FORM 2
THE PATENT ACT, 1970
(39 of 1970)
As amended by the Patents (Amendment) Act, 2005
&
The Patents Rules, 2003 As amended by the Patents (Amendment) Rules, 2006
COMPLETE SPECIFICATION (See section 10 and rule 13)
TITLE OF THE INVENTION:
A process for treating alkaline bauxite residue obtained during Bayer process.
APPLICANT:
Grasim Industries Limited, an Indian Company, having address at Aditya Birla Centre, B-Wing, 2nd Floor, S. K. Ahire Marg, Worli, Mumbai – 400030, Maharashtra, India.
PREAMBLE TO THE DESCRIPTION:
The following specification particularly describes this invention and the manner in which it is to be performed.

FIELD OF THE INVENTION
[001] The present invention relates to a process for treating alkaline bauxite residue obtained
during Bayer process. Specifically, the invention relates to a process for treating alkaline bauxite residue having a solid concentration in the range of 35-45%, obtained during Bayer process.
BACKGROUND OF THE INVENTION
[002] In the Bayer process for obtaining alumina from bauxite, bauxite is initially digested with caustic soda to extract alumina as dissolved sodium aluminate in an aqueous liquor which also contains, in suspension, undissolved particulate residue of the original bauxite. After initial separation from the process liquor, bauxite residue (BR), also known as red mud, is typically washed in a counter-current washing circuit to remove most of the caustic soda carried with the BR from the starting liquor. After the step of counter-current washing, the BR or red mud obtained is at a solid concentration in the range of 35-45%.
[003] Generally, Bayer process results in huge quantity of BR or red mud which is highly alkaline in nature with a pH in the range of 12-14. Depending upon the raw material processed, 1-2.5 tonnes of red mud is generated per tonne of alumina produced. BR or red mud, is generally considered as a waste product, although it has some present or potential uses. Chemically, it comprises, in varying amounts (depending upon the composition of the starting bauxite), hydroxide/ salts of iron, titanium, aluminium, sodalite, silica, and minor quantities of other metals.
[004] BR or red mud is made up into an aqueous slurry, which is thickened or filtered to a high solids concentration for delivery to a storage or disposal site. For disposal, red mud is generally sent to residue ponds or alternatively dry-stacked, either directly or after neutralization.

However, the current process of neutralisation and disposal of BR or red mud poses certain problems which are discussed below.
[005] First, when BR or red mud is neutralised and its slurry is stacked into the disposal sites, the leachate from such neutralised red mud still has comparatively high pH and fluoride content. This forms a major source for contamination of surface as well as ground water especially during rainy season.
[006] Second, the process of neutralisation of BR or red mud having a solid concentration of 35-45%, as known in the prior art leads to loss of huge quantity of alkali thereby decreasing the process efficiency.
[007] Third, huge quantity of waste water is generated during the prior art process which has to be discarded again resulting in process inefficiency. Lastly, the process in prior art does not result in complete recovery of alumina from the neutralised BR or red mud that is finally disposed.
[008] A typical illustration of the process of neutralisation of BR or red mud having a solid concentration of 35-45% as known in the prior art is shown in Figure 1 which depicts (step 1) taking BR or red mud having 35-45% solid concentration, obtained during Bayer process. (Step 2) Neutralisation of the BR or red mud by a mineral acid such as sulphuric acid. At this step, alkali is lost and a huge quantity of waste water is generated which is required to be discarded. (Step 3) Filtration of the neutralised BR or red mud slurry to obtain a red mud cake and a filtrate. (Step 4) Disposal of the red mud cake at storage sites. The filtrate is also further treated to recover salts which can be re-used in the industry. The BR or red mud obtained at this stage still contains toxicity and the leachate coming out of it, especially during rainy season, has high pH and fluoride thereby contaminating the ground as well as surface water. Moreover, the red mud cake which is disposed still contains certain quantities of alumina, which can be recovered.

