Technical Investigation Report on the Application of Reverse Osmosis Membranes in Lithium Extraction from Salt Lake Brine

Executive Summary

This report aims to systematically investigate the current application status, technical principles, process positioning, advantages and disadvantages, and development trends of reverse osmosis (RO) membrane technology in the field of lithium extraction from salt lake brine. The core conclusion indicates that reverse osmosis membranes are not directly used for "selective lithium extraction" but rather serve as a crucial "pre-concentration" and "purification" unit. In modern salt lake lithium extraction processes, particularly within adsorption or solvent extraction process chains, they play an indispensable supporting role. Their core value lies in efficiently and continuously increasing the lithium ion concentration in the brine while removing impurities, thereby creating optimal feed conditions for the downstream core lithium extraction unit and significantly reducing overall energy consumption and costs.

1. Technical Background and Core Logic

The core challenge of lithium extraction from salt lake brine lies in economically and efficiently separating and enriching low-concentration lithium ions from a complex brine rich in high concentrations of ions such as magnesium, sodium, potassium, and boron.

• Fundamental Difference between RO and NF: It is essential to clarify that reverse osmosis membranes are primarily used for desalination (retaining most ions), while nanofiltration (NF) membranes can achieve selective separation of monovalent/divalent ions based on the Donnan effect. Therefore, reverse osmosis membranes do not possess the capability for direct magnesium-lithium separation. Their role positioning in lithium extraction is fundamentally different from that of NF.

• Core Functional Logic of RO: Leveraging the high rejection rate (close to 99%) of ions (including Li⁺) by RO membranes, their core function is "dewatering and concentration". By subjecting pre-treated brine to reverse osmosis, a major portion of water molecules is removed, thereby significantly increasing the concentration of all ions in the brine, including the target lithium ions, and achieving a drastic reduction in brine volume (typically concentrated by a factor of 2-5). This lays the foundation for subsequent process steps (e.g., adsorption, precipitation) to handle a smaller volume of higher concentration feed, greatly saving on equipment scale, reagent consumption, and subsequent evaporation energy requirements.

2. Application Processes and Process Positioning

Reverse osmosis membranes are primarily integrated into the following two mainstream salt lake lithium extraction process routes:

Route 1: RO Pre-concentration + Adsorption/Solvent Extraction Method (Mainstream High-Efficiency Route)

This route is currently the mainstream choice for lithium extraction from high Mg/Li ratio salt lakes (e.g., Qinghai salt lakes) and low-grade salt lakes.

1. Pretreatment: Raw brine undergoes steps such as aeration, calcium/magnesium removal, and filtration to remove suspended solids, calcium, magnesium ions, and other scaling-prone components, meeting RO feed water requirements (SDI<5, very low hardness).

2. RO Pre-concentration Unit: The pre-treated, low-concentration brine (Li⁺ concentration approx. 0.2-0.5 g/L) is pumped into the reverse osmosis system. Driven by high pressure (typically 5-8 MPa), water molecules permeate through the membrane, while lithium and other ions are retained. The produced "lithium-enriched concentrate" can have its lithium concentration increased to 1-3 g/L, with volume reduced to 20%-50% of the original feed.

3. Core Lithium Extraction Unit: The concentrate enters subsequent adsorption columns (using aluminum-based, titanium-based adsorbents) or extraction units for selective extraction and enrichment of lithium ions. The efficiency of this step is significantly enhanced due to the greatly increased feed lithium concentration.

4. Deep Purification and Precipitation: The post-adsorption solution or stripped solution undergoes further purification before battery-grade lithium carbonate is precipitated by adding sodium carbonate.

Route 2: RO Deep Purification + Membrane Hybrid Process

1. NF Pre-desalting: Raw brine first passes through a nanofiltration membrane for preliminary magnesium-lithium separation, yielding a lithium-rich retentate with a low Mg/Li ratio.

2. RO Deep Concentration and Purification: The NF permeate (still containing Na⁺, K⁺, B³⁺, etc.) enters the reverse osmosis unit for further water removal and lithium ion concentration, while simultaneously retaining impurities like boron, yielding a higher purity, higher concentration lithium solution suitable for subsequent evaporation crystallization or direct precipitation.

