ONLINE EXCLUSIVE: Recycling Flue Gas Cleaning Residue

June 1, 2004
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Experiments have shown that recycling contaminated flue gas cleaning residue can be a feasible alternative to landfill disposal

Residue from the scrubbing process is milled in this system, and the resulting fine material is transported by a pneumatic conveying system to the raw material preparation area. A pressure vessel ensures that a sufficient amount of material is collected so that it can be transported by compressed air over longer distances.
Most brick are fired in tunnel kilns at temperatures of 1000-1100

Figure 1. Schematic of the process used to recycle the residue in brick production.

Recycling the Residue in Brick Production

The first option that was investigated was the simple recycling of the residue into the brick making process (see Figure 1). The trials were carried out on a small scale with varying amounts of residue (up to 5%) added to the clay. At higher addition levels, some quality problems occurred-including the appearance of white spots on the surface, scumming, blooming and color alteration in red brick-primarily due to the increased sulfate levels in the brick. Although most of these problems did not occur or were minimal at lower addition levels (0.2% and 0.5%), researchers were still careful to control the occurrence of scumming and blooming by grinding the residue particles to a maximum size of 0.5 mm. This also prevented the formation of white spots and lime blowing. The ground residue was mixed thoroughly with the clay, and the formed product was dried and fired in a normal cycle.

To assess the environmental consequences of this recycling option, leaching tests were carried out on the fired brick. From these tests, researchers concluded that the only component of concern is sulfate, which tended to leach at additions above 0.5%.

Based on these small-scale firing and leaching experiments, researchers concluded that using the limestone residue in brick production is feasible, as long as the residue addition doesn't exceed 0.5%. For most plants, the residue addition will not exceed 0.3%, even if all of the residue from the plant is recycled. However, a major uncertainty with this recycling option was whether concentrations of fluoride and sulfur dioxide/sulfate would build up during the firing process in a production operation. Such a buildup could affect emissions, product quality (through increased scumming, color changes, white spots, etc.) and the leaching rates of contaminants from the brick.

Researchers launched a second investigative phase consisting of large-scale trials in a British brick manufacturing plant. The results of the large-scale tests have shown that no concentration buildup appears in the firing process due to the 10-20% of unreacted calcium carbonate (30-50% when no peeling system is used) that is in the residue. This limestone also absorbed any flue gas contaminants that appeared during the firing process.

Figure 2. Schematic of the washing process.

Washing and Reusing the Residue

Researchers also developed a washing/crystallization process that can remove contaminants from the residue. In this process, which is illustrated in Figure 2, calcium sulfate is converted to calcium carbonate by adding caustic soda and carbon dioxide. The remaining sulfate is recovered in the form of sodium sulfate crystals, which are pure enough to be marketed. The chemical reactions are:

CaSO4(s) + CO2 + 2 NaOH í CaCO3(s) + 2 Na+ + SO42- + H2O

2 Na+ + SO42- _ Na2SO4 (s)

The process is capable of removing more than 80% of the sulfates from the residue, as well as most of the chlorides and bromides, which are subsequently purged through a wastewater stream. Although other environmentally relevant components, such as selenium, zinc and fluoride, aren't removed, they stay in a solid phase. The washed residue can be used as an additive in brick manufacturing or as a cement-bound building material.

The washing and crystallization process will be tested on a pilot scale in the next phase of the project. At the same time, brick production trials will also be performed on a larger scale with the washed residue. This will enable a comparison of product quality between the brick prepared with unwashed residue (as described in the first investigation) and washed residue.

Figure 3. The mass balance for immobilizing the washed residue with cement.

Immobilizing the Residue with Cement and/or Sulfur

Researchers next investigated whether flue gas cleaning residue treated with cement, molten sulfur or a combination of these components could be used to produce building materials. From a civil engineering point of view, cement immobilization of the residue produced suitable products with high compressive strength. A schematic of this process is shown in Figure 3. However, such products did not meet the leaching rate demands of the Dutch Building Materials Decree (BMD) due to the sulfate and selenium content of the residue. The use of washed residue in cement immobilization seems promising but has not yet resulted in a product that meets all leaching demands.

Figure 4. The mass balance for granulation and subsequent impregnation with sulfur.
Experiments with sulfur immobilization have shown that the resulting products are unstable and lose their strength due to crack formation, making them unsuitable for use as building materials.

By using a combination of cement and sulfur, artificial gravel with adequate strength can be prepared, as shown in Figure 4. However, once again the leaching rate exceeds the limits of the BMD. Although using washed residue significantly reduces the leaching rate, it is still not sufficient to meet the BMD's limits. As a result, using the residue in artificial gravel does not appear to be an environmentally sound option.

This equipment was used for the trials in the British brick manufacturing plant.

A Viable Option

These experiments have shown that immobilizing untreated (unwashed) residue with cement or sulfur to produce building materials or artificial gravel is not a viable option due to leaching problems and product quality issues. While cement immobilization of washed residue seems promising, more research is needed to further optimize the process.

However, unwashed flue gas cleaning residue in small quantities can be used in brick production without any adverse effects on product quality or the environment. While a washing process does not seem economically feasible at this point, it is possible that further developments in this area could result in even higher product quality with higher allowable additions of residue to the clay.

For more information about environmental controls, contact Hellmich GmbG & Co. KG, Holtkampweg 13, 32278 Kirchlengern, Germany; (49) 5223-75770; fax (49) 5223-757730; e-mail info@hellmich.com ; or visit http://www.hellmich.com . TNO's Department of Chemical Engineering can be reached at P.O. Box 342, 7300 AH Apeldoorn, The Netherlands; (31) 55-5493919; fax (31) 55-5493201; or http://www.mep.tno.nl .

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