PHASE PHENOMENA IN CALCITE-SEAWATER SYSTEM 



195 



O 



o 



0) 



o 

 E 

 o 



u 



E 



62.5 125 187.5 250 312.5 375 437.5 



TIME, min 



Fig. 3 The effect of pressure drop on steady state. 



precipitate on all the available analytical calcite surfaces therein. The system is in 

 steady state at the left border of the figure, as total C0 2 of the incoming 

 seawater is the same as the total C0 2 of the effluent seawater. If the pressure is 

 now dropped quickly from some nominal value below 3 34.1 atm to a lower 

 value, precipitation begins upon exposed calcite. This action results in a drop in 

 the total C0 2 of the effluent seawater, and 125 min are required to complete 

 the covering of the older phase with a new phase that is stable under the 

 imposed conditions of pressure, temperature, and seawater composition. If the 

 pressure is again dropped after the 125-min period necessary to lay down a 

 stable phase, a new phase will be formed in the same manner. Thus, for repeated 

 drops in pressure, with seawater flow rate held constant and steady state 

 reachieved before each new lower pressure is set, the original calcite particles 

 containing 100 ppM magnesium are coated with one layer after another of a 

 precipitate whose surface is stable at the pressure obtaining during its formation. 

 If the operations are performed in reverse order, with the pressures raised in 

 steps instead of being dropped, completely different results are obtained 

 (Fig. 4). Starting at a low pressure, with the system in steady state and, for 

 simplicity, only the phase stable at that pressure present on the original calcite 

 particles, an increase in the pressure to any point less than 3 34.1 atm will cause a 

 rapid dissolution of the precipitated phase followed by an irregular return to 

 steady state over a 150- to 180-min period. The same results are obtained if the 

 pressure is raised again after steady state has been achieved for the second time, 



