TECHNICAL REVIEW
Sulfur Compounas in Alkaline Pulping Richard G. Barker Union Camp Corporatmn, Princeton, New Jersey
Richard G. Barker is Director, Research & Deuelopment Projects for Union Camp Corporation, Princeton, N.J. He received his B.A. from Hamilton College (1958) and M.S. (1960) and Ph.D. degrees (1963) from the Institute Of Paper Chemistry. He joined Union Camp in 1962 a s a research scientist in Pulping research. He is the author Of a number Of papers in the Pulping, and papermaking areas, and has five patents in ‘the pulping Process field. He is a member of the American Chemical Society and the Technical Association Of the and Paper Industry.
1
Introduction In the paper industry today one of the most active fields of research is research in pulping. The reasons for this are twofold the major cost in the production of pulp is the cost of the raw material-wood-which is escalating very rapidly, and there is increased pressure to reduce the emissions of sulfur compounds which can occur during the pulping and recovery of the chemicals used in pulping. Kraft is the major pulping process because the chemicals used, sodium hydroxide and sodium sulfide, can produce pulp from any wood species, can produce the strongest pulp, and are easily recovered after concentration and burning of the spent liquors. However, these chemicals are not very specific for removing the lignin from wood and releasing the cellulosic fibers, and in producing kraft pulp there is considerable degradation and solution of polysaccharide components in the wood. Hence the yield of pulp is not as high as desired, which becomes more important as the cost of wood increases. In addition, during the pulping, evaporation, and recovery in the kraft process there can be emissions of hydrogen sulfide and organic sulfur compounds if the pro-
Research in kraft pulping is primarily directed toward methods of increasing the yield of cellulosic fiber from wood and reducina the emissions of sulfur from the recovery process. The two major methods that have been advanced for increasing the yield of kraft pulp are hydrogen sulfide pretreatment and polysulfide pulping. Progress in elucidating the mechanisms of the stabilization of polysaccharides to alkaline attack by these modified sulfur systems is described in this paper, along with methods of generating the pulping liquors.
Table I. Polymers Lost in Kraft Pulping
Qy3
ComDonents lost. % on wood H
-
I
L
-
O
a
-
CelluGalactoLignin, Arabino- glucolose % xylan, % mannan, % %
-
H-C-
Low-temperature losses High-temperature losses
I OR
Figure 1. Major linkage in lignin. radation of the lignin preceding solubilization. This bond can be cleaved by hydroxide but the reason the kraft process is preferred over the soda process is that the hydrosulfide ion is a stronger nucleophile than the hydroxide. These &aryl ether bonds are cleaved much more rapidly in kraft pulping than in soda pulping. The major reactions of the sulfide and hydroxide with lignin have been summarized recently (Gierer, 1970). Gierer also summarized the condensation reactions of the partly degraded lignin which can take place and counteract the lignin cleavage reaction. Various carbanions formed in the degraded lignin could compete with the nucleophiles present in the cooking liquor (such as HS-) for the reactive sites in the lignin degradation products generated during pulping. These carbanions could compete particularly for methylenequinone intermediates. Thus again, the reason for the hydrosulfide, the strong nucleophile can further promote delignification during kraft pulping by protecting the intermediate methylenequinone structures from such condensations. In addition to reactions with lignin, hydroxide reacts with the polysaccharides of wood leading to yield loss. This happens early in the kraft pulping sequence as indicated in Table I, taken from a recent study (Matthews, 1973). Early in kraft pulping before any lignin of significance is lost, about 13% of the wood weight as polysaccharide (galactoglucomannan and cellulose) is lost. Most of this is lost by an end group attack a t the terminal reducing unit of polysaccharides as illustrated in Figure 2. This is commonly called the peeling reaction of polysaccharides. After isomerization of the terminal reducing sugar unit, a fructose terminal unit is formed, thus putting the glycoxyl group of the chain in the P-position to the carbonyl. The chain can then be eliminated, thus being one sugar unit shorter and the free unit undergoes further reactions in alkali to an cw,@-dicarbonyl intermediate, followed by a benzoic acid type of rearrangement to form, in this case, the isosaccharinic acid (Meller, 1965). The new chain is one unit shorter and has a new terminal reducing unit which is also subject to attack. There is thus the peeling down of the chain until a stabilization reaction occurs. There is another reaction which proceeds a t a much slower rate which can lead to a stable metasaccharinic acid unit attached to the chain. This involves elimination of a hydroxyl ion from C-3 which is in the P-position to the carbonyl group in the terminal unit.
