Using Alkali-Resistant Castable in a Cement Preheater: Alkali Attack Criteria and Zone-by-Zone Selection

Using Alkali-Resistant Castable in a Cement Preheater: Alkali Attack Criteria and Zone-by-Zone Selection

Alkali attack in a cement preheater starts with one fact: the potassium, sodium and sulfur in the raw meal are modest, but they volatilize in the kiln and condense in the cooler stages, and after repeated circulation the alkali load in cyclones C3 and below can reach five to ten times the raw meal level. Between 800 and 1000 degrees C the alkalis react with the silica and alumina in the lining, forming feldspar and leucite phases that expand by well over ten percent in volume, so the hot face blisters, cracks and peels away layer by layer with the coating. A preheater lining is therefore not a simple "wear" problem: each step down toward C5 shifts the emphasis toward alkali.

What Alkali Attack Looks Like: Three Site Criteria

First, watch the coating: when buildup in the cone and downcomer needs cleaning far more often and grows back within days, the alkali cycle has matured. Second, look at the hot face during a shutdown: layered spalling with flakes 20-50 mm thick, and a fracture face showing a dense reaction layer over a porous intermediate one, is the classic alkali profile. Third, watch the running parameters: growing fluctuation in C5 outlet temperature and pressure and steadily rising draft resistance also point to debris shed from the lining. When two of the three show up, the affected zones should move to an alkali-resistant grade rather than waiting out another campaign in plain high-alumina castable.

Zone-by-Zone Selection: Temperature Sets the Grade

Use three grades by working temperature. C1, C2 and the cooler riser ducts, roughly 600-800 degrees C, take standard alkali-resistant castable: a matrix of 30-40% alumina and 55-65% silica keeps the alkali reaction at the surface, where it forms a dense glazed protective skin. C3-C5, the pinch points and horizontal ducts at 800-1000 degrees C see the heaviest enrichment plus material flow, so they take the high-strength or wear-resistant alkali grade with 5-15% silicon carbide for density and lower coating adhesion. The kiln inlet chamber and the root of the riser run above 1100 degrees C, beyond what an alkali-resistant matrix carries - use high-alumina or corundum grades there instead. Set the water ceiling grade by grade as well: 5.5-6.5% for the standard grade, 5.0-5.8% for low-cement alkali grades. Every 1% over ceiling costs about 15% of cold strength, and coating cleanup knocks are exactly what the cooler stages must survive.

Placement Controls: Three Things Easily Skipped in Elevated Ducts

First, anchors: Y- or L-shaped, 6-8 mm diameter, pitched at 150-250 mm, 304 stainless as a floor and 310S above 900 degrees C, each one coated with bitumen or taped to leave expansion and venting paths at the weld. Second, bays: cast the ducts and shells top-down in sections no taller than 1.5 m with forms braced solid - a blown form on elevated work costs three times the pour in scaffolding hours. Third, venting and curing: stagger vent holes in the forms, then cover and cure at least 24 hours, more under cold-weather rules. Each item is one line in the briefing; skip any of them and the inspection passes, but the lining tells on you within six months of startup.

Working With Coating Cleanup

Choosing the right grade is only the defense; cleanup method decides the service life. Prefer compressed-air blowing and pick-point chipping with a pneumatic lance, and forbid repeated sledgehammer blows - the hammer shatters the coating while loosening the lining, so every cleanup is a small destructive test. Manage pressure drop quantitatively: log stage outlet differential pressures every shift and trigger blowdown at 0.5-1.0 kPa above baseline rather than waiting for a thick slab to chisel. At shutdowns, decide replacement by residual thickness: below 30-50 mm on the working layer (of a cast 80-120 mm) or a through-going reaction layer goes on the next maintenance list - do not wait for breakthrough and alkali-sulfate corrosion of the shell plate.

A One-Page Checklist

Before each shutdown, verify five items: a temperature-zone map with the installed grade matched zone by zone; anchor pitch, material and coating records; water metering with the addition ceiling posted next to the mixer; the differential-pressure baseline at the control room; and the residual-thickness log from the previous outage. With those five in order, the account changes from "what castable we bought" to "what grade at what temperature, how it was placed and how it is maintained" - and settled properly, that account routinely buys one to two extra lining campaigns out of the same kiln.