Process Flow Scheme for Separating Indium Ions from Polyamic Acid

Core Idea: Hydrometallurgy + Organic/Inorganic Separation

Objective: Efficiently recover high-purity indium (as metal or indium oxide) while minimizing interference from polyamic acid organics.

Overall Process Flow Diagram

flowchart TD
    A[Indium-containing Polyamic Acid<br>Feedstock] --> B{Pre-treatment Decision}
    
    B -->|Path One: Decomposition| C1[High-Temp Decomposition/Calcination]
    C1 --> D1[In-containing Inorganic Ash]
    D1 --> E[Acid Leaching]
    
    B -->|Path Two: Dissolution-Precipitation| C2[Alkali/Acid Treatment]
    C2 --> D2[Liquid-Solid Separation]
    D2 --> D2_Solid[PAA Solid Phase<br>For Treatment or Disposal]
    D2 --> D2_Liquid[In-containing Liquid Phase]
    D2_Liquid --> E

    E --> F[Purification & Concentration]
    F --> G[Displacement/Electrolysis]
    G --> H[Product: High-Purity Indium Ingot]
    
    F --> I[Precipitation & Calcination]
    I --> J[Product: Indium Oxide<br>In₂O₃]

Step 1: Pre-treatment and Indium Liberation (Critical Step)

The goal of this step is to dissociate indium from the polymeric organic matrix. There are two main technical pathways:

Path A: Decomposition Method (Direct, thorough, but PAA is lost)

• Operation: Subject the In-containing PAA to high-temperature calcination/ashing in a controlled atmosphere (e.g., air or nitrogen).

• Temperature: Programmed heating to 600-800°C to ensure complete decomposition of PAA into gases (CO₂, H₂O, nitrogen oxides), leaving indium as indium oxide in the ash.

• Advantages: Completely destroys organics; indium exists as a simple inorganic compound, simplifying subsequent processing.

• Disadvantages: PAA is completely destroyed and cannot be reused; high temperature may cause partial indium loss via volatilization (indium oxides have significant vapor pressure at high temps) - off-gas treatment is important.

• Applicability: Suitable for waste recovery scenarios where PAA retention is not required.

Path B: Dissolution-Precipitation Method (Milder conditions, PAA backbone potentially retainable)

• Operation: Treat PAA with chemical reagents to cause its precipitation or dissolution, separating it from indium.

  • Option B1 (Acid Treatment): Treat PAA solution or solid with dilute acid (e.g., 1-3M HCl or H₂SO₄). H⁺ ions compete with In³⁺ for carboxylate sites, displacing some indium. Simultaneously, PAA may precipitate in strong acid. Preliminary separation is achieved via filtration.

  • Option B2 (Alkali Treatment): Treat with dilute alkali (e.g., NaOH). PAA dissolves to form a polyamate salt, while In³⁺ precipitates as In(OH)₃ at elevated pH. Via filtration, indium is enriched in the solid phase as indium hydroxide, and the polyamate salt remains in the liquid phase. This method effectively enriches indium into the solid.

• Advantages: Mild conditions, selective separation, potential for PAA recovery or treatment.

• Disadvantages: Separation may be incomplete; requires optimization of pH, concentration, etc.; introduces additional salts.

• Applicability: Scenarios requiring mild treatment or preliminary indium enrichment.

Pre-treatment Output: Whether it's ash from Path A or the In-containing solution/precipitate from Path B, the goal is to direct the process towards an indium-rich acidic aqueous solution (for In(OH)₃ from B2, redissolution in acid is required) for the next stage.

Step 2: Indium Purification and Concentration

The obtained acidic leach solution containing indium (likely with other metal impurities and minor organic decomposition products) requires further purification and concentration.

