EXTRACTION OF ALUMINA

Field of invention

This invention relates to alumina extraction.

Definition

In the context of the present invention, the term "Caustic" means oxides, hydroxides and carbonates of sodium, potassium and lithium.

Background of Invention

Bayer's process is employed worldwide to extract alumina from bauxite ore. It is a wet process involving the use of alkali. Bayer's process consists of various steps including digesting bauxite in alkali, clarification and removal of red mud, precipitation of alumina and subsequent calcinations to get pure alumina.

Bauxites, the aluminous ores, are digested in caustic at elevated temperatures to extract available alumina from the ore producing slurry containing enriched caustic aluminate liquor called as green liquor and an ore residue called as red mud. The green liquor is supersaturated with alumina. Then the slurry from the digestion operation is flash cooled to atmospheric conditions and subjected to clarification and/or filtration operations separating the supersaturated liquor and insoluble ore residue. The ore residue or red mud is washed to recover the entrained caustic aluminate liquor and then discarded. The supersaturated caustic aluminate liquor is further cooled and the alumina is precipitated as alumina trihydrate with or without seeding with previously precipitated alumina trihydrate. In

normal operation, 50% of the dissolved alumina contents in the supersaturated caustic aluminate liquor is precipitated as alumina trihydrate, and the remaining portion of alumina is retained by the liquor which is recycled to digestion as spent caustic aluminate liquor called spent liquor. The precipitated alumina trihydrate is filtered, washed with water and calcined at about 1000 deg C to 1200 deg C to produce 99% pure alumina. One of the most important step of Bayer's process is digestion, wherein the maximum possible amount of alumina from bauxite should be extracted. This in turn depends on quality and composition of the bauxite. Bauxite occurs in various forms depending on geological conditions of its formation. Out of this, three main forms of bauxite with respect to a) the number of water of hydration molecules; and b) the crystalline structure are Gibbsite, Boehmite and Diaspore. Gibbsite is an aluminium hydroxide having trihydrate form. Boehmite and Diaspore are aluminium-oxide-hydroxides having mono hydrate form. The basic properties of these forms are as shown in table 1. Table 1

The conditions of digestion (caustic concentration, digestion temperature and pressure) are set according to the composition of bauxite. Ores with a high gibbsite content can be digested at 140 deg C where as boehmite requires 200 -240 deg C.

Although higher temperatures and high caustic concentrations are often theoretically advantageous for maximum digestion efficiency, there are several disadvantages like corrosion problems; possibility of the digestion of other bauxite components along with higher energy costs. This increases the cost of aluminium production. Therefore low temperature digestion (100 to 150 deg C) is utilized for bauxite containing less than 5% alumina monohydrate. Since 90 to 97% alumina trihydrate is extracted at these conditions discarding alumina monohydrate in red mud to maintain lower recovery costs. However, if the bauxite contains more than 5% alumina monohydrate, a high temperature digestion unit operating at 200 to 300 deg C is employed to extract 90 to 97% of total alumina (monohydrate and trihydrate).

It is preferred that the bauxite ore used be rich in the Gibbsite content. However, due to its depletion the ore are becoming richer in Bohemite phase. Thus the digestion process is further modified to add an additional digestion step , one favoring the extraction at low temperatures - Gibbsitic and the other at a higher temperature for Bohemite. This is referred to as double digestion. It is a challenge for the alumina industry to reduce the cost of alumina production from bauxite. This is partly being attempted by benefication of the ore at the mine itself to reduce transportation and waste handling cost.

Alumina from the ore such as Bauxite could be simultaneously induced for structural and chemical changes by treatment with electromagnetic radiation of the ore in the presence of water, alkali or acidic salt and salt solution or gases individually or collectively. Microwave technology helps to achieve cleaner and more efficient chemical reactions with higher yield compared to conventional hydrothermal methods

The patent or literature for structural collapse and reactivity of bauxite in presence of NaOH and microwave radiation are not reported. However, microwaves are reported for its ease of grinding application in metal ores; Shaw, Raymond Walter. (Technological Resources Pty. Limited, Australia). Microwave absorption of various materials is different and it depends on the corresponding dielectric constants. Taken separately, the microwave absorption capacity and its subsequent heating effect for various compounds in microwave are given in following table 1.

