Effect of urea on decomposition of sodium aluminate solution

摘要

Aluminium hydroxide particles were synthesized from sodium aluminate liquor by urea decomposition method at elevated temperatures in presence and absence of seed. Different parameters such as alumina to caustic (A:C) ratio, temperature, time period of precipitation, urea concentration, etc. were studied. Characterisations of the products were also carried out using X-ray diffraction, scanning electron microscopy, TG-DTA, etc. In general, higher precipitation ratios were observed with the addition of urea in sodium aluminate solution. Temperature was found to have a significant role in precipitating aluminium hydroxide with higher yield enhancement ratio compared to that obtained at lower temperatures. Higher precipitation ratios were also obtained with urea at elevated temperatures under available supersaturation. In the presence of urea, the precipitation ratios on yield enhancement front were observed to increase with decrease in A:C ratio. It has been observed that the liquor concentration reached below the equilibrium solubility limit after 30 h of precipitation when 460 mmol/L urea is added. The synthesized aluminium hydroxide particles showed gibbsitic nature and globular morphology, with agglomerates of mostly hexagonal platelets, as revealed through XRD and SEM studies, respectively. Calcination of the product at 1000 °C results in weight loss similar to that observed with gibbsite. Endothermic peaks at 325 °C and 550 °C revealed through DTA study indicated boehmite and chi-alumina transformations, respectively. The possible mechanism of urea decomposition method is also discussed.