Environmental Dimension

The ecological hazard of abrasive grits themselves is minor (Table 2.8 left), but some problems arise from the grit production emissions and waste. Emissions from the production of conventional abrasive grits are likely to consist primarily of particulate matter (PM), and carbon monoxide (CO) from the furnaces [EPA94].

In the sintering process of SiC from quartz and coal (with salt and sawdust as auxiliary materials), a considerable amount of greenhouse gas CO2 is produced

Fig. 2.26 Economic correlations for abrasive grits, left price and manufacturing pressure [LUCE13], right price and hardness [UAMA09]

(Eq. 2.10). Salt and sawdust are likely to produce chlorides and volatile organic compounds (VOC) [EPA94]. Moreover, the SO2 and dust in the waste gases cause pollution [LIET08]. Furnaces collect and trap the reaction gases, so that CO can be purified and used [LIET08]. The gases can be desulfurized so that elemental sulfur is obtained as well [LIET08].

The production of corundum is likely to emit fluorides, sulfides, and metal constitutes of the raw materials. Sol-gel processing of sol-gel corundum emits NOx [EPA94].

The re-processing steps of conventional abrasives like cleaning and refinement also use diverse acids and complex machinery. Grit crushing emits PM [EPA94]. Also in the synthesis process of superabrasives, metallic catalysts and refractories remain as waste material. Moreover, metallic grit coatings are common to improve retention and heat-flow in resin and vitrified bondings, but may involve hazards in their production.

Because of the different synthesis routes for the different grit types, the energy consumption to produce a gram of abrasives is different (Table 2.9). Furthermore, the energy consumption per machined part material will be different due to the different wear resistances (see Sect. 8.1 “Case Study on Conventional Abrasives vs. Superabrasives for Vitrified Bonded Tools”).

Solid or chemical waste from grit production processes needs to be classified and may be treated as hazardous waste. Some manufacturing approaches re-use scrap material. For example, one grit manufacturer fuses recycled aluminum oxide into high quality fused aluminum oxide, so that a completely closed loop process is possible recycling 100 % of the spent material [WASH12]. In another example, grit producers invented a process using alumina/zirconia/silica scrap, because zirconia for the production of zirconium corundum has a high cost [ASHL96]. By adding a reducing agent, the silica content of the final abrasive grit can be kept below 0.8 % by weight [ASHL96].