WO2009066308A2

WO2009066308A2 - Extraction of alumina

Info

Publication number: WO2009066308A2
Application number: PCT/IN2008/000442
Authority: WO
Inventors: Mickey Puri Prashant; Sadguru Kulkarni; Sanjaya Mohapatra; Anant Samdani; Rahul Jadhav; Mayur Kapse; Seema Salvi
Original assignee: Aditya Birla Science & Technology Co. Ltd; Hindalgo Industries Ltd.
Priority date: 2007-07-09
Filing date: 2008-07-09
Publication date: 2009-05-28
Also published as: WO2009066308A3; AR074034A1; PE20100332A1; TW200909356A

Abstract

A process for extraction of alumina from aluminum containing ores is disclosed. The process comprises the following steps, determining the alumina and silica content in the ore; treating the ore with caustic and heat, leaching the intermediate mass so produced with water to obtain a filtrate containing aluminate and silicate, vanadate and other soluble impurities; separating silicate and vanadate and other impurities as residue from the filtrate to obtain beneficiated aluminate liquor; and extracting alumina from the beneficiated aluminate liquor by a conventional method.

Description

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 and prior art:

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 solution at elevated temperatures to extract available alumina from the ore producing slurry containing enriched caustic aluminate liquor called as green liquor, which is supersaturated with alumina and an ore residue called as red mud. Then the slurry from the digester is flash cooled and subjected to clarification and/or filtration operations for separating the supersaturated liquor and insoluble ore residue. The ore residue or red mud is washed to recover the entrained sodium 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. The filtrate having some dissolved alumina after separating precipitated alumina trihydrate is reused and it is known as spent liquor. The precipitated alumina trihydrate is filtered, washed with water and calcined at about 1000 deg C to 1200 deg C to produce pure alumina.

Alumina extraction efficiency 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 the number of water of hydration molecules 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, etc.) are set according to the composition of bauxite. Ores with high gibbsite content can be digested at 140 deg C, where as boehmite requires more than 240 deg C. Although higher temperatures and high caustic concentrations are often theoretically advantageous for maximum digestion efficiency, there are several disadvantages like corrosion; impurities due to reaction of bauxite components such as quartz, reactive silica, vanadate and the like and energy cost.

Therefore low temperature digestion of 100 to 150 deg C is utilized for bauxite containing less than 5% alumina monohydrate. However, a high temperature digestion unit operating at 200 to 300 deg C is employed if the bauxite contains more than 6% alumina monohydrate. Preferably, bauxite rich in the Gibbsite content is used. However, due to its depletion the ore are becoming richer in Boehmite phase. It is a challenge for the alumina industry to reduce the cost of alumina production from bauxite. This is partly being attempted by beneficiation of the ore at the mine itself to reduce transportation and waste handling cost. There are few attempts made to minmise the process cost by fine tuning the process parameter for digestion and desilication.

Thus in some plant 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 Boehmite, which is known as double digestion.

International application no: PCT/AU95/00526 describes process for improvements in double digestion to provide improved extraction of alumina from mixed gibbsitic/boehmitic bauxite using acceptable processing conditions, whilst minimizing the detrimental effects of DSP formation, including losses of caustic and uncontrolled scale formation. US patent 5112349 describes a process improvement for extracting alumina from gibbsitic bauxites only by maintaining reactive hydroxide ion concentration of liquid phase of the slurry leaving the digested liquor at below 6gm/ltr, most preferably in between 0-2 gm/ltr.

Another important step in alumina extraction process is known as desilication. Generally in most plants pre-desilication is carried out, where bauxite is held at a temperature of lOOdegC for 6 to 18hrs. The purpose is to convert a large portion of reactive silica to sodalite type sodium alumino silicate, which will than act as a seed to rapidly convert the remaining reactive silica to sodalite type sodium aluminum silicate during digestion. The condition under which pre-desilication is conducted is low caustic and alumina concentration and also to ensure that only small portion of total reactive silica is in solution at any given time. Some plants also carry post desilication, where the solid silicate materials are seeded to get silica out as insoluble materials. Some processes follow both pre desilication and post desilication. AU the desilication process is carried out typically at 70 - 100 deg C for long hours of 8 to 20 hrs. The known plant processes have separate individual processes for removing impurities such as reactive silica, fluoride, vanadium, phosphates, organics and the like

Patent No. DE4036448 discloses a process for alumina ore digestion in a ball mill for fine grinding, leaching and silica removal, without A12O3 and Na2O losses during silica removal and red mud formation. Grinding is carried out at 88-92 deg C with caustic soda and lime in an amount corresponding to the stoichiometric amount of 1.8-2.2 mole of lime per mole of SiO2. The process is suitable only for low temperature digestion for gibbsitic bauxite.

