Iron oxides/silicates, melting temperatures

As previously mentioned, silica brick can withstand temperatures close to its melting point, this may be attributed to silica s ability to form immiscible liquids in the presence of lime or iron oxide. No immiscibility, however, is present with silica in the Na20-Si02-Al203 system. With small additions of alumina or alkali oxides to silicate systems in which immiscibility occurs, the miscibility gap is narrowed and ultimately eliminated. A range of from 0.3% to 0.5% alkali or 2.5% to 3.0% A1203 is sufficient to cause this effect, specifically alkalies form low melting eutectics with silica. As a result, the presence of these oxides even in small... 

Concentration of copper is done by flotation, and roasting converts iron sulfides to iron oxides. The copper remains as the sulfide if the temperature is kept below 800 °C. Reduction of the roasted ore in a furnace at 1400 °C causes the material to melt and separate into two layers. The bottom layer, called copper matte, consists chiefly of the molten sulfides of copper and iron. The top layer is a silicate slag formed by the reaction of oxides of Fe, Ca, and A1 with Si02 (which typically is present in the ore or can be added). For example,... 

Then the roasted ore is combined with sand, powdered limestone, and some unroasted ore (containing copper(II) sulfide), and heated at 1,100°C in a reverberatory furnace. Copper(II) sulfide is reduced to copper(I) sulfide. Calcium carbonate and silica react at this temperature to form calcium silicate, CaSiOs The liquid melt of CaSiOs dissolves iron(II) oxide forming a molten slag of mixed silicate ... 

Silicon [7440-21-3], Si, from the Latin silex, silicis for flint, is the fourteenth element of the Periodic Table, has atomic wt 28.083, and a room temperature density of 2.3 gm/cm3. Silicon is brittle, has a gray, metallic luster, and melts at 1412°C. In 1787 Lavoisier suggested that silica (qv), of which flint is one form, was the oxide of an unknown element. Gay-Lussac and Thenard apparently produced elemental silicon in 1811 by reducing silicon tetrafluoride with potassium but did not recognize it as an element. In 1817 Berzelius reported evidence of silicon occurring as a precipitate in cast iron. Elemental silicon does not occur in nature. As a constituent of various minerals, eg, silica and silicates such as the feldspars and kaolins, however, silicon comprises about 28% of the earth s crust. There are three stable isotopes that occur naturally and several that can be prepared artificially and are radioactive (Table 1) (1). 

The Flux.—It will bo seen, by referring to the different qualities of the ores of iron, that mostly ah of them contain small quantities of other matters and that when these are silica and alumina, which are the most common, either separately or together, they are infusible In the blast furnace but At a temperature below that of melting iron, they will combine, with the oxides of other metals, and form with them combinations that are fhaible. The oxides of iron combine readily with silica, and form a silicate of iron which is very easily fused. If, then, a mixture of lion ore and cool he put into the blast furnace, the reactions may be represented as somewhat like the following, Suppose a mixture is taken of clay and black band, composed as under —... 

Silicon carbide is comparatively stable. The only violent reaction occurs when SiC is heated with a mixture of potassium dichromate and lead chromate. Chemical reactions do, however, take place between silicon carbide and a variety of compounds at relatively high temperatures. Sodium silicate attacks SiC above 1300°C, and SiC reacts with calcium and magnesium oxides above 1000°C and with copper oxide at 800°C to form the metal silicide. Silicon carbide decomposes in fused alkalies such as potassium chromate or sodium chromate and in fused borax or cryolite, and reacts with carbon dioxide, hydrogen, air, and steam. Silicon carbide, resistant to chlorine below 700°C, reacts to form carbon and silicon tetrachloride at high temperature. SiC dissociates in molten iron and the silicon reacts with oxides present in the melt, a reaction of use in the metallurgy of iron and steel (qv). The dense, self-bonded type of SiC has good resistance to aluminum up to about 800°C, to bismuth and zinc at 600°C, and to tin up to 400°C a new silicon nitride-bonded type exhibits improved resistance to cryolite. 

Na202 Zr, Ni Strong base and powerful oxidant good for silicates not dissolved by Na2C03. Useful for iron and chromium alloys. Because it slowly attacks crucibles, a good procedure is to coat the inside of a Ni crucible with molten Na2C03, cool, and add Na202. The peroxide melts at lower temperature than the carbonate, which shields the crucible from the melt. 

Many common substances—notably silicates, some mineral oxides, and a few iron alloys—are attacked slowly, if at all, by the methods just considered. In such cases, recourse to use of a fused-salt medium is indicated. Here, the sample is mixed with an alkali metal salt, called the jliLX, and the combination is then fused to form a water-soluble product called the melt. Fluxes decompose most substances by virtue of the high temperatures required for their use (300°C to 1000 C) and the high con-eentrations of reagents brought into contact with the sample. 

The resulting lime floats to the top of the molten mixture (an event called the lime boil), where it combines with phosphates, sulfates, and silicates. Next comes the refining process, which involves continued oxidation of carbon and other impurities. Because the melting point increases as the carbon content decreases, the bath temperatures must be increased during this phase of the operation. If the carbon content falls below that desired in the final product, coke or pig iron may be added. 

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