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Int. J. Chem. Pharm. Rev. Res. Vol (1), Issue (2), 2015, Page. 13-20 ISSN No: 2395-3306 Research Article

International Journal of Chemical and Pharmaceutical Review and Research

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A Novel Route to Recover Alumina Value from Red Mud by Sintering-leaching Process Using Carbonates of Alkaline Earth Metal and Sodium Carbonate S. N. Meher* National Aluminium Company Limited, Research & Development Department, M & R Complex, Damanjodi, Koraput, Pin-763008, Odisha, India

A R T I C L E I N F O Article history: Received 4 May 2015 Accepted 6 June 2015 Available online 14 June 2015 Keywords: Alumina, Red mud, Sintering, Leaching, Extraction.

A B S T R A C T A novel route was developed to recover alumina from red mud by sintering-leaching process using carbonates of alkaline earth metal and sodium carbonate. The chemical composition of red mud is 16.07% Al2O3, 53.75% Fe2O3, 8.25% SiO2, 3.83% Na2O, 4.24% TiO2, 1.48% CaO, 0.148% V2O5, 0.157% MnO, 0.020% MgO, 0.085% Ga2O3, 0.007% ZnO, 0.099% K2O and 11.83% LOI. In this process, the red mud was sintered with carbonates of alkaline earth metal (i.e., BaCO3) and sodium carbonate (Na2CO3) in different mass ratios at 900-1100 oC. The sinter product was leached with suitable concentration of caustic at 105 oC for 1 hr. The minerals and alumina phases present in red mud like gibbsite, gibbsite co-existing with boehmite, alumino-goethite, boehmite and sodalite are converted to soluble sodium meta-aluminate, di-barium silicate, barium ferrite and barium titanate during sintering-leaching process. The maximum alumina extraction was achieved by this novel sintering-leaching process is 99.14 %, 95.57% and 99.50% at a sintering temperature of 900oC, 1000oC and 1100oC, respectively. The formation of barium titanate and barium ferrite was confirmed by x-ray diffractometer (XRD) and scanning electron microscopy (SEM).

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1. Introduction

Red mud1 is a by-product in the manufacture of alumina that contains mainly iron oxide (54-65%) with significant amounts of silica, alumina, calcium oxide and titanium oxide in form of pervoskite, dispersed in highly alkaline and caustic liquor. The treatment and disposal of bauxite residue2,3 is a major operation and may account for 30-50% of operations in an alumina refinery. The red mud has been accumulating at a rate of 2.7 billion tonnes4 annually throughout the world.

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* Corresponding author. E‐mail address: [email protected] Present address: R & D , Department, Nalco, Damanjodi, Koraput, Pin-763008, Odisha, India.

For a given aluminium production rate, the quantity of red mud5 generated during the alumina extraction process varies significantly depending on the original properties of the bauxite and the operating conditions of the Bayer’s process, and in particular, the process temperature. The disposal of the red mud is associated with pollution problems. Disposal of any solid waste is associated with space or real estate near the industry, and the cost of disposal and pollution, which are now crucial factors. Obviously, these three factors are also associated with the red mud disposal. As the red mud contains6 a large amount of valuable chemicals, there is a need for developing a technology for the recovery of at least some of the important chemicals.

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The reuse of red mud is an urgent problem to be solved. To minimize the damage to the environment and the waste of the secondary aluminum resource, great efforts have been made to recover alkali and alumina from red mud7-12.The red mud13 was leached with organic acids and extracted 5500 mg/l Al (47%) using oxalic acid. The red mud14 was leached with sulphuric acid, citric acid and oxalic acids or as mixtures of the above acid. The highest concentration extracted was 13.53 g/l of Al (96% solubilization) using a 2:1 mass ratio of citric acid and oxalic acids and subsequent H2SO4 addition to lower the pH to 1.5. The lower concentration of aluminium extracted with H2SO4 at a pH of 1.0 (12.14 gm/l). During this process the presence of oxalate and citrate ions retarding the alumina hydrate growth in the precipitation of hydrate generally forms very fine and again during calcinations the attrition is more.

