The core of aluminum electrolysis industry (adopting Hall—Heroult process, i.e., cryolite-alumina molten salt electrolysis process) is "molten electrolyte system + aluminum raw materials + auxiliary reagents". The required chemicals revolve around electrolytic reaction, electrolyte regulation, equipment protection and impurity removal. Combined with industrial practical applications, they are sorted by function as follows (suitable for foreign trade and production filing needs, clarifying the role, specification and core purpose of each chemical):
I. Core Raw Materials (Necessary for Electrolytic Reaction, Determining Aluminum Output)
Such chemicals are the foundation of aluminum electrolysis production, directly participating in the reduction reaction of aluminum, and are the most consumed category in the production process. The core is aluminum-containing raw materials, supporting the production of metallic aluminum.
1. Aluminum Oxide (Al₂O₃)
As the core aluminum source for aluminum electrolysis, it is the only raw material for producing metallic aluminum, accounting for more than 90% of the total consumption of chemicals in aluminum electrolysis. Specification requirements: Industrial-grade aluminum oxide, purity ≥ 98.5%, ignition loss ≤ 1.0%, uniform particle size (20-80 mesh), good fluidity, easy for mechanized feeding and dissolution in molten salt. Function: After dissolving in the molten electrolyte, it is reduced to metallic aluminum through electrolytic reaction (core reaction: 2Al₂O₃ → 4Al + 3O₂↑). Its purity directly determines the quality of aluminum ingots; excessive impurities will reduce the grade of aluminum ingots. About 1.92-1.95 tons of aluminum oxide are consumed per ton of primary aluminum globally, making it the most core raw material in the aluminum electrolysis industry.
II. Core Chemicals for Electrolyte System (Regulating Electrolytic Conditions, Ensuring Smooth Reaction)
The electrolytic reaction of aluminum electrolysis must be carried out in a molten electrolyte. Such chemicals are mainly used to build a stable electrolyte system, reduce electrolysis temperature, improve conductivity, and are indispensable key reagents for the electrolysis process, with fluoride products as the core.
1. Cryolite (Na₃AlF₆)
The core solvent of the electrolyte system, irreplaceable, accounting for the main part of the consumption of electrolyte chemicals. Specification requirements: Industrial-grade cryolite, purity ≥ 98%, fluorine content ≥ 53%, sodium content ≤ 32%, moisture ≤ 0.5%; divided into granular (good fluidity, no dust, suitable for mechanized cell starting and normal production) and powdery (200-325 mesh, suitable for conventional electrolysis auxiliary supplement) according to particle size. Function: Dissolve aluminum oxide to form a cryolite-alumina molten electrolyte, reducing the melting temperature of aluminum oxide from 2050℃ to 940–980℃, greatly reducing electrolysis energy consumption; at the same time, it improves electrolyte conductivity, stabilizes the electrolysis process, protects the carbon lining of the electrolytic cell, and improves current efficiency, making it the core solvent of the Hall—Heroult process. The global annual consumption of cryolite is about 700,000 tons, more than 90% of which is used in the aluminum electrolysis industry. About 20-30 kg of cryolite is consumed per ton of primary aluminum (adjusted according to the type of electrolytic cell).
2. Aluminum Fluoride (AlF₃)
A key regulator of the electrolyte system, acting synergistically with cryolite, and an indispensable additive in aluminum electrolysis production. Specification requirements: Industrial-grade/metallurgical-grade aluminum fluoride, purity ≥ 98%, fluorine content ≥ 61%, moisture ≤ 0.5%; high-purity grade (purity ≥ 99.5%) can be used for high-end aluminum electrolysis production. Function: Regulate the molecular ratio of the cryolite-alumina molten salt system (molar ratio of NaF to AlF₃), controlling it within a reasonable range of 2.2−2.8, thereby reducing the liquidus temperature of the electrolyte, improving conductivity, reducing the frequency of anode effect, extending the service life of the electrolytic cell, and improving the purity of aluminum liquid and electrolysis efficiency. The global average consumption of aluminum fluoride per ton of primary aluminum is about 18–22 kg. In China, due to the use of wet-process products and old electrolytic cells by some enterprises, the average unit consumption is slightly higher, about 23–25 kg/ton.
