Method for metal-free tanning of a quantity of hide having initial weight (w), said quantity of hide being contained in a drum, wherein said method comprises at least one tanning treatment of said hide including at least the following operating steps: B.1 feeding aliphatic aldehyde into said drum, in a percentage ranging between 1% and 3% by weight with respect to the initial weight (w) of said hide, optionally equal to 2% by weight, and B.2 moving said drum for a time period of at least 10 min, optionally approximately 20 min at an operating speed between 1 and 8 revolutions/minute, preferably comprised between 4 and 5 revolutions/minute; B.3 feeding citric acid into said drum, in a percentage ranging between 0.1% and 0.3% by weight with respect to the initial weight (w) of said hide, optionally equal to 0.2% by weight, and B.4 moving said drum for a time period of at least other 10 min, optionally approximately 20 min at said operating speed; B.5 feeding aliphatic aldehyde into said drum, in a percentage ranging between 1% and 3% by weight with respect to the initial weight (w) of said hide, optionally equal to 2% by weight, and B.6 moving said drum for a time period of at least 150 min, optionally approximately 180 min at said operating speed; B.7 feeding sodium formate into said drum, in a percentage ranging between 0.3% and 0.7% by weight with respect to the initial weight (w) of said hide, optionally equal to 0.5% by weight, and B.8 moving said drum for a time period of at least 20 min, optionally approximately 30 min at said operating speed; B.9 feeding magnesium oxide into said drum, in a percentage ranging between 0.3% and 0.7% by weight with respect to the initial weight (w) of said hide, optionally equal to 0.5% by weight, and B.10 moving said drum for a time period of at least 60 min, optionally approximately 90 min at said operating speed; B.11 feeding water into said drum, at a temperature ranging between 30°C and 60°C, optionally equal to 50°C, in a percentage ranging between 30% and 50% by weight with respect to the initial weight (w) of said hide, optionally equal to 40% by weight, and B.12 moving said drum for a time period of at least 100 min, optionally approximately 120 min at said operating speed; B.13 feeding aliphatic aldehyde into said drum, in a percentage ranging between 1% and 5% by weight with respect to the initial weight (w) of said hide, optionally equal to 3% by weight, and B.14 moving said drum for a time period of at least 100 min, optionally approximately 120 min at said operating speed; and then B.15 moving said drum for a maintaining period comprised between 10 and 16 hours, optionally approximately 14 hours, at a first maintaining speed ranging between 1 and 5 revolutions/hour, optionally comprised between 2 and 4 revolutions/hour, even more optionally equal to 3 revolutions/hour. B.1 feeding aliphatic aldehyde into said drum, in a percentage ranging between 1% and 3% by weight with respect to the initial weight (w) of said hide, optionally equal to 2% by weight, and B.2 moving said drum for a time period of at least 10 min, optionally approximately 20 min at an operating speed between 1 and 8 revolutions/minute, preferably comprised between 4 and 5 revolutions/minute; B.3 feeding citric acid into said drum, in a percentage ranging between 0.1% and 0.3% by weight with respect to the initial weight (w) of said hide, optionally equal to 0.2% by weight, and B.4 moving said drum for a time period of at least other 10 min, optionally approximately 20 min at said operating speed; B.5 feeding aliphatic aldehyde into said drum, in a percentage ranging between 1% and 3% by weight with respect to the initial weight (w) of said hide, optionally equal to 2% by weight, and B.6 moving said drum for a time period of at least 150 min, optionally approximately 180 min at said operating speed; B.7 feeding sodium formate into said drum, in a percentage ranging between 0.3% and 0.7% by weight with respect to the initial weight (w) of said hide, optionally equal to 0.5% by weight, and B.8 moving said drum for a time period of at least 20 min, optionally approximately 30 min at said operating speed; B.9 feeding magnesium oxide into said drum, in a percentage ranging between 0.3% and 0.7% by weight with respect to the initial weight (w) of said hide, optionally equal to 0.5% by weight, and B.10 moving said drum for a time period of at least 60 min, optionally approximately 90 min at said operating speed; B.11 feeding water into said drum, at a temperature ranging between 30°C and 60°C, optionally equal to 50°C, in a percentage ranging between 30% and 50% by weight with respect to the initial weight (w) of said hide, optionally equal to 40% by weight, and B.12 moving said drum for a time period of at least 100 min, optionally approximately 120 min at said operating speed; B.13 feeding aliphatic aldehyde into said drum, in a percentage ranging between 1% and 5% by weight with respect to the initial weight (w) of said hide, optionally equal to 3% by weight, and B.14 moving said drum for a time period of at least 100 min, optionally approximately 120 min at said operating speed; and then B.15 moving said drum for a maintaining period comprised between 10 and 16 hours, optionally approximately 14 hours, at a first maintaining speed ranging between 1 and 5 revolutions/hour, optionally comprised between 2 and 4 revolutions/hour, even more optionally equal to 3 revolutions/hour.