Distribution of Arsenic and Mercury in Lime Spray Dryer Ash

Abstract

The partitioning of As and Hg in various components of lime spray dryer (LSD) ash samples from a coalfired boiler was characterized to better understand the form and fate of these elements in flue gas desulfurization byproducts. LSD ash samples, collected from the McCracken Power Plant on the Ohio State University campus, were separated by a 140-mesh (106 µm) sieve into two fractions: a fly-ash-/unburned-carbon-enriched fraction (>106 µm) and a calcium-enriched fraction (<106 µm). Unburned carbon and fly ash in the material >106 µm were subsequently separated by density using a lithium heteropolytungstate solution. The concentrations of As and Hg were significant in all fractions. The level of As was consistently greater in the calcium-enriched fraction, while Hg was evenly distributed in all components of LSD ash. Specific surface area was an important factor controlling the distribution of Hg in the different components of LSD ash, but not for As. Comparing the LSD ash data to samples collected from the economizer suggests that As was effectively captured by fly ash at 600 °C, while Hg was not. Leaching tests demonstrated that As and Hg were more stable in the calciumenriched fraction than in the fly-ash-or carbon-enriched fractions, potentially because of the greater pH of the leachate and subsequently greater stability of small amounts of calcium solids containing trace elements in these fractions.

Arsenic Chemistry

“…Potentially effective sorbents for flue gas arsenic include hydrated lime (Ca(OH) 2 , portlandite), lime (CaO), calcium carbonate, limestone, fly ash, and sometimes activated carbon ((Jadhav and Fan, 2001;López-Antón et al, 2007;Helsen and Van den Bulck, 2004), 287, 289;(Taerakul et al, 2006;Gupta et al, 2007); Chapters 5 and 7). The injection of hydrated lime is especially effective and probably removes volatile As 4 O 6 through the formation of calcium arsenates at least over a temperature range of 600-1000 • C ( (Mahuli et al, 1997); Chapter 5).…”

Waste Treatment and Remediation Technologies for Arsenic

“…Potentially effective sorbents for arsenic in flue gases include hydrated lime (portlandite), lime, calcium carbonate, limestone, and flyash (Helsen and Van den Bulck, 2004, 287, 289;Gupta et al, 2007;Chapter 5;Jadhav and Fan, 2001;Taerakul et al, 2006). The injection of hydrated lime is especially effective and probably removes volatile As 4 O 6 through the formation of calcium arsenates at least over a temperature range of 600-1000 • C (Mahuli et al, 1997;Chapter 5).…”

“…Calcium in the spray and flyash may also simultaneously remove arsenic. The resulting arsenic-bearing flyash and calcium sulfites and sulfates are then collected in baghouses or with electrostatic precipitators (ESPs) (Taerakul et al, 2006). Although ESPs are widely used at coal combustion facilities, their average capture efficiency for gaseous arsenic is relatively low (96.1 % in a study with 19 American facilities) when compared with particulate arsenic (99.2 %; (Helble, 2000, 129).…”

Arsenic in Human History and Modern Societies

“…The process produces a sludge that is rich in calcium sulfites and/or calcium sulfates (Taerakul et al, 2006). In the process of removing sulfur from flue gases, hydrated lime or other injected absorbents (such as, calcite or lime) frequently capture arsenic through the formation of calcium arsenates (Yudovich and Ketris, 2005;Mahuli et al, 1997), 186; (Taerakul et al, 2006;Sterling and Helble, 2003;Li et al, 2007); Chapter 7). Provided that arsenic and other potentially toxic trace elements are immobile, FGD sludges have applications in soil amending and commercial products, such as wallboard.…”

“…In low-calcium flyashes, arsenic is typically associated with iron oxides and iron-rich silicate glasses (Goodarzi, 2006;Mukhopadhyay, Lajeunesse and Crandlemire, 1996;Taerakul et al, 2006;Sterling and Helble, 2003;Mukhopadhyay et al, 1999;Seames and Wendt, 2000). Calcium in flyashes may react with arsenic in flue gas to form calcium arsenates on the surfaces of flyash particles (Goodarzi, 2006), 1423.…”