J O U R N A L O F C H E M I S T R Y Materials Hydrogen bonded associates in the Bayer process (in concentrated aluminate lyes): the mechanism of gibbsite nucleation A � gnes Buva�ri-Barcza, Ma�rta Ro�zsahegyi and Lajos Barcza* Institute of Inorganic and Analytical Chemistry, L . Eo� tvo�s University, Budapest-112, P.O.Box 32, 1518 Hungary The highly alkaline aluminate lye, which is produced in the first step of Bayer process, is characterized by low water activity and strong competition for water molecules.The consequence of these eVects is that two (or more) negatively charged species can be brought together and stabilized in the form of anionic hydrogen bonded complexes. Depending on temperature, total concentration and aluminate to hydroxide ratio, aluminate–hydroxide as well as aluminate–aluminate associates can be formed.Among the oligomerized aluminates, the cyclic hexamer seems to play a key role, as it contains octahedrally coordinated aluminate ions and is able to form nuclei in further polymerization for the partial precipitation of Al(OH)3. The interactions have been experimentally investigated using specially developed methods and a detailed computational analysis.Alumina is produced in several million tons per year by the CaO·Al2O3·10H2O 27 points to the existence of polymeric Al6(OH)246- species, containing six edge-linked, octahedrally Bayer process. The first step of this method is the extraction of bauxite using concentrated NaOH at high temperature.1,2 coordinated aluminate ions. Based on solid state 27Al NMR investigations,28–30 more and more data have been accumulated When the bauxite contains ‘monohydrates’ (AlOOH: boehmite and diaspore, minerals from temperate zones) the extraction on concentrated aluminate solutions containing not only tetrahedral but also octahedral species.31–33 Concerning the com- requires higher concentration (20–30% NaOH) and higher temperature (200–250 °C under 3.5 MPa pressure), while baux- position of aluminate lye, similar semi-empirical results can be deduced using molecular modelling.34 ite containing ‘trihydrates’ [Al(OH)3: gibbsite and hydrargillite, minerals from the tropics] can be extracted at lower tempera- According to our earlier work, the viscosity of a solution with strictly constant cation concentration and temperature is ture (120–140 °C) and with less concentrated caustic soda (ca. 15% NaOH).1–3 The highly alkaline solution is metastable at sensitive to any association, and the conductivity (under the same circumstances) is very sensitive to the hydroxide ion any temperature and can be decomposed to form Al(OH)3 precipitate. [The symbol Al(OH)3 represents here the concentration.35 (When either the total concentration or the temperature is changed, no theory is able to accurately predict amorphous�pseudo-boehmite�bayerite�gibbsite series.4] The separated product is always contaminated by some free their eVect on concentrated solutions.) To elucidate the diVerences between the two main versions of the Bayer process, the alkali, which is attributed to the unwashable mother-liquor.All forms of the precipitated Al(OH)34,5 consist of octahedrally investigation of the corresponding systems has been conducted, coordinated aluminiums connected by bridging hydroxide ions combining the experimental methods mentioned above with in a layered structure of six-membered rings6,7 ( like the honey- the most eYcient computer simulation. Two model series were comb structure of the carbon atoms in graphite layers), and chosen which resemble the processes used in practice: (i) a the layers are held together by hydrogen bonds.3 It is generally system with constant 6 M sodium (hydroxide) concentration at known that the production of Al(OH)3 is influenced both 25 °C and (ii ) a system with constant 4 M sodium (hydroxide) qualitatively and quantitatively by the total concentration, the concentration at 65 °C.In both series, the molar ratios (convenalumina/ sodium hydroxide ratio and the temperature of the tionally: r=Na2O/Al2O3) were varied down to rather low aluminate lye.1,2 In spite of the fact that several empirical and values (r#1.35, i.e. the concentration of aluminate in such semi-empirical rules are known concerning alkaline aluminate solutions was very high). solutions, further work is of importance.The interaction between aluminium and hydroxide ions has been mainly investigated in terms of hydrolysis, i.e. in more or Experimental less acidic solutions,10–14 where the main component is the Sodium hydroxide and water were purified to eliminate any octahedral Al(H2O)63+ ion itself, however the relative stability carbonate.Aluminate solutions were prepared by dissolving of a terdecamer cation