Trans. Nonferrous Met. Soc. China 25(2015) 615−621 Role of hydrazine and hydrogen peroxide in aluminium hydroxide precipitation from sodium aluminate solution N. K. SAHU 1 , C. K. SARANGI 2 , B. DASH 2 , B. C. TRIPATHY 1,2 , B. K. SATPATHY 3 , D. MEYRICK 4 , I. N. BHATTACHARYA 1,2 1. Academy of Scientific and Innovative Research, New Delhi 110025, India; 2. Institute of Minerals and Materials Technology, Council of Scientific and Industrial Research, Bhubaneswar 751013, India; 3. National Aluminium Company, Bhubaneswar 751013, India; 4. Chemical and Mathematical Sciences, Murdoch University, Murdoch WA 6150, Australia Received 25 March 2014; accepted 2 September 2014 Abstract: Aluminium hydroxide precipitation from synthetic sodium aluminate solution was studied in the presence of hydrazine or hydrogen peroxide. The addition of low concentration of hydrazine is found to be effective, while higher amount of hydrogen peroxide is required to generate similar effect. XRD data confirm the product phase to be gibbsitic by nature. The scanning electron micrographs (SEM) show that agglomerated products form in the presence of hydrazine while fine discrete particles are produced with hydrogen peroxide. The probable mechanism of precipitation in the presence of hydrazine and hydrogen peroxide is also discussed. Key words: aluminium hydroxide; hydrazine; hydrogen peroxide; sodium aluminate; precipitation 1 Introduction In the Bayer process, bauxite is digested with hot concentrated sodium hydroxide to produce supersaturated sodium aluminate [NaAl(OH) 4 ] solution. Aluminium hydroxide, Al(OH) 3 (s) (gibbsite), is obtained from the aluminate liquor by seeded precipitation method, and the spent caustic solution is recycled to the digestion process. The productivity of the plant depends upon the gibbsite yield which is low under typical plant operating conditions such as temperature from 60 to 75 °C, m(Al 2 O 3 ):m(Na 2 O) ratio of about 1.0, seed addition of 400 to 600 g/L and precipitation period of 48 to 72 h. The wide gap between the quantity of gibbsite precipitates and its equilibrium solubility at a specific precipitation temperature provides scope to enhance the gibbsite yield. The decomposition of sodium aluminate to precipitate aluminium hydroxide is possible through either physical or chemical method. Seeding is a physical method by which the decomposition of ], [Al(OH) 4 − takes place to form Al(OH) 3 (s). Another means of decomposition of ] [Al(OH) 4 − is by proton (H + ) assisted precipitation such as acid neutralization. Carbonisation decomposition is another example of chemical decomposition process [1]. In the present work, the use of hydrazine as an additive was investigated to observe any effect if it can impact the decomposition of aluminate ions since it is a weak base in comparison to highly alkaline sodium aluminate solution. Studies so far reported for enhancing the aluminium hydroxide precipitation have dealt with either thermal or mechanical seed activation [2−6]. Use of additives, mainly organic polymers or surfactants, for promoting productivity, has also been investigated [6−15]. The incorporation of H + ions in the form of EDTA has been demonstrated for improving the yield [12]. Although hydrogen peroxide has been utilized for regeneration of spent sodium hydroxide from aluminium washed solution [13] and for synthesis of high area alumina particles by precipitating boehmite from sodium aluminate solution [15], its use for precipitating aluminium tri-hydroxide in Bayer precipitation process Corresponding author: C. K. SARANGI; Tel: +91-674-2379386; E-mails: sarangi.ck@gmail.com; cksarangi@immt.res.in DOI: 10.1016/S1003-6326(15)63644-5