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Upgrade of lime kiln and causticizing plant in kraft mill

A look at the various equipment options to improve performance

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By: S. Schroderus, D. Carter and H. Brumme
2000-01-01
The causticizing and lime kiln department is one of the key links of the kraft recovery cycle. In the past, this area of the kraft mill has often appeared to be neglected. Needed renovations or upgrades have been delayed in favour of other more obvious uses of capital funds, resulting in equipment becoming heavily overloaded. However, with increased emphasis in recent years on individual department efficiencies and changing demands in the future for the quality and use of white liquor in the fibre line, the causticizing and lime kiln department must meet higher performance standards. Both new and mature technologies are available that can be cost-effectively retrofitted in existing facilities to meet the present and future requirements. From an over-all perspective, these requirements are summarized in Table I.

TYPICAL PROCESS FLOW SHEET

The basic causticizing plant process flow sheet has changed little in the last 30 years. However, the equipment has changed quite significantly, which has changed the plant's appearance and, more significant, its performance. Various innovative process modifications have been proposed to increase white liquor causticity or energy efficiency; for example, pressurized slaking and causticizing [1], adding fresh lime in the last causticizer [2], and causticizing at a low alkali concentration and evaporating the white liquor before sending it to cooking [3]. However, these generally have not developed beyond the trial stage.

Figure 1 shows, in block diagram form, a typical causticizing plant flow sheet. The process begins at the dissolving tank, which is normally considered a part of the recovery boiler process. The green liquor is pumped on level control from the dissolving tank to the agitated raw green liquor stabilization tank, to even out the variations in green liquor flow and density. These variations are a result of surges in the smelt flow from recovery boiler. The raw green liquor is then clarified by sedimentation or filtration in the next process step, and the clarified green liquor stored in a tank. The green liquor dregs are washed and dewatered on a filter and sent to landfill.

The next steps in the process include the control of green liquor temperature and density prior to the slaker, where lime is added to the green liquor. The slaking involves conversion of calcium oxide into calcium hydroxide, which reacts with sodium carbonate to form sodium hydroxide. This causticizing reaction takes place in the slaker and the subsequent causticizers. The coarse impurities in the lime are removed in the classifier section of the slaker and sent to landfill. The causticized liquor is then clarified by sedimentation or filtration in the next process step, and the clarified white liquor stored in a tank.

Lime mud, which is the reaction by-product from causticizing and mainly consists of calcium carbonate separated from white liquor, is then washed using dilution and subsequent sedimentation or filtration, and stored in a mud storage tank. The mud-washing process step can be omitted if a pressure disc filter is used for white liquor filtering.

The weak wash, which is weak liquor obtained from mud washing, is stored in a tank. To close the liquor loop, weak wash is added to the dissolving tank to dissolve the smelt from the recovery boiler and generate raw green liquor.

The washed mud is pumped from the mud storage to the pre-coat mud filter, where the mud receives its final wash, is dewatered and is then fed to the kiln. Calcium carbonate is converted in the kiln into calcium oxide in the calcination reaction closing the mud loop by converting lime mud back into active lime.

PROCESS EQUIPMENT

The most significant development in the causticizing process equipment has been the gradual change from sedimentation to filtration for white liquor, mud washing and green liquor. The multi-compartment clarifier, which was originally used in all these applications, was displaced by single compartment unit-type clarifiers in the early 1960s. Tube-type pressure filters were introduced to the North American market in about 1980 for white liquor and mud washing applications, and the white liquor pressure disc filter followed in the mid-90s. These types of equipment and their impact on liquor clarities and underflow or filter cake solids are summarized in Table II.

Green liquor dregs are notoriously difficult to filter, which is the reason why unit-type green liquor clarifiers with separate dregs washing and dewatering are still the most-favored choice. The typical clarity of about 100 ppm dregs in clarified liquor is still acceptable in most kraft pulp mills. Two new types of filters have been introduced in the 1990s, both producing green liquor with 20 ppm suspended solids content [4,5]. Several units are now operating, mostly in Europe.

