"The kiln is the big, l-o-n-g tube set at about a 40-degree angle."

The kilns can also be essentially horizontal. I once visited a former East German cement plant that was built to provide cement to the entire country. The plant had 4 spinning kilns about 100 yards long and 8 feet or so in diameter. The plant polluted so much that a neighboring town had to sweep all their roofs periodically to remove all the ash and other by-products that settled out of the air.