Utilization Of Steel Slag In A California Cement Plant

Scott Renfrew, RMC Pacific Materials and David Perkins, TXI

A cement production facility in the coastal California area utilizes a patented process to introduce steel slag directly into the feed end of the kiln. Steel slag shares many common characteristics of the cement feedstock and chemically combines with the entire mix. Not a raw material; it has already been heated to 3000 degrees Fahrenheit, and having a chemical composition quite similar to Portland cement clinker, the raw mill feed is proportioned to a chemical composition that will result in the correct clinker chemistry. This paper highlights plant production achievements, quality control and operational challenges associated with the usage of slag in clinker production. This paper also discusses theories of NOx and CO2 emission reductions per unit clinker produced, in addition to an overview of the slag handling and feeding system design and implementation process at the plant.

Introduction

The unique feature of this process is introduction of steelmaking slag directly into the feed end of the kiln. Because slag shares many common characteristics of the cement feedstock, it chemically combines with the entire mix. The steel slag is not a raw material; it has already been heated to 3000 degrees Fahrenheit, and has a chemical composition quite similar to Portland cement clinker. When steel slag is utilized, the roller mill feed is proportioned to a chemical composition that will result in the correct clinker chemistry.

The process reduces the emissions of NOx and CO2. Reduction of CO2 emissions are due in part to reduced limestone calcination per unit of product, and in part due to improved thermal efficiency of the process. The reduction of NOx emissions is achieved through less thermal work required per unit clinker produced based slag chemical characteristics and the fact that slag has already been thermally processed.

Technology Overview

In the process, slag - a byproduct of steel production, is added to the cement clinker manufacturing process. This slag comes from three primary sources: 1) Electric Arc Furnace slag ~ referred to as the “mini-mill” process which uses scrap as the raw material; 2) Basic Oxygen Furnace slag - from integrated operations that use pig iron produced from ore; and 3 ) Blast Furnace slag - from integrated operations smelting the raw ore material.

Due to the chemical composition of the slag and the energy-intensive nature of the steel production process, the material requires little or no additional fuel to convert it into cement clinker. In other words, the great majority of the thermal work has already been done in the steel mill and significant amounts of energy can be saved when the process is added to a cement clinker line.

The result is improved fuel efficiency - lower total fuel consumption per ton of clinker. This translates into a net reduction in of carbon dioxide (CO*) and oxides of nitrogen @OX) emissions per ton of clinker produced. In fact, process testing has shown a direct and statistically significant correlation between the technology and lowered NOx emissions, and the US.

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Environmental Protection Agency has recognized this process as alternative control technology for NOx in cement kilns (US. EPA, 2000).

Process Description

During the steel manufacturing process, lime - calcium oxide - (CaO) is added to the molten steel as a flux to remove impurities. This process, which takes place at around 3,000 degrees Fahrenheit, results in the lime combining chemically with silica, magnesium, aluminum and other oxides present in the steel. These oxides, which are considered to be undesirable in the steelmaking process, produce compounds that are also found in cement clinker - namely calcium silicates, aluminates and aluminoferrites. The slag material floats to the top of the molten steel and is ultimately poured off as the byproduct known as slag. This process, which occurs at temperatures similar to those in a cement kiln, converts the CaO to silicates that are similar to those in cement clinker. In addition, the slag incorporates mineralizers that promote clinker formation.

During the process, slag is crushed and screened to approximately 318 inch diameter, and added as a component of the raw material mix into the back or feed end of the kiln. Because of the amount of heat the slag has been subjected to during the steel making process, it has a relatively low melting temperature and does not require significant pre-processing before being placed into the kiln.

This technology generally has the potential to increase clinker production by as much as 15%, but the specific amount at a particular clinker producing line is controlled by mix design that takes into account such factors as the chemical composition of the slag, the type or types of clinker being produced, as well as the traditional raw material inputs which are used at a specific plant. Feed chemistry is adjusted to compensate for the iron, aluminum, silica and calcium in the steel slag that is being added to the kiln.

