I
D.
K. SMITH, W. J.
ALEXANDER, and R. L. MITCHELL
Eastern Research Division, Rayonier Inc., Whippany, N. J.
Controlling the Effect o f .
..
Manganese in the Viscose Process Proper use of this depolymerization catalyst reduces aging time without affecting product quality M A N G A N E S E accelerates alkaline oxidative depolymerization which occurs in the aging step of the viscose process. I n the past, variable amounts in steeping liquor or in the cellulose raw material were often the unknown and inadvertent cause of erratic viscose processing. With realization of this key effect, measurement and due control of manganese became of special interest to the cellulose producer and consumer alike. If care is not exercised, manganese may remain in, or even be introduced into the cellulose in a variable and uncontrolled amount during manufacture. Actually, a variation of only a few parts per million is critical and the resulting effect on the rate of aging may be so great that precise viscose viscosity control is impossible. For these reasons, naturally occurring manganese has come to be regarded as a very troublesome contaminant and costly steps have been taken to keep it as low and as uniform as possible. T h e presence of manganese itself in the pulp is not necessarily objectionable. I t is the variability under uncontrolled conditions that can lead to unpredictable and erratic fluctuations in finished viscose solution viscosity. Following recognition of this, the practice of adding precisely controlled amounts of manganese to the pulp has now been developed ( 5 ) and is becoming a wellestablished and a widely used industrial procedure. By this means the capacity of manganese to act as an aging accelerant has been utilized, and a former foe is converted into a valuable process aid. I n the following discussion, manganese in its new, useful role will be considered with regard to its presence in and influence on each individual viscose processing step. Manganese Enters Process as Pulp Stripe Treat. The catalytic value of manganese is exercised mainly in the alkali cellulose shredding and aging steps of the viscose process and in principle, therefore, manganese can be added at any time prior to these steps; for example, in steep liquor (7). I n
practice, however, addition to the pulp a t the time of manufacture is much to be preferred, since this makes it much easier to control accurately the amount being introduced into and carkied through the process. I t eliminates the need for frequent analyses by the viscose manufacturers as are required for additions to steep liquor. It levels out variation due to differences in adsorptivity which exist when all the catalyst must be picked up by the pulp from the steep liquor. T h e most suitable method of applying catalyst to the pulp sheet is as a narrow stripe of aqueous manganese sulfate at the time the pulp is cut or rewound. A fugitive dye is normally used to label the stripe. T h e treated sheets can then be handled in the normal manner with no extra effort on the part of the converter, as shown below. A considerable amount of work has been done to establish the equivalence of the several methods of treat application. Pulp sheets were treated with a n equal amount of manganese by various application methods ranging from the
extremes of single stripe per book to uniformly impregnated fibers within sheets. The regenerated, depolymerized celluloses prepared therefrom were evaluated for both chemical and physical properties. No significant differences were detectable among the aged celluloses representing various methods of applications or in quality of end products made therefrom. Behavior in the Steeping Operation. The first item of interest to a user of manganese-treated pulp concerns the partition of manganese between the cellulose and the caustic soda steeping liquor in an equilibrium caustic system (Figure 1). The distribution, of course, depends to a certain extent upon the ratio of pulp-to-caustic liquor. However, about 65y0of the total manganese remains with the pressed alkali cellulose even in the first steep, a n additional 5y0 goes with the drained caustic liquor, while the pressings contain 30y0 of the initial manganese under nonequilibrium conditions (Figure 1). As additional steeps of manganese-treated pulp
Salient Points of the Viscose Process 0 In the viscose process the use of pulp treated with controlled additions of manganese catalyst provides greater precision and flexibility in controlling depolymerization processes as well as substantially shortened aging cycles resulting in over-all improved viscose uniformity. 0 The rate of alkali cellulose depolymerization is increased with increasing amounts of manganese catalyst to a practical upper level of 20 p.p.m. 0 Manganese entering with the pulp is carried through substantially intact into the viscose solution, but is completely removed from the fibrous or film end product in the course of acidification, regeneration, and washing. 0 No detectable difference is found in the analytical or physical properties of cellulosic end products made with the use of manganese as compared with those depolymerized in unaccelerated aging sequences.
