Grinding Media Beads and Nanoparticle Production in Bead Mills

1. Introduction

Grinding is a process in which chemical bonds within a crystal or amorphous structure are broken, creating new surfaces. In grinding facilities, only approximately 1% of the energy transmitted to grinding equipment (mills) is consumed in the creation of new surfaces, while 99% is consumed in the form of transmission losses, friction, heat, and sound energy. In ore preparation facilities, approximately 50% of the energy consumed is spent in grinding circuits. For this reason, grinding media costs are important in reducing process costs and improving grinding efficiency. On the other hand, increasing demand for super fine/nano particle sized powders in various industrial fields such as the minerals industry, paints, plastics, pharmaceuticals, and paper has led to an increasingly widespread application of very fine wet grinding methods. The equipment used in very fine grinding generally consists of stirred (horizontal/vertical) mills. Stirred mills offer significant advantages over other mills in achieving very fine and sub-micron/nano particle sizes. In such mills, various beads made from different materials are used depending on the material to be ground. These beads can generally be classified as follows: • Plastic, • Silica, • Alumina, • Zirconium silicate, • Zirconium oxide, • Steel, • Other. The general characteristics of beads used in grinding are given in Table 1. Accordingly, their bulk densities range from 0.63–8.2 kg/l, true densities from 1.05–15, and colors are generally white, yellow, gray, and silver. According to bead classes; plastic beads reach 1.2 g/cc, silica beads 2.6 g/cc, alumina beads 4.2 g/cc, zirconium oxide beads 6 g/cc, steel beads 7.6 g/cc, and tungsten carbide beads 15 g/cc true density.

Table 1. Bead type and characteristics(a) polyester (b) polyamide (c) polycarbonate (d) sand (e) soda-lime glass (f) molded soda-lime glass (g) standard alumina (h) high-purity alumina (i) zirconium silicate (j) magnesium-containing zirconium oxide (k) cerium-containing zirconium oxide (l) yttria zirconium oxide (m) stainless steel (n) tungsten carbide

