¹Physics Engineer, Universidad del Cauca. Ciencia y Tecnología de Materiales Cerámicos Group (CYTEMAC), Department of Physics, Popayán, Colombia. cvruiz@unicauca.edu.co ² Ph.D., in Physical Sciences with emphasis on Materials. Teaching and Research, Ciencia y Tecnología de Materiales Cerámicos Group (CYTEMAC), Department of Physics, Universidad del Cauca Colombia, Popayán, Colombia. jpaez@unicauca.edu.co.

ABSTRACT

Sodium aluminate, NaAlO₂• xH₂O, is an important comercial chemical used in water treatment, as a source of aluminium in the preparation of zeolites and other catalytic materials and as an additive in paper manufacturing. Sodium aluminates were synthesised in this work by using the Al(NO₃)₃ • 9H₂O- NaOH system's controlled precipitation method. Using the controlled precipitation method enabled identifying the process's different stages from potentiometric titration of Al(NO₃)₃ • 9H₂O dissolved in water and using NaOH as precipitating agent to ensure control and reproducibility and also identify final product characteristics. Powders were characterised by infrared spectroscopy (FTIR), X-ray diffraction (XRD) and gravimetric and differential thermal analysis (GDTA). The results indicated that NaAlO2 crystallised well at 1,000°C (obtained as the main crystalline sodium aluminate phase) and that β-alumina treatment began to form at 1,500°C.

Keywords: sodium aluminate, synthesis, controlled precipitation method.

Received: jun 12th 2009

Accepted: jun 11th 2010

Introduction

Sodium aluminate NaAlO₂ X xH₂O is an inorganic compound having great industrial importance. This mixed oxide is used in water treatment and as a source of aluminium in other applications relating to the preparation of zeolites and other catalytic materials, as additives in paper manufacturing and for adjusting the pH in many applications (Misra, 1986; Kaduk and Pei, 1995).

An interesting use of sodium aluminate is related to special cement hydration at low temperature. Lota et al., (1997) investigated the effect of NaAlO2 on special cement hydration.

The results of their study indicated that hydration of this cement in 0.1 to 1.0 M aqueous solutions of NaAlO₂ was acelerated because the compound prevented the formation of the hydration boundary layer which completely surrounded the cement grains.They also observed that aluminium ions in the solution quickly reacted with calcium ions released from calcium silicate phase (when it is dissolved) and formed hydrated calcium aluminates (Lota et al., 1997). Andersen et al., (2004) found that NaAlO₂accelerated alite and velite hydration (5°C and 20°C, respectively), such result enabling the use of NaAlO₂as a cement hydration accelerator in cold conditions.

The most common for of sodium aluminate is found in the solid obtained by crystallisation which occurs in a concentrated aqueous solution from Na₂O and Al₂O₃ (Misra, 1986). In the literature, Misra has reported a phase diagram of the Al₂O₃-Na₂O-H₂O system, showing the formation of NaAlO₂ 3/2H₂O and NaAlO₂ 5/4H₂O at low temperatures (Misra, 1986).Although NaAlO₂ 3/2H₂O (PDF 2-1025), NaAlO₂ 3H₂O (PDF 29-1165) y el NaAlO₂ 5/4H₂O (PDF 41-638)diffraction patterns have been reported, the crystal structure of sodium aluminate has not been fully elucidated (Kaduk and Pei, 1995). NNaAlO₂ 5/4H₂Ohas a structure where the anions are highly polymerised, with tetrahedrally coordinated Al and a tetragonal unit cell (a = 10.530 À y c = 5.300À) (Kaduk and Pei, 1995). On the other hand, commercial sodium aluminate NaAlO₂ xH₂O has a tetragonal unit cell (a = 10.5349 À and c = 5.3358 À) that Kaduk and Pei could not fully resolve using the information obtained in their laboratory. As mentioned so far, sodium aluminates, not only have technological interest, but there is an additional motivation regarding the need to fully describe its structure; this requires using a reproducible and controllable strategy for obtaining theNaAlO₂ needed both for basic study and the technological applications mentioned above.