   

3. Technical Advantage Analysis

1. Efficient Volume Reduction, Lowering Overall Process Cost: The energy efficiency of RO is far superior to natural evaporation, enabling rapid, large-scale reduction in brine volume for treatment. This significantly reduces the capital investment and operating costs for subsequent stages like adsorption, evaporation, and precipitation.

2. Enhancing Core Unit Efficiency: Provides the adsorption process with feed of moderate concentration and reduced impurities, improving the adsorption capacity and rate of the adsorbent and shortening the adsorption-desorption cycle.

3. Continuous, Automated Production: Membrane systems are suitable for continuous, stable operation and easily integrated with automation control, enhancing the modernization level and stability of the production line.

4. Environmental Friendliness: Compared to relying solely on solar evaporation ponds, it requires significantly less land. Being a closed system, it is less affected by climate and avoids dust pollution.

4. Major Challenges

1. High Energy Consumption: High operating pressure makes it a major energy-consuming unit in the process. The concentration factor is limited by the brine's osmotic pressure limit and the membrane's pressure tolerance.

2. Stringent Pretreatment Requirements: The complex composition of salt lake brine (calcium, magnesium, sulfate, silica, organics, etc.) makes membranes highly susceptible to scaling and fouling. A comprehensive and efficient pretreatment system (e.g., ion exchange softening, chemical precipitation, precision filtration) is mandatory, increasing process complexity and cost.

3. Membrane Fouling and Service Life: Despite pretreatment, risks of organic, colloidal, and biological fouling persist during long-term operation, necessitating regular chemical cleaning. This affects continuous operation time and increases maintenance costs.

4. Brine/Concentrate Management: The disposal of the high-salinity RO concentrate (containing high concentrations of magnesium, sodium salts, etc.) is an environmental challenge. It typically needs to be returned to evaporation ponds for further solar concentration or used for extracting other by-products (e.g., potassium chloride, boric acid) to achieve resource utilization.

5. Representative Case Studies

• A Salt Lake Project in Qinghai, China: Employs a "pretreatment + two-stage reverse osmosis pre-concentration + adsorption" process. After pretreatment, raw brine is concentrated by the RO system, increasing lithium concentration from about 0.3 g/L to over 2.5 g/L and reducing volume by more than 70%, before entering the adsorption stage. This process significantly reduces lithium carbonate production costs and enables year-round continuous operation.

• South American Salt Lake Lithium Extraction Processes: Some projects utilize reverse osmosis as a refining step to further concentrate lithium and separate impurities like boron from the residual brine (bittern), ensuring final product purity.

6. Conclusions and Outlook

Reverse osmosis membrane technology plays a key role as an "efficient concentration engine" and a "purification front-end unit" in lithium extraction from salt lakes. Its value lies not in direct lithium extraction, but in paving the way for downstream highly efficient and selective core lithium extraction technologies (e.g., adsorption, solvent extraction) by substantially upgrading the feed stock quality. It is a crucial enabler for the large-scale, economical development of lithium resources from salt lakes.

Future Development Trends:

1. Development of Specialized Membrane Materials: Research and development of anti-fouling, high-pressure resistant, high-rejection RO membranes suitable for high-salinity, high-hardness, high-organic brine environments.

2. Deep Process Integration: Closer integration and optimization with nanofiltration, electrodialysis, selective adsorption, and other processes to form efficient, low-energy consumption "membrane-based lithium extraction" process packages.

3. Energy Recovery and Conservation: Wider application of high-efficiency energy recovery devices (e.g., pressure exchangers) to recover energy from the high-pressure concentrate, reducing overall system power consumption.

4. Concentrate Resource Utilization: Strengthening research on comprehensive extraction technologies for valuable elements (magnesium, boron, potassium, bromine, etc.) from RO concentrate to achieve "zero liquid discharge" and maximize resource utilization.

In summary, the reverse osmosis membrane is a pivotal, connecting element in the modern salt lake lithium extraction process chain. Although it lacks intrinsic selectivity, through its efficient concentration and purification functions, it significantly enhances the economics and feasibility of the entire lithium extraction process. It stands as one of the core technological pillars supporting the current transition of salt lake lithium extraction from traditional solar evaporation towards efficient, continuous plant-based production.