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...
Sialons have recently been obtained abroad from compounds in the system Si--AI--O-N [1-5]; they combine the main outstanding physicoehemical properties of oxides [6][7][8][9] and nonoxygen compounds* [10][11][12][13][14].Sialons are very resistant to oxidation and to the action of the metals Fe, AI, Cu, Zn, steels and also to sulfur, sulfuric acids, and alkalis.According to [15] the use of sialon for storing and transporting molten metals, including steel, is a very important application area.Abroad, sialons are synthesized mainly from AI=O~, Si3N~ and AIN [1][2][3][4], and in some cases nitrided mixtures of SiO2 and AI are used [5].Since sialons are essentially silicates or aluminosilicates in which the oxygen is partialiy replaced by nitrogen, the present authors synthesized sialon from various natural aluminosilicate materials whose compositions are shown in Table i. The carbon constituent consisted of graphite grade GL-3 (GOST 5279-74).Aluminosilicate material was mixed in a ball millwith the appropriate quantity of graphite calculated to obtain sialon with the composition 8i6_xAlxOxN6_ x , where x = 3~ The batch was used to make specimens (briquettes) at pressures of 30 and i00 MPa in the form of cylinders, 20 mm in length and diameter.The specimens and also the nonbriquetted batch (filling) were fired in an industrial muffle furnace at the Semiluksk refractories factory at 1450~ in a nitrogen atmosphere, purified to remove moisture and oxygen, at a pressure of 196 Fa. Firing was done in an open setting and also in graphite crucibles.The degree of synthesis in the sialon was evaluated on the basis of x-ray and petrographic investigations.Measurements were made of the intensity of the most typical lines with interplane distances of 0.270 and 0.331 nm (with an index of 200) for B'-sialon, and 0.362 nm for l-sialon.It was established that the sialon is synthesized from all the aluminosilicate materials studied.Judging from the intensity of the lines on the x-ray patterns corresponding to 8t and l-sialons, it is formed particularly actively in mixtures of Novoselitsk kaolin and carbon (Table 2), and furthermore with a reduction in the ratio of A1203:Si02 in the aluminosilicate materials we observe a reduction in the height of the lines for sialon (Fig. i). Firing in graphite crucibles speeds up the synthesis of sialon.The concentration of sialon expressed by the height of the line on the x-ray patterns is proportional to the amount of bonded nitrogen in the batch (Fig. 2).According to petrographic and x-ray data, in addition to sialon, all the specimens contained silicon oxynitride, residues of unreacted graphite, and glass. The sialon and silicon oxynitride have a fine-grained structure; their crystal sizes are not more than 3-8 ~m. The average refractive index of silicon oxynitride is 1.760• for the sialon it varies from 1.855 to 1.897, which indicates the variable composition of the compound.It is also established that during the nitriding of briquetted mixtures the amount of synthesized sialon is reduced, an...
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