.Quantitative assessment of red mud constituents by combined thin layer chromatography and scanning densitometry

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1 Indian Journal of Chemical Technology Vol. 18, March 2011, pp Quantitative assessment of red mud constituents by combined thin layer chromatography and scanning densitometry P A Mohamed Najar a *, M T Nimje b, M J Chaddha c & K V Ramana Rao a a Downstream Division, b Metal & Carbon Division, c Bauxite-Alumina Division, Jawaharlal Nehru Aluminium Research Development and Design Centre, Amaravati Road, Nagpur , India Received 8 July 2010; accepted 15 February 2011 Quantitative analysis of aluminium, iron, silicon and titanium present in bauxite residue (red mud) has been achieved by thin layer chromatography coupled with optical scanning densitometry. Silicon in red mud sample is detected at ph range on chromatography plates prepared with microcrystalline cellulose modified with 10% sodium molybdate. Aluminium, iron and titanium are detected on silica gel H layers impregnated with 0.2% sodium formate and developed with mixture of 10% aqueous solutions of sodium chloride and formic acid in 8:2 v/v. The chromatograms obtained for the cations are quantitatively evaluated by optical scanning densitometry by measuring absorbance at nm for aluminium, nm for iron and nm for titanium and silicon in reflectance mode. The quantitative densitometric data has been evaluated with analogous data obtained by wet analysis. Keywords: TLC, Separation, Red Mud, Quantitative densitometry Commercially aluminium is produced by the combined Bayer 1 chemical process and Hall- Heroult s 2 electrolytic process. Bayer process 3 involves the extractive hydro-metallurgical process of bauxite which results the alkali leaching of alumina in to sodium-aluminate solution. The remaining oxides of iron, titanium, sodium, silicon as well as other impurities are filtered out as waste residue which is generally called red mud. Chemically, red mud is highly alkaline with ph values in excess of The bauxite residues derived from different bauxite have a wide range of composition 4,5 : Fe 2 O %, Al 2 O %, SiO %, Na 2 O 2-10%, CaO-8%, TiO 2 traces 1-20%. The chemical composition of metal values (Al, Fe, Ti, Si and Ti) is adequately high to utilize red mud as a mineral rich secondary resource for product generation such as construction and special application bricks, polymerized wood substitute, soil amendment as well as for direct commercial exploitations such as cement and steel industries 6. The cost assessment of alumina generation as well as the Bayer plant production efficiency generally associated with the optimal utilization of raw materials 7. It is considered that the alumina and soda loss during Bayer alumina production is directly *Corresponding author ( najarp@hotmail.com) proportional to the quality of bauxite, mainly in terms of the silica content. Therefore, the alumina plants frequently monitor the composition of red mud samples generated during Bayer cycle of alumina production by various analytical procedures 8 such as classical wet analysis (CWA) as well as sophisticated instrumental methods of analysis such as X-ray diffraction analysis (XRD), X-ray fluorescence spectroscopy (XRF), inductively coupled plasma (ICP) emission spectrometry, atomic absorption spectrometry (AAS) and UV-Visible spectrophotometry. A systematic evaluation of the standard procedures used for red mud characterization revealed that the analytical procedures except XRD and XRF customarily involve dissolution 9-13 of red mud sample in a suitable reagent mixture for constituent analysis in the sample matrix. This realization inspired exploring the scope of ideal chromatography procedures for red mud analysis. Further, the literature survey reveled that no chromatographic study has been reported except our preliminary thin layer chromatographic (TLC) studies on the detection, separation and determination of some major constituents in red mud 14,15. In the recent past we have reported the utilization of TLC in combination with titrimetry and UV-Visible spectrophotometry 16 as well as optical scanning densitometry for the determination of bauxite

