Anode binder pitch

Aluminum industry, coke shortage, 193 Anode binder coke, source, 239 Anode binder pitch... 

In this context, a review is presented of the complex chain of events affecting anode performance, ranging from the properties of precursors for filler cokes and binder pitches, through production of these raw materials and their fabrication into anode carbon, and concluding with anode performance evaluation in full-size prebake and Soderberg cells of different designs. 

Coke is produced when organic matter is heated to 400-600°C, essentially in the absence of air. The organic matter used for anode binder coke has come primarily from coal tar, with minor amounts from petroleum residues. In contrast, filler coke is produced almost entirely from petroleum, with minor amounts from coal-tar pitch. Also in recent years, solvent-refined coal (SRC) filler coke has been found to produce high quality anode carbon. 

A patented process has been developed for the production of electrode binder pitch from petroleum-based materials. Carbon anodes produced from the petroleum-based pitch and coke have been used successfully on a commercial scale by the aluminum industry. One stage of the process involves the pyrolysis of a highly aromatic petroleum feedstock. To study the pyrolysis stage of the process a small, sealed tube reactor was used to pyrolyze samples of feedstock. The progress of the reaction is discussed in terms of the formation of condensed aromatic structures, defined by selective solvent extraction of the reaction product. The pyrolysis of the feedstock exhibits a temperature-dependent induction period followed by reaction sequences that can be described by first-order kinetics. Rate constants and activation energies are derived for the formation of condensed aromatic structures and coke. 

The formation of coke and coke precursors has been discussed in the context of decant oil pyrolysis. However, it should be noted that coking reactions and the compounds in binder pitch that produce coke, also play a critical part in the formation of the finished carbon anode. 

Naphthalene, anthracene, carbazole [86-74-8] phenol [108-95-2] and cresyUc acids are found in the tar. Phenol and cresyUc acids are useful as chemical and resin intermediates. The aromatic chemicals are useful in the manufacture of pharmaceuticals, dyes, fragrances, and pesticides. Various grades of pitch are made from residues of tar refining. Coal-tar pitch is used for roofing and road tar, and as a binder mixed with petroleum coke to produce anodes for the aluminum industry. 

Piebaked anodes aie produced by molding petroleum coke and coal tar pitch binder into blocks typically 70 cm x 125 cm x 50 cm, and baking to 1000—1200°C. Petroleum coke is used because of its low impurity (ash) content. The more noble impurities, such as iron and siUcon, deposit in the aluminum whereas less noble ones such as calcium and magnesium, accumulate as fluorides in the bath. Coal-based coke could be used, but extensive and expensive prepurification would be required. Steel stubs seated in the anode using cast iron support the anodes (via anode rods) in the electrolyte and conduct electric current into the anodes (Fig. 3). Electrical resistivity of prebaked anodes ranges from 5-6 Hm anode current density ranges from 0.65 to 1.3 A/crn. ... 

Coal-tar pitch is particularly valuable to anode and electrode manufacturers The mam function is to plasticize coke gnst so that formed bodies can be extmded or molded without distortion during the later stages of processing Additionally, the pitch should give a high-carbon yield and not adversely affect the overall properties of the finished article. Although coal-tar pitch remains the bmder of choice, petroleum-based binders can perform satisfactorily for the aluminum industry [18]. 

Graphite is a denser crystalline form of carbon. Graphite anodes are prepared by heating calcined petroleum coke particles with a coal tar pitch binder. The mix is then shaped as required and heated to approximately 2 800°C to convert the amorphous carbon to graphite. Graphite has now superseded amorphous carbon as a less porous and more reliable anode material, particularly in saline conditions. 

Viscosity is an important characteristic of pitches used as binders for the production of carbon and graphite electrodes. We used a Haake balance to measure SCT, petroleum and coal tar pitch viscosity, SCT pitches have viscosity between 1000-4000 cps at 160°C, A comparison of the viscosity-temperature relationship of two SCT pitches prepared by thermal and catalytic processes, a commercial petroleum and a coal tar pitch used for the production of carbon anodes is given in Figure 2. 

The aluminum industry consumes about 0.45 lb. of anode carbon for each pound of aluminum produced. The ideal carbon should have a moderately-isotropic structure with minimum oxidant-accessible surface of low, uniform reactivity, and a maximum ash content, excluding bath salts, of a few tenths of one percent. Industrial anode carbon is a baked composite usually made of calcined petroleum coke filler with a binder of coal-tar pitch coke. While there is no shortage of calcined petroleum coke, the quality is not very good and likely to become worse. Also, coke binders are subject to variability in both quality and supply. 

Concerning binder-filler compatibility, the ideal anode carbon should be a pure, homogeneous, moderately-disordered carbon structure. To this end, coal-tar pitch with 10-25% QI produces the most... 

Aluminium Production. Primary aluminium production is based on two different technologies basically using Soderberg or prebaked anodes. PAH are emitted from the carbon electrodes containing tar and pitch as binder (22). The emissions from the prebaked electrodes are usually 1-10% of that of the Soderberg electrodes. 

The carbon anodes are manufactured from a carbon source such as anthracite and a pitch binder mix. Two types have been used ... 

Self-baking anodes (known as Soderberg anodes), usually one per cell and therefore of much larger dimensions. Such anodes are fed at the top with the ground carbon and pitch binder and this bakes in situ as it gradually descends into the molten electrolyte to form a hard, dense material which acts as the anode surface. 

The electrochemical production of aluminum utilizes pre-baked or Soderberg anodes. Pre-baked anodes are manufactured by mixing petroleum or pitch coke with around 20% of electrode binder (see Chapter 13.1.2), followed by molding, as is also usual for the production of graphite electrodes. The green electrodes are baked in a ring furnace at a temperature of 1200 °C. 

Acheson process, the electrodes being made from a blend of delayed coke (the needle coke quality) and coal-tar pitch. Further, this period of time also saw the rapid growth of the aluminum-making industry using the Heroult-Hall electrochemical cell with its carbonaceous anodes made from regular delayed coke and coal-tar pitch as the binder. 

Graphite anodes are made by mixing calcined petroleum coke particles and coal tar pitch binder. The desired shape is heated at 2800°C in order to convert amorphous carbon to crystalline carbon (graphite), which is immune to chloride solutions [10-11]. These anodes are normally impregnated with linseed oil or synthetic resins for reducing porosity and spalling, hi addition, the anodes are buried in the soil and backfilled with coke breeze for a uniform distribution... 

All of the world s aluminium is produced in the Hall Heroult cell using a carbon anode, consisting of petroleum coke and about 20% pitch as a binder [2], Given the considerable importance of the process, an enormous amount of research has been carried out to ascertain the reaction mechanism. The overall reaction is thought to be ... 

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