The cooling rate of molten corundum largely influences the size of the crystals formed. In the case of the billet method, the corundum billet, weighing up to 20 tons, cools slowly. Cooling times can range from 10 to 14 days. In this way, generally larger crystals are formed than in the so-called tapping method, in which molten corundum in flat casting pans cools off relatively quickly, thereby forming fine crystalline corundum. In the case of the tilting method, crystal sizes depend on the furnace and casting sizes. The sizes of the corundum crystals can also be considerably increased by technological means.
After cooling, the billets or casts are crushed or ground. For this purpose, jaw crushers, crude or fine crushers, roller mills or also ball mills or pipe mills are utilised [COLL80]. The crushing process determines to a great extent the form of the grit. While crushing with rollers can create needles in extreme cases, crushing with impact mills produces cubical grit forms.
Especially in the case of normal corundum, physical properties such as toughness and hardness can be improved by means of thermal and mechanical finishing. Used in resin bonded grinding wheels, an additional silane coating improves the bond strength of the grit.
In the manufacture of sintered corundum, the goal is to create a strong ceramic body out of a-Al2O3 with a consistent, fine-grited structure. This fine-grited crystal structure should have a positive effect on wear resistance in the grinding process. For conventional methods for the productions of highly resistant sintered ceramics, a-Al2O3 is already available as a raw material for the sintering process. Using a suitable grinding method, the corundum which has been obtained from the melting process is reduced to an adjusted grit size and, in a further step, sintered additionally by the implementation of means such as glass phase-separating agents. a — Al2O3, which is available as a raw material for grinding, is obtained from bauxite via conventional melting methods [MUEL02, N. N.1].
For the manufacture of sintered bauxite corundum, ground raw bauxite is mixed with water, binding materials and compacting auxiliary agents. The pasty mass is then extruded, cut into lengths and finally sintered. Depending on the manufacturing method, a consistent, fine-grited grit structure is formed.
Sol-gel corundum, also belonging to the sintered corundum group, play an important role in industrial applications. They are distinguished by their homogeneous microcrystalline form and their high density, which, as opposed to sintered bauxite corundum, can even be obtained without compacting, i. e. using a sintering process with no pressure. In contrast to sintered bauxite corundum, these microcrystalline aluminium oxides are manufactured before the final sintering process by means of the costly chemical sol-gel method, from which the material also derives its name.
The first step of the sol-gel method is converting a solid to a colloidal solution by adding water. The solid particles dissolved in the medium exhibit a size ranging as a rule between 1 nm and 1 pm [N. N.82, ODIE85]. They thus fulfil the requirement of a colloidal solution, in which the disperse particles should have larger dimensions than those of simple molecules. Colloid-disperse materials in liquids are designated as colloidal solutions or also in general as sols. If water is utilised as a means of dispersion, it is called a hydrosol. To stabilise and to dissolve the agglomerates and thus to increase the degree of dispersions, a so-called peptisator is added to the mixture. Aluminium hydroxide is peptised, e. g. with hydrochloric acid or nitric acid [N. N.82]. In most cases, the hydrosol pH value lies between 2 and 3. By adding more electrolytes, the sol is dehydrated, i. e. the disperse particles polymerise and a gallert-type mass develops, the gel. In this way, a homogeneous mass with oriented alignments of the individual crystals is created. The process of gel formation can therefore be described as a controlled and simultaneously oriented flocculation [HERM73].
The manufacturing process for the abrasive sol-gel corundum is shown in Fig.
3- 3. For the production of sol-gel corundum, powdered boehmite (y-A100H) is used as a starting material. The latter is previously synthesised by means of hy-
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drolysis of aluminium alcoholates. The resulting boehmite is characterised by its high material purity, its high specific surface area (200-300 m2/g) and its small particle size (5 nm) [N. N.84, UHLM97a]. The powdery boehmite is transformed into a clear sol with the addition of water and mixing with the peptisator, this being nitric acid in most cases [N. N.82].
By means of further addition of an acid (mostly nitric acid) or a nitrate solution, the reaction to form a gel, i. e. dehydration and polymerisation, is induced. The fact that the acid is utilised on the one hand as a peptisator for the production of the sol and on the other hand to form the gel shows that the correct choice in added amounts of water, acid and boehmite is vital for the sol-gel process.
As a result of gelation, the boehmite is now distributed very homogeneously. In a final working step, the water which has been released is evaporated. The gel is rolled into thin strands and dried at a temperature of ca. 80 — 100 °C. In this way, a brittle solid is formed, which is crushed to the required grit size and filtered out in a further working step for use as an abrasive. In this stage, the individual grits are still composed of aluminium hydroxide in the boehmite phase.
Grits manufactured in this way undergo an initial heat treatment in the next working step. At firing temperatures of 450 — 550 °C, the aluminium hydroxide transforms into a transitional-phase aluminium oxide, y-Al2O3. Upon the reaction of boehmite with y-Al2O3, nitrogen is released as a residue of acid and water. This low-temperature firing is designated as calcining. The last step in the production of sol-gel corundum is the concluding unpressured sintering. During the sintering
process, the temperature is incrementally raised to values lying between 1200 und 1500 °C.
In general, abrasive grits are sorted according to size by sifting and, in the case of finer grits, by sedimentation or air sifting (table 3-1). Grit classification as well as the inspection methods for macrogrit and microgrit materials have been standardised by DIN ISO 8486-1. The dominant grit form of a batch can be determined by the bulk density of the loose abrasive grit.
Various special corundum materials are produced by varying the manufacturing process described above. Pink or red corundum is developed by adding about 0.3 % or 2 % Cr2O3 during the melting process. In this way, the chrome oxide is built into the Al2O3 lattice of the corundum. In the case of zircon corundum, up to 42 % zircon dioxide (ZrO2) is added to the molten mass. During the solidification phase of such molten masses, eutectic structures of Al2O3 and ZrO2 emerge. If, besides coke, iron sulphide is added to the molten bauxite, highly pure, monocrystalline aluminium oxide forms in a sulphidic matrix. We obtain monocrystalline corundum by forming monocrystals from the matrix. Hollow sphere corundum is created by atomising the molten Al2O3. This special corundum is characterised by a more or less regular, spherical form.
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Table 3-1. The classification of abrasives in accordance with the FEPA standard
dkm = mean grit diameter |