Manufacture of Sodium Sulfide Reduction of Sodium Sulfate to Sodium Sulfide at Temperatures below 800' C. JOHN F. M. WHITE' AND ALFRED H. WHITE University of Michigan, Ann Arbor, Mich.
HE usual method of manufacturing sodium sulfide is by reduction with carbonaceous material at temDeratures above 884" C., the melting point of sodium & f a t e . Commercial temperatures of reduction are reported to be between 900" and 1000" C. and, since there is a liquid phase present at those temperatures, reduction is rapid. Sulfur-bearing products in the exit gases cause loss of sulfur as well as disagreeable odors, and the lining of the furnace is corroded rapidly. The present work was undertaken to determine whether reduction of sodium sulfate could not be obtained at a lower temperature where there would be fewer objectionable side reactions and less corrosion of the furnace lining. Budnikov and colleagues (1,W, 3 , 4 ) state that reduction by pine charcoal starts a t 750", sugar charcoal a t 800", and Acheson graphite a t 880" C.; that the reaction commences slowly but accelerates rapidly; and that the formation of sodium sul€ide produces a lowering of the temperature of fusion. The reduction with hydrogen commences a t 700" C. Carbon monoxide acts as a slow reducing agent, but its action is catalyzed by nickel. Freeman (6,6)and Muller and Klema (7) note that the addition of sulfide reduces the temperature necessary for reduction of alkali sulfates. Muller and Klema say the temperature may be lowered to 850" C. Freeman states that pure sodium sulfide does not melt but begins to dell) studied the reduction compose a t 1200" C. White (9,10, of sulfate in the regeneration of black liquors from the sulfate pulp process and found that the addition of lime permitted a more rapid reduction at a lower temperature.
Melting Points of Mixtures of Sodium Sulfate and Sodium Sulfide The hints in the literature that there might be a low-melting mixture of sodium sulfate and sulfide were checked experimentally : Anh drous sodium sulfide was prepared from the crystallized Na2S.Qk20by dehydration in a stream of hydrogen at 750" C. The product contained 98.3 per cent sodium sulfide and also visible particles of graphite torn from the vessel in which it had been heated. Several mixtures of sulfate and s a d e were heated simultaneously in a graphite container made by cutting a graphite rod lengthwise and drillin several shallow holes along the flat surface. Small portions ofmixed sulfate and sulfide were placed in these holes, and the graphite rod was then heated in a tube furnace for 45 minutes in a stream of nitrogen and cooled in a stream of nitrogen in the furnace. Fusion was shown by the globular shape of the sample. The results given in Table I show that a t 684" C. there had been no fusion in any of the mixtures, that a t 700" C. there had been fusion of the mixtures containing 34.6 and 42.5 per 1
Preaent address, Canadian Industries, Ltd., Beloeil, Quebeo.
cent sodium sulfide, and that a t 740" C. all compositions between 15;O and 68.9 per cent sodium sulfide had shown a liquid phase. Since sodium sulfate melts at 884" and the sulfide does not melt at 1200" C., it is evident that there is a low-melting mixture which liquefies slightly below 700" C. TABLEI. MELTINGPOINTS OF SODIUM SULFATE-SODIUM SaFIDE MIXTURES AT VARIOUS TEMPERATURES Sample
Initial Compn., % NazS NazSO4
Appearance= after Heating for 45 Min at. 684" C. 700' C. ?40° C.
a 4 indicates that, a liquid phase was formed; 8 indicates that the material remained in the solid phase.
Through the courtesy of Peck (8) the series of mixtures which had been heated at 740" C. was examined petrographically. Peck reported that the optical evidence indicated a eutectic between samples VI1 and VI11 (25.1 and 30.0 per cent sodium sulfide), rather than between I11 and IV (34.6 and 42.5 per cent sodium sulfide) as was indicated by the melting point observations a t 700" C. In all samples which had been exposed to air, except pure sodium sulfate, there appeared a foreign material which was found to be a hydration product of sodium sulfide. It was not present in freshly prepared sodium sulfide but was formed on contact with damp air.
