AU2004222753A1 - Process for synthesis of layered double oxides (LDO) and layered double hydroxides (LDH) using red mud produced from the Bayer process of alumina production - Google Patents

Description

'4 TITLE OF THE INVENTION

O

0 N Process for synthesis of layered double oxides (LDO) and layered double hydroxides (LDH) using red mud produced from the Bayer process of alumina production.

0 FIELD OF THE INVENTION Cfl This invention relates to a process for synthesis of Layered Double Oxides (LDO) and V- Layered Double Hydroxides (LDH), also known as hydrotalcites, using red mud produced C from the Bayer process of alumina production.

O BACKGROUND OF THE INVENTION There are currently three methods of producing alumina in the world; namely Bayer process, roasting process and the combination of these two processes. The Bayer process was developed by Karl Bayer in 1887 and remains the most economic means of obtaining alumina, which in turn is vital for the production of aluminum metal. The main raw material for this process is bauxite which contains high concentrations of aluminum and iron. In the Bayer process, the aluminum in bauxite is firstly digested by a concentrated hot caustic soda solution. The aluminum-bearing solution is separated from the insoluble impurities and then the aluminum is precipitated out as gibbsite. It is then calcined and the final product is alumina. Bauxite residue, also known as red mud, is a by-product of the Bayer process.

The amount of red mud generated, per ton of alumina produced, varies greatly depending on the type of bauxite used, from 0.3 tons for high grade bauxite to 2.5 tons for very low grade.

The chemical and physical properties of red mud depend primarily on the bauxite used and, to a lesser extent, the manner in which it is processed. The average particle sizes of the red mud are sub-microns. It is very alkaline, its pH value can be as high as 13, and thus it is very corrosive. The following data gives some idea of the wide range in chemical composition that can be found in red mud from different bauxites; Fe 2 0 3 =30 A1 2 0 3 =10 25%, SiO 2 =3 50%, Na 2 0=2 15%, CaO=0 8% and TiO 2 =0 Red mud is the major waste from the Bayer process. Because of its complicated physical and chemical properties which make it difficult for further processing, thus it remains as the major source of contamination for the Bayer process. Worldwide, most of the alumina refineries impound their red mud. However, alkaline liquid from red mud can seep through cracks in liners of storage ponds then contaminate underground water and also increase the salinity of surrounding soil. A large amount of energy is required for dewatering and pre-drying the red mud prior to disposal. A sizable disposal site is also required. The fine dust of dried red mud is often blown around by seasonal winds and causes air pollution. The environmental impacts of red mud disposal have become well known and remain of great concern among communities and the alumina industry.

Because of the high iron contents in red mud, some companies have proposed using red mud as the raw material for iron smelting, such as in the US Patent No. US3989513. It is an expensive process with low economic returns from the final product and thus is difficult to S.compete with processes using iron ore as raw material. The Chinese Patent No. CN1117945 O suggested using red mud as a raw material for cement manufacturing. But because of the colour of the final products, this type of cement has very limited usages. Red mud was also ,1 suggested for processing into bricks as seen in the Australian Patent No. AU583731 and the European Patent No. EP0794161. There are also suggestions of using red mud as a landfill e¢ material, such as in the Australian Patent No. AU701874 and the US Patent No. US4270875.

Due to the fact that major constituents of red mud are aluminum and iron hydroxides; C therefore, theoretically, it is possible to use it as the raw material for synthesizing layered double oxides and hydrotalcites.

Hydrotalcites are also known as layered double hydroxides (LDH) which have layered structures. The basic layers are composed of divalent and trivalent metal ions hydroxides with anions and water molecules filling the spaces between layers. Thus, they are also called anion clays. Once heated, hydrotalcites can lose those inter-layered water and hydroxyl groups and become layered double oxides (LDO). These LDOs retain their original layered structure. After absorbing water, hydroxyl groups and anions from aqueous solution, they can revert back to their original LDH structures.

LDH and LDO are new synthetic minerals which have become popular in recent years. In chemistry and chemical industries, they can be used as catalysts or carriers for catalysts. As a functional material, they can absorb infrared and ultraviolet radiations and have also been used as an insulating material. They can be used as a fire retardant and PVC stabilizer in the plastic industry. They have very strong absorption capability for many harmful anions and thus have many applications in the field of waste water treatment, pollution prevention and environmental rehabilitation etc.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The objective of this invention is to provide a method for synthesis of LDOs and LDHs using red mud from the Bayer process of alumina production as a raw material.

The chemical compositions of a typical red mud are: Fe 2 03=32 39%, A1203=22 SiO 2 =16 19%, Na20=8 13%, CaO=O 2% and Ti02=7 9%.