US Patent 4518571 describes a process consists of two desilication stages, In the first stage the aluminate liquor resulting from digestion of the sinter is subjected to an autoclave treatment at 150 -170 deg C, under 6 to 12 atmospheric pressure to remove major portion of impurities of SiO2 and Fe2O3. In the second stage the aluminate liquor is treated with a desilication agent comprising a lime pulp containing oxides CaO, A12O3, SiO2, Fe2O3, and Na₂O. The two step desilication step is not process-friendly and it also consumes lots of time.

There is hardly any major successful attempt to reduce the alumina processing cost significantly from the day the Austrian chemist KARL JOSEF BAYER received a German patent 43977 in August, 1888. The main feature of the Bayer process has remained unchanged for more than 100 years, although the scale of operation has been enlarged considerably due to advances in chemical engineering.

The production cost of alumina mainly depends on the cost of energy, caustic and ore. In the present day plant process

1. Lots of energy is lost in both dry & wet grinding of bauxite ore. 2. Energy, aluminum& soda is also lost during pre-desilication which is carried out normally at 70- 100 deg C for long hours (8-20 hrs or more) prior to digestion.

3. Energy is lost during the digestion process for unnecessarily heating lots of water (about more than 60% wt of total digester composition). Use of diluted caustic solution contributes to the major water in the digester before digestion. Dilute caustic is less effective compared to concentrated caustic. Water added for washing red mud and in the precipitation process also has a negative impact.

4. There is huge loss of caustic with red mud due to the formation of various sodium salts other than sodium aluminate such as vanadate, silicate, phosphate, etc, which is present in bauxite.

The present invention overcomes at least partially the above shortcomings of the present plant process by envisaging a novel digestion and desilication process.

Objects of the invention

One of the objects of the invention is to provide a process for extraction of alumina with more alumina extraction compared to prior art processes.

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

Yet another object of the invention is to provide a process for extraction of alumina having decreased losses of soda and aluminum with the red mud. Yet another object of the invention is to provide a process for extraction of alumina with less amount of caustic.

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

Still another object of the invention is to provide a process for extraction of alumina where the impurities are separated in a single step.

Summary of the invention

In accordance of 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;

treating the ore with caustic at a temperature of 280 - 350 degree C and at a pressure of 1- 150 atm , optionally with the addition of an additive (I) to form an intermediate mass including aluminate, silicate, vanadate and other 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 and silicate, vanadate and other soluble impurities; separating silicate and vanadate and other impurities as residue from the filtrate by heating with the additive (II) at a temperature of 50 - 350 degree C and cooling to 4 - 300 degree C to obtain beneficiated aluminate liquor;and

extracting alumina from the beneficiated aluminate liquor by a conventional method.

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

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

Typically, the weight ratio of alumina in ore to caustic expressed as sodium carbonate is in a ratio between 1:0.1 and 1: 3.0. Typically, the caustic is in the form of a solid.

Alternatively, the caustic is in a form of vapor.

Preferably, the caustic is in form of slurry.

Preferably, the caustic is in the form of molten liquid .

Typically, the caustic is in the range of 50 - 99 % of the mass of the caustic slurry. Typically, the said additive (I) is selected from a group consisting of alkaline earth metals.

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

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

Typically, the water used for leaching is preferably between 4 degree C and 150 degree C.

Typically, the mole ratio of the total amount of silica in the filtrate to the additive (II) is in the range of 1 : 0.5 to 1 : 6.0.

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

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

Description of accompanying drawings

FIG 1: is a flow diagram of the process in the prior art.

FIG 2: is a flow diagram of the process in accordance with the invention.

FIG 3: is the XRD image of bauxite ore sample. FIG 4: is the XRD image of the red mud as per the prior art process.

FIG 5: is the XRD image of the red mud as per the process in accordance with the present invention.

FIG 6: is the XRD image of the impurities in the aluminate liquor obtained by the process of the present invention.

