Process Design Scheme for High-Purity Iron Electrowinning Project

1.0 Design Basis and Objectives

• Product Target: Produce high-purity iron with a purity of ≥ 99.99% (4N) and above, primarily targeting high-end electronics, aerospace, specialty alloys, and scientific research.

• Raw Material Selection:

  • Preferred Option: High-purity iron oxide (Fe₂O₃, purity ≥ 99.9%), sourced from deep purification of high-grade iron concentrate or decomposition of carbonyl iron.

  • Backup Option: Pickling waste solution from high-quality steel (for preparing ferrous chloride) or commercial high-purity iron scraps/offcuts.

• Core Technical Route: Raw Material Deep Purification → Electrolyte Precision Preparation → Electrorefining → Product Post-Treatment

• Design Philosophy: Full-process cleanliness control, multiple purification barriers, closed-loop material management to achieve ultimate purity.

2.0 Overall Process Flow Design

The core of high-purity iron production lies in the "dissolution-purification-recrystallization" process. The complete process chain is shown in the flowchart below, covering the entire journey from raw material pretreatment to the final packaged product:

flowchart TD
subgraph PreTreatment[Raw Material Deep Purification Section]
    A[High-Purity Iron Oxide Fe₂O₃] --> B[Reduction Step<br>High-Purity H₂/Carbothermal Reduction]
    B --> C[Crude Iron Anode Plate]
end

subgraph Electrolysis[Electrorefining Section - Core]
    C --> D[Electrolytic Cell Anode]
    D -- Electrochemical Dissolution --> E[Electrolytic Cell Cathode]
    E -- High-Purity Iron Deposition --> F[High-Purity Iron Starter Sheet/Product]
    G[Precision Electrolyte<br>FeCl₂/FeSO₄] --> D
    G --> E
end

subgraph PostTreatment[Product Post-Treatment Section]
    F --> H[Peeling and Washing]
    H --> I[Vacuum Drying]
    I --> J[Vacuum Arc Melting<br>Optional]
    J --> K[Ingot Product]
    L[Inert Gas Protection] --> I & J
end

subgraph Auxiliary[Auxiliary Systems]
    M[Electrolyte Purification System<br>Ion Exchange/Electrolysis] --> G
    N[High-Purity Water System] --> G
    O[Cleanroom Environment] --> PostTreatment
end

Step-by-Step Process Explanation:

I. Raw Material Deep Purification Section

The goal of this section is to prepare the high-purity crude iron anode plate used as the soluble anode.

1. Raw Material Processing: High-purity iron oxide powder is reduced at low temperatures (~600-800°C) in a high-purity hydrogen atmosphere or vacuum environment to obtain relatively high-purity sponge iron or iron powder.

   • Reaction Equation: Fe₂O₃ + 3H₂ → 2Fe + 3H₂O

2. Melting and Casting Anodes: The obtained high-purity iron powder/sponge iron is melted in a Vacuum Induction Melting (VIM) furnace. During melting, heavier impurities sink, and some gaseous impurities are removed. It is then cast into crude iron anode plates. The purity of these anode plates (e.g., 99.9%) is much higher than that of ordinary steel.

II. Electrorefining Section (Core)

This is the most critical step for enhancing iron purity, conducted within a cleanroom.

1. Electrolyte Preparation:

   • System Selection: Commonly used systems are Ferrous Chloride (FeCl₂) or Ferrous Sulfate (FeSO₄). Chloride systems have better conductivity but are more corrosive; sulfate systems are milder and easier to control for purity.

   • Solvent: Prepared using ultrapure water with a resistivity ≥ 15 MΩ·cm.

   • Additives: Add small amounts of grain refiners (e.g., saccharin, gelatin) and surfactants to make the iron deposit dense and smooth.

2. Electrolytic Cell Operation:

   • Anode: The crude iron anode plate prepared above.

   • Cathode: Can be titanium mother blanks (for subsequent peeling) or stainless steel plates. Titanium is preferred due to its smooth surface, which facilitates product peeling.