2 100 INDIAN J. CHEM. TECHNOL., MARCH 2011 constituents 16-19, impurity elements in aluminium alloys 20 and primary aluminium 21. The imprecise chemical composition of red mud due to the plant specific experimental parameters such as digestion temperature, digestion pressure, alkali concentration and presence of additives in the Bayer plants are the challenging factors for optimizing TLC parameters for red mud analysis. It is thought that the close proximity of chemical characteristics of red mud and bauxite as a convenient prospect to use the best chromatographic system reported from our laboratory for bauxite constituents Further, for the present TLC-Densitometric study of the four major constituents (Al 3+, Fe 2+, Ti 4+ and Si 4+ ) in red mud the selectivity of impregnated adsorbent layers in the TLC separation of cations on silica gel layers has been utilized for the better detection and selectivity. The method developed is shown to have reasonable competence with the existing analytical tools such as CWA as well as AAS and UV-Visible spectrophotometry for quantitative determination of red mud constituents. The TLC Densitometric analysis of red mud may find useful applications in plant laboratories and R&D laboratories as a complementary and convenient analytical tool for rapid and cost effective analysis of red mud and mineral constituents present in identical sample matrices of industrial and natural origin. Experimental Procedure Apparatus Chromatographic trials were performed with glass plates (7 2.5 cm 2, cm 2, 15 3 cm 2 ) coated with suitable adsorbents. Systronics ph meter model 335 was used for all ph measurements. Photometric measurements were carried out by Shimadzu make UV-Visible Spectrophotometer (Model 1601) and Dual Wavelength Scanning Densitometer equipped with Quanta Scan Software (Model CS-9301PC). Chemicals and reagents Silica gel G, ethyl alcohol, sodium formate, potassium ferrocyanide, aluminium sulphate, ammonium molybdate, sodium molybdate, acetic acid, silicon standard solution (E. Merck, India); titanium chloride, tiron, aluminon, microcrystalline cellulose (Loba, India); formic acid, sodium chloride, silica gel H, H 254 (Qualigens, India), pre-coated silica gel H and G plates (Analtech, USA) and potassium bromide (Spectra-Tech Inc., USA) were used. Other reagents used were of analytical grade. Test solutions 1% reference standards were prepared in double distilled water with moisture free ammonium ferrous sulphate, potassium titanium oxalate and aluminium sulphate salts. Standard silicon solution (1 mg/ml) was used as received after appropriate dilution with double distilled water. 1 g red mud sample was weighed in analytical balance and transferred in to a clean 500 ml beaker. 1% red mud sample was prepared by standard method 9-13 reported for bauxite sample preparation. Precipitated silica in the solution was separated by filtration through Whatman No. 40 ash less filter paper and subjected for alkali fusion. Fused alkali salt of silica was dissolved in double distilled water containing dilute HCl (1:1v) and transferred in to 100 ml standard flask made up to the mark. The resultant sample solution was preserved in teflon beakers for chromatographic study. Chromatographic system Stationary phases Plain cellulose microcrystalline (S 1 ) and Silica gel H impregnated with 0.2% sodium formate (HCOONa) (S 2 ) were used as adsorbent materials. The modification of cellulose for quantitative determination of Si 4+ has described in results and discussion. Mobile phases Mixture of 10% aqueous salt solutions of NaCl with 10% HCOOH ( M 1 ) in 8:2 v/v ratio and 1% sodium molybdate with 3% HCl (M 2 ) in 1:9 v/v ratio (M 2 ) were used as mobile phases. Detection reagents was detected by spraying TLC plates with 0.05% aqueous aluminon (tri-ammonium aurin tricarboxylate, C 22 H 23 N 3 O 9 ) Fe 2+ by 0.1% potassium ferrocyanide ([K 4 Fe(CN) 6 ]) and Ti 4+ (II) by 0.50% tiron (Yoes reagent, C 6 H 4 Na 2 O 8 S 2 ) respectively. Si 4+ was detected by spraying 1% ammonium molybdate [(NH 4 ) 6 Mo 7 O 24.4H 2 O] or sodium molybdate (Na 2 MoO 4.2H 2 O) at ph on semi dried plates. Al 3+ Sample purification 1% test solution of red mud containing Al, Fe and Ti prepared by the standard method was evaporated on a hot plate. The dried sample is dissolved in double distilled water containing few drops of dilute HCl (1:1v) and made up to 100 ml in a standard flask for chromatographic study.

