SCALE DEPOSIT PROBLEMS IN PULP AND PAPER MILLS

Author

Bruce SitholÚ

Company

Paprican, Pointe Claire, Quebec, Canada

email

bsithole@paprican.ca

Keywords

deposits, corrosion, cleaning, pulping, bleaching, recovery, black liquors, carbonates, oxalates, silicates

ABSTRACT

Scale deposition is a phenomenon that can occur in all pulp and paper making processes. It is an unwanted occurrence that causes a number of operational problems such as plugging of equipment, inefficient usage of chemicals, increased utility costs, lost production due to downtime, corrosion, and downgraded products from increased dirt counts. This report reviews major scale deposition problems that have been identified and provides solutions that can be implemented to overcome the problems. Scale deposits that have been identified include complexes of calcium (carbonate, oxalate, sulfate, silicates), aluminum (silicates, hydroxides, phosphates), barium sulfate, radioactive radium sulfate, and silicates of magnesium. The scale deposition problems can be mitigated by procurement of good quality wood furnish and good debarking systems. Proper mechanical design and operation modifications in chemical recovery, the digester area, and the brownstock screen room as well as proper operation in the bleach area can also help in overcoming the scale deposition problems. Addition of chemical additives in the wet-end of paper machines is also a viable option.

INTRODUCTION

Accumulation of scale is a phenomenon that can occur in all pulp and paper making processes. The large quantities of water used in the process lead to the formation of scale deposits on mill equipment. This occurs even with the purest water and state-of-the art water treatment. The scale deposits can cause a number of operational problems such as plugging of equipment, inefficient usage of chemicals, increased utility costs, lost production due to downtime, and downgraded products from increased dirt counts. For example, carbonate deposits on green liquor lines and recovery tank at a packaging mill increased exponentially, reducing a 4-inch diameter line to 1-inch and brought transfer pumps on the system rated for 260 gal/min to 60 gal/min1. The scale acts as a heat insulator and accounts for 10% loss in efficiency of heat transfer processes for every 1/16" thickness of scale.

Scale that has accumulated from one type of component can form an excellent base for other scales to grow on. For example, calcium carbonate scale on black liquor evaporators is often comprised of other scales such as sodium carbonate and burkeite (double salt of sodium sulfate and sodium carbonate).

This treatise, based on reviews of the literature, summarizes some of the major scale deposit issues that have been identified and solutions that may be used to control or inhibit the scaling problems.

MECHANISMS OF SCALE FORMATION

Several mechanisms for scale formation have been hypothesized, but the main one is based on exceeding solubility limits of the scale components. Thus if we consider the formation of calcium carbonate or oxalate scale it can be deduced that the scale formation follows a sequence of events2. The initiating step, called nucleation, involves adsorption of the scale components onto a surface. As the concentration of ions in the bulk phase of the solution increases, the adsorption process continues until ion clusters begin to form on the surface. The ion clusters eventually grow to a critical size and become stable enough to remain on the surface, usually at imperfections or rough locations on the surface itself.

Once nucleation has occurred, the rate of scaling is accelerated. Scale growth, also called crystallization, can propagate via two ways, namely, ion by ion or nucleus by nucleus. Ion by ion growth occurs as ion clusters continue to be adsorbed by the existing nuclei on the surface. Such a mechanism typically leads to formation of a smooth scale (Figure 1). In nucleus to nucleus growth, nuclei are formed in the bulk phase of the solution prior to their attachment to the surface nuclei. These nuclei are larger in size than ion cluster and consequently build-up a rougher, imperfect scale on the surface. In addition, any crystals that are formed in solution may also become entrapped within the rapidly growing scale matrix resulting in a mixed deposit scale.

The scaling process is controlled by four critical parameters: cation concentration (e.g., calcium), anion concentration (e.g., carbonate), pH and temperature. The solubility product of the cation/anion product affects the precipitation of scale. pH of the bulk solution determines the form of the various species contributing to scale; for example, carbonate dissolves in acid solution and precipitates under alkaline environment. Temperature displays a positive correlation with the rate of nucleation.

