MINERAL PROCESSING KNOWLEDGE · IRON ORE BENEFICIATION
Reverse flotation removes silica from an iron concentrate by floating quartz and silicate gangue upwards and retaining the iron minerals in the cell underflow.
Technical overview. Actual process selection depends on representative ore samples, mineralogy, testwork and site conditions.
Most flotation applied to iron ore runs in reverse. Instead of attempting to float hematite or magnetite into a froth, the circuit floats the quartz and silicate gangue and keeps the iron mineral in the cell. Where a concentrate is already substantially upgraded and the remaining problem is silica, it is more controllable to remove the unwanted mineral than to recover the wanted one.
Technical note: The reagent schemes and values below are principles only. They are not engineering designs or performance guarantees. Reagent choice, dosage and pH must be established by testwork on representative ore samples and reviewed by qualified metallurgists.
1. Why silica becomes the target
A high iron assay is not the same thing as a saleable concentrate. The iron minerals are already well enough liberated, yet the concentrate still carries fine quartz and silicate particles that dilute the grade and add silica load to smelting. By the time a hematite flowsheet reaches its magnetic stages the material has been upgraded substantially, so what remains is largely a silica problem rather than a recovery problem. Anionic reverse flotation after high-gradient magnetic separation is what lifts the described hematite concentrate above 65% Fe.
The separation depends on surface chemistry, not on density or magnetic response: in the alkaline pulp used here, quartz surfaces carry a negative charge, and the reagent scheme is built around anchoring a collector there.
2. Direct flotation versus reverse flotation
The direction of the float affects product quality. In direct flotation the iron minerals are floated. Because the froth is loaded with the valuable mineral, iron particles are easily carried into it mechanically, entrained in the water between bubbles rather than attached to them, and gangue that should have been rejected travels with them. The grade is therefore lower and less stable.
Reverse flotation reverses the burden: quartz and silicate gangue are floated while the iron concentrate stays in the underflow. The valuable mineral never attaches to a bubble, so mechanical entrainment is no longer the controlling loss mechanism and the grade is higher and more stable. The trade-off is a large froth volume to transport, wash and dewater.
3. Anionic reverse flotation
Anionic reverse flotation runs at high pH. Sodium hydroxide raises the pulp to pH 11.5–12.0, lime supplies Ca²⁺ as the quartz activator, starch or water glass depresses the iron minerals, and a fatty acid collector such as sodium oleate or tall oil makes the quartz hydrophobic.
The mechanism is a two-step adsorption. Under strongly alkaline conditions Ca²⁺ adsorbs onto the negatively charged quartz surface; the fatty acid anion then combines with the adsorbed Ca²⁺, making the quartz hydrophobic so that bubbles lift it. The iron minerals, held down by the depressant, stay in the underflow. Each reagent has a separate duty:
- Sodium hydroxide: pH control at 11.5–12.0.
- Lime (CaO): Ca²⁺ activator for quartz.
- Starch or water glass: iron depressant.
- Fatty acid collector: makes quartz hydrophobic.
4. Cationic reverse flotation
Cationic reverse flotation works in a milder regime: neutral to mildly alkaline, at pH 8–10. An amine collector such as dodecylamine or an ether amine is added with starch, which depresses the iron minerals.
The mechanism is more direct: the amine dissociates in water to the cation R–NH₃⁺, which adsorbs on the negatively charged quartz surface by electrostatic attraction alone. No lime activation is needed and the reaction is fast. The drawback is the froth, which is relatively viscous and harder to transport, wash and remove from the cell.
5. Comparing the two reagent schemes
The schemes differ in pH, in collector and in adsorption mechanism; the principal values are summarised below.
| Parameter | Anionic reverse flotation | Cationic reverse flotation |
|---|---|---|
| Pulp pH | 11.5–12.0 | 8–10 |
| Quartz activator | Lime (CaO) supplying Ca²⁺ | None required |
| Collector | Fatty acid (sodium oleate, tall oil) | Amine (dodecylamine, ether amine) |
| Iron depressant | Starch or water glass | Starch |
| Adsorption mechanism | Ca²⁺ bridges the quartz surface and the fatty acid anion | Direct electrostatic attraction of R–NH₃⁺ to the negatively charged quartz surface |
The choice should follow from the ore and the plant. Water chemistry, lime availability, gangue mineralogy and froth behaviour in the cells all bear on it.
6. Where reverse flotation sits in an iron ore flowsheet
In the hematite route, reverse flotation is not the first separation step. The ore is ground and treated by low-intensity magnetic separation, then high-gradient magnetic separation at 0.8–1.2 T. The high-gradient stage rejects a large tonnage of tailings and upgrades the material to a hematite rougher concentrate. Anionic reverse flotation then removes the remaining silicate gangue and lifts the concentrate above 65% Fe.
The sequence matters because flotation is a finishing duty, not a bulk recovery duty. The feed to the bank is a comparatively small, high-grade stream, so reagent addition, conditioning time and cell volume are matched to it rather than to the whole plant throughput. The bank is fed through a conditioning and mixing stage, and the underflow goes to thickening and filtration.
For contrast, a magnetite flowsheet needs no flotation: multi-stage grinding and two or three stages of low-intensity magnetic separation reach 85%–95% passing 200 mesh and a concentrate grading 66%–68% Fe.
Conclusion
Reverse flotation works because it treats silica as the problem and iron as the product: floating the iron invites mechanical entrainment and a lower, less stable grade, while floating quartz leaves a cleaner product in the underflow. Anionic flotation does this at pH 11.5–12.0 with lime activation and a fatty acid collector; cationic flotation does it at pH 8–10 with an amine collector and no lime, at the cost of a more viscous froth. The right route depends on the ore and the plant, and should be settled by testwork on representative samples.
How LiCheng Tech can help
LiCheng Tech supplies the equipment that surrounds a reverse flotation circuit: conditioning and reagent mixing, flotation cells with the froth handling a reverse circuit demands, and the thickening and filtration train behind them. Share your concentrate assays, gangue mineralogy and target silica specification, and we can discuss the reagent scheme you are considering and prepare an equipment proposal. Grinding, magnetic separation and flotation are treated as one flowsheet, so the bank is matched to the upstream magnetic circuit rather than specified in isolation.
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