The effect of processing parameters on particle size in ammonia-induced precipitation of zirconyl chloride under industrially relevant conditions G.A. Carter a,b , R.D. Hart b , M.R. Rowles c , C.E. Buckley b , M.I. Ogden a, ⁎ a Nanochemistry Research Institute, Curtin University of Technology, PO Box U1987, Perth, Western Australia, 6845, Australia b Centre for Materials Research, Curtin University of Technology, PO Box U1987, Perth, Western Australia, 6845, Australia c Commonwealth Scientific Industrial Research Organisation (CSIRO) Minerals Clayton South, Victoria, Australia abstract article info Article history: Received 25 June 2008 Received in revised form 2 October 2008 Accepted 17 October 2008 Available online 8 November 2008 Keywords: Zirconia Precipitation Particle size Zirconyl chloride The effect of pH of precipitation, starting solution concentration, and agitation levels on the particle size of hydrous zirconia precipitates have been investigated. It was found that all three variables affect the particle size of the hydrous zirconia. The smallest particle size is produced by a 0.81 M starting solution, precipitated at pH 12 with a high agitation level. The pH of precipitation was also found to have a significant impact on the type of hydrous zirconia produced. TGA/DTA, micro combustion and TEM/EDS were used to investigate the difference in the powders produced at pH 3 and 12. This work suggests that powders produced at pH 3 will have a structure similar to Zr[OH] 4 whilst those at pH 12 are more likely ZrO[OH] 2 . XRD and micro- combustion suggest that the powders produced at pH 3 retained ammonium chloride whilst those produced at pH 12 did not. The filtration rates for the pH 3 product were significantly faster than that of the powders made at pH 12 which is significant in the industrial production of these materials. © 2008 Elsevier B.V. All rights reserved. 1. Introduction Recent studies of hydrous zirconia for the eventual preparation of yttria stabilised zirconia have investigated the formation of hard agglomerates produced during aqueous precipitation of hydrous zirconia precursor from zirconyl chloride solutions [1]. The formation of these hard agglomerates is industrially relevant as they are a significant impediment to the use of these materials in the large scale production of ceramic powders for purposes such as solid oxide fuel cells [2,3]. Whilst not the primary focus of the reported studies, literature suggests both zirconium species and precipitate particle size can be affected by the pH of precipitation, concentration and agitation rate [4,5]. These properties are crucial in the industrial processing of these powders as they impact on parameters such as filtration rates and rheology. These effects eventually result in processing and performance issues in the ceramics produced [1]. Larsen and Gammill [6] conducted electrometric titration studies on zirconium and hafnium using zirconyl and hafnyl chlorides as the base solutions. They produced tables and curves showing precipita- tion occurs for zirconium at pH of approximately 2 and coagulation occurred pH ~6. The hafnium curve shows little difference in the precipitation points or coagulation points from the zirconyl solution. Later extensive work by Clearfield [4] suggested that the precipitation process was a hydrolytic polymerisation, and that the precipitation occurred at a low pH and was completed before the end point of neutralisation is reached. Clearfield explains that this is due to the retention of anions by the precipitate and that the amount of anion retention is dependent on pH, decreasing as the pH of precipitation increases. It was suggested that this behaviour indicates that the precipitates should be viewed as basic salts of variable composition [4]. Larsen and Gammill [6] also postulated the formation of basic salts by zirconium compounds and used this to explain differences found between the precipitation of hafnium and zirconium with respect to chloride ion content. The range of pH values for the precipitation points for differing concentrations of chloride ions was 1.88 to 2.29 [6]. These values agree with the results of Kovalenko and Bagdasaov [7] who conducted dissolution studies on what they call solid Zr(OH) 4 where they found that dissolution in nitric acid occurs at a pH of 1.9 and increases up to a pH of 1.8. They suggest that the precipitation must occur at these pH values as well. The solubility product was also calculated using a stoichiometric formula of Zr(OH) 4 although it is made clear in the paper that this formula is an assumption. Huang et al. [8] investigated the differences between zirconium hydroxide (Zr (OH) 4 ·nH 2 O) and hydrous zirconia (ZrO 2 ·nH 2 O) but did not investigate the previously suggested structure ZrO(OH) 2 [9–13]. The nature of zirconium hydroxide species precipitated under acidic conditions has also been investigated using thermal and X-ray techniques [14,15]. The suggested structure, which differs from the previous literature, is Zr 4 O 3 (OH) 10 d 6H 2 O. The authors compare this structure with α-Zr 4 (OH) 16 , β-Zr 4 O 2 (OH) 12 and γ-Zr 4 O(OH) 8 with the major difference being the coordinated water ([14] and references therein). The suggested structure for hydroxides produced at pH 4 and 3 is the same although no data are shown for the pH 3 sample [14]. Powder Technology 191 (2009) 218–226 ⁎ Corresponding author. Tel.: +61 8 9266 2483; fax: +61 8 9266 4699. E-mail address: m.ogden@exchange.curtin.edu.au (M.I. Ogden). 0032-5910/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.powtec.2008.10.021 Contents lists available at ScienceDirect Powder Technology journal homepage: www.elsevier.com/locate/powtec