Al13O4(OH)24(H2O)127+ (containing high purity (99.99%) aluminium metal in sodium hydroxide twelve [AlO6] octahedra joined together by common edges) is solution. Contamination by silicate and carbonate were care- of note.15 fully avoided: the solutions were prepared and stored in The composition of alkaline aluminate solutions and the polythene vessels under carbon dioxide free nitrogen.structure of the aluminate anion have been investigated by To check whether the properties of solutions depend on several workers,9,16–22 the definitive review is that of Eremin their preparation, freshly prepared or aged, heated and/or et al.,23 who analysed UV, IR, Raman and NMR spectra, as mixed and/or diluted solutions of identical composition were well as electrochemical, thermodynamic and kinetic properties. studied.Disregarding some solutions of very low molar ratio Similarly, the formation constants for the Al(OH)3–Al(OH)4- (r<1.3, i.e., with extremely high aluminate concentration), the equilibrium system have been measured by several workresults are very reproducible, which means that the equilibria ers;10–14 the definitive review of earlier work is that of Baes between the diVerent species must be fast and reversible.and Mesmer.12 In solid state, the Al2O(OH)62- unit (built up The viscosities were measured in Ostwald type viscometers from two [AlO4] tetrahedra) was identified in a crystalline constructed of alkali resistant glass and having capillary diam- solid24 (and detected later in concentrated solutions by IR, NMR and Raman methods25,26) while the structure of solid eters of 0.47, 0.53, 0.63 or 0.84 mm.They were calibrated with J. Mater. Chem., 1998, 8(2), 451–455 451tration (r>6 in 6.0 M and r10 in 4.0 M solutions, respectively), while a mixture of monomeric and dimeric aluminate species is assumed if r2 in 6.0 M solutions at 25 °C or r4 in 4.0 M solutions at 65 °C (which means that the aluminium concentrations are<3 or 1 M, respectively).The viscosities of solutions with higher aluminate content can be best approached (in 4.0 M solutions at 65 °C) assuming monomeric, dimeric and hexameric aluminate species, while any combination of polymeric species (including the variation mentioned as the simplest case) fits well for 6.0 M solutions, too.The measured conductivities show a very interesting trend, characteristic for complex formation.35 Namely, when we suppose a single monomeric aluminate component [i.e. Al(OH)4-] at relatively low total aluminate concentration, the equilibrium concentration of monomeric aluminate can be presumed to be equal to the total aluminium concentration (cAl): cAl=[NaAl(OH)4] (1) and we can assume: k=lNaOH [NaOH]+lNa-aluminate [NaAl(OH)4] (2) where k is the measured conductivity, l is the molar conductance of a given species (in the given media), while [ ] denotes equilibrium concentrations [eqn.(2) reflects the well known Fig. 1 Viscosities (×=every fifth data point, —=calculated curve) rule of additivity].According to our experiences, this rule is measured in 6 M solutions and at 25 °C valid also for the conductivity of rather concentrated electrolyte mixtures with constant common cation concentration, e.g. for that of NaOH–NaClO4 solutions with constant 6.0 M sodium ion concentration, or OH–K2CO3 mixtures with constant 5.0 M potassium ion concentration.35 As lNaOH can be measured in pure NaOH solutions (cAl= 0.0 M) very precisely, the ‘free’ sodium hydroxide concentrations and its conductivity [based on eqn.(2), disregarding the conductivity of sodium aluminate] can be calculated in the concentration range under discussion. The calculated, hypothetical values give a straight line, as shown in Fig. 2. The (negative) diVerence between the measured and hypothetical conductivities can not be explained by assuming completely undissociated sodium aluminate ion pairs (since the molar conductance can never be negative) but only by interactions which decrease the hydroxide ion concentration, such as the formation of a hydrogen bonded hydroxide– Fig. 2 Conductivities (×=every tenth data point, —=calculated data, B=the calculated conductivity of the free sodium hydroxide content) aluminate complex Al(OH)4-·OH-. Extremely low water measured in 6 M solution and at 25 °C activity is characteristic for aluminate lyes8,17,23 and therefore strong competition exists for water molecules, which can bring the two negatively charged species together.