VOL. 52, NO. 1 1
0
NOVEMBER 1960
905
Figure 1. Distribution of manganese in a conventional steep whose pulp was treated with 10 p.p.m. of manganese-a ratio of 15 parts liquor to 1 part pulp was used
11
IN A L K A L I /CELLULOSE
I
/
/
/
are made in the caustic liquor, an equilibrium concentration is reached. This concentration level and the number of steeps required to reach it depend primarily upon the amount of manganese applied to the pulp, the pulp-caustic ratio in use, and rate of loss from the system due to carry out in alkali cellulose or to discard liquor. I n a number of respects, attainment of manganese equilibrium is parallel to hemicellulose build-up in steeping liquor. Under equilibrium conditions, which may be achieved more rapidly by deliberately adding catalyst to the caustic system at the outset, substantially all of the manganese remains with the alkali cellulose. This is true because the equilibrium steep liquor remaining in the pressed alkali cellulose also contains manganese. For a given level of manganese content in pulp, cquilibrium concentration to which manganese builds up in a given steep liquor system depends on amount of discard from the caustic system. Manganese concentration in the pressings is slightly greater than the equilibrium manganese-content of the recycled steep liquor (Figure 2). The liquor equilibrium concentration increases linearly as the pulp treat level increases. At equilibrium, substantially all manganese going into the system on
I5k, 5 W K
0
NGS
/-RESSI
-STEEP
0
Mn
in
ilOUOR
5 10 PULP BEFORE STEEPING, PPM.
I5
Figure 2. Distribution of manganese among pressed alkali cellulose, steep liquor, and pressings for various levels of manganese added to the pulp for a conventional sheet at equilibrium
Mn in PULP BEFORE STEEPING,
PRM
Figure 3. Effect of the amount of applied treat on the distribution of manganese after steeping in a slurry system at equilibrium, where agitation causes better contact between caustic and fiber
A 50
L
b Figure 4. Effect of the amount of manganese treat on the aging requirement to produce a standard viscose viscosity level for three pulps of different initial DP
906
INDUSTRIAL AND ENGINEERING CHEMISTRY
l
\
0
PULP
DP
1600
0
PULP
DP
1000
0
PULP
DP
720
the pulp is uniformly carried out on tlic alkali cellulose. I n high discard systems perhaps 5 to 10yGwill be discarded, but the level in alkali cellulose is uniform if the discard is kept so. The situation existing in slurry steeping is somewhat modified by differences in unsegregated steep liquor handling. Figure 3 shows manganese distribution for a slurry steeping system. Because of agitation, a more intimate contact of caustic and fiber results in this type of steeping as compared with conventional sheet steeping. The equilibrium manganese content of the liquor is: therefore. higher for slurry stceping, but the manganese content of the pressed alkali cellulose is nornially the same at equilibrium for slurry or conventional sheet steeping at a given level in the incoming pulp. Manganese Catalyzes Alkali Cellulose Aging. The aging step is of utmost imporxance, since it is in this oxidative alkaline depolymerization reaction that the catalytic effect of manganese is operative (2, 3 ) . The percenrage reduction in alkali cellulose aging requirement resulting from the addition of a given amount of manganese catalyst is approximately the same for simiiar types of pulp, regardless of the initial intrinsic viscosity. Figure 4 shows pulp manganese content plotted us. aging requirement to produce a standard viscose viscosity level. The curves for the three pulps of low, medium, and high initial degree of polymerization are the same, only displaced because of differences in the initial D.P. Thus, 10 p.p.m. of manganese will reduce the aging requirement of a 1600 D.P. cellulose about 60y0and of a 720 D.P. cellulose by the same 60%. I n considering the effect of added manganese on the aging requirement of pulps of different origin, the original pulp manganese content must be taken into account. The percentage effect of progressive additions of manganese on aging requirement decreases considerably as the amount applied is increased. An untreated pulp of 720 D.P. requiring 1 3 hours of aging to give a standard viscose viscosity will require 5.5 hours if treated with 10 p,p.m. of manganese, but treatment with an additional 10 p.p.m. will reduce the aging requirement to only 4 hours. Some further reduction in aging time is obtained if the manganese is increased above 20 p.p.m., but beyond this amount the catalytic effect is slight. This accounts for better viscosity control possible with controlled addition of manganese to the pulp. ‘Thus, a 0.5 p,p,m. variation of manganese content at the 10 p.p.m. level gives only a slight variation in aging requirement, while a 0.5 p.p,m. variation at the 1 p.p.m, level results in a pronounced variation. The effect of added manganese, therefore, is
M A N G A N E S E IN VISCOSE PROCESS
A conventional steeping press i s loaded with manganese stripe-treated pulp sheets
Cans are loaded with shredded alkali cellulose prior to the aging step during which manganese exerts i t s catalytic effect
Viscose i s fibered and transferred into a pilot plant solution tank for subsequent ripening and spinning stages
In the pilot plant experimental spinning machine, all of the manganese added in the process i s removed during washing
The polarographic analysis for manganese i s suitable for accurate determination even in the presence of iron and copper
Figure
5.
