Introduction

The origin of the sulfite process is attributed to the efforts of Benjamin Chew

Tilghman, an American chemist, who was granted U.S. patent 70,485, dated

November 1867, entitled Treating Vegetable Substances for Making Paper Pulp. The

invention was based on the results of the experiments at the mills of W.W. Harding

and Sons at Manayunk, near Philadelphy, in 1866, and covers the pulping of

wood with aqueous solutions of calcium hydrogen sulfite and sulfur dioxide in

pressurized reactors. However, the first sulfite mill started its production in Europe

at Bergvik,Sweden, in 1874 under the direction of C.D. Ekman usingmagnesium

hydrogen sulfite solution, Mg(HSO3)2, as the cooking agent. The mill was equipped

with small rotating digesters heated indirectly by means of a steam jacket. In 1875,

theGerman chemist A. Mitscherlich was developing a sulfite cooking process using a

horizontal, stationary, cylindrical digester lined with brick and indirectly heated by

means of coils of lead or copper pipe. Cooking was carried out under moderate temperature

and pressure conditions. Consequently, the Mitscherlich process was characterized

by a much higher retention time as compared to the directly heated Ritter–

Kellner process, which was developed at the same time in Austria.

The sulfite process was developed around the acid calcium bisulfite process, as

mentioned in the Tilghman patent. It remained the principal process for wood

pulping because of the low costs and high availability until the beginning of the

1950s, when the need to recover the waste liquor and pulping chemicals slowly

emerged, mainly for reasons of environmental protection. Since calcium sulfite is

soluble only below pH 2.3, it can solely be used in acid bisulfite pulping in the

presence of excess SO2. At cooking temperature, the calcium hydrogen sulfite

decomposes to calcium sulfite and hydrated SO2:

Ca HSO3 _ _2__

T CaSO3 SO2 _ H2O _155_

Thus, high charges of free SO2 and low cooking temperatures must be maintained

to prevent the precipitation of calcium sulfite. A further drawback of the use of

calcium as a cation for the acid sulfite process is the formation of calcium sulfate

during the course of the recovery process. The conversion to calcium sulfite is not

practical, since a temperature above 1200 °C is required to achieve its complete

decomposition to calcium oxide and sulfur dioxide. At this high temperature the

crystal structure of calcium oxide changes, thus reducing its reactivity. Furthermore,

calcium sulfite anhydride tends to disproportionate to calcium sulfate and

calcium sulfide. For these reasons calcium has been replaced by more soluble

bases, and is now reserved for a few pulp mills with complete by-product recovery.

Today, the dominating base used in sulfite pulping is magnesium. The corre-

392 4 Chemical Pulping Processes

sponding aqueous magnesium bisulfite solutions are soluble in a pH range up to

5–6, so that acid bisulfite and bisulfite (magnefite) pulping processes in both oneand

two-stage operations can be carried out. The big advantage of the magnesium

bisulfite process compared to the calcium base system lies in its thermochemical

behavior [1]. In contrast to the calcium system, the thermal decomposition of MgSO3

occurs at a rather low temperature, generating only a small amount of sulfide. The

magnesiumsulfate obtained from the combustion of magnesiumsulfite spent liquor

can be decomposed thermally in the presence of carbon from the dissolved organic

substances to give gaseous SO2 and magnesium oxide according to Eq. (156) [2]:

2MgSO4 C __ 2SO2 2MgO CO2 _156_

In order to avoid secondary oxidation of SO2 to SO3 in the absorption unit, the

flue gas must not contain free oxygen in excess of 3%, so that the surplus of air in

the combustion process must not exceed 1.5–2.0% [2].

As alternatives to calcium and magnesium, sodium and ammonium cations are

also used in sulfite pulping. Since both monovalent cations are soluble over the

entire pH range, they can be used in acid, bisulfite (magnefite), neutral and alkaline

sulfite processes. The prevailing sulfite processes are defined according to the

pH range of the resultant cooking liquor, as shown in Tab. 4.51.

Tab. 4.51 Assigament of sulfite pulping processes according to

the different pH ranges.

