Preparation of Gelatinous Aluminium Hydroxide from Aqueous Solutions of Aluminium Salts Containing Sulphate Group with Alkali

Journal of the Ceramic Society of Japan 104 [5] 377-382 (1996)

Taichi SATO*,** and Keiichi SATO* *Faculty of Engineering , ShizuokaUniversity, 3-5-1, Johoku, Hamamatsu-shi 432 **Metallurgical Engineering Department , Queen'sUniversity, Kingston, Ontario K7L 3N6, Canada

アルカリによる硫酸根を含むアルミニウム塩の水溶液からのゲル状水酸化アルミニウムの沈殿

佐藤太一*,**・佐藤馨一* *静岡大学工学部, 432浜松市城北3-5-1 ** Metallurgical Engineering Department , Queen's University, Kingston, Ontario K7L 3N6, Canada

The precipitation of gelatinous aluminium hydroxide from aqueous solutions of aluminium sulphate and ammonium, potassium and sodium slums with sodium hydroxide solution was investigated under different conditions. The resulting precipitates were examined by X-ray diffraction study, infrared spectroscopy and thermal analysis (TG and DTA). It was determined that the composition of the precipitates depends on experimental conditions, in particular the pH value of aqueous solutions: this composition varied in the order of amorphous aluminium hydroxide, pseudoboehmite and bayerite as the pH of aqueous solutions in their formation increased, although the precipitation behavior was influenced by aluminium salts containing sulphate. The results obtained are discussed in comparison with the precipitates from aqueous solutions of chloride or nitrate of aluminium with alkali. [Received June 19, 1995; Accepted January 18, 1996]

Key-words: Gelatinous aluminium hydroxide, Aluminium sulphate, Ammonium, Potassium and sodium alums

1. Introduction the case of ammonium alum which requires relative As the composition of gelatinous aluminium ly large amounts of alkali to reach a pH above 8.5. hydroxide depends on the method and duration of The aqueous solution of sodium hydroxide in 3 preparation, it is commonly prepared empirically. mol•Edm-3 was added by agitation at the rate of 5 and Studies on the preparation of gelatinous aluminium 50cm3•Emin-1 to aqueous solutions of aluminium hydroxide were previously carried out by a number salts containing sulphate such as aluminium sulphate

of researchers under limited conditions.1 8) Investi (Al2(5O4)3•E18H2O) and ammonium, potassium and gations have also been conducted on the preparation sodium alums (NH4Al(5O4)2•E12H2O, KAl(SO4)2•E of gelatinous aluminium hydroxide from aqueous so 12H2O and NaAl(SO4)2•E12H2O) at selected pH lutions of chloride and nitrate of aluminium with alka values of 5, 6, 7, 8, 9,10,11 and 12 at temperatures li in order to obtain further information on prepara of 25 and 50•Ž. The resulting precipitates were aged tion factors which influence the composition of gelatinous aluminium hydroxide.9) This paper ex tends the earlier work to include the preparation of gelatinous aluminium hydroxide from aqueous solu tions of aluminium salts containing sulphate, such as aluminium sulphate and ammonium, potassium and sodium alums.

2. Experimental As precipitation of aluminium hydroxides from aqueous solutions containing aluminium salts by alka line solution is known to result from a neutralization reaction, the titration curves of aqueous solutions with alkaline solution have previously been exa mined (Fig. 1). These results indicate that the titra tion curves resemble each other, especially when us ing alum similar to that of sodium alum. These Fig. 1. Titration curves of the aqueous solutions of aluminium sulphate (•›) and ammonium (•¢), potassium (• ) and sodium curves further indicate that the molar ratio of [OH-]/ alums (•¤) in 0.1mol•Edm-3, respectively, with 0.6mol•Edm-3 so [Al3+] attains about three at pH 10-11, except in dium hydroxide solution.

