Precipitated calcium carbonate, commonly called lime sludge or lime mud, is produced when sulphate green liquor is causticized with lime. For many years this lime sludge was considered a waste product and was dumped into rivers or waste ponds, or used for fill around the plants. Large quantities of new lime were purchased from commercial producers to replace that lost in the sludge waste.

A number of paper manufacturers soon became aware of the savings that could be achieved by recovering the lime. Beginning in the ’20’s, efforts were directed toward lime recovery installations. Today, a proper Rotary Lime Kiln Operation is an integral part of all modern pulp mills.

How a Rotary Lime Kiln Works

Kiln Feed and Feeders

High quality lime can be produced at a uniform rate only if the kiln is fed at a constant rate with lime sludge of constant composition. In some cases the feed to the kiln consists of a pumpable sludge containing 55 to 65% water. However, in most installations a drum filter or a centrifuge is installed just ahead of the kiln to reduce the moisture content to 35 to 50%. This cake is fed to the kiln through a screw conveyor, which is water-jacketed for that portion extending into the kiln feed end connection.

A ferris wheel slurry feeder and a surge tank located ahead of the filter or centrifuge will insure a constant rate of feed to the kiln. The surge tank should be provided with efficient means of agitation and dilution control to maintain uniform consistency of the feed to the ferris wheel slurry feeder. The overflow from the feeder is returned to the surge tank. The surge tank should have a capacity of 1 to 2 hours’ kiln feed.

Dissociation of Calcium Carbonate

Calcium carbonate begins to dissociate in the kiln at a temperature of about 1500 F. Theoretically, lime sludge could be heated to this temperature and held there until dissociation was complete. However, this dissociation process is accelerated at elevated temperatures. Therefore, to facilitate complete dissociation of various sized pellets with a reasonable retention time, temperatures in excess of 1500 F are necessary. The lime is usually discharged from the kiln at approximately 2200 F, although this will vary somewhat with the size of the kiln and the capacity at which it is operated.

Increasing Thermal Efficiency

To improve the thermal efficiency of the kiln, a chain system is installed in the feed end. The chains pick up the lime sludge and expose it to the hot gases. The chains also absorb heat from the hot gases, transferring this heat when they again dip into the wet material.

The gases enter the chain system at a temperature of 1200 to 1400 F and leave at approximately 300 to 550 F. In general, the lime sludge leaving the chain system should have a moisture content of 10 to 15%. If the moisture content is permitted to go below this figure, the sludge will no longer protect the chains, with the result that they will be oxidized and disintegrated by the hot gases.

Sampling

To evaluate the performance of the chain system, samples of sludge should be obtained from the sample hole located just after the chain system. These samples should be analyzed for moisture content.

Such samples will also provide the operator with a visual check on whether satisfactory pellets are being produced. To minimize dusting, the lime should travel through the kiln in the form of pellets about ½ inch in diameter. Larger pellets are undesirable since they will be overburned on the outside. The natural rolling action of the sludge on the kiln wall will form good pellets, provided the kiln is fed at a uniform rate with sludge of constant composition.

Excessive impurities, particularly soda, will promote the formation of large balls as well as rings. Experience indicates that the soda content should be maintained below 1% reported as Na2O on a dry basis. Impurities must be controlled by proper liquor clarification, washing and filtering.

Controlling Ring Formation

Some mills with lime recovery kilns are troubled by ring formation. So many factors influence the ringing characteristics in a given installation that it is difficult to make any definite statements regarding ring control. Varying amounts of moisture, soda, free calcium oxide, iron, alumina and silica will affect ring formation. Increasing the feed rate beyond the normal capacity of the kiln will increase the tendency to ring. In some cases a change in the moisture content has greatly reduced the formation of rings.

The tendency to ring formation is more pronounced in a small diameter kiln than in a large diameter kiln. This is due to the greater arch effect in a small kiln. The smaller arch effect in a large diameter kiln will permit agglomerating material to fall away more readily, thus minimizing ring formation. Sometimes a change in kiln speed or a change in burning conditions will overcome ringing.

