Chemical Degradation of the Cathodic Electrical Contact Between Carbon and Cast Iron in Aluminum Production Cells
- PDF / 4,464,736 Bytes
- 15 Pages / 593.972 x 792 pts Page_size
- 31 Downloads / 190 Views
UCTION
THE cathodic assembly of the Hall–He´roult electrolysis cell is made out of carbon blocks in which steel bars are rodded in place. Molten cast iron is the most common rodding material but ramming paste is used in some cases.[1] This arrangement accounts for about 10 pct of the total voltage drop of the cell.[2–4] The cathode voltage drop (CVD) is also known to increase along the cell life.[2,5] The electrical contacts deterioration (aluminum–carbon and carbon–cast iron) are among the suspects for this increase. Measurements made by Haupin revealed that the electrical contact resistance between the collector bar and the carbon block decreases in the first part of the cell life but increases to up to three fold after.[2] Sodium fluoride-enriched electrolytic bath containing up to 30 pct alumina was detected at the interface and was identified as the main factor for CVD increase. Stagg reported a Fe-Al double layer on the collector bar surface.[6] Those layers were observed on collector bars rodded in monolithic cathodes. Collector bars rodded in cast iron displayed Fe-Al layer on the cast iron surface but in a lower proportion. The presence of a basic (NaF
MARTIN BRASSARD, MARTIN DE´SILETS, and GERVAIS SOUCY are with the Department of Chemical Engineering and Biotechnological Engineering, Universite´ de Sherbrooke, 2500 boul. de l’Universite´, Sherbrooke, QC, J1K 2R1, Canada. Contact email: [email protected] JEAN-FRANC¸OIS BILODEAU and MARTIN FORTE´ are with Rio Tinto, Centre de Recherche et De´veloppement Arvida, 1955 boul. Mellon, Saguenay, QC, G7N 4K8 Canada. Manuscript submitted June 23, 2016. METALLURGICAL AND MATERIALS TRANSACTIONS B
enriched) bath deposit on the bar surface is also reported. Xue et al. have published results about interface degradation over time on collector bars rodded with ramming paste.[7] They observed an increase of the surface layer thickness on 3 different collector bars from 650 to 3800 days old. An alumina-rich layer was identified on the bar surface. In the literature, the generally accepted reaction for the aluminum formation at the interface is: 3NaðgÞ þ AlF3ðsÞ ! AlðlÞ þ 3NaFðsÞ
½1
The other product of the reaction, NaF, is suspected to stay at the interface and to be part of the surface deposit or to diffuse if the local temperature allows it. Even though carbon–cast iron interface degradation is identified as the main reason for CVD increase in most articles, this is not always the case. According to the work of Lombard et al. about the use of graphitized cathode blocks, cathodic resistance is reported to be stable over time except for one smelter where sludge formation is suspected to be responsible.[5] To the author’s knowledge, no laboratory scale testing of these phenomena has been reported in the literature. Furthermore, even though the published results show tendencies related to the formation mechanisms, clear links between operational parameters and the interface degradation are still to be established. The goal of this work is to understand the chemical d
Data Loading...
