TIP 0416-15 Chloride and Potassium measurement and control in … · 2020. 1. 9. · deposits that...

TIP 0416-15

ISSUED – 2005 REVISED – 2015

©2015 TAPPI

The information and data contained in this document were prepared by a technical committee of the Association. The committee and the Association assume no liability or responsibility in connection with the use of such information or data, including but not limited to any liability under patent, copyright, or trade secret laws. The user is responsible for determining that this document is the most recent edition published.

TIP Category: Automatically Periodically Reviewed (Five-year review)

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CAUTION: This TIP may require the use, disposal, or both, of chemicals which may present serious health hazards to humans. Procedures for the handling of such substances set forth on Material Safety Data Sheets must be developed by all manufacturers and importers of potentially hazardous chemicals and maintained by all distributors of potentially hazardous chemicals. Prior to the use of this technical information sheet, the user should determine whether any of the chemicals to be used or disposed of are potentially hazardous and, if so, should follow strictly the procedures specified by both the manufacturers, as well as local, state, and federal authorities for safe use and disposal of these chemicals.

Chloride and potassium measurement and control in the pulping and chemical recovery cycle Scope Considerable research and operating experience have established industry acceptance that limiting chloride and potassium concentrations in the pulping and recovery cycle liquors is necessary to achieve recovery boiler capacity and high availability. Limiting these non-process elements can result in ash deposits on boiler heat transfer surfaces that are controllable by sootblower operation. This Technical Information Paper (TIP) will provide an understanding of the sources of chloride and potassium in the mill cycle and their effect on carryover particles and fume in the recovery boiler, the steps that can be taken to develop a mill material balance and determine the maximum acceptable concentration of these in the precipitator ash, and the various approaches that can be applied to limit the concentrations. Ash produced in many recovery boilers contains alkali carbonate compounds, in addition to the sulfates, and their effect is also addressed. Note that there are many other factors that contribute to recovery boiler fouling such as: high carryover, low ash pH, and poor sootblower operation. These issues are not covered in this document. Processes are described that are available and are being developed to remove chloride and potassium from the cycle with sufficient detailed information to permit an operating mill to make a preliminary evaluation of the most suitable process (1). Definitions A more extensive definition of the terms following has been provided by Tran (2). First melting temperature – the temperature at which the first liquid phase is formed in the material. Sticky temperature – temperature at which the material contains 15 to 20% liquid phase and becomes sticky. TSTK or T15. Radical deformation temperature – temperature above which the material contains about 70% liquid phase and is so fluid that it can run off due to its own weight (also referred to as slagging temperature). TRD or T70. Sticky temperature range– temperatures in the range between TSTK and TRD where deposits are sticky. Enrichment factor (EF) - a multiplier factor by which the chloride and potassium in the black liquor solids increase in the ash deposit. TPD – tons per day of pulp production

TIP 0416-15 Chloride and potassium measurement and control / 2 in the pulping and chemical recovery cycle

Safety precautions The application of the information in this TIP requires personnel to come in contact with the process liquid streams in the pulping and recovery cycle that must be handled with caution. Some of these are caustic at temperatures that can cause burns, such as, black liquor, green liquor and white liquor. Others are acidic such as spent acids from chlorine dioxide production and tall oil acidification. The mill should have Material Safety Data Sheets for these streams that describe procedures for sampling, use and disposal, and the protective attire to be worn, and these should be strictly adhered to. Definition of chloride and chlorine as used in this document Chloride refers to the Chloride ion as it is present in either the liquor streams or the ash. In the liquor streams Chloride would exist as a negatively charged ion (Cl-), which is typically balanced by a positively charged ion, either sodium (Na+) or potassium (K+). In the ash it would be present mostly as either sodium chloride (NaCl) or potassium chloride (KCl). When using the term chlorine (Cl), this document is referring to all forms of this element, not only the chloride ion. For example an elemental analysis indicates the amount of Cl, not the amount of chloride ion in a sample analyzed. Both amounts are exactly the same, nonetheless. Chloride and potassium in the mill cycle The emphasis in the kraft pulp mill to reduce effluent discharge results in the retention of non-process elements in the mill recovery and pulping cycle. The concentrations increase as the effluent discharges are reduced and more are retained in the pulping and recovery cycle at equilibrium. They must either be prevented from entering the mill or separated from the cycling liquor and removed from the mill, or both. Chloride and potassium are non-process elements in the liquor cycle of the mill. They can reach a concentration in the black liquor where they are troublesome for the operation of the recovery boiler. Their presence can have a negative impact on the availability of the recovery boiler to process black liquor. Increasing concentrations of chloride and potassium in black liquor have resulted in increased fouling of the heat transfer tube surfaces with deposits that can become hard and more difficult to be removed by the sootblowers. An increase in the chloride and potassium concentrations lowers the temperature at which the ash becomes sticky, but also increases the range of temperature in which deposits can accumulate between the sticky temperature (TSTK) and the radical deformation temperature (TRD), thus reducing the effectiveness of sootblowers to remove the deposits. Low first melting temperature deposition is the most significant factor contributing to corrosion of superheater tubes. Molten smelt flowing on the surface of superheater tubes generally occurs at the flue gas inlet in the lower sections of the superheater tube banks where the temperature of the gas exceeds the radical deformation, or slagging, temperature of the deposit. Considerable work on deposit properties are documented in the following reference; results of research on first melting temperature were published in 1999 (3). The impact of chloride and potassium in the deposits on recovery boiler heat transfer surfaces has been the subject of intense study and there exists today an extensive list of literature on the subject. Only a small part of the literature is cited to an extent required for determining and interpreting the results of sample analysis, sticky temperature and the melting point of ash deposits, and processes for chloride and potassium removal. The negative impact of the chloride and potassium in the black liquor and carryover particles resulting from liquor combustion requires consideration of alternative methods to control their levels in the mill cycle. The TIP is written for use in the pulp mill by the people working in the mill. It is designed to develop the information to establish a mill balance on chloride and potassium, to determine a limit of these in the mill cycle to enhance recovery boiler availability, to evaluate the best control process for removal of chloride and potassium from the precipitator ash, to recognize that the carbonate will affect the process selection and operation, to interpret results, and to know the simple tools for measurement and control of the operation.