The inverse relationship of temperature to solubility implies that areas of high temperature are particularly prone to scale deposition. Temperature can also cause formation of non-crystalline scale because heat has a tendency to bake deposits in place. Once baked in place, sludge deposits can be as troublesome as scale.

From the preceding paragraphs it can be deduced that the most effective means for controlling scale deposits are based on three basic chemical mechanisms:

1. Control of the rate of precipitation and of crystal growth.

2. Control of particle agglomeration and nature of scale deposit by affecting particle-to-particle attractive forces.

3. Crystal modification.

Scale deposits that have formed beyond control by any or a combination of these mechanisms require removal by mechanical and chemical means (boil-outs).

Let us now take a look at the occurrence and formation of scale deposits in various pulp and paper mill processes.

Scale Deposits on Scrubber Heat Exchangers

In an acid plant of a pulp mill, SO2 is manufactured and used to make ammonium bisulfite cooking acid. SO2 is also recovered from the digesting process that has a low-level SO2 scrubber and from the boilerhouse that has a high level SO2 scrubber. The primary loop contains a shell and tube heat exchanger that removes some of the heat from the scrubber neutralization process. The heat exchanger has a process side (tube side) and a shell side where there is cooling water. The tube side suffers from chronic fouling due to build-up of hard calcium sulfate scale but the shell side is not affected by scaling3.

Scale Deposits in Lime Kiln Scrubbers

A lime kiln scrubber can experience scaling usually in cases where there is low pH and low solids.

Build up of scale occurs on the underside of venetian blind plates and inside pipes. Fresh water, introduced in the scrubber inlet duct, combines with the SO2 in the gas to form sulfurous acid which then mixes with the lime solution, from the spray nozzles, to form Gypsum which settles out to cause scaling.

Scale Deposits on Liquid Evaporators

Fouling of the heat transfer surfaces is probably the major cause of reduced evaporator efficiency. Scaling may be a serious bottleneck to pulp production especially if the evaporators are undersized or if the mill is dependent on a single train of evaporators and weak liquor storage is limited4. During operation the evaporator heat transfer may be materially affected by the buildup of scale on the heat transfer surfaces. The scale originates from insoluble matter in black liquor. The major scale deposits have been identified as caramelized liquor, calcium sulfate, calcium oxalate, calcium carbonate, sodium carbonate-sodium sulfate, and aldonic acid calcium complex. The calcium oxalate plates out continuously in the sulfite process and increases significantly when the liquor pH exceeds 4.05.

Water-soluble scales can be a major problem in black liquor evaporation systems. This is due to scale deposition of the double salt "burkeit" that consist of sodium carbonate and sodium sulfate5. A region of the evaporation is reached in the burkeit, known as the critical solids zone, where deposition of sodium salts increases exponentially due to various liquor chemistry conditions such as sodium content, sulfate-to-carbonate ratio, soap content, and residual alkali. It is even different within a given pulping process, where changes to alkalinity, recausticizing efficiency, and sodium levels may swing with planned or unplanned mill operations5.

In the quest to improve pulping efficiency, some mills use quinones to improve digester yield. Anthraquinone (AQ) or its soluble form (SAQ) catalyzes the digestion of the pulp and improves the performance of the cooking process. However, the use of quinones changes the chemistry of the black liquor that may result in increased fouling in the evaporator train. For example, laboratory studies have shown that the scaling rate of a black liquor with AQ is about 30% faster than a black liquor with no AQ6. This is because the solubility of AQ in water or condensate (containing methanol) is low. Analysis of the scales shows that they may contain 60-80% AQ and 40-20% oil (methylated abietic acid). The scale problems are worse in hardwood mills than in softwood mills. This is due to the higher H factors used in cooking softwoods; as the digester H factor (cook time) increases, the amount of AQ surviving the cook decreases. Similarly, the problem is worse when higher doses of AQ or SAQ are employed in the digester7.

