Description:Field of Invention
The present invention relates to a process for reusing ground sand in the production of alumina.

Background of Invention
Aluminum (Al) is produced from its ore, bauxite. The chemical formula for bauxite is Al2O3.3H2O (gibbsite).

One of the known processes for extraction of alumina (Al2O3) from bauxite is the Bayer’s process. In Bayer’s process, bauxite (ore of aluminum) is ground and mixed with caustic soda and further subjected to aluminate liquor through digestion at temperatures as high as 245 °C.

Bauxite grinding is one of the major bottlenecks for alumina refineries which often results due to the deterioration of bauxite quality. As a result of bauxite quality deterioration, the crushing and grinding sections are usually stretched to their limit for coarser grinding of bauxite which leads to higher sand generation than normal. This higher sand generation leads to alumina losses across the bauxite residue washing circuit and also leads to failure of decanter rake mechanism, thereby increasing the caustic losses with bauxite residue.

In order to reduce the loss of aluminum from the bauxite ore, while extracting aluminum from bauxite ore, many steps have been proposed to modify Bayer’s process.

US4483830A discloses that loss of caustic soda and alumina are directly proportional to the amount of reactive silica contained within the bauxite. Thus, while processing bauxites of increased reactive silica content to recover alumina, the cost of the product alumina increases by three times. This is due to the lower effective yield of alumina available from the bauxite and the larger quantities of caustic soda which are required to be added. Because of these costs there is a limit to the amount of reactive silica allowable in bauxite such that it may be economically processed by the Bayer’s process.

Further the solutions available are limited to procuring high grade bauxites, which are softer and thereby lead to minimal sand generation. However, considering the availability and cost of the high-grade bauxites, this is not a feasible option. Secondly there are chemicals such as bauxite grinding aids, which help in processing of hard bauxite to grind thereby reducing the sand generation. However, this does not lead to higher chemical extraction and also the operating cost is high due to higher dosage and cost of chemical.

There is, therefore, a need to provide a process for reusing ground sand in the production of alumina so as to avoid additional raw materials or increasing production cost of aluminum.

Summary of the Invention
The present invention relates to a process for reusing ground sand in the production of alumina, the process comprises of adding ground sand to bauxite during pre-desilication to form a feed slurry at a predetermined temperature for recovering residual gibbsite and de-silicating the fed slurry at a predetermined temperature and predetermined time.

Figures of the Invention
Figure 1 is a flow chart diagram showing the process for reusing ground sand in the production of alumina.

Detailed Description of the Invention
The term ‘bauxite’ refers to a sedimentary rock with a relatively high aluminium content in different forms.

The term ‘gibbsite’ refers to a form of alumina, Al(OH)3, and it is one of three main phases that make up the rock bauxite.

The term ‘ground sand’ refers to sand obtained as a side-product after grinding and filtering bauxite.

The terms residual gibbsite, bauxite residue, gibbsite residue and residual bauxite as used herein refer to the gibbsite present in the ground sand. The said terms can be used interchangeably.

The term ‘red mud’ or ‘mud’ refers to the mud obtained after desilication of the bauxite slurry and is generally a side-product.

‘Bauxite residue factor’ and ‘mud factor’ are calculated as follows:
BRF = Fe2O3 in bauxite / Fe2O3 in Mud
The said terms can be used interchangeably.

The term ‘further’ as used herein refers to the additional steps for the process of production of alumina.

The term ‘less than’ as used herein refers to the amount of a component present in maximum quantity.

The present invention relates to a process for reusing ground sand in the production of alumina, the process comprises of adding ground sand to bauxite during pre-desilication to form a feed slurry at a predetermined temperature for recovering residual gibbsite and de-silicating the fed slurry at a predetermined temperature and time.

Bauxite rock is ground and filtered to prepare filtered bauxite, preferably a mixture of bauxite ore and sand is ground followed by filtering to obtain a filtered bauxite.

Ground sand is obtained from the filtered bauxite. Ground sand comprises of residual gibbsite. In other words, ground sand is obtained by grinding and filtering bauxite comprising gibbsite. Ground sand has residual gibbsite varying from 5% to 8%.

