Preparation of Nano Hematite Mineral Pigment Water Dispersions Using High-Energy Ball Milling

Abstract:This study investigates the preparation of nano-sized hematite pigment water dispersions through high-energy ball milling, using hematite as the raw material and a copolymer of maleic anhydride and vinyl acetate (PMV) as a dispersant. The research explores the impact of milling time, milling media, and dispersant dosage on particle size and Zeta potential of the dispersions. Key findings include that optimal conditions involve a ball mill rotation speed of 500 rpm, a milling time of 5 hours, a dispersant dosage of 0.25 g per gram of pigment, and a mass ratio of 2:3 for zirconia beads with diameters of 2 mm and 0.5 mm. Under these conditions, we achieve hematite pigment water dispersions with an average particle size of 230 nm and satisfactory particle size dispersion stability.

Introduction:Mineral pigments have a rich history of use due to their diverse colors resulting from variations in mineral composition. This study focuses on hematite, a common mineral, as the source material. Hematite's potential applications are enhanced when reduced to nano-sized particles. The dispersion properties of these nanoparticles play a crucial role in their performance. To achieve optimal dispersion, we explore the effects of milling parameters.

Experimental Section:

1. Materials and Equipment:

• PMV copolymer (self-made)

• Hematite (industrial grade)

• Pulverisette 7 planetary ball mill

• LS13320 laser particle size analyzer

• DHG electric constant temperature blast drying oven

• NEXUS-670 Fourier-transform infrared and Raman spectrometer

• 501 constant temperature bath

• JS94J microelectrophoresis instrument (Zeta potential instrument)

High-Energy Ball Milling of Mineral Pigments:The milling process involves mixing 0.5 g of hematite pigment with a specified amount of dispersant, zirconia beads, and 10 mL of distilled water in a milling tank. Milling proceeds at 500 rpm, and the dispersions' particle size, distribution, and stability are measured post-milling.

Particle Size and Dispersion Stability Testing:We analyze the particle size and distribution of milled hematite pigment dispersions using a laser particle size analyzer. Additionally, we take 2.0 g of dry sample, place it in a 20 mL graduated test tube, add water to reach 20 mL, and sonicate it for 15 minutes. Particle size is periodically measured to assess dispersion stability.

Zeta Potential Measurement:Zeta potential measurements help us understand particle-particle interactions. The Zeta potential is measured using a microelectrophoresis instrument (Zeta potential instrument) under specific conditions.

Results and Discussion: 

2.1 Impact of Dispersant Dosage on Average Particle Size:At a milling time of 5 hours and a mass ratio of 2:3 for zirconia beads with diameters of 2 mm and 0.5 mm, the effect of dispersant dosage on hematite pigment dispersion's average particle size is explored. Results show that as dispersant dosage increases, the average particle size decreases. At a dispersant dosage of 0.25 g per gram of pigment, the average particle size is reduced to under 300 nm. Beyond this point, further increases in dispersant dosage have minimal impact.

2.2 Influence of Milling Time on Average Particle Size:With a dispersant dosage of 0.25 g per gram of pigment and a mass ratio of 2:3 for zirconia beads with diameters of 2 mm and 0.5 mm, the effect of different milling times on the average particle size of hematite pigment dispersions is examined. Results indicate that as milling time increases, the average particle size decreases, stabilizing at around 230 nm after 5 hours.

2.3 Impact of Milling Media on Average Particle Size:The effect of different sizes and quantities of zirconia beads on the particle size of milled hematite pigment dispersions is studied. A combination of larger-diameter zirconia beads with a lower quantity and smaller-diameter zirconia beads with a higher quantity yields the best milling results.

2.4 Dispersant Stability:Dispersant quantities are tested for stability. Over time, dispersions with higher dispersant quantities exhibit smaller particle sizes and higher stability.

2.5 Influence of Dispersant on Average Particle Size:The type of dispersant also affects average particle size. Commercial dispersant #1 and PMV produce smaller average particle sizes, while a polyacrylamide copolymer results in larger particle sizes due to its high hydrophilicity.

Conclusion:This study highlights the successful preparation of nano-sized hematite pigment water dispersions through high-energy ball milling. Optimal conditions involve specific parameters: a ball mill rotation speed of 500 rpm, a milling time of 5 hours, a dispersant dosage of 0.25 g per gram of pigment, and a mass ratio of 2:3 for zirconia beads with diameters of 2 mm and 0.5 mm. These conditions produce hematite pigment water dispersions with an average particle size of 230 nm and adequate particle size dispersion stability.