Specialty chemical technologies for high-quality, sustainable and cost-effective chemical pulp production - Paper Technology International 2023 - Paper Technology International 2023 - Flipbook - Page 34

Specialty chemical technologies for high-quality, sustainable and cost-effective chemical pulp production - Paper Technology International 2023

PAPERTECHNOLOGYINTERNATIONAL Specialty chemical technologies for high-quality, sustainable and cost-effective chemical pulp production - Paper Technology International 2023
Chemical additives reduces variations giving several benefits:
•
Improved cooking liquor penetration into wood chips for
more uniform cooking
•
H-factor reduction (alkali / steam / time)
•
Extractives load reduction and stabilisation
•
Reject amount reduction, gives possibility to increase
Kappa target
•
Improved pulp physical properties
•
Reduced evaporation load
•
Yield increase
•
Reduced bleaching chemicals consumption
•
Improved BSW
To get real benefits and chemical cost payback from
chemical additives, a thorough evaluation must be done by a
chemical supplier. The dosage can take place in different positions
depending on mill conditions and process parameters, amongst
others impregnation/digester and the oxygen stage. The fibre
protection and cooking aids applications can be combined with other
pulp concepts for synergy effects, for example anti pitch agents and
fixative to achieve talc-free production.
Inorganic scaling is a growing problem in the pulp industry
Scaling is a growing problem in the pulp and paper industry.
There are many reasons for this; increased productivity, demands
on higher yields, problems with raw material supply and not least
closure of process water loops. Closure of the process water
loops will result in an increasing concentration of non-process
elements and lead to enhanced risks for precipitation and scaling of
compounds with low solubility. This is a pervasive problem where
the build-up of scale costs the industry in terms of equipment
downtime, quality problems, reduced volume capacity and heattransfer efficiency, lost production, de-scaling operations costs, and
occupational health and safety.
Pulp production uses a high amount of water; roughly 20
– 40 m3/produced ton. The tendency to reduce fresh water usage
leads to higher concentration of salts and scaling problems. When
liquor dries solids increase, the concentration of metals as iron,
manganese, calcium, copper etc gets higher and higher, until their
solubility is exceeded.
Scale, or more accurately inorganic scale, consists of
precipitated metal salts. The most common scale in pulp mills
is calcium carbonate, typical in heat exchanges, digester and
evaporation plants. In acid stages calcium oxalate and barium
sulphate are a common scale. The traditional tools for fighting scale
are acid cleaning and high pressure water cleaning, two methods
which have benefits but there are drawbacks as well. Today, there
are much more effective and cost-efficient methods available to
control the scale problem. When it comes to elimination of scaling,
the most economical way usually is to use highly efficient cleaning
methods combined with an effective scale inhibition strategy.
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When a scale control programme is implemented, the whole
system has to be included. Otherwise, in the modern pulp mills,
there is a risk that the scale problem could me moved from one
position to another. A comprehensive view of the system combined
with knowledge about how the process works is necessary for an
effective scale control programme.
Effective cleaning
“It’s much easier to keep a clean surface clean than to
prevent a dirty surface from getting dirtier”. The main problem with
mechanical or high pressure water cleaning, is that these methods
don’t really clean the metal surface, they only appear to. Scaling
is a surface-controlled chemical reaction and the precipitating ions
attached to crystals already on the metal surface. It is essential to
clean the whole surface, even the microscopic crystal growth sites to
prevent the new starting points for scaling. Efficient cleaning with a
scale cleaning agent dissolves all the scale, even the crystals within
the metal pores, and is the best possible start for a scale treatment
programme.
Continuous scale inhibition
Two of the methods available which prevent the return of the
scale problems are stoichiometric and sub-stoichiometric additives.
Stoichiometric additives are basically chelating agents, e.g. EDTA
or DTPA. They work very well, but the problem is that at least one
chelating molecule per metal ion is needed. Large quantities are
required.
Sub-stoichiometric additives work by a combination of
different mechanisms, such as crystal distortion, threshold inhibition
and dispersing. The scale inhibitor blocks the growth sites of the
crystals, increasing the chemical reaction’s activation energy. By
understanding the scale forming mechanisms, it is possible for
chemical supplier to produce and choose the best continuous scale
inhibitor in each different situation.
An efficient scale control programme provides:
•
Increased production capacity: deposits are avoided in
the digesters, in the brown stock wash, in the bleach
plant and in the evaporation plant.
•
Reduced production loss and decrease energy cost
•
Improved process control
•
Quality benefits due to more even process control:
more stable conditions in the digesters and in the
bleach plant give pulp with more stable and better
quality.
•
Maintained heat exchange: deposits are avoided in
heat exchangers and in evaporators.
•
Increased runtime between stops.
Avoiding harmful metal reactions to spare fibre properties and
reduce chemical consumption
Keeping control of metals and other cations, for example
Mn, Fe, Cu and Ca will save a lot of problems and, in the end,
money. These substances will cause scaling with decreased
production and/or quality issues as a
consequence. Especially iron manganese and
copper will react with bleaching chemicals,
such as hydrogen peroxide, and can destroy
the bleaching sequence. By using sustainable
solutions for metal chelating and sequestering,
the amount of dosed EDTA and DTPA can be
reduced or completely removed in cleaning and
bleaching applications. This chemistry can also
replace MgSO4 in oxygen delignification and
peroxide bleaching.

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