Patent GB-653733-A (GB)
Title: Improvements in or relating to growing crystals
IPC: C30B7/00
Filed: 1946-11-07
Abstract: <PICT:0653733/III/1> <PICT:0653733/III/2> <PICT:0653733/III/3> Crystals of ammonium dihydrogen phosphate and its isomorphs, of Rochelle salt, and of lithium sulphate are grown by providing a solution of the salt-to-be-crystallized chemically conditioned to inhibit crystal growth along at least one but not all of the axes of the crystal, introducing into the solution a seed body having extensions in the directions of inhibited growth about equal to those of the required crystal, and causing accelerated deposition of clear crystal material on the seed body in the direction of uninhibited growth by maintaining a substantially higher degree of supersaturation in the solution than is operable when crystallizing from a solution which is not chemically conditioned, e.g. by lowering the temperature of the conditioned solution faster than is possible with a non-conditioned solution as carried out in the prior art exemplified by the subject-matter of U.S.A. Reissue 19697.  Ferric, chromic, aluminium and barium ions are inhibitants for ammonium phosphate; the same ions except for barium and with the addition of lead are inhibitants for potassium phosphate; copper salts are inhibitants for Rochelle salt; while in the case of lithium sulphate, inhibitation is effected by pH-adjustment.  In an example, ammonium phosphate is dissolved in hot distilled water until a specific gravity of about 1.1963 at 60 DEG  C. is obtained.  Iron filings are dissolved in aqueous phosphoric acid, and the resulting solution is added to the salt solution until it contains from 0.06 to 0.12 grams of iron per litre.  The pH of the solution is adjusted (if necessary), to 4 or below, and it is poured into a preheated tray at about 43 DEG  C., in which are mounted seed plates 5 (Fig. 5), with sides 1 5/8 inch in length, cut from previously grown crystals of ammonium phosphate.  The tray is rocked so that the solution flows to and fro over the plates at right angles to the Z-axis, and the temperature is lowered rapidly to 41 DEG  C.  The rate of cooling is now reduced by supplying heat to the tray to partially compensate for the heat loss therefrom until the solution cools to about 40.5 DEG C.  Automatic control apparatus is then used to control the amount of heat supplied so as to cause the temperature of the solution to drop at a uniform rate of about 1.7 DEG  to 2 DEG  C. per day until seed crystals such as shown in Fig. 6 are produced.  These seed crystals which have a smooth hard surface are replanted in another tray.  An aqueous solution of ammonium phosphate having a specific gravity of 1.213 at 60 DEG  C. is prepared; and aqueous iron phosphate is added to it until it contains 0.06 to 0.08 grams per litre.  The pH is adjusted to within the range 3.9 to 4.1; and the solution at a temperature of about 53 DEG  C. is introduced into the tray which should be at about 46 degrees.  The tray is rocked so that the solution flows to and fro parallel to the Z-axis of the seeds, and temperature is then lowered from 49 DEG  to 22 DEG  C. at 0.4 DEG  C. per day for 3 days, 0.6 DEG  C. per day for 3 days, 0.8 DEG  C. per day for 3 days, 1 DEG  C. per day for about 3 days, and 1.2 DEG  C. per day for about 15 days to give crystals such as that shown in Fig. 1.  In order to produce lithium sulphate crystals it is desirable to provide seed material p  by growing seed crystals in a solution whose pH is below 5.3 which permits ready growth in the direction of the a axis, and then cutting seed plates from such crystals perpendicular to the a faces.  These plates are then planted in a solution with a pH above 5.9, which restricts growth in the direction of the a axis and permits accelerated growth parallel to the a faces. Specifications 653,736 and 653,737 are referred to.