Details of the synthesis process are as follows: 1. Prepare measured amounts of divalent metal ions oxide or hydroxide which are 0.5-3 times to that of red mud. Pulverize the measured material to less than 200 mesh.

Charge the pulverized material and red mud into a mixer with 2 8 times of water and mix them thoroughly.

0 C 2. Dewater the mixture and dry it at room temperature or at temperatures less than o Sinter the dried material at 450 750'C for 2 5 hours. After cooling to room O temperature, pulverize the sintered material to less than 200 mesh. The product is a LDO with a typical formula of Mm(A1, Fe 3 where M is the divalent metal ions and x m+3n/2.

Cfl 3. Measure soluble carbonate or bicarbonate, at a ratio of one mole of carbonate or Vbicarbonate to 200 300 gram of the LDO produced above. Prepare the carbonate or C bicarbonate solution with a concentration of 0.5 2.0 mole/L. Add the LDO into this Csolution and continuously stir for 5 10 hours to ensure the completion of the hydrolysis oprocess. Remove water from the hydrolysate by filtration, sedimentation or Ocentrifugation. Wash it with clean water for 2 3 times then dewater and dry it at cl temperatures below 90'C. Pulverize the dried material to less than 200 mesh. The final product is a LDH with a typical formula of [M1-,(A1,Fe31)"(OH)2] (CO3)/2 nH 2 0, where M is the divalent metal ions, x 0.5 0.17 and M/(AI+Fe 3 =1 The divalent metal ions mentioned above can be either magnesium or zinc or nickel or their combination. The hydrate or oxide of divalent metal can be either natural minerals or synthetic commodity.

The soluble carbonate and bicarbonate mentioned above can be either sodium carbonate or potassium carbonate or ammonium carbonate or sodium bicarbonate or potassium bicarbonate or ammonium bicarbonate or the combination of them.

Advantages of this method are that the process is simple, and the outlay of equipment is low; while the raw main materials required are cheap and easily obtainable. Compared to the existing co-precipitation method for synthesizing LDH/LDO, the method mentioned in this invention is simpler and adopts an industrial waste material as the raw mineral, which thus further reduces the costs. At the same time, it solves the storage and environmental pollution problems of red mud. The final products, LDO and/or hydrotalcite, have applications in many areas. For example, in chemistry and chemical industries, they can be used as catalysts or carriers for catalysts. As a functional material, they can absorb infrared and ultraviolet radiations and have also been used as an insulating material. They can be used as a fire retardant and PVC stabilizer in the plastic industry. They have very strong absorption capabilities for many harmful anions and thus have many applications in the field of waste water treatment, pollution prevention and environmental rehabilitation etc.

EXAMPLES

Theories behind this synthesis of LDOs and hydrotalcites using red mud from the Bayer process of producing alumina are based on the fact that the aluminum and iron in the red mud can react with the added divalent metal oxide or hydroxide during the high temperature 0 osintering process and form a solid solution. This solid solution can be hydrolysed into hydrotalcites.

,0 O The reaction between the divalent metal ions from the additive and trivalent metal ions from the red mud will affect the properties of the final products. Generally speaking, if the amount of the former is higher, then the final products will be structurally more stable; whereas the converse will produce better absorption capability products. It is preferred to Cfl limit the molar ratios between the divalent metal ions from the additive and trivalent metal V)ions between 1 1 and 4 1. In practice, the weight ratios between additives and red mud C are restricted between 0.5 1 and 3 1. The most common divalent metal ions oxide and C hydroxide used are magnesium oxide [MgO] and magnesium hydroxide [Mg(OH) 2 1. They can be either natural minerals, i.e. periclase [MgO] and brucite [Mg(OH) 2 or synthetic Ocommodities. They have to be pulverized to less than 200 mesh prior to the mixing.

The amount of water added into the mixture will depend on the moisture content of the red mud selected. It should be adjusted so that the slurry produced will be thick but easily flow.

Too much water added will prolong the mixing process and thus waste energy and increase wear of equipment; whereas not enough will make it difficult to homogenize the sample during the mixing. It is preferred that the amount of water added is 3 5 times of the total charge.

Due to the slightly soluble nature of divalent metal ions oxide and hydroxide, they are easily homogenized with red mud during the mixing.

Water can be removed from the mixed slurry by filtration, sedimentation or centrifugation.

The solid will preferably be air-dried or dried at temperatures less than 90'C, so that the mixture can be further aged during the drying process. The purpose of sintering is to transfer the mixture into solid solution. The resulting product from the sintering process is a LDO. It is a marketable product, or can be made into a hydrotalcite after further processing.