The sintering of red mud15 at 1100-1150oC in presence of limestone and soda ash in an oxidizing atmosphere for achieving maximum recovery of alumina. Leaching was done at 60-90 oC for 15-40 min. with liquid: solid ratio of 10-3:1. Red mud16 was sintered with lime and soda at about 1100oC and alumina extracted from the ground sintered with the wash water from Bayer’s process. The presence of Fe2O3 had a beneficial effect on alumina extraction, e.g., increase in Al2O3/Fe2O3 mole ratio 1.00-1.25 enhanced alumina extraction from 78-88.7%. to 81.40%. At temperatures below 1200 oC

(i.e. 1100 oC and 1150 oC) also lower recoveries of alumina were obtained.

In the literature17, a lot of different articles that explain the properties and the investigation of the red mud tailings can be found. One such reference investigated the red mud under caustic concentrations of 10-30% to determine the extraction of soda and alumina as a function of caustic ratio, mole ratio of CaO to SiO2, water content, temperature and reaction time. Hydrothermal treatment of the red mud with lime at high temperatures of 300 °C was found to be an effective method for the recovery of soda and alumina. Recoveries of 95% Na2O and 70% Al2O3 were obtained. It has shown that on the recovery of aluminium18 from the red mud of the Seydisehir Aluminium Company Limited. They prepared a mixture composed of 26.6 wt% red mud, 37.2 wt% CaCO3 and 36.2 wt% Na2CO3. The mixture was sintered at 900 oC for 2 hrs, then leached in water. Increasing the sintering temperature as high as 1150 oC decreased the alumina extraction efficiency.There are now a few millions of tonnes red mud stored in red mud ponds in India. These wastes have not been investigated yet in any industrial process or used for material production as an additive. The objective of this study was to investigate these muds for maximum alumina extraction by adding alkaline earth metal carbonate (BaCO3) and alkali metal carbonate (Na2CO3) by sintering and leaching process.

2. Materials and Methods

Red mud used in experimental studies were supplied from, Damanjodi, Dist.-Koraput. The BaCO3 and Na2CO3 used in experimental studies were supplied from Qualigens fine Chemicals, Bombay. The major elements were analysed by classical methods as well as X - Ray

Fluorescence Spectroscopy (XRF) (Maker: Philips, Model: PW 2400). The minor elements were analysed by Atomic Absorption Spectrometer (AAS) (Maker: Varian, Model: Spectra 220FS). The chemical composition of the red mud is shown in “Table 1”.

Table 1 . Chemical Analysis of Red Mud

Chemical Constituents

Al2O3 Fe2O3 TiO2 SiO2 Na2O CaO V2O5 MgO MnO LOI at 1000oC

(%) 16-18 51-57 3-5 8-12 4-6 0.03-2.3 0.14-0.21 0.13-0.18 0.15-0.25 11-13

2.1 Experimental

The red mud was mixed with different mass ratios of barium carbonates to sodium carbonate thoroughly and pulverised in a pulveriser. The above mixture was sintered at temperatures of 900, 1000, and 1100 °C for 1 to 4 hrs in a ceramic crucible in high temperature muffle furnace. After sintering the sinter products were

leached with 80 gpl caustic at 105oC for 1 hr in an autoclave of 2.5-litre capacity. The leached red mud was filtered after the leaching. The amount of Al2O3 in the leach solutions was determined by using a titrimetric method.

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2.2 XRD Study

X-Ray Diffractometer (XRD) analysis was done to detect the presence of different phases in the sinter red mud and leached red mud. XRD work was carried out on a Rigaku XRD, (Maker: Rigaku, Model: Dmax2200). The XRD of sinter red mud and leached red mud were taken using Cu Kα (Kα =1.54186 Å) radiation at scan speed of 1o/min. and scan step of 0.02 with 2θ value from 5o to 70o at 30 mA and 20 mV.

2.3 SEM Study

The sinter red mud and leached red mud samples were coated with gold, palladium or carbon at 18 mA for 105 seconds. The morphological behaviours of coated samples and formation of sodium barium silicate, di-barium silicate, barium titanate, barium ferrite, were studied by using Scanning Electron Microscope (SEM) (Maker: Leo Electron Microscopy, Model: 430).

