Calcium silicate, melting temperatures

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 ... 

The mixture of bone-ash or ground phosphate rock, sand and coal dust or wood charcoal is introduced continuously into a furnace heated by the electric current, passing between carbon electrodes (see p. 8). The reaction begins at about 1100° C., but a much higher temperature is required to melt the charge and slag of calcium silicate which is drawn off continuously, while the phosphorus distils at about 1300° C. The yield is said to be about 92 per cent, of the theoretical. 

Both MgO and CaO have high melting temperatures and do not combine into compounds. The only eutectic in the system CaO — MgO melts at about 2300 C. In a pure binary system, very high temperatures are required for sintering. However, natural dolomite always contains impurities (FCiOs, AI2O3, Si02) which facilitate formation of clinker at lower temperatures. The impurities partly produce a liquid phase above 1300 °C, and form silicates, ferrites and aluminates of calcium either directly or by recrystallization from the melt. However, most of the CaO and MgO remain uncombined. 

The overall glass composition is by far the most important factor in controlling the batch-free time. Simple oxide mixtures, such as those used to produce calcium aluminate glasses, often form eutectic mixtures which melt directly with very short batch-free times. Many non-silicate melts are very fluid at any temperature above the melting point of their components and rapidly dissolve all batch particles. Borate, phosphate, and germanate melts can be formed at much lower temperatures than are typically required for silicate melts. As a result, it is usually easier to decrease their viscosity by increases in temperature, e.g., an increase in temperature from 1000 to 1200 C is more easily attained than an increase from 1400 to 1600 C. 

Calcium aluminate cement with a lower or intermediate AI2O3 content cannot be produced in rotary or shaft kilns of the type common in the manufacture of Portland clinker. The temperature range between incipient melting and complete fusion of the raw mixes is too narrow to permit successful clinkerization, in which a melt and solid phases must coexist. Moreover, the viscosity of calcium aluminate (-ferrite) melts is significantly lower than that of the calcium silicate-aluminate-ferrite melts formed when Portland clinker is produced. 

The main problem in using the Hall process for silicon electrodeposition is that silicon melts at a much higher temperature than aluminum (1412°C compared to 660°C). Cryolite can not be used at this temperature due to volatization problems, so a binary or ternary melt containing SiOg had to be developed that would be stable above this temperature. Johnson (29) indicated that calcium and magnesium based silicate melts looked favorable, while other alkaline earth and alkali metal silicates were less desirable. 

It is immediately obvious that most of the iron-containing compounds melt in the area of 1200- 1350°C while most of the calcium silicate phases melt above 1485°C. It is also true that melting can typically begin below these indicated temperatures by as much as 50°C due to the effect of other impurities and due to the presence of multicomponent eutectics not seen on three-component diagrams. 

Figure 3.1 summarises how the temperature and the particle size of quartz sand determine the required residence time for batch melting of a sodium-calcium-silicate glass (described in Sect. 1.2.2.). 

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. 

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