Abstract Rice straw pulping with formic acid was studied for different temperatures, cooking times and acid concentrations. Delignification percentage of approximately 85% with a pulp yield of 44.4% was obtained under relatively mild cooking conditions (temperature, 100°C; cooking time, 60 min; formic acid concentration, 90%). Pulp chemical and mechanical properties were comparable with those found for pulp obtained in basic environments. However, the advantage of this technique compared with cooking in basic environments is that most of the silicon derivatives remain in the pulp

Gérard Avignon

Michel Delmas

Quoc Hoang

Yves Le Bigot

CiteSeerX

Industrial Crops and Products 14 (2001) 65–71
Formic acid pulping of rice straw
Hoang Quoc Lam a, Yves Le Bigot a, Michel Delmas a,*, et Gérard Avignon b
a Laboratoire de Catalyse, Chimie fine et Polymères, Institut National Polytechnique,
Ecole Nationale Supérieure de Chimie de Toulouse, 118 Route de Narbonne, 31077 Toulouse Cedex 04, France
b Compagnie Industrielle de la Matière Végétale, 62 Chemin Champs de Lassalle 47310, Estillac, France
Accepted 13 October 2000
Abstract
Rice straw pulping with formic acid was studied for different temperatures, cooking times and acid concentrations.
Delignification percentage of approximately 85% with a pulp yield of 44.4% was obtained under relatively mild
cooking conditions (temperature, 100°C; cooking time, 60 min; formic acid concentration, 90%). Pulp chemical and
mechanical properties were comparable with those found for pulp obtained in basic environments. However, the
advantage of this technique compared with cooking in basic environments is that most of the silicon derivatives
remain in the pulp. © 2001 Elsevier Science B.V. All rights reserved.
Keywords: Delignification; Formic acid; Paper-pulp; Rice straw
www.elsevier.com/locate/indcrop
1. Introduction
Many cereal straws, particularly rice straw,
have been used as raw materials for paper pro-
duction. These raw materials were gradually re-
placed with wood products after World War II.
However, straw is still a good source of raw
material in areas where wood supplies are scarce.
The main reasons for the decreased use of straw
in the paper industry are (Fineman et al., 1988;
Kulkarni et al., 1989)
 using straw pulp in paper machines is harder
than using wood pulp;
 the specific properties of the black liquors re-
sulting from straw cooking in basic environ-
ments, makes them hard to use in industrial
concentration/combustion/generation units.
These handicaps are primarily due to the pres-
ence of large amounts of pentosanes in the pulp
and black liquors, which also contain silica. The
need to add alkali to reduce or prevent silicate
deposits or install a specific device to remove
silica from the black liquors are factors currently
limiting the use of rice or other cereal straws in
paper mills, despite recent widespread progress in
the field (Pedersen, 1989).
The development of new technologies for cereal
straw pulping is the only solution to avoid the
current industrial problems and to put to better
use these generally undervalued agricultural
resources.
* Corresponding author. Tel.: +33-5-62885721;
fax: +33-5-62885797.
E-mail address: mdelmas@ensct.fr (M. Delmas).
0926-6690/01/$ - see front matter © 2001 Elsevier Science B.V. All rights reserved.