Table 1 :

Object of invention

One of the objects of the invention is to provide a process for extraction of alumina with less energy for ore crushing.

Another objects of the present invention is to provide a method for enhanced rate of reaction

Yet another object of the invention is to provide a process for extraction of alumina with decreases in the time required for heating.

Yet another object of the invention is to provide a process for extraction of alumina which is economical.

Yet another object of the invention is to provide a process for extraction of alumina is to increase the digestion efficiency.

Summary of the invention

In accordance with this invention there is provided a process for extraction of alumina from aluminum containing ores, comprising the following steps

determining the alumina and silica content in the ore;

placing the ore having alumina greater than 5% and silica less than 50% of the total mass of ore in a reaction vessel;

mixing caustic with the ore, optionally with an additive (I) to form a reaction mass;

subjecting the reaction mass to electromagnetic radiation in the range of 800 - 3000 MHz with a power of 250W - 50KW for a time period of 1 microsecond - 3600 seconds resulting in a temperature of 100 - 1500 degree C to form an intermediate mass containing sodium aluminate and sodium silicate along with impurities;

leaching the intermediate mass with water in the reaction vessel to separate insoluble red mud as residue and to obtain a filtrate containing aluminate, silicate and vanadate;

separating silicate and other impurities as residue from the filtrate by heating, optionally with an additive (II) at temperature of 50 - 350 degree C and cooling to 4 - 300 degree C to obtain beneficiated aluminate liquor; and

extracting alumina from sodium aluminate by a conventional method.

Typically, the caustic is selected from a group of alkali metals.

Typically, the alkali metals include oxides, hydroxides, carbonates of sodium, potassium and lithium.

Typically, the weight ratio of alumina to caustic expressed as Na ₂ CC> ₃ is in the range of 1 :0.3 to 1 :2.5.

Typically, the caustic is in form of a solid.

Typically, the caustic is in the form of 10 - 99 % of caustic slurry.

Typically, the caustic is in form of a vapor.

Typically, the said additive (I) is selected from a group of alkaline earth metals.

Typically,,the alkaline earth metals include oxides of calcium, barium, strontium and magnesium.

Typically,,the additive added is in the range of 0 - 20 % of the total weight of ore.

Typically, the said electromagnetic wave is microwave.

Typically, the water is in the range of 4 degree C to 150 degree C.

Typically, the weight ratio of alumina to caustic expressed as sodium carbonate is in the range of 1 : 0.3 - 1 :2.5.

Typically, the mole ratio of additive (II) to silica is in the range of 1 :2 to 1 :2.5.

Typically, the additive (II) is selected from group of alkaline earth metals.

Typically, the alkaline earth metals include oxide, hydroxide and carbonate of calcium, barium, strontium and magnesium.

Description of the accompanying drawings

X-Ray Diffraction image (XRD):-

XRD gives the intensity of various crystalline phases in the ore. The y-axis shows linear counts for peak intensity for particular crystalline phase at particular angle on x-axis.

FIG 1: is the XRD image of the bauxite ore before the ore is subjected to microwave radiations

FIG 2: is the XRD image of the bauxite after subjecting to microwave radiations

Detailed description of the invention:

In accordance with a preferred embodiment of the invention the ore is exposed to dilute NaOH solution along with microwave radiation which causes Al to form sodium aluminate and similarly to convert the silica species to sodium silicates. These formed compounds/complexes are subsequently dissolved by wetting with water or are subsequently subjected to conventional hydrothermal treatment.

In the present case exposure of ore with alkali such as NaOH along with microwave radiation can cause Al to form sodium aluminate and similarly silica species to convert to sodium silicates. These can be subsequently

dissolved by wetting with water or subsequently subjected to conventional hydrothermal treatment for recovery of alumina.