Scale Deposits in Kraft Digesters

Scale and deposit formation is most prevalent in kraft mills that use continuous instead of batch cooking8. A typical continuous digester may consist of a double feed system, top separators, high pressure feeders, steaming vessels, low pressure feeders and chip meters. Scaling tends to occur on the top separators. On a double feed system, scaling on one of the top separator baskets lets the liquor enter the main vessel with the wood chips and then the liquor flows counter current through the other top separator to satisfy the top circulation flows, and by so doing affects the capacity of that second top separator. Scaling is most common in liquor heaters due to temperature effects9.

Scale Deposits in Kraft Green and White Liquors

It has been stated in the previous paragraphs that aluminosilicates can form deposit scales in black liquor evaporators. As the aluminum and silicon concentrations increase as a consequence of system closure the management of the black liquor evaporators and the lime cycle becomes more difficult. The Al and Si species are more soluble in alkaline pulping than most other non-process elements and their dominant natural purge point from the recovery cycle is the green liquor dregs10. However, their high solubility in white liquor implies that they are likely to accumulate in the liquor due to ineffective green liquor dregs removal and dissolution of aluminum and silicon from the make-up lime.

Solutions to Scale Deposits on Kraft Digesters, Evaporators, Lime Kilns, and Heat Exchangers

Strategies for controlling calcium scales have been reported4,2,11. They can be divided into operation and cleaning processes. Operation processes include low liquor temperatures to minimize scaling, efficient soap separation, liquor recirculation pumps to prevent plugging of tubes, isolation of hardwood liquor from softwood liquor because hardwood liquors cause more scaling problems than softwood liquors, calcium deactivation, and the use of alkali and scale modifying chemicals. Cleaning operations include steam shock during water boil-out, periodical dumping of flow boxes to remove calcium flakes, periodical high-temperature washing with acid or chelate solutions, and hydroblasting.

Scale Deposits in Kraft Pulp Bleaching

The goal of achieving system closure can be attained by closing up the bleach plant. Unfortunately, a "totally closed" bleached plant is difficult to achieve because of problems with scaling. The scaling occurs at several places in the bleach plant including the first Q-stage of a 100% TCF process, acidic and neutral stages of any bleaching process, filtrate tanks, washer filter wires, shower nozzles, repulper screws, pumps, stock lines, and on-line instrumentation12-15.

As can be seen in Figure 1, calcium carbonate scale is common in bleaching stages such as extraction, peroxide, and hypochlorite where the pH exceeds 9. Calcium oxalate, on the other hand, is predominant in the chlorine dioxide and hypochlorite stages where the pH exceeds 5.

Studies by Elsander et al14 also show that formation of oxalic acid decreases along an ECF bleaching sequence as shown in Figure 2. The largest amount of oxalic acid is formed during the oxygen delignification stage but this seldom causes scaling problems because the pH (~11) is too high for the formation of calcium oxalate (calcium oxalate scale occurs between pH 2 and 9). However, inefficient washing can result in transfer of oxalic acid with the pulp into the bleach plant. In a separate part of the study, it was concluded that one third of the oxalic acid formed in the ozone bleaching stage is adsorbed onto the pulp and not removable by washing. When the pulp reaches higher pHs in a later stage of the bleaching process, the oxalic is released and becomes a potential source of scaling problems in the bleaching sequence.

Figure 1

Figure 2

Barium Sulfate Scale

Studies show that pulp lignin content (i.e., kappa number) seems to correlate with barium sulfate deposition potential16. In cases where deposition has occurred at only one of two or more parallel paper machine lines, the barium sulfate deposition is more likely on the line where the pulp has been cooked to a lesser extent, that is, to a higher kappa number. A higher kappa number indicates less delignification, and possibly the presence of natural dispersants or inhibitors for barium sulfate. The pH decrease of the stock as it proceeds toward the headbox further favors barium sulfate deposition, since dissolved lignins may be precipitating out and cannot act as natural dispersants.

