The limestone preheater is a refractory lined vessel used to dry and preheat the limestone. Limestone enters via a chute, dropping out of a stone bin onto the brick floor of the preheater. Exhaust gas is drawn out of the kiln, passing through the bed of limestone preparing it for the kiln. The heated limestone is pushed across the floor of the preheater by hydraulic rams, ultimately dropping through the feed hopper and into the rotary kiln. The range of temperatures in the preheater varies from to several hundred degrees in the gas outlet chamber to the calcining temperature (around 1800°F).
The gas outlet chamber is commonly lined with fireclay-based castable or gunning mix. EZ Cubed 2700 and Rescocast 15GM are common selections for the working lining. These products provide ease of installation and good abrasion resistance. Calcium silicate block insulation or light-weight gunning mix are in the back-up layer to provide reduced shell temperatures, here and through the entire preheater.
The roof of the preheater is usually suspended brick constructed from 45% alumina fireclay or 60% alumina special shapes. At the fully retracted position of the rams, the brick floor should feature high abrasion resistance. Typically, phosphate-bonded high-alumina brick are preferred, including Seneca 60P, and Alumex P-85-HS for more severe abrasion. Normal refractory selection for the hopper includes 40% alumina castables and gunning mixes. For outstanding product performance and longevity, R-MAX PC and R-MAX PC-QT are recognized as industry standards for this application.
The transfer chute breaches the preheater and the kiln, as preheated stone slides down its sloping brick wall and enters the rotary kiln. The brick wall requires maximum abrasion resistance, making Alumex P-85-HS the ideal refractory choice. Continuing the practice from the hopper, R-MAX PC and R-MAX PC-QT provide installation ease for complex construction as well as optimal abrasion resistance. To help manage process-related build-ups, Vibrocast 60Z PC, EZ Cubed AZS, and QuikTurn AZS-PC are ideal.