INDUSTRIAL A N D ENGINEERING CHEMISTRY
364
Vol. 1.3, KO. 4
Evaporator Design' Influence of the Properties of Aluminium Sulfate Solutions By W. L. Badger and J. S. France UNIVERSITY
OMMERCIAL aluminium sulfate-commonly
OF
MICHIGAN, ANN ARBOR,MICH.
but er- solution gave a total content of 56.5 per cent and a freezing roneously called "aIum"-is ordinarily made by con- point of 68" C., but it was too viscous to get a density readcentrating to such high densities that the resulting ing. It is believed that it was about 1.79 sp. gr., in which solution solidifies, when cold, to a cake hard enough to grind. case the curve for total solids is not drawn steep enough. This calls for concentration to about 60" BB. The solution The results are purposely plotted on a small scale, to prevent is usually boiled in open lead-lined tanks, heated by lead coils. any tendency to read the curves closer than the accuracy of Before attempting to design an evaporator for such work, it the work warrants. was necessary to obtain data on the relation between density It isevident from the data presented that a vacuum evaporaand total solids in strong solutions of aluminium sulfate. A tor cannot be applied to this problem, since a t 55" BB. the difficult point in the process is the fact that these strong solu- solution freezes a t 85" C. These solutions are so viscous tions contain less than enough water to supply water of crys- near their freezing point that the margin between freezing tallization for the salt they contain, and therefore they and boiling points a t atmospheric pressure is necessary to solidify a t rather high temperatures; hence, freezing points permit handling the material. If the solution is boiled were needed. Elevation of boiling point was also of interest at atmosQheric pressure, steam of at least 25 Ibs. gage in designing this evaporator. would have to be used for heating; and if a greater temperaA search of the literature revealed very little information. ture drop could be obtained, it would be an advantage. No data on freezing points or boiling points could be found. All handbooks and reference works quoted determinations of 6b specific gravity by Preuss,2 who only worked up to 25 per I 60 cent solids a t 45" C.; and by Gerlach,3 who worked only at 15" C. The only other information available was a stateSO ment4 that commercial aluminium sulfate for water purificaI 45 tion should contain not over 47 per cent water. 40 The time available for this work was limited, and approxi35 mate results would be of use if they could be obtained at l l l l l l l l l l once. It has not been feasible to repeat this work with more I l l care, and the results are therefore published for the convenience of those who would rather have approximate data than none a t all. It is hoped that someone may be interested enough to repeat this work more accurately. Solutions were made from a laboratory grade of aluminium sulfate of better color than the commercial article-hence, probably fairly free from iron-but not an analyzed reagent. The solution was put in a large test tube of Pyrex glas~.(2~/2 x 10 in.) and heated by a glycerol bath. Boiling and 65 freezing points were read with an uncalibrated thermometer, 60 uncorrected for emergent stem. Density was read at or 6. near the boiling point with an uncalibrated hydrometer J'o spindle, and was not corrected for temperature. I 45 The solutions were frozen by bubbling air through them, I 40 which probably concentrated them somewhat, but this has i JS not been allowed for, as the samples for analysis were removed 102 104 M 6 I06 1/0 /le ii4 //6 JIB I Z O ' C Boiling P o i n t before freezing. Analyses were made by heating a weighed sample just below red heat. Even with small samples and All fittings of this evaporator must be very simple, and the large crucibles, the samples puffed up into bulky and friable discharge would be best handled by a steam-jacketed pipe crusts which extended over the sides of the crucibles and leading straight to the cooling floor. One obvious possicould not be handled without loss. The purpose for which bility is a multiple-effectfevaporator operating with backward the work was done did not warrant the expenditure of time feed-i.e., thin liquor fed to the last effect and finished in or funds on a better analysis. the first. The objection to this is that the small temperaIt is believed that boiling points and freezing points have ture drop regularly occurringIin the first effect of a multiplean absolute accuracy of * 1O C., with the results probably low effect evaporator will not be sufficient to keep such viscous rather than high. Their relative accuracy is considerably material moving. The loss of available temperature drop greater. Total solids are accurate to * 1per cent, and density from elevation in boiling point of the solution in the first to * 0.05 per cent. Boiling points seem to indicate a change effect would be too great. Hence, the only practical way of in phase at about 1.50 sp. gr., though this is not confirmed by handling this material is to concentrate part way in a multiplethe other curves. The freezing point of the densest solution effect, preferably with backward feed, and finish in a singlemeans little, as the solution was extremely viscous, and even effect pan, heated by steam a t as high pressure as possible. when hot the hydrometer reading was questionable. One The properties of solutions of aluminium sulfate will prob1 Received August 19, 1922. ably not be studied again in the near future, so that the 2 Bev., 17 (1884),288. field is open to anyone who cares to repeat this work using 8 Z. anal. Chem., 28 (1889),493. more accurate methods. 4 Anon., Mu?zic. J. Eng., 31 (1911),263; C.A , , 6 (1911), 3487.
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