This article describes the proposed processing route for the Pastos Grandes Project, based on solar evaporation ponds and a lithium carbonate plant, as outlined in a 2022 Preliminary Economic Assessment.
Report context
The Preliminary Economic Assessment (Technical Report N° 209020-00051-000-GE-TEN-0001_1) for the Pastos Grandes Project, Salta Province, Argentina, was completed in 2022. The report presents a proposed design for the processing facilities, including production wells, evaporation ponds, and a lithium carbonate plant, all designed to produce 25,000 tonnes per year of lithium carbonate (Li₂CO₃). The information below is derived solely from Section 17 (Recovery Methods) of that report.
Processing route
Proposed design: brine extraction and solar evaporation
The proposed design calls for brine extraction wells to be drilled in the Pastos Grandes brine aquifer area, as defined by the resource drilling. Brine is pumped to halite evaporation ponds, where solar evaporation causes salt super-saturation, precipitating halite salts while lithium concentrates in the remaining brine. At a point to be defined between the initial fill and the end of the halite ponds, the brine is sent to a liming plant, where calcium hydroxide reacts with magnesium present in the brine to precipitate magnesium hydroxide, further concentrating the lithium.
After the halite precipitation stage, as the brine approaches saturation in potassium salts, the brine is fed to sylvinite evaporation ponds, where potassium salts precipitate and the brine is concentrated until it reaches a lithium concentration of 1.5%. Estimated pond lithium recovery efficiency is 80% to 85%.
An estimate for the treated brine in the ponds is 15,300,000 tonnes per year. The solar evaporation ponds are concentration and partial purification facilities based on natural evaporation. They are designed with a large area and low depth to create a high natural evaporation rate. The chemical equilibrium and natural saturation property of the brine allow the precipitation of unwanted salts, which are harvested and stocked in piles outside the ponds.
For pond construction, material from the area is used to build walls and platforms. The inside base and sides of the ponds are covered by geomembranes (waterproof plastic membranes) to avoid brine leakage. Geotextiles can be installed beneath the geomembrane if soil mechanics require.
The brine is fed from one interconnected solar pond to another downstream, preferably driven by evaporation and concentrate extraction.
Proposed design: liming plant
In the proposed liming plant, brine from the well is fed to a distribution tank and then to liming reactors with hydrated (slaked) lime. The slurry is fed to a clarifier, where solids are separated from the brine, with solids sent directly to a discard pond, while decanted brine is pumped to the sylvinite ponds. After the sylvinite ponds, lithium concentrated brine is sent to the Lithium Carbonate Plant, optimally by way of a reservoir or final “cook-up” pond.
Proposed design: lithium carbonate plant
The report describes the lithium carbonate plant as a chemical facility that receives brine concentrate from the solar ponds. The proposed plant expected lithium recovery efficiency is approximately 75% to 80%, resulting in total process efficiency of 60% or higher. The plant is assumed to operate at 90% availability.
The first stage of the proposed lithium carbonate plant is the extraction of boron, carried out in a solvent extraction (SX) stage. This stage is conducted at an acidic pH (pH = 3), yielding a boron-free refined brine. The boron-rich solution is fed to a re-extraction (or stripping) stage, where a change to basic pH (pH = 12) is achieved by recirculating part of the mother liquor and adding sodium hydroxide. This eliminates boron in the form of sodium borate and recovers the extraction solution for re-use.
The report notes that the suggested process for salar brine processing through solvent extraction is technology that has already been implemented and is proven in different plants in the lithium carbonate production industry. A footnote in the report states that the number of stages for SX defined in the diagram is only referential and that the required number of stages must be determined by laboratory tests.
The boron-free brine is then fed to a primary carbonation stage, where, through recirculation of mother liquor (solution recovered from filtration and centrifugation) and adding soda ash as required, contaminants such as calcium and magnesium precipitate as calcium carbonate and magnesium carbonate. These are pumped directly to a discard pond.