Proper Rotary Lime Kiln Combustion

The uniformity of the kiln product, as well as kiln operation as a whole, depends first on controlled feed and second on proper combustion control. Good control of both feed and combustion will establish and maintain a definite temperature gradient throughout the length of the kiln. Any fluctuations in combustion or kiln feed will cause corresponding changes in this temperature gradient, with the result that the lime will not be uniformly calcined.

A constant draft at the firing hood is necessary for good combustion control. An automatic draft controller should be installed. This controller will regulate a damper in the exhaust system to maintain a constant draft at the firing hood. The usual draft at the firing hood will range to 0.05 in. water column.

The fuel should be burned with about 5 to 10% excess air to insure complete combustion. Too much air, as well as too little air, wastes fuel. Periodic Orsat analysis of the exhaust gases should be made to determine whether the correct draft is being maintained at the firing hood for proper combustion conditions.

The burner should be adjusted so that the flame is confined to a long, narrow cone with no impingement on the brick lining, since impingement will greatly reduce brick life.

Rotary Lime Kiln Control Panel

To properly record, interpret and control the kiln performance, there should be available to the operator a record of the exhaust gas temperature, the burning zone temperature, the rate of fuel flow, and the draft at the feed end of the kiln as well as at the firing hood. All of these instruments, together with the necessary gauges, ammeters, voltmeters, and motor starter pushbuttons, should be mounted on a control panel located on the burning floor.

Dust Collection of Rotary Lime Kiln 

The chain system in a lime sludge kiln is an excellent dust arrester. However, a small amount of dust will be carried out with the exhaust gases, making a dust collecting system desirable. Returning the collected dust to the system will reduce the amount of make-up lime required.

The form in which the collected dust can be handled most conveniently in a particular mill must be decided before selecting the dust collecting equipment. Dust can be collected dry in a simple cyclone or in a commercially manufactured collector, pugged with water and flushed to waste or returned to the system. Pugging is necessary to permit ready transportation and return to the system. Sometimes difficulty is encountered in pugging this fine dust with water. A wet type collector will collect the dust in a form immediately convenient for further use.

HOW TO START A ROTARY LIME KILN

CAUTION: Brick lining must be thoroughly dried before kiln is started up for the first time and immediately after extensive lining repairs have been made. Failure to do so may result in disintegration of the lining when kiln is brought to operating temperature.

The lining can be dried by keeping a small fire going in the kiln for approximately 48 hours. The kiln should be rotated 180 degrees every half hour during this period.

Lime Kiln Startup Procedure

• a. Start exhaust fan with damper completely closed. When fan has reached operating speed, open damper to provide sufficient draft at the firing hood to prevent smoke from puffing out around the hood when the oil is ignited. The amount of draft required will vary, depending upon the particular installation, but in most cases a draft of 0.05 to 0.10 in. water column will be sufficient.
• b. Start oil burning system at a low rate in accordance with instructions supplied by manufacturer of the burning equipment.
• c. Reset exhaust fan damper to maintain sufficient draft to prevent hot gases from leaking out around firing hood. This will require a draft of about 0.00 to 0.05 in. water column.
• d. Adjust flame to avoid impingement on brick.
• e. Rotate kiln 180 degrees every 15 minutes for 1 hour.
• f. Start sludge feed at low rate and run kiln at low speed for next hour.
• g. Increase oil rate gradually, but keep exhaust gas temperature below 550 F. The kiln should be raised to a bright heat before any product is discharged. The amount of oil required both for starting and for normal operation can be determined only by experience.
• h. When the kiln is hot and discharging lime, increase the speed and the feed rate one step at a time to the desired rate. This must be done gradually to insure high quality lime throughout the entire starting-up period.
• i. The fuel rate and firing hood draft must be adjusted with each change in kiln speed or feed rate. If an automatic draft controller is used, the draft can now be set at the desired value. The controller will then automatically maintain the desired and preset value.

OPERATING A LIME KILN

Exhaust Gas Temperature

The exhaust gas temperature in chain equipped kilns ranges from 300 to 550 F, depending on the particular installation. A few days’ operation will indicate the average temperature for normal operation. Watch the recorder. Any unusual temperature rise may indicate that the kiln feed has stopped. The temperature will change with changes in kiln speed or moisture content of feed.

