Abstract
Most of the cost and embodied energy of precursors for the early transition metals, aluminium, silicon and carbon arises in refining to a specified grade rather than in extraction. Industrial and agricultural residues contain these elements, yet their use is usually assessed as a recovery problem rather than a specification problem. This review develops a specification and cost-based framework for determining when waste-derived precursors can substitute for primary feedstocks. Purification ladders linking form, attainable purity and price are compiled for each element; for titanium, indicative price rises more than tenfold from ilmenite to sponge metal and again to classified powder. Impurity budgets for the principal end-uses are used to evaluate recovery routes from metallurgical slags, bauxite residue, spent catalysts, aluminium dross, silica fume, rice-husk ash and carbonaceous residues. At equal specification, provenance alone does not determine performance or cost; secondary feedstocks are competitive when the residue has already undergone part of the primary processing, when the end-use accepts feed from a lower rung of the ladder, or when avoided disposal costs are material. These conditions are met industrially for vanadium from steelmaking slag and for secondary aluminium, and in principle for carbides made carbothermically from oxide feed. Layered ternary carbides and nitrides (MAX phases) are examined as a stringent specification case: their impurity tolerance is site-specific, and carbide or oxycarbide intermediates are more compatible with residue-derived titanium than titanium dioxide. For several end-uses, adoption appears limited less by attainable purity than by the absence of certified, batch-consistent specifications