FIG 7: is the graphical representation of silica content in the aluminate liquor and residue obtained thereof in y-axis at various temperatures on x- axis; and

FIG 8: is a graphical representation of the affect of additives, silica content in the aluminate liquor and residue obtained thereof in y- axis to various set of experiments consisting of varying reaction time, temperature and amount additive (II) in x-axis.

Detailed description of the invention

In the normal plant process bauxite digestion or heating is carried out at 150 to 265 deg C for 20 - 60 min depending on the type of bauxite and amount of solid content of material present in the digester. Typically, in the normal plant process, the solid content of digestion is around 40-50% by mass. In the process according to the present invention, digestion is carried out at more than 150 deg C and more preferably at a melting point of sodium hydroxide that is in the range of 300 -320 deg C for about 5- 30 minute depending on the type of bauxite and the amount of solid content of the material present in the digester. The process of digestion in accordance with this invention is more than 60% and preferably in between 85 to 100%. In the present process digestion efficiency is more than the normal plant process using the same or less additive. In the present case caustic along with 0% to 10% (wt) lime is used during heating. Lime not only the increases the digestion efficiency but also decreases the sodium content in the red mud of the invented process.

With reference to FIG 1, in a normal plant process bauxite ore is dry ground and crushed and then wet ground with 25% sodium hydroxide solution in a ball mill for 12 hr at 90 deg C to about 60 mesh, where silica is removed. This is known as predesilication. Then predesilicated slurry is charged to a digester then remaining caustic is added to the slurry to form a mixture. The mixture in the digester are heated is 265 deg C for 30 minutes and then cooled. Red mud is separated by filtration and is washed to recover a small amount of alumina. The filtrate is pure sodium aluminate liquor contains the alumina. This is further sent for aluminum trihydrate precipitation.

In the preferred embodiment of the 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.

Referring to FIG 2, the ore is dry ground to about 60 mesh. The ground ore is taken in a reaction vessel and treated with a caustic preferably, with sodium hydroxide powder of purity 95% to form a reaction mass. The caustic is selected from a 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 1:0.1 to 1: 3.0. To this mixture 95 % of lime is added to the reaction mass as additive (I) along with or without water and heated to a temperature of 280 - 350 degree C for 10 min to form an intermediate mass containing sodium aluminate, sodium silicate and vanadium along with other impurities as red mud. 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 separate sodium aluminate from 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 benefϊciated aluminate liquor. Typically, the additive (II) is selected from a 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 the filtrate to the additive (II) to is in the range of 1 :0.5 to 1 :6.0.

The beneficiated aluminate liquor is further sent for alumina extraction by conventional process. In a conventional process alumina is further sent for aluminium trihydrate precipitation. This is subjected to calcination to extract alumina. Major benefits of the present process over the normal plant process are given below

1) Higher alumina extraction using same or less caustic.

2) Caustic soda loss with red mud is less.

3) All impurities such as vanadate, phosphate, carbonate, and the like are removed in one go during post desilication and before alumina precipitation.

4) No separate process is required for treating spent liquor for the removal of vanadium & other impurities.

5) Caustic loss with impurities are less.

6) Relatively pure alumina can be obtained as it will be precipitated from pure sodium aluminate solution.

7) Highly concentrated sodium aluminate liquor, which is beneficial for faster precipitation, can be obtained by adjusting the leaching & washing water amount.

8) The amount of red mud generated per ton of bauxite is low and the amount of caustic and aluminum present is also low, therefore it is more eco-fiϊendly and more suitable for further processing to make valuable product for iron extraction, titanium oxide extraction, cement making & tile making and the like.

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

Example 1 was carried out as per the existing knowledge. 100 gm of bauxite ore was determined for alumina and silicate content. The ore contained 50% (wt) alumina and 2.5% (wt) silica. The ore was dry ground and then wet ground with 150 gm of 25% sodium hydroxide solution in a ball mill for 12 hr at 90 deg C to about 60 mesh, to separate silica. The ground slurry of ore and sodium hydroxide was charged to a reaction vessel to form a reaction mass. Further 38.5 gm of 50 % sodium hydroxide solution was added to reaction mass. The mass was heated to 265 deg C for 30 minute and then was cooled to 60 degree C. The alumina to caustic (as Na₂CO₃) weight ratio was maintained at 0.7. Insoluble red mud was separated as a residue from the intermediate product by filtration to obtain aluminums liquor as filtrate. Red mud was then washed by 30.75 gm of water. The red mud contained 8 gm of alumina. 265 gm of filtrate obtained contained 42 gm of alumina. The filtrate, containing sodium aluminate was sent for alumina extraction by a conventional process. The total alumina extraction efficiency from the ore was found to be 84%.