Draft

Use only enough draft to provide air for combustion and to prevent smoke from showing up at the stack. Too much draft chills discharge end and also wastes fuel. The kiln exhaust gas should be checked frequently with an Orsat gas analyzer. The oxygen content should be held to 1.5% or less, under which conditions there should be no combustibles in the exhaust gas.

Sludge Composition

The moisture content of the sludge to the filter or centrifuge should be maintained constant with a consistency regulator. It is also important to maintain the percent of impurities in the sludge at a constant value.

Feed Rate

The ferris wheel slurry feeder is driven by a four-speed motor which is electrically interlocked with the four-speed kiln drive motor to insure a constant rate of feed per kiln revolution. In addition, there is a speed changer which enables the operator to determine the optimum feed rate to the kiln for a given kiln speed.

Steps should be taken in the initial installation to provide a way of determining the rate of solids to kiln. A calibrating tank following the ferris wheel slurry feeder will enable the operator to make periodic checks on the rate of slurry to the filter or centrifuge. If the moisture content of the slurry is known, the rate of solids to the kiln can be readily determined. The rate of solids to the filter must be maintained constant at all times.

While the slurry feeder will insure a constant volume of sludge to the kiln per kiln revolution, it is possible for the rate of solids to the kiln to vary unless the composition of the sludge is maintained constant as pointed out under “Sludge Composition,” above.

Flame

Best results are obtained with a long, steady flame. Do not permit flame to impinge on lining or material.

Sludge Samples

Hourly samples of sludge should be taken from the sample holes located just after the chain system. These samples should be analyzed by the laboratory for moisture, soda and free lime, and the results should be listed on the kiln data sheet. This information is necessary for a proper study of kiln performance.

These samples will also enable the operator to see whether satisfactory pellets are being produced.

Lime Samples

Hourly samples of lime should be taken from the discharge end of the kiln for determination of “available lime” (CaO). Good kiln operation should result in a product with an available lime content of 90%.

Daily Reports

A ruled form should be printed for the purpose of recording pertinent data in connection with the lime burning operation. The report forms most commonly used provide spaces designated Shift 1, Shift 2, and Shift 3 for the signatures of the three men in charge of these shifts so that the responsibility for irregularities on any shift can easily be placed.

Data that should be recorded hourly on the report is given in Table I.

Even though recording instruments are used, the use of a daily report form requiring the hourly entry of significant figures by the operator insures better control of the process.

The daily reports are sent to the supervisor for use in compiling the monthly report. They will also bring to his attention quickly any abnormal condition in the operation.

HOW TO STOP A ROTARY LIME KILN

KILN Shutting Down Procedure

• a. Stop oil flow, kiln feed and exhaust fan, and close fan damper. (However, to prevent dust from blowing out around the firing hood, it may be necessary to open the fan damper slightly.)
• b. Continue to rotate kiln at lowest speed for 5 minutes and then stop kiln completely.
• c. Rotate kiln exactly 180 degrees every 5 minutes for next hour.
• d. Rotate kiln exactly 180 degrees every 10 minutes for the second hour.
• e. Rotate kiln exactly 180 degrees every 30 minutes until cool.
• f. Do not start exhaust fan until kiln has cooled below red heat. Then open door in firing hood and start exhaust fan to draw cooling air through kiln. Air flow should be regulated with fan outlet damper to prevent overloading of fan motor.
• g. In the event of a power failure, the kiln should be rotated with the auxiliary gasoline engine drive in accordance with steps a, b, c, d, and e above.
• h. Sometimes a breakdown in the equipment preceding the kiln will necessitate a short period of operation with no kiln feed. Under these circumstances the oil flow rate and exhaust fan settings can be left unchanged if water is run into the feed end of the kiln. The rate of flow should be equal to the rate at which water was entering with the kiln feed. This will impose approximately the same heat load on the feed end of the kiln and maintain the same exhaust gas temperature. Such action will prevent the chain system from being damaged by hot exhaust gases.

ROTARY LIME KILN MAINTENANCE