In extractive metallurgy, what is 'speisses' best described as? Choose the description that correctly identifies its typical chemical nature during smelting operations.

Introduction:
During smelting and matte-forming operations, various immiscible phases can appear: slag, matte, speiss, and metal. Understanding what each phase contains is essential for predicting element distribution, especially for semimetals such as arsenic and antimony that do not behave like typical chalcophiles or siderophiles under furnace conditions.

Given Data / Assumptions:

• We focus on the definition of speiss in metallurgical practice.
• Smelting involves complex multicomponent melts with temperature-dependent equilibria.
• Heavy metals (e.g., Ni, Co, Fe, Cu) can form intermetallic-like compounds with semimetals.

Concept / Approach:
Speiss is classically the phase consisting predominantly of arsenides and antimonides of heavy metals. It forms under certain reducing conditions where arsenic and antimony preferentially combine with metals rather than entering sulfide matte or oxide slags. The phase is dense and can separate from both slag and matte, complicating metal recovery and impurity control.

Step-by-Step Solution:
Identify the elements involved: arsenic and antimony (semimetals) plus heavy metals.Recognize that 'speiss' denotes arsenide/antimonide phases, not oxides or sulfides.Select the option that includes both arsenides and antimonides.

Verification / Alternative check:
Classic metallurgy texts and smelter operations manuals describe speiss formation in arsenic/antimony-bearing feeds, noting its distinct behavior and the need for appropriate disposal or reprocessing.

Why Other Options Are Wrong:

• Arsenides only or antimonides only: Incomplete; speiss commonly contains both.
• Iron, cobalt and nickel metal alloy: Describes a metallic phase, not speiss.
• Silicate slag: That is the oxide phase, distinct from speiss.

Common Pitfalls:
Confusing matte (sulfide-rich) with speiss; they differ in chemistry and density, affecting separation and downstream handling.

Final Answer:
Arsenides & antimonides of heavy metals