4
0
9
4
19
2
2
1
3-Deoxy-D-glucosone is the a,@-dicarbonylintermediate in the formation of the metasaccharinic acid. Most pulping studies within the context of the kraft system are aimed at reducing this peeling reaction so that less cellulose and hemicelluloses are lost during kraft pulping while still having the hydrosulfide ion available for the increased rate of delignification. There have been two major approaches to achieving this through modification of the sodium-sulfur system. The first is that of pretreatment of wood with hydrogen sulfide before kraft pulping while the second is pulping with polysulfide instead of sulfide. Stabilization by Hydrogen Sulfide P r e t r e a t m e n t Over the past 5 years, a new method which was developed for increasing the yield of kraft pulp consisted of pretreatment of wood with hydrogen sulfide in the presence of an alkaline buffer (Vinje and Worster, 1969; Hartler and Olsson, 1972). This is a process which has now been carried successfully through pilot plant operation (Cox and Worster, 1972). Studies (Procter and Wiekenkamp, 1969a; Procter, 1971; Wilson and Procter, 1972) have been directed at the mechanism of the stabilization treatment. These studies indicated that the majority of the pulp yield increase after hydrogen sulfide pretreatment was due to a considerable reduction in the degradation of glucomannan, the major hemicellulose in softwood. Procter’s generalized mechanism for the reductive thiolation of the reducing end group of a polysaccharide is shown in Figure 3. Reaction of hydrogen sulfide with a reducing end unit leads to the formation of l-thioD-glucitol as an end unit attached to the chain. Reaction of the simple sugar D-glucose with hydrogen sulfide has been studied under similar conditions through the wood stabilization treatment and led to the isolation of 1-thio-D-glucitol. This modified end group is much more stable to alkaline attack than is the simple reducing end group. The system has been studied with different cations present and it was found that the bivalent cations are much more effective in the reductive thiolation than is sodium. The proposed mechanism could account for the catalytic effect of the divalent metal ion. Thiols can form strong salts with metal ions, and structure 111, the gem-dithiol, could influence the stabilization of this structure and the equilibrium reaction with compound 11. Thus, if the concentration of the dithiol intermediate was a rate-controlling step in the overall reductive thiolation, this would explain the catalytic effect of the divalent cation. In the overall reaction mechanism shown in Figure 3, structure 11, the hydroxythiol, would be in equilibrium with Ind. Eng. Chem., Prod. Res. Dev., Vol. 15,No. 1, 1976
19
H
I
c =o I
HCOH
--t
HO-4H
I
HC-Ojchaln)
1
I
I
HCOH
CH20H
I
C%OH
CHZOH
@H
- __ OH-
K c - H
HA0 (chain)
1
"p"
C5OH
H I C==O
H M I
I
COH
b
HC-O(cha1n)
C=O
coo I _c
I
HP
Hh(chaln)
I
I
HCOH
HCOH
CI V H
kH+H
CH(0H)
I
HY
Hb-O(chaIn)
I
HCOH CHgH Metasaccharlnlc Acid
Figure 2. Alkaline reactions at terminal reducing unit of polysaccharide (3).
II
I
4.' I
RO
Neither of these ions will, of course, have any effect in stabilization of polysaccharides and increasing the pulp yield. For the mechanism of the alkaline degradation of polysulfide, Teder (Gustafsson and Teder, 1969) drew an analogy between the alkali-induced cleavage of the S-S bond in organic disulfides. With organic disulfides, the initial reaction as in reaction 2 would be the splitting off of hydrogen to give a sulfenic acid. R-S-S-R
+ OH-
G
+ RS-
RS-OH
(2)
This then rearranges to disulfide and sulfonic acid. For a sodium polysulfide ion, there would initially be nucleophillic attack on the S-S bond by the hydroxide as in reaction 3. SX2-
k-
+ OH- * S,-1 SO2- + HS-
(3)
The sulfenic acid derivative would then be converted to thiosulfate such as by reactions 4 and 5.
S,-1S02-
V
+ OH-
+
S,-2SOz2-
+ HS-
(4)
Figure 3. Reaction mechanism for reductive thiolation (9).
the aldehyde end group. The gem-dithiol (structure 111) would be in equilibrium with the thioaldehyde shown as structure IV. The two electron reducing steps with the thioaldehyde then leads to the 1-thio-D-glucitol end group. The thioalditol is not susceptible to the alkaline peeling attack.
