Why Are Carbonaceous Reducing Agents Like Coke Or Coal Powder Added During The Reduction Treatment Of Copper Smelting Slag?

The primary function of adding carbonaceous reducing agents like coke or coal powder is to fundamentally alter the chemical and physical state of copper smelting slag. These agents chemically reduce high-valence magnetite (Fe3O4) into low-valence ferrous oxide (FeO), a transformation that significantly lowers the slag’s viscosity and density to release trapped metal.

The presence of magnetite in smelting slag creates a thick, viscous environment that traps valuable copper. By using carbon to convert this magnetite into ferrous oxide, you create a fluid slag that allows copper droplets to settle and be recovered efficiently.

The Chemistry of Slag Reduction

Targeting Magnetite

The core objective of adding carbon is to address the accumulation of magnetite (Fe3O4). In copper smelting, this high-valence iron oxide is a byproduct that negatively impacts the fluidity of the melt.

The Reduction Process

Carbonaceous agents act as chemical scavengers. They strip oxygen from the magnetite, converting it into ferrous oxide (FeO). This reduction from a high-valence state to a low-valence state is the critical first step in optimizing the slag.

Improving Physical Properties

Lowering Viscosity

The presence of solid or complex magnetite structures makes molten slag thick and sluggish. Converting this to ferrous oxide significantly reduces the viscosity, making the slag much more fluid.

Decreasing Density

The chemical transformation also lowers the overall density of the slag phase. This increases the density difference between the waste slag and the valuable copper, which is essential for gravity-based separation.

Mechanisms of Copper Recovery

Disrupting Encapsulation

High-viscosity slag tends to form encapsulated structures that physically trap copper matte or metallic copper droplets. The reduction process breaks these structures down, effectively unlocking the trapped metal.

Enhancing Sedimentation

Once the slag is fluid and less dense, gravity takes over. Metallic copper droplets that were previously suspended can now settle through the molten layer with minimal resistance. This sedimentation is the primary driver for improving the overall copper recovery rate.

The Pitfall of Unchecked Magnetite

The Viscosity Trap

If carbonaceous agents are not used or are dosed incorrectly, magnetite levels remain high. This results in a slag so viscous that it behaves like a thick paste rather than a liquid, preventing physical separation.

Mechanical Entrapment

Without the reduction to ferrous oxide, copper droplets remain mechanically suspended in the slag. This leads to significant yield loss, as valuable metal is discarded along with the waste slag.

Making the Right Choice for Your Process

Understanding the relationship between slag chemistry and physical behavior is key to high recovery rates.

If your primary focus is Maximizing Fluidity: Ensure your reduction process aggressively targets the conversion of Fe3O4 to FeO to minimize viscosity.
If your primary focus is Minimizing Copper Loss: Prioritize breaking encapsulated structures to allow the gravity settling of trapped copper droplets.

Control the chemistry of your iron oxides, and the physical separation of your copper will follow.

Summary Table:

Mechanism	Chemical Change	Physical Effect	Outcome
Reduction	Fe3O4 → FeO	Decreased Viscosity	Fluid slag allows metal movement
Phase Change	High-valence to Low-valence	Decreased Density	Enhanced gravity separation
Recovery	Breaking Encapsulation	Fluidity Increase	Settling of trapped copper droplets

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