Carbon Ramming Paste

From a materials science point of view, carbon ramming paste between the carbon cathode blocks and in the seam lining is the same carbon cathode material. The finished properties of baked ramming paste are the same, and the testing is almost the same. Yet it is an unfinished product that will receive the properties only after preheating and startup. 

The decision to use carbon ramming paste between the carbon cathode blocks and in the peripheral seam sometimes is met with some puzzlement among the refractory producers, who know that in ferrous metallurgy, this technical decision was ended many years ago. Yet this decision has served aluminium producers for many years, and it doesn t seem they are going to introduce something new. 

Initially, hot ramming pastes were used in repair shops of the smelter, but cold ramming pastes give fewer problems due to ecological issues. Certain effects are made to reduce the polyaromatic hydrocarbons in the binders in order to avoid fumes at preheating. 

The processing of ramming paste is very much like the preparation of anthracite graphite carbon cathode blocks—only without shaping and baking parts (Fig. 2.19). The raw materials are preheated and mixed in the mixers with pitch-based preheated binders and then packed into packs or barrels. 

When considering the final properties the ramming material receives during preheating of the cell and startup, it is necessary to remember that the temperature of carburization goes from the surface of the rammed material to the lower end of the shaft or seam, and the process takes a rather long time. 

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The gaps between the blocks are 30-40 mm, while the peripheral seams between the end of the cathode blocks and the peripheral side lining are up to 150 mm. They are filled with carbon ramming paste. Today there are alternatives to the manual ramming of gaps with hand rammers and the machined ramming with special devices they are more convenient because of the elimination of the human factor. 

Carbon ramming paste a semifinished product used to fill in the gaps between the end of carbon cathode blocks and the refractory side lining. The raw materials are the same as for carbon cathode blocks, but the material receives its final properties during the preheating and startup. 

These last numerical results show that the plastic strain is obviously greatly influenced by the presence of the creep/relaxation phenomenon. In fact, the level of the plastic strain was considerably reduced from 0.004 to 0.0018, i.e a reduction ratio of 2.2 based on the reference case (Figure 6), which does not take into accormt creep behaviour of the ramming paste. Also, the anelastic strain level at the end of the simulation (i = 40 hours) is almost negligible compared to the other strains (e.g., plastic, thermal, etc.). This result directly ensues from the assumption that the baked ( = 1) ramming p>aste creep/relaxation behaviour is similar to that of the carbon cathode block (Richard et al., 2006). This case study thus shows the importance of taking all the relevant phenomena including creep behaviour into account in similar problems. A similar analysis could be done for all other deformations (chemical, thermal, plastic, etc.). 

Depending on the method, the preheating lasts from 24 to 72 h. During preheating the carbonization of the ramming paste in the gaps takes place, and the cathode becomes monolithic. 

The dry penetration of sodium to the refractory layers due to the diffusion of sodium through carbon cathode blocks really should be taken into account. The consequence of such diffusirm is the appearance of free silicon in the form of balls of ore drops or globules, having a diameter from 1 to 4 sm, which appear due to the reaction of sodium with silica or silica-containing compounds. However, the dry penetration of sodium doesn t seriously deteriorate the refractory lining, which cannot be said about wet penetration or the capillary flow of electrolyte through permeable pores of carbon cathode blocks and rammed carbon of ramming paste. 

Fig. 2.54 Expansion/shrinkage/dilatation curves of ramming paste in comparison with expansion of carbon cathode block...

As we stated previously, there are currently two variants in the side linings of reduction cells an N-SiC side lining with rammed carbon paste, and a combiblock (Fig. 2.70)—N-SiC block with glued carbon block (or artificial side ledge). 

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