THE CURRENT EFFICIENCY FOR ALUMINIUM DEPOSITION FROM CRYOLITEALUMINAMELTS AT HIGH CURRENT DENSITY

Abstract

A laboratory study was performed to investigate first and foremost the effects of cathodic current density on the current efficiency with respect to aluminium electrolysis in cryolite-aluminamelts. Effects of NaF/AlF3 ratio and impurity concentration on CE were also investigated at a high current density of 1.5A/cm2.

The laboratory cell used in the present study has been designed specifically to ensure evenly distributed current density on the aluminium cathode surface and to ensure good and reproducible convective mass transport conditions in the bulk phase of the electrolyte.

The current efficiency was determined as the percentage ratio of the aluminium produced to the theoretical amount determined using the Faraday’s law.

The current efficiency was measured by performing Al electrowinning using the said laboratorycell with graphite as the anode material which was cylindrically shaped with a vertical hole made through the centre. Two holes normal to the vertical hole and normal to each other were also drilled. The cathode was a stainless steel plate on which the aluminium was produced. This gave an even current distribution over the cathode surface. All experiments were conducted at atemperature of 9800C.

The CE was found to increase with increasing cathodic current density, from 93.53% to 95.45%for current densities of 0.85A/cm2 and 1.5A/cm2 respectively.

The dependence of NaF/AlF3 ratio and impurity concentration was also investigated at high current density of 1.5A/cm2. CE was found to decrease with increasing of both NaF/AlF3 ratio and impurity concentration. CE values of 92.97%, 95.45% and 96.1% were measured forNaF/AlF3 ratios of 3, 2.5 and 2 respectively.

Also when 100ppm of AlPO4 was added the CE measured dropped from 95.45% to 94.45% at 1.5A/cm2 and 93.53% to92.25% at 0.85A/cm2.

The discussion and conclusion of this work was made by comparing the results obtained to the literature available and a similar work conducted by Solli et.al [1] though their work were mostly done at a lower current density of 0.85A/cm2