Microwave exists in the broad range of frequencies from 900 MHz - 3000 MHz. however, in most of the countries; the permitted frequency for the industrial and household usage is 2450 MHz. Microwaves are available in various wattages upto 50 KW.

In one of the preferred embodiment there is provided a process for extraction of alumina from aluminum containing ores in accordance with this invention. The ore is determined for its alumina and silicate content. Preferably, ore having alumina greater than 5% and silica less than 50% of the total mass of ore is used for the present process. The ore is dry or wet ground to about 60 mesh. The ground ore is taken in a first reaction vessel and treated with a caustic preferably, with sodium hydroxide powder of purity 97% to form a reaction mass optionally along with the an additive (I). The caustic is selected from group of alkali metal including oxides, hydroxides, carbonates of sodium, potassium and lithium. The alumina in ore to caustic expressed as sodium carbonate is in the ratio of 1 :0.3 to 1 :2.5. The reaction mass (ore with dilute/concentrated NaOH) is subjected to electromagnetic radiation, preferably microwave in the range of 800 - 3000 MHz with a power of 250W - 50KW for a time period of 1 micro second to 1 hour to form an intermediate product containing sodium aluminate and sodium silicate along with other impurities as red mud. FIG 1 and 2 shows the XRD image of the bauxite ore in red mud, before and after subjecting it to red mud. FIG 2 is more distinctive, which enhances the reactivity.

Typically, the said additive (I) is selected from a group of alkaline earth metals which include oxides, hydroxides, carbonates of calcium, barium, strontium and magnesium.The additive (I) added is in the range of 0 - 20 % of the total weight of ore.

The intermediate mass is then leached in the reaction vessel with hot water to dissolve sodium aluminate and the sodium silicate, vanadium and other impurities from the filtrate. The obtained filtrate contains aluminate liquor. The red mud is then washed with hot water at 100 degree C. Lime as an additive (II) is added to the aluminate liquor at 200 degree C for 30 minutes to remove silica. The liquor is then cooled to 40 degree C, which separates impurities like silicate and vanadate to form beneficiated aluminate liquor. Typically, the additive (II) is selected from group of alkaline earth metals which include oxides, hydroxides and carbonates of calcium, barium, strontium and magnesium. Typically, the mole ratio of total amount of silica in filtrate to the additive (II) to is in the range of 1 :0.3 to 1 :2.5.

The beneficiated aluminate liquor was further sent for alumina extraction by conventional process. In a conventional process alumina was further sent for aluminium trihydrate precipitation. This is subjected to calcination for alumina extraction.

Advantages of microwave effects:

1) Unlike conventional method, in case of microwave heating several localized hot spots are created which results in enhanced rate of reaction.

2) The timescales of transfer of Microwave energy is in 10 ^"9 sec. while the kinetic molecular relaxation from this energy is approximately 10 ^"5 sec. The faster rate at which it heats, leads to enhancement in reaction rates and product yields.

3) Microwave radiation results in the generation of surface defects in solid particles. The generation of these surface defects enables increased ion motion, and hence helps in easy excitation of these ions to higher energy levels.

4) Microwave radiation can be attributed to the superheating of solvents such as water (increase in boiling point of solvent or solution) and or with water dissolved with salts and or acid, which enables the reaction to be performed at higher temperatures and results in increase in the rate of the of the reaction and or reactivity

5) Microwave can be used in Bayer's digestion with or without using additives like oxides and hydroxides of calcium or Magnesium, organic complexing agents, etc.

6) Microwave treatment can be given to both bauxite ore and caustic mixture without water or with water, which is just sufficient enough to wet the ore particles and then doing alumina extraction.

7) Microwave treatment to the bauxite ore particles may break the structure of alumina hydrates from crystalline to amorphous and increase the alumina

extraction efficiency as the leaching efficiency of amorphous alumina from bauxite ore is more compared to its crystalline form.