3 NAJAR et al.: QUANTITATIVE ASSESSMENT OF RED MUD BY TLC & SCANNING DENSITOMETRY 101 Buffer solution Acetate buffer of ph~6.98 was prepared by dissolving 25 g ammonium acetate in 50 ml hot distilled water followed by the addition of glacial acetic acid (~3 ml). The mixture was diluted to 100 ml in standard flask. Preparation of TLC plates TLC plates were prepared by mixing silica gel H or microcrystalline cellulose and double distilled water in 1:3 or 1:4 w/v. The slurry obtained was shaken mechanically for 5 min after which it was spread over polished glass plates (3 15 cm 2 for qualitative studies and cm 2 for quantitative studies). The layer thickness on glass plate was physically measured and maintained 0.25 mm (average) for optimization. The plates were dried at room temperature and activated at 100 ± 5 C for 1 h in an electric oven. The activated plates were stored in closed chamber until used. The experimental conditions remain the same for the preparation of cellulose plates where the activation temperature is 60~80 C. Chromatography procedure Standard chromatographic procedure has been used for sample application on TLC plates, chromatogram development, visualization of separated constituents and characterization based on the respective R F values, calculated from the equation R F = R L + R T /2 where R L is the R F of leading front and R T is the R F of trailing front. Compound identification and Separation The representative chromatographic system for the TLC study of red mud constituents was derived from earlier reported studies on bauxite samples. For the compound identification and separation of the cations of interest, a synthetic mixture containing of Al 3+, Fe 2+ and Ti 4+ were prepared in 40:50:10 v/v from their respective 1% aqueous salt solutions. Approximately 10 µl of the solution (ph 6.65) was loaded on the chromatographic plate coated silica gel H impregnated with 0.2% HCOONa. 1% aqueous sodium silicate solution was loaded on modified microcrystalline cellulose for the detection of Si 4+. The plates were developed with M 1 and 5% M 2 respectively for achieving the detection of Al 3+, Fe 2+ and Ti 4+ and Si 4+. Al 3+ appeared as light pink coloured spot (R F = 0.90) with 0.05% aqueous aluminon (tri-ammonium aurin tri-carboxylate, C 22 H 23 N 3 O 9 ), Fe 2+ was detected as blue colored spot (R F ~0.34) with 0.50% potassium ferrocyanide ([K 4 Fe(CN) 6 ]) and Ti 4+ gave yellow spot (R F ~0.01) with 0.50% tiron (Yoes reagent, C 6 H 4 Na 2 O 8 S 2 ). The detection of Si 4+ was achieved by spraying 10% hydrochloric acid on cellulose coated TLC plates pre-developed with 10% sodium molybdate (Na 2 MoO 4.2H 2 O). Si 4+ detected as bright yellow spots. It is noticed that the yellow sodium molybdosilicate spots on S 2 are as bright and compact as the spots on undeveloped TLC plates and show zero mobility (R F ~ 0.04) after development with 3% M 1. Chromatography was performed with TLC plates loaded with 10 µl solutions of red mud samples under the same experimental conditions described for compound identification with synthetic sample. Well resolved compact spots of Al 3+ (R F = 0.82), Fe 2+ (R F = 0.40) and Ti 4+ (R F = 0.02) were obtained on sodium formate impregnated silica gel H plates developed with mobile phase (M 1 ). Si 4+ was detected (R F =0.03) on cellulose plates developed with mobile phase M 1 (5%). The detection efficiency and separation possibilities of all the cations were confirmed with that present in red mud samples of various experimental origins as shown in Table 1. Quantitative studies: Densitometry analysis The quantitative determination of Al 3+, Fe 2+, Ti 4+ and Si 4+ in red mud was carried out by scanning densitometry with deuterium source at the scanning rate of 20 mm s -1 at the respective λ max values ( nm for Al 3+, nm for Fe 2+, nm for Ti 4+ and Si 4+ ) in ascending and descending concentrations. The beam size was size with a pitch 0.04 (Delta Y). Sample Table 1 Chemical compositions of representative red mud sample % Composition of red mud constituents derived from wet analysis Al 2 O 3 Fe 2 O 3 TiO 2 SiO 2 Na 2 O CaO LOI* Total Sample-I Sample-II Sample-III Sample-IV *Loss on ignition

