A procedure of minimum environmental impact and maximum lithium recovery for obtaining concentrated brines with minimal impurity content from brines that embed natural salt flats and salt marshes, the procedure wherein the following stages are comprised: a) building fractional crystallization ponds by solar evaporation; b) filling the ponds with natural brine; c) initially pre-concentrating natural brine to the maximum possible lithium concentration in the liquid phase without precipitating lithium-containing salts; d) cooling the pre-concentrated brine obtained in c) ensuring maximum precipitation of salts containing sulfate anion; e) chemically pre-treating the liquid phase of brine separated from precipitated salts by cooling to minimize sulfate anions in the liquid phase after cooling; f) finally pre-concentrating the pre-treated liquid phase to the maximum possible lithium concentration in it without precipitating lithium-containing salts; g) chemically treating the liquid phase of brine separated from precipitated salts at the stage f) to minimize the concentration of magnesium, calcium, boron and sulfate in the liquid phase; and h) concentrating the liquid phase obtained at the stage g).