Nomenclature Initital pH range at 25 °C Base alternatives Acitve reagents

Acid bisulfite 1–2 Ca2+, Mg2+, Na+, NH4

+ H+, HSO3

–

Bisulfite (Magnefite) 3–5 Mg2+, Na+, NH4

+ (H+), HSO3

–

Neutral sulfite 6–9 Na+, NH4

+ HSO3

–, SO3

2–

Alkaline sulfite 10–13.5 Na+ SO3

2–, OH–

The use of monovalent cations, especially ammonium, tends to increase the

rate of delignification at given process conditions [3]. Mill experience indicates

that maximum temperature could be decreased by 5 °C when changing from calcium

to ammonium base while keeping the cooking cycle constant [4]. The reason

for the more rapid delignification in the cooks on soluble cations is not entirely

known. According to the Donnan law, it appears that acidity in the solid phase

decreases as the affinity of the cation to the solid phase increases. It is assumed

that the concentration of the lignosulfonate groups in the solid phase equals about

0.3 N, corresponding to a pH level below 1.0 in the absence of cations other than

protons [5]. The affinity for the solid phase is increasing in the order [6]:

H+ < Na+ < NH4

+ < Mg2+ < Ca2+ < Al3+

4.3 Sulfite Chemical Pulping 393

The acidity of the solid phase should therefore be lower in the presence of aluminum

ions, and highest in the presence of sodium ions. It is likely that the higher

acidity of the solid phase in the case of monovalent bases contributes to a slightly

higher extent of carbohydrate hydrolysis and somewhat greater velocity of delignification.

Although the differences in rate and selectivity of delignification are not

significant, mill application has revealed several advantages, such as higher pulp

yield, viscosity and alphacellulose content at a given kappa number and a lower

amount of rejects [7,8]. These advantages can be attributed to better penetration

with cooking chemicals and a more uniform cook when changing from calcium

to magnesium, ammonium, or sodium base. The brightness of the unbleached

pulps is, however, clearly impaired in the case of ammonium-based pulps. There,

the lower brightness is probably due to a selective reaction between the ammonium

ion and carbonyl groups of lignin. This reaction is also responsible for a

much darker color of the ammonium-based spent liquors. However, no differences

in the bleachability of ammonium-based pulps in comparison to other sulfite

pulps can be observed.

Despite some clear advantages of the monovalent over the bivalent bases with

respect to flexibility (entire pH range available) and pulping operations (more

homogeneous impregnation, higher rate of delignification), their use in sulfite

cooking processes has been limited to a few applications, mainly due to deficiencies

in recovery of the cooking chemicals. For ammonium sulfite waste liquor no

economically feasible solution exists to recover the base. Ammonia recovery processes

based on ion exchange have been developed to the mill level, but have not

gained acceptance in praxis because of high costs. The use of ammonium base in

particular has been shown to be advantageous for the production of highly reactive

dissolving pulp where mill scale operations still exist. Sodium base is predominantly

used in neutral and alkaline sulfite processes. The recovery of sodiumbased

sulfite processes combines the use of a kraft-type furnace and the conversion

of the resulting sodium sulfide to sodium sulfite using carbonation processes

(e.g., liberation of hydrogen sulfide from the smelt by the addition of CO2 from

the flue gas, oxidizing hydrogen sulfide to SO2, reaction of SO2 with sodium carbonate

to give sodium sulfite). The technology employing carbonation of green

liquor was developed in the 1950s and 1960s, but since then no decisive improvements

of this recovery concept have been made. Thus, the recovery of the sodiumbased

sulfite cooking chemicals is significantly less efficient than the sodiumbased

kraft process, and this may be the main reason for the comparatively limited

application of the sodium-based sulfite processes.

