How is cement made?

Ron Weiss, maintenance manager, Essroc: "The raw materials are things like limestone, clay and sand. We heat them in kilns until they form new chemical compounds."

How hot does it get inside the kilns?

"At the lower end, where the coal fire is, temperatures will be about 2,700 degrees Fahrenheit."

Cement kilns are among the world's largest moving manufacturing machines. The ones at Essroc's facility are cylinders 450 feet long and 12 feet in diameter, running 24 hours a day, 7 days a week.

Lower end?

"Our kilns are long - in our case, about one and a half times the length of a football field - cylinders that are slightly tilted. The upper end is about 28 feet higher than the lower end."

What's the reason for that?

"It allows us to feed in raw materials continuously at the top. The whole kiln turns slowly, at about 70 revolutions per hour.

"We're constantly taking out product at the bottom. Except for maintenance or repairs, we can produce cement 24 hours a day, 7 days a week."

How much cement does your facility make?

"We'll produce about 621,000 tons of clinker this year."

Clinker?

"That's what comes out the end. The materials partially melt and react, and the turning of the kiln forms them into small spheres. "We grind up the clinker, add some gypsum, and the result is 'cement.' Our production will be about 780,000 tons of cement this year."

The kiln produces "clinker," which is ground and mixed with gypsum to make "cement." Cement is then mixed with other materials, including sand or aggregate, to produce "concrete."

How much fuel do you use?

"Normally, we'd use over five tons of coal an hour. That's why we're so interested in burning tires."

With the coal?

"No. The tires are put in quite a ways up the kiln. The temperature there is about 1,800 degrees, which allows us to put the tires in whole, without having to cut them up. That saves a lot of work and time."

What about smoke and fumes?

"If you think about what tires are made of, it's a lot like putting in #2 fuel oil. The kiln is so hot that combustion is nearly complete."



What about the steel cords?

"The iron becomes part of the clinker. In fact, iron is one of the components of cement, and when we use tires, we are able to adjust the chemistry of the raw materials."

How do you get the tires into the kiln?

"We built a special lift device that can tip up an entire trailer to empty it. Conveyors carry the tires to the feed fork system."

What does that do?

Tom Castellino, tire supply associate, Cadence Environmental Energy, Inc.: "It's called a 'fork' because it looks a bit like one. Two forks, actually. One is at the end of the conveyor. The other is mounted on the kiln, and turns with it.

"Each time the kiln revolves, the 'tines' of the kiln fork pass through the 'tines' of the conveyor fork, transferring the tires to the kiln fork.

"Then, as the kiln fork comes straight up, doors open, and the tires drop into the kiln. The doors close as the kiln continues turning. The process repeats with each revolution of the kiln."

The Cadence feed fork actually uses two forks, transferring fuel from one to the other, and then into the kiln.

How many tires go in at once?

"It might be just one, especially if it's a truck tire, but with smaller tires, it could be two or three at a time. It's all governed by weight and computer-controlled."

What are you controlling with the weight?

Ron Weiss: "We're regulating the burn rate and oxygen content in the kiln, to ensure good quality, proper combustion, and emission control."

Does burning tires produce less pollution?

Tom Castellino: "Studies indicate, with Mid-Kiln Technology, whole scrap tires burn every bit as cleanly as coal, and reduce nitrogen oxide emissions."

Are there other benefits?

Ron Weiss: "For one thing, scrap tire dumps are an eyesore, and, they're a threat to the environment. There's always the chance of an uncontrolled fire that can result in both air and water pollution from the smoke and runoff of tire by-products.

"That's not the case in a cement plant like ours, where the burning is at a very high temperature and very well-controlled."

How many tires can you use?

"We estimate we'll burn over 1.5 million tires this year." And how many scrap tires are out there? Tom Castellino: "The industry estimates there are about 2 billion scrap tires already, and that we're adding about 250 million per year - about one scrap tire per year for every man, woman and child in America.

"If every cement kiln that could do so would start substituting scrap tires as part of its fuel, we could make a serious dent in our scrap tire problem."