Thursday, March 19, 2015

Cement Industry

Karl Rethwisch posted
This is a 136 car loaded cement train in the siding at Baxter, CO.
Had to be over 18,000 tons !!
The active ingredient in concrete is Portland cement. A cement plant, like a coal mine, needs railroad service to be economically viable because of the bulk and weight of its output. It is manufactured as a chemical combination of calcium, silicon, aluminum, and iron []. The plants are generally located next to limestone quarries, which provides the calcium. Clay, shale and sand is added to provide the silica, aluminum and iron. Other "alternative" raw material such as mill scale, fly ash and slag may be used if the industries that produce these waste products are nearby or they are needed for additional iron and/or aluminum. [WBCSD]

Initially, a "wet process" was used. This mixed the raw materials with water to create a slurry to help achieve a homogenous mix of the raw materials. After equipment was developed to thoroughly mix the raw materials without water, the "dry process", it replaced the wet process because a lot more energy (heat) is needed by the wet process to get rid of the water in the slurry.

The following two videos describe how cement is made.

(new window)   Of the videos I watched, I think this one is the best description.

(new window)   This video has more details, but I found the segments with just music are hard to set through.

To summarize the videos, the limestone is crushed and blended with the other raw materials. Then it is milled to a fine powder, dried, and "cooked" in a preheater and a rotating kiln to make clinkers (calcium silicates [WBCSD]). The clinkers are then cooled and ground in a ball mill with a little gypsum (calcium sulfate, [understanding]) added to extend the setting time. "Cement is so fine that 1 pound of cement contains 150 billion grains []. Another way to express the fineness of the cement powder is that it can pass through a sieve that won't pass water.

The calcination of limestone (CaCO3) to CaO+CO2 produces 60% of the CO2 emissions from the kiln. The combustion of the fuel generates the rest. [WBCSD] I did not realize the significance of creating CO2 from calcination as well as from burning a lot a fuel until I saw the figure that the cement industry was "source of about 8% of the world's carbon dioxide (CO2) emissions" in 2016. [BBC] I know there is a chemical reaction when concrete sets because it generates heat. Evidently the reaction absorbs CO2. [BBC] So if they could capture and store the CO2 produced when making cement, the cement industry would remove CO2 rather than produce 8% of the worlds emissions. Negative perception killed the nuclear energy industry in the 20th century. And negative perception is killing coal-powered electricity generation in the 21st century. Will the cement industry escape negative perception? The BBC article indicates alternatives to calcination such as bacteria that use a room-temperature process similar to the one that creates coral. Replacing cement with fly ash also helps reduce the carbon footprint.

The original "kiln" was a kitchen stove: "Bricklayer Joseph Aspdin of Leeds, England first made Portland cement early in the 19th century by burning powdered limestone and clay on his kitchen stove." []
Now "the kiln is the world's largest piece of industrial equipment." [WBCSD] It is 12-feet in diameter and 200-feet long. The interior is lined with fire brick. The 2000 degree (C) (3632F) flame blown into the low end of the kiln is so hot that it can burn about anything including "alternative fuels such as plastic, solvents, waste oil or meat and bone meal." [WBCSD] To put 3632F in perspective, steel melts at 2500F. In fact, the long atmosphere of extremely hot gases in a rotary kiln has established a secondary business of burning hazardous waste. This is a win-win because it reduces the need for fossil fuels and it breaks down hazardous substances and gets them out of our atmosphere. [ScienceDirect]

CementKilns has photos and a lot more information on rotary kilns.

Update: a video of BNSF train hauling a serious quantity of cement through Hastings, NE. Too bad one can't tell where a train is coming from and going to just by looking at it. It does illustrate that cement is hauled in 2-bay covered hoppers.

whitecap (source)
Portland cement was patented in 1824 by Joseph Aspdin, a mason in England. He named it “portland” because concrete made with it was similar in color to natural limestone from the Isle of Portland. Although Aspdin was the first to patent a formula for cement, natural cements produced by heating natural minerals had been used for centuries. The Greeks and Romans used lime mortars that were given hydraulic properties by the addition of volcanic ash and other natural pozzolans. The first recorded shipments of portland cement to the U.S. came from Europe in 1868, and it was first manufactured in the U.S. in 1871 (Design and Control of Concrete Mixtures 1).

But we still don't really know how the Romans made such good cement. We now know what makes their concrete strong: "crystals of a layered material called 'aluminous tobermorite' provided structural strength." But we don't know how they made those crystals at normal temperatures. Learning their secret would allow concrete to reduce the CO2 levels rather than be a major contributor to CO2 pollution. (Not only is a lot of CO2 emitted to heat the kiln, the chemical reaction that turns limestone (CaCO3) into lime (CaO) releases CO2.) 
safe_image for X-Ray Studies Hint At The Romans' Secret To Stopping Climate Change