Manufacturing Energy of a Vitrified Bond

Vitrified bonded tools are manufactured through mixing of the components, molding, pressing, sintering, pre-processing, and quality control (see Sect. 3.2 “Vitrified Bonds”). This study leaves out the embodied energy in the tooling equipment and assumes that a sufficiently large number of grinding wheels are

Table 8.1 Bond ingredients for a representative bond Bond ingredient [BOTS05] Formula Proportion in bond (w%) [BOTS05] Density (g/cm3) [GRAN12, GEST12] Embodied energy (MJ/kg) [GRAN12] Silicon oxide SiO2 56.88 2.65 37.4-41.4 Boron oxide B2O3 16.61 2.46 Estimated 50-75 Aluminum oxide Al2O3 10.01 3.94 49.5-54.7 Calcium oxide CaO 8.14 3.37 Estimated 50-75 Sodium oxide Na2O 4.62 2.27 Estimated 50-75 Potassium oxide K2O 3.52 2.32 Estimated 50-75 Magnesium oxide MgO 0.22 3.58 120-133

Table 8.2 Volumetric structural composition of the grinding wheels Corundum wheel (V%) CBN wheel (V%) Bond volume 15 30 Grit volume 55 45 Pore volume 30 25

produced, so that the equipment accounts for a negligible amount of embodied energy per grinding wheel. The proportion of grit, bond and pore volume defines the structure and hardness of a grinding tool. Close to industrial practice, this study assumes the structural compositions as in Table 8.2.

The raw materials need to be mixed and pressed before the actual sintering process can take place. For the analysis of the mixing energy two representative mixing machines were chosen. The total amount of raw material for the production of a conventional grinding wheel greatly differs from the amount used for superabrasives. Table 8.3 provides basic information for each mixer and possible production rates. The larger mixer has a maximum capacity of material for 26 conventional grinding wheels whereas the small mixer can contain material for the abrasive layer of 15 superabrasive wheels.

Table 8.3 Mixer characteristics and energy consumption [WAB09] Corundum wheel CBN wheel Volume 50 l 2 l Material capacity 5-26 wheels 2-15 wheels Power 1.1 kW 0.18 kW Mixing time 1 h 1 h Total mixing energy (power over time) 3.96 MJ 0.648 MJ Mixing energy per grinding wheel (at max. capacity) 0.152 MJ 0.0432 MJ

Table 8.4 Molding press characteristics and energy consumption, adapted from [SCHT11] Corundum wheel CBN wheel Table size 710 x 800 mm 400 x 400 mm Plunger size 500 x 630 mm 320 x 320 mm Max. press capacity 2500 kN 125 kN Power 22 kW 4 kW Molding time 5 min 5 min Molding and pressing energy per grinding wheel 6.60 MJ 1.20 MJ

The total mixing time can be up to one hour in which the material is mixed by a three-dimensional movement in a sealed chamber. An advantage of this method is that no potentially hazardous dust can exhaust during the mixing process. Table 8.3 gives the consumed energy for the mixing time of 1 h in total and per grinding wheel. The overall energy consumption for the mixing of one grinding wheel is 0.152 MJ for the conventional type and 0.0432 MJ for the superabrasive. In the case that the wheels are produced in smaller batches, the mixing energy per wheel will be higher.

In the next step, the homogenous tool mixture needs to be molded into the appropriate form. To achieve a certain porosity, a preselected pressure is applied to the mixture in the mold until the mixture reaches a predefined volume. The pressure depends on the mixture itself, its volume, and the desired porosity of the final abrasive layer. The segments for a superabrasive grinding wheel require a com­parably lower pressure and are sometimes even molded manually. In contrast, the compressive force for a complete vitrified bonded grinding wheel can range from 500 up to 45,000 kN. For this study, two hydraulic single column presses were selected according to Table 8.4. Selecting a molding time of 5 min for both grinding wheels results in total molding energy of 6.60 MJ for a conventional grinding wheel and 1.20 MJ for the segments on a superabrasive wheel.

For the sintering process, two example industrial furnaces are chosen with the same maximum temperature of 1600 °C, but different chamber sizes (Table 8.5).

Table 8.5 Furnace data and consumed energy during sintering, adapted from [NABE12] Corundum wheel CBN wheel Dimensions of working chamber 500 mm x 550 mm x 550 mm 150 mm x 150 mm x 150 mm Segments produced per cycle — 360 Max. number of wheels produced per cycle 22 9 Max. furnace power Pmax 21.0 kJ/s 5.2 kJ/s Max. temperature 1600 °C 1600 °C Sintering temperature 1250 °C 1250 °C Sintering time 100 h 100 h Consumed energy for one sintering process 4162.62 MJ 1030.77 MJ Consumed sintering energy for one wheel (at max. capacity) 189.21 MJ 114.53 MJ

Fig. 8.3 Sintering temperature profile

The CBN segments are sintered in a smaller furnace. A total amount of 22 con­ventional wheels can be stacked including a 5 mm thick spacer between each layer in the larger furnace. The smaller furnace holds 15 layers of superabrasive material segments. Each layer consists of 24 individual segments. In total, an amount of 360 segments can be stacked in the small furnace. This is enough material for nine superabrasive grinding wheels.

Both furnaces feature a maximum temperature of 1600 °C at the maximum heating power, Pmax. As the sintering process requires a lower sintering temperature of 1250 °C, the maximum sintering power is only 80 % of Pmax. The sintering temperature profile in Fig. 8.3 left is representative and allows for calculating the sintering energy for both grinding tools. In the first 30 h of the heating cycle the furnaces, containing the abrasive material are heated-up linearly from room tem­perature to the sintering temperature of 1250 °C. The wheels are then soaked at this sintering temperature for 40 about hours. After the soaking period, the grinding wheels and segments are cooled down linearly to room temperature in about 30 h. The same heating cycle applies to both wheel type and segments, because sintering temperature and time does not depend on the volume of the sintered material, but on the material and chemical reactions.

For simplification, it is assumed that the power consumption runs linearly to the temperature. The total consumed energy during the sintering process is the power over time. For the large furnace with conventional grinding wheels, energy accounts to 4162.62 MJ, leading to an energy consumption of 189.21 MJ for one conventional grinding wheel. The smaller furnace uses 1030.77 MJ, which results in 114.53 MJ for the segments needed for one superabrasive grinding wheel.