*2.3. Fe2O3*

The Fe2O3 used is found in tailings from the Altonorte Smelting Plant. Its size is in a range of −75 + 53 μm. The methods used to determine its chemical and mineralogical composition are the same as those used in marine nodules. Figure 2 and Table 3 shows the chemical species that use QEMSCAN (QEMSCAN has a database, which has the elemental composition, and density of the minerals that are detected. With this information, it is possible to obtain the elementary contribution of the measured sample), and several iron-containing phases are presented, while the Fe content is estimated at 41.9%.

**Figure 2.** Detailed modal mineralogy.

**Table 3.** Shows the mineralogical composition of the tailings. The Fe in it was mainly in the form of magnetite.


#### *2.4. Steel (FeC)*

A low carbon steel sheet (FeC; 0.25% C) from the steel supplier company Salomon Sack was used. This sample was reduced in size with the use of a cone crusher at laboratory level and later a pulverizer until reaching a size range between −75 + 53 μm.

#### *2.5. Ferrous Ions*

The ferrous ions used for this investigation (FeSO4 × 7H2O) were the WINKLER brand (Santiago, Chile), with a molecular weight of 278.01 g/mol.

#### *2.6. Reactor and Leaching Tests*

The sulfuric acid used for the leaching tests was grade P.A., with 95–97% purity, a density of 1.84 kg/L and a molecular weight of 98.8 g/mol. The leaching tests were carried out in a 50 mL glass reactor with a 0.01 solid/liquid ratio in leaching solution. A total of 200 mg of Mn nodules were maintained in agitation and suspension with the use of a 5 position magnetic stirrer (IKA ROS, CEP 13087-534, Campinas, Brazil) at a speed of 600 rpm. The tests were conducted at a room temperature of 25 ◦C, with variations in additives, particle size and leaching time. The tests performed in duplicate, measurements (or analyses) carried on 5 mL undiluted samples using atomic absorption spectrometry with a coefficient of variation ≤5% and a relative error between 5% and 10%. Measurements of pH and oxidation-reduction potential (ORP) of leach solutions were made using a pH-ORP meter (HANNA HI-4222, St. Louis, MO, USA). The solution ORP was measured in a combination ORP electrode cell composed of a platinum working electrode and a saturated Ag/AgCl reference electrode.
