Nucleation of Crystalline Phases from Sea Water and Sea Water Concentrates L. H. Shafferl and R . A. Knight Research Division, American Machine & Foundry Co., 689 Hope St., Stamford, Conn. 06907
When sea water is heated rapidly in a pressure vessel, a whisker-like crystalline phase forms at 180' C. These whiskers redissolve rapidly at high temperatures if the reaction vessel is cooled immediately; but, if the temperature is held at 140" to 180" C., they break LIP into short fat needles, which redissolve slowly at room temperature. X-ray analysis showed that the needles recovered from a tjpical experiment were anhydrite (CaSOJ.
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o make exploratory observations on the nucleation of crystalline phases in sea water and sea water concentrates, a small pressure vessel (-0.2 cc.) equipped with a viewing window was constructed (Figure 1). The vessel was heated on a microscope hot stage equipped with a temperature controller. The optical setup shown in Figure 2 was used to observe and photograph the interior of the vessel. Observations were made on sea water and on mixtures of sea water with a brine obtained by concentrating sea water about six times (nominal 6 X brine. Shaffer. 1967). When sea water was heated rapidly (1 to 4" per minute). very long thin needles would form in the liquid at 185' C. (Figure 3. A and B). When brine samples were heated. these needles were not always seen. but often a shorter fatter type of crystal appeared. If samples containing the very long thin needles were cooled immediately, these needles redissolved rapidly at temperatures ranging from 131" C. (sea water) to 149' C. (3X brine). If the samples were repeatedly reheated, or if they were held at 185" to 190" C. for an appreciable length of time, a different kind of crystal formed (Figure 3, C through F). The new material was also needle-like. but the needles were relatively short and fat and did not redissolve rapidly when the sample was cooled. By taking advantage of this in a separate experiment, a small quantity of the more stable crystals could be recovered. X-ray examination showed that they were an-
Figure 1. Nucleation test cell To scale: cell diameter 1.03 an.
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Figure 2. Optical system for nucleation experiments Volume I , Number 8 , August 1967 661
1 IO Crystals did dissolve
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Figure 4. Saturation temperatures for anhydrite in brines made by mixing sea water with a 6 X concentrate
662 Environmental Science and Technology
Figur.e 3. Nucleation experiment Sea vrater at 190" C. Thesc! photographs are time exposures of about 1-min. duration A . Interior of vessel after 1 to 2 min. Note extremely long whitrkers B. After 5 min. Note extremelJ- long whiskers: the blurs are (:rystals which are moving or falling to the bottom of the vessel C. After 15 min. D. After 20 min. Crystals are beginning to convert to a second form E. After 30 min. Partial conversion to second form F. After 50 min. Conversion appears to be completed
hydrite. The authors found no way to recover the extremely long thin needles which formed at first, and therefore no definitive identification was possible. In a series of experiments in which the fat needles were induced to form in various brines having chloride contents
Table I. Brine.
ZCl-
a
Solubility Products for Anhydrite in Sea Water SatuMolal Soluration Calcium Sulfate Temperbility Molalit, Molality ature Product
1.66 1.63
0.913 X 2.54 x 1.17 X lo-? 2.72 x
4.90 4.96
1.325 X lo-? 6.51 X lo-* 1.72 X 6.84 X lo-*
106.5 106.5
2.32 X lo-@ 3.18 X
92 92
8.63 X lo-'' 11.7 X lo-@
Deduced from compositions of test brines (Shaffer, 1967). CalcuIated from cross plot of data reported by Hara et a [ . (1932,
1934).
ranging from 1.66 to 4 . 9 0 z , the temperature at which the crystals would just dissolve in approximately 16 hours was observed. The saturation temperatures thus determined are plotted as a function of the chlorinity of the brine in Figure 4. The points at 1.66 and 4 , 9 z C1 correspond to compositions that also were studied by Hara et al. (1932, 1934). For comparison, the solubility products were calculated from the known composition of the authors' brines, and the results are compared with the data reported by Hara et al. in Table I. The authors' solubility product values are somewhat lower than the values reported by Hara et ai. It appears desirable, therefore, to remeasure the actual solubilities of anhydrite in sea water and sea water concentrates. Literature Cited Hara, R., Nakamura, K., Higashi, K . , Technol. Rept. Toliok-ii Imp. Univ. 10, 433 (1932). Hara, R., Tanaka, Y . :Nakamura, K . , Technol. Rept. Toliokir Imp. Uniu. 11, 199 (1934). Shaffer, L. H., J . Chem. Eng. Data 12, 183 (1967). Received for review Ma). 1, 1967. Accepted July 24, 1967. Volume 1, Number 8, August 1967
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