Dregs are typically filtered on a lime mud pre-coat on a vacuum drum filter. Two new types of green liquor filtration systems have been introduced recently which use no pre-coat. The dryness of the dregs cake from the filter is greater than 40% solids. One objective of this kind of dregs filter operation is to burn the dregs in power boiler with hog fuel, to reduce the mass and the volume of the dregs and to combine the residue in boiler ash for less costly disposal.

White liquor sedimentation clarifiers have been largely displaced, starting in the mid-70s, by pressure tube filters, because the filter can produce very clean liquor containing less than 20 ppm suspended solids. Pressure disc filters were introduced in the late-80s [6] ,and have since become the favored choice in new mills and major causticizing plant renovations. A white liquor disc filter is capable of discharging the mud at 70% solids, compared to the 35 to 45% solids of pressure tube filters or clarifiers. In addition, water showers on the disc perform some displacement washing. Thus, there is much less residual alkali in the mud discharged from the filter, which gives the following benefits:

• No additional mud washing is required before the pre-coat mud filter feeding the kiln.

• The alkali recycle in the causticizing plant liquor loop is reduced, resulting in 10 to 15% lower flows of green liquor and causticized liquor at the same white liquor production rate.

• A significant portion of the dilution to the dissolving tank can be from sources other than weak wash produced in the causticizing plant itself. This provides opportunities for closed-cycle process schemes in the future.

The mud pre-coat filters have been gradually improved and increased in size over the years, and 75% mud dryness is the norm when a properly sized filter is used. The benefits of higher mud dryness include higher lime kiln capacity and lower TRS emissions from the kiln. Perhaps the most radical single improvement recently was the continuous pre-coat renewal and wire washing, which results in a slight increase in capacity and eliminates the disturbance caused to kiln operation by the change of pre-coat. Disturbances in kiln operation can cause high peaks in TRS emissions from the kiln stack, which may now be avoided.

Lime kilns have increasingly been equipped with insulating refractory and dams, which were nearly unknown in pulp mill lime kiln in North America until the early 1980s. That design change, combined with higher dryness of kiln feed and optimized chain section design, has allowed lime kiln capacities to increase. Significant developments in lime kiln technology in the 1990s include the new-generation flash drying of mud with kiln flue gases and the new designs of lime product coolers.

PROCESS CONTROLS

The causticizing plant process is a confluence of three process cycles: the lime cycle; the kraft process liquor cycle which involves cooking, washing, evaporation and recovery boiler; and the weak wash cycle. The white liquor delivered to the fibre line must have uniform alkali concentration, and a low content of suspended solids and dead load. The causticizing plant must have high uptime to meet the production requirements, meet the emission standards and have low operating costs. The best over-all results are achieved when each of the processes in the causticizing plant is controlled. The most critical process controls are summarized in Table III.

UPGRADE OPTIONS

The typical causticizing plant design has a lot of flexibility, which often permits relatively low-cost means of upgrading the capacity and performance. The first step is to define the short- and long-term objectives of a plant upgrade. The short-term objectives may involve an improvement in the performance at the present capacity level or a modest increase in the production capacity. Long-term objectives must fit with the mill's other long-term development plans, which may involve a significant capacity increase or changes in the fibre-line process. For example, the sulphidity might increase to much higher levels in the future or planned use of oxidized white liquor for oxygen delignification and bleach plant caustic replacement may affect future capacity. Therefore, it is important to prepare causticizing plant balances for the present and future production rates and process conditions for the existing equipment. The balances can then be used to calculate equipment loading to identify which pieces of equipment are, or will be, overloaded to the point that upgrading or replacement is necessary.

The next step in contemplating a causticizing plant upgrade is to review the existing plant configuration. It likely differs little, if at all, from the standard flow sheet described earlier, but it might show multiple pieces of equipment in various process locations, perhaps as a result of previous upgrades. Thus, rather than adding to the complexity of the process and layout, a radical rejuvenation of the plant might be in order. The mechanical condition of equipment, plant layout and space availability should also be carefully reviewed, because they are important in finding the right solution.

