Lime Recovery Kiln

Figure 1. Refractory Zones in a lime recovery kiln.

Figure 2. Alkali spalling in an insulated high alumina brick lining, Products.

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, the 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 the 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 lime 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 kiln.

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 is much more compatible with lime, resist flexing, and offer extended lining reliability. Selection should be based on desired shell temperatures, frequency of cycling, and cost.

GREENFREE™ 92 is the 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. 

RESCOMAG 85 is the option for the harshest conditions in the lime kiln burning zone. RESCOMAG 85 features a 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 chain systems that have more severe abrasion, R-Max G QT offers higher strength and better durability.

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.