Example 2

Example 2 was carried in accordance with this invention. 100 gm of Bauxite ore was determined for its alumina and silicate content. The ore contained 50% (wt) of alumina and 2.5 % (wt) silicate. The ore was dry ground to about 60 mesh. The ground ore was taken in a reaction vessel and heated with 49.65 gm sodium hydroxide powder of purity 95% to form a reaction mass. 5 gm of 95 % of lime as additive (I) was added to the reaction mass as additive along with 15 gm of water and was heated to a temperature of 300 degree C for 10 min to form an intermediate product containing sodium aluminate and sodium silicate. The alumina to caustic (as Na₂COs) weight ratio was maintained at 0.8. The intermediate product was leached using 145 gm of hot water to separate soluble sodium aluminate 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. the residue contained 2.75 gm of alumina. 270 gm of aluminate liquor that is the filtrate obtained was reacted with 4 gm of lime as additive (II) at 200 degree C for 30 minutes to remove silica, vanadium and other impurities. It was then cooled to 60 degree C and then impurities like silicate, vanadate were separated as residue. 1.26 gm of alumina was found in these impurities. The total alumina in pure benefϊciated aluminate liquor was found to be 46gm and was further sent for alumina extraction by conventional process. The alumina extraction efficiency was found to be 92 %.

Examples 1 and 2 shows that in the present invented process, 7.1 gm (12.5% by wt) less caustic was used compared to prior art process and 4gm (8% by wt) more alumina was extracted. The higher extraction efficiency in case of bauxite processed as per process of the present invention compared to prior art process is seen from XRD of bauxite, as shown in Fig 3, 4 and 5 where the gibbsitic, boehmitic and diasporic peak for red mud of the process of present invention is much smaller than the red mud of prior art process. 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.

This signifies the presence of these phases in red mud of process of present invention is less compared to that of bauxite and red mud of prior art process. This shows that unlike the prior art process there is no need for a separate process for separation of vanadium & other impurities after alumina precipitation from sodium aluminate liquor. Vanadium can be removed during desilication process with silica & other impurities before alumina precipitation from pure sodium aluminate liquor in invented process. Presence of vanadate and silicate phases can be seen in XRD as shown in Fig 6 in invented process. The chemical analysis of the residue was carried out and was found to be Ca(OH)₂=44.5%; A1₂O₃.3H2O=22.5%; V2O₅=5%; SiO₂=I 3.5%; NaOH=0.5%; Water=12%; Others=2% of total weight.

The process in Example 3 and 4 were carried out with 50 gm of bauxite with an alumina content of 46.7 wt % and silica of 2.5 wt %

Example 3

The process in Example 3 was carried out as per example 2, wherein the alumina to caustic (expressed as Na₂COs) wt ratio was maintained at 0.70 and the reaction temperature was maintained at 245 degree C without the addition of additive (I) to reaction mass. Alumina extraction efficiency was found to be 85.78%.

Example 4

The process in Example 4 was carried out as per example 2, wherein the alumina to caustic (expressed as Na₂CO₃) wt ratio was maintained at 0.70 and the reaction temperature was maintained at 300 degree C without the addition of additive (I) to reaction mass. Alumina extraction efficiency was found to be 94.75%. Examples 3 and 4 shows that percentage of alumina extraction efficiency calculated from aluminum (Al) content in red mud is higher for reaction temperature at 300 degree C compared to 245 degree C.

Example 5

The process in Example 5 was carried out as per example 2, wherein 50 gm of bauxite having alumina of 50.7% and silica of 2.66% was reacted with 23.69 sodium hydroxide of 95% purity, 7.5 gm of water and varying amount of lime (as shown in table 2) in the reaction vessel for 10 minutes at 300 deg C. The alumina to caustic (expressed as Na₂COa) ratio was maintained at 0.85.

Table 2

The results in table 2 shows that the alumina extraction efficiency is more when weight ratio of lime (95%) to ore is in the range of 0.04 to 0.07 and CaO: SiO2 mole ratio is in the range of 1.4 to 2.5.

