Challenges for refractories in rotary kilns
Surveying the non-captive lime industry reveals there are approximately 120 rotary kilns producing pebble lime in the US. Just over half these rotary kilns have stone preheaters that substantially improve their fuel efficiency. Rotary pebble lime kilns typically have four refractory zones: charging, calcining, burning, and discharge.
Figure 1. Rotary lime kilns have four refractory zones: discharge, burning, calcining and charging.
Figure 2. A highly abrasion-resistant R-MAX© C lining in a metallic heat exchanger section of a long pebble limestone kiln still shows the form marks after 16 months service.
Conditions that the refractory must withstand in the charging zone vary with the process type. In preheater kilns, the stone feed can be partially calcined as it falls through the transfer chute into the rotary kiln. Partial calcination softens the abrasiveness of the feed: and high alumina brick with moderate abrasion resistance, including burned 70% alumina brick, can withstand the moderate abrasion.
Long rotary kilns without preheaters require abrasion-resistant refractory in the charging zone, where abrasion of the stone can be particularly severe. Typically, phosphate-bonded high alumina refractory brick lines the charging zone. Phosphate-bonded brick exhibits high-strength, and outstanding abrasion resistance.
The standard refractory test in North America for abrasion resistance is the ASTM C-704 procedure. In this test, specially sized silicon carbide grit is blasted onto a weighted refractory specimen at 90° angle. The volume loss from the scouring abrasion is calculated from the loss in weight of the sample and reported in cc’s. Thus, a smaller volume loss indicates better abrasion resistance. Typically, abrasion loss for 70% alumina brick is approximately 8 to 10 cc loss, while phosphate-bonded 80% alumina brick shows less than 4cc loss.
Long pebble lime kilns may have internal devices to facilitate heat exchange, like metallic lifters in the feed ends. In such cases, installation of refractory brickwork between the heat exchangers can be time consuming, and a monolithic refractory is more suited, so as to reduce installation time. For such applications, a special abrasion-resistant monolithic refractory was developed to solve abrasion problems.
This monolithic refractory uses advanced particle packing and a special bonding matrix to achieve exceptional strength and abrasion resistance. The family name is R-MAX©, and R-MAX© castable can exhibit abrasion losses of less than 3.0 cc after heating to 816°C(1500°F) in the ASTM C-704 test. R-MAX© castable come in a variety of consistencies, depending on the intended use, including versions for pneumatic gunning. In one cast installation between metallic heat exchangers in a long kiln feed end, the performance R-MAX© C (casting grade) surpassed the wear resistance of other low-cement castable used previously (Figure 2). R-MAX© castable have been widely used in many other lime kiln applications, including transfer chute and preheater linings.
Figure 3. Installation of a cross-type refractory heat exchanger promotes heat transfer in the rotary lime kiln.
The calcining zone prepares the stone feed for the burning zone, where the decarbonization reaction is driven to completion. For the calcining zone linin, 70% alumina class brick refractory is most commonly used in North America. Internals are frequently located in the calcining zone, including lifters refractory heat exchangers (refractory crosses), and dams. The two former structures increase mixing within the charge, while the latter increases the residence time of the charge in the kiln. Refractory heat exchangers also absorb heat energy from the flue gas and return it to the charge by radiative and conductive heat transfer.
Cross-type refractory heat exchangers have either three or four legs, and are fabricated from refractory castable that are heat-treated (Figure 3)
Figure 4. Lime-filled refractory cup tests of a high alumina (60% Al2O3) brick show that fluxing starts to occur between 2350°F (1288°C) and 2400°F (1316°C) by the appearance of a dark gray coating inside the cup.
Figure 5. Magnesite-spinel brick (RESCOMAG® 85) burning zone dam uses a relatively thick refractory construction.
When an alumina-silica refractory is fluxed by lime in a rotary kiln, the lining appears abraded or washed away. An individual refractory brick exhibits two hallmark features: a greatly varying remnant thickness and a thin, dark grey hot face coating. Based on laboratory refractory cup tests, high alumina brick can react with the lime charge between 2450°F (1288°C) and 2400°F (1316°C). Figure 3 illustrates this, as a dark grey coating has formed inside the refractory cup.
