Behavior of Aqueous Electrolytes in Steam Cycles - The Final Report on the Solubility and Volatility of copper(I) and Copper(II) Oxides

摘要

Measurements were completed on the solubility of cupric and cuprous oxides in liquid water and steam at controlled pH conditions from 25 to 400 C (77 to 752 F). The results of this study have been combined with those reported from this laboratory in two previous EPRI reports to provide a complete description of the solubility of these oxides and the speciation of copper dissolved in liquid water and steam as a function of oxidation state, temperature, pH, and in the case of steam, pressure. These constitute the first set of reliable data for cuprous oxide solubility over this range of conditions. For the more intensively studied CuO case, agreement was found between our results and those of previous studies of its solubility in steam, whereas only partial agreement was evident for its solubility in liquid water. For both oxides this disagreement often amounted to orders of magnitude. The solubility of cuprous oxide is somewhat lower than that of CuO at ambient conditions, except as very high pH. However, by 350 C (662 F), Cu{sub 2}O is the more soluble phase. At 100 C (212 F) and above, the logarithm of the solubility of both phases decreases linearly with increasing pH to a minimum value then sharply increases linearly with pH. In other words, above 100 C the solubility of both oxides become highly pH dependent. In fact at constant pH during startup, very high copper concentrations can be reached in the boiler water, more than an order of magnitude above those at ambient or operating temperatures. The enhancing effect of added ammonia on the solubility of both oxides is most significant at low temperatures and is much greater for cuprous oxide. Consequently, the mobility of copper is affected significantly under AVT startup conditions. The oxidation of copper metal and presumably cuprous oxide by addition of air-saturated makeup water can lead to much higher copper concentrations than equilibrium with cupric oxide would allow, but the presence of both copper metal and cuprous oxide provides an effective scavenger for oxygen, even at room temperature, with copper levels consistent with those in equilibrium with cuprous oxide. The solubilities of Cu{sub 2}O and CuO in steam are quite similar and are virtually temperature independent at the 1 to 2 ppb level, respectively, although at supercritical conditions, both solubilities increase with increasing pressure and temperature. The species that partition to the vapor are believed to be the neutrally charged molecules, Cu(OH){sup 0} and Cu(OH){sub 2}{sup 0}, for the reduced and oxidized forms, respectively, and their concentrations in the vapor are therefore independent of the pH of the liquid water phase from which they originate.