Journal of Scientific & Industrial Research
Vol. 74, April 2015, pp. 236-244
Controlling Barium Sulphate scale deposition problems in an
Unbleached Kraft Paper Mill
B Sithole1*, M Doucette2 and R Fiolet3
*1
Forestry and Forest Products Research Centre, Discipline of Chemical Engineering,
University of KwaZulu-Natal, CSIR, Durban, South Africa
2
Tolko Manitoba Kraft Papers, The Pas, Manitoba, R9A 1L4, Canada
3
Ashland Hercules Water Technologies, The Pas, Manitoba, R9A 1L4, Canada
Received 27 July 2012; revised 19 November 2013; accepted 30 December 2014
Troubleshooting of scale deposits and defects in paper samples showed that the problem was caused by barium sulphate
and calcium sulphate scales. However, it was ascertained that barium sulphate was more of a concern than calcium sulphate
as it is more tenacious than the calcium sulphate scale. A study of the inputs of the scale components found that the major
inputs of barium into the mill system were the wood chips and bark contamination. The major inputs of the sulphate
component of the scale were sulphuric acid used for pH control and alum used for rosin sizing.Evaluation of several options
for mitigating the scale deposition problem showed that bark removal and continuous application of a scale inhibitor
comprised of phosphates/phosphonates and polyacrylates were effective in reducing and controlling the scale deposit
problems. Institution of these measures led to a cleaner paper machine that required far fewer boil outs than before. In
addition, productivity improved and the fisheye defects disappeared.A rapid method was developed for on-site estimation of
barium levels in mill process samples. The data were used to adjust the dosage of the scale inhibitor to ensure optimum
effectiveness of the programme.
Keywords: Scale, barium sulphate, deposition, alum, pH control, scale inhibitors
Introduction
The formation of sulphate scale deposits is a huge
problem in pulp and papermaking operations. The
deposits cause a number of problems such as
deposition on mill equipment that can grow to cause
process upset and blockages of pipes1-3. A particularly
problematic issue if the breakage of scale deposits to
small particles that appear as fish eyes in the final
pulp and paper product. Several approaches can be
used to avoid or reduce scale deposition4,5, but barium
sulphate forms tenacious deposits that are very
difficult to remove and control6,7.Several years ago we
worked with a sack kraft mill to help in its conversion
from rosin to alkyl ketene dimer (AKD) sizing in
order to improve runnability. AKD is normally used
in alkaline paper and board grades, but not in sack
kraft grades. The conversion was a success and the
mill was the first sack kraft mill in North America to
achieve this. However, about two years later the mill
reverted back to rosin sizing due to the much higher
cost of AKD versus rosin size. A few months after the
——————
*Author for correspondence
E-mail:[email protected]
switch the mill experienced serious deposits that
accrued on the Uhle box strips, headbox, LEXAN®
sheets, header, and slice. To cope with these
problems, the mill washed the headbox with caustic
and chelant every month, and even more frequently at
times, in contrast to a competitor mill where the
headbox required only two washes per year. With
time, the boil outs were ineffective in removing the
deposits. In addition, a substantial portion of the paper
produced was off-grade and plagued with fisheye
defects.The deposits were very expensive for the mill
and, at the time, cost upwards of $0.5 million per
annum for boil outs, downtime for cleaning,
replacement of LEXAN sheets, reduced efficiency
due to poor machine and winder runnability, and
off-grade and/or rejected paper products.A team was
set-up and called in to troubleshoot the deposition
problem and help recommend methods to eliminate or
control the problem. We found that the deposit was an
inorganic scale comprised largely of barium sulphate
and some calcium sulphate. The inputs of the
principal scale components were determined and used
to develop methods to reduce or control the scale
deposits.