The controlled precipitation method was used in this work for ynthesising sodium aluminate. This method allows the synthesis of ceramic powders from a solution(Arai, 1996; Ganguli and chatterjee, 1997; Ring, 1996). The corresponding precursors,dissolved in a solvent, are precipitated by adding a chemical compound (precipitating agent) that reacts with the solution; the other alternative is to add the precursor solution to the solution containing the precipitating agent in excess. In both cases there is a decrease in solubility leading to the recipitation of metal. The hydrolysis and condensation reactions of metallic salts occurring in aqueous solutions in this process can be expressed so (Fernández,2003):

M+z(aq)+zOH-(aq)→M(OH)_z(s) (1)

M-OH→[M(OH)(H₂0)_N-1]^(z-1)+(2)

The stages of the process are: nucleation, being the formation of embryos and later cores (seen a slight turbidity in the solution), the growth of cores (turbidity is more intense), sedimentation and aging.

The increase in metal ion concentration would increase the number of cores, induce a high degree of supersaturation and reduce particle size. The pH of the system (it may appear compound) may have several polymorphic forms at different pH. Incomplete precipitation to take a low pH must be avoided, or otherwise soluble complexes may be formed by using a high pH. Temperature affects salt solubility. Mixing speed prevents the occurrence of agglomerates and treatment temperature is important in obtainingthe compound of interest.

Some of the problems with this synthesis method provide for the presence of impurities in the products due to the precipitating agent,by an incomplete or non-uniform precipitation or incorrectlaundering operation that may cause further material losses with consequential stoichiometric deviations (Fernández, 2003). Sodium aluminate powders were synthesised by using the controlled precipitation method. The titration curve for the Al(NO₃)₃ 9H₂-H₂O system was obtained for understanding the system's evolution and identifying the process steps when adding a NaOH solution, at 2M concentration, to an Al(NO₃)₃ 9H₂O-H₂O solution having 0.1M concentration. Powders were characterised using infrared spectroscopy (FTIR) to ascertain the main functional groups, X-ray diffraction (XRD) was used for identifying the system's crystalline phases and gravimetric and differential thermal analysis (GDTA) for determining the effect of heat treatment on the solid.

Experimental procedure

Sodium aluminate synthesis using controlled precipitation

Solutions of 0.1M aluminium nitrate (Al(NO₃)₃ 9H₂O-Merck 95%)and 2M sodium hydroxide aOH-Mallinckrodt 98.7%) were prepared separately in 50 ml and 100 ml volumes of distilled water, respectively. The aluminium nitrate solution was stirred continuously at room temperature (728 Stirrer Metrohm). The sodium hydroxide solution was then added to the nitrate solution using a dispenser (Metrohm Dosimat 685) at 0.0046 ml/s (0.084 ml every 18 s) addition rate. Potentiometric titration curves were obtained by recording the pH values (pH-metro Metrohm 744) when adding the NaOH; the system was brought to 11.24 pH.Figure 1. gives an outline of the synthesis process. The resulting suspension was dried in an oven at 70°C for 24 hours and the powder was macerated using an agate mortar. This powder was heat-treated at different temperatures and after being characterised.

Characterising the ceramic powders so obtained

The powders obtained by controlled precipitation were initially characterised with infrared spectroscopy (Termo Nicolet IR200 Spectrometer) to identify the major functional groups present in the sample. The changes in functional groups considering different heat treatment made to the solid precipitate were then analysed.

X-ray diffraction (XRD) was used for determining the crystalline phases existing in the solids. CuK_α (1.54056 Å) radiation equipment was used for this (10 to 70° range). Differential thermal analysis (DTA) (DTA-50 Shimadzu) was used for ascertaining the different physical-chemical events occurring when the solids being synthesised were thermically treated (dry air atmosphere, 7 l / min flow and 10°C/min heating rate) and subjected to thermogravimetric analysis (TG).