Barium sulfate deposition often occurs in mills that switch from chlorine to ECF bleaching. The deposition is common on washer wires, inside washer vats and in the dilution zone of the Do stage of ECF bleach plants17. Changing from elemental chlorine to ECF bleaching is accompanied by a pH increase in the first stage from 2 to above 3. The final pH in the Do stage is influenced by several parameters including the amount of alkali in the pulp liquor carryover from the brownstock screenroom (a function of washing efficiency), chlorine dioxide charge (the ClO2 is acidic but releases additional hydrogen ions upon reaction with pulp), and buffering capacity of carbonate, dissolved organic acids and carboxylic acids present on pulp. Another parameter is the amount of acid added for pH control. A recent survey shows that a majority of Canadian mills control the final pH of the Do stage to 2.5 via the addition of sulfuric acid18. pH 2.5 is an optimum value because a lower pH would require more acid thus increasing the concentration of sulfate (and oxalic acid too). Higher pH does not lend itself to effective washing of calcium from the pulp and can lead to formation of calcium carbonate scale in the Eo stage. The pH of the Do stage is controlled because it determines the relative proportions of ClO2, chlorite, chlorate and Cl2 in the ClO2-pulp mixture. The amounts of these compounds ultimately determine the delignification, brightening and selectivity in ECF bleaching. They also affect the formation of AOX19. The optimum pH is where the concentrations of chlorite and chlorate are minimized.

Sulfuric acid or excess sodium sesquisulfate (Na3H(SO4)2), a by-product of chlorine dioxide generation, is used for pH control. Unfortunately, this introduces more sulfate into the bleach plant and exacerbates BaSO4 scaling. Sodium sesquisulfate has four times the sulfate of sulfuric acid for an equivalent drop in pH.

Solutions to Scale Deposits in the Kraft Mill

Solutions that can be employed in preventing/controlling scale problems in the bleaching area can be divided into three categories, namely, reducing the input of materials that contribute to scale; removal of scale build up; and prevention/control of scale formation.

Radium Sulfate Scale Deposition

It has been reported that some barium scales contain radium sulfate that may pose a safety hazard to personnel. In one case the level of radium sulfate in a barium scale on a washer drum wire was so high that the wire could not be disposed of due to its high radioactivity. Also, the level of radium sulfate was so high that when the washer drum was being removed not one person was allowed to work on it for more than 8 hours a day. It is estimated that 250 g of barium sulfate scale contained 1/10 of the allowable 8 hour radiation exposure20. This problem has been observed in other industries (e.g., the oil industry) where the radium sulfate has deposited onto seals and fiberglass surfaces.

Naturally occurring radioactive materials (NORM) that originate from the earth's crust include radium and its decay products. The NORM can be found in trees and groundwater because water percolating through rock or soil dissolves some of the radium and when this water is used in an industrial process the pH, temperature, sulfate ion and barium ion conditions may be sufficient to induce its precipitation from solution20. The solubility of radium sulfate is very low but it forms a stable colloidal suspension that does not settle. Since NORM co-deposits with barium, it can be found wherever barium scale accumulates. Indeed, the commercial production of radium is achieved by enrichment via the addition of barium to a suspension of radium sulfate that results in almost quantitative recovery of radium. International Paper conducted a survey of piping and equipment in three of its mills and identified three mill areas where NORM levels exceeded state regulatory limits: the paper machine headbox, the bleach washer drum, and the drop leg of the bleach washer drum20. A special license is required to dispose of or decontaminate wastes containing NORM.

Solutions that work for barium sulfate scale should help eliminate radium sulfate scaling.

