ENCYCLOPÉDIE DE LA RECHERCHE SUR L’ALUMINIUM AU QUÉBEC 2013 | Page 39

NOUVEAUX PRODUITS ET MATÉRIAUX À BASE D'ALUMINIUM
NEW ALUMINIUM BASED
Protection of prebaked anodes PRODUCTS AND MATERIALS

37

Protection of prebaked anodes
against air burning
against air burningL’ANODE CONTRE LA COMBUSTION À L’AIR
PROTECTION DE

Protection des anodes précuites
Protection des anodes précuites
contre l’oxydationPROTECTION OF ANODE AGAINST AIR BURNING
à l’air
contre l’oxydation à l’air
R. Ishak1 ,H. Alamdari1,2, G. Laroche1, M. Fafard2 , D. Ziegler3

Auteur 1, Author2

1Department

of Mining, Metallurgical and Materials Engineering, 1065 avenue de la Médecine
Université Laval, Département et Institution 1
1 Québec, QC, G1V 0A6, Canada
2NSERC/Alcoa Industrial Research Chair MACE3 and Aluminium Research Centre – REGAL
Université Laval,departmentG1V 0A6, Canada 2.
2 Québec, QC, and Institution
3Alcoa Primary Metals Alcoa Technical Center 100 Technical Drive Alcoa Center, PA 15069-0001 USA

1. Introduction
Several reactions occurring during the electrolysis of alumina in
the Hall-Heroult process result in a significant overconsumption
of carbon. Material oxidation reaction i.e. C(s)+ O2(g) → CO2(g)
.
is one of such reactions. Boron is well known to decrease the
rate of this reaction in graphite [1]. The effect of boron on
graphite oxidation has been attributed to several phenomena.
The inhibition is a consequence of blockage of surface active
sites by the boron oxide.

This inhibition occurs because boron lowers the electron
density on reactive carbon atoms and has a suppression effect
on O2 chemisorption. There is also a catalytic effect on the
desorption of CO and CO2 due to the presence of boron. The
inhibition of this reaction by applying protective layers on the
material could be an effective technique to delay the material.
The application and usage of boron oxide has been
investigated in this work to exploit the blockage effect of boron
oxide and to get an effective depth of coating in the anode.

1. PERFORMANCE OF THE ALUMINIUM ELECTROLYSIS AND CARBON ANODE DUSTING

2. Objectives

Electrolysis
Bath,Tordai,2007

• Understanding the mechanism by which boron inhibits the air
reaction to reduce the excessive consumption of anode.
• Optimizing the coating parameters and maximizing the
coating efficiency.
• Impregnation of fine coke with boron oxide;
Binder matrix (fine coke + pitch) is supposed to be the more

3. Methodology
1. Sample preparing :
• Anode Sample was impregnated in
a solution of boric acid with two
different concentrations.
• Crushing anode sample with high
energy ball mill for five minutes.
• TGA analysis to evaluate mass loss
of fine particles (-200 US Mesh).

Size range (US Mesh)

Size range (mm )

Boron oxide content (ppm)

-100+200

0.074 – 0.149

60; 90; 100

-200

Smaller than 0.074

60; 90; 100

Fig.1.17 Anode dusting related to preferential attack of the binder matrix

In molecules with
On the anode side immersed in the bath happens a visible selective burning, which is no
different elements on the bottom working face. The loosening carbon particles have a tendency
longer apparent of
to move up to the bath surface.
symmetry,
certain
When the gas permeability is high, there is a strong devel