Also, the red mud cake contains around 50-500 ppm of fluoride which does not make it safe for disposal and the quantity of fluoride can be further reduced.
[009] Hence, a strong need exists for an alternate process for treating alkaline bauxite residue which solves some of the problems present in the prior art as mentioned above.
SUMMARY OF THE INVENTION
[010] According to an embodiment of the present invention, there is provided a process for treating alkaline bauxite residue having a solid concentration of 35-45%, obtained during Bayer process, the process comprising the steps of filtering said alkaline bauxite residue to obtain bauxite residue cake having a solid concentration of 60-80% and alkaline filtrate respectively; re-slurring the bauxite residue cake to obtain a bauxite residue slurry having a solid concentration of 35-45%; neutralizing the bauxite residue slurry with an acid resulting in decrease in the pH of said bauxite residue and sorption of fluoride on the neutralized bauxite residue; filtering the neutralized bauxite residue to obtain neutralized bauxite residue cake and filtrate containing salts of alkali; and handling the filtrate containing salts of alkali by further treatment or recirculating it back to the process in step (c).
BRIEF DESCRIPTION OF THE DRAWINGS:
[011] Figure 1 depicts a flow chart showing the process steps for neutralisation of alkaline BR or red mud slurry having a solid concentration of 35-45%, in accordance with the process in prior art;
[012] Figure 2 depicts a flow chart showing the process steps for neutralisation of alkaline BR or red mud slurry having a solid concentration of 35-45%, according to an embodiment of the present invention;

[013] Figure 3 illustrates the process of neutralisation of alkaline BR or red mud slurry having a solid concentration of 35-45% as shown in Figure 2, according to another embodiment of the present invention;
[014] Figure 4 shows the process of neutralisation of alkaline BR or red mud slurry having a solid concentration of 35-45%, according to an embodiment of the present invention; and
[015] Figure 5 illustrates a graph showing the impact of neutralisation of alkaline BR or red mud on filterability, according to an embodiment of the present invention.
DETAILED DESCRIPTION OF THE EMBODIMENTS
[016] As set out in the claims, the present invention eliminates or reduces the aforementioned problems of the prior art by providing a two stage filtration process for neutralisation of alkaline bauxite residue having a solid concentration of 35-45%, obtained during the Bayer process.
[017] According to an embodiment of the invention, there is provided a process for neutralisation of alkaline bauxite residue having a solid concentration of 35-45% comprising the following sequential steps: (1) filtration, (2) re-slurring, (3) neutralisation, (4) filtration, and (5) handling of filtrate and disposal of red mud cake, as more clearly illustrated in Figure 2.
[018] (Step 1) Filtering alkaline bauxite residue having a solid concentration of 35-45%, obtained during Bayer process, to obtain bauxite residue cake having a solid concentration of 60-80% and high TDS alkaline filtrate respectively. In an embodiment, the high TDS alkaline filtrate is further wherein the alkaline filtrate obtained in step (a) is further treated to obtain (20-40) g/L alkali as sodium oxide (Na2O), dissolved sodium aluminate (NaAlO2) and other dissolved salts.

[019] (Step 2) The bauxite residue cake having a solid concentration of 60-80% is re-slurried to obtain a bauxite residue slurry having a solid concentration of 35-45%. In an embodiment, re-slurring can be done either by adding fresh water or by adding fluoride rich water stream.
[020] (Step 3) Thereafter, the bauxite residue slurry is neutralised with an acid resulting in decrease in the pH of said bauxite residue and sorption of fluoride on the neutralized bauxite residue. In an embodiment, the acid can be a mineral acid such as hydrochloric acid, sulphuric acid or the likes or an organic acid such as carboxylic acids. Preferably, the alkaline bauxite residue slurry is neutralised by hydrochloric acid. In a preferred embodiment, the pH of the neutralised bauxite residue is in the range of 6.0 to 9.0.
[021] (Step 4) The neutralized bauxite residue is thereafter filtered to obtain neutralized red mud cake and filtrate containing salts of alkali.
[022] (Step 5) The neutralised red mud cake is further sent to disposal sites for stacking/disposal. This neutralised red mud is free of any toxicity and can be further used to construct bricks. In another embodiment, the neutralised red mud cake can be further used in de-fluoridation of fluoride rich water. In a preferred embodiment, the filtrate containing salts of alkali can be handled by further treatment or recirculating it back to the process in step 2. In an embodiment, treatment of filtrate containing salts of alkali includes evaporating the filtrate to obtain concentrated salts or re-using it in brine cycle for caustic production.
[023] In a preferred embodiment, during recirculation of the filtrate obtained in step 5, pre-determined quantity of the filtrate is purged in order to maintain the volumetric balance of the process and wherein the purged filtrate can be further treated to remove impurities and obtain clear salt water for use in brine cycle.