Figure 1. Appearance of various beads

The detailed characteristics of some of the beads used as grinding media are provided below: Polyester: Non-abrasive, spherical in shape. Can be produced in small sizes down to 100 microns. Low density but hard and resistant to wear. Available in porous, hardened, and narrow size distribution forms. Polyurethane: High-density beads used for mixing and blending powders. Allows for very long wear periods. The smallest size is 4–5 mm. Sand: Still used due to its low cost but can be expensive in the long term. Due to its irregular shape, it abrades the mill because it is more needle-like compared to spheres and the tip ends are prone to breaking off. Alternatives are glass and mullite. Soda-lime glass: The most popular glass sphere used as grinding media. Some types are made from virgin glass while others are made from recycled glass. Air entrapment also determines bead durability, showing variability that can determine bead life. An excellent bead for low-viscosity materials or low-temperature heat treatments. Low-alkali / Borosilicate glass: Borosilicate glass beads come in various qualities. Used for low-alkali applications as well as food and pharmaceutical uses. Additionally, they have high crush strength, more wear-resistant than soda-lime glass. More expensive than soda-lime glass, but more valuable. Borosilicate glass is also available but is extremely expensive. Gravel: A resource becoming increasingly difficult to find in its natural state. Essentially has a density similar to quartz but is hardened with irregular shapes and surfaces. The advantage of gravel is its aspect ratio, thus providing more surface area for contact in the mill. Can be used but becomes expensive. Steatite: This grinding medium is a fused magnesium silicate composition consisting of 62% SiO₂. Cylindrical and spherical in shape with a minimum size of 6 mm. An excellent alternative to gravel or large glass beads. Although it has the same density, it lasts longer than glass. Mullite: This bead contains approximately 35% SiO₂. Has the advantage of higher density than glass and lower density than other aluminas. Beads can be produced in various alumina ratios and densities with or without bauxite. Lasts longer than glass beads and is not as abrasive as other alumina beads. Standard alumina: The most commonly used alumina grinding medium for reducing particle size in both powders and liquids. Some sizes are available in both dry-pressed and isostatically-pressed forms. Beads can be abrasive. High-density alumina: Higher density, so coating hardness should be considered before use. 93% alumina beads are spherical and excellent for use in both liquids and powders. High-purity alumina: Higher-purity alumina is used in grinding materials that cannot tolerate contamination other than alumina. Can be much more expensive and more brittle compared to other alumina formulations. Can be spherical and cylindrical in shape. Zirconium silicate: A popular medium-density, sintered bead. It has a hard outer shell and soft inner core. Sizes above 3 mm are impractical due to the natural structure that weakens the bead and causes it to crack/break. Can be abrasive in the mill. Zirconium silica: A popular medium-density bead that looks almost identical to zirconium silicate. This bead is fused and homogeneous from shell to core. Lasts longer than a sintered bead but should not be used in sizes above 2 mm due to internal air entrapment known as voids. Can be brittle and cause wear problems in the mill. High-density zirconium silica: A fused zirconia silica containing a higher amount of zirconia compared to the more common formulation. Has excellent performance and value. Magnesium-containing zirconium oxide: A very popular high-density and cylindrical-shaped ceramic bead for all types of grinding. Cerium-containing zirconium oxide: A high-density ceramic bead. Very popular due to durability and value pricing. Does not crack or break. Available in cylindrical and spherical shapes. Yttria zirconium oxide: This is the grinding medium with the highest density, longest life, and greatest durability. Very hard and non-porous, thus unbreakable. Produced in spherical and cylindrical shapes in a wide variety of sizes. Stainless steel: These beads can be expensive but are typically used when other types of beads are unacceptable. Tungsten carbide: Continues to attract interest due to its high density. Mills using this grinding medium must be manufactured to withstand high density. Spherical ones are quite expensive. 2. How to Select the Right Bead? i. Chemical compatibility: Will the grinding medium cause any chemical contamination problems? All grinding media can cause some degree of contamination of the product to be ground, even if the contamination amount is in the ppm or ppb range. Although steel grinding medium is attractive due to its relative density and low cost, it can particularly cause graying of a white or light-colored product. ii. Physical compatibility: The relative hardness and abrasiveness of the grinding medium should be considered in relation to mill and product properties. iii. Bead density: For grinding media available on the market, specific gravity values range from 1 for plastic beads to 15 for tungsten carbide. A very light bead should not be selected for a highly viscous system, nor should a high-density bead be selected for an aqueous suspension with low solids content. iv. Bead diameter: Generally, as the feed material particle size increases, the bead diameter required for grinding also increases. Additionally, for finer grinding, the bead must be smaller. Other size considerations include whether screening is used to separate the grinding medium from the product, and as a rule, the smallest bead diameter should be three times the screen opening. v. Budget: There are many considerations here: a. Initial cost: Generally, beads with the best wear properties are more expensive than other options. Additionally, the smaller the grinding medium, the more expensive it is. b. Bead life: Beads must be replaced as they wear. The longer a grinding medium lasts, the more economical it can be in the long term due to reduced downtime, new bead costs, disposal, and labor. c. Mill wear: Inexpensive, hard beads can lead to costly equipment repairs. d. Product quality: Grinding media with less waste (small sizes or defective pieces) have longer service life than cheaper materials that may contain such defects. For stirred mills performing high-density grinding, the ideal grinding medium type should have some consistent and repeatable characteristics as shown below: • Size distribution and largest bead size, • Chemical composition, • Hardness (related to chemical composition and bead size), • Density, • High sphericity, • High roundness, • Integrity (mechanical soundness).

