Linoleic Acid And Amine Sub-process Of Sodium Aluminate Solution

The seeded precipitation of sodium aluminate solution is one of the two key work steps in the Bayer process. How to take effective measures to improve the seeded precipitation of sodium aluminate solution is critical to the future of the Aluminium industry in China. In recent hot researches,it has been proved feasible and simple with low cost increase that the application of some special organic additives to enhance the seeded precipitation of sodium aluminate solution. Linoleic acid, Ethanolamine, Diethanolamine and Triethanolamine were selected as additives in this paper. By analyzing the data of the precipitation ratio, the particle size distribution,the morphology and the crystal characteristics of Al(OH)₃, the effects of Linoleic acid, Ethanolamine, Diethanolamine and Triethanolamine on the seeded precipitation of sodium aluminate solution were investigated at different concentration of the additives, different caustic concentration of the liquor and different temperature. The mechanism of the interaction between the alkylol amine and the Al(OH)₃ was suggetted by using the Dmol3 program in software Material Studio4.0.Conclusions were made as follows. That long carbon chain fatty acids as additive affected the seeded precipitation of sodium aluminate solution could be related to the existence of the C = C in the molecular structure of the fatty acids.Compared to the blank ,the change of the precipitation ratio was small with low concentration of Linoleic acid. But the agglomeration extent of the fine paticles less than 20μm was promoted significantly so that the product particle size was increased obviously. High concentration of Linoleic acid strongly inhibited the precipitation of sodium aluminate solution and caused fine Al(OH)₃ parcicles on some surfaces of the seeds.High concentration of Ethanolamine could enhance the precipitation of sodium aluminate solution continuously and promoted the agglomeration of the fine seed particles,but the degree of agglomeration did not increase when more Ethanolamine was added. On the contrary,low concentration of Ethanolamine had no notable effect. The morphology of the product changed a little with Ethanolamine was added in.Diethanolamine had a threshold quantity to the precipitation process. Small dosage could increase the precipitation ratio slightly,otherwise inhibited the precipitation of sodium aluminate solution. The volume percentage of the particles in each size range decreased or increased more or less,when the amount of Diethanolamine increased. The morphology of the Al(OH)₃ obtained with Diethanolamine added seemed the same.Triethanolamine had no apparent effect on the precipitation of the sodium aluminate solution. The agglomeration of those particles less than 30μm was promoted a lot and the product particles size was increased only when the concentration of Triethanolamine was low. Triethanolamine could not change the morphology of the product distinctly. But the Al(OH)₃ crystals had clearer edges and corners,and more (001) surfaces revealed.The X-ray diffraction pattern of the product suggested that Linoleic acid,Ethanolamine,Diethanolamine and Triethanolamine could not change the crystal pattern of aluminum hydroxide.Linoleic acid as surfactant,could be absorbed on the surface of the seed by the functional group of -COOH^-,and made the physical and chemistrical properties of the seed surface change,then affected the precipitation process of Al(OH)₄^- from the sodium aluminate solution. The molecules of Ethanolamine, Diethanolamine and Triethanolamine had different space structure,so they might affect the precipitation of sodium aluminate solution in different ways.Quantum chemistry calculation results showed that the absorption of ethanolamine, diethanolamine and triethanolamine on the (001) surface of aluminum hydroxide crystal may be in different patterns. Small molecule of amine alcohol as the additive using the amidocyanogen by hydrogen bonds to get involved into the decomposition process seemed unlikely. While large molecule of alcohol amine was absorbed mainly through the hydroxyl forming hydrogen bonding with the surfaces of the seeds to impact on the decomposition of sodium aluminate solution.