SITHOLE et al: CONTROLLING BaSO4 SCALE DEPOSITION PROBLEMS
The options that were investigated included
pre-acidification of pulp before washing and use of
scale control additives. Advantages of preacidification include:
 improved pH control in the wet end – a stable wet
end pH requires less acid and makes correction at
the silo easier;
 less “shock” at the alum addition point resulting in
reduced deposition, i.e., reduced formation of
aluminum hydroxide and barium sulphate;
 soluble “anionic trash” is partially neutralized by
pH reduction – leads to lower cationic demand;
greater alum/size efficiency also results in reduced
sulphate and barium from wood
 other benefits - reduced size demand, improved
refining and strength
Observations at the mill
A quick review of mill operations showed the use
of fresher chips than prior to the deposition problems
and visible bark in the chip supply. The chip supply
had changed from a well known furnish from the
nearby saw mill to unknown furnish of variable
quality from places distant from the mill. In addition,
brownstock washing at the mill was much poorer than
at a competitor mill. Additives used at the mill were
wet-strength resin, biocide, oil-based defoamer,
cationic wet-end and surface starches, calcium
carbonate, rosin size, alum, retention and drainage
aids.
Experimental Approach
Analysis of deposits
The approach developed was to conduct thorough
analyses of the deposits to determine their
composition and to trace the source or cause(s) of the
deposition. Initially, deposit samples from the
headbox and LEXAN sheets were collected and
analysed for comparison with the deposits on the
paper samples. Later, samples of deposits as well as
pulps were collected at several unit operations across
the mill to profile the levels of major components
suspected to be the cause of the deposits.The samples
were freeze-dried and portions were analysed for ash
content. The samples and ashes were then
characterized by Fourier transform infra-red
spectroscopy (FTIR) to obtain information about the
functional groups present and by energy dispersive
spectroscopy (EDS) to obtain information about the
inorganic elements present. Portions of the dried
deposits were extracted sequentially with acetone and
237
chloroform and the extracts were dried and
characterized by FTIR and gas chromatography to
determine the organic components extracted from the
deposits. The metals present were determined by
inductively coupled plasma - optical emission
spectrometry (Perkin Elmer, USA). The analytical
techniques used have been described before8.
Profile of scale components along the fibre line
This study was conducted to ascertain which unit
operations were more prone to deposition than others.
Time-sequenced samples were collected along the
fibre line and analysed for ash, barium, calcium and
sulphur content. The sampling points were weak
black liquor tank (WBL), raw stock chest (RSC),
decker stock (DS), before the twin roll press (BTRP),
after the twin roll press (ATRP), after the high
consistency refiner (AHCR), machine chest outlet
(MCO), white water silo (WWS), and headbox (HB).
The samples were collected over 2 weeks except for
the headbox sample where the samples were collected
over three weeks.
Pre-acidification of stock before washing
Laboratory experiments were conducted to assess
the best conditions for pre-acidification of pulp stock
with acid to remove metal ions from the pulp. The
pre-acidification studies were conducted as follows:
Pulp stock was vacuum filtered. The pH of the
filtrates was measured and the filtrates were then
analysed for metals. Distilled water (same volume as
the filtrate) was added to the filtered pad, and the pH
was adjusted from an initial value of 9.5 to lower
values using nitric acid. The mixture was shaken for 5
minutes and filtered as before. The process was
repeated down to pH 3. All the filtrates were analysed
for metal content.A control study was done by
washing with distilled water with no pH adjustment.
Pulp stock was vacuum filtered. The volume of the
filtrate was measured and the filtrate was analysed for
metals. Distilled water (same volume as filtrate) was
added to pad with no pH adjustment. The sample was
processed as described before. The washing was
repeated twice with fresh water volumes. All the
filtrates were analysed for metal content.The pulps
used in these studies were Raw Stock, Brown Stock,
and Machine Chest Outlet.
