INDUSTRIAL A N D ENGINEERING CBBMISTRY
676
Vol. 16, No. 7
T h e Manufacture of Sodium Hyposulfite’ By L. A. Pratt MERRIIACCHEMICAL CO.,
T
HE use of sodium hyposulfite as a reducing agent in the application of dyestuffs has made this product of unusual importance. The manufacture of hyposulfite in the form of a 10 to 12 per cent solution is easily effected by treating a solution of sodium bisulfite with powdered zinc or by passing sulfur dioxide gas (free from oxygen) into water containing powdered zinc in suspension. The zinc hyposulfite thus formed is converted into the sodium salt by the addition of soda ash. Many concerns employing large amounts of hyposulfite produce their requirements by one or the other of these two methods. The solution formed in this manner has the disadvantage of being unstable and must be prepared only in sufficient quantity for immediate use. The stable anhydrous sodium hyposulfite powder has become a popular commodity because of its convenience and high strength. Its manufacture is accompanied by difficulties, however, and requires a considerable plant investment. The process of manufacture may be briefly described as follows : GASSING Finely powdered zinc testing 93 per cent Zn or better is suspended in water in a lead container equipped with means for cooling and a stirrer or pump for agitation. Liquid sulfur dioxide contained in a weighing cylinder is gasified and passed into the lead container through a distributor located near the bottom of the apparatus. The gas can be passed rapidly at first, although it is best not to allow the temperature to rise above 30” C. As the reaction proceeds the rate at which the gas is passed must be progressivelydecreased. The amount of sulfur dioxide employed is weighed for the purpose of following a definite rate schedule, but the exact end point is determined by the color of the solution. The color changes from that of metallic zinc in the beginning to black, then to gray, and finally to cream. A yellow color must be avoided, as it indicates decomposition of zinc hyposulfite into free hyposulfurous acid, which is very unstable. The time required for the “gassing” with sulfur dioxide depends upon the particular design of the apparatus employed, but in the plant being described is from 3 to 4 hours. 1 Presented before the Division of Dye Chemistry a t the 67th Meeting of the American Chemical Society, Washington, D. C , ApriI 21 t o 26, 1924.
WATER-
-
BOSTON, MASS.
SODATION The zinc hyposulfite solution is rapidly pumped to a leadlined steel tank provided with a stirrer, and a previously prepared sodium carbonate solution is added in slight excess over the amount necessary to precipitate the zinc. The solution and the suspended zinc carbonate are dropped upon a suction filter located directly below the “sodation tank.” If the precipitation has been carefully carried out, the filtration proceeds rapidly. The cake is washed with a small amount of water to replace the strong sodium hyposulfite liquor and the combined filtrate, and wash water is pumped to a calibrated tank called the “salting-out tank.” The zinc carbonate cake is subsequently washed more thoroughly and the wash water is employed in the next batch. S a L T I N G OUT
This operation is carried out in a jacketed steel tank equipped with a stirrer. The volume of the solution in the tank is noted and a sample taken for analysis. The solution should contain about 15 per cent sodium hyposulfite by weight. A good grade of salt (NaC1) is added to the salting-out tank in an amount corresponding to 0.29-0.30 kg. for each liter of solution. The hydrated crystals of sodium hyposulfite (Na&04.2HzO) separate from the solution as the salt dissolves. The mass is then heated rapidly to 60’ C. and the temperature maintained at this point until all the crystals have changed to the anhydrous form (Xa2S201). This point may be easily determined by examination of a sample withdrawn in a large test tube. The hydrated crystals are small needles which remain suspended for some time in the solution, while the anhydrous form consists of heavy, sandy crystals which settle out rapidly. When the change has been effected the temperature is lowered to 55” C. and the stirrer is stopped to allow the crystals to settle. The transition of hydrated crystals to anhydrous crystals is 52” C., and it is best not to drop the temperature quite to this point. The supernatant liquor is siphoned off, and hot denatured alcohol is added to the salting-out tank. The alcohol and hyposulfite crystals are agitated for several minutes by means of the stirrer and then dropped through the bottom outlet into the drier.