Once the short- and long-term objectives have been defined, and the equipment capabilities identified, the pathway from the present situation to the future goal can be developed. The task is often complex and several options will usually have to be considered to identify the most cost-effective approach. The pathway in the plant upgrade from the present to the ultimate goal might involve several phases to minimize downtime and to gain the best value from the existing equipment. The following discussion is a review of options that we have typically considered or implemented in kiln and causticizing plant upgrade projects over the last decade.

The basic options for upgrade of green liquor clarification are as follows:

• Improve dissolving tank density control;

• Add raw green liquor stabilization tank;

• Replace by new clarifier(s);

• Add new clarifier;

• Convert existing white liquor clarifier or mud washer for green liquor duty;

• Replace by or add green liquor filter.

Conversion of existing clarifiers is cost-effective, because only minor modifications are usually required. In addition, the white liquor clarifier and mud washer might be overloaded already, and both might be replaced by new equipment. A remaining clarifier might be used as a storage tank and prepared for use as a backup clarifier. If pressure tube filters are used for white liquor filtering and mud washing, a new or additional green liquor clarifier might be the most cost-effective solution for improved green liquor clarity. Selection of green liquor filtering in a causticizing plant upgrade is, however, a complicated matter and requires a careful study of available options and justification of the benefits the green liquor filter would give. The low dregs content in filtered green liquor is very desirable for the efficient operation of the kiln and causticizing plant. However, the full value of the clean green liquor is achieved in closed-cycle operation, because the non-process elements with low solubility in alkaline liquors can be effectively purged in green liquor dregs. Thus, a green liquor filter could be a good fit if the mill plans to pursue process closure, and can already invest funds for that purpose.

Upgrade of the slaker and causticizers is often difficult because of layout constraints. The best approach from the process and control point of view is to stay with a single slaker-causticizer line, although the replacement of an existing slaker by a larger unit might be a complicated undertaking. Possible options include replacement of only the slaker bowl, or replacement with a slaker containing a deep bowl in cases where there is a limit to the available length or diameter.

Increased retention time in causticizing is usually required if a change to filtration technology is planned. This can be achieved by adding one or more new causticizing vessels. It is desirable that the slurry flow by gravity -- from the slaker through to the last causticizer -- and pump only the causticized liquor. At times, existing small causticizers might be arranged in parallel flow, which connect to the large new causticizing vessels. The existing causticizers may be located such that the addition of a new causticizer after the last existing unit does not make sense because the volume of the new causticizer would become small. If the slaker can be raised, new causticizer vessels could possibly be added ahead of the existing vessels. Complete replacement of the existing causticizers with new large units, which can be a stacked type, should be seriously considered if an upgrade of existing causticizers would result in layout complications and in a system consisting of several small vessels.

As indicated earlier, installation of a new disc filter for white liquor filtering is an attractive upgrade option because a separate mud washing step is no longer required. In addition, the green liquor and causticized liquor flows are reduced, relative to other white liquor filtering or clarification processes, which could save equipment costs. Other options must, however, be considered because of the high capital cost of a disc-filter system. A minor capacity upgrade of pressure tube filters may be possible by extending filtration tubes. A major increase is possible by operating existing white liquor and weak wash filters in parallel and adding a new large filter or clarifier. Although a disc filter can be considered as the ultimate goal for white liquor filtering and mud washing with present causticizing technology, investment in a new pressure tube filter or clarifier may still make economic sense, if circumstances warrant it. Yet, it is necessary to prepare a cost estimate and a summary of technical and other merits for each causticizing plant option to identify the most cost-effective approach.

Upgrade of the capacity of a lime kiln is a complex task that must address the following items as a single entity:

• Emissions of particulate matter and TRS;

• Back end devices (flue gas scrubber, precipitator, cyclone, flash dryer);

• Mud pre-coat filter

• Lime kiln internals (refractories, chain section);

• Lime kiln auxiliaries.