• Solvent Extraction Method (Most common and effective):

  1. Extraction: Mix the leachate with an organic solvent containing an extractant (e.g., Di(2-ethylhexyl)phosphoric acid, D2EHPA or P204 in kerosene) at pH 2.0-3.0. Indium ions are selectively extracted into the organic phase, leaving most impurities in the aqueous phase.

  2. Washing: Wash the indium-loaded organic phase with dilute acid to remove co-extracted minor impurities (e.g., Fe³⁺).

  3. Stripping: Strip the washed organic phase with a high-concentration hydrochloric acid solution (e.g., 6M HCl). Indium is efficiently back-extracted (stripped) into the HCl solution. At this point, indium is highly concentrated and purified.

• Ion Exchange Method (Suitable for cleaner, lower concentration systems):

  • Pass the indium-containing solution through a column packed with cation exchange resin or specialized chelating resin to adsorb indium ions. Then, selectively elute indium using a suitable eluant (e.g., HCl of varying concentrations).

• Precipitation Method (Simple, but lower purity):

  • Adjust the solution pH to 3.5-4.0 to hydrolyze and precipitate indium as In(OH)₃, which can be separated from some impurities that precipitate at higher or lower pH. This method offers limited separation efficiency.

Step 3: Final Indium Recovery

Recover metallic indium from the purified, concentrated indium chloride solution.

• Zinc Cementation (Classic method):

  • Immerse high-purity zinc plates (or powder) into the purified InCl₃ solution. The displacement reaction occurs: 2In³⁺ + 3Zn → 2In + 3Zn²⁺.

  • Spongy metallic indium deposits on the zinc surface or at the bottom. Collect and melt it into an indium ingot. This method is simple, but product purity is affected by zinc and impurities.

• Electrolytic Refining (For high-purity indium):

  • Use the purified InCl₃ solution as the electrolyte, with insoluble anodes (e.g., Ti coated with precious metal) and cathodes (e.g., Ti or stainless-steel plates).

  • Electrolyze at a suitable current density. High-purity metallic indium deposits on the cathode. This is the standard method for producing 4N (99.99%+) high-purity indium.

• Preparation of Indium Oxide:

  • Add ammonia or alkali to the purified indium salt solution to generate high-purity In(OH)₃ precipitate. After filtration, washing, and drying, calcine it at high temperature (>600°C) to obtain high-purity indium oxide (In₂O₃) powder.

Recommended Integrated Process Flow

For recovering indium from polyamic acid waste, the most reliable and industrially proven combination is:

1. Pre-treatment: Adopt "Path A: Low-temperature decomposition/calcination" to thoroughly decompose organics and avoid their interference in subsequent hydrometallurgy.

2. Leaching: Leach the ash with 2-4M Hydrochloric Acid to bring indium into solution as InCl₃. Filter to remove insoluble residues.

3. Purification & Concentration: Employ the "P204 Solvent Extraction" process, including extraction, washing, and stripping, to obtain a high-concentration, high-purity indium chloride strip solution.

4. Final Recovery: Adopt a "Cementation + Electrolysis" combination. First, use zinc cementation to obtain crude indium. Then, use the crude indium as an anode for electrolytic refining to obtain a high-purity indium ingot. Alternatively, directly electrolyze the strip solution to obtain high-purity indium.

Key Considerations

• Safety & Environment: The entire process involves acids, alkalis, organic solvents, and potentially toxic gases (during calcination). It requires a complete ventilation, off-gas scrubbing (e.g., alkali spray), and wastewater treatment system. Organic extractants need a closed-loop system to prevent leaks.

• Process Optimization: Optimal acid concentration, extraction pH, phase ratio (O/A), stripping acid concentration, etc., must be determined through laboratory bench-scale testing.

• PAA Characteristics: The specific solid content, molecular weight, and solvent type (typically DMAC, NMP, etc.) of the PAA need to be clarified, as they affect the choice of pre-treatment method.

This scheme provides a technical framework from principle to practice. Systematic laboratory and pilot-scale testing is essential before implementation.