Basically everywhere you look in modern society, you see something made of concrete. Often we see trucks traveling down the road, drums spinning to keep the cargo mixing on their way to a job site in a subdivision or business park. It’s so common, in fact, that most people barely give it a second thought, but the process that turns limestone into towers of concrete and steel is one that creates its own challenges for valves in a cement plant, especially those in the slurry line.
One of the largest cement companies in the world is Lafarge, which produces cement for residential and commercial construction and for oil wells. Ed Kunkel worked for Lafarge at its plant in southwestern Ontario for more than 30 years, and he provided much of the process line and valve specification information for this article. We also spoke Ed Holtgraver of QTRCO, Inc., Tomball, TX, who provided us with additional information about valves and actuation.
While much modern cement manufacture today uses the “dry” process, some plants throughout North America use the wet process. The two processes are essentially alike; the only difference is that in the wet process, the raw materials are ground with water, creating slurry, before being fed into a kiln.
The Slurry Process
“We used many different valves in the plant,” said Kunkel. “In the slurry lines they’re mostly plug and gate valves as well as a few pinch valves. But no matter what they are, they take a beating.”
The largest portion of slurry is ground limestone, which, once it comes into the plant from the quarry, goes to the crushers. There are generally two: a primary or cone crusher, and then to a secondary, hammer crusher or impacter. When the limestone comes out of the cone crusher, the chunks can be up to six inches, sometimes a bit larger. They are then fed into the secondary (hammer) crusher. From the secondary crusher the pieces go through a screen house that allows the ¾ inch and smaller aggregate through. That is sent to feed the raw mills. Larger pieces are diverted back into the secondary crusher.
From the screen house, the ¾ inch pieces are sent to a storage bin. From there, a clam bucket or some other conveyance takes the limestone to the raw mills, also called slurry mills.
The mills are divided into three compartments, each with different sized hardened steel balls inside. Those balls are agitated by the motion of the mill, which motion is governed by huge bearings at each end of the mill. To keep those bearings operating, they are water cooled. One inch globe valves are used to control the flow of water going to the bearings; water pressure coming into the lines is 150 lbs. The valves in the Lafarge plant Kunkel worked at were all rated 250 lb. “It was a very simple system,” he said. “The valves would control the amount of water flowing through the bearings, and they were also used for on/off functions.”
Once inside the ball mills, the limestone is made finer in each compartment as it passes through 3-inch steel balls in the first compartment, 2-inch balls in the second and finally 1-inch balls in the third compartment. When the limestone exits the discharge end of the ball mills, it is a fine powder that becomes the slurry.
In many modern plants, the ball mills have been replaced by a roller press, which has the same result. The limestone becomes a fine powder.
Next, pyrite or other materials is added on its way via conveyor belt to the slurry mill. Until it arrives at the slurry, or raw, mill, it is completely dry.
According to Kunkel, in the earlier days of operation, the quarry at the plant where he worked was still relatively shallow, so silica was added to the mixture in order to strengthen the cement and make it more resistant to abrasion. As the quarry got deeper, there was enough naturally occurring silica with the limestone that it was no longer necessary to add it.
Depending on the location of a quarry and where the cement will be used, other added materials could include shells, chalk or marl combined with shale, clay, slate, blast furnace slag or iron ore.
Once the limestone slurry is mixed with the required materials, it is fed into a wet process mill (slurry mill) where cold water is added to it. Water used in this process passes through a manually operated globe valve used as an on/off valve that also controls the rate of flow of water into the slurry mill.
When the product comes out of the discharge end of the slurry mill, it is a heavy, slippery, very abrasive slurry that is gravity fed into a tank.
A 10-inch gear-operated plug valve acts as on/off control of the slurry from the mill to the slurry pump. An 8-inch plug valve functions as on/off control at the discharge end of the slurry pump, heading to the slurry tank. From this tank, the slurry is again pumped, this time to a large slurry basin that has a large air-fed agitator in it to keep the slurry from settling. Once at the slurry basin, the same gear-operated 10-inch plug valves are used for open/close control to send the slurry from the basin through the slurry pumps to the kiln feeder.
In the case of the Beachville, Ontario plant, early in its operation, the slurry line fed the kiln at full flow; the valves were simply on/off, and any overflow was caught by a ferris-wheel feeder with a tank. From there it was diverted back to the slurry basin by gravity. The speed of the wheel regulated the amount of slurry that went into the kiln. Later, air-operated pinch valves were used as flow control, eliminating the need for the ferris-wheel feeder and the overflow tank. That is the system generally used in modern wet process plants.
Between the slurry mill and the kiln feed, different grades of slurries for different cement mixtures are controlled by outlets from the slurry tanks. Flow of the slurry is again controlled by plug valves, manually controlled in earlier days but later pneumatically. “The plug valves were open/close,” said Kunkel. “The largest plug valve was 8 inches, not gear operated. Instead of having a handle, those valves simply had a 2-inch-square shaft sticking out of the body that was opened or closed with a 3-, 4- or 5-foot-long handled wrench. They would need that much force because they fill up with an aggregate of grime and over time they become difficult to operate.”
The big challenge for cement slurry is the abrasiveness. Holtgraver said that the slurry can even erode pipe where there is a bend, so everything must be particularly robust to handle the abrasiveness.
Shutdowns for maintenance present their own challenges to valves in a slurry line. According to Kunkel, at one point they were using pinch valves coming out of the pumps that were pumping slurry out of the slurry tanks into the kilns, but when the process was stopped, the slurry would settle and harden in the valve. “They would have to disassemble the valve and basically chip away at the hardened slurry to get it running again,” said Kunkel. “With the plug valves, we could shim them to loosen the plug from the body, so that it would be easier to operate. They would almost seize up with the aggregate, but the shim would allow a little more space in the valve body and the plug, making it easier to operate.”
Also, according to Kunkel, the maintenance team could force some hardened slurry out of a plug valve by opening and closing it, causing the slurry to break apart so that it could be forced out of the system when the system started up again. “Of course you couldn’t do that with a pinch valve,” he said. “They were used only on the discharge end of the pumps.’”
For operation, Holtgraver said that in modern plants, there would be some electric, but just as often, pneumatic actuation if the valves were automated. “However,” he said, “They are just as likely to be hand-operated with a handle and gearbox because of the expense. Some of the pinch valves would be air operated.”
While the technology and requirements for valves needed for producing cement have not changed drastically in the last 20 years or so, efforts to make the process more environmentally friendly continue. In many modernized wet process plants, a slurry drier can be added to dry the slurry before entering the kiln, using waste heat from the kiln. This reduces energy consumption considerably while increasing productivity. Other improvements include recycling water, capturing waste heat for power production, and lowering dust emissions.
The End of the Line
Once the product enters the kiln that is the end of the slurry line, but obviously, not the end of the product.
One of the crowning achievements of the Lafarge plant at which Kunkel worked was to provide the cement required for construction of Canada’s CN Tower – a concrete behemoth that rises more than 1,800 feet above the Toronto skyline. The core of the structure is a 1,100-foot-tall concrete shaft, a hexagonal core with three curved support arms. More than 59,000 cubic yards of concrete was poured continuously into a massive mold, or “slipform,” to build the column. The slipform, supported by a ring of climbing jacks powered by hydraulic pressure, moved upwards at the rate of approximately 20 feet per day as the concrete below set. Construction took 40 months.