It is also envisaged that the process of microwave treatment of minerals, illustrated above with the case of bauxite, will also be applicable to other minerals. The above treatment is also claimed to be useful for enrichment of the other metal components in bauxite such as TiO2, Iron oxide etc.

The patent or literature document for structural collapse and reactivity of bauxite in presence of NaOH and microwave radiation is not reported. However, microwaves are reported for its ease of grind/crushing application in metal ores.

The invention will now be described with respect to the following examples which do not limit the invention in any way and only exemplify the invention.

Example 1

40 gm of Bauxite ore was determined for its alumina and silicate content. The ore contained 45.5 % alumina and 2.3 % silicate. The ore was dry ground to about 60 mesh. The ground ore was taken in a reaction vessel and mixed with caustic of purity 97% to form a reaction mass. The alumina to caustic (as Na ₂ COs) weight ratio was maintained at 0.5. The reaction mass was subjected to electromagnetic radiation of frequency 2450 MHz with a power of 900W for a time period of 9 min resulting in a temperature of 289 degree C. to form an intermediate product containing sodium aluminate and sodium silicate along with other impurities. The intermediate product was

then leached in the reaction vessel using 145 gm of hot water to dissolve soluble sodium aluminate and silicate along with other impurities from the intermediate product as filtrate and red mud as residue. Insoluble red mud was washed with 40 gm of hot water at 100 degree C. Resulting aluminate liquor was reacted with 4 gm of lime as an additive (II) at 200 degree C for 30 minutes to separate silica. It was then cooled to remove impurities like silicates. The aluminate liquor was sent for alumina recovery by conventional process. The alumina extraction efficiency based on alumina content of red mud was found to be 85.91%.

Example 2

Example 2 was carried out as per example 1, wherein the reaction mass was subjected to microwave for 6 min, resulting in a temperature of 196 degree C. The alumina to caustic (as Na ₂ CO ₃ ) weight ratio was maintained at 0.50. The alumina extraction efficiency was found to be 87.67 %

Example 3

Example 3 was carried out as per example 1, wherein the reaction mass was subjected to microwave for 3 min, resulting in a temperature of 161 degree C. The alumina to caustic (as Na ₂ CO ₃ ) weight ratio was maintained at 0.50. The alumina extraction efficiency was found to be 82.40 %

Example 4

Example 4 was carried out as per example 1, wherein the reaction mass was subjected to microwave for 9 min, resulting in a temperature of 383 degree C. The alumina to caustic (as Na ₂ CO ₃ ) weight ratio was maintained at 0.70. The alumina extraction efficiency was found to be 80 %

Example 5

Example 5 was carried out as per example 1, wherein the reaction mass was subjected to microwave for 9 min, resulting in a temperature of 332 degree C. The alumina to caustic (as Na ₂ COs) weight ratio was maintained at 0.85. The alumina extraction efficiency was found to be 63 %

The microwave treatment was carried out in multiple stages. The red mud of already microwaved reaction mixture after leaching, was again subjected to microwaves with caustic distributed in these stages. It can be observed that the multiple stages enhances the extraction efficiency of the process.

Examples 6 to 10 were carried out in two stages.

Example 6

Example 6 was carried out as per example 1, wherein the reaction mass was subjected to microwave for 18 min, resulting in a temperature of 700 degree C. The alumina to caustic (as Na ₂ CO ₃ ) weight ratio was maintained at 0.51. The alumina extraction efficiency was found to be 95.86%

Example 7

Example 7 was carried out as per example 1, wherein the reaction mass was subjected to microwave for 12 min, resulting in a temperature of 390 degree C. The alumina to caustic (as Na ₂ CO ₃ ) weight ratio was maintained at 0.51. The alumina extraction efficiency was found to be 90.72%

Example 8

Example 8 was carried out as per example 1, wherein the reaction mass was subjected to microwave for 6 min, resulting in a temperature of 343 degree C. The alumina to caustic (as Na ₂ CO ₃ ) weight ratio was maintained at 0.51. The alumina extraction efficiency was found to be 86.50%