Examples 6, 7 and 8 with laterite samples

Generally laterite the low grade gibbsitic bauxite has high silica content. The ore is not used in the normal prior art process plants due to high content of silica. Extraction of alumina from laterite was carried out by both the prior art process and as well as the process in accordance with this invention, wherein 50 gm of laterite with an alumina content of 45.64 wt % and silica of 6 wt % was used. 3.72 gm of lime was added as additive (I).

Example 6

Example 6 was carried out as per example 2, wherein the alumina to caustic (expessed as Na₂COs )wt ratio was maintained at 0.85. The percentage of alumina extraction efficiency calculated from aluminium (Al) present in

25.5 gm of red mud was found to be 83.50%.

Example 7

Example 7 was carried out as per example 2, wherein the alumina to caustic (expessed as Na₂CO₃ )wt ratio was maintained at 0.70. The percentage of alumina extraction efficiency calculated from aluminium (Al) present in

23.6 gm of red mud was found to be 87.83%.

Example 8

Example 8 was carried out as per example 1, wherein the alumina to caustic (expessed as Na₂CO₃ )wt ratio was maintained at 0.70. The percentage of alumina extraction efficiency calculated from aluminium (Al) present in 27.7 gm of red mud was found to be 70%.

Examples 6, 7 and 8 shows percentage of alumina extraction efficiency- calculated from aluminium (Al) present in the red mud of laterite sample is much higher in process of the present invention compared to prior art process.

Example 9, 10 and 11 were carried out with 50 gm of bauxite with an alumina content of 50 wt % and silica of 2 wt % without the addition of additive (I)

Example 9

The process in Example 9 was carried out as per example 2, wherein the alumina to caustic ratio (calculated as Alumna (wt)/Caustic (wt) as Na₂COs) was maintained at 0.70. Percentage of alumina extraction efficiency was calculated from aluminium (Al) present in 23.6 gm of red mud and was found to be 94.70%.

Example 10

The process in Example 10 was carried out as per example 2, wherein the alumina to caustic ratio (calculated as Alumna (wt)/Caustic (wt) as Na2CO3) was maintained at 0.80. Percentage of alumina extraction efficiency was calculated from aluminium (Al) present in 23.6 gm of red mud and was found to be 93.07% Example 11

The process in Example 11 was carried out as per example 2, wherein the alumina to caustic ratio (calculated as Alumna (wt)/Caustic (wt) as Na2CO3) was maintained at 0.97. Percentage of alumina extraction efficiency was calculated from aluminium (Al) present in 23.6 gm of red mud and was found to be 78.79 %

Examples 9, 10 and 11 shows that decrease in alumina to caustic ratio increase the extraction efficiency.

The process in Example 12, 13 and 14 were carried out with 50 gm of bauxite with an alumina content of 45 wt % and silica of 2.5 wt % without the addition of additive (I)₅ wherein the alumina to caustic ratio (calculated as Alumna (wt)/Caustic (wt) as Na2CO3) was maintained at 0.70.

Example 12

The process in Example 12 was carried out as per example 2, wherein the reaction time was in the first reaction vessel was 10 min. Percentage of alumina extraction efficiency was calculated from aluminum (Al) present in 17 gm of red mud and was found to be 93.03 %

Example 13

The process in Example 13 was carried out as per example 2, wherein the reaction time was in the first reaction vessel was 20 min. Percentage of alumina extraction efficiency was calculated from aluminum (Al) present in 21 gm of red mud and was found to be 84.88 % Example 14

The process in Example 14 was carried out as per example 2, wherein the reaction time was in the first reaction vessel was 60 min. Percentage of alumina extraction efficiency was calculated from aluminum (Al) present in 25 gm of red mud and was found to be 78.13 %

The process in Examples 12, 13 and 14 show that digestion carried out at 300 deg C for 10 min has more extraction efficiency than experiments carried out for more time. This is due to precipitation of dissolved alumina with red mud in the form of desilicated product.

The process of Example 15, 16 and 17 were carried out with 50 gm of bauxite with an alumina content of 47 wt % and silica of 2.5 wt % without the addition of additive (I), wherein the alumina to caustic ratio (calculated as Alumna (wt)/Caustic (wt) as Na2CO3) was maintained at 0.70.