4 102 INDIAN J. CHEM. TECHNOL., MARCH 2011 Al 3+, Fe 2+ and Ti 4+ were separated on silica gel H coated TLC plates (S 1 ). For the determination of Al 3+ and Fe 2+, 20 ml red mud sample (Sample-IV) was evaporated on a hot plate at 100 C. The dried sample was cooled at room temperature and subsequently dissolved 5 ml double distilled water. The sample solution was diluted to 10 ml with acetate buffer (ph 6.98) in 1:1 v/v ratio. Chromatography was performed by loading 4, 6, 8 and 10 µl of the solutions by using micropipette on S 1 plates. The TLC plates were developed with M 1 and the ions were detected (R F ~Fe 2+ : 0.34, Al 3+: 0.92), on separate TLC plates and subjected for densitometric scanning. Calibration curves were constructed for the determination of Al 2+ and Fe 2+ containing 6.26, 9.40, 12.52, µg Al 2+ and 14.4, , µg of Fe 2+. The blue spot of Fe 2+ was scanned at 623 nm and light pink spot of Al 3+ was scanned at 525 nm respectively. The determination of Ti 4+ and Si 4+ was carried out similarly by evaporating 20 ml each of respective samples followed by dilution in to 10 mlwith double distilled water without buffer addition. Subsequently 4, 6, 8 and 10 µl of the samples containing 3.56, 5.34, 7.13, 8.91 µg of Ti 4+ and 4.58, 6.88, 9.17, µg of Si 4+ were loaded separately on S 1 and S 2.. The sets of S 1 chromatographic plates were developed with M 1 and Ti 4+ spots were visualized (R F ~Ti 4+ : 0.01) by spraying tiron. The dull yellow spots of Ti 4+ appeared on the chromatograms were exposed for densitometry at 410 nm. The bright yellow colored spot (R F ~Si 4+ : 0.04) of sodium molybdosilicate appeared on the TLC plate (S 2 ) by spraying with 10% hydrochloric acid. The spot colour remained stable over a period of 2 h. Since the silica sample prepared is free from interference of other cations in red mud, the chromatograms were subjected directly for densitometric evaluation (without development) at 395 nm for the determination of Si 4+. From the peak area correspond to the concentration range of cations loaded on the TLC plates, calibration curves were constructed and the concentration Si 4+ in the standard aluminium samples was determined. Repeated trials were carried out for checking the reproducibility and accuracy of the results. The peak area correspond to the cations present in the samples (I-III) were determined by the simultaneous loading of Sample IV and samples (I-III) side by side on the same TLC plate and chromatography was performed under the same experimental conditions described for standard sample. From the peak area of standard and sample spots, the recovery percentage of unknown cations of interest was determined. Results and Discussion The composition of titanium, iron and silicon in red mud varies in wide range depend on the alumina production parameters (Bayer plant parameters) and quality of raw material input of alumina production. The broad variation in the chemical composition generally restricts limited use of specific chromatographic systems for constituent analysis. This demands minor modifications in the sample preparation as well as chromatographic systems for improving spot compactness and colour stability, better resolution and reproducibility in quantitative assessment. Sample composition and nature of chromatographic system The effect of sample composition on the retardation factor (R F ) of major constituents in red mud was studied with S 1 -M 1 and S 2 -M 2. The R F values recorded for the cations revealed that the variation of chemical composition does not affect the mutual separation of Al-Fe-Ti obtained for different samples. Ti 4+ and Si 4+ remained at the point of sample application showing little mobility on silica gel H and microcrystalline cellulose respectively. In comparison with Fe 2+ and Ti 4+, Al 3+ spot appeared slightly diffused on plain silica gel H plates. The impregnation of silica gel H with 0.2% sodium formate considerably improved the spot compactness as explained in Fig. 1. This lead using impregnated silica gel H plate for all quantitative studies. The zero mobility of sodium molybdosilicate complex presents the choice of using undeveloped plates for quantitative assessment of Si 4+. Fig. 1 Variation of spot size on plain and impregnated palates