3. Results and Discussion

3.1 Principle of Sintering

Sintering is a thermal treatment, below the melting temperature of the main constituent material, which transforms a metallic or ceramic powder (or a powder compact) into a bulk material containing, in most cases, residual porosity.The process of sintering brings about certain physical as well as chemical changes in the material. The chemical changes can be illustrated as:

a) Change in composition or decomposition b) New phase formation or decomposition followed by phase change c) New phase formation due to chemical changes

The physical changes that take place are: - Change of grain size - Change of pore shape and pore size All these changes bring about the complete change in microstructure, which bring about the complete change in the properties of the material. Change in grain size is brought about by re-crystallization whereas densification or solid state sintering is responsible for change of pore shape and size. The various stages in the process of sintering are: 1. Primary re-crystallization 2. Grain growth 3. Secondary re-crystallization 3.1.1 Primary Re-crystallization It is the process by which a new set of grains are formed from a previously deformed matrix. The grains get deformed in a particular direction. Hence, all these grains are strained grains or it is a strained matrix. If there is a strain in the material then that portion is slightly warm as it possesses strain energy of the order of 0.5 to 1 cal/gram, which may lead to re-crystallization and grain growth. When a body is mechanically warmed the grains are elongated and when cooled a new set of grains are formed. Primary re-crystallization is dependant on primary deformation. Ceramic materials are formed from powders, which are sintered, and hence they undergo negligible mechanical deformation. Driving force for re-crystallization is stored energy. The schematic representation of sintering technique is shown in figure 1.

Figure 1: Schematic Representation of Sintering

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3.1.2 Grain Growth

This process is defined as nucleation and grain growth of the grains without changing the overall distribution of grains. Particles on heating come to form common boundary or grains start growing. Driving force for grain growth is the difference between grain areas and hence difference between surface energy. As grain growth occurs surface area decreases. Smaller is the grain size, more is the surface area. The difference in surface energy or grain boundary energy is responsible for grain growth.

Factors affecting grain size during this period:

1. Temperature 2. Time 3. Presence of inclusions

Due to change of surface curvature due to the presence of inclusions, the grain boundary energy decreases. So extra energy is required to overcome this barrier and move due to which movement decreases and the grain growth is not that fast. Sometimes pore spaces act as inclusions. When a ceramic product is sintered from powders, initially the porosity is high and hence the rate of grain growth is low but when porosity is brought down to less than 15% then grain growth starts increasing.

3.1.3 Secondary Re-crystallization

This is a process where a few grains grow abnormally or discontinually at the expense of other grains. If grains are too small at places due to inclusions, at some other places there will be abnormal growth and once

this starts they will consume the normal sized or uniform grains and grow. If the inclusions are high or there is segregated grain growth or the grains are very fine in size then there are chances of secondary re-crystallization. Initial requirement for secondary re-crystallization is presence of some embryos. It is unwanted as it deteriorates the mechanical properties of the material.

Sintering (firing) of ceramic materials is the method involving consolidation of ceramic powder particles by heating the “green” compact part to a high temperature below the melting point, when the material of the separate particles diffused to the neighbouring powder particles. The driving force of sintering process is reduction of surface energy of the particles caused by decreasing their vapour-solid interfaces.During the diffusion process the pores, taking place in the “green compact”, diminish or even close up, resulting in densification of the part, improvement of its mechanical properties.

Decrease of the porosity, caused by the sintering process, is determined by the level of the initial porosity of the “green” compact, sintering temperature and time. Sintering is enhanced if a liquid phase takes part in the process (liquid phase sintering). Sintering (firing) of pure oxide ceramics requires relatively long time and high temperature because the diffusion proceeds in solid state. Applying pressure decreases sintering time and the resulted porosity. The figure of sintering and agglomeration technique is shown in figure 2.

Fig. 2: Sintering and Agglomeration technique

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3.2 Sintering and leaching chemistry

The process involves by adding between alkaline earth metal carbonate (i.e., BaCO3) and alkaline metal carbonate (i.e., Na2CO3) to the red mud and sintering at temperatures of 900-1100 °C. The silica in the red mud reacts with BaCO3 at temperatures of 900-1100 °C to form the relatively inert di-barium silicate and release CO2 as shown in Equation 1. The sodium19, which is in the red mud via soda ash or sodium carbonate, reacts with the alumina and forms the soluble sodium aluminate as given in Equation 2. The Fe2O3 contents of red mud react with sodium carbonate to form sodium

ferrite as given in Equation 3. The TiO2, which is in the red mud, reacts with sodium carbonate forming sodium titanate as given in Equation 4. The sodium titanate reacts with BaCO3 forming barium titanate and Na2CO3 as given in Equation 5. The sodium ferrite reacts with barium carbonate to form barium ferrite and Na2CO3 as given in Equation 6. The product is then leached in an alkaline solution or water as given in Equation 7 and the sodium aluminate solution is directed to the precipitation stage of the Bayer’s process.

