Research On Dissolution Behavior Of Alumina In Cryolite-based Electrolyte

Alumina is the raw material in the aluminum electrolysis industry. The dissolution property of alumina in cryolite-based electrolyte is an important index for aluminum electrolysis process. Generally, faster dissolution rate and higher solubility are required. Dissolution rate of alumina in cryolitic electrolyte relates to its physicochemical properties such as particle size distribution, density, surface area, alpha phase content, LOI (loss on ignition) and condition of the electrolyte. LOI is one of the most important production quotas in Bayer Process and alumina grade. LOI is closely related to the other physicochemical properties formed in calcinations. Therefore, LOI can greatly affect the alumina dissolution performance.In this paper, the dissolution behavior of five types of industrial alumina samples in cryolite-based melts was studied by using a quartz transparent cell. The whole dissolution process was observed and recorded with a camera. Furthermore, the influencing factors on the dissolution rate and solubility of alumina are analyzed and discussed. It is found that the dissolution phenomena of the fluorinated alumina is quite different from the other industrial alumina, and the dissolution rate and solubility of the fluorinated alumina are approximately twice that of the other industrial alumina. In order to investigate the effect of composition fluctuation of the electrolyte on alumina dissolution, the dissolution phenomenon of5types of industrial alumina samples in electrolyte of different addition agent concentration were compared. It is found that the higher addition agent concentration of the electrolyte will causes slower dissolution of the alumina.Gibbsite produced in Bayer Process is calcined at different temperatures to prepare alumina samples of different LOI. The transparent cell was used to observe the dissolution phenomena of alumina in cryolite-based melts. The dissolution rates of different samples are compared and it is found that LOI affect alumina dissolution rate by changing of alumina surface area. When the LOI rises from0to3, alumina dissolution rate increases linearly with LOI. The floating time of alumina on the surface of the electrolyte increases with the LOI firstly and reaches to the top at the LOI of1.01. Then it gradually decreases as the LOI rise. Besides, effect of alumina crystal is also studied. Alumina of gamma phase takes a long time to float on the surface of the electrolyte and dissolves quickly, while alpha phase alumina sinks to the bottom as soon as it is added and dissolves slowly at the bottom. The dissolution rate of intermediate phase or mixed phase alumina located between the two phases. Since the recovery efficiency of aluminum plant can reach up to more than95%, alumina with higher LOI should be considered as raw for low temperature aluminum electrolysis to avoid the waste of energy.