A feed slurry is formed by adding ground sand to the filtered bauxite in the presence of sodium hydroxide.

The feed slurry is processed to pre-desilication (PDS) step. The temperature of the feed slurry is in a range from 78°C to 79 °C.

Preferably, the ground sand is added to the slurry at approximately 10% to 15% of bauxite weight during pre-desilication. Preferably, the amount of ground sand added is in a range from 10% to 15% of the bauxite charged to desilication. Addition of ground sand to pre-desilication leads to a reduction in the total quantity of bauxite residue generated. This is mainly because the quantity of ground sand added to the process gets adjusted against the bauxite charge to ball mill. Ground sand added to the bauxite charged to desilication leads to an increase in the chemical extraction of alumina.

The pre-desilication step seeks to transform reactive silica in bauxite into desilication product (DSP) prior to digestion, thereby reducing post-digestion precipitation and scaling.
In an embodiment of the invention, the addition of ground sand to bauxite during pre-desilication to form a feed slurry is done at a predetermined temperature in the range of 85 oC to 95 oC, preferably at 78 oC to 79 oC for recovering residual gibbsite.

The desilication of the fed slurry is carried out at a predetermined temperature from 80 °C to 90 °C and time in a range from 12 hours to 16 hours in presence of lime: 1:6 moles of lime as CaO per mole of P2O5 in bauxite.

Figure 1 is a flow chart diagram showing the process for reusing ground sand in the production of alumina.

The de-silicated slurry comprising bauxite and ground sand is digested under low temperature digestion conditions, preferably, in a range from 138 °C to 145 °C. Addition of ground sand to pre-desilication followed by desilication and digestion leads to reduction in silica content of liquor. Preferably, the silica content in the slurry is reduced from 1.8 g/l to less than 0.9 g/l. Digestion of ground sand also leads to reduction in caustic losses by elimination of washer failures. Recovery of entire residual gibbsite from ground sand is also obtained after digestion.

The digested slurry is treated with flocculants selected from polyacrylate, hydroxy-polyacrylamide or combination thereof which is followed by filtration to obtain aluminum.

The residual gibbsite in ground sand is thus extracted fully, and therefore it does not contribute significantly to the overall load to thickeners / washers. Hence the bauxite residue generation to thickener / washers also comes down. The bauxite residue factor is reduced by 5% to 10%.

In an aspect, the process comprises.
i. grinding of bauxite comprising gibbsite to form ground bauxite.
ii. filtering the ground bauxite obtained in step (i) to provide filtered bauxite and ground sand comprising residual gibbsite.
iii. processing the filtered bauxite to pre-desilication.
iv. adding ground sand to the filtered bauxite during pre-desilication to form a feed slurry.
v. heating the feed slurry to a temperature in the range from 78 °C to 79 °C.
vi. de-silicating the fed slurry at a temperature in a range from 80 °C to 90 °C and time in a range from 12 hours to 16 hours; and
vii. digesting and treating the fed slurry with flocculants.

The present invention is useful for the alumina refineries processing low grade bauxites even with higher sand generation without causing any impact on plant operations. The process is useful to recover residual gibbsite from ground sand leading to higher chemical extraction instead of dumping the sand in bauxite residue disposal area (BRDA).

The process provides an increase in the amount of aluminum obtained by at least 10 %, without increasing the cost of extraction.

Example
Bauxite samples were collected on different days from the sand classifier outlet. The samples were analyzed for their elemental composition which is shown in Table 1 below.