Spot test for measuring barium levels in process samples
Normally, the concentration of barium ions in
process samples can be determined with accuracy by
spectrometric methods. However, the methods are not
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J SCI IND RES VOL 74 APRIL 2015
rapid enough to yield data that could be used to make
important mill decisions. Consequently, a spot test for
rapid “ball park” estimation of barium levels was
developed and instituted at the mill. The method was
based on formation of barium sulphate precipitates by
mixing barium laden samples with a copper sulphate
solution. Since copper sulphate is soluble in water, it
can react with barium ion to produce a thick white
precipitate of barium sulfate:
Ba2+(aq) + CuSO4 → BaSO4(s) + Cu2+(aq)
A 1,000 ppm stock solution of BaCl2 was prepared
in distilled water and then diluted to make solutions
that contained 0.5 to 10 ppm of barium. 5 mL of each
solution were mixed with 1 mL of a 50,000 ppm
aqueous copper sulphate solution in clear conical
polypropylene centrifuge tubes. The samples were
mixed well, allowed to sit for 5 minutes, and then
centrifuged on a portable microcentrifuge for 1
minute. The tubes were placed on a rack to be used as
standards for estimating the concentrations of
precipitated barium in process samples. To measure
barium levels in process samples, 100 g of pulp stock
were acidified to pH 3 with nitric acid and then
centrifuged to collect the aqueous fraction. 5 mL of
the liquid were reacted with copper sulphate as
previously described in the preparation of barium
sulphate precipitate standards. The concentration of
barium in the precipitates was estimated by visual
comparison of the amount of precipitates with the
known barium sulphate precipitates in the calibration
standards.
mills that use rosin size are plagued with aluminium
and/or calcium rosinate deposits9-11, but this was not
the case in this mill. Analysis of the extracts by gas
chromatography showed the presence of wood resin
but no rosin size or oils. These results indicated that the
deposition problem, and consequently, the fisheye
defects in the paper, were not attributable to wood pitch
or improperly retained rosin size. Indeed, the
extractives content of one problem paper, grade A,
were lower than those of the good paper.
Ash content and characterization of the ash
The ash contents were very high (58 -88%) in the
headbox and LEXAN sheet deposit samples but low
(less than 1%) in the final paper samples. Analysis of
the ash by energy dispersive spectroscopy (EDS)
showed that the major elements present in the
headbox and LEXAN sheet deposit samples were
aluminum, barium, calcium and sulphur, whereas the
fisheye defects showed the presence of mainly barium
and sulphur. The presence of aluminium is not
unexpected since alum was used for rosin sizing at the
mill, but the presence of barium, calcium and sulphur
were of concern. Further analysis of the ash samples
by FTIR showed that they contained large amounts of
barium sulphate and calcium sulphate. Calcium
sulphate was dominant in the whitewater silo ash
whereas barium sulphate was more dominant in the
LEXAN sheet and headbox deposit. Typical FTIR
spectra are shown in Figure 1. From these results it
Results and Discussions
The headbox deposit samples were brown flaky
material, whereas the deposits in the paper products
were in the form of fisheye defects. The deposits on
mill equipment were tenacious and not easily
removed by scraping with a metal blade. However,
deposits in the whitewater silo were easier to scrape
off than the ones in the headbox.
Solvent extracts
Acetone extractives provide quantitative data on the
presence of wood resin, hydrocarbon oils, defoamer
oils or rosin size whereas chloroform extractives are
used to quantify the concentrations of multivalent
metal soaps of wood resin and/or rosin size. The values
of the acetone solvent extracts were low and were
within the normal range for unbleached kraft papers.
The chloroform extracts were negligible indicating
that there was no deposition of metal soaps. At times,
Fig. 1—FTIR spectrum of headbox ash samples showed a good
match with barium sulphate.
SITHOLE et al: CONTROLLING BaSO4 SCALE DEPOSITION PROBLEMS
239
was deduced that the fisheye defects in the final
products were caused by contamination from barium
sulphate deposited on the LEXAN sheets. The high
levels of ash (barium sulphate) in the headbox
samples resulted in deposition of barium sulphate on
the LEXAN sheets.
the northeast USA where barium is present in water
and soil13. Later reports show that the problem was
endemic in the kraft bleach plant, especially since the
advent of chlorine dioxide bleaching. The information
from the literature is further summarized in the
sections that follow.
