The Magnesium-to-Lithium Ratio: Why This Number Defines Process Technology

What the Mg/Li ratio measures and why it matters so much

The magnesium-to-lithium ratio (Mg/Li) expresses how many grams of magnesium there are per gram of dissolved lithium in a brine. It is one of the variables that most shapes the design of a plant, because magnesium and lithium behave chemically in similar ways: both are cations that precipitate and act alike during concentration and purification. Separating them demands energy, reagents and time, and that cost scales directly with the amount of magnesium present.

The industry uses indicative ranges: an Mg/Li ratio below 6 is considered favorable, between 6 and 20 implies a more demanding process, and above 20 the treatment becomes costly and technically delicate. There is no single threshold that decides everything, but the ratio works as a first traffic light that anticipates a project's complexity even before sizing a plant.

Magnesium never comes alone: sulfates, boron and calcium

The Mg/Li ratio is the most cited figure, but it rarely acts alone. Brines in the Puna typically contain sulfates, boron, calcium and potassium in varying proportions, and each of these impurities adds a treatment step. Boron, for example, requires extraction with solvents or specific resins; calcium and sulfates determine which precipitating reagents to use and in what order. A brine with a good Mg/Li ratio but a high sulfate load may end up as complex as one with more magnesium.

That is why a full chemical analysis, rather than a single indicator, is what defines the flow diagram. The sequence of precipitation, pH adjustment and selective removal is designed to fit each salar. Two projects a few kilometers apart may require different configurations simply because their impurity matrix differs.

Evaporation: the traditional route and its limits

The classic method in low-cost brines is solar evaporation in ponds, where the sun concentrates the brine over months and precipitates unwanted salts in a staged manner. It works very well when the Mg/Li ratio is low and the climate cooperates with high radiation and low humidity, conditions typical of the Puna. It is intensive in surface area and water, but requires little external energy.

The problem appears when magnesium is high: during evaporation, magnesium salts form and drag lithium along, generating recovery losses that can fall in a range of 40 to 60 percent of the contained lithium. The higher the Mg/Li ratio, the more lithium is lost in discarded salts and the more reagents are needed for subsequent purification. In those cases, evaporation stops being the most efficient option.

DLE: when the ratio pushes toward direct extraction

Direct lithium extraction (DLE) groups technologies —adsorption, ion exchange, solvent extraction— that selectively capture lithium without relying on massive evaporation. Its great advantage is selectivity: by targeting lithium directly, they tolerate high Mg/Li brines better and achieve recoveries that can exceed 70 or 80 percent, while shortening production timelines from months to hours or days.

The trade-off is cost and maturity. DLE usually demands more energy, treated water and capital than evaporation, and several of its variants are still being validated at industrial scale. The choice between evaporation, DLE or hybrid schemes is not ideological: it emerges from crossing the Mg/Li ratio, the rest of the impurities, water and energy availability, and the target price of the product.

The impact on final product quality

The ratio also affects the output specification. The battery market requires battery-grade lithium carbonate or hydroxide, with purities on the order of 99.5 percent and strict limits on magnesium, sodium, calcium and sulfates in parts per million. The higher the impurity load in the source brine, the more purification and crystallization stages are needed to reach that specification.

This creates a direct link between geochemistry and value: a clean brine can reach battery grade with fewer steps, while a loaded brine may be more economical to process to technical grade. The Mg/Li ratio, ultimately, defines not only the cost of the process, but also which product is worth making.

Argentina's Puna: diverse salars, diverse solutions

Argentina, the world's fifth-largest lithium producer, is betting on the brines of the Puna for their relatively low cost and high solar radiation, ideal conditions for evaporation. But the Puna is not homogeneous: the salars of Jujuy, Salta and Catamarca show very different Mg/Li ratios and impurity profiles, which explains why traditional evaporation projects coexist with developments that incorporate DLE or hybrid schemes.

For investors and operators, understanding this ratio is key when evaluating assets and estimating capital and operating costs, especially in a context of incentives such as the RIGI in force since 2024. There is no universal winning technology: there are concrete brines that dictate the most efficient solution. Reading the geochemistry of each salar carefully is, today, the indispensable first step before any investment decision.