Table 1: elemental analysis of different sand samples:
Samples Sample 1
(%) Sample 2
(%) Sample 3
(%) Sample 4
(%)
Loss of Mass (LOM) 13.2 12.2 13.4 12.6
Elemental analysis by X-ray Fluorescence (XRF)
SiO2 7.83 8.26 6.78 7.62
TiO2 3.29 3.40 3.67 3.62
Fe2O3 48.7 50.7 43.4 44.6
Al2O3 22.7 20.8 25.4 23.3
V2O5 0.29 0.32 0.33 0.32
P2O5 0.24 0.24 0.22 0.25
CaO 0.28 0.52 0.44 0.64
Phase Analysis %
Al2O3 (g) by DSC 7.50 5.00 5.50 8.33

Sample 1 was ground and filtered to obtain Ground sand which comprised residual gibbsite in a range from 5-8 %. Bauxite to be charged for achieving target ratio of 0.690 was calculated. Ground sand was added to the filtered Sample 1 to form a feed slurry. Ground Sand was added at 12% of the bauxite weight taken for predesilication. The feed slurry comprising bauxite and sand sample is represented as Example 1.

Analysis of Sample 1, ground sand and Example 1 separately are given below in Table 2.

Table 2: elemental analysis of ground sand, Sample 1 and Example 1
Ground sand Sample 1 Example 1 Weighted avg. (sample 1 100%, ground sand 12%)
Loss of Mass LOM 21.8% 13.2% 20.9%
Elemental analysis by X-ray Fluorescence (XRF)
SiO2 5.61% 7.83% 5.85%
TiO2 4.37% 3.29% 4.25%
Fe2O3 25.0% 48.7% 27.5%
Al2O3 42.3% 22.7% 40.2%
V2O5 0.16% 0.29% 0.17%
P2O5 0.22% 0.24% 0.23%
CaO 0.02% 0.28% 0.05%
Na2O 0.0% 2.46% 0.26%
Phase Analysis
Al150 30.7% 7.50%* 28.21%
Si150 5.13% 5.87% 5.21%
*Al2O3 (g) by DSC

Liquor Analysis
Mixed liquor was collected, kept overnight, decanted and added in Example 1. Thus, a feed slurry was formed. Analysis of mixed liquor is given in Table 3:

Table 3:
Sample 5
Caustic concentration (NaOH) 244.5 g/L
TTS (Total Titratable Soda) 286.4 g/L
Causticity 85.4%
Al2O3 concentration 89.5 g/L
A/C ratio
(Aluminum / Caustic soda (NaOH) ratio) 0.366
SiO2 concentration 0.84 g/L

Low Temperature Digestion Test

To evaluate the feasibility of extracting the residual gibbsite in ground sand, low temperature digestion tests were conducted without and with ground sand addition to the slurry. Also, the effect of sand addition on the settling behavior under liquor decanter conditions was studied as follows:

Low temperature digestion tests with ground sand addition (Example 1) and without ground sand addition (Example 2) were carried out under the following conditions:

The tests were carried out under two conditions twice - ‘without ground sand addition (Example 2) and with ground sand addition (Example 1)’.

Predesilication and Desilication
The feed slurry comprising bauxite and ground sand was heated to a temperature in a range from 78 °C to 79 °C. During de-silicating, the temperature was increased to 90 °C, and the desilication was carried out for a time period of 16 hours in presence of lime: 1:6 moles of lime as CaO per mole of P2O5 in bauxite.

Digestion
Temperature of the slurry was increased to 145 °C and the slurry was digested for a time period of 30 minutes. Further, a combination of polyacrylate and hydroxy polyacrylamide as flocculants were added.

Results of low temperature digestion are enclosed in Table 4.