Profile of scale components along the fibre line
Barium sulphate is a recalcitrant scale
The results for the profiles of scale components
along the fibre line are showed that large amounts of
ash, and therefore by inference, scale deposition,
occurred in the whitewater silo and on the headbox.
Indeed, the data for barium and sulphate concentrations
confirm this, as cab ne seen in Figure 2. The trend
observed here agrees with literature reports of barium
profiles across a kraft mill4.Although the scale
deposits contained barium sulphate and calcium
sulphate, it was deduced that it was worthwhile to
focus more on barium sulphate scale than on calcium
sulphate scale since the barium sulphate scale is more
tenacious and difficult to control and remove than the
calcium sulphate scale. Finding a solution or solutions
to the scale deposition problem required an
understanding of barium sulphate scale precipitation:
its mechanism, inputs of scale components, and scale
removal strategies.
Review of barium sulphate scaling
A review of the literature on barium sulphate
scaling in pulp and papermaking reveals that the
problem can be widespread12. A paper from the 70s
states that the deposition problem was not isolated to
any particular geographic region, since it had been
observed in groundwood mills, fine paper machines
and unbleached kraft machines in all parts of the
USA4. However, a paper from the 90s states that the
problems occurred in the southeast and some parts of
Fig. 2—Profile of barium content along the fibre line (values
magnified x100).
Of the three mineral scales encountered in pulp and
papermaking, (barium sulphate, calcium oxalate, and
calcium carbonate), barium sulphate is the most
difficult to remove and the most difficult to
prevent2,14. Barium sulphate scale is difficult to
remove because sulphate is a strong acid and the
dianion sulphate persists to a pH below 2. It requires a
strong acid to remove and the acid used cannot be
sulphuric acid. Preventing barium sulphate scales is
difficult because the sulphate ion is ubiquitous in a
pulp mill and in the bleach plant. A solubility product
plot of barium sulphate shows that at any realistic
sulphate level, the solubility product of barium
sulphate is exceeded in the presence of very small
amounts of barium ions4. Barium sulphate deposition
correlates with pulp lignin content. On two parallel
lines, the deposition is more likely on lower kappa
number pulp4. This is probably because higher kappa
pulps have more lignin content that acts as a
dispersant15 or inhibitor for the barium sulphate scale.
Decreasing of the pH of the stock towards the
headbox further favours barium sulphate deposition,
and the loss of the lignin (a natural dispersant) due to
precipitation of dissolved lignins may be a
contributing factor.
Location and description
Barium sulphate scale builds up on headboxes,
flow spreaders, or organ tubes of the paper machine6.
It is also found on table-roll deflectors, in stock lines,
rectifier rolls, vacuum boxes, etc. The scale varies in
colour and consistency, being very thin, bright, and
rough in the headboxes and greyish-to-brown, hard
and pasty in other areas. Two types of barium
sulphate crystals occur that differ in size and shape:
rhombic and dendritic. It has been observed that
scaling is most severe at points of cascades and
pressure drops, but both forms of crystals have been
found in whitewater systems of kraft paper
machines16. Barium sulphate has been blamed for
corrosion problems, since under-deposit corrosion can
result from the presence of sulphate-reducing bacteria
which exist in the anaerobic space beneath the
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J SCI IND RES VOL 74 APRIL 2015
deposit13. The by-products of theses microbes can be
highly corrosive to even stainless steel, causing
pitting of the metal and a roughened surface that will
be more prone to deposition after cleaning.
Mechanism
Scaling tendency (also called the saturation level or
ratio) is a measure of a system’s level of saturation
with respect to a solid phase chemical species, in this
case BaSO4. The scaling tendency indicates the
magnitude of the driving force for the dissolved
species to precipitate. The value is thermodynamically
based on the ratio of the ion activity product to the
equilibrium constant of the precipitation reaction or
the solubility product17-18.