3 / Chloride and potassium measurement and control TIP 0416-15 in the pulping and chemical recovery cycle

Effect of chloride and potassium on boiler availability The availability of the recovery boiler to process black liquor for chemical recovery is compromised by increased concentrations in the black liquor of Cl and K (2). The amounts of Cl and K in the deposits on boiler heat transfer surface and in the precipitator ash increase with an increase in the Cl and K concentrations in the black liquor. Interrupting the chemical recovery process to clean the boiler heat transfer surfaces can result in an incremental decrease in pulp production. Likewise, outage time to repair superheater tubes that have thinned due to corrosion can decrease production. The desirable recovery boiler operation is to be on-line from one scheduled outage to the next, generally one year, with the only cleaning required being successfully provided by the sootblowers removing ash deposits from surfaces with blowing steam jets. The information in this TIP is intended as a guide to determining for a specific set of mill conditions the Cl and K concentration in the deposits on superheater and generating bank surface below which they are easily removed by the impact of sootblower steam and above which the deposits become sticky and/or hard and are not effectively removed, to establish the related Cl and K in the precipitator as a measurement for control, and to select control measures to operate accordingly. Sticky deposits are more difficult to remove from surfaces than non-sticky deposits. In principle, if the deposit is below TSTK, it is easily removed. The temperature at which deposits are sticky is lowered by increasing amounts of Cl and K. If the deposit on heat transfer surface is above TRD, the material is fluid and does not build on the surface; but flows down and off the tubes. The objective is to operate the mill with equilibrium Cl and K concentrations that minimize the fraction of the convection heat transfer surface covered in sticky deposits and most importantly, to insure that the generating bank does not operate in the sticky temperature range (4). This critical temperature range usually occurs in the superheater area. The effectiveness of sootblowers to clean the heat transfer surfaces can be further impeded by sintering of the fine fume particles that collect on the tubes. Densification of porous fume deposits results in hardening that makes more difficult the removal with sootblowers. Sintering rate of deposits from fume particles is shown to correlate with both first melting temperature, and independently, the chloride content of the fine particles (5). Increased frequency of sootblower operation may be required to remove the deposits before they harden to an extent that they cannot be removed.

Fig. 1. Effect of chloride on carryover deposit TSTK and TRD (2)

Figure 1 defines that a range of temperature exists over which ash deposited on the convection surface is sticky and will tend to accumulate; these deposits are difficult to remove with the sootblower steam jets. In this figure, the

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Slagging region

Radical deformationtemperature

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First melting temperature

Cl/(Na+K) mole%0 102 124 146 168 18 20

Sticky region

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(oC

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5 mole% K/(Na+K)


sticky temperature at 5 mole percent potassium varies with the concentration of the chloride component of the ash. There is a family of curves for other potassium concentrations. A low first melting temperature can result in the presence of liquid smelt at the outside surface of the superheater tubes. The combined effect of fluid smelt and a high tube surface temperature (primarily a result of the steam temperature in the tube) can accelerate metal loss due to corrosion. However, corrosion also occurs on a superheater tube at a moderate steam temperature where char particle carryover deposits on the tube result in local reducing conditions; it is well known that this can decrease the susceptibility of the tubes to corrosion (6). The designer can select tube metallurgy and arrange the flow path of steam through the superheater for optimum operation, but it will still be necessary to limit the Cl and K to control corrosion. An extreme case of recovery boiler corrosion experience in Pacific coast mills with chloride concentrations in white liquor ranging from 10 g/L to 100 g/l, most of it at about 30 g/L is described in this reference (8). The experience cited is with boilers operating at relatively low pressure and steam temperature conditions of 4.24 MPa to 4.93 MPa (600 – 700 psig) and 370ºC to 400ºC (700 - 750°F). High chloride concentrations would result in more severe corrosion at higher pressures and temperatures. Another reference describes the experience with a closed effluent free concept in a bleached pulp mill at Thunder Bay, Ontario, where the concentration of white liquor averaged 20 g/L to 30 g/L (8). The recovery boiler operating at 454ºC to 466ºC (850 - 870ºF) steam temperature had severe superheater corrosion after 13 months. These are cases with extreme concentrations of Cl and K that emphasize the corrosion threat of chloride as steam temperature increases. Boilers are now producing steam at >11.5 MPa (1650 psig) and >495°C (925°F) and in the future, are being installed for yet higher pressure and temperature. Control of Cl and K in the black liquor is critical for recovery boilers, and even more so for those designed to operate at high temperature and pressure for electrical generation. Sources of chloride and potassium The principal sources of Cl and K are wood supply and makeup chemicals entering the mill cycle. Their concentration in the pulp wood is very much dependent on the mill location and species. Hardwood typically contains a higher concentration of K than softwood (9). Chips typically have 200 … 600 mg/kg potassium per dry wood. Because the bark has typically 1400 … 2200 mg/kg potassium per dry bark then poor debarking can increase potassium input to the mill. Wood harvested from coastal areas generally has a higher content of Cl than wood from inland sources. It is a practice in some coastal areas to transport logs to the pulp mill by water rafting and storage with an obvious effect of increasing the chloride concentration in the wood supply. However, in general, the pulp mill has no control of potassium or chlorides in the pulp wood. The concentration of potassium in the cycle is largely established by the content in the wood supply. The water supply is infrequently a source of Cl and K. However, there are exceptions. There is one mill where water from drilled wells was high in Cl because the wells extended under the ocean and there was seepage of salt water into the wells. Makeup chemical is purchased to provide sodium and sulfur to replace these chemicals that are lost from the mill cycle in the pulp product, to the sewer or discharged from the recovery boiler stack. Caustic soda (NaOH), or “caustic,” is available with various levels of chloride; chemical grade caustic is available with very low chloride content. Salt cake (Na2SO4) is also sourced with different degrees of purity. Sodium hydrosulfide (NaHS), which is used widely by bleached pulp mills for sulfidity control, is available as a chemical plant product or as a byproduct of oil refineries, with the refinery byproduct containing a wide range of non-process elements. Other sources of sodium and sulfur for makeup are at times available from nearby industrial plants looking for opportunities to dispose of their waste streams ranging from utility power station stack scrubber effluent to caustic used in the manufacture of carpet yarn. The mill must exercise control over the chloride content of makeup chemicals entering the cycle. Makeup caustic is frequently a major source of chloride. Chemical with lower Cl concentration can be purchased albeit at higher cost. Caustic makeup from diaphragm processes can have a residual Cl content of 1 to 2% (10,000 to 20,000 ppm) as compared to caustic from a membrane process or mercury cell with less than 50 ppm chloride. The economics of purchasing higher quality chemical for control can be compared with the cost of removing the Cl and K from the mill cycle.


