Small Molecular Organic Acid Impurities And Silicon Sodium Aluminate Solution Sub-process

Many impurities such as carboxylic acids, dicarboxylic acids and silicon in the concentrated sodium aluminate solution influence the seeded precipitation in Bayer digestion process. To increase the seeded precipitation rate and the size of Al(OH)₃ particales we must first fully understand the influence mechanism of these impurities on the process.According to the practical conditions of alumina production, three kinds of carboxylic acids (formic acid, acetic acid and propionic acid), three kinds of dicarboxylic acids (oxalic acid, succinic acid and glutaric acid) and silicon acid are taken as the impurities to investigate their influence on the rate of seeded precipitaton, the particle size distribution (PSD), the TG/DTG and the X-Ray diffraction (XRD) under different temperature conditions. Combined with quantum chemistry calculation, the influence mechanism of carboxylic acids, dicarboxylic acids and silicon on the seeded precipitation is discussed and the conclusions are drawn as follows:1. Silicon acid observably inhibits the precipitation and decreases the size of Al(OH)₃ particales. When temperature is low formic acids and propionic acids slow down the precipitation and decrease the size of Al(OH)₃ particales. Acetic acid noticeably increase the precipitation rate and decreases the size of Al(OH)₃ particales at low temperature. When temperature is high oxalic acid, succinic acid and glutaric acid inhibit the precipitation and decreases the size of Al(OH)₃ particales.2. Whether all the inpurities coexist together or occurred separately, their influence on the precipitation and the size of Al(OH)₃ particales shows no synergistic effect.3. Carboxylic acids and dicarboxylic acids dissociate to their conjugate base in the sodium aluminate solution, which makes the adsorption on the seed surface easier. Chemisorption happens between silicate anions and the surface of Al(OH)₃ by forming Al-O-Si bond. Silicate anions then occupy the active sites of seed surface and inhibit the precipitation.The results of quantum chemistry calculation show that: The total energy of (100) surface of gibbsite decreases in a sequence of acetic acid , glutaric acid and silicon acid in the adsorption system, while the energy gap broadens a little at first, and then narrows. The adsorption of the three compounds on (001) surface is also estimated. According to the sequence mentioned above, the Fermi energy increases at first and then decreases, while the energy gap narrows continuously. The results of theoretical calculation are in good agreement to the experimental outcome, which displays a performance from promotion to inhibition.