SCALE DEPOSITS IN THE PAPER MACHINE AREA

Barium Sulfate Scales

The problem of barium sulfate deposition is not limited to kraft mills since it has been observed in groundwood mills, fine paper machines and linerboard grades16,21. The deposits have also occurred in sulfite mills and recycled paper mills. The severity of the problem is variable from mill to mill, and the deposits have been found on screens, cleaners, fan pumps, organ tubes, headboxes, rectifier rolls, headbox lips and slices, and on Fourdrinier foils16. The problem is more severe when hardwood furnish is present since this furnish contains 10 times more barium than its softwood counterpart. The problems emanate in the form of plugged screens, fiber bundles that break loose to cause crush-outs in the press section, and poor formation.

Aluminum-based Scales

Alum is used in acid papermaking for several applications such as pH control, pitch control, retention, water clarification, and rosin sizing. Some alum is frequently used in alkaline papermaking to improve sizing by increasing fines retention and also to improve retention and drainage22. When the pH of a system exceeds 5.0, the alum hydrolyses to form insoluble aluminum hydroxide that can cause serious deposit problems. The deposits occur in various places such as primary screens, fan pump blades, machine chests, headboxes, wire return rolls, foil blades, vacuum boxes and couch rolls23,24. Other aluminum-based deposits that develop include aluminum phosphate and aluminum-size deposits of both hydrolyzed ASA and AKD25.

Phosphoric acid is used to acidify certain broke streams of high pH to reduce brightness reversion because it can react on the surface of calcium carbonate particles forming a coating of calcium phosphate25. This coating protects the particle from additional attack by acid. The calcium phosphate is often not well retained and can result in severe felt filling.

Solutions to Scale Deposits on Paper Machines

Avoiding conditions where alum would experience high pH, long dwell times, and heat is one way to solve aluminum scale deposition23. The concentration of soluble aluminum in the headbox should be measured to ensure that it is not greater than twice the alum addition rate in kg/t24.

Chelants can be used to tie up free metal ions that contribute to scale. However, they can be expensive as they generally work at stoichiometric levels and large amounts of the chelants are required to tie up the metal ions. It may be better to use threshold inhibitors that have the ability to inhibit crystal growth at significantly less than stoichiometric amounts. These inhibitors are capable of maintaining ion pairs in solutions well beyond their solubility limit. For example, polyphosphates can be used in the presence of calcium and carbonate ions to prevent calcium carbonate formation. In cases where crystal formation cannot be totally inhibited, a dispersant can be used to prevent the crystal from depositing on a surface. Anionic polymers such as sodium polyacrylate impart an increased negative charge to the particle as well as providing a steric barrier, thus preventing formation of the deposit25.

Replacing alum with an organic cationic donor can help reduce aluminum-based scale deposits, as this will eliminate one source of aluminum ions in the system. Alternatively, other forms of aluminum compounds could be used. For example, polyaluminum chloride (PAC) can be used to retain as much wood resin as possible to the sheet. Barium sulfate scaling is believed to occur via a monomolecular layer of resin (pitch) that first attaches to the metal surfaces and then attracts the BaSO4 crystals to the layer21. The use of PAC thus avoids or reduces formation of the resin layer. Concurrent reduction of the sulfate input is also important.

SCALE DEPOSITS DUE TO FILLERS

High levels of inorganic fillers such as clay, calcium carbonate, and titanium dioxide are employed in a variety of paper and board making processes. Poor filler retention can result in build up of filler in the white water eventually leading to deposit problems such as felt and wire filling. The fillers can also cause pitch deposition problems due to complexation with wood resin components26,27. Fillers are much smaller in size than fibers and therefore difficult to retain. In addition, the fillers are negatively charged in paper machine whitewaters and thus are less likely to be attracted to negatively charged fibers. The situation is made worse by the use of anionic dispersants that are used to stabilize the filler suspensions. For example, recent reports from Paprican have shown that use of certain calcined clay fillers stabilized with trisodium polyphosphate can lead to the formation of aluminum soaps complexes that cause deposition problems on press rolls28,29.

