Limonite is a secondary iron ore composed mainly of hydrated iron(III) oxide, chemically represented as FeO(OH)·nH₂O. It is not a single mineral but a mixture of hydrated iron oxides formed by weathering and oxidation of primary iron minerals.
Often called brown iron ore, limonite typically contains 30–55% iron and appears yellowish-brown to dark brown. Although considered low-to-medium grade, it remains an important iron resource due to its abundance and versatility.
Chemical Composition & Mineralogical Nature
| Component | Percentage (%) |
|---|---|
| Iron (Fe) | 55 – 60 |
| Oxygen (O) | 30 – 35 |
| Combined Water (H₂O) | 5 – 15 |
| Silica (SiO₂) | 2 – 10 |
| Alumina (Al₂O₃) | 1 – 6 |
| Other Impurities | Trace |
Mineralogical Constituents:
Goethite (α-FeO(OH))
Lepidocrocite (γ-FeO(OH))
Amorphous iron hydroxides
Physical Properties
| Property | Typical Value |
|---|---|
| Color | Yellow-brown to dark brown |
| Streak | Yellowish-brown |
| Density | 3.6 – 4.0 g/cm³ |
| Hardness (Mohs) | 4.0 – 5.5 |
| Porosity | High |
| Magnetic Behavior | Non-magnetic to weak |
Mechanical Properties
| Property | Description |
|---|---|
| Compressive Strength | Low to moderate |
| Tensile Strength | Very low |
| Fracture Behavior | Brittle, friable |
| Wear Resistance | Low |
| Impact Resistance | Very poor |
Metallurgical Behavior
Upon heating, limonite undergoes dehydration and phase transformation:
100–300 °C: Loss of free and bound water
300–500 °C: Conversion to hematite (Fe₂O₃)
Due to high moisture and gangue content, limonite requires pre-treatment such as drying or calcination before ironmaking.
Key Characteristics
✔ Hydrated and porous structure
✔ Lower iron content
✔ High moisture and LOI
✔ Abundant and widely distributed
Available Forms
| Form | Description |
|---|---|
| Run-of-Mine Ore | Natural weathered ore |
| Lumps | Low-grade iron feed |
| Fines | Beneficiation feedstock |
| Calcined Limonite | Upgraded iron oxide |
| Pigment Grade | Yellow iron oxide |
Applications
Secondary iron source for sintering and blending
Natural yellow iron oxide pigments
Water purification and adsorption media
Soil remediation and environmental use
Decorative stone and earth pigments
Advantages
✔ Abundant and low-cost
✔ Easy surface mining
✔ High reactivity after calcination
✔ Valuable pigment and adsorbent
Why Choose Limonite?
Limonite is chosen as a cost-effective secondary iron resource, particularly for blending, sintering, pigment production, and environmental applications. While inferior to hematite and magnetite for direct ironmaking, its abundance and versatility make it strategically important.