Removal of gaseous mercury by ZnS adsorbent prepared from zinc smelter byproducts

The zinc smelting industry is a major source of atmospheric mercury emissions in China. It is crucial to develop a low-cost technology to reduce mercury pollution emissions for the green development of zinc smelting. Thus, based on the concept of “using waste to treat waste”, this study uses high-sulfur slag and secondary zinc oxide, which are produced in the zinc smelting process as raw materials, to prepare zinc sulfide adsorbent by high-temperature sulfurization in an inert atmosphere. The zinc sulfide adsorbents prepared under different roasting temperatures and time were characterized and analyzed by various analysis methods, including X-ray diffraction (XRD), Raman spectroscopy (Raman), scanning electron microscopy (SEM) and specific surface area analysis (BET). The effects of various reaction times and flue gas components on the mercury removal performance were studied. The experimental results reveal that the zinc sulfide adsorbent can be successfully prepared when the roasting temperature and holding time are more than 250 ℃ and 60 min, respectively. Under this condition, many mesoporous holes are formed. The prepared ZnS–250 ℃–90 min adsorbent has the best performance for Hg⁰ adsorption. Moreover, gaseous elemental mercury (Hg⁰) has an average adsorption efficiency above 98% at flue gas temperatures below 150 ℃, and it exhibits high resistance to O₂, SO₂, and H₂O. When the Hg⁰ adsorption efficiency decreased to 50%, the Hg⁰ adsorption capacity of ZnS–250 ℃–90 min was 3.04 mg·g⁻¹ in the simulated zinc smelting flue gas atmosphere, which is superior to conventional metal sulfides for Hg⁰ adsorption. Furthermore, the Hg⁰ adsorption on the surface of ZnS adsorbent is a chemical adsorption process, and the unsaturated short-chain sulfur on the surface of ZnS adsorbent is the critical active site, which can react with Hg⁰ to form stable HgS, thus realizing the high-efficiency purification of Hg⁰ from smelting flue gas. Furthermore, the spent ZnS adsorbent can be regenerated using thermal desorption and polysulfide impregnation. An excellent cycle performance for Hg⁰ capture is realized, and the Hg⁰ adsorption efficiency remains above 99% after five cycles. Simultaneously, the desorbed mercury can be recycled as elemental mercury by condensation during the regeneration process of ZnS adsorbent. Thus, this study not only provides a low-cost method for mercury pollution control and resource recovery from the zinc smelting industry but also supports the cyclic utilization of typical waste produced in the zinc smelting industry, promoting the green production of the zinc smelting industry.