Article pubs.acs.org/IECR
A More Ecofriendly Cold Ramming Paste for an Aluminum Electrolysis Cell with Phenol−Formaldehyde Resin As Binder Lin Tian,*,†,‡ Fu shun Xu,† Gang Xie,*,‡,§ Rong xing Li,‡ and Shang yong Li‡ †
Yunnan Metallurgical Group Co., LTD., Kunming 650224, PR China Faculty of Metallurgical and Energy Engineering, Kunming University of Science and Technology, Kunming 650093, PR China § Kunming Metallurgical Research Institute, Kunming 650031, PR China ‡
ABSTRACT: A cold ramming paste for aluminum electrolysis cells was synthesized with electro-calcined anthracite and artificial graphite as aggregate and phenol−formaldehyde resin as binder. The synthesized pastes were characterized by thermogravimetry, infrared spectroscopy, scanning electron microscopy−energy dispersive spectroscopy, and gas chromatography−mass spectrum. The results show that the synthesized cold ramming paste is an ecofriendly material and it has some superior physical properties, such as low electrical resistivity, high compressive strength, appropriate sodium penetration, and thermal expansion, all of which can meet industry requirements. The mass loss (3.4%) and linear shrinkage reach the maximum during the third stage (from 360 to 600 °C) based on thermal analysis, so a lower heating rate should be used in this stage. Moreover, the resin functional groups disappear more completely at higher carbonization temperature.
1. INTRODUCTION
temperature and content of PAHs, especially genotoxic PAHs. Such a paste is urgently needed. As graphitic or graphitized carbon blocks are used, the paste is considered as a key material in the cell.7−9 Inferior paste or improper ramming operation may shorten cell life or cause early cell failure. Therefore the paste synthesized with an ecofriendly binder must have the appropriate properties. The objective of this research is to synthesize an ecofriendly cold ramming paste that can be rammed at room temperature, using phenol−formaldehyde (PF) resin as binder. The properties of the synthesized paste must also meet industry standards.
Cold ramming paste basically contains a mix of dry aggregate (mainly electrocalcined anthracite (ECA) and artificial graphite) and a binder (mainly a coal tar pitch). It is widely used in aluminum electrolysis cells to fill the gaps between and around the cathode bottom blocks. Many polycyclic aromatic hydrocarbons (PAHs) can escape when cold ramming paste is being tamped and the cell is started, especially coal tar pitch-based pastes. Some of these are very dangerous to the environment and worker’s health, so the aluminum industry is confronted with increasing pressure as greater emphasis is placed on occupational health and environment. Some scientists have realized this serious problem.1−6 For example, Kvam et al.1 studied coal tar pitch and resin as binder, respectively, to synthesize ramming paste and found that the amount of PAHs decreased obviously using resin as binder. Paulus and Meseguer4 studied six binders, such as coal tar pitch, petroleum pitch, and Novolak resin, for use in cold ramming pastes, and found that paste F (it was unknown until now) was an effective compromise, offering superior health characteristics and good properties. Lacroix5 studied cold ramming pastes synthesized with coal tar pitch and resin as binders, and concluded that the toxicity of the paste using resin as binder was very low. These authors presented a new approach,1,4,5 in that the cold ramming paste could be synthesized with resin as a binder. However, its ramming temperature (35−50 °C) was above ambient, meaning that the paste could not be tamped into the cathode block gaps at room temperature (20 °C). Instead, these pastes had to be preheated before ramming to get the correct paste properties, which increases the energy consumption. Allard et al.6 introduced a new paste called NeO2 and pointed out that the new paste had low hazardous PAHs and a wide temperature window, but some of its properties still needed to be improved such as compressive strength during the baking. In this work, we synthesize a new paste which has both low ramming © 2012 American Chemical Society
2. EXPERIMENTAL SECTION 2.1. Materials. Taking the paste between carbon blocks as an example, the typical particle size distribution of aggregate and basic physical properties of ECA are shown in Tables 1 and 2, respectively. The carbon content of artificial graphite is 98%. Table 1. Typical Particle Size Distribution of Aggregate particle size (mm):
5−3
3−1
1−0.074
