Carbon technology for aluminum

Anode materials for aluminum (carbon anode material for aluminium electrolysis) ⑴ Classification Anode materials can be divided into two main categories: anode pastes and pre-baked anode blocks. The anode paste is used directly as an anode in self-baking aluminum electrolytic cells without being baked ； The anode carbon blocks have been shaped and calcined for use as anodes in pre-baked aluminum electrolysis cells. 　　A continuous self-baking anode composed mainly of an anode paste can operate continuously without the need for replacement; it uses the heat from the electrolytic cell to bake the anode, thereby saving energy, and it does not require pressure or baking equipment, which reduces investment costs. However, since asphalt fumes are emitted directly above the electrolyzer, it causes severe environmental pollution and poses difficulties in the purification of flue gases and the automation of aluminum electrolysis processes. In addition, the limited cross-sectional area of self-baked anodes hinders an increase in the capacity of the electrolyzers; the operation of these anodes is more complex than that of pre-baked anodes, their resistivity is higher, and they result in higher power consumption. 　　Pre-baked anodes, which are constructed with anode carbon blocks as their core, are relatively simple to operate; they exhibit a lower anode voltage drop compared to self-baked anodes. They facilitate mechanization and automation, and they eliminate the problem of pitch fumes during the electrolysis process, which helps to enable the development of electrolyzers with larger capacities. However, manufacturing anode carbon blocks requires processes such as shaping and baking, resulting in a long production process, high costs, and investment levels that are much higher than those required for producing anode paste. 　　⑵ Characteristic requirements: Aluminum production is an electrolytic process of the molten cryolite-alumina system. Carbon anode materials conduct current into the electrolyzer and participate in electrochemical reactions. The carbon anode is installed at the top of the electrolyzer; a strong direct current (30–300 kA) is passed through the carbon anode into the aluminum electrolyzer, where complex electrochemical reactions (anodic reactions) occur to decompose alumina at the bottom of the carbon anode. The final products of these reactions are CO and CO2. In aluminum electrolysis production, the carbon anode participates in the reaction and is gradually consumed; 420–650 kg of carbon anode are used up per ton of aluminum produced. During production, it is necessary to regularly add new anode paste to the electrolyzer (for self-baking anodes) or replace the anode blocks (for pre-baked anodes) in order to maintain normal continuous operation. Aluminum electrolysis production imposes the following requirements on carbon anode materials: high purity. In aluminum electrolysis production, the carbon anode material is gradually consumed by the electrolytic reaction, and the ash impurities contained in it end up in the molten aluminum, contaminating its quality. Therefore, it is required that the impurity content in carbon anode materials be as low as possible; generally, the ash content should not exceed 0.5%. 　　It has good electrical conductivity. In aluminum electrolysis cells, the carbon anode is involved in conducting electricity; the voltage drop across the carbon anode ranges from 0.35 to 0.5 V. Approximately 1,500 to 2,000 kW·h of electrical energy is consumed per ton of aluminum produced, which accounts for 10% to 15% of the total electrical energy used in aluminum production. Therefore, reducing the resistivity of the anode material is very important for lowering aluminum production costs. The resistivity of the anode carbon blocks should not exceed 60 μΩ·m, and the resistivity of the sintered anode paste should not exceed 80 μΩ·m. 　　Sufficient mechanical strength. The anode in an aluminum electrolyzer weighs dozens of tons and must also withstand electrical and thermal stresses, therefore it requires sufficient mechanical strength. The compressive strength of the anode paste sintered body and the anode carbon block should not be less than 27 MPa. 　　It has good resistance to reaction with CO2. 　　⑶ Raw materials: The raw materials used to produce anode carbon materials for aluminum production include petroleum coke, pitch coke, residual poles from pre-baked anodes, and a small amount of additives; the binder is coal tar pitch (the performance specifications of these raw materials are shown in Tables 4, 5, and 6). The raw coke is calcined, crushed, and graded; it is then mixed with coal tar pitch according to a specific formula, and after cooling it becomes a paste ； The kneaded paste is shaped and baked to become anode carbon blocks. The formulations of the anode paste and the anode carbon blocks are generally different. Table 4 Quality specifications for delayed petroleum coke and pitch coke (ZB E 44002–88 and GB 3070–82)
Specification 1# Petroleum Coke 2# Petroleum Coke 3# Petroleum Coke Pitch Coke
A B A B A B
Ash content/% ≤ 0.3 0.5 0.5 0.5 0.8 1.2 0.5
Sulfur content/% ≤ 0.5 0.8 1.0 1.5 2.0 3.0 0.5
Volatile matter/% ≤ 10 12 12 15 16 18 1.2
Moisture content/% ≤ 3 3 3 3 3 3 3
True density (g/cm³) ≥ 2.08 2.08 2.08 2.08 —— 1.96
Powdered coke content/% ≤ 3.0 3.0 3.0 3.0 —— 4.0
Note: The true density of delayed petroleum coke refers to that after calcination at 1,300 °C for 5 hours ； 　　　　The fine coke content for petroleum coke refers to particles of 8 mm or less, while for pitch coke it refers to particles of 25 mm or less. Table 5 Quality specifications for coal tar pitch (GB2290-80)
Specifications: Medium-temperature pitch, High-temperature pitch
Softening point (Ring method)/°C: 75–90, 95–120
Toluene-insoluble content (Extraction method)/%: 15–25
Quinoline-insoluble content/%: <10
Ash content/%: ≤ 0.3
Volatile matter/%: 60–70
Moisture content/%: ≤ 5.0
Coking value/%: ≥