2-
[Ba 2 ][SO 4 ]
Saturation index  log 10
K sp BaSO 4
where [Ba2+] is the concentration of free barium ions,
[SO42-] is the concentration of free sulphate ions, and
KspBaSO4 is the solubility product for the precipitation
reaction. Remember that the activity of the solid
species, the product in the chemical reaction equation,
is equal to one. All of the quantities on the right side
of the above equation are adjusted for pH,
temperature and ionic strength. Calculation and use of
the saturation index, and generation of the data from
which it is derived, are best done by computational
programmes such as WaterCycle, ScaleChem; Avista
Advisor19. When the scaling tendency = 0, barium
sulphate is in equilibrium with the solution. When it is
< 0, the mineral is undersaturated, and when it is > 0,
the mineral is oversaturated. Therefore, when the
solubility product is exceeded, the scaling tendency
is high as the reaction is in a supersaturation
state. Consequently, deposition of BaSO4 occurs.
The Ksp for barium sulphate is 1.0810-10 at 20°C
and 1.9810-10 at 25°C. To put this solubility product
into perspective, barium sulphate scale is 25 times
less soluble than calcium carbonate and 35 times
less soluble than calcium oxalate7.Deposition can
also occur via nucleation, whereby clusters of
barium and sulfate reach “critical nucleus size” and
crystals begin to form and build. This mechanism is
initiated by BaSO4 seed crystals, shocks to the system
(pH, thermal, shear), or over time (induction
time).The deposition can also occur via crystal
growth: after nucleation, BaSO4 crystals build with
new molecules being added to the crystal surface or
face of the initial crystals.
Inputs of barium and sulphate ions
Inputs of barium and sulphate into the pulp and
papermaking process in an unbleached kraft paper
mill include: acidification of pulp with sulphuric acid,
carryover from Na2S in white liquor that has been
oxidised to sulphate, use of alum for rosin sizing, and
naturally occurring barium in wood and/or water.
Softwoods contain 8-15 ppm barium whereas
hardwoods contain 5 times more barium20-21. The
barium levels are higher in bark than in wood. It has
been reported that 50% of the barium in wood input
into the digester remains bound to carboxylic acids on
the pulp5. The amount of barium on pulp is little
affected by brownstock washing. The sulphate anion
is the net product of bound sulphur on brown stock
pulp, sulphur carryover from brownstock washing,
sulphuric acid used for pH adjustment, and alum used
for rosin sizing.
Formation of barium sulphate scale
Very minute amounts of barium can eventually
result in large quantities of deposit13. The surface of
the deposit is very rough, and as a result traps fibres
and other filler particles. Eventually the deposit grows
large enough to plug consistency regulators and cause
flow control problems, such as consistency and basis
weight variations. With more time, the deposits will
grow large enough and become unstable, and pieces
will break off and cause sheet spots, breaks and press
end crushing. Precipitation or agglomeration of
BaSO4 into small or large particles dictates how they
will partition between fibres and filtrates. This is
dependent on the type of washing equipment used.
For example, in drum filters, barium sulphate particles
that are less than 50 µm are retained in the pulp,
whereas particles that are <1 µm exit with the filtrate5.
pH is a critical parameter in BaSO4 scale formation.
Deposition occurs most at points of pH shock, such as
white water dilution and acid/alum addition points. In
general, the pH of pulp stock decreases from 10 at the
brownstock washers to 5 at the paper machine headbox.
The pH directly affects the distribution in solution of the
various ions and complexes in the system.
Solutions to barium sulphate scale
In general, solutions to barium scale deposition can
be divided into four groups:
Reduce the input of scale materials
Where possible, switching from hardwoods to
softwoods will help greatly in minimizing barium
241
SITHOLE et al: CONTROLLING BaSO4 SCALE DEPOSITION PROBLEMS
scale deposition. Good debarking is a good way to
reduce input of barium into the system because trace
metals are at significantly higher concentrations in the
bark than they are in clean wood. Good brownstock
washing will reduce inputs of barium. Inputs of
sulphate can be reduced by good white liquor
clarification and avoiding the use of sulphuric acid for
acidification of pulp. Reid et al.5 concluded that mills
using wood species with high levels of barium
(>25 ppm on pulp) can only minimize BaSO4 scaling,
whereas it is possible to avoid BaSO4 scaling in mills
that have pulps with lower barium concentrations.