The substitution of soda ash (Na2CO3) for caustic as the source of sodium makeup is an alternative that warrants consideration. This could be an attractive approach in a mill that has excess causticizing system and lime kiln capacity. However, this process substitution would not be considered in a pulp mill where the lime kiln capacity limits production. The financial evaluation of Na2CO3 as makeup requires consideration of the increased amount of auxiliary fuel that would be used in the lime kiln. Most bleached pulp mills generate chlorine dioxide (ClO2) on the mill site. The process effluent is a spent acid containing sodium and sulfur that is neutralized and used as a source of makeup chemical in the mill cycle. The effluent normally contains low levels of Cl, but higher concentrations are possible if the ClO2 generation is not optimal or the system requires maintenance. There are other sources of sodium and sulfur in individual mills that are frequently added to the black liquor incinerated in the recovery furnace, such as, soap, acidic tall oil brine and effluent from a power boiler scrubber. The tall oil brine and the ClO2 plant effluent are usually neutralized with caustic, which may include chlorides. A common process for ClO2 production is the R8 process that reacts sulfuric acid and methanol with salt cake (Na2SO4). An effluent by-product is sodium sesquisulfate acid [Na3H(SO4)2]. For each ton of ClO2 produced, the effluent is an equivalent of 1.3 tons of Na3H(SO4)2 solution, in which there is residual chloride. The newer R10 process operates with less Cl in the byproduct having about 1.1 ton of Na2SO4 crystals per ton of product ClO2. The R10 process also has an advantage of a lower sulfur to sodium ratio in that each mole of sodium in the crystals carries with it one-half mole of sulfur whereas the R8 effluent brings to the cycle two-thirds of a mole of sulfur for each mole of sodium. The effect is that the R10 byproduct used as makeup puts less pressure on elevating the sulfidity than does the R8 product. The R10 process offers additional advantages to the mass balance in the mill pulping and recovery cycle. A further reduction in chlorides can be achieved by washing the crystals before mixing them into the black liquor. Also, as the byproduct does not need to be neutralized as is the requirement with the Na3H(SO4)2 solution, the amount of caustic required for neutralization is eliminated, thus eliminating a source of chloride. There is another alternative to investigate for the R8 process. This is to substitute hydrogen peroxide for the methanol that is widely used as the reducing agent in the ClO2 generators (10). The result of using the hydrogen peroxide is a decrease in the quantity of salt cake available for mill makeup. This could be particularly attractive for a mill already generating hydrogen peroxide for the bleach plant. Obtaining samples for determination of chloride and potassium concentration Ash samples for the determination of chloride and potassium values that are used in a mill balance and to establish the requirements for the removal process are generally preferred to be collected at one of the electrostatic precipitator discharge chutes. The sample point should be located downstream of the seal valve that prevents a back flow of air into the precipitator. The practice is to install a small door or a short pipe nipple with a large valve at the sample location. The opening should be large enough that a ladle or cup can be inserted into the chute to “catch” the sample. The quantity of sample required for analysis is about 150-200 ml (1 pint). Where conditions make it possible, a sample should be collected from each of the precipitator chambers and the equal quantities mixed thoroughly to prepare a representative sample for analysis. However, other locations are acceptable, such as, the discharge from an economizer hopper ash conveyor. Avoid taking a sample from the final electrical field of a chamber as the fine ash at this point may be enriched in chloride. There is no steadfast rule for sampling and analysis frequency. Some considerations for obtaining ash samples for analysis are:

• Obtain samples when the boiler has been operating steady at a constant liquor flow for several hours (4 hours is good).

• Once a month obtain samples during operating periods when the sootblowers are effective in maintaining clean surfaces. Surfaces that are accumulating deposits can be observed to have gas side increasing pressure drop and/or decreasing superheater steam temperature.

• For a mill producing both hardwood and softwood pulp, occasionally analyze samples for both.


• At any time that a black liquor sample is to be collected and analyzed for elemental composition, for example, for boiler performance calculations, obtain an ash sample. The relative Cl and K concentrations between the precipitator ash and black liquor yields an enrichment factor, which if abnormally high, suggests to make changes in the black liquor burning. An enrichment factor will be needed for the mass balance of the cycle to calculate alternative processes.

• If there is an increase in the mill sulfidity level, more SO2 may be formed in the furnace. • With high SO2 levels (>50 ppm), it may be necessary to make more frequent analysis of the black liquor or

the white liquor for Cl content as a control measure. Variations in SO2 will cause variability in the Cl content of the precipitator ash due to reactions between SO2 and alkali chlorides (NaCl and KCl) to form HCl (11). Therefore, with high SO2, an assessment of the mills requirement for Cl and K removal should be based on the Cl levels in actual recovery boiler deposits as precipitator ash chloride levels may not be well correlated with deposit chloride levels.

Measurement of Cl in white and green liquors Monitoring the chloride levels in liquors and inferring the chloride content of recovery boiler carryover and electrostatic precipitator (ESP) ash levels (and the resultant sticky temperature) can be done effectively by using either the chloride content of green liquor or white liquor. There are a variety of ways to do this by using either wet chemistry methods or by the use of ion specific electrodes. As pointed out above this will eliminate the variability due to fluctuations in SO2 levels in a recovery boiler and will also reduce the variability due to short term changes on the recovery boiler. Cl and K in ESP ash Table 1 shows data on chloride (Cl) and potassium (K) levels in ESP ash from 30 different kraft pulp mills in Canada, the United States, Brazil and Scandinavia, in the ascending order of Cl. The Cl content in the ash varies significantly from mill to mill, from less than 1 wt% Cl (Mills #1 to 5) to over 10 wt% Cl for coastal mills (Mills #27 to 30). The potassium content also varies but not as much as chloride. It typically ranges between about 4 to 10 wt% K, depending on the wood species used in individual mills. ESP ash from hardwood mills tends to contain more K than that from softwood mills. ESP ash consists of mostly sodium (Na), potassium (K), chloride (Cl), sulfate (SO4) and carbonate (CO3). The total amount of these constituents should be close to 100 wt%, with a tolerance of ± 2 wt%. Furthermore, in a perfect ionic system, the negatively charged ions (anions) should be balanced by the positively charged ions (cations). This means that the anions-to-cations (-/+) molar ratio of the ash should not deviate from 1 more than ± 0.05. Impurities in the ash can cause some deviation in the total. The variance in the charge balance has some scatter, possibly reflecting the cumulative deviation for analyzing each constituent. Calculation of -/+ molar ratio Below is an example for calculating the -/+ molar ratio of the precipitator ash from Mill #16 which has a typical Cl content of about 2.5 wt% Cl. Note that since Na+, K+ and Cl- are monovalent (single-charged), their atomic weights must be multiplied by 2 to obtain a molecular weight that matches the divalent (double-charged) SO42- and CO32-. The molecular weights of Na2, K2 and Cl2 are respectively 23 x 2 = 46, 39.1 x 2 = 78.2, and 35.5 x 2 = 71. Thus,

− +⁄ molar ratio =∑ anions∑ cations

= Cl2 + SO4 + CO3

Na2 + K2=

2.53/71 + 53.1/96 + 7.50/6031.1/46 + 4.96/78.2

= 0.96


Table 1. Composition of ESP ash from 30 kraft pulp mills

Mill # Na K Cl SO4 CO3 Total - / + Cl/(Na+K) K/(Na+K) wt% wt% wt% wt% wt% wt% molar ratio mol% mol%

