In hydrometallurgical processes, removing cobalt from zinc sulfate solutions is crucial for producing high-purity zinc. Here are four methods commonly used for this purpose:

1. Zinc Powder-xanthate Method

The Zinc Powder-xanthate Method involves the selective precipitation of cobalt from zinc sulfate solutions using zinc powder and xanthate ions:

1. Principle: Xanthate is an organic reagent that includes compounds such as potassium ethyl xanthate (C2H5OCSSK), sodium ethyl xanthate (C2H5OCSSNa), potassium butyl xanthate (C4H9OCSSK), and sodium butyl xanthate (C4H9OCSSNa). These xanthates form complexes with cobalt ions in solution.

2. Process:

   • Step 1: Zinc powder is added to the zinc sulfate solution to replace and remove copper and cadmium through a displacement reaction.
   • Step 2: Xanthate reagent is then added to selectively precipitate cobalt ions by forming stable cobalt xanthate complexes.

3. Advantages:

   • Selectivity: The method selectively removes cobalt due to the specific complex formation with xanthate ions.
   • Efficiency: It effectively purifies the solution to produce high-purity zinc.

2. Arsenic Salt Method

The Arsenic Salt Method uses arsenic salts and zinc powder in a two-stage purification process to remove cobalt and other impurities:

1. Principle: Arsenic salts, such as arsenic trioxide (As2O3), activate zinc powder to enhance its ability to remove cobalt, copper, nickel, and cadmium from the solution.

2. Process:

   • Stage 1: At elevated temperatures (353~368K), arsenic salt (As2O3), zinc powder, and CuSO4 are added to the zinc sulfate solution.
     • Reaction: CuSO4 reacts with zinc powder, leading to copper deposition on the zinc surface and forming a Cu-Zn micro battery. Cobalt ions (Co2+) and other impurities precipitate onto the copper cathode, forming a Zn-Cu-Co alloy.
   • Stage 2: Without additional heating, more zinc powder is added to remove residual cadmium.
     • Reduction Mechanism: As2O3 facilitates the reduction of As3+ ions on the cathode, forming a Zn-Cu-Co-As alloy that stabilizes cobalt and reduces Co2+ to acceptable levels for electroplating.

3. Advantages:

   • Selective Removal: Effectively removes cobalt, copper, nickel, and cadmium.
   • Efficiency: Two-stage purification ensures high-purity zinc products.

3. Antimony Salt + Zinc Powder Method

The Antimony Salt + Zinc Powder Method involves the use of antimony salts to remove cobalt under high-temperature conditions:

1. Principle: Antimony salts, such as antimony trioxide (Sb2O3), are used in combination with zinc powder to selectively precipitate cobalt from zinc sulfate solutions.

2. Process:

   • Preparation: Antimony salts and zinc powder are added to the zinc sulfate solution under high-temperature conditions.
   • Reaction: The antimony salts react with cobalt ions, leading to the formation of insoluble cobalt-antimony compounds.
   • Separation: The precipitated cobalt-antimony compounds are separated from the solution using filtration or other solid-liquid separation methods.

3. Advantages:

   • Effective Precipitation: Removes cobalt by forming stable cobalt-antimony compounds.
   • High Temperature Efficiency: Utilizes high temperatures to enhance the reaction and precipitation processes.

4. Nitroso-β-naphthol Method

The Nitroso-β-naphthol Method is commonly used internationally for cobalt removal:

1. Principle: Nitroso-β-naphthol is an organic reagent that forms a complex with cobalt ions in solution, facilitating their removal.

2. Process:

   • Addition: Nitroso-β-naphthol is added to the zinc sulfate solution containing cobalt ions.
   • Complex Formation: The reagent reacts with cobalt ions to form a colored complex that is insoluble in the aqueous phase.
   • Separation: The insoluble cobalt-nitroso-β-naphthol complex is separated from the solution using filtration or other solid-liquid separation methods.

3. Advantages:

   • Selectivity: Specifically targets cobalt ions, forming a stable complex.
   • International Usage: Widely used and recognized for effective cobalt removal.

Each of these methods offers a practical approach for removing cobalt from zinc sulfate solutions, contributing to the production of high-purity zinc products suitable for various industrial applications. Careful management of reaction conditions and impurities is essential to optimize their effectiveness in zinc refining processes.