PII: S 0926 -6690 (00 )00089 -3
H.Q. Lam et al. / Industrial Crops and Products 14 (2001) 65–7166
Table 1
Effect of different parameters on yield and kappa index (values in parentheses) of pulps obtained from rice straw in formic acid
Acid concentration (% v/v)Temperature (°C)Time (min)
70 80 90 98
69.8 (66.7) 56.9 (52.0)30 46.2 (33.6)90 46.9 (36.7)
100 45.6 (54.3) 45.1 (38.0) 45.5 (29.5) 45.8 (29.5)
44.7 (36.4) 43.7 (25.8) 41.4 (21.7)110 40.2 (23.4)
42.2 (27.9) 42.0 (24.4)115 40.9 (17.6) 40.1 (19.9)
56.6 (50.6) 46.0 (37.4)60 46.6 (31.9)90 42.3 (34.3)
46.6 (34.9) 46.4 (31.4)100 44.4 (25.1) 41.2 (26.1)
110 44.0 (34.0) 42.1 (23.6) 39.2 (19.6) 38.8 (19.6)
42.3 (27.8) 38.5 (19.8)115 38.2 (16.6) 40.2 (19.9)
9090 49.6 (48.0) 46.7 (35.8) 42.0 (28.3) 41.8 (28.2)
41.3 (36.3) 39.1 (25.8)100 38.9 (23.4) 38.5 (23.3)
40.0 (26.1) 38.3 (21.5)110 36.0 (18.3) 38.8 (18.2)
37.5 (20.8) 36.5 (17.6) 35.8 (16.3)115 37.2 (17.5)
45.0 (46.8) 41.5 (32.0)90 40.2 (19.3)120 40.9 (20.5)
100 40.5 (32.1) 37.2 (21.3) 39.3 (17.6) 39.9 (19.6)
39.4 (21.6) 37.0 (18.5)110 36.5 (15.2) 37.2 (17.6)
115 33.0 (16.6) 32.1 (14.6) 34.2 (12.2) 37.0 (14.6)
In this article, we present processing techniques
and pulp properties obtained through cooking
rice straw with formic acid. This procedure makes
it possible both to selectively separate the main
chemical compounds of the plant material and to
preserve most of the silicon derivatives in the
cellulose fibers.
2. Material and methods
The raw material used was a rice straw from
Vietnam (Oryza satia). This straw has a water
content of 9%. Its chemical composition is, 37.0%
cellulose, 18.3% lignin, 22.0% pentosanes, 14.6%
ash and 7.0% silica (based on dry straw).
Cooking was performed in a 1-l glass reactor at
atmospheric pressure. Rice straw was cut into 0.3
cm long pieces then soaked in the cooking liquor
at 50°C for 30 min. Heating took 20 min. The
liquor/dry straw ratio (L/S) was 12/1. A 40 g of
straw was used in each assay.
After soaking, cooking was performed under
the conditions described for each assay.
The resulting pulp was filtered, pressed, washed
twice in formic acid, then in hot water and finally
again carefully in cold water (on a 200 mesh
sieve), then dried and analyzed. The cooking
liquor was evaporated to recycle the formic acid.
Lignin was, then, precipitated by adding water,
filtered, washed several times in water and dried.
The water-soluble sugars were obtained as syrup.
Pulp was first refined in a Valley beater (°SR=
45), then transformed into sheets using a Frank
apparatus with two dryers, according to AFNOR
standards NF Q50-002 (Rapid–Köthen method)
to test its mechanical properties. Measurements of
mechanical properties were performed using stan-
dard procedures.
Fig. 1. Effect of formic acid concentration on dissolved matter
concentration during rice straw cooking.
H.Q. Lam et al. / Industrial Crops and Products 14 (2001) 65–71 67
Fig. 2. Influence of acid concentration on pulp mechanical and chemical properties (cooking conditions: temperature, 100°C; time,
60 min).
Fig. 3. Hydrogenous linkages between (a) dehydrated and (b) hydrated fibers (De Pascoal Neto, 1992).
Fig. 4. Effect of cooking time on pulp chemical and mechanical properties (cooking conditions: temperature, 100°C; acid
concentration, 90%).
Ash and silica percentages were obtained ac-
cording to the Chinese standard GB 2677.3-81.
Unbleached pulps were first transformed into
holocelluloses, using a sodium hypochlorite solu-
tion in a buffer (acetic acid and soda, pH 4.9), to
measure viscosity. Holocelluloses viscosity was
measured according to the NFT 12-005 standard.
Dissolved matter in the cooking solution was
evaluated by calculating the difference between
pulp and initial straw contents.
H.Q. Lam et al. / Industrial Crops and Products 14 (2001) 65–7168
Fig. 5. Effect of cooking temperature on pulp chemical and mechanical properties (cooking conditions: acid concentration, 90%;
time, 60 min).
Table 2
Acid and basic pulping characteristics
Kappa numberMedium Breaking length (m)Yield (%) Tear index (mN m2/g Burst index (Kpa m2/g
Acidica 46.6 39.4 4524 4.78 2.49
25.1 391944.4 3.95Acidicb 2.39
Basicc 12.159.4 3900 3.30 2.40
a Formic acid/water, 70/30 (% v/v); liquor dry matter ratio, 12/1; temperature (°C), 100; cooking time (min), 60.
b Formic acid/water, 90/10 (% v/v); liquor dry matter ratio, 12/1; temperature (°C), 100; cooking time (min), 60.
c NaOH (Patel et al., 1988), liquor/dry matter ratio, 6/1; temperature (°C), 140; cooking time (min), 90.