Reduction of Sodium Sulfate by Hydrogen Experiments on the rate of reduction of sodium sulfate in a stream of hydrogen are given in Table I1and are readily explained by the appearance of a mixture melting slightly below 700" C. At 645" C. there is no liquid phase, and reduction is extremely slow because of the low temperature. At 704" C. the initial reduction takes place rather rapidly by hydrogen diffusing through the solid phase. As soon as sufficient sodium sulfide is present to liquefy the mass, the extent of surface exposed to the hydrogen decreases and the reaction becomes slower. In the experiment at 704" C . when the graphite boat was removed after the furnace had cooled, it was found to contain a cylinder of fused salt about 50 mm. in length. The end toward the incoming hydrogen (A, Table 11) which was gray, hard, and rather stony showed 68.0 per cent reduction. The center portion which was pink and
FEBRUARY, 1936
INDUSTRIAL AND ENGINEERING CHEMISTRY
245
S e v e n g r a m s of sugar charcoal 8o were h e a t e d i n vacuum to 725 " C. for 12 hours. This d e g a s s e d TABLE11. REDUCTION OF SODIUM SULFATE BY HYDROGEN charcoal p r o v e d g Time Condition of Product Reduction Temp. to be as rapid a w40 C. Hr. % reducing agent as 18 No sign of fusion 15.6 645 that w h i c h h a d 704 3 Three distinct divisiona: not had its gases A Herd gray 68.0 B.' Gloss;, pink, fused 94.0 removed. D a t a C. Less colored than B 45.1 a r e given i n a 755 1 Single globule, pink 30.4 subsequent paragraph. FIGURE1. RESULTS OF ONE-HOUR HEATING TESTS It had been preReduction by Carbon Monoxide viously observed The rate of reduction of sodium sulfate by carbon monoxide by one of the writers that lime accelerated the rate of reducwas tested. Gas was circulated a t 650" C. with the changes tion, and this observation was retested on a group of mixtures in percentage composition noted in the following table; the in which lime was incorporated. In order to avoid hydration test indicated that carbon monoxide is a slower reducing of the lime, the final grinding of the mixtures was carried out agent than hydrogen: in a small porcelain ball mill; the time of grinding was -Compn., Vol. %FCompn., Val. %usually 2 hours. The mixtures were in the molal ratios of 1 After After sodium sulfate to 4 carbon. When lime was added, it was in Con135 min. Con135 min. stituent Initial at 650' C. atituent Initial at 650° c. the proportion of 2.5 lime to 1 sodium sulfate. There was cor 6.2 17.0 NI 8.0 8.5 thus always an excess of the carbon and of lime. 01 0.0 0.0 H2S 0.0 0.0 GO 85.8 74.5 The results of the tests with solid carbonaceous materials when heated for one hour are given in Figure 1, where twentyReduction of Sodium Sulfate by Solid Carbothree results are shown. The percentage reduction is denaceous Materials h e d as the ratio of sulfide sulfur to total sulfur. The material as drawn from the furnace showed no evidence of fusion The mechanism of the reduction of sodium sulfate by solid unless the temperature had been above 700" C. The reduccarbonaceous materials may be predicted on the basis of the tion by graphite was barely observable after one hour at preceding discussion. When mixtures of sulfate and pure 700" C. and was slight even a t 750" C. Admixtures with carbon are fist heated to temperatures below the melting lime raised it from 2 to 15 per cent after one hour a t 725". point of the sulfate, the reaction will be between two solid Sugar charcoal and wood charcoal did not react rapidly in one phases and will be slow. At temperatures below 700" C. it hour even a t 700" C.; sugar charcoal showed 2.9 per cent will remain slow. Above 700" as soon as the liquid phase apreduction a t 700 " and wood charcoal 1.57 per cent reduction a t pears, the reaction will be much accelerated since the solid 703". These samples where reduction was slight showed no carbon particles will be wetted by a liquid containing unreevidence of fusion. The reaction was much more rapid a t duced sulfate. somewhat higher temperatures; sugar charcoal a t 722 " Experiments coniirmed this theory of the mechanism of the showed 56.3 per cent reduction and wood charcoal at 725" reduction