Carbonation of Sodium Aluminate/Sodium Carbonate Solutions for Precipitation of Alumina Hydrates—Avoiding Dawsonite Formation

Abstract

Experimental work has been performed to investigate the precipitation mechanism of aluminum hydroxide phases from sodium aluminate/sodium carbonate pregnant solutions by carbon dioxide gas purging. Such solutions result from leaching calcium aluminate slags with sodium carbonate solutions, in accordance with the Pedersen process, which is an alternative process for alumina production. The concentration of carbonate ions in the pregnant solution is revealed as a key factor in controlling the nature of the precipitating phase. Synthetic aluminate solutions of varying sodium carbonate concentrations, ranging from 20 to 160 g/L, were carbonated, and the resulting precipitating phases were characterized by X-ray diffraction analysis. Based on the results of the previous carbonation tests, a series of experiments were performed in which the duration of carbonation and the aging period of the precipitates varied. For this work, a synthetic aluminate solution containing 20 g/L free Na2CO3 was used. The precipitates were characterized with X-ray diffraction analysis and Fourier-transform infrared spectroscopy.

Simulating the Use of a Smelter Off-Gas in the Precipitation Stage of the Pedersen Process

“…An increased precipitation temperature seems to improve the aluminum recovery rate and also enhances the formation of other phases than bayerite, which is undesirable, as shown in the XRD analysis, from Table 5. The same has been observed previously [5,6], with the formation of other oxides, such as dawsonite.…”

Investigating Aluminum Tri-Hydroxide Production from Sodium Aluminate Solutions in the Pedersen Process

“…5, this perhaps can be attributed to the use of a solution with a Na2CO3 concentration of 80 g/L. This has also been reported to favor dawsonite formation [24]. Regardless, going forward, it is recommended that all tests (carbonation and aging) be conducted with 60 g/L Na2CO3 concentration at temperatures less than 30 °C.…”

“…The most notable outcomes from the ENSUREAL studies were the observations of the formation of a calcium-containing passivation layer around the slag grains, limiting the extraction of Al during leaching [21]. Further, leaching solutions with a Na 2 CO 3 concentration higher than 100 g/L were observed to promote dawsonite [NaAlCO 3 (OH) 2 ] precipitation over the desired Al tri-hydroxides gibbsite and bayerite [22][23][24]. Additionally, it was noted that the precipitation process had to be a combination of carbonation and aging (allowing the precipitate to sit in solution for at least 24 h after carbonation) to convert boehmite to bayerite and gibbsite [9,23,24].…”

“…Further, leaching solutions with a Na 2 CO 3 concentration higher than 100 g/L were observed to promote dawsonite [NaAlCO 3 (OH) 2 ] precipitation over the desired Al tri-hydroxides gibbsite and bayerite [22][23][24]. Additionally, it was noted that the precipitation process had to be a combination of carbonation and aging (allowing the precipitate to sit in solution for at least 24 h after carbonation) to convert boehmite to bayerite and gibbsite [9,23,24]. Furthermore, other studies [17,25] have reported that elevated temperatures exceeding 60 • C promote gibbsite formation.…”

One‐Step Synthesis of Pseudo‐Boehmite by Carbonation in a Microchannel Reactor

“…The carbonation of sodium aluminate solution method is to purge carbon dioxide into sodium aluminate solution under certain conditions to generate aluminum hydroxide gel [8]. The synthesis of pseudo-boehmite by traditional carbonation is an intermittent operation.…”