Solutions to Scale Deposits Caused by Fillers

Retention aids are used to overcome the repulsive forces that keep the filler particles dispersed. The retention aids should agglomerate the fillers and fines such that they will be retained but not over agglomerated as this results in poor formation. Fillers should be stabilized with dispersants that do not cause scale problems. For example, avoiding trisodium polyphosphate dispersants in calcined clay fillers avoids deposition problems on press rolls in newsprint making29.

CONCLUSIONS

The types, sources and place of occurrence of scale that can accumulate in pulp and paper mill processes are quite diverse and are summarized in Table 1. This report shows that scale problems can occur in all pulp and paper making processes. Controlling inorganic scale in pulp and paper mills includes procurement of good quality wood furnish, good debarking systems, proper mechanical design and operation modifications in chemical recovery, the digester area, and the brownstock screen room. In the bleach plant, pH and thermal shock should be minimized while washing efficiency should be maximized. Continual build up of scale can be moderated by application of deposit control agents. Surfactants can be applied in the paper machine system to maintain control over pitch and carbonate in the system and in the paper, and to reduce attraction between deposition components and machine equipment.

While increase in system closure may exacerbate scale problems, technologies have been developed that can be used to overcome the problems. For example, computer simulation studies have been used, with success, to evaluate the impact of various mill scale components on system closure.

Table 1. A summary of types, sources, location and causes of scale deposits in pulp and paper mills.

Scale

Source

Place of occurrence

Conditions for scaling

Aluminum hydroxides

Aluminum: wood, alum, sodium aluminate, polyaluminum chloride, rosin sizing

Paper machine, deinking plants

Temperature, high total dissolved solids (TDS)

Aluminum phosphates

Aluminum: wood, alum, sodium aluminate, polyaluminum chloride, rosin sizing

Phosphates: starch, filler stabilizers

Paper machine, size press

Excess phosphoric acid

Aluminosilicates

Aluminum: wood, alum, sodium aluminate, polyaluminum chloride, rosin sizing

Silicate: peroxide stabilizer

Recovery area

High residual alkali

Barium sulfate

Barium: wood

Sulfate: sulfuric acid, white liquor, alum

Bleach plant, paper machine

Oxidation of sulfite, addition of sulfate, poor white liquor clarification, temperature, high TDS

Calcium carbonate

Calcium: wood, white liquor, calcium carbonate filler, hard water

Carbonate: white liquor, calcium carbonate filler, formation in caustic extraction stages

Digesters, heat exchangers, evaporators, lime kilns, brownstock screen room, bleach plant, paper machine, deinking plants

Alkaline conditions, high temperature, high TDS

Calcium oxalate

Calcium: wood, white liquor, calcium carbonate filler, hard water

Oxalate: wood, pulping , delignification

Recovery area, digesters, bleach area, deinking plants, BCTMP mills, fermentors

Acid conditions, high TDS

Calcium sulfate

Calcium: wood, white liquor, calcium carbonate filler, hard water

Sulfate: sulfuric acid, white liquor, alum

Recovery area, digesters, bleach area

Oxidation of sulfite, addition of sulfate, poor white liquor clarification, temperature, high TDS

Calcium silicate

Calcium: wood, white liquor, calcium carbonate filler, hard water

Silicate: peroxide stabilizer

Pulp brightening processes, refiner plates, deink plants

Recycling of bleach effluents, drop in pH, low Mg/Ca ratio in pressate, carryover of deinking gents

Burkeit

Sodium: wood, water, process additives

Carbonate: white liquor, calcium carbonate filler

Black liquor evaporators

Sulfate:carbonate ratio, residual alkali

Magnesium silicate

Magnesium: wood, peroxide stabilizer

Silicate: peroxide stabilizer

Pulp brightening processes, refiner plates

Recycling of bleach effluents, drop in pH, low Mg/Ca ratio in pressate

Radium sulfate

Radium: wood, water

Sulfate: sulfuric acid, alum

Associated with barium sulfate scale

Same as for barium sulfate


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