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

Abstract

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 NH<inf>4</inf>⁺ 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 (CaSO<inf>4</inf>), MnNH<inf>4</inf>PO<inf>4</inf>·H<inf>2</inf>O, and struvite (MgNH<inf>4</inf>PO<inf>4</inf>∙6 H<inf>2</inf>O), leading to the stabilization of NH<inf>4</inf>⁺ and Mn²⁺ in the EMR-P. Leaching tests of EMR-P indicated that NH<inf>4</inf>⁺, 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 CaHPO<inf>4</inf>∙2 H<inf>2</inf>O) and sulfate (from the EMR) to form calcium sulfate to improve the stability and unconfined compressive strength of cementitious materials (EMR-P).