Ways to Improve the Production Technology of Artificial Corundum Abrasive Materials

V. M. Vityugin, V. V. Karlin, V. A. Lyutov, I. P. Chashchin

(Presented by the scientific seminar of the Department of General Chemical Technology)

The development of the metalworking and machine-building branches of industry in our country is placing ever higher demands on the quality of abrasive tools and, in particular, on wheels for rough grinding.

An increase in the operating efficiency of wheels of this type can be achieved by increasing speeds and force loads. To manufacture wheels that would withstand greater force loads and speeds, high-quality abrasive grain is required first of all. Electrofused corundum grain, from which wheels for rough grinding are currently manufactured, has a high cost and an imperfect production technology, which over its 75-year existence has not undergone substantial changes. The technological process remains periodic, low-productivity, and energy-intensive. The periodicity of the process can also be explained by the fact that, in order to obtain a normal crystalline structure throughout the entire block of electrocorundum, a long time is required for the crystallization processes to proceed and be completed. In addition, stringent requirements are imposed on the quality of the starting raw material (bauxite), both in chemical and in granulometric composition. Finely ground bauxite promotes sintering of the charge, as a result of which the gas permeability of the furnace shaft deteriorates; this leads to disruption of furnace operation, excessive energy consumption, and worsens furnace servicing conditions. To prevent these undesirable phenomena, an additional operation of bauxite agglomeration is introduced, which undoubtedly affects the increase in the cost of the finished product.

Ensuring the high melting temperature of electrocorundum requires significant energy expenditures and, moreover, during the melting process losses of charge materials (up to 4–6%) due to dust formation are unavoidable. The costs associated with converting large blocks of electrocorundum into grain and powders are extremely high, as are the unavoidable product losses due to overgrinding. A major disadvantage is also that, during crushing of the material, the monolithic nature of the grain structure is disrupted, which in turn has an adverse effect on the mechanical strength both of individual grains and of rough-grinding wheels.

Recently, abroad, a new, more progressive method of producing abrasive material has become widespread. The essence of this method is that the raw material is first formed into cylindrical granules and then fired at temperatures of 1400–1600°C. The fired granules are superior in quality to ordinary electrocorundum grain.

The present study is devoted to the development of a method for obtaining calibrated abrasive material using domestic bauxite and dust wastes from electrocorundum production.

As the starting raw material, bauxite was used; its chemical composition is given in Table 1. The bauxite was crushed and then ground in a laboratory vibratory mill by the wet method to a particle size of 0–100 microns. The largest particles in the ground bauxite consisted mainly of silicon dioxide and were removed by elutriation in water. The suspension was filtered; the precipitate was partially

Table 1

Chemical composition of bauxite

dewatered by drying to a residual moisture content of 8%. The resulting molding semi-finished product was fed into a vertically arranged screw press, where it was forced through a die-filter with a hole diameter of 2–3 mm. At the outlet from the press, elongated cylindrical rods were obtained; to prevent them from sticking together, they were blown with hot air, as a result of which they were partially dried from the surface and already acquired sufficient mechanical strength. Drying of the granules was carried out at a temperature of 150–200°C for 10–15 minutes. The dry granules were fired in a laboratory kryptol furnace. Firing is the most critical stage in the technology for obtaining calibrated abrasive grains and, it may be said, the principal characteristics of the product will depend to a significant extent on the firing regime. As a result of the investigations, the following firing regime was selected:

working firing temperature — 1450–1500°C;
time of heating to the working temperature — 25–30 min;
time of isothermal holding at the working temperature — 10–15 min;
cooling — quenching from high temperatures in air.

According to the developed regime, granules were made from a charge with the addition of up to 30% white electrocorundum sludge. Since sludge is a lean material, up to 2% bentonite was introduced into the mass to increase plasticity.

Qualitative spectral analysis showed that the material has the composition of normal electrocorundum. Qualitative mineralogical analysis showed that the section of a granule consists of a corundum field of fine crystallization and dense packing, with separate inclusions of hematite grains. The abrasive ability, evaluated on abrasive paper No. 16 in comparison with monocrystalline corundum, is 50–60%; the mechanical crushing strength of individual grains is 60–70 kg. It is important to note that granules containing white electrocorundum sludge possess considerably greater mechanical strength and abrasive ability than granules made only from bauxite. This provides grounds for the use of sludge from electrocorundum production, which at present finds no application. The proposed technology for obtaining calibrated abrasive grain is comparatively simple in its implementation and does not require special and costly equipment. The process for producing calibrated grain is continuous and more highly productive than the existing method.

LITERATURE

1. A. S. Polubeslova, V. N. Krylov, V. V. Karlin, I. S. Efimova. Production of Abrasive Materials. Mashinostroenie Publishing House, L., 1968.