3. Results and discussion
3.1. Delignification
The effect of variables, such as temperature,
acid concentration and cooking time on pulping
and on the pulp’s chemical and mechanical prop-
erties were studied (Table 1).
The results given in the table demonstrate that
the delignification of rice straw can be successfully
performed in formic acid. The kappa index values
and pulp yields also progressed simultaneously.
These values decreased, when the following
increased.
 cooking time;
 oil bath temperature;
 acid concentration. The best values were ob-
tained at a 90% acid concentration.
The formic acid concentration affects cooking
quality as follows. Kappa index values indicate
that rice straw pulping in formic acid reaches a
Table 3
Ash and silica distribution after cooking rice straw in formic
acida
Measured parameters Straw Pulp
Pulp yield (%) 44.4100
14.6Ash content (%) 16.4
Ash yield (%) 100 49.9
47.8 89.5Silica content (% ratio to ash)
100 93.4Silica yield (%)
Lignin alone
18.3Lignin content 15.2
100Lignin yield (%) 83.1
Ash content (%) – 3.0
3.1Ash yield (%) –
Water soluble substances
Yield (%) 37.7
17.1–Ash content (%)
44.2–Ash yield (%)
–Silica yield (%) 0.16
a Cooking conditions: acid concentration (%,v/v),90; cook-
ing time (min), 60; temperature (°C), 100; liquor/dry matter
ratio, 12/1).
H.Q. Lam et al. / Industrial Crops and Products 14 (2001) 65–71 69
maximum at a 90% acid concentration. A high
acid concentration helps break down matter. This
was further confirmed by the fact that the quan-
tity of dissolved matter in formic acid (e.g. lignin
and pentosanes) was higher during cooking at
acid concentrations of 90%, than when cooking
was performed at lower acid concentrations (Fig.
1).
These results differ from those obtained by
Deineko and Kostyukevich (1989)during acetic
acid pulping of wood. These authors found an
optimal acetic acid concentration of 70% volume.
This was due to the fact that the results obtained
were naturally closely linked to experimental con-
ditions, particularly acid and raw material struc-
ture used in cooking.
Cooking time affected pulping and pulp yields
in the same way as cooking temperature. A lower
cooking temperature required longer contact times
to obtain good matter breakdown. For example,
at 90% acid concentration, an increase of 30–120
min in cooking time at 90°C reduced the pulp
kappa index by 14.3 U (from 33.6 to 19.3). The
kappa index only decreased by 5.4 U, when cook-
ing was performed at 115°C (from 17.6 to 12.2).
Temperature plays a vital role in rice straw
cooking in formic acid. Increased cooking temper-
atures (from 90 to 115°C) always, considerably,
sped up plant matter breakdown and, conse-
quently, significantly reduced kappa index. In-
creasing cooking temperature also led to
decreased pulp yield as complete pulping was
achieved.
The severe effect temperature has on delignifica-
tion and pulp yield during rice straw cooking in
formic acid can be explained by the positive role
of increased temperature on
 environment acidity (De Groote et al., 1993);
 availability of formic acid to active plant mat-
ter sites;
 reaction speeds leading to matter breakdown
(primarily hemicellulose attack);
 lignin solubility in formic acid.
Temperature also has a stronger effect on pulp
yield and kappa index, when cooking conditions
are less efficient, particularly when cooking is
performed at relatively low formic acid
concentrations.
Table 1 illustrates that cooking temperature
must be chosen carefully as
 too low a temperature means increased contact
time;
 too high a temperature means reducing contact
time to avoid carbohydrate degradation.
A 90% formic acid concentration was shown to
be the most efficient.
3.2. Pulp chemical and mechanical properties
The chemical and mechanical properties of un-
bleached rice straw pulp obtained after cooking in
formic acid were determined. Analytical results
are shown in Fig. 2 and Figs. 4 and 5.
Increasing the formic acid concentration from
70 to 90% during cooking at 100°C for 60 min has
the following effect on pulp chemistry (Fig. 2)
 a sharp reduction in pentosanes content;
 a gradual increase in viscosity.
