Abstract: Metallic arsenic is a critical raw material in the semiconductor industry. Arsenic-alkali slag from antimony smelting contains a high concentration of arsenic and has a complex composition. The key to preparing metallic arsenic lies in the efficient separation of arsenic from various impurities. In this study, based on the geochemical mineralization principles of arsenic, we propose an innovative theory of high-precision mineralization and precipitation of arsenate complex salts. We developed a key technology involving one-step reduction roasting of arsenate complex salt precursors for metallic arsenic production. This approach overcomes the challenges of efficiently separating arsenic and alkali in high alkali/salt solutions and similar issues with impurity separation, enabling a shortened process for converting arsenic-containing solid waste into high-purity metallic arsenic. Our findings show that oxidative leaching using hydrogen peroxide enables effective and selective removal of arsenic from arsenic-alkali residue. The liquid-solid ratio, temperature, and hydrogen peroxide dosage significantly influence the leaching rate. Under optimal reaction conditions, the leachate contains high concentration of alkali, arsenic, and sulfur. Carbonation of the leachate allows for alkali recovery, yielding a product with an alkali content of up to 98.79% and a uniform particle size distribution. Arsenate salt mineralization and precipitation achieve selective separation of arsenate from bicarbonate and alkali. With increased dosage of ammonium salts and magnesium sources, the arsenic removal rate improves. The arsenic content in slag increases with magnesium salt dosage and then decreases. Reaction time positively influences arsenic removal, while higher temperatures reduce both the arsenic removal rate and the arsenic grade in slag, the latter reaching 29.75%. Through reduction roasting of high-arsenic slag using carbon powder, metallic arsenic with 99.81% purity was obtained. The monomeric arsenic contained only 0.03% antimony and 0.16% sulfur impurities. Analysis of the reduction roasting and condensation processes, considering the temperature and Gibbs free energy, indicates that to maintain the quality of the metallic arsenic monomers, sulfate reduction should occur at 620 °C. Increasing the carbon powder dosage or elevating the roasting temperature promotes reduction volatilization and lowers sulfur impurity content in the final product. This study provides both a theoretical basis for the resource-efficient disposal of arsenic-containing solid waste and technical support for the efficient preparation of metallic arsenic.