Smelter Slags: Chemical Characterization and Leaching Behavior of Slags Piled on an Old Smelter in Chihuahua City, Mexico

Smelter slags are a waste product from the pyrometallurgical processing, typically stored in piles or stocks around or near the smelter site. Research on slags has focused on potential environmental issues related to the weathering of slag dumps or on its utility as a construction material (as road ballast, road base or sub-base material), or reprocessing for secondary metal recovery. Chihuahua city, located in northern Mexico, has a rich mining history going back to the 18^th century. A lead smelter operated in Avalos (SE of Chihuahua city) has left behind considerable slag piles after its closure in 1998. In this study, slag was chemically analyzed and used in "tank tests" experiments (EA-NEN-7375-2004 tank test standard from Netherlands), to assess its leaching behavior. This test was selected because it allows the use of coarse material. The slags contain Zn (15-35 wt.%), Pb (0.5 - 4 wt.%), As (0.6 wt.%), Sn (888 ppb) and Hg (170 ppb). Several minerals were identified by XRD, such as hardystonite (Ca₂ZnSi₂O₇), melanotekite (Pb₂Fe³ ₃O₂Si₂O₇), kentrolite (Pb₂Mn₂Si₂O₉) and sphalerite (ZnS) or trapped in the glass. Major elements are present in phases such as monticellite (CaMgSiO₄), kirschsteinite (CaFe² SiO₄), hedenbergite (CaFeSi₂O₆), babingtonite (Fe₂Si₃O₉), magnetite (Fe₃O₄), and calcite (CaCO₃). Leaching experiments were performed for 6, 24, 168 and 360 hours in mixtures of 30 and 50% of slags with natural gravel material using distilled water at a pH 5 and 8 to recreate acidic and alkaline waters. Amounts of leached Pb range from 0.1 to 0.5 ppm, Zn from 0.1 to 0.6 ppm, As from 0 to 0.09 ppm, and Ca from 40 to 180 ppm, being the acidic experiments the ones that leached out the highest amounts of metals. Based on the results of analyses and experiments, to redistribute the pile material, it is recommended to use the waste material (slags) as aggregates in road construction, but only as minor constituent (<30%) in base materials. Research into characterizing slag and its environmental and health effects (to vulnerable exposed populations) should remain to be a relevant topic, whether the goal is its reuse, recycling, or remediation.