The recent developments in the production of large-sized and intricate refractory products by casting from aqueous suspensions [1][2][3][4][5] call for an improved technology of obtaining the casting suspensions and an increased strength of the unfinished castings.The conventional method of obtaining A1203 suspensions includes milling the powder in steel mills, subsequently washing off the milled iron contaminant, and suspending the milled products in the optimum pH range [4][5][6]. However, this method involves a lengthy process (multiple decantation requiring a total duration up to 30 days) and a significant consumption of hydrochloric acid (up to 0.5 liter per i kg of material). Besides this, the castings obtained from such suspensions exhibit a relatively high shrinkage during the process of drying (up to 3-4%) and low strength [7,8] because of which it is not possible to produce large-sized intricate components. Furthermore, the low strength makes machining of the unfinished (raw) products difficult. Machining of the underfired or sintered products leads to an increased consumption of diamonds.Furthermore, the conditions have been established [9-13] for obtaining ceramic suspensions possessing considerable binding properties that ensure high strength of castings. Thus, a new method was proposed [13] for obtaining heat-resistant binders (including those based on A1203) by introducing small amounts of highly active additives having low values of ionic potential (BaO, SrO, CaO) into the composition of the low-activity materials (having high values of ionic potential) and for obtaining the suspensions by wet milling the materials in corundum-lined mills using corundum grinding bodies at predetermined pH values. The ultimate bend strength of the raw castings based on the additiveless GK alumina that was prepared using the new method was found to be 2-4 MPa and after drying it increased up to 5-8 MPa [13].Wet milling at elevated temperatures (owing to the occurrence of heat liberating processes) and optimum (for thinning) pH values forms the main condition for obtaining such suspensions. This makes it possible to carry out the process at the maximum possible concentration of the solid phase. In this case, one obtains polydispersed granular composition (size distribution) and a low content of the bound (bonded) liquid that subsequently determine the density of the semifinished product. This paper deals with the evaluation of three methods of preparing the suspensions: the commonly used method of suspending a powder milled in a vibrational mill and subsequently washed for removing the iron contaminant; suspending a powder milled in a corundum-lined vibrational mill using corundum balls; and suspending a powder wet-milled in a corundumlined ball mill using corundum balls under the optimum values of the moisture content and pH. In all the cases, the material:balls ratio was maintained at 1:6; and during wet milling, the material:balls:water ratio was equal to 1:6:0.5.An aluminous frit (sinter) containing a glass phase...

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The development of the electronic and semiconductor branches of industry --the creation of various design elements for the new technology --has extended the field of use of single leucosapphire crystals, which have a wide range of transmission in the UV, visible, and IR regions of the spectrum combined with excellent thermomechanical properties.The large crystals were zone-grown from a melt of aluminum oxide blanks in special boats [i, 2].In order to obtain large single crystals of high purity, the aluminum oxide blanks must have a sufficiently high density and a minimum amount of impurity.The concentration of A1203 in the blanks should be not less than 99%.* The use for this purpose of sintered corundum plates made from microlite (TsM-332) measuring 7•215 mm, whose production method has been described in [8], does not meet the increasing demand from industry; it requires a significant amount of manual labor to stack them in a container and in several cases leads to the boiling of the melt when the crystals are grown.Therefore, the aim of the present study was to develop the manufacturing technology for corundum plates from 7 to 35-40 mm thick as the blanks for growing single crystals of leucosapphire of a size equal to that of the container.Corundum, deep-purified from alkalis, was used to produce the high-purity corundum [3].For this purpose, technical alumina, grade G-00, containing 99.64% of A1203, 0.02% of SiO2, 0.01% of TiO2, 0.01% of CaO, 0.01% of MgO, 0.03% of Fe203, and 0.28-0.33% of Na20, were mixed together in a ball mill with an addition of MgCI2"6H20 calculated as 0.2% of MgO [4]; the materials were fired in a batch or tunnel furnace under pilot-plant and industrial conditions. During the firing the density of the alumina increased from 3.665 to 3.95-3.99 g/cm 3 while the concentration of A1203 grew from 99.74-99.75% with a reduction in the amount of impurity Fe203 to 0.02% and of Na20 to 0.007-0.04%; the concentration of MgO in the corundum varied between 0.14-0.16%.Since the deep-purified corundum in the form of aqueous suspensions has been used up to the present time after milling and washing in HCI with seven decantations for products cast in gypsum molds [5], we studied the milling kinetics of corundum; the hammering on of iron; and the effect of these on the sintering of pressed products (no washing) and also on the concentration of impurity iron oxide in the products.The study of the milling kinetics of the deep purified corundum (dry method) in a laboratory vibromill (capacity i0 liter) showed that intense dispersion continues for ~ 4 h and the number of fractions of < 1 or 1-2 Dm increases significantly) the concentration of fractions of 2-5, 5-10, and 10-20 ~m is reduced (Fig. i). With prolonged milling for > 4 h, the concentration of all grains and, correspondingly, the specific surface and average diameter do not change (Fig. 2).Here and elsewhere mass fractions are given.