In order to develop a strategy of alumina supplying the electrolysis cell, a change of the alumina content dissolved in cryolite electrolyte during long-term aluminum electrolysis was studied under different modes of alumina feeding. The experiment was carried out using a lab setup providing the electrolyte convection due to the anode gas evolution. The electrolyte composition was close to the conventional one: NaF-AlF3 with CR=2.7, CaF2 5 wt%. The initial content of Al2O3 in the electrolyte was 4 wt%. The electrolyte samples were withdrawn during electrolysis in order to determine the Al2O3 content by means of an Oxygen Analyzer. The alumina dissolution rate in the electrolyte was found by comparing the values of the dissolved alumina content, determined in the electrolyte sample, and the loss of alumina consumed in the electrochemical reaction of the aluminum production. Measurements of the side ledge thickness formed due to solidification of the electrolyte during electrolysis were conducted. Under the conditions of electrolysis, at a rate of the alumina addition of 10 g/h, 36% of the added alumina was spent on increasing the alumina content in the melt; 44.6% was depleted on the electrochemical reaction; 19.4% did not dissolve in the electrolyte, but was involved in the formation of the side ledge. Thus, the content of dissolved alumina in the electrolyte increased and the side ledge was observed, which indicated an incorrect mode of the alumina supply. A decrease in the alumina feeding rate to an average value of 5.88 g/h contributed to the stabilization of electrolysis. In this case the side ledge was not formed and the content of dissolved alumina in the melt was constituted during electrolysis. That is, the conditions for optimal alumina feeding have been found for the electrolysis process under studied conditions.

An experimental setup is created to study the dynamic behavior of the side ledge under the electrolysis in a cryolite-alumina electrolyte. The installation has a built-in window on the side panel, which allows the lining material and, hence, the heat flow to be varied. The dynamic formation of a side ledge is experimentally studied as a function of temperature, the electrolyte velocity, and the heat flow during the electrolysis of aluminum. The rate of the ledge formation and the ledge thickness are determined by the heat flow caused by the electrolyte temperature, the cell side temperature, and the liquidus temperature. As in an industrial electrolysis cell, the side ledge profile formed in the experimental cell can be conventionally divided into the following three zones: the ledge at the bottom in contact with liquid aluminum, the ledge at metal/electrolyte interface, and the ledge at the electrolyte level. If the temperature of the inner wall is higher than the liquidus temperature, the side ledge does not form; if it is lower, the side ledge forms until these temperatures become the same. Therefore, the stability of the side ledge depends on the heat flow from the center to the walls; however, the dynamic behavior (solidification/melting) of the ledge in the metal zone occurs slowly and differs from the behavior of the side ledge.

A model unit simulating the actual conditions of electrolytic aluminum production was used to conduct an experimental study of ledge to determine its dynamic behavior (formation/dissolution) depending on the electrolyte overheating temperature, lining thermal resistance and cryolite-alumina electrolyte composition. A window was mounted in the front wall of the unit housing to change the lining material. Ledge is formed due to the heat flow generated by the temperature difference between the electrolyte and electrolyzer walls. The electrolyte cryolite ratio (CR) varied in the range of 2.1–2.5. The alumina concentration in the electrolyte did not exceed 4.5 wt.%. Shape change in the electrolyzer working space during electrolysis was determined by the thickness of the formed ledge on the walls and bottom. The dynamic ledge formation in the experimental cell begins at the overheating of 3–4 degrees. It was found that with a decrease in the thermal resistance of the lining material from 16 to 14 m2/W at the same overheating temperature, the side ledge with a greater thickness was formed, however, the decrease in the thermal resistance hardly affected its thickness when the ledge has been already formed. As in the industrial electrolyzer, the ledge profile formed in the experimental cell can be conditionally divided into three zones: bottom ledge, metal/electrolyte interface ledge and side ledge. The dynamic behavior of the side ledge was different from the bottom ledge: the higher the CR, the thicker the side ledge and the thinner the bottom ledge. Chemical analysis of components in the dry knockout showed that the CR and Al2O3 concentration increase throughout the cell height from top to bottom. It was concluded that the side ledge has a heterogeneous composition depending on the electrolyte composition and cooling rate.

The dynamic behavior (formation/dissolution) of the ledge, depending on the electrolyte overheating temperature, thermal resistance of the lining material, and composite of the cryolite–alumina electrolyte, is studied experimentally using a model installation mimicking the actual conditions of the electrolytic aluminum production. A window that enabled the change of the lining material is mounted into the front wall of the installation case. The ledge formation occurs due to a heat flow formed due to the temperature difference of the electrolyte and electrolyzer walls. The electrolyte cryolite ratio (CR) is varied in a range of 2.1–2.5. The alumina concentration in the electrolyte does not exceed 4.5 wt %. The change in the shape of the working space in the electrolyzer during the electrolysis is determined by the ledge thickness. The active ledge formation in the experimental cell starts upon overheating by 3–4 K. It is shown that a thicker ledge is formed at the same overheating temperature with a decrease in the thermal resistance of the lining material from 16 to 14 m 2 /W, but a decrease in the thermal resistance in the already formed ledge almost does not affect its thickness. Similarly to the industrial electrolyzer, the ledge profile formed in the experimental cell can be conditionally divided into three zones, notably, bottom ledge, ledge at the metal/electrode interface, and side ledge. The dynamic behavior of the side ledge differs from the bottom ledge; notably, the side ledge is thicker at a high electrolyte CR, while the bottom ledge is thinner. The chemical analysis of components in the dry knock out shows that the CR and Al 2 O 3 concentrations increase over the cell height from top to bottom. It is concluded that the side ledge has a heterogeneous composition that depends on the electrolyte composition and cooling rate.