1 28.96 6.27 0.29 64.80 0.14 100.5 0.96 0.58 11.30 2 31.25 3.94 0.60 61.04 3.31 100.1 0.96 1.15 6.90 3 28.22 7.14 0.71 60.51 4.08 100.7 1.00 1.43 12.96 4 29.86 4.07 0.79 60.92 2.91 98.6 0.99 1.59 7.42 5 31.39 3.22 0.80 64.41 1.26 101.1 0.97 1.56 5.69 6 29.75 5.89 1.22 60.66 2.73 100.2 0.96 2.37 10.43 7 30.78 3.78 1.26 63.94 0.63 100.4 0.97 2.47 6.74 8 30.25 5.94 1.70 51.20 10.04 99.1 0.99 3.26 10.35 9 29.82 4.41 1.50 61.69 1.07 98.5 0.97 3.00 8.00 10 29.14 5.81 1.75 62.50 0.29 99.5 0.96 3.48 10.49 11 31.23 9.23 2.26 38.67 19.25 100.6 0.95 3.99 14.81 12 29.83 5.00 2.27 62.00 1.39 100.5 0.98 4.48 8.97 13 32.83 4.74 2.35 46.49 13.54 99.9 0.96 4.27 7.83 14 30.00 3.80 2.40 60.30 3.00 99.5 1.02 4.82 6.93 15 31.60 3.33 2.52 54.66 7.39 99.5 1.00 4.86 5.84 16 31.10 4.98 2.53 53.10 7.50 99.2 0.96 4.82 8.60 17 31.08 7.11 2.54 47.97 11.03 99.7 0.94 4.66 11.86 18 28.06 8.52 2.75 53.35 5.90 98.6 0.96 5.38 15.16 19 29.42 7.11 2.99 52.73 7.34 99.6 0.98 5.76 12.44 20 30.78 5.76 3.24 54.02 5.90 99.7 0.95 6.15 9.92 21 31.00 3.49 3.59 59.89 2.50 100.5 1.00 7.03 6.21 22 29.31 5.30 3.88 55.56 4.20 98.3 1.00 7.75 9.61 23 32.42 3.63 4.17 55.92 3.84 100.0 0.94 7.82 6.17 24 31.79 3.75 6.11 57.09 2.11 100.9 0.97 11.65 6.49 25 33.07 6.90 6.23 36.39 17.92 100.5 0.95 10.87 10.92 26 25.90 11.15 6.40 50.80 5.95 100.2 1.02 12.78 20.21 27 31.40 4.94 10.12 50.88 2.44 99.8 0.96 19.11 8.47 28 31.86 4.03 12.36 51.70 0.60 100.6 0.97 23.40 6.93 29 33.6 4.35 13.59 43.7 5.5 100.7 0.94 24.37 7.09 30 30.2 7.74 13.61 41.9 6.3 99.8 0.97 25.37 13.11

Calculation of mole% Cl/(Na+K) and mole% K/(Na+K) Similarly the mole% Cl/(Na+K) and mole% K/(Na+K) of ash from Mill #8 can be calculated as follows:

mole% Cl (Na + K)⁄ = Cl2

Na2 + K2× 100% =

2.53/7131.1/46 + 4.96/78.2

× 100 = 4.82

mole% K (Na + K)⁄ = K2

Na2 + K2× 100% =

4.96/78.231.1/46 + 4.96/78.2

× 100 = 8.60


Note that since Cl, K and Na are all monovalent, there is no need to double their atomic weight. The above formulas can be simplified as:

mole% Cl (Na + K)⁄ = Cl

Na + K× 100% =

2.53/35.531.1/23 + 4.96/39.1

× 100 = 4.82

mole% K (Na + K)⁄ = K

Na + K× 100% =

4.96/39.131.1/23 + 4.96/39.1

× 100 = 8.60

Cl and K enrichment factors of precipitator ash Cl and K enrichment factors, EFCl and EFK of a precipitator ash are defined as the ratio of mole% Cl/(Na+K) in ash to the mole% Cl/(Na+K) in as-fired black liquor, and the ratio of mole% K/(Na+K) in ash to the mole% K/(Na+K) in as-fired black liquor, respectively, i.e.

EFCl = mole% Cl (Na + K)⁄ in ash

mole% Cl (Na + K)⁄ in as fired black liquor

EFK = mole% K (Na + K)⁄ in ash

mole% K (Na + K)⁄ in as fired black liquor

The mole% values for Cl and K in the precipitator ash can be calculated in the same way as above. To obtain the mole% values for Cl and K in the as-fired black liquor, however, the Cl, K and Na contents of the liquor are needed. Table 2 shows the elemental analysis of as-fired black liquor from Mill #8 collected at about the same time when the precipitator ash was collected.

Table 2. Elemental analysis of as-fired black liquor from Mill #8

Element Wt% Carbon, C 33.40 Hydrogen, H 3.42 Nitrogen, N 0.13 Sulfur, S 4.92 Sodium, Na 19.70 Potassium, K 2.10 Chlorine, Cl 0.55 Inerts 0.21 Oxygen (by difference) 34.13 Total 100.00

The mole% values for Cl and K in the as-fired black liquor from Mill #1 are:

mole% Cl (Na + K)⁄ = Cl

Na + K× 100% =

0.55/35.519.7/23 + 2.1/39.1

× 100 = 1.86

mole% K (Na + K)⁄ = 𝐾𝐾

𝑁𝑁𝑁𝑁 + 𝐾𝐾× 100% =

2.1/39.119.7/23 + 2.1/39.1

× 100 = 5.90


The enrichment factors are:

EFCl = 4.821.86

= 2.6

EFK = 8.605.90

= 1.5

For this example of calculation, the enrichments of Cl and K in the ash are consistent with the typical values of 2.5 for Cl and 1.5 for K suggested by Hupa (12). Effect of Cl and K on sticky temperature of carryover deposits Considerable research on deposit properties has been conducted. A summary of this work has been published (2). Figure 3 shows the effect of Cl content on the deposit TSTK at varying potassium levels. The Cl and K concentrations in carryover deposits at the superheater region and at the boiler bank inlet are reflected in the figure. It is not intended to be applied for precipitator ash that has Cl and K enrichment values associated with fume particles. It is a part of this TIP because it provides valuable insight into the relative impact of Cl and K on the sticky temperature range.

Figure 3. Effect of Cl on the deposit TSTK for a range of potassium mole ratios

The figure illustrates the considerable effect on TSTK of reducing Cl below 8 mole %, and that higher concentrations have little effect. Further, as the Cl deceases, the sticky temperature range decreases signifying that the section of the superheater bank subject to the sticky temperature range is decreasing. The potassium concentration will remain relatively constant as the chloride is reduced entering the cycle. For example: by changing to a higher quality of caustic soda makeup. Most of the potassium enters the mill with the wood. There is usually no practical way to intervene with the wood entering the pulp mill to reduce potassium. However, there will be some opportunity to reduce potassium along with removal of chloride depending upon the process selected for chloride control. An important consideration is that a very efficient chloride removal strategy