During the kiln pyro process, the slag mixes with the raw materials and a number of chemical reactions take place. Because the melting point of slag is typically between 1,260 and 1,315 degrees Celsius, it begins to react with the other raw feed components before the clinkering temperature zone. In the liquid phase, the slag becomes extremely reactive and will seek other material with which to combine, i.e., the raw feed being supplied to the kiln. As the process moves through the kiln, mineralizers present in the slag, such as manganese, help to catalyze clinker formation reactions. In addition, dicalcium silicate, which is also present in varying amounts in slag, is converted into tricalcium silicate by combining with the calcined limestone (CaO) from the raw feed. This exothermic reaction results in slag being an energy efficient and highly effective addition to the cement manufacturing process.

Discussion of NOx and CO1 Reduetion

CO2 Emissions CO2 is a significant emission product of the cement manufacturing process. COz, while not currently regulated in the US, may be regulated as a pollutant in the future. Therefore, reduction of CO2 emissions by economical means is of great interest to the cement industry. Calcination is a necessary step in the production of Portland cement from limestone, and CO2 is the by-product of calcining limestone to CaO. It may be assumed that all of the calcium carbonate, CaCO3, in the limestone calcines, releasing the CO2 by-product. Hence, for every 100

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pounds of limestone added to the kiln, up to 44 pounds of CO2 will be produced. This CO2 is a by-product of the cement-making process that is released to the atmosphere. Therefore, the reduction of CO2 emissions from calcination is not possible without a reduction of Portland cement production.

CO2 emissions are also a direct result of carbon-based fuel use. Therefore, for a given fuel and limestone feed rate, changes in CO2 emissions are a direct indicator of changes in fuel consumption. CO2 emissions therefore can be reduced through any method that improves the overall thermal efficiency of the cement making process. Fuel substitution with fuels having lower carbon content per BTU than coal can reduce emissions, but this is economically difficult unless there is access to a low cost substitute fuel.

Methods that can improve thermal efficiency of cement production have the double benefit of reducing CO2 emissions and reducing the cost of manufacturing cement. Therefore, these methods of reducing CO2 emissions are considered very attractive.

The characteristics of steel slag make it a much more thermally efficient material for making clinker as well as a more efficient source of calcium. Calcination is not necessary for steel slag, which results in lower CO2 emissions than if the calcium was provided from limestone. Based on the slag used in testing at the TXI cement plant in Midlothian, Texas, 172 pounds of slag provides about the same amount of calcium as 200 pounds or more of limestone. And, 200 pounds of limestone will contribute up to 88 pounds of CO2 emissions to the atmosphere during calcination. Therefore, for each ton of slag used in place of an equivalent amount of limestone (in terms of calcium provided for the clinker) 0.512 tons less CO2 will be emitted (Andover Technologies, 1999). Moreover the thermal transformation of steel slag to clinker requires less energy than for the processing of limestone and sand to clinker. This mitigates the amount of fuel generated COz.

NOx Emissions NOx is an air pollutant that contributes to acid rain and to ground level ozone (smog). NOx includes nitric oxide (NO) and nitrogen dioxide (N02).

NOx is formed in a combustion system from oxidation of the nitrogen in the combustion air (thermal NOx) or oxidation of nitrogen that is in the fuel and subsequent oxidation of the HCN intermediary (fuel NOx). At the high temperatures of the kiln, thermal NOx is formed in large quantities. Since coal (which contains significant fuel-bound nitrogen) is normally the primary fuel, substantial fuel NOx may be fomied as well. But thermal NOx is normally the dominant NOx formation pathway in the kiln. In the pyro-processing’s precalciner section, fuel NOx is more important.

Thermal NOx formation is promoted by high oxygen content, high temperature, and long residence time at temperature. It is important to note that thermal NOX will be generated regardless of how heat is added. Heating air with electric heaters to a sufficiently high temperature will produce thermal NOx. At high temperatures, where reactions occur very quickly, the NO concentration will tend rapidly toward the thermodynamic limit. If this heated gas is then cooled, the chemical reactions that would reduce NOx slow down so that the NOx emissions are effectively “frozen” at the higher levels produced at the higher temperatures. This is what happens in a kiln. As the gases quickly cool, the NOx stays about the same, although equilibrium NOx drops. This is because the reactions that would reduce the thermal NOx towards equilibrium levels are too slow at the lower temperatures. By contrast, if oxygen were

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reduced while maintaining a very high temperature, the NOx level would rapidly reduce toward the equilibrium level.