Figure 1. Experimental procedure using the controlled precipitation method

Results

Potentiometric titration

Four regions can be identified in the titration curve obtained for the 0.1M aluminium nitrate and 2M sodium hydroxide solution (Figure 2)showing the stages of precipitate formation. There wasOH^- consumption and pH variation in region 1 (Figure 2), not as representative as in region 3 where the slope of the curve indicated rapid change in pH values. Region 2 was a plateau indicating little change in pH for the system and represented high OH^- consumption. Region 4 corresponded to the solution's saturation region(Cobo, 2005)

There is total solubility of Al(NO₃)₃ 9H₂O in water; the solution obtained is transparent and homogeneous. If complete dissociation of the precursor is considered, then this reaction can be expressed as (Cobo, 2005):

This reaction normally occurs with partial hydrolysis of the chemical species of Al, where Al(NO₃)₃ becomes partially dissociated, forming aluminium nitrate aquo species, as proposed in the following reactions (Cobo, 2005);

[Al(NO₃)₃]_(ac)+2H₂O → Al(NO₃)₂(H₂O)₂]⁺NO₃^-(4)

[Al(NO₃)₂(H₂O)₂]⁺_(ac)+2H₂O→ Al(NO₃)₂(H₂O)₄]⁺²NO₃^-(5)

[Al(NO₃)(H₂O)₄]⁺²_(ac)+2H₂O → Al(H₂O)₆]⁺³_(ac)+NO₃ (6)

Figure 2,Potentiometric titration curve for the 0.1M Al(NO₃)3•9H₂O/ 2M NaOH system

As water has a high solvation power, each species produced through the above reactions experienced hydrolysis reactions favouring [Al(H₂O)₆]⁺³, formation (equation (5)), where Al presented an environment coordinated with six (6) water molecules as first neighbours. On the other hand, the formation of aquo hydroxo aluminium nitrate should be considered, as shown in the following reactions (Cobo, 2005);

[Al(NO₃)₂(H₂O)(OH)]_(ac)+H₂O→ [Al(NO₃)₂(OH)₂]^-_(ac)+ H₃O⁺ (8)

[Al(NO₃)₂(OH)₂]^-_(ac)+ 2H₂O →[Al(NO₃)(OH)₄]^-2_(ac)+ 2H₃O ⁺_(ac)+NO ₃^- (9)

Observing region 1 in Figure 2, the precipitating agent interacted with the chemical species in the aluminium nitrate solution, slight turbidity being observed in the system. This effect indicated the formation of colloidal aggregates. The slight increase in pH value in this region may have been due to the neutralisation of acid species for the OH^- resulting from NaOH dissociation. The protons (H⁺) generated during mononuclear species formation (equations 3 to 5) were also neutralised. The hydrolysis and condensation reactions in region 2 would permit the forming of mono, polynuclear and polymeric species. This species allowed the formation of solid phase embryos inside the solution and when they acquired a critical size the solid phase nucleus would become consolidated in the system. As the solution contained nitrate ions, it was possible to form dimers containing this anion (Al₂(OH)₂(OH₂)₂(NO₃)₆)^-2•[Al₂(OH)₂(OH₂)₈]⁺⁴, [Al₁₃O₄(OH)₂₄(OH₂)₁₂]⁺⁷ and [Al₃(OH)₄(OH₂)₉]⁺⁵, polynuclear species may be obtained. These chemical species can lead to the formation of [Al₂(OH)₂(H₂O)₈](NO₃)₄ in the system (Cobo, 2005).

The curve presented a marked change in pH in region 3, Figure 2.This behaviour indicated that the process' effectiveness for forming the aluminium complex and nucleus formation became reduced. The low OH^- consumption for the system was related to the system's pH increase (Cobo, 2005).

Region 4 of the curve, Figure 2, corresponded to the saturation of the system. The white suspension was more intense; this indicated that chemical weathering and formation of colloidal particle agglomerates were the principal phenomena in the process (Cobo,2005). The NaOH used as precipitating agent was the source of sodium which could be expected to become incorporated inside the solid phase matrix obtained. This process is vital for shaping sodium aluminates.

Fourier transform infrared spectroscopy (FTIR)

Figure 3 shows the IR spectra for ceramic powders treated at different temperatures which were obtained by controlled precipitation at pH 11.24.

Figure 3. Infrared spectra of powders obtained by precipitation and treated at different temperatures.