Example 9

Example 9 was carried out as per example 1 , wherein the reaction mass was subjected to microwave for 18 min, resulting in a temperature of 285 degree C. The alumina to caustic (as Na ₂ CO ₃ ) weight ratio was maintained at 0.70. The alumina extraction efficiency was found to be 83.62%

Example 10

Example 10 was carried out as per example 1, wherein the reaction mass was subjected to microwave for 18 min, resulting in a temperature of 390 degree C. The alumina to caustic (as Na ₂ CO ₃ ) weight ratio was maintained at 0.85. The alumina extraction efficiency was found to be 84.23%

Example 11

Example 11 was carried out as per example 1, wherein 100 gm of bauxite was used. The reaction mass was subjected to microwave for 9 min, resulting in a temperature of 192 degree C. In this example the process of subjecting the reaction mass was carried out in three stages with caustic distributed in these stages. The alumina to caustic (as Na ₂ CO ₃ ) weight ratio was maintained at 0.85. The alumina extraction efficiency was found to be 85.24%.

Example 12, 13 and 14 was carried out as per example 1 in combination with other conventional techniques, wherein microwaved reaction mixture was hydrothermally treated, wherein the separated red mud was heated to a temperature 240 degree C. It can be absorbed in these examples that the digestion efficiency is increased inspite of increase in bauxite loading.

Example 12

Example 12 was carried out as per example 1, wherein 100 gm of bauxite was reacted with caustic. The reaction mass was subjected to microwave for 8 min, resulting in a temperature of 390 degree C. The alumina to caustic (as Na ₂ CO ₃ ) weight ratio was maintained at 0.7. The digestion efficiency was found to be 86.47%

Example 13

Example 13 was carried out as per example 1, wherein 500 gm of bauxite was reacted with caustic. The reaction mass was subjected to microwave for 25 min, resulting in a temperature of 306 degree C. The alumina to caustic (as Na ₂ CO ₃ ) weight ratio was maintained at 0.7. The digestion efficiency was found to be 86.40%

Example 14

Example 14 was carried out as per example 1, wherein the reaction mass was subjected to microwave for 5 min, resulting in a temperature of 445 degree C. The alumina to caustic (as Na ₂ CO ₃ ) weight ratio was maintained at 0.7. The digestion efficiency was found to be 87.50%

Example 15 and 16 were carried out with laterite which has a higher silica content compared to bauxite. 40 gm of laterite ore was used with silica content of 6-8 wt %

Example 15

Example 15 was carried out as per example 1, wherein the reaction mass was subjected to microwave for 12 min, resulting in a temperature of 285 degree C. The alumina to caustic (as Na ₂ COa) weight ratio was maintained at 0.7. The digestion efficiency was found to be 69.88%

Example 16

Example 16 was carried out as per example 1, wherein the reaction mass was subjected to microwave for 8 min in single stroke, resulting in a temperature of 350 degree C. The alumina to caustic (as Na ₂ COs) weight ratio was maintained at 0.7. The digestion efficiency was found to be 74.04%

This shows that even high silica content; the ore can be subjected to microwave treatment with an alkali.

Example 17

Example 17 was carried out as per example 1, wherein the reaction mass was obtained by heating caustic (in solid form) with the ore at a temperature of 290 - 320 degree C to form a intermediate product as said in example 1. This intermediate product was subjected to microwave for 9 min, resulting in a temperature of 400 degree C. The alumina to caustic (as Na ₂ COs)

weight ratio was maintained at 0.7. The digestion efficiency was found to be 95.12%

While considerable emphasis has been placed herein on the specific steps of the preferred embodiment, it will be appreciated that many alterations can be made and that many modifications can be made in the preferred embodiment without departing from the principles of the invention. These and other changes in the preferred embodiment as well as other embodiments of the invention will be apparent to those skilled in the art from the disclosure herein, whereby it is to be distinctly understood that the foregoing descriptive matter is to be interpreted merely as illustrative of the invention and not as a limitation.