K/(Na+K) mole%

Cl/(Na+K) mole%0 102 124 146 168 18 20

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(oC

)


renders the potassium influence on TSTK and TRD to be negligible because the sticky temperature range is rendered to be small. As a general guide for the evaluation of chloride removal, a concentration of chloride of less than 1.5% by weight in ash (or 0.35 wt% Cl in as-fired black liquor) with a low potassium concentration may provide the desired one-year of continuous recovery boiler operation without interruption for cleaning. A method for predicting the ash composition and melting properties based on the black liquor elemental analysis, single droplet pyrolysis tests and chemical equilibrium calculations is described in the following reference (13). The predicted ash composition agreed well with the composition of the collected samples of precipitator ash. The method is said to be useful for evaluating the effect of chloride and potassium concentrations in the liquor and the change required in these concentrations to increase the sticky temperature, TSTK, of the ash. Determining the maximum chloride level for boiler operation The TIP uses the term “ash” broadly for describing both the carryover material from the furnace and the precipitator catch that is estimated to contain about 95% fume transported by the flue gas through the boiler to the precipitator (2). The deposits in the superheater, and at times the generating bank inlet, are mostly formed by carryover in the flue gas i.e. physically entrained smelt and /or partially burned black liquor droplets. It is in these carryover deposits that the control of Cl and K concentrations is essential. A measurement system for control by the operator must be simple and it would be unrealistic to base it upon samples of deposits from the superheater. The concentrations of Cl and K in the precipitator ash are used as a surrogate. In order for this approach to be effective care must be taken in determining the relationship between superheater deposit chloride and potassium levels to precipitator ash chloride and potassium levels The approach to control of these two process elements is to determine the maximum level in a given mill where the recovery boiler can operate for an extended period with cleaning only by sootblowers. Each recovery boiler has a unique characteristic of operating conditions and location for carryover and fume deposits on the convection surfaces that build over time until they limit the boiler operation. Deposits that cannot be controlled with sootblower operation can block the passage of gaseous products of combustion to the stack and/or reduce the steam temperature of steam below a level required for turbine operation. Conditions can require taking the boiler out of service for cleaning. The Cl and K content of the liquor being fired and of the ash can be determined by analysis of samples of these two streams. The operating conditions associated with these analyses can be recorded. The ash enrichment factor can be determined by studying the change in Cl and K from that in the liquor to that in the ash. The mole ratio of Cl and K, respectively, to the sum of Na and K, can be calculated for both the ash and the liquor. The enrichment is then the mole ratio of each individually in the ash divided by that of the liquor. This enrichment provides a factor to be used in the mill balance discussed in a later section of this TIP. Mill balance – establishing the foundation for selection of a control process There is no one process for achieving the desired chloride and potassium level in the mill cycle that maximizes recovery boiler availability with the most economical solution. Each mill pulping and recovery cycle has its own unique conditions to be investigated. However, one element is common to all - a mass balance must be developed for the mill! As a minimum, the balance should establish the flow rate of sodium, sulfur and chloride at all the principal locations of material entering the cycle. Potassium, because of the limited sources, is of lesser importance but should be determined for the wood supply and water supply. The mass balance will define the concentrations of chloride and potassium entering the recovery furnace. It will further define the sources of these constituents where they enter the cycle. The sodium and sulfur flows will establish the excess of these that are entering the cycle and must be removed in a bleached pulp mill. Recommended reading is a report of a kraft mill study to selectively removing chloride and potassium (14). The authors evaluated in two pulp mills processes for the removal of Cl and K based on sampling data. A material balance study for removal of Cl and K from a mill cycle reported by Tran et al also provides good background information for undertaking a mass balance (1). A portion of the Cl and K leave the liquor cycle in the pulp and in various effluent streams. The relative impact of a stream on the mass balance should be investigated and data collected for those of significance in removing pulping


chemicals (sodium and sulfur) and the Cl and K from the cycle. Some of the more significant streams are white water, biosludge, bleach plant effluent, and dregs, as well as the pulp/paper product being marketed. Biosludge is particularly high in chloride content. The balance makes possible the study of alternative sources of material supply to the mill and their impact on the ash chemistry in the recovery boiler. Process changes can also be evaluated, such as, substituting white liquor for caustic used in neutralizing the acid streams. It is important to realize that the inventory of chemical in the pulping and recovery cycle results in the effect of change requiring a time delay to reach equilibrium. The inventory is reflected in storage tank size and it is necessary to determine the hold-up time and account for equilibrium conditions existing for taking data and samples. It is also desirable to change makeup chemical amounts slowly to maintain process stability. The boundaries for reviewing the alternatives need to be established. Key boundaries to be considered are:

1. Establish a chloride removal efficiency at desired end chloride level. A removal efficiency of equal or greater than 85% is recommended.

2. What efficiency for sodium recovery is required? For a bleached pulp mill, this will almost always be well below 100% because the sodium in the effluent from the ClO2 production exceeds the sodium losses.

3. What is the upper limit for white liquor sulfidity acceptable in the pulp mill? 4. How much ash needs to be treated to achieve the target level of chloride in the ash? The ash flow rate to be

treated will increase as the target level of Cl and K decreases. The systems that are evaluated should be based on each resulting in an equivalent net chloride removal. There will be a different flow rate of ash to be treated for each process that needs to be considered. In effect, each should be evaluated to achieve the same predetermined target TSTK. The effect on the operation of decreasing the Cl level should also be taken into account. Considerations for selection of a method or process to remove Cl and K The evaluation of the various methods for removing Cl and K from the pulping and recovery cycle should consider the following list of questions; some will not be applicable to every situation.

• Current concentrations of chloride and potassium in the precipitator ash that are to be reduced? • Concentrations of chloride and potassium acceptable in the ash to accomplish the desired boiler

availability? • The removal efficiency for chloride and potassium, and recovery efficiency for sulfate and carbonate

compounds? • The portion of the total ash that must be purged or treated to accomplish the acceptable chloride and

potassium concentrations in the ash. • Feasibility to integrate an ash-treatment evaporator/crystallizer into the multiple effect black liquor

evaporator train for steam economy? • Is there multiple effect evaporator capacity available to handle the evaporation load from an aqueous fluid

stream of chemical makeup from a Cl and K removal process? • Are wet bottom ash systems being used, if so then this reduces the penalty of using some types of systems • What are the costs and benefits to selectively remove the Cl and K from the recovery cycle compared to

sewering a portion of the precipitator ash? • In a bleached pulp mill, the costs and benefits of returning all of the ClO2 system by-product to the black

liquor for chemical makeup should be evaluated against sewering more ash and some portion of the ClO2 system acid. This may have a benefit considering that the ClO2 plant effluent is free of potassium and generally has a low level of chloride.

• Determine capital and operating costs for each of the competing processes being evaluated. Operating costs should include material that will require replacement from time to time, such as, resin for ion exchange.


Purging One method for control of both Cl and K in the mill pulping and recovery cycle is to purge a quantity of ash to the sewer. The purge rate is selected to limit the Cl and K in the cycle to a level that minimizes problems that can be caused with higher concentrations. This is an effective control method employed by a number of mills, but not necessarily an efficient approach. The downside is that:

1. The installed ash systems for recovery of hopper and precipitator ash are not suited to selective ash discharge adjustment. For example, it is frequently necessary to discharge the total catch of a precipitator chamber to the sewer on a continuous or intermittent basis. It has been referred to as a “binge and purge” approach, and it tends to upset green liquor density control and other downstream processes.

2. The purge of ash for chloride and potassium control carries with it quantities of sodium and sulfur compounds requiring significantly higher flow rates of makeup with its associated cost.

3. Sodium make-up must be added to the mill cycle at the rate chemical is discharged to sewer, plus compensating for other losses. This makeup would generally be in the form of caustic soda (NaOH). The caustic makeup is offset by other sources of sodium makeup such as the effluent from ClO2 production.

4. The industry can expect in time that environmental authorities will eliminate, or at least restrict, the quantity of chemical that can be discharged to the sewer.

