Grinding Media and their Manufacture 2.1
Tested Grinding Media – SiLibeads
Three different types of grinding media and two different bead sizes for each type were used in the tests. The grinding media were classified according to bead material as follows: – Yttria stabilized zirconia (Type ZY-Premium, production method sintering process); – Yttria stabilized zirconia (Type ZY Standard like Type ZYPremium, but with different raw materials and production method sintering process), and – Zirconium silicate (Type ZS, sintering process production method).
The bead types used and their properties are detailed in Tab. 1. The specific weight of the yttria stabilized zirconia beads varies from 6.00–6.06 kg L–1 due to the different raw materials used. The specific weight of Type ZS 4.1 kg L–1 is lower than the yttria stabilized zirconia beads. All of these beads were manufactured by a sintering process.
For a better understanding of grinding media apart from glass and ceramic materials, a general overview of possible production methods and their influence on the bead quality is given in the next section.
2.2 Possible Production Methods of Ceramic Beads and Glass Beads
In general, with regard to ceramic grinding media production methods, two manufacturing processes can be distinguished. One method is the sintering process where the shape of the beads is formed in a previous step, e.g., by a sol-gel-process, granulation process or pressing process. Afterwards, densification of the beads is brought about by high temperatures in a furnace, i.e., a sintering process, and their final mechanical properties are determined. The necessary temperature range and temperature schedule of the sintering process depends on the raw material and material formulation and can vary, e.g., from 1350–1700 °C. During this heating treatment, the densification of the beads takes place by material transport or diffusion at the grain boundaries linked with shrinkage of the Table 1. List of SiLibeads grinding media used in the milling tests and their properties.
The second possible production method of grinding media is the fused process. All necessary raw materials are smelted and homogenized in a kiln at high temperatures of more than 2000 °C. The hot material smelt leaves the kiln and the bead formation process from the smelt takes place whereby droplets are generated and fly from a certain height to the bottom of the kiln. Due to the surface tension of the hot liquid droplets, the shape changes to form round spheres. During the flight through the atmosphere, the spheres cool down and become stable and solidify before reaching the bottom. Beads formed by the fused process have the following disadvantages:
– During production, it is unavoidable that air bubbles are included in the beads, and therefore, the specific weight is reduced. Such air bubbles form pores in the inner structure of the bead, as can be seen in Fig. 1, and cause bead breakage during the milling process. Bead breakage contaminates the milling product and can clog the outlet of the mill, which can bring production to a standstill.
– During the formation process, both large and small beads collide with each other and stick together, resulting in the formation of and so-called satellites, as shown in Fig. 2. During the later application of the milling process, these satellites are separated from the larger beads. The small satellites can contaminate the milling product or can clog the outlet of the mill and the grinding process cannot continue.
– After formation and cooling of the bead, inhomogeneous material areas can form, caused by destabilization of different material phases, which can initiate defects in the internal structure of the beads, as shown in Fig. 3. Such flaws can weaken the mechanical properties, which may lead to some problems in the milling process, e.g., beads with lockedin air bubbles.
All the aforementioned negative influences and problems of the fused bead manufacturing process can largely be avoided when the beads are produced by the sintering process. The sin tering manufacturing process requires more production steps, in particular, material preparation and bead formation, before the beads can be compressed by sintering at high temperatures. Consequently, a higher technical expenditure is necessary, but beads with a very good internal structure, i.e., almost without any defects, can be produced, especially when the sol-gel process is used for bead formation. The very dense inner structure of the sintered yttria stabilized zirconia grinding media SiLibeads Type ZY is obvious from Fig. 4, and the internal structure of the sintered zirconium silicate grinding media SiLibeads Type ZS is shown in Fig. 5. A more consistent and homogeneous inner structure of the beads ensures better wear and tear resistance in the later applications of the milling process. Naturally, some other parameters such as quality of the raw materials used, i.e., chemistry, particle size, and specific surface area, etc., necessary additives, material composition and firing schedule of the sintering, influence the bead quality with regard to the milling efficiency and bead lifetime. The specific weight of the grinding media is characterized by the material composition, but can also be influenced by the production method. Glass grinding media can be produced by different methods. One possible method is the smelting process where all raw materials are fused in a glass furnace. The smelting temperature depends on the glass composition and temperatures up to 1400–1600 °C are usually necessary. This method is comparable with the aforementioned fused production process of ceramic beads, but in addition, it is possible to manufacture viscose glass smelt into beads using moulds and press technologies using this method. Another possible method for producing glass grinding media is the processing of glass granules in rotary kilns. In this method, the glass granules are transported to the rotary kiln and, due to the high temperature (range 800–1100 °C depending on the glass formulation), the surface of the glass granules soften and, by the rotation of the kiln, the granules are formed into spheres. All of the common glass grinding media have an amorphous internal structure and are transparent. The color of the beads depends on the glass composition and the atmosphere in the kiln during production.
All test runs were performed in a Drais Laboratory Perl Mill PM1. The working volume of the mill was 0.96 L. The material of the mill chamber as well as the stirrer and discs were lined with polyurethane. A screen cartridge with a screen size of 0.1 mm was used to separate the beads and slurry at the outlet of the mill. All tests were performed with a grinding media filling grade of 80 % (mill working volume). Firstly, the stirrer tip speed was set at 12 m s–1 for all tests and afterwards it was adjusted to 8 m s–1. Due to the tip speed of 12 m s–1 achieved, only grinding media in the size range 0.4–0.5 mm were tested at 8 m s–1 to reduce the experimental expenditure.
A red organic color pigment powder (Heubach, Germany) was used as the milling product. The water-based slurry formulation was composed of: 30 % red organic pigment Heuco Rot (Heubach, Germany), 35 % binder Joncryl HP 96E – liquid styrol-acrylate soluble in water (BASF, Germany), and 35 % deionised water. All milling tests were performed in recirculation mode with a slurry flow rate of 50 L h–1.
4 Results and Discussion
4.1 Results of the Milling Tests
The following parameters were monitored and measured during the tests:
– The slurry flow rate was controlled and fixed at 50 L h–1.
– The stirrer tip speed was measured in revolutions per min (rpm) and converted into m s–1. The tests were performed with tip speeds of 8 and 12 m s–1.
– The real active electrical motor power input (kW) was measured. – The milling time was registered and pigment size measurements were taken at times 0, 5, 15, 30, 60, 90 and 120 min.
The particle size measurements in the suspension were undertaken using a Cilas-1064 particle size analyzer, which works according to the laser light scattering method.
– The dependence of the color change of the pigment during the particle comminution process on the particle size reduction at a given time was measured. For the investigation of the color change, a Konica Minolta, CM3500-D device and measuring system was used, which coordinates to the colorimetric technology by using the CIELAB color space model. All the color measurements were performed using sample illumination by both daylight and UV-light (D65). The CIELAB color space model describes the color and brightness in the 3D coordinate system of a sphere, as shown in Fig. 6. The two area axes a and b characterize the location of the color. Axis a is defined as the green-red-value. The values can vary between –60 and +60, where the value –60 is set as green and +60 is set as red. The second axis b with possible values between –60 and +60 is defined as the blue-yellow-value. The value of –60 is set as blue and +60 is set as yellow. The third axis, named L, is defined as the brightness and is situated vertical to the color axes a and b. The value of brightness L can vary between 0, which characterizes black and a value of 100, which defines white. Colors, which have the same green-redvalue a and blue-yellow-value b, are identical in color, but they can be different in brightness and differ from the value of L.
FROM DOI: 10.1002/ceat.201000064
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Post time: Dec-27-2019