[024] The process illustrating another embodiment of the present invention is shown in Figure 3. Here, the filtrate containing salts of alkali is not circulated back in the process. In this embodiment, the filtrate is further treated which includes evaporating the filtrate to obtain concentrated salts or re-using it in brine cycle for caustic production.
[025] In an embodiment, the fluoride content in alkaline bauxite residue obtained during Bayer process is in the range of 20-1000 ppm which decreases to less than 2ppm once the bauxite residue is neutralised in step 3 in the present process.
[026] In an embodiment, filtration is done by Rotary Vaccum Drum Filter (RVDF), filter press, decanter centrifuge or belt press or the likes. According to an embodiment of present invention, as shown in Figure 4, the process of neutralisation of alkaline bauxite residue or red mud is done by using double drum filtration technology. Moreover, the figure shows recirculation of filtrate containing salts of alkali at process step 2.
[027] Figure 5 illustrates a graph showing the impact of neutralisation on filterability of alkaline red mud versus that of neutralised red mud. The results show that the rate of filtration is almost 1.5 times better for neutralized BR or red mud. Since alkali has a tendency to retain water, the neutralization of the same made it easier for de-watering of the neutralised BR.
[028] The process for neutralisation of alkaline bauxite residue as described in the present invention is a two stage filtration process where first stage recovers most of the alkali and second stage neutralises the residual alkali in the bauxite residue. Moreover, recirculation of filtrate results in around 30 times less generation of waste water. Furthermore, the neutralised red mud cake obtained at the end of the process does not contain any toxicity and can be utilised in construction of bricks or can be disposed-off without affecting the quality of soil or drinking water thereby meeting the disposal norms of red mud and avoiding environmental issues.

EXAMPLES
[029] Advantages and benefits of the process of neutralisation of alkaline bauxite reside according to the embodiments of the present invention would become more apparent from the below experimental details to a person skilled in the art.
[030] Experimental Data 1:
a) 600 g of red mud cake was dispersed in 350 g water to have a 40-45% solid concentration of red mud slurry and the same was neutralised with 20g conc. Hydrochloric acid (HCl). Resultant slurry was agitated for 10 minute to ensure the completion of the neutralization.
b) Slurry was filtered and filtrate of first step was taken to disperse fresh red mud in set 2 and neutralised with required quantity of HCl. This process was repeated for 20 sets of experiment (Table 1).
c) TDS of filtrate initially increased and later on stabilised in the range of 35-45 g/l.
Table-1

Set PH TDS of filtrate, g/l Sludge solid, %
1 7.1 17.1 63.1
2 7.0 20.4 62.0
3 6.8 26.1 63.4
4 6.7 31.9 63.0
5 6.7 35.5 63.8
6 6.6 37.3 64.6
7 6.8 44.1 63.8
8 6.7 43.2 64.3
9 6.6 46.0 64.4
10 6.6 39.4 64.5
11 6.6 34.5 63.8
12 6.9 44.5 64.7
13 6.9 41.6 64.1
14 6.8 43.9 64.0
15 6.8 38.5 63.4
16 6.9 38.8 65.0
17 6.8 38.9 64.5
18 6.8 38.9 63.8
19 6.8 37.6 63.7
20 6.8 38.9 63.7

