Background

The cement manufacturing process is noted for wear-intensive unit operations, including mining of raw materials, crushing, blending, milling and calcining; followed by crushing and milling; and, finally, conveying. Wear conditions range from high impact and some abrasion (such as in primary breakers) to low impact and high abrasion (such as in pulverizing and final grinding).

Materials Preparation and Finishing Systems — Materials Wear

Over the years, several wear-resistant cast materials have been utilized in the cement industry for applications involving crushing and grinding. These materials range from manganese steel, water-hardening alloy steels, air-hardening alloy steels, to alloy white irons, in order of increasing hardness and decreasing toughness. For abrasive wear, rubbers, plastics, epoxy and refractory coatings, and ceramics are used. Service conditions dictate which materials may be used for a particular application.

What follows are examples of issues, problems, and questions you may be asked to solve or answer while working in the cement industry:

1. What is a casting overlay? Is this different than a bi-metallic wear casting? What metals can be used in a casting overlay? How is it produced?

2. What is clinker? What is the hardness of clinker? How does this compare with the hardness of cement? Is cement known more for abrasion or hardness?

3. What are the main disadvantages of manganese (Mn) steel? Where can Mn steel be used in the cement plant and why?

4. In alumina ceramics, what is the function of the ceramic component? The alumina component? How are these two combined? How is the alumina ceramic specified? (Hint: It's not by alumina concentration). Why does that specification determine the application?

5. What are the temperatures in the major process areas (raw mill grinding, blending, pyroprocessing, crushing, milling, conveying)? Are these temperatures typically given for materials or gases? In which of these areas could plastic wear-resistant materials be used? How could these materials be tested in the laboratory for cement plant conditions?

6. What is “hardfacing”? For what wear conditions in a cement plant would be this deposition technique be used? Under what conditions is there a heat distortion of the piece being treated?

7. How is the wear resistance in epoxies improved? What properties, in addition to wear resistance, does the addition of alamide fibers to the epoxy enhance?

8. Which cement process equipment pieces are cast, and which are made of other means? What are the reasons for the varying methods of equipment production?

9. Based on your understanding of the cement manufacturing process, which pieces of process equipment could be composed of concrete? (Hint: Consider functions at a continuous process facility other than those pictured above.)

Recommended References:

(Copies of all references are available from the Portland Cement Association)

• Alsop, P.A., and Post, J.W., “The Cement Plant Operations Handbook,” Tradeship Publications Ltd., Surrey, U.K., 1995.

• Firestone, S.E, “Alternate Wear Resistant Material for Cement Plants - Plastics,” Mill Session Papers, Portland Cement Association, Skokie, Illinois, 1996.

• Heath, G.R, and Skora, J., “New Materials and Process Developments for Preventative Maintenance and Repair in Cement Plants,” 40th Cement Industry Technical Conference, IEEE/PCA, ISSN No. 1079-9931, 1998.

• Manganon, P.L., “The Principles of Materials Selection for Engineering Design,” Prentice-Hall, Inc., Upper Saddle River, New Jersey, 1999.

• Stafford, J. and Bergner, J., “Wear Resistant Casting Materials and Overlays Used in Crushing and Grinding Applications in the Cement Industry, Mill Session Papers, Portland Cement Association, Skokie, Illinois, 1996.

• Yandora, P.A., “Alternate Wear Resistant Material for Cement Plants - Ceramic,” Mill Session Papers, Portland Cement Association, Skokie, Illinois, 1996.

Background

In simple rotary kiln systems, some finely divided particles of raw mix, calcined kiln feed, clinker dust, and volatile constituents (e.g., potassium sulfate) are entrained in the exiting gas stream. These particles are almost entirely removed from the gas streams before the combustion products are vented to this atmosphere.

The powder that is collected from the kiln exhaust gases is known as cement dust kiln (CKD). CKD is captured in baghouse filters and electrostatic precipitators. Most plants return all or a portion of the CKD to the process; others completely remove it from the process.

The chemical composition and physical state of the CKD depend on the type of pyroprocessing system, the chemical composition of the raw materials and fuel, and the state of the process at any given time.

Specifications for portland cement often contain limitations on the quantity of sodium and potassium, also known as alkalies. Since the volatile oxides and salts of these metals tend to partition to the CKD, a portion or all of the CKD is sometimes removed from the system to meet product quality standards. Dust from the preheater tower has the same general chemical and physical composition as the kiln feed, and is captured and returned to the process.

Pyroprocessing System — Air Emission Characterization

The principal gaseous emissions from the cement pyroprocessing system in a typical descending order are nitrogen, CO₂, water, oxygen, NOx, SO₂, CO, and hydrocarbons. The volumetric composition range of these constituents is from about 73% to less than 10 ppm. CO₂ and the last four listed gases are the primary constituents of environmental concern. Emission rates of SO₂ and NOx display a wide range of values throughout the industry. Each individual pyroprocessing system has its own emission characteristics, and the SO₂ and NOx emissions are difficult to predict from untested or proposed systems. Efforts to control emissions of SO₂ and NOx from cement plants are becoming more prevalent and are meeting with success.

What follows are examples of issues, problems, and questions you may be asked to solve or answer while working in the cement industry:

1. Why is dust captured from the preheater tower of the same composition as the raw feed? (Hint: See question 6.)

2. Is CKD chemically or physically different from the cement product?

3. Are electrostatic precipitators operated based on chemical or physical processes? Are baghouse filters operated based on chemical or physical processes? How could either of these unit operations be changed to incorporate other types of mechanisms, either physical or chemical?

4. What is the difference between a direct- and an indirect-fired coal system? Does the concentration of NOx in the kiln gas increase or decrease with increased temperature at the flame?

5. What is the optimum temperature at which to inject urea or ammonia into the kiln gases? Where is this temperature located in a dry system rotary kiln? In a precalciner system (with vertical reaction vessels)? In a wet system rotary kiln? Does it make sense to inject urea or ammonia into a wet system rotary kiln — why or why not?

6. What effect does the chemical form of the input sulfur (in either fuel or raw feed) have on the emitted gas concentration? In the baghouse, why doesn't the dust (formed of CaCo₃ and other minerals) preferentially absorb the SO₂?

7. How is CO₂ gaseous emission concentration calculated from raw feed mass flow of CaCO₃ and air inflow? Gases flow countercurrent to mass flow in the cement manufacturing process. What are the ranges of gas flow rates for precalciner systems? For dry system rotary kiln? Assuming a precalciner system, what are four possible ways of reducing the CO₂ gaseous emissions?

8. What are the sources of hydrocarbons in the kiln gases - fuel or feed? What is a typical hydrocarbon ambient concentration for an urban metropolis with traffic jams? (Not in references, please consult local regulatory agency.) How can the ambient concentration be higher than the emitted concentration of gases from the kiln?

Recommended References

(Copies of all references are available from Portland Cement Association)

• Alsop, P.A., and Post, J.W., “The Cement Plant Operations Handbook,” Tradeship Publications Ltd., Surrey, U.K., 1995.

• C. Bech and L. Gundtoft, “Study of NOx, SOx and CO Mechanisms Based on Actual Plant Data,” 40th Cement Industry Technical Conference, IEEE/PCA, ISSN No. 1079-9931, 1998.

• Greer, W.L., Dougherty, A., and Sweeney, D.M., Chapter on Portland Cement, “Air Pollution Engineering Manual,” 2nd edition, John Wiley and Sons, Inc., New York, 2000, pp. 664-681.

• M. Terry, “SO₂ Removal in Pre-heater and Long Kilns,” Mill Session Papers, Portland Cement Association, Skokie, Illinois, 1994.