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Iron ore was studied as a CO2 absorbent. Carbonation was carried out by mechanochemical and high temperature–high pressure (HTHP) reactions. Kinetics of the carbonation reactions was studied for the two methods. In the mechanochemical process, it was analyzed as a function of the CO2 pressure and the rotation speed of the planetary ball mill, while in the HTHP process, the kinetics was studied as a function of pressure and temperature. The highest CO2 capture capacities achieved were 3.7341 mmol of CO2/g of sorbent in ball milling (30 bar of CO2 pressure, 400 rpm, 20 h) and 5.4392 mmol of CO2/g of absorbent in HTHP (50 bar of CO2 pressure, 100 °C and 4 h). To overcome the kinetics limitations, water was introduced to all carbonation experiments. The calcination reactions were studied in Argon atmosphere using thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) analysis. Siderite can be decomposed at the same temperature range (100 °C to 420 °C) for the samples produced by both methods. This range reaches higher temperatures compared with pure iron oxides due to decomposition temperature increase with decreasing purity. Calcination reactions yield magnetite and carbon. A comparison of recyclability (use of the same material in several cycles of carbonation–calcination), kinetics, spent energy, and the amounts of initial material needed to capture 1 ton of CO2, revealed the advantages of the mechanochemical process compared with HTHP.