Sodium Hydroxide Recovery Technology in Tungsten Metallurgy Industry

Tungsten, as a critical strategic metal, plays an irreplaceable role in numerous fields such as aerospace, electronics, and metallurgy. In tungsten metallurgy, sodium hydroxide (NaOH) is commonly used as a decomposing agent to break down tungsten ore raw materials. However, the amount of alkali added in tungsten smelting is typically 2.8–4 times the theoretical quantity, resulting in a mass ratio of excess NaOH to WO₃ in crude sodium tungstate solution reaching 0.7–1.2. Failing to recover this excess NaOH not only increases production costs but also causes adverse effects in subsequent processes and requires neutralization with sulfuric acid, leading to environmental pollution. Therefore, recovering NaOH in tungsten metallurgy is of significant economic and environmental importance. Membrane separation technologies, as novel separation methods, have demonstrated great application potential in NaOH recovery for tungsten metallurgy.

1. Principles of Membrane Separation Recovery Technology

(1) Principles of Membrane Separation

Membrane separation is a process that drives ions through a semipermeable membrane using a concentration gradient. In NaOH recovery for tungsten metallurgy, the waste solution containing NaOH and water are placed on either side of a cation-exchange membrane (CEM). Due to the concentration gradient, Na⁺ and OH⁻ in the NaOH solution tend to diffuse toward the water side. The CEM allows Na⁺ to pass through smoothly, while WO₃²⁻ (from sodium tungstate) cannot permeate the membrane. Meanwhile, OH⁻ more easily crosses the membrane to combine with Na⁺ on the water side, forming NaOH and achieving separation of alkali and salt.

(2) Principles of Bipolar Membrane Electrodialysis (BMED)

BMED is a technology that utilizes the selective permeability of bipolar membranes (BPMs), cation-exchange membranes (CEMs), and anion-exchange membranes (AEMs) to separate ions under an electric field. BPMs can dissociate water into H⁺ and OH⁻ under direct current (DC) electric fields. In NaOH recovery for tungsten metallurgy, the waste solution containing NaOH enters a specific membrane compartment. Driven by the electric field, Na⁺ migrates through the CEM to the opposite side, while OH⁻ is replenished by OH⁻ generated from BPM water dissociation, enabling enrichment and recovery of NaOH.

2. Applications of Membrane Separation Recovery Technology in NaOH Recovery for Tungsten Metallurgy

(1) Applications of Membrane Separation

In tungsten smelting processes, the crude sodium tungstate solution obtained after pressure filtration of alkali-pressure cooked slurry contains excess NaOH. This solution is used as the feed for membrane separation, where it is separated from water in a membrane device. By optimizing operating conditions such as solution flow rate and temperature, NaOH recovery efficiency can be improved. For example, under certain conditions, membrane separation can effectively separate NaOH from the crude sodium tungstate solution, producing a low-concentration tungsten-containing solution and a high-concentration NaOH solution that can be recycled into the tungsten smelting process.

(2) Applications of Bipolar Membrane Electrodialysis

Key variables affecting BMED were tested using economic and technical indicators such as free alkali (NaOH) migration rate, power consumption (E/kWh/T), current efficiency (η/%), dialyzer voltage, and temperature as objective functions. Results show an inverse relationship between power consumption and current efficiency; the initial concentration of free NaOH and current density are the main parameters influencing these indicators. Increasing the initial free alkali concentration improves both current efficiency and reduces power consumption. However, when the initial concentration exceeds 2.0 mol/L, an inflection point occurs: current efficiency slightly decreases, while power consumption remains constant. For a 6-stage parallel dialyzer, setting the current density at 73.3 mA/cm² and initial free alkali concentration at 2.0–2.5 mol/L results in power consumption below 2500–3000 kWh/t NaOH solution. Rational control of these parameters enables efficient NaOH recovery via BMED in tungsten metallurgy.

3. Factors Influencing Membrane Separation Recovery Efficiency

(1) Membrane Performance

Properties of the membrane—such as selective permeability, alkali resistance, and mechanical strength—directly affect recovery efficiency. High-performance membranes enable better alkali-salt separation and improve NaOH recovery rate and purity. For example, a CEM with high selective permeability can effectively block WO₃²⁻ while allowing Na⁺ to migrate smoothly.

(2) Operating Conditions

Operating parameters such as solution flow rate, temperature, and concentration significantly impact the membrane separation process. Excessively high flow rates may cause concentration polarization on the membrane surface, reducing separation efficiency; too low flow rates decrease processing capacity. Elevated temperatures enhance ion diffusion rates but may degrade membrane performance if too high. Solution concentration also affects ion migration driven by concentration gradients and electric fields, influencing recovery efficiency.

(3) Impact of Impurities

Tungsten metallurgy waste solutions contain various impurities, such as tungstate ions (WO₃²⁻) and heavy metal ions, which may deposit on the membrane surface, causing fouling and reducing membrane flux and separation efficiency. Therefore, proper pretreatment of the waste solution to remove partial impurities is necessary before membrane separation recovery.

4. Challenges and Development Directions for Membrane Separation Recovery Technology

(1) Challenges

• High membrane costs: Currently, high-performance membrane materials are expensive, increasing the cost of membrane separation recovery technology and limiting its large-scale application.
• Membrane fouling: Impurities in tungsten metallurgy waste solutions easily deposit on membrane surfaces, leading to fouling and requiring frequent membrane cleaning and replacement, which increases operational costs and maintenance difficulties.
• Need for improved technical integration: Membrane separation recovery technology needs integration with other processes (e.g., pretreatment, post-treatment) to optimize the entire tungsten metallurgy process. Current technical integration levels are insufficient and require further research and development.

(2) Development Directions

• Development of new membrane materials: Increase R&D efforts to create new membrane materials with high selective permeability, excellent alkali resistance, and low cost, thereby reducing the cost of membrane separation recovery technology.
• Optimization of membrane cleaning and regeneration technologies: Investigate effective membrane cleaning and regeneration methods to extend membrane lifespan and mitigate the impact of fouling on recovery efficiency.
• Enhanced technical integration: Deeply integrate membrane separation recovery technology with other tungsten metallurgy processes to achieve process optimization and automated control, improving production efficiency and product quality.

 

Membrane separation recovery technology holds significant application value in NaOH recovery for tungsten metallurgy. Methods such as conventional membrane separation and BMED can effectively separate NaOH from tungsten metallurgy waste solutions, enabling resource recycling. However, the technology still faces challenges such as high membrane costs, fouling, and low integration levels. In the future, strengthening R&D of new membrane materials, optimizing cleaning/regeneration techniques, and enhancing technical integration will promote the widespread adoption of membrane separation recovery technology in the tungsten metallurgy industry, facilitating its sustainable development.