Stabilization by Reaction with Polysulfide If the sulfide in kraft liquor is converted to polysulfide or replaced by polysulfide, then there is less degradation of the polysaccharides in pulping and increased yields are obtained (Sanyer and Laundrie, 1964; Teder, 1969). Under certain circumstances, sodium polysulfide is not stable. These circumstances coincide with the conditions utilized in alkaline pulping: high temperature and high hydroxide concentration. Hence if sodium polysulfide is heated in caustic, it is converted as shown in reaction 1 to sulfide and thiosulfate. SX2-
20
+
(X
- 2)OH-
--*
("-') SH- +
Ind. Eng. Chem., Prod. Res. Dev., Vol. 15, No. 1, 1976
Sanyer (Sanyer and Laundrie, 1964) followed the composition of the sulfur compounds without wood present and with wood present. He demonstrated that in the presence of wood components there is a higher proportion of the polysulfide converted to sulfide and less to thiosulfate than if wood is not present. Thus in the presence of wood, polysulfide must be reacting as an oxidizing agent. The reaction first suggested (Olsson and Samuelson, 1966) would be a direct oxidation of the reducing end group to an aldonic acid. This is shown in reaction 6. (X
- 1)RCHO
+ SX2- + ( (X
2 -~ 3)OH- 1)RCOO- + xSH-
+
+
(X
- 2)H20 (6)
If the mechanism of polysulfide stabilization of polysaccharides were a simple direct oxidation of the end group then if glucose were treated with polysulfide it should be converted to gluconic acid. However, it has been shown that when glucose is treated with polysulfide the saccharinic acids are obtained in addition to erythronic acid, arabinonic, ribonic, and mannonic acid (Abenius et al., 1967). If cellobiose or hydrocellulose are used, then the end group units are converted to all of the above acids plus mannonic acid (Ahlgren et al., 1968). When polysulfide pulps were
analyzed (Alfredsson and Samuelson, 1969), the major end group on the pulp was mannonic acid, hence the reaction is far from a simple oxidation. Theander (Abenius et al., 1967) suggested that the primary oxidation would be D-glucosone. For a cellulose end group this would be as shown in reaction 7. CH?OH -01
Table 11. Hydroxide Formed in Sulfide Oxidation ~
____
Na,S, g/l. NaOH, g/l. Na,S oxidized, mol/l. NaOH formed, mol/l. NaOH predicted, mol/l.
Initial concn
Final concn
79.3 5.8
21.1 64.9 0.746 1.48 1.49
...
...
reaction with additional sulfide in the liquor to give the polysulfide as shown in reactions 10 and 11. xNazS This could then go to mannonic acid on rearrangement or arabinonic acid by fragmentation. These would be more stable than the reducing end group to alkaline attack. Formation of Sodium Polysulfide Sodium polysulfide can be formed simply by dissolving sulfur in a kraft liquor solution containing sodium sulfide as in reaction 8. (x
- 1)s+ Na2S
-
NazS,
-
-
(9)
(10) (11)
This is a feasible process since the spent manganese oxidant is insoluble in the polysulfide liquor and can be physically separated. It can then be regenerated to a higher oxide form for reuse simply by reaction with oxygen in air as shown in reaction 12.
(8)
This was how polysulfide was first formed for the initial pulping studies which demonstrated that yield could be increased over the kraft process. However, as mentioned earlier, after polysulfide reacts with wood, it is converted to either sulfide or thiosulfate. When this spent liquor stream after pulping is then concentrated and burned in a conventional kraft recovery cycle, all of the added sulfur is then converted to sodium sulfide. Hence the sodium/sulfur balance of the kraft system has been upset. On the next cycle, in order to have polysulfide pulping, additional sulfur would have to be added. This leads to a gradual increase in the sulfur/sodium ratio and gradually increasing emissions of hydrogen sulfide and mercaptans in pulping and recovery of chemicals. Because of the concern for emissions, this is not an acceptable commercial process in this country. Basically, two approaches have been taken to conducting polysulfide without having an increase in sulfur/sodium ratio. One approach is to stay with sulfur addition to white liquor but remove hydrogen sulfide a t a later point in the process, such as from the spent liquor or black liquor directly after pulping (Fogman, 1972). Alternatively, the hydrogen sulfide could be removed from the green liquor, which is the liquor obtained after solution of the smelt from the kraft recovery cycle (Ferrigan 1965). Either of these could then convert the hydrogen sulfide to free sulfur by the well known technology of the Claus process. The alternative approach is to directly convert the sulfide present in kraft liquor to a polysulfide form by some specific oxidizing means. Electrolytic oxidation of alkaline sulfide was one approach taken (Sanyer, 1968), but it was found that essentially a pure sulfide solution had to be employed without the presence of the caustic which exists in kraft liquor. This is a feasible route, but a high capital process and high power demand process. The most specific oxidant known at this time for converting sodium sulfide to sodium polysulfide with little thiosulfate formation is manganese dioxide (Barker, 1970). A process based on manganese dioxide oxidation of the sulfide in kraft liquor to polysulfide, and regeneration of the manganese oxidant, was taken through the pilot plant stage (Barker and Ma, 1973). The overall oxidation reaction can be formulated as shown in reaction 9, or alternatively this could be broken down to an initial oxidation of some of the sulfide in the kraft liquor to elemental sulfur followed by