These chemical characteristics remain constant,
if the acid concentration is over 90%. This demon-
strates that the cellulose macromolecules are not
broken down, even when acid concentration is
quite high. The selective separation of lignin,
hemicelluloses and cellulose occurs, when acid
concentration is high (the resulting pulp has high
viscosity along with low pentosanes and lignin
contents).
Pulp mechanical properties decrease, when for-
mic acid concentration increases. These results can
be surprising considering how stable cellulose is
under these conditions. However, this phe-
nomenon can be explained by the fact that the
linkages between amorphous cellulose chains and
water (H-bonds) are destroyed during cooking.
When cooking is performed in highly concen-
trated formic acid, part of the formic acid
molecules between the cellulose chains latch onto
the water molecules during cooling, which pro-
duces direct H-bonds between cellulose macro-
molecule hydroxyl groups (Fig. 3a). Fibrous
blocks are relatively uninflatable; the blocks are
stiff, breakable, inflexible and easily cut during
refining, as opposed to hydrated fibers (Fig. 3b).
Saake et al. (1995), as well as Seisto and Pop-
pius-Levlin (1997a) proved that reducing pen-
tosanes content was the main cause for the
H.Q. Lam et al. / Industrial Crops and Products 14 (2001) 65–7170
decrease in pulp mechanical properties, when pulp
was obtained after cooking at extreme tempera-
tures, cooking times and acid concentrations.
The mechanical properties of unbleached pulp
obtained after cooking in 90–98% formic acid
were lower than those found after cooking in 70%
formic acid. These results are in agreement with
those obtained later (Pan et al., 1998) during rice
straw cooking in acetic acid, water and sulfuric
acid.
The results shown in Fig. 4 demonstrate that
increasing the cooking times gradually reduced
pulp pentosanes content, while viscosity reached a
maximum level (867 cm3/g) after 60 min. Viscosity
decreased beyond this threshold, indicating cellu-
lose fiber breakdown. Break length and tear index
of unbleached pulp decreased progressively along
with increased cooking time, while the burst index
remained relatively constant.
A similar reduction in pulp mechanical and
chemical properties was observed, when tempera-
ture was increased (Fig. 5).
It should be noted that the chemical and me-
chanical properties of pulp obtained using formic
acid were as good (Table 2) as those found in rice
straw pulp obtained using alkali or alkaline/an-
thraquinone (Patel et al., 1988).
3.3. Ash and silica distribution in different
fractions obtained after cooking
Ash and silicon contents were determined for
each of the different fractions obtained, pulp,
lignin water-soluble substances (Table 3).
These results indicate that
 ash content in pulp and water soluble sub-
stances was very high (16.4 and 17.1, respec-
tively), while content in lignin was low (3.0%);
 silicon derivatives were the main components
of rice straw ash and were primarily retained in
pulp (93.4% compared with silicon derivative
content in initial straw).
These results are very important, as they prove
that cooking rice straw in formic acid helps retain
most of the silicon derivatives in the pulp. This
considerably simplifies cooking liquor treatment
(Seisto and Poppius-Levlin, 1997b).
These results also demonstrate that the biomass
recovery rate (pulp, lignin alone, water-soluble
substances) during pulp making from rice straw
using formic acid is very high (97.3%). Biomass
loss is nearly null, but can occur when pulp is
washed with water.
4. Conclusion
Rice straw pulping can be successfully per-
formed in formic acid at atmospheric pressure.
The pulp obtained has
 a moderate degree of polymerization;
 a low lignin and pentosanes content;
 sufficient mechanical properties.
Temperature, acid concentration and contact
time significantly affect pulping and hemicellu-
loses hydrolysis. Transforming pulp into paper
requires mild conditions to maintain a reasonable
pentosanes content in the pulp.
Most acid soluble substances dissolve during
rice straw cooking in formic acid. Silicon deriva-
tives remain primarily in pulp. Recycling formic
acid can be achieved through simple distillation.
The numerous advantages of this procedure
compared with using basic environment systems
led us to focus on the following research
directions
 localization of silicon derivatives in unbleached
pulp and definition of their chemical structure;
 understanding the linkage types found between
silicon derivatives, carbon and organic matter;
 examination of derivative behavior during
refining and bleaching;
 characterization and utilization of lignins and
sugars from the pulping processes.
The above themes are currently part of our
research program. Results from our experiments
will be presented shortly.
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