   • Condition Control:

     • Temperature: 50 - 70°C (stable, conducive to deposition)

     • pH Value: Strictly controlled (e.g., 2.5-3.5) to prevent Fe²⁺ oxidation and hydrolysis.

     • Current Density: Use a relatively low cathode current density (e.g., 200 - 500 A/m²) to favor the formation of large, dense grains and reduce impurity entrapment.

     • Cell Voltage: Approximately 0.5 - 1.0 V.

3. Purification Mechanism:

   • Upon energization, the anode crude iron undergoes oxidation (Fe → Fe²⁺ + 2e⁻), and iron ions enter the solution.

   • Metal impurities more active than iron (e.g., Zn, Mn) will also dissolve but will not deposit at the cathode due to their more negative deposition potential.

   • Metal impurities less active than iron (e.g., Cu, Ni, Pb) do not dissolve at the anode and settle as anode slime at the bottom of the cell.

   • Finally, only Fe²⁺ ions, with a potential close to iron and high purity, are reduced at the cathode (Fe²⁺ + 2e⁻ → Fe), forming a 99.99%+ high-purity iron deposit.

III. Product Post-Treatment Section

1. Peeling and Washing: The cathode plate deposited with high-purity iron is removed. The iron deposit (starter sheet) is carefully peeled off under ultrapure water spray.

2. Vacuum Drying: The wet iron sheets are immediately transferred to a vacuum drying oven and dried at low temperature (e.g., 80°C) to prevent oxidation.

3. Vacuum Melting (Optional but highly recommended): To obtain dense, defect-free ingots and remove trace residual gaseous impurities (O, H, N), Vacuum Arc Remelting (VAR) or Electron Beam Melting (EBM) is performed. This step can push the final product purity to 5N (99.999%) or even higher.

4. Packaging: Vacuum packaging is carried out in an inert gas (argon) glove box to prevent product surface oxidation.

IV. Auxiliary Systems

1. Continuous Electrolyte Purification: The circulating electrolyte needs to pass through ion exchange resin columns to deeply remove impurity cations (e.g., Cu²⁺, Ni²⁺).

2. Ultrapure Water System: Ultrapure water is prepared using a "RO + EDI + Polished Mixed Bed" process.

3. Environmental Control: Key processes are carried out in a cleanroom (at least Class 1000) to prevent contamination from airborne particles.

3.0 Key Process Parameters and Equipment Selection

4.0 Scheme Advantages and Challenges

Advantages:

1. Extremely High Purity: Electrorefining is one of the most reliable and mature methods for producing high-purity metals at 4N grade and above.

2. Controllable Impurities: Effective separation of impurity elements can be achieved by controlling the potential.

3. Diverse Product Forms: Can produce high-purity iron in various forms such as sheets, blocks, and ingots.

Challenges and Countermeasures:

1. Fe²⁺ Oxidation: Highly susceptible to air oxidation to Fe³⁺, affecting deposition quality.

   • Countermeasure: Sealed electrolyte circulation, covering the cell surface with inert balls or blanketing with inert gas (e.g., Ar).

2. High Energy Consumption: Both electrolysis and vacuum melting are energy-intensive processes.

   • Countermeasure: Optimize current efficiency, use high-efficiency rectifier power supplies, and implement heat recovery systems.

3. Equipment Corrosion: Especially in chloride systems.

   • Countermeasure: Strict selection of corrosion-resistant materials (titanium, special alloys, polymer materials).

Summary

This scheme outlines a technical pathway centered on "Raw Material Purification - Electrorefining - Vacuum Melting," systematically addressing impurity control challenges in the production of high-purity iron. The key to successful implementation lies in cleanliness control throughout the entire process, precise management of electrolytic process parameters, and advanced vacuum melting technology. This scheme provides a solid technical foundation for establishing an industrial-scale high-purity iron production project. Detailed pilot testing is necessary before actual construction to optimize all parameters.