During the first 50 years of chemical pulp production, the sulfite process was

the dominating technology, due mainly to the high initial brightness and the easy

bleachability of the sulfite pulps. With the developments of both a reductive recovery

boiler for the regeneration of kraft spent liquor by Tomlinson and chlorine

dioxide as a bleaching agent to ensure selective bleaching to full brightness in the

mid-1940s, the kraft pulping technology became the preferred method because of

better energy economy, better paper strength properties, and lower sensitivity

towards different wood species and wood quality. In the meantime, efficient

394 4 Chemical Pulping Processes

chemical recovery systems have been developed especially to use magnesium as a

base. The high sensitivity to the wood raw material still constitutes a problem in

the case of acid sulfite pulping. Most softwoods except spruce, such as pines,

larches and Douglas fir, are considered less suitable for sulfite pulping. A certain

part of the extractives of phenolic character such as pinosylvin, taxifolin (Douglas

fir) as well as the tannins of bark-damaged spruce and oaks give rise to condensation

reactions with reactive lignin moieties in the presence of acid sulfite cooking

solutions.

Since the 1960s the basic and applied research has been directed almost exclusively

towards alkaline pulping technologies, with kraft pulping as the key technology,

due to the higher overall economic potential. Consequently, the kraft process

has become increasingly important and is now the principal pulping process,

accounting for far more than 90% of world pulp production. For the production of

most paper-grade pulps, the strength properties are of utmost importance. Kraft

pulps show clearly better strength properties, especially with regard to the tear

strength as compared to sulfite pulps. Consequently, new installations for the production

of paper-grade pulps are almost exclusively based on kraft pulping technology.

Unlike paper-grade pulping, the acid sulfite process is the dominant technology

for the production of dissolving pulps and accounts for approximately 70%

of the total world production. Although a clear niche product, the dissolving pulp

production is a firmly established pulp market with a predicted annual growth

rate of about 5% within the next five years. The strong position of sulfite technology

in dissolving pulp production of low-purity grades sufficient for regenerated

fiber manufacture is based on a favorable economy, because of higher pulp yield,

better bleachability and higher reactivity as compared to a corresponding prehydrolysis-

kraft pulp. Therefore, the following presentation of acid sulfite pulping

technology is predominantly oriented toward dissolving pulp production.

4.3.2

Cooking Chemicals and Equilibria

In practice, the terms total, combined and free SO2 are used to characterize sulfite

cooking liquors. The content of stoichiometrically base bound hydrogen sulfite

and sulfite ions is referred to as combined SO2. The difference between the total

SO2 and the combined SO2 is then free SO2. According to this definition, half of

the pure hydrogen sulfite solution is free and the other is bound SO2. The following

definition is rather misleading from a chemistry point of view as no free sulfur

dioxide is present in a pure hydrogen sulfite solution:

2HSO_3 __ SO2_ 3 SO2 H2O _157_

In the central European acid sulfite industry the technical sulfite solutions are

characterized more closely to the chemical state of the constituents. There, the

combined SO2 is equal to the pure hydrogen sulfite, whereas the (excess or true)

free SO2 accounts solely for the sulfur dioxide in its hydrated form (SO2.H2O) and

4.3 Sulfite Chemical Pulping 395

can be calculated as the difference between the total and the combined SO2. The

combined SO2 is calculated from the active base content, usually expressed as oxide

(e.g., CaO, MgO or Na2O). The following equation demonstrates the definition

of combined SO2 using magnesium hydrogen sulfite as an example:

Mg_HSO3_2__ MgO 2SO2 H2O _158_

According to Eq. (158), 1 mol MgO accounts for 2 mol combined SO2. On a weight

basis, one part of MgO corresponds to 3.179 parts of combined SO2. A typical acid

sulfite cooking liquor contains 17.5% total SO2 and 2.20% MgO, equal to 7.0%

combined SO2 on o.d. wood, respectively. The amount of free SO2 calculates to

10.5% (range 7.0–17.5), representing 60% of the total SO2 as free SO2.

For characterization of the composition of the sulfite cooking liquor, the latter

definition will be used.

To better exemplify the differences between the two ways of cooking acid specification

a comparison is provided on the basis of a typical acid sulfite cooking

liquor composition, as depicted in Tab. 4.52.

Tab. 4.52 Specification of a typical acid sulfite cooking liquor

expressed in two different terms (actual definition related the

more actual species concentrations as used in this book vs. the

Palmrose definition according to TAPPI Standard T604 pm-79).

Parameter units Actual definition

used in this booka)