377 378 Preparation of Gelatinous Aluminium Hydroxide from Aqueous Solutions of Aluminium Salts Containing Sulphate Group with Alkali

for 24h at selected temperatures, centrifugalized, precipitation is carried out at 25•Ž, amorphous washed with distilled water as free as possible from aluminium hydroxide alone is formed at a pH below alkali and anions, and then dried with acetone. The 9; pseudoboehmite is precipitated in the range of pH concentration of aluminium ion in aqueous solution 10 to 11, in particular at pH 10; precipitates at pH 11 was determined by back-titration of EDTA using XO to 12 are made of crystalline bayerite, but hydrargil (Xylenol Orange) as the indicator. Determination of lite is not formed. The amount of pseudoboehmite the sulphate ion concentration was carried out as fol formed increased as the temperature of precipitation lows: after the sample was dissolved in concentrated or the concentration in the aqueous solution in hydrochloric acid, the sulphate ion in aqueous solu creased: when precipitation was carried out at 50•Ž tion was precipitated by the addition of excess lead or at the rate of 5cm3•Emin-1, pseudoboehmite was nitrate; its precipitate was then dissolved in ammoni formed at pH 9. In addition, when alkali was added um hydroxide solution; the amount of lead ion was slowly at the rate of 5cm3•Emin-1(generally, 50cm3•E then surveyed by the EDTA titration at pH 10 using min-1), the composition of the precipitate resembled the indicator BT (Eriochrome Black T). that of a precipitate prepared with increased temper The materials obtained were examined by X-ray ature. The X-ray diffraction diagrams of the diffraction study, infrared (IR) spectroscopy and precipitates at 25•Ž are shown in Fig. 2. In Fig. 3, thermal analysis (TG and DTA).10) that the IR spectra exhibit the following absorptions:

precipitates at pH below 9 show OH stretching 3. Results and discussion bands at 3500 and 1650cm-1, attributed to the 3.1 Preparation from aluminium sulphate solu presence of adhesive water, and a broad band cen tion tered at around 1120 and 980cm-1 due to the sul In Table 1, it is observed that the composition of phate; for the precipitate at pH 10, OH bending the precipitates depends on experimental conditions, bands of pseudoboehmite appear at 1150 and 1070 in particular the pH value of aqueous solutions: when cm-1 in addition to the absorption of adhesive water,

Table 1. Precipitates from Aqueous Aluminium Sulphate Solutions on Addition of Alkali at Various pH

(a) Precipitates were aged at room temperature for 24h. (b) A, Bo and B represent amorphous aluminium hydroxide, pseudoboehmite and bayerite, respectively. (c) Parentheses indicate the small amount of aluminium hydroxide present. Taichi SATO et al. Journal of the Ceramic Society of Japan 104 [5] 1996 379

Fig. 2. X-ray diffraction diagrams of precipitates from aqueous aluminium sulphate solution on addition of alkali at various pH (numerals on curves represent the pH values).

Fig. 4. TG curves of precipitates from aqueous aluminium sul phate solution on addition of alkali at various pH (numerals on curves represent the pH values).

Fig. 3. Infrared spectra of precipitates from aqueous aluminium sulphate solution on addition of alkali at various pH (•ª and represent the absorptions due to pseudoboehmite and bayerite, respectively; numerals on curves represent the pH values).

Fig. 5. DTA curves of precipitates from aqueous aluminium sul while the OH stretching band of pseudoboehmite is phate solution on addition of alkali at various pH (numerals on not clear because of the overlap to that of adhesive curves represent the pH values). water; at a higher pH, absorption due to bayerite is clearly observed as OH stretching bands at 3700 3400cm-1 [3600 (shoulder), 3540 (shoulder) and •` 300•Ž, to it-alumina at •`500•Ž, while bayerite-‡U