5 NAJAR et al.: QUANTITATIVE ASSESSMENT OF RED MUD BY TLC & SCANNING DENSITOMETRY 103 Effect of ph ph of sample and mobile phase plays an important role in the detection and mobility of the cations. It was noticed that a decrease in ph due to the addition of 1% formic acid either in the sample solution or in the mobile phase composition of M 1 harm the separation possibility due to tailing (R L -R T 0.30, were observed where R L is R F of leading front of the spot and R T is R F of trailing front of the spot) of Al 3+ and Fe 2+. The detection efficiency of Al 3+ and Fe 2+ was found best at ph range The spots were compact and brighter and the change of increase of ph from acidic to neutral range do not hampered the detection of Ti 4+. Therefore, the quantitative studies of Al 3+ and Fe 2+ were carried out with the sample solution diluted with acetate buffer in 1:1 v/v. The detection of Si 4+ is highly ph sensitive and the detection and stability of yellow molybdosilicate was achieved only in the ph range of The sample solution needs to be acidified to ph , prior to loading on TLC plate for achieving the detection of Si 4+. Surprisingly, ph effect has found little effect in the detection and resolution of Ti 4+ in red mud sample. Effect of calcium and sodium The presence of calcium and sodium in red mud are the result of their deliberate addition in the Bayer process cycle. In order to study the effect of these inclusion elements on the chromatographic behavior of Al 3+, Fe 2+, Ti 4+ and Si 4+, the sample solution of red mud was mixed with 1% aqueous salt solutions of sodium and calcium (chloride salts) in 7:3, 8:2 and 9:1 v/v ratio. 5 µl of these solutions were loaded on TLC plates and performed the chromatography in S 1 - M 1 and S 2 -M 2. No change in R F values was noticed for Al 3+, Fe 2+, Ti 4+ and Si 4+ in all the three concentration ranges of impurities added. This reproducible R F values in the presence of excess sodium and calcium indicates that these impurity elements has no effect on the chromatographic detection and determination of major inorganic constituents (Al 3+, Fe 2+, Ti 4+ and Si 4+ ) present in red mud. Role of organic constituents Organic matter in Bayer liquor (sodium aluminate extract) has been classified into three main groups: (i) low molecular weight acid salts such as formate, acetate, oxalate, succinate and less amounts of hydroxy acids such as lactic, glycollic and glyceric, (ii) salts of intermediate molecular weight acids, including hydroxy aliphatic acids, benzene, carboxylic acids and phenolic acids with molecular weights of less than 500 and (iii) humic matter of high molecular weights (over 500). Fortunately, the Indian bauxites are characterized with low level of organic constituents (Table 2) and their concentration varies within the range percentages. During the prayer process, the organic components in the bauxite get accumulated in the sodium aluminate liquor and the bauxite residue (red mud) generally left with traces of organics in the order percentage which has no significance in terms of TLC procedures. Limit of detection The detection limits of Al 3+, Fe 2+ and Ti 4+ were determined by spotting different volumes of standard red mud sample solutions on TLC plates (S 1 ) followed by development of the plates with mobile phase (M 1 ). The procedure was repeated with successive