Environmental concerns and economic issues have resulted in mills looking for alternative approaches to reducing the Cl and K concentrations. Processes for chloride and potassium removal The various commercially available processes all treat ash collected in the recovery boiler electrostatic precipitator to remove chloride and potassium from the mill cycle. The processes separate the chloride and potassium constituents from an ash feed stream dissolved in water by taking advantage of the difference in solubility of the alkali sulfates and the alkali chlorides. There is one exception; one process uses a proprietary ion exchange resin that selectively adsorbs NaCl. The constituents of the precipitator ash are alkali compounds of K and Na are shown in Table 3.

Table 3. Constituents of the precipitator ash

Sodium compounds Potassium compounds

Chloride NaCl KCl

Sulfate Na2SO4 K2SO4

Carbonate Na2CO3 K2CO3

The removal of NaCl depends on the relative solubility of the NaCl and Na2SO4. Pure solutions of each in water have similar solubility properties above 30ºC (15). Below 30ºC, the solubility of NaCl remains constant while the solubilities of Na2CO3 and Na2SO4 solutions decrease rapidly as temperature is decreased. One process takes advantage of this difference in solubility at low temperature. The others operate at higher temperature. The dissolution of ash in water results in a multi-constituent solution that shifts the solubility in favor of a more soluble NaCl. Generally for these salts that are ionized in solution, the principle of the common ion effect applies “where the same ion is formed from each of two ionized compounds, the solubility of each is diminished by the presence of the other” (16). This characteristic applies for the Na and K compounds in ash. .Leaching and crystallization processes both take advantage of the relatively low solubility of the Na2SO4 and K2SO4 to preferentially dissolve NaCl and KCl. The presence of carbonate in the ash increases the sodium and potassium losses as both Na2CO3 and K2CO3 are very soluble. K2CO3 in particular has a solubility that is at 20°C two to three


times that of Na2CO3 or Na2SO4. The recovery boilers that are firing high solids black liquor result in ash recovered that can have a significant portion of carbonate. In general, at solids concentrations below 70-72%, there is little if any carbonate in the ash; the ratio of carbonate to sulfate in the ash increases as the firing solids are increased. The performance projections for potassium and chloride removal and for sodium sulfate recovery that are a part of the description for the various processes are provided by the respective suppliers. None of the supplier information available for preparing this TIP addressed the Na2CO3 or K2CO3. Leaching Leaching takes advantage of the high solubility of NaCl and KCl compared to the less soluble Na2SO4. The leaching process consists of two main unit operations. In the first step, precipitator ash is combined with water and recycled leachate to form a slurry. The amount of water/leachate used must be carefully controlled to promote the dissolution of Cl and K salts without dissolving Na2SO4. A higher temperature improves the selectivity of the process as more Cl salts and less sulfate salts dissolve in the system. After mixing in a stirred tank, the slurry is passed to the separation step, typically a filter or centrifuge. The solids fraction, primarily sodium sulfate, is returned to the kraft liquor cycle, while the leachate that is rich in Cl and K is recycled back to the first step leaching tank. A portion of the leachate is purged from the system as the means of Cl and K removal. The solids fraction is mixed with the black liquor. Figure 4 is a schematic of a leaching system that would currently be installed.

Figure 4. Ash leaching system A leaching system for precipitator ash was placed in service in 1973 at MacMillan Bloedel’s mill at Harmac, BC, to recover some of the Na2SO4 being lost when dumping ash for chloride control (7). NaCl was removed preferentially from the ash and a stream sewered with a high ratio of NaCl to Na2SO4. The system material balance diagram shows 90% removal of NaCl while recovering 92% of the Na2SO4. The ash stream to the leach tank included approximately 4% by weight of Na2CO3 of which 94% went to the sewer. The system was abandoned in 1976 reportedly because of severe corrosion and operating problems. The experience of starting up in 2002 an ash leaching system in Brazil at the Fibria Aracruz mill is reported in the following reference (17). The system was installed as an alternative to dumping recovery boiler ash. Prior to start-up, the mill was dumping 12 metric tons/day of ash resulting in 7% by weight K in the precipitator ash. The ash leaching system is reported to be designed to decrease the K to 4% by weight. The fate of Na2CO3 in the leaching process has not been reported by more recent researchers or suppliers. The reason may be that there is limited operating experience. Considering that sodium carbonate is soluble in water to a degree comparable to sodium sulfate (almost identical curves for the pure solutions in water; the solubility is a maximum and the same at 40ºC, and at higher temperature, both solubility’s decrease about 6% from the maximum),


a large percentage of the sodium associated with Na2CO3 can be expected to be retained in the leachate.. That is, the solution is saturated in sulfate but not in carbonate. Leaching of ash having a carbonate content may well result in a large portion of the carbonate being sewered with the leachate. Additionally, the presence of carbonate does lower the solubility of the sulfate. Carbonate is less soluble in a saturated sulfate solution than in water. Note that high carbonate ash has been shown to lead to poor solids-liquid separation and hence poor efficiency. Many systems dealing with high CO3 ash end up using H2SO4 to neutralize the ash before treatment (18, 19). The basic difference in the two systems is in the second step of separation in which the former uses a centrifuge and the latter uses a belt filter. Each piece of separation equipment has its merits and problems. High carbonate has been a problem in some cases. The efficiency of removal of the K and Cl from the ash that can be accomplished by the first system supplier is advised to be: Chloride 75 to 95% Potassium 55 to 95% Recovery of Na2SO4 typically ranges from 55 to 85%. The second supplier projects Cl and K removal at 80% of each based on pilot plant data. The recovery and removal of Cl and K depends on many factors of which the most important are (1) the concentration of these non-process elements and (2) the mill chemical balance. The removal efficiency for K is very much affected by concentration. As the concentration of K in the ash increases, the removal efficiency decreases as necessary to maintain a high level of Na2SO4 recovery. Figure 5 is based on data from an ash leaching system operating in Brazil. The ash leaching system is reported to operate within the range shown. If the operational point is moved to the left, the recovery efficiency for sodium sulfate is increased at the expense of reducing the efficiency for Cl and K removal

Figure 5. Operational results for a mill in Brazil

Freeze crystallization Freeze crystallization is a modification of the leaching process to operate at a reduced temperature. Freeze Crystallization takes advantage of the formation at low temperature of sodium sulfate decahydrate (Na2SO4 •