Table 4: Results of Low Temperature Digestion Studies
Bauxite Size -35 mesh
Charging A/C Ratio
(Alumina (A) to Caustic (C) ratio) 0.690 0.701
Sample number and Test number Without sand (Example 2) With sand
(Example 1)
Test 1 Test 2 Test 1 Test 2
Analysis Blow-off liquor Caustic concentration, g/L 217.5 216.1 215.6 215.7
Total Titratable Soda, g/L 263.0 260.5 261.8 263.2
Causticity, % 82.7 83.0 82.4 82.0
Alumina concentration, g/L 147.6 148.4 152.6 151.1
A/C ratio 0.679 0.687 0.708 0.701
SiO2, g/L 0.61 0.65 0.65 0.62
Mud Loss of Mass
LOM, % 9.90 9.90 9.84 9.86
SiO2, % 9.42 9.39 9.43 9.32
TiO2, % 7.29 7.41 6.76 6.76
Fe2O3, % 45.8 45.5 46.9 47.2
Al2O3, % 20.4 20.5 20.0 19.7
Na2O, % 4.73 4.78 4.58 4.62
P2O5, % 0.27 0.28 0.29 0.30
V2O5, % 0.31 0.30 0.30 0.30
CaO, % 0.98 1.03 0.93 0.97
Results Mud Factor 0.54 0.55 0.59 0.58
Extraction, % 101.4 101.0 100.8 101.7
Silica solubility, g/L 0.51 0.51 0.52 0.51
Silica ratio 1.20 1.28 1.25 1.20
Silica supersaturation, g/L 0.10 0.14 0.13 0.11
Sodalite factor 0.86 0.87 0.88 0.88

From the above table, it can be observed that the amount of alumina in Example 1 is less as compared to Example 2. That is, the amount of alumina in blow off liquor is more in Example 1 than in Example 2. This shows that the residual gibbsite (alumina) from ground sand was extracted in Example 1 and therefore, the amount of alumina in Example 2 is less.

Usually, addition of ground sand leads to lower extraction of alumina due to an increase in the mud factor.

Settling Tests
The blow-off slurry generated under digestion conditions without and with ground sand addition was further distributed evenly in 1-litre cylinders and the settling test was conducted.

The results of the settling test performed are presented in Tables 5 and 6.

Table 5: Results of Low Temperature Settling Tests without Ground Sand Addition to Pre-desilication (Example 2)
Test no. Feed
Solids
Concentration

g/L Liquor
Caustic
Concentration

g/L Flocculant
Dosage
g/t of Mud Settling
Rate

m/h O/f (Overflow) Solids
after
30 minutes
mg/L Filtration
Rate

L/h Compaction
after
20 minutes

mL U/f (Underflow)
Solids
%
F-4995
(Flomin, flocculant) Hx-300
(hydroximate, flocculant)
1 84.0 240.0 12 190 1.4
2 12 214 5.4 126 6.7 180 28.7
3 12 224 10.8 158 6.7
4 13 190 8.5 172 7.2 170 30.2
5 13 200 15.6 116 8.2

Table 6: Results of Low Temperature Settling Tests with Ground Sand Addition to Pre-desilication (Example 1)
Test no. Feed
Solids
Concentration

g/L Liquor
Caustic
Concentration

g/L Flocculant
Dosage
g/t of Mud Settling
Rate

m/h O/f
(Overflow) Solids
after
30 minutes

mg/L Filtration
Rate

L/h Compaction
after
20 minutes

mL U/f
(Underflow)
Solids
%
F-4995
(Flomin, flocculant) Hx-300
(hydroximate, flocculant)
1 103.7 239.0 11 162 V. slow
2 12 174 2.7 110 6.7
3 12 193 1.6 116 8.2
4 15 174 5.9 178 7.2 210 30.5
5 15 185 5.1 140 7.2 210 31.0

From the above Tables 5 and 6, it can be observed that Example 1 (with ground sand addition to pre-desilication), showed only a marginal (negligible) change in the flocculent consumption, even at higher feed solids content. However, there was no effect on liquor filtration rate in Example 2 filtration.

Effect on Sand Cyclone / Mud Thickeners
Table 7 shows results of mud generated by Example 1:
Table 7: Effect of Ground Sand Addition in Pre-desilication (PDS) on Mud Load to Thickener / Washer
Without Ground Sand Addition to PDS With Ground Sand Addition to PDS
Bauxite charge t/h 100 Bauxite charge t/h 92
Ground sand t/h 12
THA (Trihydrate Alumina) in bauxite % 30.7 THA (Trihydrate Alumina) in bauxite % 30.7
k Silica in bauxite % 5.13 k Silica in bauxite % 5.13
THA in ground sand % 7.50
k silica in ground sand % 5.87
Extraction of alumina as gibbsite % 94 Extraction of alumina as gibbsite % 94
Weight average THA % 29.1
Weight average k silica % 5.4
Mud factor 0.56 Mud factor 0.59
Mud generation t/h 56 Mud generation t/h* 54
Sand generation t/h 11 Sand generation t/h 11
Production t/h 44 Production t/h 44
* The residual gibbsite in sand is extracted fully, thus not contributing to extra mud load.

