Molten Light Metal Processing: Part Two

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Fluxing of Copper Alloys

In the processing of copper alloys, fluxing practices are tailored to address a range of challenges, including gas absorption, impurity removal, and melt refinement.

Oxidizing fluxes, such as those containing cupric oxide or manganese dioxide (MnO₂), are used to regulate hydrogen gas content. These fluxes decompose at copper alloy melting temperatures, generating oxygen to prevent hydrogen absorption and mitigate steam reactions. Neutral cover fluxes, often made from borax, boric acid, or glass, form fluid slag covers that reduce metal loss, prevent zinc flaring, and absorb nonmetallic impurities like oxides and lubricants.

Reducing fluxes, incorporating carbonaceous materials like charcoal or graphite, help lower oxygen absorption and melt loss in high-copper, low-zinc alloys. However, these fluxes lack the ability to clean impurities from the melt. Refining fluxes are utilized to remove specific metallic impurities, with their efficacy highly dependent on the alloy system being processed. For example, silicate fluxes are effective in removing lead, while sodium carbonate fluxes manage sulfur and other harmful impurities. Additionally, semi-continuous casting mold fluxes are employed to lubricate and control surface structures during casting processes.

Impurity Removal in Copper Alloys

The removal of impurities in copper alloys varies depending on the alloy composition and specific contaminants. Fire refining efficiently removes elements like aluminum, manganese, silicon, phosphorus, iron, zinc, tin, and lead. Lead, a common contaminant, can be extracted using silicate fluxes or phosphate slag covers combined with thorough stirring. Sulfur impurities, which are particularly harmful in copper-nickel or nickel silver alloys, can be mitigated using sodium carbonate fluxes or through the addition of manganese or magnesium.

Aluminum, often a contaminant in copper alloys, causes porosity and compromises pressure tightness even at low concentrations. It can be removed using fluxes with oxidizing agents and fluoride salts, which render aluminum oxides removable. Silicon impurities are similarly addressed after aluminum has been removed.

Fluxing of Zinc Alloys

Zinc alloy processing involves specific fluxing practices aimed at minimizing melt loss and cleansing charge materials. Clean, pure zinc and its alloys generally require minimal fluxing, as zinc does not readily oxidize or absorb hydrogen at typical melting temperatures. However, chloride-based cover fluxes, when used carefully, can form fluid slag covers that protect the melt and reduce melt loss.

In situations involving dirty scrap or contaminated materials, reactive fluxes are employed to separate entrapped metal from oxides. Exothermic dressing fluxes, containing nitrate salts and silicofluoride double salts, aid in recovering zinc from dross by chemically reducing zinc oxide and increasing the fluidity of the melt. This reaction, combined with mechanical rabbling or raking, improves the efficiency of zinc recovery.

Flux Injection

Flux injection is a modern and efficient technique for introducing flux compounds into molten metal baths. Using an inert gas carrier, flux materials are submerged deep into the molten metal, ensuring uniform distribution and enhanced reactivity. This approach addresses the limitations of manual fluxing, which is typically restricted to surface or near-surface treatment.

By bubbling flux compounds throughout the molten bath, flux injection promotes better impurity removal and optimizes the refining process. Additionally, it reduces the labor intensity and safety risks associated with manual fluxing methods, while achieving superior results in dross phase treatment through localized temperature increases and reduced surface tension.

Advantages of Flux Injection

Flux injection provides several distinct advantages over traditional manual methods. By ensuring thorough mixing and uniform treatment of the molten bath, it achieves more effective impurity removal and refining. Furthermore, it enables efficient treatment of dross phases by increasing local temperatures, enhancing fluidity, and reducing oxide skin tension. This innovative technique streamlines the fluxing process, minimizes manual intervention, and improves overall operational efficiency.