Behavior of MgO as a CO 2 Scavenger at the Waste Isolation Pilot Plant (WIPP), Carlsbad, New Mexico
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155 Mat. Res. Soc. Symp. Proc. Vol. 608 © 2000 Materials Research Society
indicated by the upward diagonal trace of the arrow from the Mg(OH)2 in Fig. 1. The timing of their appearance may affect brine availability as hydrous phases first form and then invert to anhydrous MgCO 3. These changes will also have a large impact on the physical properties of the backfill. Thus, putting the CO 2 scavenging ability of the backfill in its proper perspective requires consideration of all of the evolutionary steps that may lead to the final production of the end-stage magnesite. MgOD
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Figure 1 Chemical Evolution MgO backfill. HYDRATION EFFECTS Under repository conditions the hydration of MgO may occur in two ways; by contact with brine and by exposure to water vapor. Brine hydration rates are relatively well understood but the vapor-phase hydration studies are less advanced. Simpson (3) investigated MgO hydration in water vapor at temperatures from 20' to 1500 C. At 20 0C and 100% humidity, over reaction times up to about 80 days, only half the material was hydrated. At 600 C, the impact of particle size became noticeable, and complete hydration was observed in the 31-62.5 pm-sized fraction, but not for the 250 - 350 gtm-sized fraction. Simpson also noted that the vapor pressure of the brine has a significant effect on hydration rates, although thermodynamically Mg(OH) 2 was the stable phase over all the brines studied. Our recent studies confirmed this by using various salt-saturated solutions to provide a range of relative humidities. At the highest relative humidities the vapor phase hydration progresses in a linear manner but for lower relative humidities about 1%converts rapidly. After this, the hydration ceased for the remaining 185 days of the test. This pronounced drop in reaction rate occurs at a humidity greater than that of a saturated NaCG solution so it will have a large effect on predicting the in situ hydration rate of the MgO backfill. Hydration rates in brines were measured for a variety of conditions and demonstrated that hydration rates depended on several experimental factors: brine chemistry, notably the Mg content of the brine, the solid to brine ratio, and the temperature of the experiment. This latter feature was exploited in many cases to accelerate reactions that would require many decades (or centuries) at the anticipated repository temperature (about 28' C). Brine influx to the WIPP workings may either take place by the (very) slow oozing of fluid from the rock salt adjacent to the repository - or as a consequence of drilling activities that penetrate both the repository and a brine reservoir that may exist below the repository. Samples from other deep brine pockets suggest that this latter fluid would be closer to a NaCI-Na 2SO 4 brine and contain significantly less Mg than brines from the Salado rock salt in which the facility is located.
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These studies also found that hydration in brines did not proceed linearly wi
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