Carbon solubility as carbide in calcium silicate melts
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Communications Carbon Solubility as Carbide in Calcium Silicate Melts R.A. BERRYMAN and I.D. SOMMERVILLE
CaC2 + (A1203) = (CaO) + 2Alr~ + 2CO
A number of workers have measured carbon solubility in calcium aluminates. I]-4] Swisher I1] also presented resuits showing carbon solubility across a range of liquids from CaO. A1203 to CaO. SIO2, as shown in Figure 1. A sharp rise in measured carbon solubility occurs toward the silica-rich edge of the graph. Turkdogan I5,6] noted that this trend did not agree with the accepted model for carbidic dissolution discussed below, which involves consumption of free oxide ions. Another source .of data for carbon solubility in liquid silicates is a study in the CaO-AI203-SiO2 system by Ponomarenko and Kozlov. [4] No compositions containing only CaO and SiO2 were reported, but intermediate compositions close to those used by Swisher in Figure 1 were reported to have carbon solubilities even higher than those reported by Swisher. Also, their results for silica-free melts indicated solubilities up to 5 times higher than those for other work in calcium aluminates, [',2,3] suggesting that their data may not be reliable. Carbide capacity, Co, is derived from the carbide dissolution reaction (x + y / 2 ) C(gr) + y / 2 0 = = C~ + y/2CO(g)
[1]
If the activity of the carbide ion is proportional to its concentration, if the activity of graphite is unity, and if the value of y is 2, then carbide capacity can be defined
[4]
and that the low recovery of carbon in the melt occurred because most of the calcium carbide reacted with the alumina. The C22- ion is the form generally accepted by other workers, rather than a C 2- ion. Schwerdtfeger and Schubert tS} noted that evidence also existed for the existence of a C 4- ion in aluminum oxycarbide. A gas-liquid-solid equilibration technique was selected for the measurements. Premelted samples, 3 g in mass, were placed in crucibles (LECO* hydrogen de*LECO is a trademark o f LECO Corporation, St. Louis, MI.
termination, high-purity graphite) having dimensions 10-mm ID and 30-mm high. Four such crucibles were placed inside a larger graphite crucible (32-mm ID and 76-mm high) with a lid. The thermocouple and gas inlet tube terminated between the smaller crucibles at the level of the liquid samples, as shown in Figure 2. The graphite container crucible was suspended inside a closed end alumina tube (48-mm ID) which was held in a larger vertical tube furnace. During the time taken (4 hours) for lowering the apparatus into the furnace, an Ar-5 pct H2 gas mixture was passed through the apparatus. Once
as
Cc = (Pct C) (Pco) = K,(ao=)/(fc)
[21
This parameter is an expression of the oxide ion availability but also includes the effect of the activity coefficient of carbide, ft. In Reaction [1], the value of x is not specified. Although this is unimportant in the definition and use of carbide capacity, it would be useful to know its value to understand the dissolution chemistry more fully. Calcium carbide, which releases acetylene (CzH2) on contact with
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