[031] Experimental Data 2:
a) Plant having capacity to handle 250 MT red mud was run in continuous mode. Red
mud slurry was fed at 12-16 m3/h flow rate to neutralization tank.
b) HCl was fed at 0.5 m3/h flow rate in red mud slurry prior to neutralization tank for continuous neutralization of alkali.
c) Amount of filtrate equivalent to HCl added was purged out continuously to maintain volumetric flow in the system and rest of the filtrate was fed back for re-slurring the red mud.
d) Initial Total Dissolved Solids (TDS) of filtrate after 1st cycle was 12 gpl which increased to ~40 gpl after 5 cycles and TDS remained 37±3 gpl during rest of the trial.
e) It was also possible to fix fluoride with neutralised RM at near to neutral pH and average concentration was less than 2 ppm, whereas the same was 50 – 500 ppm in alkaline filtrate.

Filtrate recycle, no. Alkaline RM Slurry Neutral RM Slurry Filtrate

Soda, gpl pH Soda, gpl pH pH TDS, mg/l F, mg/l
1 30.6 13.1 0.3 8.3 8.7 11.8 --
5 27.7 13.1 0.2 7.8 6.3 41.3 0.92
9 25.5 13.0 0.2 8.1 7.6 33.5 1.96
17 28.2 12.8 0.1 7.5 8.1 34.1 --
29 35.0 12.8 0.1 7.3 7.7 37.5 --
37 32.6 12.7 0.1 7.3 7.9 34.9 1.60
39 30.2 13.0 0.2 7.9 8.0 35.5 2.07
48 -- -- -- -- 8.3 37.4 3.47
52 34.3 13.0 0.2 7.05 7.0 40.6 0.96
[032] The foregoing description of specific embodiments of the present invention has been presented for purposes of description. They are not intended to be exhaustive or to limit the present invention to the precise forms disclosed, and obvious modifications and variations are possible in light of the above teaching.

We Claim:
1. A process for treating alkaline bauxite residue having a solid concentration of 35-45%,
obtained during Bayer process, the process comprising the steps of:
a) filtering said alkaline bauxite residue to obtain bauxite residue cake having a solid concentration of 60-80% and alkaline filtrate respectively;
b) re-slurring the bauxite residue cake to obtain a bauxite residue slurry having a solid concentration of 35-45%;
c) neutralizing the bauxite residue slurry with an acid resulting in decrease in the pH of said bauxite residue and sorption of fluoride on the neutralized bauxite residue;
d) filtering the neutralized bauxite residue to obtain neutralized bauxite residue cake and filtrate containing salts of alkali; and
e) handling the filtrate containing salts of alkali by further treatment or recirculating it back to the process in step (c).

2. The process as claimed in claim 1, wherein the filtration can be done by Rotary Vaccum Drum Filter (RVDF), filter press, decanter centrifuge or belt press or the likes.
3. The process as claimed in claim 1, wherein the alkaline filtrate obtained in step (a) is further treated to obtain (20-40) g/L alkali as sodium oxide (Na2O), dissolved sodium aluminate (NaAlO2) or the likes.
4. The process as claimed in claim 1, wherein re-slurring of the bauxite residue cake in step (b) can be done by either pure water or fluoride rich water.
5. The process as claimed in claim 1, wherein the acid can be a mineral acid such as hydrochloric acid, sulphuric acid or the likes or an organic acid such as carboxylic acids.
6. The process as claimed in claim 1, wherein the pH of bauxite residue after neutralization in step (c) is in the range of 6.0 to 9.0.

7. The process as claimed in claim 1, wherein the treatment of filtrate containing salts of alkali in step (e) includes evaporating the filtrate to obtain concentrated salts or re-using it in brine cycle for caustic production.
8. The process as claimed in claim 1, wherein during recirculation of the filtrate in step (e), pre-determined quantity of the filtrate is purged in order to maintain the volumetric balance of the process and wherein the purged filtrate can be further treated to remove impurities and obtain clear salt water for use in brine cycle.