Proceso Kraft

operation, and advanced technologies that have been
developed to overcome the problems.

THE KRAFT CHEMCIAL RECOVERY
PROCESS

Fibre

Honghi Tran
Pulp & Paper Centre
University of Toronto
Toronto, Ontario, CANADA

Lignin
Fibre

+

Esa K. Vakkilainnen
Pöyry Forest Industry Oy
Vantaa, FINLAND

White
Liquor

(NaOH + Na2S)
Wood
Black Liquor

ABSTRACT
Figure 1. KraftPulping Process

The kraft chemical recovery process is a mature, effective
technology that provides for recycling of the pulping
chemicals, efficient generation of steam and electrical
power from the fuel value of the black liquor, and
effective disposal of dissolved wood substances. The
process now faces significant challenges with respect to
air emissions, effluent discharge, and the needfor
improved energy recovery and utilization. Means for
dealing with these issues are available, but they can costly.
New technology may result in significant energy benefits.

THE RECOVERY PROCESS
Figure 2 shows a simple flowsheet of the kraft chemical
recovery process. The process has three main functions: i)
minimizing the environmental impact of waste material
(black liquor) from thepulping process; ii) recycling
pulping chemicals, NaOH and Na2S; and iii) co-generating
steam and power.
Wood
Wood

Lime Kiln

INTRODUCTION
The kraft process, which uses sodium hydroxide (NaOH)
and sodium sulphide (Na2S) to pulp wood, is the dominant
pulping process in the pulp and paper industry. About 130
million tons/year of kraft pulp are produced globally,
accounting for two-thirdsof the world’s virgin pulp
production and for over 90% of chemical pulp. The high
strength of kraft pulp, the ability of the process to handle
almost all species of softwood and hardwood, and the
favourable economics due to high chemical recovery
efficiency (about 97%) give the kraft process an advantage
over other pulping processes.

Lime
Mud

Digester
Pulping

Lime

PulpWhite
Liquor

Washing
Washing
Weak
Black Liquor

Causticizing
Causticizing
Plant
Plant

Evaporators

Green Water
Liquor
Smelt

Recovery
Boiler

Heavy
Black Liquor

Figure 2. Kraft Recovery Process
The magnitude of the recovery process is often not fully
appreciated. Globally over 1.3 billion tons per year of
weak black liquor are processed; about 200 million tons
per yearof black liquor dry solids are burned in recovery
boilers to recover 50 million tons of cooking chemicals as
Na2O, and to produce 700 million tons of high pressure
steam. This makes black liquor the fifth most important
fuel in the world, next to coal, oil, natural gas, and
gasoline [1]. Since black liquor is derived from wood, it is
the most important renewable bio-fuel, particularly inSweden and Finland.

In the kraft process, about half of the wood is dissolved,
and together with the spent pulping chemicals, forms a
liquid stream called weak black liquor (Figure 1). The
weak black liquor is separated from the pulp by washing,
and is sent to the kraft recovery system, where the
inorganic pulping chemicals are recovered for reuse, while
the dissolved organics are used as afuel to make steam
and power.
For every ton of pulp produced, the kraft pulping process
produces about 10 tons of weak black liquor or about 1.5
tons of black liquor dry solids that need to be processed
through the chemical recovery process. This paper
discusses the basic steps in the kraft recovery process,
problems encountered and challenges in recovery

The chemical, physical andcombustion properties of
black liquor vary from mill to mill depending on many
factors, including mill location (inland or coastal), digester

1.1-1

conditions, pulp yield, wood species, white liquor
properties, chemicals-to-wood ratio and brownstock
washing efficiency. In general, hardwood pulping requires
less chemicals, has a higher pulp yield, and consequently,
generates less black...