Electrolytic Manganese Metal Technology from Low-Grade Oxidized Manganese Ore

Introduction

Manganese is a critical raw material for the steel industry, battery manufacturing, and chemical sectors. With the gradual depletion of high-grade manganese resources, the efficient utilization of low-grade ores has become a priority. American Manganese Inc. (AMI) developed an innovative electrolytic manganese metal (EMM) production process for low-grade oxidized manganese ore (average Mn content: 4%) from the Artillery Peak deposit in Arizona, USA. This process, centered on sulfur dioxide (SO₂) reduction leaching, integrates energy-saving and eco-friendly designs, offering a technical model for industrializing low-grade manganese ores. This article systematically analyzes the technical scheme, covering ore pretreatment, leaching, purification, electrolysis, and byproduct recovery.

I. Ore Pretreatment and Crushing

1. Ore Characteristics
   The Artillery Peak ore primarily consists of pyrolusite, psilomelane, and wad (a mixture of manganese oxides/hydroxides). The soft and friable nature of the ore eliminates the need for intensive grinding.

2. Crushing Process
   Run-of-mine ore is screened via a grizzly, with oversized lumps crushed by a bulldozer. Subsequent jaw crushers and trommel screens reduce the ore to below 30 mm, minimizing capital and energy costs for low-grade ore processing.

II. Leaching Process

1. Sulfuric Acid Pre-Leaching
   Crushed ore is mixed with manganese-bearing wash solution (12%~20% pulp density) and sulfuric acid to remove acid-consuming impurities (e.g., calcium), forming insoluble sulfates.

2. SO₂ Reduction Leaching

• SO₂ Generation: Liquid sulfur is atomized and combusted to produce gas containing 17.5% SO₂. Waste heat recovery generates 7.2 MW of electricity, achieving partial energy self-sufficiency.

• Counter-Current Leaching: Ore pulp reacts with SO₂ in multi-stage tanks, reducing Mn⁴⁺ (MnO₂) to soluble Mn²⁺ (MnSO₄).

• Optimized Conditions: At 10% pulp density, 6% SO₂ concentration, and 0.1 M H₂SO₄, manganese recovery reaches 98%, with dithionate (MnS₂O₆) byproduct limited to 12.7 g/L.

III. Solid-Liquid Separation and Purification

1. Solid-Liquid Separation
   Leachate is thickened, with overflow (50 g/L Mn) sent to purification and underflow to a counter-current decantation (CCD) circuit. Non-ionic flocculant Percol 351 accelerates settling to 73 cm/min, achieving 99.5% Mn recovery.

2. Impurity Removal

• pH Adjustment & Oxidation: Raising pH to 5.5 and aerating oxidizes Fe²⁺ to Fe³⁺, precipitating Fe, Al, and As hydroxides (<1 ppm).

• Sulfide Precipitation: Neutral pH conditions facilitate Zn and Co removal via sulfide precipitation, avoiding sulfur contamination in final products.

IV. Dithionate Removal and Acid Balance Control

1. Manganese Carbonate Precipitation
   Sodium carbonate (Na₂CO₃) is added to convert MnSO₄ and MnS₂O₆ into MnCO₃ precipitate, separating manganese from dithionate:

   $${\text{MnSO}}_4+{\text{Na}}_2{\text{CO}}_3\to {\text{MnCO}}_3+{\text{Na}}_2{\text{SO}}_4$$$${\text{MnS}}_2{\text{O}}_6+{\text{Na}}_2{\text{CO}}_3\to {\text{MnCO}}_3+{\text{Na}}_2{\text{S}}_2{\text{O}}_6$$

2. Acid Dissolution for Electrolyte
   MnCO₃ is dissolved in sulfuric acid-rich anolyte to regenerate MnSO₄ electrolyte:

   $${\text{MnCO}}_3+{\text{H}}_2{\text{SO}}_4\to {\text{MnSO}}_4+{\text{CO}}_2\uparrow$$

   This step simultaneously addresses dithionate removal and acid balance.

V. Electrolytic Manganese Metal Production

1. Electrolysis Parameters
   Purified electrolyte is electrolyzed using stainless steel cathodes and lead-alloy anodes at 400–500 A/m², 40–50°C, and pH 7–8.

2. Product Specifications
   EMM purity exceeds 99.5%, with 67% current efficiency. Consumption rates: 0.64 t sulfur and 1.93 t Na₂CO₃ per ton of EMM.

VI. Byproduct Recovery and Resource Circulation

1. Anhydrous Sodium Sulfate Production

• Freeze Crystallization: Na₂SO₄ and Na₂S₂O₆ crystallize at 0°C.

• Thermal Decomposition: Heating to 267°C decomposes Na₂S₂O₆ into Na₂SO₄ and SO₂ (recycled to leaching).

• Nanofiltration: 64.7% of water is reclaimed via nanofiltration, achieving zero liquid discharge.

2. Economic Benefits
   The process yields 12.9 t/a anhydrous Na₂SO₄ alongside 50,000 t/a EMM, with a production cost of $1,278/t, demonstrating strong market competitiveness.

VII. Innovations and Environmental Advantages

1. Energy Efficiency
   Waste heat recovery covers 60% of power demand; the freeze-nanofiltration system reduces energy use by 94% compared to evaporation.

2. Eco-Friendliness

• Tailings pass EPA toxicity tests for safe backfilling.

• Closed-loop water system eliminates wastewater discharge.

3. Technical Prospects
   Patented in the U.S., this process is undergoing pilot-scale validation and may set a benchmark for low-grade manganese ore utilization.

Conclusion

AMI’s electrolytic manganese process achieves high efficiency in low-grade ore processing through SO₂ reduction leaching, MnCO₃ precipitation, and resource circulation. Its energy-saving and eco-friendly features, coupled with low costs, provide a sustainable pathway for the global manganese industry. With pilot plant optimization, this technology is poised to advance manganese mining and smelting into a greener, smarter era.