Research on Optimization of Regrinding and Reflotation Technology for Large Flake Graphite Ore

Abstract

Large flake graphite, with its unique layered structure and exceptional physicochemical properties, is widely used in high-tech fields such as aerospace, nuclear reactors, and new energy materials. However, China’s reserves of large flake graphite are limited, and balancing the improvement of fixed carbon content with the protection of large flake yield remains a technical challenge. This study focuses on a large flake graphite ore in Gansu Province, optimizing the regrinding and reflotation process through mineralogical analysis, separation efficiency modeling, stirred mill comparisons, and grinding kinetics. Results show that the optimized process combining high-pressure grinding rolls (HPGR) and impeller-stirred mills achieves a concentrate fixed carbon content of 96.66%, with a +0.355 mm yield 8.49% higher than conventional methods, effectively preserving large flakes.

Keywords: Large flake graphite; Regrinding and reflotation; Separation efficiency; Grinding kinetics; Particle breakage

Introduction

Graphite is a critical strategic resource, and large flake graphite is particularly valued for its conductivity, lubricity, and thermal stability. Despite China’s abundant graphite reserves, large flake graphite accounts for only 15.76% of global resources. Traditional beneficiation processes face challenges such as lengthy flowsheets, multiple regrinding stages, and structural damage to large flakes. This study addresses these issues by optimizing regrinding technology for a Gansu graphite ore, aiming to enhance both grade and flake integrity.

Ore Characteristics and Challenges

1. Mineralogical Analysis

The ore contains 3.24% fixed carbon, with quartz (55-60%), plagioclase (25-30%), and limonite (3-5%) as main gangue minerals. Graphite flakes are predominantly +180 μm (80.44%) in size and 20-50 μm (81.29%) in thickness, intergrown with muscovite and limonite.

2. Key Conflicts in Beneficiation

Conventional flotation requires multiple regrinding stages to liberate gangue, but mechanical forces inevitably damage large flakes. For instance, extending regrinding by 10 minutes reduces +0.150 mm yield by 5-10%. Thus, balancing liberation efficiency and flake protection is critical.

Separation Efficiency Model

To quantify the trade-off between fixed carbon improvement and flake loss, a separation efficiency on sieve (SI+x) model is established based on the Douglas formula:

SI+x=Qy×Qc=(γ1β1γ2β2−γ1γ2)×(100−β1β2−β1)

Here,  Qy (quantity efficiency) and Qc (quality efficiency) evaluate flake recovery and carbon enrichment, respectively. This model guides equipment selection and parameter optimization.

Optimization of Regrinding Equipment

1. HPGR vs. Ball Mill (BM)

·         HPGR Advantages: Interparticle crushing exposes gangue at flake surfaces, achieving 70% fixed carbon distribution in +0.355 mm fractions.

·         BM Limitations: Impact grinding results in tight intergrowth, requiring higher energy for liberation.

2. Stirred Mill Comparisons

·         HPGR Fine High-Carbon Product: Impeller-stirred mills yield the highestSI+0.150 of 61.76%.

·         BM Fine High-Carbon Product: Disc-stirred mills perform better, with SI+0.150 at 39.72%.

·         Coarse Low-Carbon Product: Two-stage impeller (HPGR) or squirrel cage (BM) regrinding elevates fixed carbon above 95%.

3. Industrial Validation

At a graphite plant in Jixi, Heilongjiang, optimized squirrel cage mills improved SI+0.150 by 19.31% in the first regrinding stage, matching the performance of the sixth regrinding stage in the original process.

Grinding Kinetics and Liberation Mechanisms

1. Grinding Rate Analysis

·         HPGR Products: Initial regrinding (0-10 min) focuses on gangue liberation with minimal flake damage; prolonged grinding (>15 min) requires strict fineness control.

·         BM Products: Fluctuating grinding rates necessitate 25-minute regrinding for effective liberation.

2. SEM Observations

·         HPGR Products: Gangue concentrates at flake edges, easily removed by shear forces (Fig. 1a).

·         BM Products: Gangue embeds between flake layers, demanding higher energy for liberation (Fig. 1b).

Process Optimization and Benefits

Adopting particle breakage-classification instead of conventional ball milling achieved:

1.    Concentrate Quality: 96.66% fixed carbon, 97.56% recovery, and 52.85% +0.355 mm yield, surpassing conventional methods by 8.49%.

2.    Cost Efficiency: One fewer regrinding stage reduces energy consumption by 25% and media wear by 50%.

3.    Sustainability: Closed-circuit flowsheets minimize tailings discharge, aligning with green mining standards.

Conclusions and Prospects

1.    Conclusions

o    The SI+x model effectively balances carbon enrichment and flake protection.

o    HPGR combined with impeller mills delivers optimal concentrate quality.

o    Grinding kinetics reveal liberation patterns, guiding process design.

2.    Future Directions

o    Integrate nanobubble flotation and ultrasonic-assisted grinding to reduce regrinding stages.

o    Extend classification processes to microcrystalline graphite ores for holistic resource utilization.