The 3,440 cm^-1 band in the spectra was assigned to a vibrational mode of H-O-H, while the 1,630 cm^-1 band could have been associated with water in the system. For the spectrum corresponding to the sample without heat treatment, Figure 3, bands can be identified as being associated with NO₃^- free, N-O bonds in 1,383 cm^-1 doubly degenerate tension (strong bond), and were located at 830 and 750 cm^-1 which corresponded to deformation modes (Cobo,2005). The band located at 525 cm^-1 would have been associated with the vibration of Al-O bond (Nakamoto, 1962). The following bands were not associated with NO3 - in the spectra of the samples treated at 1,000°C, 1,200°C and 1,500°C ( Figure 3,): the 1383 cm^{- 1} band characteristic of this functional group and bands appearing at 811 cm^-1, 806 cm^-1 and 803 cm^-1, which may have correspondded to the formation of O-O triangular species bonds. Bands at 627 cm^-1 and 558 cm^-1 must have corresponded to the vibrations of the Al-O bond (Nakamato, 1962). The shift of the band at 627 cm^-1 present in the spectrum of the sample treated at 1,000°C, at lower wave numbers, for treatments at 1,200 °C and 1,500 °C, indicated that the binding energy and stability of the functional groups became greater when samples were treated at high temperatures.

Bands associated with O-Na-O bonds were identified at 456 cm^-1 at 1,000°C, 448 cm^-1 at 1,200°C, and 447 cm^-1, at 1,500°C for the spectra of thermally-treated samples ( Figure 3) (Londoño, 2004).

X- ray diffraction (XRD)

The main crystalline phases present in the solids obtained by the synthesis method used in this study of the Al(NO₃)₃ • 9H₂O-NaOH system were subjected to different thermal treatments (Figure 4).

Figure 4.XRD patterns for solids from the [2M] NaOH/[0.1M] Al(NO3)3, system pH 11.24, treated at 1,000°C, 1,200°C and 1,500°C for 1 hou.

The diffractograms in Figure 4 show that the majority phase of well-crystallised sodium aluminate (NaAlO₂) (PDF33-1200) was presented at 1,000°C. Sodium aluminate phase was still present as main crystalline when treating the sample at 1,200°C where representative peaks were more defined. The sample treated at 1,500°C presented a mixture of phases, predominantly sodium aluminate (NaAlO₂). There were also peaks corresponding to β-alumina phase(Na₂O(Al₂O₃)₁₁).

Gravimetric and differential thermal analysis GDTA

Figure 5. GDTA curves for the [2M] sample in NaOH / [0.1M] of Al(NO₃)₃ obtained by controlled precipitation

This curve presented two endothermic peaks around 86°C and 112°C which may have been associated with the volatilisation of physiadsorbed water and crystallisation, respectively. An endothermic peak appeared around 258°C, associated with a slight weight loss in GDTA curve which might have been mainly associated with the dehydroxylation of NaOH (Arai, 1996). The intense endothermic peak at ~306°C may have corresponded to the de-hydroxylation of Al-OH type species present in the sample, which would have led to the formation of Al-O bonds. On the other hand, there could have been progressive elimination of the nitrate ion, because rapid weight loss was shown from ~ 632°C which was associated with an endothermic peak on the GDTA curve.

The sample had constant weight at ~838°C and endothermic peaks presented at ~917°C and ~1,100°C could have been associated with the formation and crystallisation of sodium aluminate which was the majority phase observed in the diffractograms (Figure 4) from 1,000° C to 1,200°C .

Conclusions

Potentiometric titration identified the stages which occur during synthesis (four regions) for which the main physicochemical phenomena which can occur have been detailed. This allowed a means of control structure for the process and to ensure reproducibility.

Both the results of infrared spectroscopy and XRD indicated that NaAlO2 training is already evident from 1,000°C onwards. Infrared spectroscopy identified bands corresponding to the vibrations of the Al-O bond and the band associated with the O-Na-O bond,located between 456 cm^-1 and 448 cm^-1. Samples heat treated at temperatures above 1,000°C had bands associated with Al-O bonds and O-Na-O shifted towards lower wave numbers, indicating greater bond strength and therefore greater stability for the compound so formed.

The X-ray diffractograms of the samples studied revealed that the main crystalline phase present in the samples wasNaAlO₂(PDF33- 1200) and Na₂O(Al₂O₃)₁₁ (PDF19-1177) as a minority phase, specifically in the sample treated at 1,500°C.

The results will form the basis for future research which will discuss the use of sodium aluminate in obtaining catalytic and cementitious materials and beta alumina (Londoño, 2004; Ruiz, 2008),mainly considering their use as solid electrolyte and gas sensor.

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