Electrolytic manganese residue-based cement for manganese ore pit backfilling: Performance and mechanism

Slag backfilling with electrolytic manganese residue (EMR) is an economical and environmentally-friendly method. However, high ammonium-nitrogen and manganese ions in EMRs limit this practice. In this study, a method of highly efficient simultaneous stabilization/solidification of ultrafine EMR by making EMR-based cementitious material (named EMR-P) was proposed and tested via single-factor and response surface optimization experiments. Results show that the stabilization efficiency of NH4+ and Mn2+ were above 95%, and the unconfined compressive strength of the EMR-P was 18.85 MPa (megapascal = N/mm2). The mechanistic study concluded that the soluble manganese sulfate and ammonium sulfate in EMR were converted into the insoluble precipitates of manganite (MnOOH), gypsum (CaSO4), MnNH4PO4?H2O, and struvite (MgNH4PO4.6 H2O), leading to the stabilization of NH4+ and Mn2+ in the EMR-P. Leaching tests of EMR-P indicated that NH4+, Mn2+, and others heavy metals in the leachate were within the permitted level of the GB/T8978-1996. The novelty of this study includes the addition of phosphate and magnesium ions to precipitate ammonium-nitrogen and the combination between calcium ions (from CaHPO4.2 H2O) and sulfate (from the EMR) to form calcium sulfate to improve the stability and unconfined compressive strength of cementitious materials (EMR-P).