3420cm-1], OH bending bands at 1020 and 975 (fine) dehydrates directly to ƒÅ-alumina at •`300•Ž. cm-1, and a broad band centered around 770-720 Thus it is seen that bayerite-‡U is formed at pH 11 cm-1, due to the Al-OH group. For both bayerites and bayerite-‡T at pH 12. In addition, an endother precipitated at pH 11 and 12, no characteristic differ mic peak due to dehydration of adhesive water is ob ence in X-ray diffraction peaks is revealed, and the served at 120•Ž in the DTA curve for the precipitate IR spectra exhibit similar absorption patterns, as at pH 11, but this peak disappears at pH 12. These shown in Figs. 2 and 3. results imply that when the pH value of the aqueous However, the results of thermal analyses (Figs. 4 solution is increased, the particle size of the sample and 5) suggest that the dehydration behavior of increases in size and then bayerite is transformed bayerite formed at pH 12 is different from that at from type ‡U to ‡T. For amorphous aluminium pH 11: the DTA curves exhibited three endothermic hydroxide prepared at pH 6, the DTA curve exhibit peaks (at about 200, 300 and 500•Ž) for the former ed a large endothermic peak at •`130•Ž due to the and one (at about 300•Ž) for the latter; these peaks release of adhesive water, and a very broad endother occur at points near the change in shape of the TG mic band centered around 300•Ž, the resulting from curves. As reported previously,11),12) bayerite-‡T the thermal decomposition of aluminium hydroxide

(coarse) dehydrates to a mixture of boehmite and to amorphous alumina,13) while the TG curve shows bayerite at 200•Ž, to boehmite and ƒÅ-alumina at a gradual weight-loss on heating. In the DTA curve 380 Preparation of Gelatinous Aluminium Hydroxide from Aqueous Solutions of Aluminium Salts Containing Sulphate Group with Alkali

for the precipitate at pH 10, however, the endother 3.2 Preparation from aqueous solutions of am mic peak at •`130•Ž, due to the presence of adhesive monium, potassium and sodium alums water, decreased while simultaneously a broad en The results of the preparation of gelatinous dothermic peak, ascribed to the dehydration of pseu aluminium hydroxides from aqueous solutions of am doboehmite to ƒÁ-alumina,13) appeared at •`450•Ž. monium, potassium and sodium alums with alkali are

However, since the IR spectra of precipitates from given in Tables 3-5. When precipitating aqueous so aqueous solutions of aluminium sulphate exhibited lutions of ammonium alum (Table 3), amorphous absorption bands due to the sulphate group, it was aluminium hydroxide always formed at a pH below possible to determine the amount of the sulphate 7. At 25•Ž, the precipitate formed at pH 8 contained group contained in the precipitates (Table 2). These a small amount of pseudoboehmite in addition to results indicate that precipitates from an aqueous so amorphous aluminium hydroxide, but pseudoboeh lution of aluminium sulphate contain an amount of mite alone was precipitated when the temperature sulphate less than 1%, although this amount was increased to 50•Ž. Pseudoboehmite always decreases in the order of amorphous aluminium formed at pH 9; at pH 10 a small amount of bayerite hydroxide, pseudoboehmite and bayerite. went to boehmite. The amount of bayerite increased when the concentration of ammonium alum in aque-

Table 2. Amounts of Sulphate Group Contained in Precipitates from Aqueous Aluminium Sulphate Solutions with Alkali

Table 3. Precipitates from Aqueous Ammonium Alum Solutions on Addition of Alkali at Various pH Taichi SATO et al. Journal of the Ceramic Society of Japan 104 [5] 1996 381

Table 4. Precipitates from Aqueous Potassium Alum Solutions on Addition of Alkali at Various pH

Table 5. Precipitates from Aqueous Sodium Alum Solutions on Addition of Alkali at Various pH