-
+ ( x - 1)MnOz + ( x - 1)H20 Na2S, + ( x - 1)MnO + (2x - 2)NaOH NazS + MnOz + H 2 0 S + 2NaOH + MnO ( x - 1)s+ NazS NazS,
MnO
+ '/~02
-
MnOz
(12)
As shown in Table 11, the hydroxide formed from an oxidation of sodium sulfide with manganese dioxide is that predicted by reaction 10. Recent work (Morud and Rolland, 1972) has shown that cupric oxide can also be somewhat effective in converting a kraft liquor system to a polysulfide system, The chemistry of the copper oxide system is quite different from the manganese oxide system, as is shown by reactions 13 through 15. CuO
+ NaaS + HzO 2cus Na2S
-
-
CuS
s
CUZS -k
-
+ ( x - 1)s
+ 2NaOH
(13)
(14)
NaZS,
(15)
The cupric oxide reacts with sodium sulfide to be converted first to cupric sulfide which then decomposes to cuprous sulfide and sulfur. The sulfur can then react with additional sulfide to give the polysulfide. However, in this case, 50% of the reacted sulfide has been converted to the insoluble cuprous sulfide. This is then removed from the system. In order to have a regenerative process, the cuprous oxide can be roasted to give cupric oxide, but in the process sulfur is converted to sulfur dioxide and is removed from the system as shown in reaction 16. cuzs
+ 20.2
-
2cuo
+ so*
(16)
Two other oxidants that can convert sodium sulfide to sodium polysulfide are ferricyanide and persulfate. As shown in reactions 17 and 18, each of these reagents is capable of oxidizing sulfide to sulfur which can then react with additional sulfide to form polysulfide.
-+ s*-+ s*o**-s +
S2-+ 2Fe(CN)&
S 2Fe(CN)6*2s042-
(17) (18)
Table I11 presents sulfide oxidation studies wherein the hydroxide concentration was varied. It can be seen that with both ferricyanide and persulfate, in the presence of the same concentration of hydroxide as is normally present in kraft white liquor, the efficiency of the oxidation (as measured by the conversion of sulfide to polysulfide divided by the sulfide oxidized) has decreased considerably. Thus with high hydroxide, the oxidants are probably not as specific Ind. Eng. Chem., Prod. Res. Dev., Vol.
15,No. 1, 1976 21
Table 111. Sulfide Oxidation Initial Na,S
Effective alkali,
Oxidant
gPl
gPl
Ferricyanide Ferricyanide Persulfate Persulfate
40 39
15.9 87.8 15.9 83.3
40
37
Polysulfide sulfur, gP1
Final Na,S, gP1
Sulfide to polysulfide/ sulfide oxidized
6.8 3.7 7.4 4.3
13.1 16.0 17.2 18.9
60.8 37.4 78.8 57.8
Table IV. Catalytic Air Oxidation
Catalyst MnO, MnS NiS ~
Initial Na,S, gPl 38 38 32.7
~~
Oxidation time, hr 3 1.5
4
Final polysulfide sulfur, gPl 6 7.5 7.1
1 50-
1
"1
60
\, Kraft
\
O
o
\
1 0
Oxldatlon Durlng Impregnat ion Control w i t h Black Llquor
"'1 Figure 4. Effect of persulfate addition t o kraft pulping.
+ 8Fe(CN),j3- + 60H+
2s2- 4s208'-
+ 60H-
-
+
+
+
S ~ 0 3 ~ -3Hz0 8 F e ( c N ) ~ ~ -(19)
sz03'-
+ 8S042- + 3Hz0
can be utilized to obtain an increase in pulp yield. However, this is not an efficient process and a high total sulfur level must be utilized as considerable quantities of the sulfide are oxidized to thiosulfate (Landmark, 1968). The oxygen in air can oxidize sulfide to sulfur as in reaction 21 2NazS + 0
Catalytic air oxidation has also been examined as a means for converting a kraft liquor to a polysulfide liquor. If small amounts of metal oxides or sulfides are present during the air oxidation of a sulfide liquor, polysulfide sulfur can be formed as shown in Table IV. A somewhat similar process was recently described with the major differences being that the air and the sulfide streams are separated by a finely divided carbon impregnated with metal salts on a Teflon support (Smith and Saunders, 1972). It was indicated that sulfide could be converted to polysulfide with little thiosulfate formation through this means. Oxygen does not react with sulfide in kraft white liquor at a very high rate unless a catalyst is present. However, in the presence of degraded wood components such as spent liquor or black liquor, the oxidation can proceed quite readily. If spent kraft liquor and white liquor are mixed and the mixture is oxidized with air, the resulting liquor Ind. Eng. Chem., Prod. Res. Dev., Vol. 15, No. 1, 1976
2
-
+ 2H20
2s
+ 4NaOH
(21)
but because of its nonspecificity, it could also oxidize either polysulfide to thiosulfate as shown in reactions 22 and 23.
