666.762.11 In view of the high cost and scarcity of technical alumina it is important to increase the use of natural raw materials in order to develop the production of high-alumina refractories. It is therefore of interest to use nontraditional types of raw material, in particular those with a predominant concentration of topaz.Topaz is considered to be a rare gemstone. Topaz-containing rocks of the common sort do not generally contain jewelry topaz, and so are not counted as an assessory component -cassiterite, various rare-earth minerals, fluorite, etc. [1]. At the same time, the significant concentration of alumina in topaz suggests that it is possible to make high-alumina refractories from it.Topaz is classified with the insulator fluoroaluminosilicates, and has the composition A12SiO4(F 2, OH) 2. The mass concentration of alumina varies in the 48-62%* range, silica 28-39%, fluorine 12-20%, and H20 up to 3%. It is possible to note the substitution of aluminum by ions of iron, chromium, magnesium, titanium, vanadium and others, but the total content of impurities is usually less than 1% [2, p. 274-290]. The ratio of alumina to silica in topaz is almost the same as in kyanite (distene), which has the formula A12SiO 5. When pure topaz is heated, starting at 850-900~ a substantial mass loss occurs due to the removal of fluorine as silicofluoride (SiF4) with a fall in density from 3.36 to 3,08 g/cm3; and after firing at 1500~ the topaz practically consists of pure mullite [3]. Thus, calcination results in the material being enriched with alumina to 71%.The use of topaz for making heat-resistant materials has been known for quite a long time [4, 5]. In the mid-1930s at the worked-out gold mine at Brever (USA, South Carolina) deposits of topaz rock were found, and it began to be mined. The rock was enriched in heavy suspensions and by flotation. The raw (numerator) and enriched at 1350~ concentrate (demoninator) contained, %: A1203 55.7/71.6, SiO 2 33.1/27.8, Fe203 0.7/0.9, TiO 2 0.2/0.3, F 13,9/0.2, H20 2.1/-, and its cost varied from 15 to 40-50 dollars per ton [1]. A large number of articles was published at that time dealing with investigations of topaz. Most popular was the use of topaz for producing high-alumina refractories on the basis of kyanite, with the replacement of up to 40% of the kyanite; and moreover, an improvement was noted in the working properties of the final product [6, 7]. Lightweight refractories from kyanite, raw topaz, kaolin and anthracite, ftred at 1600~ and containing 56-58% A1203 and with an apparent density of 1.3-1.5 g/cm 3, had a refractoriness of above 1820~ and a high spaUing resistance; their after-contraction at 160WC was 0.15-0.3% [8]. The after contraction for lightweights obtained in such conditions on the basis of technical alumina (aluminous chamotte) is 0.4-0.8% [9]. With an addition to alumina-calcium cement of about 5% topaz concentrate, the cement's strength after calcination at 880~with an improvement in the spalling resistance, was increased 3-6 times, compared w...
In our and other countries the share of high-alumina refractories in the total production of aluminosilicate ones has a regular tendency toward an increase.In connection with this broadening of the production of high-alumina refractories on the basis of natural raw material, particularly bauxites, becomes more important, especially as the result of the shortage and comparatively high cost of commercial alumina, which at present is used by domestic industry for a large portion of high-alumina production.
White electrocorundum melted from commercial alumina is used in the refractory industry for production of corundum and particularly high-alumina refractory parts and compounds. In addition to white electrocorundum the abrasive industry also produces normal electrocortmdum (types 13A-15A to OST MT 71-5-84) produced by melting of bauxites. It is frequently called black because of the dark color caused by the presence of about 5 % impurities, including titanium dioxide and iron oxides. Such corundum, containing 94-96% A1203, is little used in the refractory industry primarily because of the increased content of fluxes and to some extent because of lack of production of it in the industry.Taking into consideration the shortage and very high cost of commercial alumina, the production from bauxites of electrocorundum for production of refractories, the requirements for which differ from the requirements for abrasive corundum, is a pressing problem. The latter must contain a minimum of calcium oxide as the most harmful, impurity and 1.5-2%* titanium dioxide. For refractory corundum, calcium oxide is less harmful, while titanium dioxide is very undesirable. Therefore in melting of black electrocorundum the abrasive industry uses (or used) primarily imported bauxites with a low CaP content. Domestic bauxites, particularly of the Northern Ortega group of deposits of Arkhangel Oblast, contain from 0.25-0.35 to 0.5-1.0% CaP. At present, in this group Iksinsk deposit is the raw material base of the Northern Onega Bauxite Mine and is characterized by the presence of bauxites with a comparatively low iron oxide content (an average of 6-8 %) with a significant alumina content (60-70%).Taking into consideration these differences in the requirements for corundum, independent tests have been made of production of electrocorundum from bauxites for the refractory industry. The principle of reduction melting of bauxite used in the abrasive industry remains common. To the bauxite is added coke (anthracite, etc.) to reduce the oxides of silicon, iron, titanium, etc. Arc furnaces with powers of 9-10 and 16.5 MW with two tap holes for separate tapping of the upper layer of molten material -corundum -and the lower -ferroatloy (ferrosilicon) -are used. The energy consumption per ton of corundum is about 3000 kWh and increases with an increase in the weight portion of silica in the bauxite. The capacity of the 10.5 MW furnace is about 70 tons per day. The temperature of the molten corundum reaches 2400-2500 K and it is tapped into metal molds lined with chromite-periclase refractories.In melting, reduction and removal into the metal alloy of oxides of silicon, iron, titanium, etc. occurs. There exist data [1] on obtaining etetrocorundum with 98.9% A1203 from bauxite. The degree of reduction is determined by the thermodynamic stability of the oxides, the process temperature, the heating rate, the concentrations of carbon and oxides in the charge, the composition of the ferroally incidentally obtained, and other factors. Under the action of CO...
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