Example 15

Example 15 was carried out as per example 2, wherein the reaction water level in the first reaction vessel was 0 and leaching water level was 90 gm and 10 gm water was used for red mud washing. Percentage of alumina extraction efficiency was calculated from aluminum (Al) present in 19.60 gm of red mud and was found to be 87.70 %

Example 16

Example 16 was carried out as per example 2, wherein the reaction water level in the first reaction vessel was 7.5 gm and leaching water was 82.5 gm and 10 gm water was used for red mud washing. Percentage of alumina extraction efficiency was calculated from aluminum (Al) present in 17.20 gm of red mud and was found to be 94.50 %

Example 17

Example 17 was carried out as per example 2, wherein the reaction water level in the first reaction vessel was 21.5 gm and leaching water level was 68.5 gm and 10 gm water was used for red mud washing. Percentage of alumina extraction efficiency was calculated from aluminum (Al) present in 16.60 gm of red mud and was found to be 91.77 %

Examples 15, 16 and 17 show that extraction efficiency can be maximized by adjusting water amount during digestion.

Example 18

The process in Example 18 was carried out as per example 2, wherein the filtrate obtained after the removal of red mud having silica content 4gm/ltr was heated to various temperatures for 1 minute and then was cooled to below 50 deg C. Residue containing mainly silicates were filtered out. Results are as shown in table 3 for dry wt of residue obtained and amount of silica present in filtrate.

Table 3

22

It was found that the removal of silica is increased by heating the liquor to a high temperature, under pressure without adding any additive during the desilication. It can be observed that there is a decrease in silica content of liquor at higher temperature (300 degree C ) as shown in FIG 7.

Example 19

Example 19 was carried out as per example 2, wherein the filtrate obtained after the removal of red mud having silica content 4grn/ltr was reacted with different amount of lime to various temperatures, with varying reaction time and then it was cooled below 50 deg C. Residue containing mainly silicates were filtered out. Results are as shown in table 4 for dry wt of residue obtained and amount of silica present in filtrate.

Table 4

It was found that the silica in liquor can be controlled or eliminated by maintaining the right amount of lime. It can be observed that there is absolute zero silica content in aluminate liquor at higher temperature with the addition of the additive (II) as shown in FIG 8. 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.

Claims

Claims:

1. 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;

treating the ore with caustic at a temperature of 280 - 350 degree C and at a pressure of 1- 150 atm , optionally with the addition of an additive (I) to form an intermediate mass including aluminate and silicate, vanadate and other 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 and silicate, vanadate and other soluble impurities;

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

2. A process as claimed in claim 1, wherein the caustic is selected from the group of alkali metals.

3. A process as claimed in claim 1, wherein the caustic is selected from oxides, hydroxides, carbonates of sodium, potassium and lithium.

4. A process as claimed in claim 1, wherein the weight ratio of alumina in the ore to caustic expressed as sodium carbonate is in a ratio between 1 :0.1 and 1: 3.0.

5. A process as claimed in claim 1, wherein the mass of alumina in the ore to caustic expressed as sodium carbonate is in a ratio between 1 :0.7 and 1 :1.

6. A process as claimed in claim 1, wherein the caustic is in form of a solid.

7. A process as claimed in claim 1, wherein the caustic is in form of a slurry.

8. A process as claimed in claim 1, wherein the caustic is 50 - 99 % caustic slurry.

9. A process as claimed in claim 1, wherein the caustic is the form of a vapor.

10. A process as claimed in claim 1, wherein the caustic is in the form of a molten liquid.

1 LA process as claimed in claim 1, wherein the additive (I) is selected from a group of alkaline earth metals.

12.A process as claimed in claim 1, wherein the additive (I) is selected from oxides, hydroxides, carbonates of calcium, barium, strontium and magnesium.

13. A process as claimed in claim 1, wherein the additive (I) is in the range of 0 - 20 % of the total weight of ore.

14.A process as claimed in claim 1, wherein the water used for leaching is at a temperature of 4 degree C to 150 degree C.

15.A process as claimed in claim 1, wherein the mole ratio of total amount of silica in filtrate to additive (II) to is in the range of 1 :0.5 to 1 :6.0.

16.A process as claimed in claim 1, wherein the additive (II) is selected from group of alkaline earth metals.

17.A process as claimed in claim 1, wherein the additive (II) is selected from oxides, hydroxides and carbonates of calcium, barium, strontium and magnesium.