Course
of manganese removal in processing viscose into yarn, during the spinning step where viscose i s in contact with concentrated sulfuric acid
to mask differences in natural manganese content either in pulp or caustic soda. The effect of manganese on the uniformity of depolymerization has been the subject of considerable work. Present data indicate that manganese functions as a true catalyst, affecting only the rate of depolymerization and having no influence on the nature of the reaction. The data in the table support this view. Herein various properties are listed for cellulose aged under both normal and accelerated conditions. Chain length distribution of cellulose regenerated from a catalytically-aged sample is the same as from pulp-aged without treatment. Also, alpha, beta, and gamma cellulose analyses corroborate this view, as does the carboxyl analysis. Manganese has no appreciable effect, either beneficial or adverse, upon xanthation, mixing, filtration, or viscose ripening. These operations proceed in the usual manner whether the pulp is aged catalytically or not. While it is well recognized that depolymerization does occur in xanthation in the presence of oxygen (4), the consumption is normally so fast that all available oxygen is consumed regardless of the presence or absence of manganese. Manganese Completely Removed in Finishing Operations. In the spinning step, viscose is contacted with strong sulfuric acid solution in the coagulation and regeneration stages. During this and subsequent washing and finishing
processes, manganese is completely removed from cellulose (Figure 5). A standard viscose made from pulp containing 10 p.p.m. of manganese-based on cellulose- was spun in a conventional high tenacity tire yarn system and samples of the yarn were collected at various stages of the operation. The first sample was taken after several inches of bath travel. The catalyst was extracted from the regenerated cellulose by acid and the manganese level of the yarn was reduced from 10 to 9.5 p.p.m. during this phase of the spinning operation. The manganese content of the spin bath approaches an equilibrium level also, but a t such a low concentration that no complications are encountered. Analysis of the regenerated yarn shows that the manganese level was reduced to 7.7 p,p,m. just prior to the washing step. The manganese level was rapidly lowered in acid washing and was at zero concentration at completion of the washing. These data were obtained under conditions comparable to typical, current industrial practices and, based on the corresponding sodium and zinc analyses, the washing procedure used was actually conservative with respect to the use of wash water. I n addition, it was found that close to 200 p.p.m. of manganesebased on cellulose-had to be added to the viscose before a significant amount of manganese was detectable in the finished product. These data serve to establish
the fact that viscose rayon produced from cellulose containing commercially useful concentrations of manganese aging accelerant are completely free from residual manganese in the finished state. A similar situation exists in casting viscose into cellophane film. The casting operation itself reduces the manganese content from 10 p.p.m.-based on cellulose-in the original viscose down to about 7.8 p.p.m. Here also an equilibrium content of manganese in the casting bath is approached but again the level is so low that no processing difficulties are encountered. The subsequent early washing steps markedly reduce the manganese level to less than 0.1 p.p.m. The remaining finishing and late washing operations further reduce the manganese until none remains in the finished film. Experimental
While several methods for manganese analysis were explored, a colorimetric procedure was employed in obtaining most of the data for this paper. This method involved ashing the sample, isolating the manganese along with heavy metals as the 8-hydroxyquinoline derivative, then developing the permanganate color with periodate oxidation, and comparing with color standards prepared from solutions of known manganese content. A combination dry and wet ashing technique was found more reliable than repeated dry ashing, particularly in the analysis of caustic solutions containing hemicellulose. I n this technique, the water-insoluble residue from an initial dry ash step was re-ashed by treatment with mixed sulfuric, nitric, and perchloric acids. Alkali cellulose and samples of caustic containing hemicellulose were acidified and dried in a forced draft oven prior to ashing. Sample weights were selected so that the analyses were sensitive to less than 0.1 p.p.m. of manganese. Some work was also done with the polarograph, and it was established that such techniques are suitable for accurate measurements of manganese even in the presence of other ions such as iron and copper. literature Cited
Chemical and Physical Properties of Tire Cord Prepared from Manganese Treated Pulp
Analytical
Alpha
97.9 97.9 97.8
1.4 1.3 1.4
Treatment Application
None
Stripped Dipped
908
Car-
boxyl meq./ Beta Gamma kg.
0.7 0.8 0.8
29.4 29.2 29.5
INDUSTRIAL AND ENGINEERING CHEMISTRY
Chain Length Uniformity DP
0.5 0.6 0.5
1.1 1.2 1.1
< 60