In the secondary carbonation stage, lithium carbonate precipitates through the addition of soda ash at high temperatures (80°C). The precipitated lithium carbonate is separated from the solution through filtration and centrifugation, and fed to a final purification stage to ensure production of technical and battery grade lithium carbonate. Through absorption, ionic exchange, and desorption, remaining contaminants are removed, complying with customer lithium carbonate specifications.
Finally, the product is dried, sized, and packaged, and delivered to clients according to their individual requirements.
Proposed design: reagents
The report details reagent consumption for the proposed process. One of the main reagents is soda ash (Na₂CO₃), used in primary and secondary carbonation stages, with annual consumption of 46,166 tonnes. Soda ash will be prepared in a dissolution plant, transformed from solid state to pulp. Recycled water from the process is used for preparation to a saturated concentration (28%), with temperature controlled for efficient dilution.
Calcium hydroxide is used in the liming plant within the ponds, aiding precipitation of magnesium and sulphates. The process considers consumption of 130,000 tonnes per year. Preparation requires slaking (addition of water) in dedicated equipment, with the slaking plant installed near the evaporation ponds.
Other reagents include CaO (80% purity) at 108,275 TPY for liming in ponds, HCl (33% purity) at 5,410 TPY for solvent extraction, NaOH (100%) at 483 TPY for solvent extraction, CO₂ (100%) at 1,341 TPY for purification, H₂SO₄ (100%) at 575 TPY for solvent extraction, and diluent plus organic reagents at 191 TPY for solvent extraction.
Proposed design: water purification and equipment cleaning
Although the proposed process considers re-utilization of water in various stages, fresh water injection is necessary in some steps, including final product washing. This fresh water will be treated in a water treatment plant to reduce contaminants.
Due to brine characteristics and contained salts/elements, the system generates salt deposits and incrustations inside equipment. These must be periodically cleaned using sulfuric acid solution at 18% concentration at intervals defined by the operations area.
Proposed design: solid waste management
Besides solids discards generated in evaporation ponds, solid discards are generated in the lithium carbonate plant. Borates and impurities from purification stages are solutions sent to the discard pond, with solution re-pumped to halite ponds to aid brine transfer. Magnesium carbonate and calcium carbonate generated in the primary carbonation stage must be sent to the discard pond, where solids decant and liquid is reused in evaporation ponds.
Key reported parameters
| Parameter | Value | Basis |
|---|---|---|
| Lithium carbonate production capacity | 25,000 TPY | Proposed design |
| Plant availability | 90% | Proposed design assumption |
| Brine feed to ponds | 15,300,000 ton/year | Proposed design estimate |
| Target lithium concentration after sylvinite ponds | 1.5% Li | Proposed design specification |
| Pond lithium recovery efficiency | 80% to 85% | Proposed design estimate |
| Plant lithium recovery efficiency | 75% to 80% | Proposed design estimate |
| Total process efficiency | 60% or higher | Proposed design estimate |
| Secondary carbonation temperature | >80°C | Proposed design specification |
| Primary carbonation temperature | 40–60°C | Proposed design specification |
| Solvent extraction pH | 3 | Proposed design specification |
| Stripping stage pH | 12 | Proposed design specification |
| Soda ash concentration for preparation | 28% (saturated) | Proposed design specification |
| Equipment cleaning acid concentration | 18% H₂SO₄ | Proposed design specification |
Project website: https://www.lithium-argentina.com/projects/pastos-grandes
Technical qualifications
The report states that the suggested process for salar brine processing through solvent extraction is technology that has already been implemented and is proven in different plants in the industry of lithium carbonate production. The report also notes a specific limitation: the number of stages for solvent extraction defined in the diagram is only referential and the required number of stages must be determined by laboratory tests.
The estimated pond lithium recovery efficiency of 80% to 85% is described as being current at the time of the report, and the plant expected lithium recovery efficiency of approximately 75% to 80% is provided, resulting in total process efficiency of 60% or higher.
Source: Section 17, Recovery Methods, Preliminary Economic Assessment of the Pastos Grandes Project, Salta Province, Argentina, Technical Report N° 209020-00051-000-GE-TEN-0001_1, 2022.