Methods to reduce NOx formation in other combustion systems, such as boilers, have been directed toward reducing oxygen, temperature and residence time. Hence, to minimize NOX the kiln should be operated with the lowest oxygen level consistent with making a cement product of good quality. However, these methods cannot be applied so easily to cement plants because the chemistry and thermodynamics of the cement making process are so sensitive to the oxygen concentration, temperature and time at temperature.

For this technology, the temperature effect on thermal NOx is the most important. If oxygen is kept constant, formation of thermal NO increases exponentially with temperature. Therefore, a small change in the combustion intensity of the kiln can result in a substantial reduction in the formation of NOx. For most kilns it is not possible to reduce the oxygen much beyond current levels because local reducing zones would be established in the kiln that would reduce product quality and potentially increase emissions of unburned fuel.

Because the process is capable of reducing the required heat intensity in the high temperature end of the kiln, a reduction in primary flame temperature can be achieved. A reduction in flame temperature will reduce the amount of NOx that is formed. Because of the exponential effect of temperature on the NOx formation rate, the impact of reduced combustion intensity per unit of material can be pronounced.

Advantages of utilizing the Process at RMC Pacific Materials

Some of the advantages of utilizing the process at the RMC Pacific Materials Davenport Facility:

1. Steel slag has already been subjected to the high temperatures of the steel furnace. The fuel needed to convert it into clinker is low because most of the required chemical reactions have already taken place. Increases in fuel cost when utilizing slag is minimal, lowering the fuel used per ton of clinker produced.

2. The emissions per ton of clinker produced are reduced from slag usage because it has already been heated to 3000 degrees Fahrenheit.

3. Since the chemical composition of the roller mill product will be altered to compensate for the steel slag addition, an expense from the reduction or elimination of other purchased raw materials is expected.

4. Steel siag is delivered to the Davenport Plant via railcars. The plant was utilizing truck delivery for much of the necessary iron containing materials. The introduction of steel slag has reduced a portion of plant’s and surrounding area’s truck traffic.

5. Production increase is expected to contribute an additional 13% clinker with no significant impact on current emissions or fuel rates. Current rates of approximately 7% utilization are demonstrated.

Results at RMC Pacific Material’s Davenport Plant

RMC Pacific Materials operates a cement manufacturing facility in Davenport California, approximately 10 miles north of the city of Santa Cruz. The plant’s four stage preheater / precalciner arrangement includes a 13’ x 185’ kiln and 3 reciprocating grate cooler. The area is environmentally sensitive with respect to all land, air and water issues. The plant currently has

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one of the most stringent NOxemission limits in the country, if not the world The mass flow NOx limit at the exhaust stack is 250 1 b . h averaged over a rolling 24 hour period.

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The Davenport Cement Plant obtained a permit from Monterey Bay Unified Air Pollution Control District to perform a pilot test of steel slag usage. The pilot test was concluded on June 1,2001. The major observations made during the testing phase are specifically listed below:

* As was demonstrated during the pilot test phase, it should be noted that there is a direct relationship between the kiln feed chemistry and the kiln’s operation. Since the kiln feed chemistry is modified to lower iron percentages based on slag iron content, the pyro- processing system cannot be quickly “switched from slag usage and non-slag usage.

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The pilot test on the effect of slag usage has not shown an increase in NOx emissions. The slag actually helped to stabilize the kiln feed consistency while lowering the heat requirement of the clinker process per unit produced. The pilot test has shown that steel slag allows the plant to continue efficient operations with respect to emission, quality and production goals. Continued steel slag at the plant will lower CO2 missions per unit produced, and help lower truck traffic levels associated with off-site raw materials. This, in addition to the ability to consistently meet its production limit, will allow RMC Pacific Materials operational flexibility in conserving process fuel.

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Summary of Operations for 2001 at Davenport Plant

A revision to the plant's operating permits in August 2001 allowed continuous utilization of iron slag. This marked the start of continuous operations with slag for the Davenport Plant, with optimization and maximization of slag being the primary goals. The rate of slag addition would make up approximately 5 percent of clinker production. For the year 2001, slag addition yielded a contribution of 24,000 tons clinker produced, allowing the plant to reach it's second highest annual clinker production of 855,135 tons.