(Of course, the glycerol solutions of known concentration and viscosity.36 The flow-time varied from 60 to 150 s, measured with an accuracy associate can bind some water molecules, also, but the total quantity of bound water is surely decreased.The role of low of±0.05 s. Conductivity data were recorded with a Radiometer CDM-2d type conductometer using CDC-104 or other, water activity has been previously discussed by Scotford and Glastonbury17 and by Eremin et al.23) specially adapted electrodes.36,37 The reproducibility of measured data was better than ±0.5%.The temperature was kept The oligomerization equilibria of tetrahydroxo aluminate anions and their background (OH- here) can be explained strictly constant (within ±0.02 °C), as was the sodium ion concentration (variation <±0.1%).similarly. Further, both possibilities have been considered. Since the formation constant of the Al(OH)4- ion (b4= The data were measured in diVerent ranges of molar ratios (r>6, r=6–3, r=3–2 and r=2–1.35), i.e. starting with low [Al(OH)4-]/[Al3+][OH-]4) is extremely high (at 25 °C and at ionic strength extrapolated to 0.0: log b43313,14), it must aluminate content and increasing the concentration up to the highest values.The measurements were repeated several times. be regarded as the basic component for all species containing aluminium in alkaline solution: Some of the viscosity data measured in 6.000 M solutions and at 25.0 °C are presented in Fig. 1 and those of conductivities pAl(OH)4-+q OH-=(Al(OH)4-)p(OH-)q (3) in Fig. 2. (The data measured in 4.000 M solutions and at 65.0 °C show no special characteristics, the trends of data The general definition of the formation constant for this equilibrium is as follows: are similar.) bpq=[(Al(OH)4-)p(OH-)q]/[Al(OH)4-]p [OH-]q (4) Results We can define a parameter Y 35,36 (in general form, similarly to the simpler case represented by eqn.(2), at constant cation The measured viscosity data could be fitted in a computer simulation [based on equations similar to eqn.(2) and (5),36,37 concentration and temperature) as with coeYcient(s) characterising the contribution(s) of the Y=.. fpqbpq [Al(OH)4-]p [OH-]q (5) individual species to the viscosity measured] assuming monomeric species alone only at relatively low aluminium concen- where fpq symbolizes the linear contribution of a ( p,q) species 452 J.Mater. Chem., 1998, 8(2), 451–455to the given property. It follows that fpq is a constant valid only for the given electrolyte and temperature (such as molar conductance in the case of conductometric measurements). According to eqn. (5) and using the expressions of mass balances: cAl=.. p bpq [Al(OH)4-]p [OH-]q (6) cOH-=..q bpq [Al(OH)4-]p [OH-]q (7) both the viscosity and conductivity data can be evaluated in parallel and step by step in the whole concentration range investigated. With the aid of eqn. (6) and (7), some relations, used earlier, can be defined more exactly, as cNa+=cOH-+cAl=const (8) Fig. 3 Molar speciation in 6 M solutions at 25 °C as a function of the and total aluminium concentration (ci expressed as molar concentration of Al in the form of the given species; —=Al(OH)4-; – –= r=cNa+/cAl (9) [Al(OH)4]22-;— —=Al6(OH)246-; E=Al(OH)4-·OH-; In systems of low aluminate content (at higher r values: r6 I=[Al2(OH)82-·OH-]) or r10, respectively, for [Na+]=6 or 4 M) the viscosity data can be interpreted by monomeric aluminate species [which can be either Al(OH)4- or Al(OH)4-·OH-; viz.(1,0) and (1,1) species]. In the same concentration range, the conductivity data indicate both (1,0) and (1,1) species and their stability constants can be computed using eqn. (5)–(7). Correlating the viscosity data with these constants using eqn. (5)–(7) leads to a perfect fit of measured and calculated data. (We should mention here that constants calculated in the range of lower aluminate concentrations were kept unchanged for the subsequent computer calculations.) The evaluation of viscosity data in themselves shows the increasing presence of dimeric species with increasing cAl (with decreasing r, between r=6–2 and r=10–4, respectively for [Na+]=6 and 4 M), corresponding to Al2O(OH)62-, indicated here as {Al(OH)4-}2 or (2,0).[However, the formation of an Al(OH)4-·Al(OH)4- species could also be rationalized similarly to the formation of Al(OH)4-·OH- species, and it may Fig. 4 Molar speciation in 4 M solutions at 65 °C as a function of the have a similar Raman spectrum to that of Al2O(OH)62-.34] total aluminium concentration (see Fig. 3 caption for key) Computation of the conductivity data does not give a good fit assuming only (1,0), (1,1) and (2,0) species, and hydroxide reasonable considering the temperatures and the mainly hydroassociate (2,1) has to be introduced.It is remarkable that no gen bonded character of the interactions. The mole fractions (2,2) complex exists:38 repulsion among the four negatively of diVerent species as a function of cAl are presented in Figs. 3 charged ions (two of them being