ous solution decreased, and bayerite alone was amorphous aluminium hydroxide always formed at a formed at pH 11. When precipitating aqueous solu pH below 8 except when the temperature rose to tions of potassium alum (Table 4), amorphous 50•Ž a small amount of pseudoboehmite formed. At aluminium hydroxide always formed at a pH below 8 pH 9, pseudoboehmite was formed except when the except when the phenomenon occurred that a small concentration of sodium alum in aqueous solutions in amount of pseudoboehmite formed when the temper creased a small amount of pseudoboehmite went to ature was increased to 50•Ž. Pseudoboehmite always amorphous aluminium hydroxide. Pseudoboehmite formed at pH 9, and at pH 10 a small amount of always formed at pH 10 except when the concentra bayerite went to pseudoboehmite. The amount of tion of sodium alum in aqueous solution decreased a bayerite increased when the rate of alkali added small amount of bayerite formed. At pH 11, a small decreased (from 50 to 5 cm3•Emin-1), and bayerite amount of hydrargillite was accompanied by a mix alone was formed at pH 11. During precipitation ture of bayerite and pseudoboehmite except when from aqueous solutions of sodium alum (Table 5), the concentration of sodium alum decreased a small 382 Preparation of Gelatinous Aluminium Hydroxide from Aqueous Solutions of Aluminium Salts Containing Sulphate Group with Alkali amount of pseudoboehmite went to bayerite; a mix and bayerite as the pH of aqueous solutions in their ture of bayerite and hydrargillite formed at pH 12. formation increases, although their precipitation be Furthermore, when precipitations of aluminium havior is influenced by aluminium salts containing hydroxide from aqueous solutions of aluminium salts sulphate. In addition, the amount of pseudoboehmite containing the sulphate group with alkali were com formed increases as the temperature of precipitation pared with identical experimental conditions, the fol or the concentration in the aqueous solution is in lowing characteristic behavior was observed: for the creased and/or when the alkali is added slowly. In precipitation from aqueous solutions of ammonium contrast, it was observed that the results obtained alum, pseudoboehmite formed in a wide range at low corresponded essentially to those for precipitations pH; for that of aluminium sulphate, bayerite did not from aqueous solutions of chloride or nitrate of form at a pH below 11; the behavior of sodium alum aluminium with alkali, but hydrargillite was not resembled that of potassium alum, although hydrar formed during precipitations from aqueous solutions gillite formed at a pH above 11. When comparing of aluminium salts containing sulphate, with the ex these results with the precipitation from aqueous so ception of sodium alum. lutions of chloride or nitrate of aluminium with alkali,9) it was observed that although the precipita Acknowledgement The authorswish to thank Mr. H. Otsuki tion from aqueous solutions of aluminium salts con for his assistancewith the experimentalwork. taining sulphate formed crystalline aluminium References hydroxide at a higher pH and was rather difficult to 1) R. Fricke and K. Meyring,Z. anorg.allgem. Chem., 214, form hydrargillite, the pH value dependence of the 269-74 (1933). latter was fundamentally similar to that of the form 2) V. Kohlschuter,Kolloid-Z., 96, 237-44 (1941). er. 3) S. Geilingand R. Glocker,Z. Elektrochem.angew. physik. Chem.,49, 269-73 (1943). 4. Conclusions 4) H. B. Weiserand W. O.Milligan, J. Phys. Chem.,36, 3010 -29 (1932);ibid., 38, 1175-82(1934). The factors influencing preparation of precipitates 5) W. O.Milligan, J. Phys. Colloid.Chem., 55, 497-507(1951). from aqueous solutions of aluminium salts contain 6) E. Calvet,P. Boivinet,M. Noel,H. Thibon,A. Maillardand ing sulphate have been investigated under different R. Tertian,Bull. soc.chim. France, 99-108 (1953). conditions. These included such factors as the kind 7) B. Imelik,M. V. Mathieu, M. Prettre and S. Teichner,J. and concentration of the aqueous solutions, the pH chim.phys., 51, 651-62 (1954). 8) D. Papee, R. Tertian and R. Biais,Bull. soc.chim. France, value, the temperature and the addition rate of alkali 1301-10 (1958). for the precipitate to form. The results determined 9) T. Sato,Z. anorg.allgem. Chem., 391, 69-78 (1972). that the composition of the precipitates depends on 10) T. Sato,J. Jpn. Inst. Light Metals,38, 731-39 (1988). the experimental conditions, especially the pH value 11) T. Sato,J. Appl.Chem., 9, 331-40 (1959). of the aqueous solutions; that it varies in the order of 12) T. Sato, ThermochimicaActa, 88, 69-84 (1985). 13) T. Sato,Z. anorg.allgem. Chem., 391, 167-73 (1972). amorphous aluminium hydroxide, pseudoboehmite