventional methods in the Bayer process.₃(Givezite), including rapid (flash) firing. This step is carried out industrially to produce activated alumina fine particles and special powders capable of rehydration. The simplified reaction is shown in reaction (1). Alumina produced in this first step is referred to as flash-fired alumina (FCA).

2Al (OH)₃+ Heat-Al₂O₃+ 3H₂O (1) Other aluminas, such as those produced by hydrolysis of aluminum alkoxides, precipitation of alumina sources or hydrothermal treatment, are reinforced according to the present invention as long as these aluminas are sufficiently reactive with ammonium carbonate reagents. Can be easily used as a raw material for producing. Generally at least 50m²Having a BET surface area of / g and some rehydratability is an important condition for compatibility to serve as an alumina raw material.

The alumina powder produced in this first step is mixed with solid ammonium carbonate in a blender with the addition of a small amount of water. Ammonium bicarbonate NH₄HCO₃Is a preferred carbonate, but other ammonium carbonates such as (NH₄)₂CO₃And sesquicarbonates or carbamates and the like can also be used. The mixture heats itself without external heat, but moderate heating and curing may be applied to accelerate reaction (2), resulting in hydroxyammonium carbonate aluminum (dersonite). Type) NH₄AlCO₃(OH)₂Intermediates are prepared. Two different hydroxycarbonate intermediates have been identified, depending on the conditions of this reaction and the curing process.

Curing at longer times and higher temperatures yields the classic ammonium dosonite identified by X-ray diffraction patterns, while at lower temperatures and shorter contact times, another unknown hydroxy Ammonium aluminum carbonate is produced. This is named "HYCARB" for the purposes of the present invention. Both the X-ray pattern and the FTIR (Fourier Transform Infrared) spectrum show significant differences between HYCARB and the classical ammonium dosonite.

Al₂O₃+ 2NH₄HCO₃+ H₂O = 2NH₄AlCO₃(OH)₂ (2) This reaction (2) may be carried out to various degrees as long as it is necessary to obtain the characteristics of the reinforced alumina. Partial conversion of raw material alumina to reinforced alumina and practically complete conversion can be achieved. Repeating step 2 with the addition of a new portion of ammonium carbonate speeds up the alumina conversion process. The use of a small amount, typically 2-10% by weight, of hydroxyammonium carbonate aluminum powder in step 2 also facilitates the conversion of the raw material alumina to hydroxycarbonate.

The third step involves the decomposition of ammonium dosonite, ie the reaction (3), to obtain fortified alumina (EA) with specific morphology and nano-sized material characteristics.

2NH₄AlCO₃(OH)₂+ Heat = Al₂O₃(EA) + 2CO₂+ 2NH₃+ 3H₂O (3) This disassembly step can be performed in various devices. A device capable of reusing decomposition products is preferable. Normally, a temperature of 130 ° C to 320 ° C is sufficient to completely decompose the hydroxycarbonate intermediate, but the lower range is particularly high if step 2 of the method results in HYCARB formation. preferable. Further heat treatment may be performed as required for the application. If the final product should have gamma alumina as the mail alumina crystal phase, it is treated at a temperature of 600 ° C.

The particle morphology of the final alumina is significantly different from the particle morphology of the raw material. The main difference is the appearance of a large number of rod-shaped subparticles that occur and are firmly fixed in the original particles. The width of the rod is usually less than 100 nanometers, while the length is limited to a few micrometers.

Carbon dioxide and ammonia may be recovered from reaction (3) and reused in the preparation of ammonium bicarbonate used as a raw material for reaction (2). The present invention differs from known methods for the production of alumina powders, such as hydrolysis of aluminum alkoxides and various precipitation pathways. It is also different from the method using rehydration of flash-fired alumina (FCA), which is usually followed by a combination of acid treatment, autoclave treatment and heat treatment. No significant liquid or release is involved. The carbonate additive may be used in a dry form, and the NH generated at the decomposition step may be used.₃And CO₂Can be easily recovered for reuse by supplementing.

<p> Example 1 100 g of A-300 alumina is mixed with 150 g of solid ammonium carbonate in a blender with the addition of 24.5 g of water as described above. The material was heated overnight at 71 ° C. in a sealed container. The resulting powder is then calcined at 400 ° C. for 1 hour in a muffle furnace to produce reinforced alumina with improved thermal stability.</p><p> Example 2 The experiment of Example 1 is repeated, but 34.5 g of colloidal silica Nalco 1130 is used instead of water.</p><p> Example 3 Comparative A-300 powder was treated with the equipment described in Example 1. No bicarbonate was added, but only 30.4 g of deionized water was added. The sample was then calcined at 400 ° C. for 1 hour.</p><p> Example 4 The conditions of Example 1 were applied, but the duration of the firing step at 400 ° C was 16 hours.</p><p> Example 5 The conditions were the same as in Example 4, but the firing temperature was 600 ° C.</p><p> Example 6 The conditions were the same as in Example 4, but the firing temperature was 800 ° C.</p><p> The calcined samples of Examples 1-6 were analyzed by cold nitrogen adsorption using common methods to determine BET surface area and pore volume. In addition, the selected samples were analyzed for thermal stability using DTA (Differential Thermal Analyzer), air flow, and temperature programming rate of 5 ° C / min. The temperature of the particular exothermic effect indicating the transition to the alpha-alumina phase was recorded as a measure of thermal stability. The data obtained are summarized in a table, but the sample according to Example 3 represents a comparative sample prepared without any addition of ammonium carbonate.</p><p><tables num="1"><img file="JP5425928B2_D0001.tif" /></tables></p><p> The data in the table show that the present invention produces alumina with a much higher BET surface area, pore volume and thermal stability compared to the raw material alumina.</p>