"火法化验" can be translated as "pyrometallurgical assay" or "fire assay" in English.
"Pyrometallurgical assay" emphasizes the use of high-temperature processes (pyrometallurgy) in the analytical method.
"Fire assay" is a more colloquial and widely recognized term in the mining and metallurgical industries for the specific process of assaying precious metals by fire.
Both are acceptable, but "fire assay" is more commonly used in practical contexts.