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The removal of molybdenum (Mo) impurities from sodium tungstate (Na?WO?) is a critical step in the purification process of tungsten chemicals, especially for producing high-purity tungsten products or intermediates such as APT (Ammonium Paratungstate) and AMT (Ammonium Metatungstate). Since both molybdenum and tungsten belong to Group VI B of the periodic table and exhibit similar chemical properties, their separation is particularly challenging. Below are commonly used methods and the principles behind them:

I. Forms of Molybdenum Impurities

In alkaline conditions, both tungsten and molybdenum exist in the form of polyoxoanions:

Tungstate ion: WO?2?
Molybdate ion: MoO?2?

Because of their similar structures in alkaline solutions, conventional separation is difficult and requires the use of redox reactions, potential differences, or differences in complex stability.

II. Common Removal Methods

Sulfide Precipitation Method (H?S Precipitation Method)

Principle: Molybdate ions react with H?S under acidic or neutral conditions to form insoluble MoS?, while tungstate ions remain soluble.
Process Steps:
1. Adjust the pH of the sodium tungstate solution to a weakly acidic range (pH 1.5–3.0);
2. Introduce H?S gas;
3. Mo forms a black precipitate (MoS?), which is filtered out;
4. Tungstate remains in the supernatant.
Advantages: Simple equipment and convenient operation.
Disadvantages: Toxic H?S gas; sulfur-containing waste needs proper treatment.

Solvent Extraction Method

Principle: Organic extractants with selectivity for Mo complex with Mo to extract it into the organic phase, while W stays in the aqueous phase.
Common Extractants:
- Oxo-phosphine compounds (e.g., TOPO)
- Organic phosphate esters (e.g., TBP)
- Trioctyl phosphate (TOP)
Operating Conditions:
- Optimal pH range: 1.5–2.5
- Multistage extraction enhances separation efficiency.
Advantages: High separation accuracy, suitable for continuous operation.
Disadvantages: High cost; organic solvent recovery required.

Reduction Method (Selective Reduction)

Principle: Under controlled conditions, Mo is reduced to an insoluble lower-valence oxide (e.g., MoO?) and removed by precipitation, while W remains in a soluble, higher-valence form.
Common Reductants:
- Metallic zinc powder
- Reducing organic compounds (e.g., sulfites, formaldehyde)
Process Control:
- Typical temperature: 60–90°C
- Precise control of pH and reductant dosage is critical to avoid reducing W.
Advantages: Thorough separation and low cost.
Disadvantages: Complex process control; risk of introducing new impurities.

Ion Exchange Method

Principle: Separation based on the difference in affinity of Mo and W ions toward ion exchange resins under different pH conditions.
Types of Resins:
- Strongly basic anion exchange resins
- Special chelating-type resins
Control Conditions:
- Requires precise regulation of pH, temperature, and flow rate
- Typically suitable for small-scale, high-purity preparations
Advantages: High product purity
Disadvantages: Complex operation and high cost

Calcium Salt Precipitation Method

Principle: Under certain pH conditions, Mo forms insoluble CaMoO? precipitates while W remains soluble.
Application: Limited separation precision; usually used as a preliminary pretreatment step or in combination with other methods.

III. Recommended Industrial Flow (Combined Methods)

Adjust the pH of the alkaline sodium tungstate solution to 2–3;
Introduce H?S gas or add sodium sulfide to precipitate Mo;
Filter out MoS? precipitate;
Perform solvent extraction for further Mo removal;
Use ion exchange for final purification if necessary.

Summary and Comparison

Method	Selectivity	Process Complexity	Cost	Industrial Applicability
Sulfide Precipitation	Medium-High	Low	Low	Common for coarse separation
Solvent Extraction	High	Medium	Medium-High	Suitable for large-scale operations
Reduction Method	High	Medium	Medium	Suitable for refining processes
Ion Exchange	Very High	High	High	For high-purity preparation
Calcium Precipitation	Low	Low	Low	Used in pretreatment