10H2O) that has a low solubility and readily precipitates. Precipitating along with the sodium sulfate decahydrate is sodium carbonate decahydrate (Na2CO3 • 10H2O) which has a similar characteristic solubility curve as the Na2SO4·10H2O. Below about 32°C (90°F) and in the absence of Na2CO3, Na2SO4 precipitates as large decahydrate crystals leaving the Cl and K in solution (20). The presence of carbonate decreases the solubility of the sulfate. Freeze crystallization systems have been installed in pulp mills in Japan. The precipitator ash is mixed with water to dissolve the chloride and potassium and a portion of the sodium sulfate and sodium carbonate. Sulfuric acid is added to improve sodium recovery by converting the Na2CO3 to Na2SO4 thereby decreasing the solubility of these two sodium compounds. The slurry is then transferred to a precipitation tank into which ice is added to decrease the temperature to the range of 10-15°C (50-59°F). At this temperature, the Na2SO4 in solution crystallizes as Na2SO4·10H2O. The slurry, including undissolved Na2SO4, is separated from the liquid waste containing the Cl and K, and the dewatered solid material added into the weak black liquor. A portion of the waste stream is recirculated to the slurry tank. The removal efficiency is reported to be 90% of the chloride and 75% of the potassium coincident with 70% recovery of sodium. The addition of acid to the slurry tank would be disadvantageous in a bleached mill where a mass balance shows sulfur to be in excess without the additional constituent from this process. The effect on sodium recovery of operating without acidification is not defined. Crystallization processes Crystallization processes take advantage of the relatively low solubility of Na2SO4 as compared to the NaCl and KCl. The precipitator ash is dissolved in water or recycled process condensate and then evaporated. The lower solubility Na2SO4 crystallizes first. The presence of sodium ions associated with the chloride ions acts to decrease further the solubility of the Na2SO4. The purified Na2SO4 is filtered and returned as a solid to the black liquor cycle. Most of the mother liquor rich in Cl and K is returned to the crystallizer; a small amount is sewered to purge the Cl and K. There are two crystallization processes known to be commercially available:

1. Ash recrystallization process (ARC) 2. Chloride removal process (CRP)

The principle of operation is similar for each of them. All use steam to evaporate water from the aqueous Na2SO4 feed stream to the crystallizer. The number of effects is dependent upon steam economy needs of the pulp mill and the degree of integration into the black liquor evaporator vapor system. Ash recrystallization process (ARC) The ARC process uses a falling-film type evaporator to concentrate the ash solution. Ash from the electrostatic precipitator is metered into an agitated ash dissolving tank where it is mixed with evaporator clean condensate. The slurry is pumped to the filtrate tank serving the Na2SO4 crystal vacuum filter, and then fed to the recirculation line on the falling film evaporator body. A stream of crystal slurry is transferred to the rotary drum filter where the sodium sulfate crystals are removed and this product mixed with the heavy black liquor. The portion of the chloride and potassium solution captured in the filter vacuum pump receiver is purged. An example of the performance assumes a feed stock having 1 kg of Cl and 2 kg of K per ADT of pulp. The target level for these non-process elements in the as-fired black liquor dry solids is 0.3 % Cl and 1.0% K. The total mill losses are established to be 5 kg Na for each TPD of air-dried pulp. With these conditions, 52 % of the total ash is treated with a Cl and K removal efficiency of 90% each. This would be accomplished with a sodium loss of 21% corresponding to 5 kg Na/TPD. See the schematic (Figure 6).


Figure 6. Ash re-crystallization process schematic (21) Chloride removal process (CRP) This process first implemented at the Champion International mill in Canton, North Carolina, utilizes the principles of re-crystallization for purification of precipitator ash and subsequent removal of chloride and potassium (22). The evaporator feed stock is precipitator ash dissolved in hot evaporator clean condensate or water in a mix tank. The dissolved ash is re-crystallized as sodium sulfate and sodium carbonate in the chloride removal system, and a chloride-rich brine is then purged from the liquor cycle. The CRP is schematically depicted in Figure 7. The crystallizer in this process is a forced circulation unit using an external tubular heat exchanger and a vapor separation body. The vapor body incorporates an internal wash column, or elutriation leg, in which the solid crystals are separated from the liquor at 93-104°C (200-220 °F). The Na2SO4 and Na2CO3 crystallize and are withdrawn from the wash column at 40% crystal concentration. The product is then centrifuged to achieve a higher solids concentration and to improve Cl removal and Na2SO4 recovery. The liquid removed from the crystals in the centrifuge is recycled to the suction side of the crystallizer circulation pump. The crystal product is typically sluiced with intermediate liquor and introduced back into the liquor cycle.

Figure 7. Chloride removal process schematic

Hot Water

Mix Tank Ash Solution

Feed Tank

RB Ash Steam

Cooling Water

Filter Vacuum Receiver

Recovery Cycle

Dissolver Hot Water

Condensate

Evaporator

Cl rich Solutio

Dissolved Crystals


The Cl and K are removed in a purge stream at the top of the vapor body as this is where the crystals are least likely to be suspended in the liquor. Alternatively, the purge flow piping can be connected to the recirculation pipe upstream of the pump suction. The crystallizer generally operates under a vacuum. The chloride removal efficiency is shown to be 99% with 90% sulfate recovery. Potassium removal varies considerably depending on the amount of K in the precipitator ash and the desired Na and S recovery. An increased sodium recovery is at the expense of potassium removal, that is, potassium removal decreases. The economics favor a single crystallizer body for this process to control the capital cost. A reasonable operating cost is dependent on integrating the body into the evaporator vapor stream to obtain some degree of multiple effect economy for the crystallizer operation. Ion exchange (precipitator dust purification system) The Precipitator dust purification system (PDP) is an ion exchange system developed to remove chloride from the mill cycle. The specialized ion exchange resin contains both cation exchange and anion-exchange groups on each particle to remove both ions simultaneously. This amphoteric resin has a high selectivity to remove the sodium chloride over sodium sulfate, sodium carbonate and sodium hydroxide from solutions of recovery boiler ash or caustic. This technology has been adapted into a salt separation unit (SSU) that is the heart of the PDP System. The precipitator ash solution must be filtered upstream of the ion exchange treatment to minimize fouling of the sensitive resin. Resin regeneration to remove NaCl from the resin uses water; no chemicals are required. The purified sodium sulfate is returned to the liquor cycle as a solution. See Figure 8

Figure 8. Precipitator dust purification system

The PDP system is supplied as a skid-mounted equipment package.. Precipitator ash is metered into an agitated dissolving tank with warm water to maintain a desired density. The feed solution at a temperature in the range of 35oC and 60oC (95oF – 140oF) is pumped through a pressure filter to remove insoluble solids such as unburned carbon and non-process metal oxides. The filtered solution overflows into the SSU feed tank. From there it is pumped through a cartridge filter to remove any suspended solids that escape capture in the pressure filter, and then


up through the SSU resin bed and into the sulfate product tank. From the product tank, it is transferred to be mixed with the black liquor. The SSU resin bed is regenerated on a five-minute cycle. There are two strokes in an operating cycle. An upstroke, filtered feed solution is pumped up through the resin bed where NaCl and KCl are removed. During the subsequent down stroke, water passed down through the bed to wash the chloride from the resin. The first part of the displacement from the SSU is feed solution that is reintroduced into the feed tank, and the second part is diverted to the sewer. To maximize chemical recovery and minimize water usage, a portion of the chloride rich waste can be recycled to the dissolving tank. Due to the cyclic operation of both the filter and the resin bed, the tanks are sized to allow continuous flows from the process to the PDP system and back to the process. See Figure 9.