Remove built-up scale
This is usually accomplished by mechanical means,
viz, hydroblasting and/or by chemical means that can
be boil-outs with chelants or acid washes.
Hydroblasting entails fracturing and removing
deposits with intense, high-pressure, water washing. It
is “akin to trying to demolish a concrete highway with
a pressure washer”2.
Prevent scale build-up
Equipment made of stainless steel and Teflon
equipment is less prone to scale deposition. Process
changes, such as the addition of dispersants and
crystal modifiers to prevent crystal growth, can also
be used to prevent the build-up of barium sulphate
scale.
Replace alum with polyaluminium chloride (PAC)
No sulphate ions are introduced by PAC but the
kraft (sulphate) pulping process introduces such a
high sulphate ion concentration, that the sulfate
reduction from replacing alum may have a negligible
effect on BaSO4 deposition. In addition, PAC
introduces chloride ion into the white water system
that may be of some concern in chlorine stress
corrosion cracking. However, the chloride ion
concentration introduced with PAC would probably
be too low to cause a problem.
Solutions implemented at the mill
The solutions listed in the previous section were
explored to ascertain which ones could best be
implemented at the mill. The first obvious one was
bark content in the chips. The mill improved their
chip screening procedures and emphasized the
importance of minimal bark content in the chips in
discussions with their chip suppliers.An initial profile
was conducted to ascertain which mill unit operations
were most prone to the accumulation of barium
sulphate scale. This was done by determining the ash
content of pulps from 3 unit operations, viz., the stock
tower, the machine chest and the headbox. Four
samples were collected over four days and analysed
for ash content as an indicator of barium scale
content. The results demonstrated that the ash content
increased along the fibre line to the paper machine
and was highest at the headbox where the ash content
was 7-10 times higher than at the stock tower and
machine chest unit operations. Quantitative measurements of the actual concentrations of barium and
sulphate in a thick stock sample, white water silo
sample, and a headbox sample and their calculated
scaling tendencies clearly indicated that the scaling
tendency was very high, as illustrated in Table 1. At
the time of investigation, the mill was practicing
Option 2 in the form of caustic washes of the headbox
every month (sooner in some instances). However,
this was an expensive option; in a competitor’s mill,
such washes were necessary only twice a year. The
barium sulphate scale had been increasing steadily
over the previous six months. To make matters worse,
the last boil-out had not been 100% effective in
removing the barium sulphate scale. These results
indicated that a solution to the scale problem required
institution of measures to reduce the inputs of barium
sulphate scale components to the headbox in order
to reduce the frequency of boil-out episodes, or
methods to facilitate and improve the efficiency of the
boil-outs.Option 4, replacement of alum with PAC,
was examined. Some facts to consider about PAC are:
its basicity is important; higher basicity translates to a
more stable and less corrosive additive; PAC consists
of cationic aluminum polymers, which cause
coagulation, but it is important not to overfeed. The
performance of PAC is affected by system
temperature, pH, alkalinity, system charge demand,
feed point (mixing energy and time), and dilution
water. It is important to not pre-dilute prior to
addition and the dilution should be at the injection
point only.
The pros for using PAC include:
 Has a higher charge and can be more effective
than alum
Table 1—Concentrations of barium and sulphate moieties in
pulp samples and their effect on scaling tendency
(temperature corrected to 50oC).