LDOs can be hydrolyzed into hydrotalcites in carbonates aqueous solution. A typical reaction in the solution can be seen as: Mg 6 (A1,Fe) 2 0 9 9H120 C03 2 [Mg 3 (A1,Fe)(OH) 8 2

CO

3 nH 2 0 2(OH) In the above reaction, the molar ratio of LDO and carbonate is 1 1. Practically, in order to ensure the complete hydrolysis, the moles of carbonate added should be 1.2 1.5 times more than that of LDO. There is not much difference in the final properties as a result of using different kind of carbonates. The sodium carbonate is preferred for economical reasons. In order to remove the residue of carbonate, the hydrolyzed product should be washed with water 2 3 times after it has been initially dewatered.

Examples of the process of this invention are described as follows: 0 o EXAMPLE 1: Synthesizing layered double oxide (LDO) using brucite (Mg(OH) 2 and C red mud ,0 O 1. Measure 50kg of brucite and grind it to <200 mesh. Measure 5kg of red mud and charge into the mixer with brucite and 35kg of water, then mix them thoroughly.

2. Filter the slurry and air-dry it. Sinter the dried mixture at 500C for 3 hours. Cool it to room temperature and grind to <200 mesh for future use.

e¢3 rn EXAMPLE 2: Synthesizing layered double oxide (LDO) using magnesite (MgCO 3 and C red mud 1. Measure 15kg magnesite, and calcine it at 400 0 C for 3 hours, so it will decompose to 0 o MgO.

C 2. Measure 5kg of red mud, and charge it with the MgO from Step 1 into a mixture with of water and mix thoroughly.

3. Filter the slurry and air-dry it. Sinter the dried mixture at 450'C for 3 hours. Cool it to room temperature and grind to <200 mesh for future use.

EXAMPLE 3: Synthesizing layered double oxide (LDO) using red mud and zinc oxide (ZnO) commodity 1. Measure 6kg of zinc oxide and charge it into a mixer with 10 kg of red mud and 60kg of water. Mix them thoroughly.

2. Filter the slurry and air-dry it. Sinter the dried mixture at 750 0 C for 4 hours. Cool it to room temperature and grind to <200 mesh for future use.

EXAMPLE 4: Synthesizing layered double oxide (LDO) using red mud and nickel oxide (NiO) commodity 1. Measure 6kg of nickel oxide and charge it into a mixer with 10 kg of red mud and of water. Mix them thoroughly.

2. Filter the slurry and air-dry it. Sinter the mixture at 500C for 2 hours. Cool it to room temperature and grind to <200 mesh for future use.

EXAMPLE 5: Synthesizing layered double hydroxide (LDH) from layered double oxide

(LDO)

1. Measure 1.5kg of sodium carbonate and dissolve it in 10kg of water. Charge 4kg of LDO into the solution and continuously stir for 6 hours to ensure the completion of hydrolysis process.

2. After filtration and dewatering, add another 10kg of water and mix properly. Filter it again. Repeat this step once more.

3. Air-dry or dry it at temperatures less than 80'C, then grind it to <200 mesh for future use.

EXAMPLE 6: Synthesizing layered double hydroxide (LDH) from layered double oxide NC (LDO) O 1. Measure 1.5kg of sodium bicarbonate and dissolve it in 10kg of water. Charge 3kg of LDO into the solution and continuously stir for 8 hours to ensure the completion of hydrolysis process.

2. After filtration and dewatering, add another 10kg of water and mix properly. Filter eC again. Repeat this step once more.

3. Air-dry or dry it at temperatures less than 80'C, then grind it to <200 mesh and pack it for C,1 future use.

EXAMPLE 7: Synthesizing layered double hydroxide (LDH) from layered double oxide

(LDO)

1. Measure 1.5kg of ammonium carbonate and dissolve it in 10kg of water. Charge 3kg of LDO into the solution and continuously stir for 7 hours to ensure the completion of hydrolysis process.

2. After filtration and dewatering, add another 10kg of water with mix properly. Filter again. Repeat this step once more.

3. Air-dry or dry it at temperatures less than 80'C, then grind it to <200 mesh and pack it for future use.

EXAMPLE 8: Synthesizing layered double hydroxide (LDH) from layered double oxide

(LDO)

4. Measure 1.5kg of ammonium bicarbonate and dissolve it in 10kg of water. Charge of LDO into the solution and continuously stir for 8 hours to ensure the completion of hydrolysis process.

After filtration and de-watering, add another 10kg of water with mix properly. Filter again. Repeat this step once more.

6. Air-dry or dry it at temperatures less than 80'C, then grind it to <200 mesh and pack it for future use.