Figure 9. SSU resin bed operating cycle

The product sulfate solution at approximately 25 to 28% solids requires evaporation when introduced into the recovery cycle. This can be accomplished in a number of ways. If the mill black liquor evaporator system has spare capacity, the simplest and most economical method is to add the solution to the weak liquor feed to the evaporator train. Whenever makeup water exists in causticizing, (typically the case when pressure disk filters are employed) then the product sulfate solution may replace a portion of the water makeup, reducing evaporator load A pilot plant operation reported chloride removal to be 97% with minimal losses of sodium – 6%, sulfate – 1%, and carbonate – 0.5%. The potassium removal is low, on the order of 5%. A desired sticky temperature (TSTK) can be achieved while retaining potassium simply by removing approximately 10% more chloride (2). It is desirable to retain potassium since it is equivalent to sodium in pulping, and any losses of potassium must be made-up as sodium. There are seven operating PDP systems at the time of writing, operating on ashes from both wet bottom (23) and dry bottom (20) electrostatic precipitators. The resin has proved very robust with the alpha unit operating on the original resin after some 6 years. Low process losses are characteristic of the PDP system, as well as ability to operate with ash of any level of carbonate. As with all processes exposed to precipitator dust solutions, care in design is required to avoid excessive scaling. Use of softened water or evaporator process condensate has proved beneficial in this regard for PDP systems. In frequent washing with an organic acid is typical. Disclaimer The processes described in this TIP are intended to include all types of chloride and potassium removal systems. Other suppliers of systems that accomplish the objectives of chloride and potassium removal should submit a written request with system description for inclusion in this TIP. The request should reference TAPPI TIP 0416-15 and be submitted to the TAPPI Quality and Standards Department.


Keywords Recovery furnaces, Precipitators, Ash, Chlorides, Potassium, Carbonates, Deposits, Removal, Systems Additional Information Effective date of issue: August 27, 2015 Working Group Members: A.K. Jones, Chair, International Paper Company H.N. Tran, Pulp & Paper Centre, University of Toronto E.K. Vakkilainen, Lappeenranta University of Technology K. Salmenoja, Andritz Literature cited 1. Tran, H. and Earl, P.F., “Chloride and Potassium Removal Processes for Kraft Pulp Mills,” Proceedings,

2004 International Chemical Recovery Conference, Charleston, SC, TAPPI Press. The content of this paper is used extensively throughout the TIP.

2. Tran, H.N., “Kraft Recovery Boilers – Chapter 9, Upper Furnace Deposition and Plugging,” edited by T.N.

Adams, TAPPI Press, pp. 247-282 (1997). 3. Tran, H.N., Gonsko, M., and Mao, X., “Effect of Composition on the First Melting Point Temperature of

Fireside Deposits in Recovery Boilers,” TAPPI J. 82(9):93-100 (1999). 4. Tran, H., Mao, X., Kuhn, D.C., Beckman, R. and Hupa, M., “The Sticky Temperature of Recovery Boiler

Fireside Deposits,” Pulp Paper Can. 103(9):T233-237 (2002). 5. Frederick, W.J., Jr., Lien, S.J., Vakkilainen, E.K., and Tran, H.N., “A Method to Predict the Conditions of

Boiler Bank Plugging by Sub-Micron Sodium Salt (Fume) Particles,” 2001 International Chemical Recovery Conf., Oral Presentation Proceedings, Pulp & Paper Assoc. of Canada, Montreal, pp. 311-321.

6. LaFond, J.F., Verloop, A., and Walsh, A.R., “Engineering Analysis of Recovery Boiler Superheater

Corrosion,” 1991 Engineering Conference, Proceedings, TAPPI, Atlanta, pp.223-231. 7. Blackwell, B. & Hitzroth, A., “Recycle of Bleach Plant Extraction Stage Effluent to the Kraft Liquor Cycle:

A Theoretical Analysis,” Proceedings, 1992 TAPPI International Chemical Recovery Conference, Seattle, WA, TAPPI Press, pp. 329-350.

8. Reeve, D.W., Pryke, D.C., Lukes, J.A., Donovan, D.A., Valiquette, G. and Yemchuk, E.M., “Chemical

Recovery in the Closed Cycle Mill-Part I: Superheater Corrosion,” Pulp & Paper Canada 84(1):58-62 (1983).

9. Tran, H.N., Barham, D. and Reeve, D.W., “Chloride and Potassium in the Kraft Chemical Recovery Cycle,”

Pulp & Paper Canada, 91(5):55-62 (1990). 10. Patrick, K., “New Dynamics Drive ClO2 Technology Developments in Post Cluster Rule Era,” Paper Age,

July/August 2004, pp. 26-29. 11. Someshware, A.V. and Jain, A.K., “Emissions of Hydrochloric Acid from Kraft Recovery Furnaces,”

Proceedings of the 1992 TAPPI International Chemical Recovery Conference, (Seattle, June 1992), pp. 329-350. pp. 351-363.

12. Hupa, M., Kraft Recovery Boilers – Chapter 2, Recovery Boiler Chemistry,” edited by T.N. Adams, TAPPI

Press, pp. 41-57 (1997).


13. Hupa, M., Backman, R., Skrifvars, B.-J., and Forssen, M., “Liquor-to-Liquor Differences in Combustion and Gasification Processes: Dust Composition and Melting Properties,” Journal of Pulp & Paper Science 27(12):418-422 (2001).

14. Ferreira, L.M.G.A., Soares, M.A.R., Egas, A.P.V., and Castro, J.A.A.M., “Selective Removal of Chloride

and Potassium in Kraft Pulp Mills,” Tappi Journal, 2(4): 21-25 (2003). 15. Dean, J.A., “Lange’s Handbook of Chemistry,” 15th Edition, McGraw-Hill Ltd., pp. 5.17-5.21. 16. Hougen, O.A., Watson, K.M. and Ragatz, R.A., Chemical Process Principles-Part I, Material and Energy

Balances, Second Edition, John Wiley & Sons, Inc., New York, 152-156 (1959). 17. Knutsson, M., Eriksson, M., Satorio, L.C., Hilbertt, V.R., and Filho, O.M., “Experiences from First Start-up

Ash Leaching System,” Proceedings, 2002 Fall Technical Conf., TAPPI, Atlanta, pp. 75-81. 18. Goncalves, C., Tran, H.N., Braz, S., Puig, F., and Shenassa, R., “Chloride and Potassium Removal

Efficiency of a Recovery Boiler Ash Leaching System,“ Pulp & Paper Canada, 109:3, p 33-38 (2008) 19. Gonçalves, C., Tran, H.N., and Shenassa, R., “Factors Affecting the Removal Efficiency of Cl and K from

Recovery Boiler Precipitator Ash", Pulp & Paper Canada, January/February Issue (2010). 20. Futterer, M. and Mathis, T. PEERS Conference Proceedings, TAPPI, September 15-18, Green Bay, 2013,

paper S09.1 21. Koskiniemi, J., Vakkilainen, E., and Paju, R., “Removal of chlorides from chemical circulation in the kraft

pulp mill,” Appita Journal, Vol.52, No.6, 1999, pp. 459 – 463. 22. Ferguson, K.R., “Champion Closure Plans Take Shape with BFR,” PPI, June 1996, pp:73-78. 23. Lockhart, J, Bucher, W. and Wearing, J. EPE Conference Proceedings, TAPPI, October 11-14, Memphis,

2009, paper 44.2.

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