Structural Changes and Strength Considerations of Cathode Carbon During Electrolysis

J. G. Hop SINTEF Applied chemistry

Sem Scelandsvei 12, 7465 Trondheim, Norway H. A. Oye

Institute of Inorganic Chemistry, Norwegian University of Science and Technology Sem Scelandsvei 12, 7491 Trondheim, Norway

I n t r o d u c t i o n

In the early stages of electrolysis the cathode carbon is exposed to severe thermal stresses and sodium expansion. Figure 1 shows how bending strength varies with time for laboratory electrolysis experiments [1]. The anthracitic carbon looses more than half the bending strength after 0.5-1 hours of electrolysis. From one to six hours the strength recover some. The same variations are measured on semi graphitic carbon in basic melt. But when the melt is acidic the semi graphitic carbon will keep it's strength while the amorphous will give the same down-up curve with smaller end values. The semi graphitized material keep it's strength independent of the cryolite ratio. Similar strength variations are found by Welch [2].

- - - - • • Anthracitic I Semi graphitic

I A _~ , ' ,m i t a r a n h r l ~ i t i z e _ _ _ @ : 2 5

i N 5 ................................................ ~ ........................... ! .......................... i ........................... i . . . . . . . . . . . . . . . . . . . ,-" i I l l ~ ::

1 o . . . . . . . : . . . . . . . . . . . i . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . i . . . . . . . . . . . .

o ~ L ................................................ ....................................................

0 60 120 180 240 300 360

Electrolysis tim e (m in.)

Figure 1. Bending strength versus electrolysis time for three different cathode materials. The CR (cryolite ratio = NaF/A1F3) is 4 and the current density is 0.2 A]cm 2. (Redrawn from S0rg~rd [ 1 ]).

The aim of this paper is to understand the strength differences by the use of surface photos.

E x p e r i m e n t a l

Cylindrical core samples (0 25 mm) were drilled out, the bottom cross section polished, and pictured with a 20X

magnification using a digital camera. After a given time of electrolysis the samples were cooled to room temperature, carefully polished, and photographed again. The images do not show exactly the same piece of the material as the surface was polished between each electrolysis experiment. However, it gives a good indication of the main changes.

The investigated materials were anthracitic, semi graphitic and semi graphitized.

Other experimental conditions: Current density : 0.35-0.40 A/cm 2 Cryolite ratio : 4 Temperature : 980°C. Inert gas : Nitrogen

R e s u l t s / d i s c u s s i o n

The typical damages due to the electrolysis are shown in Figure 2. After half an hour of electrolysis new cracks appear in the grains. The cracks appear both as extensions of original cracks as well as in intact material. After one hour of electrolysis the cracks already present in the material grow wider and further cracks appear. The change in structural defects parallels the change in bending strength (Fig. 1A versus Fig. 1C). No increase in number of cracks appears from 1 to 6 hours but they get thinner. The thinner cracks can be explained by the expansion of the grain. When the number of cracks remains steady and the grain expands, the cracks will be thinner.

Cracks in the binder region are sometimes observed. These cracks seem to be an extension of cracks from anthracite grains. They follow to some degree open porosity and the grain edges.

Similar pictures are taken of graphitic and semi graphitized cathode carbon. No cracks are observed at this magnification. The bending strength varies for the semi graphitic material during electrolysis for CR=4, but new cracks are not found. The bending strength only varies

7 1 8

with basic melts for anthracitic and semi graphitic carbon [1].

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• ' ...... ~<"<"~:;~.~'.~i ~ : , ;; " :"%~,: : :

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~:...... ::....:....~:.:.:::..:;: ~ . .: : . . ::: .>.,:.:.,:..... ::: : ...

~ . ~

, ~ "

~i~ '<':::.:

C D . . . ... ' . . ~ : . :.

Figure 2. The crack development in anthracite grain after 0 (A), 0,5 (B), 1 (C) and 6 (D) hours of electrolysis.

SEM backscatter mappings of the electrolyte in the cracks in anthracite grains were also studied.

I

7%< > : " ~ : ~ ..... ~ 1t : ~ : ~ - ~ : ~'"!~"~':.:" " . . . . ~,.. ~ . ,~ ......

. . ~. ~ .~ . . , /~ . : : . , ... ;-~. ~'~.,~.~:.~..<" • : ~ * ~ i . ~ . . : ¢ ~ . - : . . . .:::: ~ ~ - - ~ ......

C P , - - - - 5 u m ~ l N = , - - - 5 z m

:. i~i:i¢:~i~!l!!' . . . . : "

5 I , I I i F ' , , , , , ~ 5 i ,m I I ~ C ~ ~ l l i l l i l l t l l - - - - 5 u lm l /

Figure 3. An element mapping of a crack in an anthracite grain after 1 hour of electrolysis.

No particular tendencies were observed between the different electrolysis times, but the images often show Na and F together and O and A1 together (Figure 3).

This is also found by Welch [2], which proposed the absence of the carbide and the overlap of O and A1 with the reaction

A14C3 + 6H20 ----> 2A1203 + 3CH4

The NaF is believed to come from the reaction

Na3A1F6 + 3Na(in C) + 0.75C ~ 0.25A14C3(s) + 6NaF

The sodium front

The sodium front is considered to be a weak part of the carbon due to the different expansion on both sides. An amorphous sample with a horizontal notch sawed into the carbon was set under a constant load (1600 N) in a Rapoport apparatus (Figure 4). The electrolysis was started with basic melt and the expansion was measured. When the sample broke, one could read this on the dilation computer. After 200 minutes of electrolysis the sample broke with a fracture pointing downwards shown in Figure 4. The sodium height was located 0,5 to 1 cm above the provoked fracture. The surface of the fracture showed some areas with NaF crystals.

: - . . . . . . <.;_ . . . . . . . . . . . . . . .

Ftapopor t p ressu re rod

~ S t e e l suppo r t

, - " ' S o d i u m h igh t af ter f ract t

~ " ~ N o t c h s a w e d into the

samp le

Frac ture

Figure 4. The principle of the set-up for the sodium front test.

Acknowledgement

Financial support from the Norwegian Research Council and the Norwegian aluminium industry is gratefully acknowledged.

References

[1] SOrg~rd W, Oye HA, Eurocarbon, Strasbourg, France, Post dead-line communications (supplementary references), 1998, X.7.

[2] Welch B J, Hyland MM, Utley M, Tricklebank SB., Metson JB, Light Metals, 1991, 727-733.

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