From the above Tables 4 to 7, it can be inferred that by addition of ground sand to predesilication, the total quantity of mud generated for a given bauxite charge reduced. This was due to the quantity of ground sand added to the process, wherein the ground sand adjusted against the bauxite charge to ball mill and the residual gibbsite in ground sand was extracted fully, thus not contributing significantly to the overall mud load to thickeners / washers. In fact, the mud generation to thickener / washer was reduced/decreased.

Another factor which is to be considered is reduction in temperature profile across predesilication tank. Since the ground sand is at a temperature of ~78 °C to 79 °C, the temperature of fine slurry to digester comes down.

Thus, it can be concluded that this process allows complete extraction of residual gibbsite from ground sand along with extraction of gibbsite and decreasing the overall mud load to thickeners / washers.

This process is useful where sand generation is higher due to poor quality bauxite leading to a reduction in bauxite charge to ball mills. The reduction in bauxite charge can be compensated by the addition of ground sand to predesilication. Thus, the losses associated with higher sand generation can be minimized.

The foregoing description of the invention has been set merely to illustrate the invention and is not intended to be limiting. Since modifications of the disclosed embodiments incorporating the spirit and substance of the invention may occur to a person skilled in the art, the invention should be construed to include everything within the scope of the disclosure.
, Claims:We Claim:

1. A process for the production of alumina, the process comprising
adding ground sand to bauxite during pre-desilication to form a feed slurry at a predetermined temperature for recovering residual gibbsite and de-silicating the fed slurry at a predetermined temperature and time .

2. The process as claimed in claim 1, wherein the addition of ground sand to bauxite during pre-desilication to form a feed slurry is done at a temperature in the range of 85 oC to 95 oC, preferably at 78 oC to 79 oC for recovering residual gibbsite.

3. The process as claimed in claim 1, wherein desilication of the fed slurry is carried out at a predetermined temperature from 80 °C to 90 °C and time in a range from 12 hours to 16 hours in presence of lime: 1:6 moles of lime as CaO per mole of P2O5 in bauxite.

4. The process as claimed in claim 1, wherein the ground sand is obtained by grinding bauxite comprising gibbsite and filtering the same.

5. The process as claimed in claim 1, wherein the process further comprises of digesting the de-silicated slurry under low temperature in a range from 138 °C to 145 °C.

6. The process as claimed in claim 3, wherein the process further comprises of treating the digested slurry with flocculants selected from polyacrylate, hydroxy-polyacrylamide flocculants or combination thereof.

7. The process as claimed in claim 1, wherein the bauxite residue factor is reduced by 5% to 10%.

8. The process as claimed in claim 1, wherein the amount of ground sand added is in a range from 10% to 15% of the bauxite charged to desilication.

9. The process as claimed in claim 3, wherein during digestion, silica content in the slurry is reduced from 1.8 g/l to less than 0.9 g/l.

10. The process as claimed in any one of the claims 1 to 7, the process comprises
i. grinding of bauxite comprising gibbsite to form ground bauxite.
ii. filtering the ground bauxite obtained in step (i) to provide filtered bauxite and to obtain ground sand comprising residual gibbsite.
iii. processing the filtered bauxite to pre-desilication.
iv. adding ground sand to the filtered bauxite during pre-desilication to form a feed slurry.
v. heating the feed slurry to a temperature in a range from 78 °C to 79 °C;
vi. de-silicating the fed slurry at a temperature in a range from 80 °C to 90 °C and time in a range from 12 hours to 16 hours; and
vii. digesting and treating the fed slurry with flocculants.

Dated 06th of March 2024
Hindalco Industries Limited
By their Agent & Attorney

(Nisha Austine)
of Khaitan & Co
Reg No IN/PA-1390