SiO2 +2BaCO3 → Ba2SiO4+ 2CO2 (1)

Al2O3 + Na2CO3 → 2NaAlO2 +CO2 (2)

Fe2O3 + Na2CO3 → 2NaFeO2 +CO2 (3)

TiO2 + Na2CO3 → Na2TiO3 +CO2 (4)

Na2TiO3 + BaCO3 → BaTiO3 + Na2CO3 (5)

2NaFeO2 + 2 BaCO3 → Ba2Fe2O5 + 2Na2CO3 (6)

NaAlO2(s) + Ba2SiO4(s) + 2NaFeO2(s) + BaTiO3 (s) + Ba2Fe2O5 (s) +3H2O →

NaAl(OH)4(aq)+2NaOH(aq)+Ba2SiO4(s)↓+ Fe2O3(s)↓+ BaTiO3 (s)↓+ Ba2Fe2O5(s) ↓ (7)

Na2O.Al2O3.2SiO2.nH2O(Sodalite)+2BaCO3+4NaOH(aq.)→

2Na2BaSiO4(s)↓+2NaAlO2.(n+2)H2O(aq.)+2CO2 (8)

However, thermodynamic calculations and laboratory tests20 show that MSiO3 does not form under sinter conditions. From the present experiments it was thought that the following barium carbonate sinter reaction was feasible due to the presence of sodalite in the red mud formed by the reaction of kaolin and sodium aluminates liquor. A similar lime sinter process21 is referred to in a paper discussing the

integration of coal combustion with lime sintering. Reaction (8) shows that soluble sodium aluminate and sodium barium silicates are produced. The objective of this project was to recover alumina with maximum alumina extraction from red mud by sintering-leaching process using carbonates of alkaline earth metal and sodium carbonate.

3.3 Study on recovery of alumina from red mud followed by formation of Ba2Fe2O5 and BaTiO3.

The extraction efficiency of sintering-leaching process using carbonates of alkaline earth metal and alkali

metal carbonate at different conditions is shown in Table 2.

Table 2. Extraction Efficiency of barium carbonate and sodium carbonate sinter process at different condition.

Sample

No.

(RM+ BaCO3+ Na2CO3 )

in mass ratio

Temperature

in oC

Time

in hours

Alumina Extraction

Efficiency (%)

1 1:0.1288:0.25 900 1 99.14

2 1:0.1932:0.25 1000 4 99.50

3 1:0.2576:0.25 1100 1 95.57

4 1:0.2576:0.10 1100 3 32.56

In the sintering-leaching process using BaCO3 and Na2CO3, calcination at 900 °C for 1 hr with red mud,

BaCO3 and Na2CO3 in a mass ratio of 1:0.1288:0.25 was found to produce the most stable sodium barium

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silicates, di-barium silicates, barium ferrite (SEM Fig. 4A and 4B) and barium titanate (SEM Fig.5A and 5B) confirmed by XRD (as shown in Figure 3 and Table 3) which maximized at 99.14% the extraction of alumina. In the sintering-leaching process using BaCO3 and Na2CO3, calcination at 1000 °C for 4 hrs with red mud, BaCO3 and Na2CO3 in a mass ratio of 1:0.1932:0.25 was found to produce the most stable sodium barium silicates, di-barium silicates, barium ferrite (SEM Figure 4A and 4B) and barium titanate (SEM Figure 5A

and 5B) confirmed by XRD (as shown in Table 3) which maximized at 99.50% the extraction of alumina. In the sintering-leaching process using BaCO3 and Na2CO3, calcination at 1100 °C for 1 hr with red mud, BaCO3 and Na2CO3 in a mass ratio of 1:0.2576:0.25 was found to produce the most stable sodium barium silicates, di-barium silicates, barium ferrite (SEM Figure 4A and 4B) and barium titanate (SEM Figure 5A and 5B) confirmed by XRD (as shown in Table 3) which maximized at 95.57% the extraction.

Fig. 3: XRD of Sintered Product at 1000oC.