In general, the capacity can be raised by reducing the amount of water in the mud feed to the kiln, thereby minimizing the radiation heat loss and optimizing the retention time and temperature profile in the kiln system. The amount of water to the kiln can be reduced by increasing the size of the mud filter. The obvious options to upgrade a small mud pre-coat filter is to either consider the installation of a second filter or a complete replacement by a larger unit. The building layout, kiln downtime and any planned upgrades to the kiln itself are major considerations in the selection. If a new mud pre-coat filter is necessary, it might be worthwhile to consider whether the installation of a flash dryer is a realistic option in the near future. If so, the new mud filter should perhaps be sized accordingly and the building modified to suit the installation of the filter in a position that also accommodates the addition of flash dryer later.

Further reduction in the amount of water can be successively achieved in the following cases:

• Add a flue gas cyclone with dry mud return to the kiln (if the kiln has a flue gas scrubber);

• Use an electrostatic precipitator with dry mud return to the kiln;

• Add a flash dryer.

The radiation heat loss can be minimized by using an insulated refractory lining, which results in a higher flame temperature and improved heat transfer in the kiln. The retention time is optimized by a refractory dam and a proper kiln speed. The chain section design must suit the heat transfer requirements. The auxiliary equipment and the mechanical condition of the kiln must also be reviewed to ensure they also meet the new requirements.

The capacity of a lime kiln can also be increased by lengthening the kiln. However, this approach tends to result in too high of a capital cost to be attractive, mainly because of extensive modifications required for buildings and equipment and, as well, long downtime.

UPGRADE CASES

The following examples briefly describe two causticizing plant upgrade projects. In each case, the final solution was a result of examination of several alternatives:

Case I -- Problem:

• White liquor clarifiers suffering from serious corrosion problems;

• Inadequate white liquor clarity;

• Too high soda loss with green liquor dregs;

• Inadequate capacity for planned production increase.

Final project scope:

• Install a new dregs filter (eliminate old dregs washers);

• Re-use existing raw green liquor stabilization tank as dregs holding tank;

• Install a new, larger raw green liquor stabilization tank;

• Replace slaker in each of the two causticizing lines by a larger unit (deferred);

• Add two causticizing vessels in each of the two causticizing lines;

• Install two new white liquor pressure tube filters and mud mixing tanks.

Case II -- Problem:

• Inadequate capacity for planned production increase;

• Inadequate green liquor clarity;

• High particulate matter emissions from lime kiln.

Final project scope:

• Convert existing mud washer into a second green liquor clarifier;

• Install a new mud mixing tank;

• Install a new mud washer;

• Add a new mud storage tank;

• Install a cyclone on flue gas from the lime kiln;

• Replace the lime kiln flue gas scrubber.

Résumé: La technologie moderne associée au four à chaux et à la caustification est examinée et comparée à une technologie plus ancienne pour soulinger l'amélioration de la performance. Une application sélective de la plus récente technologie à l'amélioration des installations existantes est illustrée par des exemples. Les exigences de capacité pour l'atelier de caustification et le four à chaux, ansi que les critères de performance, sont discutés à court et à long termes.

Abstract: The state-of-the art lime kiln and causticizing technology is reviewed and compared to older technology to highlight advances in performance characteristics. Selective application of the latest technology in upgrading of existing facilities is illustrated by examples. Lime kiln and causticizing plant capacity requirements and equipment performance criteria are discussed in view of short- and long-term requirements.

Reference: SCHRODERUS, S., CARTER, D., BRUMME, H. Upgrade of lime kiln and causticizing plant in kraft mill. Pulp Paper Can. 101(1): T26-30 (January 2000). Paper presented at the 84th Annual Meeting of Technical Section, CPPA, in Montreal, QC, on January 27 to 30, 1998. Not to be reproduced without permission of PAPTAC. Manuscript received on November 14, 1997. Revised manuscript approved for publication by the Review Panel, August 17, 1999.