(20)
22
I 60
5s
Figure 5. Effect of oxidation during impregnation.
and thiosulfate is formed such as by reactions 19 and 20. As shown in Figure 4, however, if persulfate is added to the normal kraft liquor before pulping, a yield increase over kraft at an equivalent degree of delignification can be obtained. 2S2-
Total W e l d , X
510
2NazS2 + 302 2NazS
+ 2 0 2 + HzO
2NazS203
Na2S203
+ 2NaOH
(22)
(23)
Any sulfite which might be produced in an air oxidation would immediately react with the polysulfide to remove additional polysulfide from the system as in reaction 24. (x
- 1)Na2S03 + NaZS,
F?
NazS
+ ( x - l)Na2Sz03
(24)
This led to the concept of directly introducing oxygen into the alkaline sulfide mixture while it was in contact with wood (Barker, 1973). By this means, the polysulfide would have the opportunity of reacting with wood components as soon as it was formed. As shown in Figure 5 , this direct oxidation of the system while in contact with the wood leads to increased screened fiber as a given total yield. Literature Cited Abenius. P. H.. Ishizu, A,, Lindberg, E., Theander, 0..Svensk Papperstid., 70, 612 (1967).
Ahlgren, P., Ishizu, A., Szabo, I., Theander, O., Svensk Papperstid., 71, 355
(1968). Aifredsson, 8.. Samuelson, O., Svensk Papperstid., 72, 361 (1969). Barker, R. G.. Tappi, 53 (6),1087 (1970). Barker, R. G.. Ma, J. L., Tappi, 56 ( 5 ) . 112 (1973). Barker, R. G., U.S. Patent 3 723 242 (1973). Cox, L. A,. Worster, H. E., Pulp Paper Mag. Can., 73 (9),106 (1972). Ferrigan, J. J., Coppick, S., US. Patent 3 210 235 (1965). Fogman. C. D., Paper Presented at 1972 Alkaline Pulping Conference, Memphis, Sept 1972. Gierer, J., Svensk Papperstid., 73 (le),571 (1970). Gustafsson. L., Teder, A., Svensk Papperstid., 72, 249 (1969). Hartler. N., Olsson, L. A., Svensk Papperstid., 75 (13),559 (1972). Landmark, P., ATlPRev., 22 (l),87 (1968). Matthews, C. H., "Carbohydrate Losses at High Temperature in Kraft Pulping", Paper Presented at ACS Meeting, Chicago, Aug 1973. Meller, A., Tappi, 48, 231 (1965). Morud, B., Rolland, K.. Norsk Skogind., No. 4, 1 (1972). Olsson, J. E.,Samuelson, O., Svensk Papperstid., 69,703 (1966).
Procter, A. R., J. Appl. Poly. Sci., 15, l(1971). Procter, A. R., Wiekenkamp, R. H., Carbohydr. Res., I O , 459 (1969a). Procter, A. R., Wiekenkamp, R. H., J. Polym. Sci. (C. Polym. Symp.) No. 28,
1 (l969b). Sanyer, N., Tappi, 51, (8), 48A (1968). Sanyer, N., Laundrie, J. F., Tappi, 47, (lo),640 (1964). Smith, G. C., Sanders, F. W., German Patent No. 2 151 465 (1972). Teder, A., Svensk Papperstid., 72 (9),294 (1969). Vinje. M. G..Worster, H. E., Tappi, 52 (7).1341 (1969). Wilson, G., Procter, A. R., Pulp PaperMag. Can., 73 (l), T9 (1972).
Received for review June 23, 1975 Accepted September 23,1975 Presented at the Symposium on New Sulfur Chemistry, Division of Petroleum Chemistry, 167th National Meeting of the American Chemical Society, Los Angeles, Calif., Apr 12, 1974.
Ind. Eng. Chem., Prod. Res. Dev., Vol. 15,No. 1. 1976
23