and gave some evidence on the reactivity of the varishowed 64.6 per cent. The residue of these tests showed ous forms of commercial carbonaceous materials. The carbonaceous materials tested were wood charcoal, sugar charcoal, complete fusion. At 750" C. there was 83.7 per cent reducbituminous coal, lampblack, and Acheson graphite. The mation in one hour. The results for the two kinds of charcoal terials were all from commercial sources except sugar charcoal fall so closely on a single curve that no distinction is made which was prepared by heating sugar to 760" C. in a graphite between the two in Figure 1. crucible. The materials were all ground separately in an agate mortar and mixed just prior to use. The graphite container When lime was added to the mixtures of wood charcoal and with the mixed powders was placed in the cool end zone of a tube sulfate, the reaction a t the lower temperatures was much furnace and, when the furnace reached the desired temperature, more rapid, as shown in Figure 1. At 678" C. there was 14.5 was pushed into the hot zone. At the end of the allotted time it was pulled back into the cool zone and then removed from the per cent reduction, whereas without lime only a trace of sulfurnace. All of the mixtures were in the mole ratio of 4 carbon to fide was formed. At 702" with lime the rediction was 41.9 1 sodium sulfate. per cent with no evidence of The initial rate of reduction by Acheson graphite was very fusion, whereas without lime the X CHARCOAL much less than that with any of the other materials. The reduction was only 1.6 per cent. GRAPHITE most reactive materials were bituminous coal and charcoal. At higher temperatures where The coal contained 38.1 per cent of volatile matter, softened the liquid phase appeared in all A LAMPBLACK below 500" C., and coked to a considerable degree below the tests, the advantage of the addifurnace temperature of 680" C. The greater rate of reduction tion of lime was less marked. a 60 w with the coal may be due to intimacy of mixture obtained At 750" the relative figures for during the coking process, to the hydrogen evolved during reduction piere 83.7 for charcoal coking, or to specific reactivity of the carbon itself. Gases alone and 97.9 for charcoal with evolved from the carbonaceous material might also have lime. The dissociation of calplayed a part in the reduction by lampblack, wood charcoal, c i u m c a r b o n a t e a t 750" is and sugar charcoal. The catalytic influence of the ash consufficient to give a partial presstituents in the cases of wood charcoal and of coal had to be sure of ,carbon dioxide of 0.1 taken into account also. atmosphere. The a d v a n t a g e An answer to the question of influence of reducing gases of an addition of lime is slight was sought by the preparation of degassed sugar charcoal. a t temperatures above 725" C.
vitreous showed 94.0 per cent reduction, and the end farthest from the inlet hydrogen had also been fused but showed only 45.1 per cent reduction. It is probable that liquidation caused these variations in composition.
(L
INDUSTRIAL AND ENGINEERING CHEMISTRY
246
and becomes a disadvantage a t higher temperatures. The curve shows that the reduction in one hour a t 780" is less than that a t 753" and substantially the same as at 726". Bituminous coal with added lime proved the best reducing agent as shown in Figure 1; the reduction in one hour was 70 per cent at 700" and 100 per cent at 752". The effect of heating mixtures longer than an hour is shown in Figure 2, where the time of heating is 2 hours. Graphite gave only a trace of reduction in 2 hours at 705" C., and, even when the mixtures were ground in a ball mill for 4 hours and heated to 700" for 4.5hours, there was only 4.4 per cent reduction. With added lime the reduction became 7.6 per cent. Lampblack in 2 hours a t 687" gave only 1.0 per cent reduction, the reaction produced being a loose mass, but a t 712"with the same length of heating the product was fused and the reduction 55.4 per cent. Wood charcoal gave 82.7 per cent reduction and sugar charcoal 100.0 per cent reduction a t 700" in 2 hours. Bituminous coal gave 13.2 per cent reduction at 660 in 2 hours.