3. Nanoparticle Production

Nano-sized particles play an important role in nanotechnology applications due to their unique functional properties. Nanoparticle technology serves as a bridge between new nanotechnologies and traditional powder production technologies. The use of sub-micron (nano) particles to achieve higher product homogeneity, strength, or solubility particularly provides higher product quality. Stirred media mills (Figure 2) are generally used for grinding and dispersion of nanostructured particles to obtain stable suspensions of dimensions finer than 100 nm in pigments, fine coatings, inks, cosmetics, biotechnology, and similar applications. Fine particle production in stirred media mills is possible by using very small grinding beads. The product size distribution obtained is a function of bead size. As a general rule, the average product particle size (D50) obtained corresponds to 1/1000th of the bead used. For this reason, to theoretically obtain a suspension with a D50 of 100 nm, a 100 μm bead would be required. Grinding operations have been conducted with these beads for approximately ten years, but in practice, two fundamental problems are encountered when grinding materials to nano dimensions: i. The first problem relates to dispersion chemistry. As mill particles rapidly become finer and particle size decreases, Van der Waals forces increase, which leads to agglomeration or flocculation. Flocculation increases the apparent viscosity of the suspension. Flocculation is generally an undesired condition in suspensions. Flocculated particles, for example in paints, generally do not exhibit the desired level of color fastness, transparency, and gloss. ii. The viscosity created by flocculation also causes a problem in separating grinding beads from the medium. This is the second major problem. As higher viscosity is observed inside the mill, a more viscous pulp is created. When the pulp is pumped out of the mill, the grinding medium beads become part of the pulp, increasing the solids content of the suspension. There are two ways to prevent the above problems: i. To prevent flocculation of fine particles, chemical substances (grinding aids) that will stabilize the suspension should be used. Different surfactants are recommended for nano suspensions. ii. Using beads smaller than 300 μm increases efficiency for micron/nano suspensions. In terms of the steepness of particle size distribution, the use of small beads in wet grinding is the most effective process parameter in bead grinding, and narrower particle size distribution can be obtained. The use of smaller beads (50–300 μm) is particularly effective for ore preparation processes under wet grinding conditions. In practice, bead size can be reduced to 20 μm. Table 3 specifies bead sizes and weight ratios. Smaller beads mean more beads per unit mill volume. For example, in a 1-liter grinding vessel, approximately 1.1 million 1 mm beads fit, while approximately 9.4 billion 0.05 mm (50 μm) beads are used. In this way, the probability of contact between particles and beads increases significantly. Again, as bead size decreases, the space between beads also decreases, so the beads act like a filter material that holds large agglomerates. This spacing between beads is approximately 44 μm for 1 mm beads, while for 0.05 mm beads it is approximately 2 μm. The stirring speed in mills is also quite important. At high stirring speeds (13 m/s), agglomerates cannot escape the resulting impact and stress, enabling more efficient grinding and dispersion. Table 4 shows different applications of grinding medium beads tested in newly developed stirred media mills.

4. Conclusion

The use of sub-micron (nano) particles to achieve higher product homogeneity, strength, or solubility provides higher product quality. Stirred media mills are generally used for grinding and dispersion of nanostructured particles to obtain stable suspensions finer than 100 nm in pigments, fine coatings, inks, cosmetics, biotechnology, and similar applications. In these mills, bead size can range from 3–4 mm down to 20 μm in practice. Prof. Dr. Öner Yusuf Toraman / Industrial Raw Materials and Building Materials Application and Research Center / Ömer Halisdemir University Harun Köse / Research and Development Center Director / Niğtaş Ltd. Assoc. Prof. Dr. Orkun Ersoy / Industrial Raw Materials and Building Materials Application and Research Center / Ömer Halisdemir University Bedrana Bitirmiş / Research and Development Center Deputy Director / Niğtaş Ltd. References 1. Clermont B., Philippe, C., 2012, Do's and Don'ts of measuring ceramic beads wear in the lab and in the industry. http://www. magotteaux.com/wp-content/ uploads/Do%E2%80%99s-and-Don%E2%80%99tsof- measuring-ceramic-beads-wear-in-the- lab-and-inthe- industry.pdf 2. Collins, D., 2011, Nano milling with micro beads, http://connect.innovateuk.org/web/nanotechnologyand- coatings-industry- 2011/speakers 3. Lichter, J, Davey, G., 2002, Selection and sizing of ultrafine and stirred grinding mills, SME Mineral Processing Plant Design Symposium, Vancouver, Canada. 4. Mende, S., Schwedes, J., Mechanical production and stabilization of nanoparticles by wet comminution in stirred media mills, Powder Handling & Processing, Vol.18, No. 6, 2006, S. 366-373. 5. Mende, S., Grinding and dispersion in the field of nanotechnology, Ceramic Forum International, Ber. DKG 82, 2005, No.9,S. E39-E43. 6. Mende, S., Kolb, G., Enderle, U., Nanoparticle grinding and dispersing, http://www.ceramicindustry. com, August 2006. 7. Shemanski, B., Miranda, S., Nano sizing via wet or dry grinding, http://powderbulksolids.com 8. Stenger, F., Mende, S., Schwedes, J., Peukert, W., 2005, The influence of suspension properties on the grinding behaviour of alumina particles in the submicron size range in stirred media mills, Powder Technology, 156, s.103-110. 9. Toraman, Ö.Y., 2012, Karıştırmalı değirmenlerde çok ince (100 μm) öğütücü bilyalar kullanılarak nano partikül üretimi, TurkChem, September/October 2012, Year:5 Issue:30, p.130-134. 10. http://www.norstoneinc.com/our-products/beads/ grinding-media-depot/grinding-media-definitions/ 11.http://www.glenmills. com/grinding-media/