The lowest alumina extraction efficiency of 32.56% obtained in the BaCO3 Na2CO3 sinter process with red mud BaCO3 Na2CO3 mass ratio (1:0.2576:0.1) sintered at 1100 oC for 3 hrs is due to the formation of insoluble barium aluminium silicate hydrate and sodium aluminium silicate22. The formation of small needles like structure agglomerated to each other and whisker like structure indicates the formation of barium

ferrite22-24 as shown in Figure 4A and 4B at 241X and 840X magnification. The formation of large single tetragonal like structure indicates the formation of barium titanate as shown in Figure 5A and 5B at 3.88KX and 1.79KX magnification. The chemical analysis of di-valent alkaline earth metal soda ash sinter process before and after leaching is shown in Table 4.

4A 4B

Fig. 4A and 4B: Formation of sodium barium silicates, di-barium silicates and barium ferrite.

RM+BaCO3+Na2CO3(1:0.15:0.25)-1000-4

1.63

2.13

2.23 2.

42

3.85

3.36

4.26

2.95

2.78

2.63

800

1800

2800

1.50 2.00 2.50 3.00 3.50 4.00 4.50 5.00 5.50

d-value

Inte

nsity

(cps

)

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5 A 5B

Fig. 5A and 5B: Formation of barium titanate.

Table 4. Chemical analysis of BaCO3 and Na2CO3 sinter process analysed by XRF.

4. Conclusions

By adoption of the sintering leaching method using BaCO3 and Na2CO3 followed by alkaline leaching, the alumina extraction achieved was 99.14% at sintering temperature of 900 oC with red mud, BaCO3, Na2CO3

mass ratio of 1: 0.1288: 0.25. In the BaCO3 and Na2CO3 sinter process alumina extraction achieved was 99.50% with red mud, BaCO3, Na2CO3 mass ratio of 1: 0.1932: 0.25 at sintering temperature of 1000 oC. In the BaCO3 and Na2CO3 sinter process, the extraction efficiency achieved was 95.57% with red mud, BaCO3

and Na2CO3 mass ratio of 1: 0.2576: 0.25 at sintering temperature of 1100 oC. The maximum alumina extraction achieved by this sintering leaching process is 99.50% using BaCO3 and Na2CO3 at sintering temperature of 1100 oC. The impurities like SiO2, V2O5 and MnO2 are removed from the Bayer’s liquor. These impurities are removed from the crystal lattice of hematite as well as sodalite and cancrinite to form insoluble materials like BaSiO3 (d value= 2.78 Å, 3.15 Å), BaV2O6 (d value= 2.95 Å) and BaMnO3 (d value= 3.15 Å) confirmed by XRD.The formation of sodium barium silicates (d value= 2.63 Å, 3.85 Å, 4.26 Å), di-barium silicates (d value= 2.78 Å, 3.15 Å), barium

ferrite (d value= 1.63 Å, 2.43 Å, 2.63 Å, 2.78 Å) and barium titanate (d value= 2.13 Å, 2.24 Å) enhance recovery alumina from red mud in BaCO3 and Na2CO3 sinter and leaching process. But from an economic point of view, the BaCO3 and Na2CO3 sintered process with sintering temperature of 900 oC and time of 1 hr having an extraction efficiency of 99.14 % is best, suitable and novel method for recovering the remaining alumina from red mud. The residual mud can be suitable for making of cement, refractory bricks, tile and filling of land due to low sodium content. The residual mud can be used as good adsorbents for heavy metals like Cu, As, Hg and Cd from ground water due to its high BET surface area.

Acknowledgements

The author is very much thankful to Management of National Aluminium Company Limited for their kind permission for publishing this research article.

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Sl.No. Condition CHEMICAL CONSTITUENTS IN PERCENTAGE (%)

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1

Sinter red mud 14.53 43.22 3.83 7.45 20.91 1.03 0.14 0.01 0.13 8.73 -

Leached red mud 0.12 66.88 5.27 10.25 2.09 1.41 0.19 0.01 0.17 8.73 4.86

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Sinter red mud 14.32 45.46 3.77 7.34 20.91 1.31 0.14 0.01 0.10 6.62 -

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Sinter red mud 14.14 47.38 3.73 7.25 20.91 1.30 0.13 0.01 0.14 4.51 -

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