Keywords: LIME KILNS, UPGRADES, KRAFT MILLS, CAPACITY, PROCESS CONTROL, PLANNING

CONCLUSIONS

The challenges to the causticizing and kiln department are as follows:

• To produce white liquor at a sufficient rate for the fibre line operation;

• To maintain a high and uniform liquor quality for efficient operation of the fibre line and the evaporation and recovery boiler processes;

• To meet the environmental requirements; and

• To have low operating costs.

These challenges have been tightening in recent years, a trend that will continue in the future. New technologies introduced in the last 10 years can be retrofitted in existing plants to meet most stringent liquor quality and causticizing plant operating requirements.

The upgrade of an existing facility for higher production and quality requirements requires a judicious study of options that include combining existing and new equipment for a cost-effective solution. A critical foundation for the study is a clearly defined long-term development strategy for a mill to help plan the right investment in the right technology at the right time.

LITERATURE

1. SANDVOLD, B. Energisparing genom kausticering vid forhojt tryck. SPCI mixerisymposium (1982).

2. LEIVISKA, K. RCL-processen, en ny mixeriprocess. SPCI:s mixeri - mesaugnssymposium nr. 3 (1986).

3. LUNDBERG, S. Ekonomisk analys av kausticering vid lag styrka kombinerat med vitlutsindustning. SPCI mixerisymposium (1982).

4. ENGDAHL, H., TORMIKOSKI, P. Clarification of green liquor by a falling film cross-flow filtration method. Paperi ja Puu 76 (5): 326-329 (1994).

5. KIISKILA, E., LINDBERG, H. Metal management with the recausticizing department. Proc., International Chemical Recovery Conference, Toronto, ON B159-163 (1995).

6. SANDVOLD, B., PETTERSSON, B. Pressure disc filters in the recausticizing process. Pulp Paper Can 92(5): T130-T134 (May 1991).

TABLE I. Lime kiln and causticizing plant -- general requirements.
ItemRequirement
White liquor productionTo consistently meet the
capacityrequirements of the fibre line
operation.
White liquor qualityStable composition and
concentration, low suspended
solids, low dead load,
minimum nonprocess
elements.
Emissions of TRS, SO2To meet emission regulations
and particulate matter
from lime kiln to
atmosphere.
Solid waste, effluentDischarges to environment to
dischargesbe minimized
Operating costsTo be minimized

TABLE II. Sedimentation and filtering processes.

Process unitApplicationSuspendedUnderflow orRemarks

solids infilter cake solids

liquor

Multi-compartmentgreen liquor>100 ppmmud 35-40%• difficult to operate

clarifierwhite liquordregs <5%

mud washing

Unit-typegreen liquor<100 ppmmud 35-45%• easy to operate

clarifierwhite liquordregs 5-10%

mud washing
Pressurewhite liquor<20 ppmmud 35-45%• very good WL clarity
tube filtermud washing

Disc filterwhite liquor<20 ppmmud 70%• mud washing not
required
• reduced green
liquor flow

TABLE III. Summary of critical process controls in a causticizing plant.

Controlled parameterControl methodObjective of control
Green liquor density• weak wash addition to dissolving tank• efficient green liquor
and/or TTA• raw green liquor stabilization tankclarification
• weak wash addition for trim control• better slaking control
before slaker• alkali concentration
Green liquor temperature• heating or cooling in heat exchanger• optimum slaking
• heating with direct steamtemperature
• cooling by flashing under vacuum
Lime addition (slaker control)• manual control• high causticity
• lime addition based on slaker• low excess lime
temperatureaddition for good lime

• advanced control systems usingmud settleability/

conductivity or other sensors or titratorfilterability

to determine alkali concentration in liquors

Mud solids in white liquor• slurry density control• minimize alkali to mud

filter or clarifier underflowwashing and weak wash

cycle

Mud solids in weak wash• slurry density control• minimize alkali to mud

filter or mud washerprecoat filter

underflow

Photos

FIG. 1. Block diagram flow sheet of causticizing plant.
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Caption: FIG. 1. Block diagram flow sheet of causticizing plant.
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