Mechanism of consumption and influencing factors of carbon anodes for aluminum

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A true classic. Lu Zhongsheng (Northeastern University, Shenyang 110004, Liaoning). Abstract: This paper reviews the consumption mechanism of carbon anodes used in aluminum production, the factors affecting the consumption of these anodes, as well as the approaches and methods to reduce the net consumption of anodes and thereby improve the economic efficiency of aluminum plants. Keywords: aluminum electrolysis ； Anode consumption Chinese Library Classification Number: TF821 ； TQ127.1 1 Document code: A Article ID: 1001—3741(2002)03—0038—3 THE CONSUMPTION MECHANISM OF CARBON ANODES IN ALUMINIUM ELECTROLYSIS AND ITS INFLUENCING FACTORS LU Zhong-sheng (Northeastern University, Liaoning Shenyang 110004, China) Abstract: This paper reviews the consumption mechanism of carbon anodes in aluminum electrolysis as well as the factors that influence their consumption. To enhance the economic efficiency of aluminum production plants, various methods for reducing the net consumption of carbon anodes are discussed in detail. Keywords: Aluminum electrolysis ； In aluminum electrolysis, the consumable carbon anode (referred to as an anode) serves the purpose of conducting electricity, as well as participating in electrochemical and chemical reactions. The theoretical consumption of the anode is 334 kg/t, while the actual consumption of the anode, or the net anode consumption, is generally between 390 and 480 kg/t. The difference between the net anode consumption and the theoretical anode consumption is called the excess anode consumption. Reducing the net anode consumption or the excess anode consumption will lower the production cost of aluminum; therefore, it is useful to understand the theories related to anode consumption. 1 Anode consumption mechanism Norwegian scholars believe that…: The anode consumption mechanism can be divided into electrochemical consumption, chemical consumption, and mechanical consumption. 1.1 Anodic electrochemical consumption The anodic electrochemical reaction can be expressed in the following two forms: 2Al2O3 + 3e⁻ → 4Al + 3CO₂ (1) Al2O3 + 3e⁻ → 2Al + 3CO (2) The theoretical anodic consumption is calculated based on equation (1), under the assumption that the current efficiency of the aluminum electrolysis process is 100% and the CO₂ concentration is 100%. 1.2 Consumption due to anodic chemical reactions The consumption due to anodic chemical reactions includes the consumption resulting from the anodic air oxidation reaction, the consumption from the anodic Budar reaction, and the consumption caused by side reactions in aluminum electrolysis. 1) Anodic air oxidation reaction: The anodic air oxidation reaction takes place on the top and side surfaces of the anode. The oxidation reactions are as follows: BaH)+O2 → O32 (3) 2C (anode) + O2 → 2CO (4). When the temperature is below 700°C, equation (3) dominates ； When the temperature is above 700°C, equation (4) dominates. 2) Anodic Buda reaction: The anodic Buda reaction primarily occurs on the working surface of the anode, as well as on the side of the anode below the electrolyte. The reaction is as follows: C(BaH)+O3²⁻ = 2CO (5) 3) Side reactions in aluminum electrolysis: These side reactions refer mainly to the interaction between aluminum dissolved in the electrolyte and O3²⁻ in the gases at the anode. The chemical reaction is as follows: 3O3²⁻ + 2Al = Al2O3 + 3CO (6) This reaction does not cause direct anode consumption, but it leads to anode consumption indirectly. 1.3 Mechanical consumption of the anode: The anode is usually composed of calcined petroleum coke and pitch coke. Studies have shown that the oxidation rate of pitch coke is greater than that of calcined petroleum coke; this phenomenon is known as anodic preferential oxidation or selective oxidation. As a result, some calcined petroleum coke particles separate from the anode body; this phenomenon is commonly known as slagging. Slagging is also related to factors such as low-temperature roasting of the binder phase and electrolyte erosion. 2 Factors affecting the net consumption of carbon anodes: Both the quality of the anodes and the conditions under which they are used have an impact on their net consumption to varying degrees. The higher the volumetric density of the anode, the lower its net consumption. In 1991, W. K. Fisher and others from Swiss Aluminium Company derived the following equation relating the net anode consumption to the electrolysis process parameters and anode quality parameters【3J: N = c + 1.2(￡–960) – 1.7CM + E·9.3Ap + 82 – 1.5AM ». In this equation, N represents the net anode consumption, in kg per ton ； C —— Electrolyzer coefficient ； CE — Current efficiency, % ； £—— Electrolysis temperature,℃ ； CRR — Anode O32 reaction rate remainder, % ； AP — Anode air permeability, nPm ； —— Anode thermal conductivity, W/(m·K) ； ARR — Anode air reaction rate remainder, %. It can be seen from equation (7) that N is inversely proportional to CE; improving current efficiency can reduce the net anode consumption. Above 960℃, N increases ； Below 960°C, N decreases. CM and AM decrease, N increases ； CM and AM increase while N decreases; in other words, CM and AM are inversely proportional to N. AP and t are proportional to N. Furthermore, Soviet researchers such as M. M. Virovkov argued that the consumption of carbon anodes is inversely proportional to the current density, or that the two are reciprocals of each other. 3 Theory on Reducing the Impact of Net Anode Consumption on the Economic Efficiency of Aluminum Plants 3.1 Reducing net anode consumption can improve current efficiency. Equation (7) above shows that there is an inverse relationship between net anode consumption (N) and the current efficiency of aluminum electrolysis (CE): reducing N increases CE. 3.2 Reducing the net anode consumption increases aluminum production. The relationship between aluminum production and current efficiency is given by: P=8.052×J×CE×Σ(Nt)×10^-1. (8) In equation (8): P — the aluminum production of the aluminum electrolyzer series, in t ； 8.052 —— Product of the aluminum electrochemical constant and the number of hours per day and night, g/A ； Average current of Bu Yi, A ； CE — Current efficiency, % ； Σ(Nt) —— Total number of days and nights in the slot ； N— — Number of trees ； £— — Number of days and nights the trough is in operation. As can be seen from equation (8), the aluminum production of the electrolyzer is proportional to the current efficiency; in other words, the higher the CE, the greater P. It is known that reducing N increases CE; from this, it can be inferred that reducing N will increase P. 3.3 Reducing the net anode consumption lowers the power consumption in aluminum electrolysis. The relationship between the power consumption in aluminum electrolysis (referred to as power consumption) and the average cell voltage as well as current efficiency is given by the formula: W = 2980 × (9) × L / Where: – Power consumption, in kW·h/t ； — — Average voltage of the slot, V ； CE — Current efficiency, %. From equation (9), it can be seen that when remains constant, is inversely proportional to CE; in other words, increasing CE reduces . It is also known that reducing N can increase CE. It can be inferred that reducing N can reduce . The above two conclusions are supported by the production practices of Guizhou Aluminum Plant and the American company Ormet in reducing the net anode consumption, thereby improving current efficiency, lowering power consumption, and increasing aluminum output. 4 Summary 1) The consumption mechanisms of aluminum anodes can be divided into electrochemical consumption, chemical consumption, and mechanical consumption. 2) The factors affecting the net consumption of the anode are related not only to the quality of the anode itself but also to its operating conditions in the aluminum electrolyzer. 3) Reducing the net anode consumption improves current efficiency, increases aluminum production, and lowers power consumption; the two conclusions in (8) and (9) have been confirmed by production practices both domestically and internationally.