The grade of brine: why lithium concentration defines a project

What the "grade" of a brine means

In brine-based lithium mining, the "grade" is the concentration of the dissolved metal in the fluid, usually expressed in milligrams of lithium per liter (mg/L). Unlike hard-rock lithium, where one speaks of the percentage of lithium oxide in the ore, in salt flats the resource is dissolved in the brine filling the pores of the subsurface. That seemingly simple number is the first indicator a geologist, an engineer or an investor checks when evaluating a project.

The reason is straightforward: the more lithium each liter of brine holds, the less volume must be pumped, evaporated and processed to obtain one tonne of product. Concentration acts as a multiplier that runs through the entire chain, from the design of evaporation ponds to energy consumption and water footprint. That is why, before discussing total reserves, it is worth understanding the quality of each liter.

Typical ranges: from marginal to world-class

As a practical reference, brines below 300–400 mg/L are usually considered marginal and can hardly sustain a project on their own with conventional technologies. Between 400 and 700 mg/L lies an intermediate range, viable depending on the other variables. Salt flats regarded as world-class exceed 700 mg/L, and some of the richest deposits in the world reach values close to or above 1,000 mg/L.

These ranges are not rigid boundaries. A salt flat with a medium concentration can prove more attractive than a richer one if it has better chemistry, a higher pumping rate or nearby infrastructure. Concentration is the most visible input variable, but never the only one that decides a project's fate.

The impact on costs and technology

Concentration translates into money in an almost linear way in the early stages. A more diluted brine forces you to handle larger volumes for the same output: more extraction wells, more pond surface, more evaporation time and more energy. In arid regions, where solar evaporation is the engine of the process, a low grade lengthens the cycles and raises operating costs.

It also shapes the choice of technology. Rich brines with good chemistry lend themselves to the traditional pond-evaporation method, which has low operating costs though it is time- and land-intensive. More diluted brines, or those with unfavorable chemistry, push toward direct lithium extraction (DLE), a set of technologies that reduces timelines and water footprint but demands higher upfront investment and energy use. The grade, then, does not just measure richness: it guides the engineering of the project.

Impurities: the other side of the equation

A lithium-rich brine can turn out to be unattractive if it carries impurities that complicate processing. The most closely watched indicator is the magnesium/lithium ratio (Mg/Li), because magnesium behaves chemically much like lithium and is costly to separate. Mg/Li ratios below 6 are considered favorable; much higher values make production more expensive and complex, requiring more reagents and purification steps.

Sulfate, calcium, potassium and boron complete the picture. Potassium and boron can become valuable by-products, while sulfate and calcium usually add treatment steps. That is why a serious evaluation of a salt flat crosses lithium concentration with the full impurity profile: two brines with the same grade can have very different economics.

Concentration, reserves and mine life

Concentration interacts with two other variables to define the resource: the volume of stored brine and the effective porosity of the aquifer, that is, how much brine can actually be extracted. A salt flat may show a high grade but low permeability, which limits the pumping rate and therefore the achievable annual production.

Moreover, concentration can vary within the same salt flat, both laterally and with depth, and tends to evolve over the project's life as brine is extracted and recharge or dilution processes occur. Hydrogeological models seek to anticipate that dynamic in order to correctly size the plant and the deposit's mine life.

The Argentine Puna: why this variable matters

Argentina, the world's fifth-largest lithium producer, concentrates its potential in the salt flats of the Puna, in Jujuy, Salta and Catamarca. The region combines high solar radiation, vast salt-flat extensions and brines that in several cases show competitive concentrations and manageable chemistry, which has sustained its reputation as a low-cost zone within the so-called Lithium Triangle.

In a context of growing investment, driven by instruments such as the RIGI in force since 2024, understanding the grade of the brine is key to reading the map of projects with sound judgment. Behind every reserve announcement it is worth looking at concentration, the Mg/Li ratio and pumping capacity: it is those fundamentals, more than the headlines, that separate a viable project from a marginal promise.