Sample
Thick stock
Headbox
Whitewater silo
Barium, ppm Sulphate, ppm Scaling tendency
0.82
0.52
0.32
380.2
650.2
470.1
149.3 x saturation
162.3 x saturation
58.8 x saturation
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J SCI IND RES VOL 74 APRIL 2015
Retains higher charge density at higher pH
Does not depress pH as much as alum
Does not cationize anionic fibers as much as alum
Forms cationic micelles in the water phase of a
stock system which combines with pitch particles
and anionic trash - micelles deposit on the anionic
fibers and are carried out with the sheet
 PAC treated stock slurry contains less anionic
trash and free fines that will improve drainage
(more pronounced in low shear systems)
The cons include:

Can cause deposits

Is more expensive than alum

Is more corrosive than alum

Behaves differently (in terms of charge and
reaction time) under different dilution scenarios
From this review, it was decided that the
disadvantages of using PAC far-outweighed the
benefits and so other options for mitigating the scale
deposition problem were explored.Option 1 was then
examined. Among the choices in this option were
chemistry changes such as: pre-acidification of stock,
higher pH at the headbox, and reduced alum, or PAC
substitution, for size development. The one
considered feasible was good brownstock washing of
pulp via pre-acidification of stock as studies have
demonstrated that barium that is bound to fiber (via
carboxylic acid groups) and is released under acid
environments22. For example, the barium content of
pulp varies from 6 ppm under alkaline conditions to
less than 0.1 ppm under acid conditions; the
corresponding values for calcium ions are 900 and
less than 10 ppm, respectively.




Results for pre-acidification studies
The results for pulp washing studies, conducted in
the laboratory indicated that washing with tap water is
not effective in removing metal ions from the pulp.
The effect of washing with pH adjusted water,
however, is very dramatic as illustrated in Figure 3.
Washing with alkaline water was not effective in
removing metal ions from the pulp but washing under
acid conditions was effective. From this, it can be
surmised that countercurrent washing operations will
not be effective in removing metal ions from
brownstock pulps: as long as the return wash waters
are alkaline, insignificant metal ions will be removed
from the washed pulps. The results demonstrate
that pre-acidification of pulps will be beneficial
in removing significant inputs of barium into
Fig. 3—Effect of pH on wash removal of metal ions from
brownstock pulp (actual concentration of barium has been scaled
up x100).
the headbox system.The results show that
pre-acidification washing of machine chest (MC)
outlet stock was more advantageous than
pre-acidification washing of raw stock pulp, i.e.,
more metal ions were removed in the washing of
MC pulp than in the washing of raw stock pulp.
However, for practical reasons, it makes sense to
apply the procedure to raw stock pulp and not
the machine chest outlet pulp. Using pre-acidification
before pulp washing can help to lower scale
deposition on mill equipment. A proviso is that the
wash effluents need to be sewered so as to avoid
recirculation of the removed metal ions.A mill trial of
the pre-acidification strategy was conducted over
four days. Samples were collected at three points
and analysed for ash content. The results indicated
that the ash content in the headbox decreased to
about 25% (versus >30% before pre-acidification).
The improvement was not impressive and not as great
as expected. This is most probably because sulphuric
acid was used for pre-acidification and, also, the acid
filtrates were not sewered as the mill had no facilities
to do this. Non-sulphate containing acids were not
considered due to budget restrictions. Consequently,
the improvement was not good enough to warrant full
scale application.
Scale inhibitor trial
The next option studied was use of additives to
control and manage the scale deposits. A barium
inhibitor program was trialed: this was a continuous
treatment program using chemicals that operated on
all three mechanisms of scale inhibition: threshold
(nucleation) inhibition, dispersion (inhibition of
crystal growth Inhibition), and crystal modification.
Information about the inhibitor, as provided by the
supplier, included:
SITHOLE et al: CONTROLLING BaSO4 SCALE DEPOSITION PROBLEMS
The inhibitor has been shown to be effective for
scale control
 The additives in the product were a blend of
polyphosphates, organic phosphonates and
polyacrylate dispersants
 The dispersant components use charge and
molecular structure to interfere with the
combination of BaSO4 molecules
 The polyphosphates and organic phosphonates
function as nucleation inhibitors
 The components in the inhibitor degrade slowly
in paper machine environments
 But they can be affected by pH and are more
effective at relatively higher pH (above 4.5 to 5.0)
 The ingredients interfere with the barium sulphate
reaction (nucleation) and increase the apparent
solubility product of the scale
 However, they degrade (become ineffective) with
time and with exposure to higher temperatures in
the system
 The inhibitor is used at sub-stoichiometric levels
and minimum concentrations are, thus, required
 The dosages used vary with scaling tendency
 The inhibitor is prone to interference from
aluminum ions.
