May 1, 2008
The advantages of using the highest specification grinding media in milling operations are well documented and understood. Higher density provides the potential to increase productivity, often significantly over other traditional ceramic and glass materials. Improved toughness and durability result in extended bead lifetimes and machine maintenance periods.
Yttria-Stabilized ZirconiaGenerally accepted as the best option is yttria-stabilized zirconia. This material is an engineering-grade ceramic, with controlled stabilization of the zirconia phase and fine crystal grain size. The resulting beads have increased strength, fracture toughness and wear resistance.
The material tends to be favoured over other high-density forms; particularly due to its compatibility with all machine types, reduced abrasion of mill parts and above all, its consistency of performance. The material has found particular application in demanding and high-specification operations, where precision, control and contamination are of primary importance. It has not, however, been generally adopted as an industrial standard. The main reason for this has been the high purchase price and the subsequent prohibitive cost of filling production machines.
The cost of the media is a function of the complicated chemical and powder processing technology required to produce a high-quality product. To give an idea of the requirements, a simplified production flow sheet is detailed in Figure 1.
Various attempts have been made to limit the level of processing in order to restrict overall media cost. Reduction in the degree of micronization and lower sintering temperatures have been the main target. But these have generally resulted in the production of inferior materials. An alternative has been the substitution of the stabilizing ion (yttria) with other oxides, such as magnesia and ceria. Some of the produced materials have had merit and have resulted in beads, which have found niche applications. In general, however, these beads have never matched the high performance of a top-quality yttria zirconia media.
Zirmil.2The challenge to develop a cost-effective, high-performance, yttria-stabilized bead has been the focus of a two-year project at the central laboratories of Saint-Gobain ZirPro. The engineers at ZirPro had an ideal basis for the investigation. The company is one of the world’s largest suppliers of zirconium oxide and was a pioneer in introducing ceramic micro-grinding media into the market place some 30 years ago.
The result of the project is the newly launched product, Zirmil.2. With an advanced ceramic formulation, this engineered media maintains the highest standards for this type of material. The bead has superior density, sphericity and size specification, with excellent wear and lifetime characteristics. The media is detailed in Figure 2.
Extensive testing has been carried out on the new material to ensure that the high-performance expectations are achieved. One series of tests, which particularly compares the performance of the media, is the ‘exaggerated wear test’. Here a small horizontal mill is operated at high speed with a standard 85% fill of media, using only water. This type of operation is very aggressive and results in accelerated media wear. Comparative data was collected for similar sized beads; some of the results are detailed in Figure 3.
It can be seen from Figure 3 that Zirmil.2 far outperforms ceria-based media and has comparable performance to the established premier grade of yttria zirconia beads. The laboratory results have been confirmed in industrial trials with the media providing excellent productivity and grind quality at very low levels of media consumption.
Operating Cost ReductionThe performance of Zirmil.2 is established and satisfactory. The significant development with the new material is that it is available at pricing levels, which should dramatically boost performance in terms of overall economics and allow the general use of the media in the majority of standard milling applications. Figure 4 details the typical media cost evolution for a standard 45-litre horizontal mill application.
It can be seen that initially the cost of charging the mill with Zirmil.2 is much higher than with traditional Zircon beads, but only marginally more expensive than ceria-based media. This is actually a dramatic step; previously the cost of yttria zirconia material would have been expected to be two or three times more than ceria products. From the graph it can be clearly seen that after a relatively short period of time, Zirmil.2 provides a continued operating cost advantage. The superior wear resistance of the material reduces the need for top-up and extends the periods between charge replacements. Actual savings will of course depend on individual applications and conditions; however Zirmil.2 certainly offers the potential for significant operating cost reductions. Importantly, the product provides a major development by reducing the cost barrier associated with introducing this type of media into larger mills.
ConclusionNow there is a real potential for high-performance milling to be generally used in industrial micro-milling applications, without the usual necessity for considerable capital investment in new equipment. Standard large-volume mills will accommodate the new media, with subsequent potential to improve productivity and grind quality from the existing mill park. The material is readily available at cost effective pricing levels, which are acceptable for initial fill and provide long-term solutions for efficient milling operations.
For more information on this new product or to review the suitability and cost effectiveness for particular applications, contact Saint-Gobain ZirPro at + 33 4 90 39 99 14, or visit www.zirpro.com.