A
m
July, 1924
I S D U S T R I A L A N D ENGINEERIXG CHEMISTRY DRIER
A Buffalo cast-iron, vacuum crystallizing panis employedfor the drying operation. A high vacuum is important in order to avoid loss by decomposition. As soon as the crystals have settled, the hot alcohol wash is siphoned off and a second and third hot alcohol treatment is given the crystals. After each alcohol addition the stirrer is started and the contents of the drier are thoroughly agitated. The crystals are allowed to settle and the alcohol siphoned off as in the case of the first wash. The drier is then closed, the stirrer placed in motion, and the vacuum applied. Low-pressure steam (5 pounds) is admitted to the jacket and the drying proceeds until no alcohol is seen in the sight glass beyond the condenser. At this point the temperature in the drier is about 90" C. and the material has become somewhat dusty. The steam is turned off and cooling water circulated until the product
677
has been cooled to 40" C. or lower. The vacuum is then released and the contents of the drierare discharged into s h i p ping containers and sealed. The product is a light gray, sandy material testing 85 to 90 per cent Na&04. Special precautions must be taken to avoid alcohol losses in the recovery and rectification. The mother liquor from the salting-out process contains considerable sodium hyposulfite and must be further treated, if the highest efficiency is to be obtained. Ac KN OWLED GB.IENT The writer acknowledges his great indebtedness to his colleagues in the Research, Engineering, and Manufacturing Departments of the Merrimac Chemical Company who played an important part ip the development of the process which has been described.
T h e Microchemistry of W h i t e Pigments and Inerts as T h e y Occur Mixed in Paints' By Henry Green N G W JERSEY ZINC
c o . , PALMERTON, PA.
be found in the literature,Z HE development of a A microchemistry of pigments is helpful in making paint analyses, but the best plan for the bernicr o c h e m i s t r y of and also in studying the nature of complex pigments, such as lithoginner who wishes to idenp a i n t pigments is pone and leaded zinc oxides. tify pigments with the mijustifiable for a t least two I n this paper are described the mefhodsfor the necessary preparacroscope is to mount them reasons: first, it is an aid tion of the sample for analysis; microchemical tests on individual and examine for himself as to the paint chemist in pigments; and a scheme for the qualitatioe determination (by the use many different specimens as making paint analyses; and of the indioidual tests) of the components of a theoretical mixture of he is able to secure. second, it is invaluable to all the white .Digments and inerts ordinarily found in paints. I n pigment microchmthe pigment chemist for istry tests are made for the studying the nature of compigments themselves and not for their component parts, thus plex pigments, such as lithopone and leaded zinc oxides. The microscopist who has studied the sizes and shapes making it somewhat distinctive from the usual form of microof various pigment particles finds no great difficulty in iden- chemistry. For instance, tests are made for basic lead carbontifying a pigment if it occurs alone, but an entirely different ate or basic lead sulfate and not for Pb, COZ, or SOa. The pigstate of affairs arises when mixtures of pigments must be ments must not be destroyed by solution unless it is desired to eliminate them, as explained further on. considered, as is usually the case in paints. Since this is perhaps the first attempt to formulate a microIf we are working with a mixture that contains but two pigments, say, in the proportion of 50 per cent of one to 50 chemistry of pigments, this paper will necessarily be of an per cent of the other, then it is often easy to identify each elementary nature and, no doubt, somewhat unsatisfactory pigment by simple microscopic inspection, without resorting as far as completeness is concerned. For instance, though to microchemistry; but if the mixture should be 95 per cent it is possible to detect ultramicroscopic layers of zinc oxide of one and 5 per cent of the other, then the 5 per cent ingredient on the surface of lithopone particles, yet no methods exist is likely to be overlooked, unless it possesses an unusually large for the detection of layers of moisture, adsorbed gascs, and water-soluble salts. There are also a few combinations of percentage of characteristic particles. Microscopically, a pigment is composed of two kinds of pigments (and also inerts) which are practically impossible particles-one, characteristic, and the other, characterless. t o analyze microchemically. Notwithstanding these shortThe chitracteristic particles serve as earmarks for the identi- comings, however, the subject is sufficiently far advanced fication of the pigment to which they belong. The character- to be useful as it is. less particles are small and roundish, and in appearance very (a) PREPARATORY PROCESSES much the same in all pigments. Consequently, if a mixture contains but a small percentage of one pigment, then the Four preparatory procesqes are necessary (as a rule) for number of characteristic particles of that ingredient present a complete analysis : will be so few that they might not be found under the micro1-The removal of the vehicle from the pigment of a paint. scope, and so the pigment remains unidentified unless the Place on a microscope slide a small drop of the paint to be investigator resorts to microchemical means. examined. Add to it a drop or two of redistilled turpentine Descriptions of some of the characteristic particles can and rub out, with a glass rod, to a thin, even layer extending from one end of the slide to the other. Place the slide on 1 Presented under the title "The Microscopy of White Pigments as They Occur in Paints," before the Section of Paint and Varnlsh Chemlsa hot plate (about 150" C.) and evaporate the turpentine.
T
t r y a t the 67th Meeting of the American Chemical Society, Washington, D. C., April 21 to 26, 1924.
2
Green, Chem. Met. Eng., 28, 53 (1923).