Table 6 Quality specifications for modified pitch (YB/T5134-33)
Specifications: Grade 1, Grade 2
Softening point (Ring method)/°C: 100–115, 100–120
Toluene-insoluble content (Extraction method)/%: 28–34, >26
Quinoline-insoluble content/%: 3–14, 6–15
Resin content/%: ≤ 13, 16
Coking value/%: ≤ 54, 50
Ash content/%: ≤ 0.3, 0.3
Moisture content/%: ≤ 5, 5

Anode materials require low impurity levels; therefore, raw materials with low ash content are used. Its impurity content is generally not more than 0.5% (electrolyte components are present in the residual polar material, so the requirements for ash content can be relaxed accordingly). 　　Residual poles are the remaining parts of anode carbon blocks after use in electrolyzers; their surfaces are covered with alumina and fluoride salts. After these substances are removed, they can be reused as raw materials for producing anode materials. Coal tar pitch used for manufacturing anodes mainly includes medium-temperature coal tar pitch and high-temperature coal tar pitch or modified coal tar pitch. Modified petroleum asphalt can also be used as a binder for manufacturing anodes. 　　⑷ Production processes and equipment. The production process of the anode material includes pre-crushing of raw materials, calcination, crushing, screening and grading, proportioning; pretreatment of the binder, kneading; shaping of the kneaded paste, baking, and cleaning processes. The process flow diagram is shown in Figure 3. 　　Calcination is a process in which carbonaceous raw materials are subjected to high-temperature treatment in an atmosphere free from air. The purpose of calcination is to remove the volatiles from the raw material and improve its thermal stability, density, mechanical strength, electrical conductivity, and oxidation resistance. The true density of calcined petroleum coke is 1.99–2.03 g/cm3, and the resistivity of its powder is less than 650 μΩ·m. The calcination of petroleum coke in our country is mostly carried out in carbon factories or aluminum plants. 　　The main equipment for calcining petroleum coke in China includes tank-type calciners and rotary kilns. Based on the direction of movement of the material and the heating air flow, tank-type calciners can be divided into co-current tank-type calciners and counter-current calciners. The coke produced by batch furnace calcination has stable quality, low oxidation loss, and saves external energy. 　　The rotary kilns used in the carbon industry have a structure similar to that of rotary kilns in other industries, but they are usually equipped with secondary and tertiary air systems. The rotary kiln has a high production capacity. 　　Crushing and screening involve breaking down calcined petroleum coke, then screening and grinding it according to the specified requirements to produce materials of different particle sizes, which are subsequently stored in respective silos. Crushing machinery typically includes jaw, roller, hammer, and impact crushers. Grinding is usually carried out using ball mills, Raymond mills, etc. 　　Ingredients: The ingredients are used according to the established recipe. Formulas are developed through scientific experiments and industrial practice, based on the raw materials, product types, and required performance characteristics. The formulas for different products vary. The typical formula for the anode paste is shown in Table 7: the asphalt content accounts for (28±3%) of the total paste. Table 7 Typical formulation of anode paste Particle energy/mm +4 -4+2 -2+1 -1 Powder content/% ＜2 23±3 13±3 Balance 483