Lime Recovery Kiln
Figure 1. Refractory Zones in a lime recovery kiln.
Figure 2. Alkali spalling in an insulated high alumina brick lining.
Rotary lime recovery kilns typically have four refractory zones (figure 1). These wet process kilns use chain systems in the charging area to dry the slurry feed. In the preheating zone, the mud feed is heated in preparation for the burning zone, and refractories range from fireclay (40% Al2O3) brick up to 70% alumina brick
The burning zone is the hottest location in a lime recovery kiln. Here dissociation of calcium carbonate occurs at high temperatures, producing the lime product. Depending on the process, burning temperature can vary from below 2000°F (1100°C) to nearly 2500°F (1370°C), and this will dictate refractory selection for the burning zone. The discharge zone of a modern lime kiln contains a dam construction to increase retention time of the lime in the kiln.
More energy efficient lime reburning kilns were introduced to the North American paper industry in the 1980s. Efficient kilns feature high-density chain systems, kiln internals to increase heat exchange, dams for greater retention time, and two-layer insulated linings. These features allow reduced burning temperature but retained longer in the kiln. Because of the insulating brick the alumina brick hot face layer must have greater alkali resistance due to the soda present in the mud feed (Figure 2)
Temperatures above 2250°F (1230°C) are common in lime kilns that have less efficient heat exchange. High calcining temperatures require basic burning zone refractories that are more compatible with the lime (CaO) charge than high alumina brick. Since the 1980s, magnesia-spinel refractories, composed of magnesite (MgO) and magnesia-alumina spinel (MGO-Al2O3), have become the preferred basic brick refractory for the rotary lime kiln.
Figure 3. SENECA 60P
FIgure 4. 60% alumina SENECA 60P has the positive reheat expansion and resistance to spalling necessary for lime recovery kiln linings.
Alumina Brick for Preheating and Burning Zones
60% to 70% alumina brick linings are often used in preheating zones and burning zones of many lime recover kilns. 60% alumina brick are the preferred selection for insulated kiln burning zone linings. 60% alumina brick offer increased resistance to the expansive alkali reactions that can cause alkali spalling.
SENECA™ 60P exhibits excellent alkali resistance from the mullite (3 Al2O3-2 SiO2) primary raw material (Figure 3). A positive reheat property in SENECA™ 60P improves the stability of the lining, as it tends to tighten during service. Phosphate-bonding gives SENECA™ 60P good mechanical strength.
For more traditional one-component recovery kiln linings, RESCAL™ 70D has been widely used in lime kiln linings and offers consistent, economical performance. For a 70% alumina class brick, RESCAL™ 70D maintains good alkali resistance and a positive reheat property for improved lining stability
Basic Brick for the Burning Zone
For kilns that have not undergone improvements in heat exchange components, basic brick presents the capability for higher calcining temperatures. At elevated temperatures, lime can react with high alumina brick to cause fluxing that leads to the “duck nesting” appearance. Basic brick are much more compatible with lime, resist fluxing and offer extended lining reliability. Selection should be based on desired shell temperatures, frequency of cycling, and cost.
Figure 5. RESCOMAG® 85 is a magnesite-spinel option for lime recovery kiln burning zones that operate at elevated temperatures.
GREENFREE™ 92 is a 1st generation magnesite-spinel refractory brick with a medium content of in-situ spinel. Widely used in lime kilns, GREENFREE™ 92 is Resco’s most economical basic brick for higher burning temperatures.
GREENFREE™ KF, a derivative of GREENFREE™ 92, features a high content of spinel and gives significantly lower shell temperatures (approximately 100°F lower than GREENFREE™ 92) due to improved thermal conductivity.
RESCOMAG 85 is the option for the harshest conditions in the lime kiln burning zone. RESCOMAG 85 features fused spinel and an enhanced matrix for high corrosion resistance, good thermal conductivity and strong thermal shock resistance.
The End Zones: Chains and the Discharge
Monolithic (unshaped)refractories are often used in these zones to simplify refractory construction. Castable refractories are used in the chain system for linings around the chain hangers. RESCOCAST® 15GM is a versatile, high cement content castable refractory that can be conventionally mixed and poured, pumped for a large lining section, or gunned for chain section maintenance between major relines. RESCOCAST® 15GM features high strength and good abrasion resistance.
For discharge zones, RESCORAM® 70 BLUE plastic can be used in discharge dam construction and features good low temperature strength, exceptionally good installation characteristics and ease of dry-out.
RESCOCAST® 15GM develops excellent strength for vibration-casting nose rings and dams. Resco’s new QUIKTURN™ castable products reduce curing time and are especially suitable for temporary repairs, thick pours and when dry-outs cannot be controlled.
The Process Boilers
The chemical recovery cycle in a Kraft-process paper mill includes a black liquor recovery boiler. Refractory installers often place monolithic refractories in the boiler bottoms to help resist the erosion of the smelt. QUIKTURN™ 56SCG is a 55% silicon carbide content refractory that is installed by pneumatic gunning in the boiler lower sidewalls and bottoms of recovery boilers. RESCOBOND® 3000 has also been utilized in recovery boiler applications, including the smelt spouts.
Other areas in the process and power generation boilers, including burner block and seals, may require 90% alumina castable. RESCOCAST® 17A is a high-purity 95% alumina castable for very high temperature applications such as ash hoppers, RESCOCAST® 15GM is selected for installation versatility and high strength.