Slag injection offers additional benefits to process operation and stability, which were better understood during 2001. Theoretically, the chemical makeup of the material allows for adjustments in clinker C3S and C3A without having to alter kiln feed chemistry, and this was confirmed through operations during the remainder of 2001 Also, the material's chemistry is perfectly suited to ease chemical changes between the plant's two clinker products, Types I and V. The slag injection also has proven to increase process stabilization as opposed to non-slag kiln operation, and operational observations during this period continued to confirm this relationship. Overall, this stability in combination with the increased production rate maintains the plant permitted gas mass flow emissions rates (NOx and SOX) and ultimately lowered these emissions per clinker ton produced during 2001.

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Challenges in 2002 at Davenaort Plant

Although the year 2001 presented the plant with many tools in conjunction with slag utilization, it also brought a few challenges. Top most of these are the injection point of the slag into the system. As originally designed, the slag was injected next to and above the kiln feed flow onto and into the feed shelf and kiln, respectively. This higher position allowed the slag to be swept up into the kiln exit gases, where it impacted material buildup in the preheater riser and alkali bypass sections. From this increased buildup, process and alkali reduction optimization was hindered.

Slag Injection into Process

To address this challenge, the plant engineered a solution with TXI to inject slag directly into the kiln feed pipe just prior to the flow entering the kiln feed shelf and kiln. This modified positioning has enabled the iron slag to be conveyed into the rotating kiln without allowing material to become entrained into the kiln exit gas stream, thus aiding the process and preventing increased material riser buildup.

In addition to the feeding of the material, storage and conveyance into the pyro-prqcessing system raised some challenges. The iron slag is stored and conveyed into the system using a portion of the existing coal storage and reclaim system, with additions of newly added conveyors and surge bin. The four (4) material feeders are located under three (3) sided covered storage. This area was once utilized exclusively for coal, and must now be shared between both coal and slag. The close proximity that the two materials have to each other has presented some operational challenges to plant personnel. Specifically, the common unloading and conveying systems have made the plant susceptible to material cross-contamination. The worst of this material contamination has caused increased wear and premature failure of the coal mill system's components. Plant personnel have planned to incorporate material separation and conveyor upgrade projects, in addition with best practices, to eliminate contamination susceptibility, control increased coal mill wear and alleviate other material handling issues.

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One of the added benefits that slag utilization hoped to accomplish was a lowering of fuel (coal) per ton of clinker. The plant has not realized the expected fuel savings but also has not seen a measurable fuel increase. The repositioning of the slag introduction point should help lower fuel usage per ton clinker based on lower dust loss. A reduction in the amount of CO2 per ton clinker produced will only be fully realized with the lowering of fuel usage.

The slag used at Davenport contains a low C3S, which necessitates a higher potential C3S chemical makeup for the raw kiln feed. Thus, a higher limestone usage (CaC03 content) per ton of kiln feed will increase CO2 emissions during the calcination process, may negate a lower CO2 emission per ton clinker produced.

Results of 2003

The iron slag /kiln feed chute modifications made in 2002 met the objectives of limiting slag entrainment in the kiln exit gas stream. As the kiln feed stream carried the slag into the kiln, ring buildup associated with slag was greatly reduced. The slag introduction into the kiln feed pipe did cause excessive wear in the feed pipe lining.

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Conclusion

Slag injection into the Davenport Plant pyro-processing line was a low capital cost project that immediately increased production over 5 percent. Since 2002, the plant has maintained a slag utilization rate of approximately 7 percent clinker. Clinker and slag compositions may allow up to 13 percent utilization rates. The goal is to maximize slag addition, continue increased fuel efficiencies per tons clinker produced, and limit the amount of costly raw mix additives.

A major benefit that slag utilization hoped to accomplish was a lowering of fuel (coal) per ton of clinker. Issues including refractory wear in the kiln feed pipe contributed higher than normal waste dust rates, limiting expected fuel consumption savings. With these issues behind, the second half of 2002 and 2003 has seen fuel consumption rate fall from 3.05 to approximately 2.92 MM Btu / short ton.

The RMC Pacific Materials Davenport Plant has very constrictive mass flow emission limits. Any project based on optimization of the process and / or production must fully consider environmental ramifications. The process has thus far met quality and production goals with little or no increase combustion emissions.

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