Attempts to Measure Equilibrium Conditions for Reactions Involving Sodium Sulfide A number of attempts were made to obtain entropy of sodium sulfide in the region 600" to 800" C. The reactions studied were:
++ +
+ +
NalSOa X C = Naps CO a n d COZ Na2S04 4C0 = Naps f 4C02 Na2S04 4Hz = Naps 4HtO
(1) (2)
(3)
I n the case of reactions 1 and 2, the attempts were unsuccessful because of the slowness of the reaction below 700", the failure of the system to reach a constant pressure, and the side reactions giving free sulfur above 700". In the case of reaction 3 no suitable material for the reaction vessel could be secured. Among the materials tested for their resistance to attack by sodium sulfide at a temperature of 750" were nickel, porcelain, Chrome1 A, alundum, aluminum, quartz, monel metal, tungsten, tantalum, and molybdenum. All of these materials were attacked to a considerable extent by sodium sulfide. In the case of porcelain and alundum there was a reaction giving hydrogen sulfide and sodium aluminate. The stainless steel KA2 (Resistol) was blackened by the sulfide. Graphite was the only material tried that did not react with sodium sulfide, but graphite could not be used for the reaction vessel in the experiments with hydrogen because of its reaction with sodium sulfate.
VOL. 28, NO. 2
The addition of quicklime increases the rate of reaction with all of the solid reducing agents at temperatures below 750" C. because of the removal of the carbon dioxide as solid calcium carbonate. Above 750" the dissociation of the calcium carbonate becomes sufficiently marked to prevent useful reduction of the partial pressure of carbon dioxide. Complete reduction of sodium sulfate may be obtained in 2 hours a t 700" C. and by bituminous coal and quicklime in 1 hour a t 750" C. At 700" there are no complicating side reactions, and, if an excem of carbon is present, the reduction may be carried to completion. Even a t 750" C. fused sodium sulfide corrodes many metals and refractory materials. Graphite is the most satisfactory material, It is evident that complete reduction can be obtained a t much lower temperatures than those reported in commercial practice.
Literature Cited (1) Bodnikov, P., Compt. rend., 197, 60-3 (1933). (2) Bodnikov, P., 2.angew. Chem., 39, 1398-1402 (1926). (3) Bodnikov, P., and Shilov, E., J. SOC.Chem. Ind., 47, 111-13T (1933). (4) Bodnikov, P., and Sisoyev, A. N., 2 . anorg. allgem. Chem., 170, 225-32 (1928). (5) Freeman, H., Can. Chem. Met., 9, 151-2 (1925). (0) Freeman, H., and Canada Carbide Co., Ltd., Britiah Patent 211,488 (Feb. 14, 1924). (7) Muller, W. J., and Klema, F., Austrian Patent 111,838 (Aug. 15, 1928). (8) Peck, A. B., private communication. (9) White, A. H., U. S.Patent 1,565,300 (Dec. 15, 1925). (10) Ibid., 1,575,473 (March 2, 1926). (11) Ibid.,1,580,269 (April 13, 1926). RECEIVED August 3, 1935. Abstracted from a dissertation presented as part of the requirements for the degree of doctor of philosophy in the Department of Chemical Engineering, University of Michigan.
Conclusions It has been shown that there is a low-melting mixture with 30 to 40 per cent sodium sulfide and 70 to 60 per cent sodium sulfate which melts somewhat below 700" C. The reduction of sodium sulfate by hydrogen proceeds rapidly as long as only a solid phase is present but becomes slower when the formation of the liquid phase hinders diffusion of the gas into the mass. Carbon monoxide is not as rapid a reducing agent as hydrogen, Solid carbonaceous materials act slowly until a liquid phase is established after which the reaction becomes much more rapid owing to wetting of the carbon surface by a liquid carrying sulfate. The most active of the solid reducing agents tested was bituminous coal. It it uncertain whether its activity is due to the intimate mixture formed when the coal fuses during the coking process, to the hydrogen and other gases evolved in coking, or to the specific activity of the coke. Charcoal ranks next and its activity seems due to its surface since wood charcoal, sugar charcoal, and degassed sugar charcoal are equally active. Lampblack is less active and graphite the least reactive of the materials tested.
Courtesy, Dow Chemical Cornpanu
CAUSTICSODA FINISHING PLANT