3. Auxiliary Electrolyte Regulators (Optional, Optimizing Electrolytic Performance)
According to the working conditions of the electrolytic cell and product requirements, a small amount of auxiliary regulators can be added to further optimize electrolyte performance, reduce energy consumption and improve efficiency:
- Sodium Fluoride (NaF): Regulate the electrolyte molecular ratio and supplement sodium element, added when the molecular ratio is too low. Specification: Purity ≥ 98%, fluorine content ≥ 45%.
- Magnesium Fluoride (MgF₂): Reduce the liquidus temperature of the electrolyte, improve electrolyte fluidity, and reduce electrolyte volatilization. Specification: Purity ≥ 98%, fluorine content ≥ 50%, dosage per ton of primary aluminum is about 2-5 kg.
- Calcium Fluoride (CaF₂): Improve electrolyte stability, extend the service life of the electrolytic cell, and inhibit anode effect. Specification: Purity ≥ 97%, fluorine content ≥ 48%, dosage adjusted according to the scale of the electrolytic cell.
III. Electrode-Related Chemicals (Ensuring Electrode Performance, Supporting Electrolytic Reaction)
Aluminum electrolysis relies on electrodes to complete current conduction. Such chemicals are used to prepare electrodes or protect electrodes to ensure the stable progress of the electrolytic reaction.
1. Raw Materials for Prebaked Anodes (Carbon Anodes)
Prebaked anodes are the core electrodes (anodes) of aluminum electrolysis. The required chemicals are used to prepare anodes to ensure their conductivity and high-temperature resistance:
- Petroleum Coke: Core raw material, purity ≥ 98%, sulfur content ≤ 0.5%, used to prepare the carbon matrix of the anode and provide conductivity.
- Pitch Coke: Auxiliary raw material, used to bond petroleum coke particles and improve anode strength. Specification: Softening point 80-120℃.
- Coal Tar Pitch: Binder, used to bond petroleum coke and pitch coke particles into shape. Specification: Grade 1 coal tar pitch, ash content ≤ 0.3%.
2. Raw Materials for Cathode Paste (Carbon Cathodes)
Cathode paste is used to build the cathode of the electrolytic cell to ensure the conductivity and corrosion resistance of the cathode. Required chemicals:
- Graphite Powder: Improve cathode conductivity. Specification: Purity ≥ 95%, particle size 100-200 mesh.
- Anthracite: Core raw material, fixed carbon ≥ 85%, sulfur content ≤ 0.3%, used to prepare the cathode paste matrix.
- Coal Tar Pitch: Binder, mixed with graphite powder and anthracite to form shape, same requirements as coal tar pitch for prebaked anodes.
IV. Auxiliary Chemicals (Equipment Protection, Impurity Removal, Safety Protection)
Such chemicals do not directly participate in the electrolytic reaction, but are used to ensure production safety, equipment stability and product quality, and are the auxiliary support for aluminum electrolysis production.
1. Equipment Protection and Anti-Corrosion Chemicals
- Refractory Auxiliaries: Used for the maintenance of the electrolytic cell lining, such as sodium silicate (Na₂SiO₃), which improves the fire resistance and sealing of the lining and prevents electrolyte leakage.
- Anti-Corrosion Coating Raw Materials: Used for anti-corrosion of equipment around the electrolytic cell, such as epoxy resin and polyvinyl fluoride resin, which resist corrosion by sulfuric acid and fluorides.
2. Impurity Removal and Flue Gas Treatment Chemicals
- Calcium Hydroxide (Ca(OH)₂): Used for flue gas desulfurization and defluorination, removing hydrogen fluoride (HF) and sulfur dioxide (SO₂) generated during electrolysis. Specification: Purity ≥ 90%, particle size above 200 mesh.
- Activated Carbon: Used to adsorb harmful impurities in flue gas and purify waste gas. Specification: Powdered, adsorption value ≥ 800mg/g.
3. Safety and Auxiliary Reagents
- Sulfuric Acid (H₂SO₄): Used for equipment cleaning and anodizing treatment. Specification: Industrial-grade 98% concentrated sulfuric acid.
- Sodium Hydroxide (NaOH): Used for wastewater neutralization and equipment cleaning. Specification: Industrial-grade flake caustic soda, purity ≥ 96%.
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