hydroxide ions) seems to be and 4.too strong. It should be mentioned that the existence of tri-, tetra-, Both viscosity and conductivity data can be well fitted down penta-, hepta- and higher oligomeric species can be neglected to r=2.1 (6 M, 25°C, cAl2.86 M) or r=4.3 (4 M, 65°C, as minor components, when we characterize the system cAl0.93 M), respectively, with the series of (1,0), (1,1), (2,0) assuming a minimum number of species present, however, the and (2,1) species.Below these r values (at higher cAl), many possibility of their existence at low concentrations is not ruled combinations were tried but the simplest (i.e. using the lowest out, in contrast to (1,2) Al(OH)63- or (2,-1) Al2 (OH)7- number of constants) and best fit was achieved when a species, which were, among others, also tested. hexameric (6,0) species was added to the series.The calculated stability constants are summarized in Table 1. Discussion It must be emphasized that these values are only valid for the given environment: for the constant cation concentration and The key species in the aluminate lye seems to be the (cyclic) temperature (and may also involve ion pair and other formahexamer, since this is the smallest oligomer where octahedral tion constants).The association constants are much higher in coordination can be fulfilled for every aluminium (Fig. 5). The 6 M NaOH (25 °C) than in 4 M (65 °C), the diVerences being ring contains six octahedrally coordinated aluminiums connected by two bridging hydroxide ions and the aluminium atoms are in a planar arrangement.This structure is identical with Table 1 Main species in aluminate lyes and their formation constants the basic unit of both bayerite and gibbsite3,6,7 determined in stability constant, bpq the solid phase, therefore the olation type polymerization by O2- bridging ligand seems less probable. p,q species 6 M, 25°C 4M, 65°C If we assume that (four) monomeric Al(OH)4- units are able to attach in-plane to the hexameric aluminate (see Fig. 5), 1,0 Al(OH)4- 1 1 0,1 OH- 1 1 a second six-membered ring could be formed (cf. naphthalene 1,1 Al(OH)4-·OH- 3.9±0.2 0.25±0.01 C10H8) with formula Al10(OH)388-. Similarly, a condensed 2,0 [Al(OH)4]22- 1.9±0.2 0.38±0.04 three ring system ( like anthracene or phenanthrene, C14H10): 2,1 [Al(OH)4]22-·OH- 26.7±3.1 1.0±0.1 Al14(OH)5210- could also be formed. The oligomer correspond- 6,0 [Al(OH)4]66- 33.0±4.3 15.0±2.0 ing to coronene (C24H12), consisting of seven rings can also be J.Mater. Chem., 1998, 8(2), 451–455 453range in 6.0 M NaOH solution (at 25 °C). It follows that the important hexameric species is only formed in a rather narrow range of cAl, where [Al(OH)4-] is very low.The consequence is that the on-plane polymerization will be predominant, leading to very small crystallite particles termed ‘mealy’ product industrially (with relatively high unwashable caustic soda content). Lower total concentration and higher temperature (4.0 M and 65 °C) hinder hydrogen bonded associations (Fig. 4): oligomerization is predominant over nearly the whole concentration range, but [Al(OH)4-] is high enough to promote in plane polymerization.The main process is therefore the growth of nuclei, which occurs also at lower aluminate concentration Fig. 5 The conventional atom-and-bond (a) and coordination poly- resulting in the precipitation of larger particles (of ‘sandy’ hedral (b) model (omitting the charges) of the Al6(OH)246- ion texture).Financial support of this work from the Hungarian Research supposed: Al24(OH)8412-. (Theoretically this compound can exist, but it is mentioned only for demonstrating the relative Foundation (OTKA T 019493) is gratefully acknowledged. decrease of charge as a function of the degree of in-plane polymerization.) The water solubility of such types of oligomer References is surely influenced by their size and charge, and associates containing three to four hexameric units are probably at the 1 T.G. Pearson, T he Chemical Background to the Aluminium limit of water-solubility. In-plane polymerization results in Industry, Royal Institute of Chemistry, London, 1955. structures identical with the structure of layers in crystalline 2 S.I. Kuznetsov and V. A. Derevyakin, T he Physical Chemistry of Alumina Production in the Bayer Method, Metallurgizdat, gibbsite.3,6,7 Moscow, 1964. On the other hand, the growth of the hexameric core is 3 N. N. Greenwood and A. Earnshaw, Chemistry of the Elements, possible not only in-plane but also on-plane. Two hexamers Pergamon Press, London–New York, 1984, p.273.could attach by hydrogen bonds, since the shape of their 4 N. Dezelic, N. Dikinski and R. H. Wolf, J. Inorg. Nucl. 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