Brick hot face temperatures in the burning zone commonly exceed 1260°C (2300°F) necessitating the use of basic brick. Basic refractories are chemically more compatible with the lime charge than high alumina brick. Magnesite-spinel bricks are most commonly selected for the burning zone. Magnesite-spinel bricks feature excellent spalling resistance and great durability for burning zone service.
It is not uncommon for rotary lime kilns in North America to operate with burning zone shell temperatures exceeding 800°F (427°C). Some operators have resorted to two-layer brick constructions in the burning zone to manage shell temperature issues. There is a great difference in the coefficient of thermal expansion between magnesia (the hot-face layer) and alumina-silica (the back-up layer) brick. This mismatch can cause instability of the hot face layer.
Magnesite-spinel bricks are binary combinations of dead-burned magnesite (MgO) and magnesia-alumina spinel (MgO-Al2O3). The alumina (Al2O3) content is a marker for the magnesia-alumina spinel in the refractory. Spinel is the more insulating component. Generally, the higher the alumina content, the greater the insulating value of the brick and, consequently, the lower the kiln shell temperature.
The second step to address high burning zone shell temperatures is to install a thicker lining. Common practice for a rotary kiln lining in North America is 220 mm (8.66in.) thickness. Burning zone linings of 250 mm (9.84in.) are becoming more frequent. Historically, in larger diameter kilns, burning zone linings as thick as 305 mm (12 in.) were used. It is unclear what the limit of thickness for a basic brick lining is, as a brick lining of 457 mm (18 in.) has performed successfully in a burning zone dam construction. (Figure 5)
Figure 6. Discharge grate/dam area of a lime recovery kiln uses a 60% alumina QUIKTURN™ castable to facilitate the initial heating of a thick refractory construction. This photograph shows the condition of the construction after one year of service.
Monolithic refractories can be used in the discharge end also, including dam constructions. As rotary kilns age, the steel shell in the discharge area can weaken and deform, causing brick constructions, including dams, to become unstable and loosen. Lime kilns, especially in the paper industry, can use more complex constructions to incorporate grates or ports for planetary coolers (figure 6). Refractory construction in both conditions is facilitated by using monolithic refractories.
Damn constructions can have great lining thickness at their peaks, and this presents difficulties in the initial heating and dry-out of the monolithic refractory. QUIKTURN castable and gunning mixes have been developed for such difficult to dry-out applications. Most low-cement castable use increased permeability to reduce the incidence of steam-spalling during dry out. QUICKTURN castable incorporate several other technologies to reduce dry-out time, in some cases to a quarter of the time required for a conventional low-cement castable. These advanced materials reduce the risk and expedite the initial heating of thick monolithic constructions
Refractory placement in the rotary pebble lime kiln requires an assessment of the conditions of service and the method of installation. Abrasion resistance is the prime consideration for the charging zone because of the abrasive nature of the stone feed, especially in long kilns that do not have preheaters. For the most severe conditions of abrasion, 80% alumina phosphate-bonded brick extends refractory life. Special extreme abrasion-resistant castable are available to be used in place of brick construction where brick installation would be too lengthy or difficult. Additionally, monolithic refractories are anchored to the steel shell, reducing the risk of lining looseness in difficult brick constructions.
In the calcining zone, 70% alumina refractory bricks are widely used. The calcining zone may also contain various kiln internals to promote heat transfer. Advancements in castable technology have made it possible to fabricate lifters and cross-type heat exchangers that provide good longevity of service.
High kiln shell temperatures in the burning zone challenge operators. The selection of magnesite-spinel brick with a consideration of the insulating value (low thermal conductivity) can help manage the hot kiln shells. Thicker brick linings in the burning zone provide better stability that two-layer refractory constructions and help to further reduce shell temperatures.
Dam construction is the discharge zone can be either brick or monolithic. Specially formulated castable refractories call QUIKTURN™ meet the challenges of thermal shock resistance, and moderate abrasion resistance and stability for the dam construction, while reducing the risk of any difficulty in the initial dry-out of a very thick construction.
Written by: Christopher Macey - Resco Products Inc.