Steel mill dust is difficult to handle. The particles are as fine as flour, yet the surface repels water, and pellets pressed from it crumble at a touch. Take the same dust, pre-wet it, cure it in a stockpile for a period, and then pelletize — and the resulting pellets survive nearly twenty drops. What changes in between? Current research points to one answer: the dust is made to grow its own binder.

1. The Contradiction: Fine, But Not Water-Wettable

Zinc-bearing dust comes mainly from blast furnace baghouse dust and from the dedusting systems of BOF and EAF steelmaking. It consists largely of iron oxides, with zinc, lead, potassium and sodium present as well. This creates a fundamental conflict for pelletizing: an extremely fine particle size should favour balling, but the particle surface has poor hydrophilicity. Water does not spread on it — it beads up. Continuous liquid bridges between particles cannot form, green balls fail to grow, and the balling rate stays low.

The practical approach is to treat wettability as a measurable and modifiable property: contact angle indicates how much wettability has improved, while the temperature rise of the dust indicates how far the reaction has progressed.

2. Mechanism: One Hydration Reaction, Three Effects

Zinc-bearing dust contains a certain amount of calcium oxide, much of it in an unhydrated "dead-burnt" state — one of the reasons the dust absorbs water slowly and balls poorly. Pre-wetting followed by stockpile curing essentially moves the hydration reaction upstream, completing it before balling begins:

CaO + H₂O → Ca(OH)₂ + heat

This reaction produces three linked effects that form the theoretical basis of the whole process:

●Colloidal binder generated in situ. Calcium hydroxide forms a colloidal phase with genuine binding capacity — in effect, part of the binder is synthesized inside the raw material itself.

●Exothermic heating drives moisture diffusion. Hydration of calcium oxide and calcium silicates releases heat, raising the bed temperature. Water molecules move faster and diffuse further, so particles are wetted more thoroughly.

●Improved hydrophilicity, lower contact angle. Once the particle surface holds sufficient moisture, wettability improves and the water film spreads continuously — the necessary condition for balling.

Treating this as nothing more than "adding water and mixing" misses the point. The temperature rise during curing is direct evidence that the reaction is proceeding, and a readily available indicator of whether pre-treatment is complete.

3. Binder System: Inorganic Backbone, Organic Bridging

Hydration colloids alone are not sufficient. The paper adopts a composite binder route; the research group's related patents set out the specific concept: bentonite as the main component, with a small addition of polyacrylamide, total dosage held below 1%.

Bentonite is an inorganic binder with good thermal stability. It maintains the pellet skeleton through drying and high-temperature reduction. Polyacrylamide has a high molecular weight and long chains, forming polymeric bridges between particles and markedly raising the initial strength of green pellets. Combining the two compensates precisely for the weakness of either used alone — thermal stability on one side, binding power on the other.

The dosage is equally noteworthy. Conventional single binders such as bentonite or starch are typically added at 1%–8%; this route holds the total below 1%, reducing dilution of the product.

4. From Mechanism to Measured Performance

5. Key Takeaways

Turning a liability into a resource. Free calcium oxide is normally a disadvantage — it slows water absorption and impairs balling. Pre-wetting converts it into a binder source. This approach is more economical than simply adding external binder.

Pre-treatment is often undervalued. Attention in the balling circuit usually focuses on machine parameters. This work suggests that the condition of the raw material before it reaches the balling disc may matter more to final pellet quality.

Mechanism must be measurable. Using contact angle to characterize wettability and temperature rise to characterize reaction progress converts operator feel into monitorable parameters — and gives pre-treatment a basis for standardization and closed-loop control.