lowering of sample concentration loaded on TLC plates until no peak was observed during the scanning of spot by densitometer at the respective λ max values. For Si 4+ the trials were carried out on S 2 pre-developed with sodium molybdate followed by development with 3% M 3 as well as without development of TLC plate. The accuracy and reproducibility of detection limit were confirmed with different aqueous salt solutions of respective cations and red mud samples (Table 3). Table 2 Organic content in some Indian bauxites Sl.No. Bauxite Source TOC as Corg (%) 1 Nilgiris range Mainpat East Coast (Panchpatmali) Kodinganamali TOC: Total Organic Carbon Sample Table 3 Detection limits of cations Detection limit (µg*) Al 3+ Fe 2+ Ti 4+ Si 4+ Potassium titanium oxalate 2.50 Ferric chloride 2.20 Aluminium sulphate 2.00 Sodium silicate 3.50 Red mud (sample II) ** *Average of five consecutive trials under standard chromatographic conditions **Without development of TLC plates

6 104 INDIAN J. CHEM. TECHNOL., MARCH 2011 Densitometry In order to achieve the accuracy and reproducibility of densitometric determination of red mud constituents, minor modifications were made in the routine TLC detection and development procedures. With respect to the constituent elements of interest, the spot (Si 4+ ) was detected on pre-developed plates or the plates were developed with mobile phases containing specific concentration of respective detection reagents to ensure consistency in R F during the repeated trials, least spot diffusion and effective separation. In certain cases, the spot (Al 3+ and Fe 2+ ) mobility was controlled with selective addition of buffers. Further, the laboratory made TLC plates were prepared in optimized experimental conditions (Table 4) for achieving consistency in R F values, spot size and development time. The densitograms of Al 3+, Fe 2+, Ti 4+ and Si 4+ obtained under the optimized experimental conditions are summarized in Figs 2 and 3. Fig. 2 Densitogram of Al 3+ and Fe 2+ in red mud under optimized chromatographic conditions

7 NAJAR et al.: QUANTITATIVE ASSESSMENT OF RED MUD BY TLC & SCANNING DENSITOMETRY 105 Fig. 3 Densitogram of Al 3+ and Fe 2+ in red mud under optimized chromatographic conditions Table 4 Conditions for quantitative TLC analysis of red mud Item Optimized condition Sample Nature Acid digested Concentration % Sample loading volume 5-10 µl Potassium ferrocyanide 0.10% aqueous Aluminon 0.05% aqueous Tiron 0.50% aqueous Sodium molybdate 0.50% aqueous Al 3+, Fe 2+ and Ti 4+ S 1 impregnated with 0.20 HCOONa Si 4+ S 2 impregnated with 10.0 % Na 2 MoO 4.2H 2 O Solid liquid ratio for slurry 1:4 (w/v ) for S 1 and 1:3 (w/v) for S 2 Plate size cm 2 Slurry volume for plate 10 ml coating Coating layer thickness Approx. 2-3 mm Plate activation temperature 100 ± 5 C for S 1 and C for S 2 Plate activation time 60 ± 5 min. Sample loading temperature 30 ~ 40 C (room temperature) 10% NaCl + 10% HCOOH 8:2 v/v Mobile phase volume 10 ml Mobile phase ascent 7 cm Conclusions The present TLC method dedicated to the analysis of red mud sample permits the separation and determination of its constituents within 15 min, which allows a high throughput of samples. The method was performed for quite a few months in the laboratory under different climatic conditions and has clearly proven its reliability irrespective of chromatographic variations with respect to humidity and temperature. The investigations revealed that TLC procedure can be successfully modified and coupled with sophisticated instrumental methods for quantitative determination of trace elements in natural samples. Further, the optimised conditions for quantitative TLC analysis of red mud has found identical for that reported for bauxite constituents and reiterate the utility of TLC-Densitometry procedure for trace level detection of cations including silica in red mud samples as

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