Another benefit of using this additive chemistry
is that phosphonates and polyacrylates are also
effective in controlling calcium sulphate scale23-24.
The scale inhibitor was to be fed continuously, neat
in an area of good mixing after the alum and
rosin size dosage points and close to the headbox.
Parameters for success for the trial were a reduction
of barium sulphate scale deposition, improved
runnability/machine efficiency, improved basis
weight and moisture profiles, reduced down time for
cleaning mill equipment, fewer and more effective
boil outs (the additive was supposed to soften the
scale deposits thus making them easier to clean from
metal surfaces), and improved reliability and
operation of metering and regulating devices. A
chelant boil out was performed before the trial to
ensure minimum barium sulphate buildup Fig. 4.
243

Results for spot tests for barium levels in process samples
Results for barium levels showed measurable levels
of barium at the beginning of the trial on samples
collected after the addition point of the scale inhibitor
in the headbox. However, after 3 days no barium
precipitates were evident in samples collected at this
point. Samples collected (before the scale inhibitor
Fig. 4—Effect of acid washing on the removal of metal ions
(actual concentration of barium has been scaled up x100).
addition point) at the twin roll press showed
consistent levels of barium precipitates that
corresponded to about 3 ppm free barium. These
results demonstrated that the inhibitor was effective in
removing barium from the system.
Results for on-line deposition coupon tests
Deposition on test coupons was negligible
throughout the trial period and varied from 0.02 to 0.6
g for regular one month exposure tests. The low
measurements contrasted with values that exceeded 5
g in pre-trial tests. This confirmed the effectiveness of
the scale inhibitor programme in preventing the
deposition of scale on mill equipment.
Results of scale inhibitor trial
The 3 week trial appeared to be a success in that
visual inspection of the headbox, foils, LEXAN
sheets, Uhle boxes showed that they were clean.
There were no fisheye defects in the paper produced
during the trial. Prior to the trial, these points would
be encrusted with significant amounts of scale
deposits after running for 3 weeks. Almost all of the
criteria for success were achieved and the costs
associated with use of the inhibitor were deemed to be
cost-effective. Calculations indicated that the costs
would be recouped in less than a year. The mill was
satisfied with the results and the scale inhibitor
programme was implemented on a full time basis. In
addition to this, the mill improved chip screening
operations to reduce the amount of residual bark in
the chips.
Conclusions
Troubleshooting of scale deposits and defects in
paper samples showed that the problem was caused
244
J SCI IND RES VOL 74 APRIL 2015
by barium sulphate and calcium sulphate scales.
However, it was ascertained that barium sulphate was
more of a concern than calcium sulphate as it is more
tenacious than the calcium sulphate scale. A study of
the inputs of the scale components found that the
major inputs of barium into the mill system were the
wood chips and bark contamination. The major inputs
of the sulphate component of the scale were sulphuric
acid used for pH control and alum used for rosin
sizing.Evaluation of several options for mitigating the
scale deposition problem showed that bark removal
and continuous application of a scale inhibitor
comprised
of
phosphates/phosphonates
and
polyacrylates were effective in reducing and
controlling the scale deposit problems. Institution of
these measures led to a cleaner paper machine that
required far fewer boil outs than before. In addition,
productivity improved and the fisheye defects
disappeared.A rapid method was developed for onsite estimation of barium levels in mill process
samples. The data were used to adjust the dosage of
the scale inhibitor to ensure optimum effectiveness of
the programme.
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9
10
11
12
13
14
15
16
17
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