PCDD/F Formation in Milled Fly Ash: Metal Chloride Catalysis

Abstract

Mechanochemical treatment reduces the particle size of fly ash and disperses catalytic metals, raising the potential reactivity of fly ash to form and destroy 'dioxins', i.e., polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD + PCDF or PCDD/F). To test this issue, model fly ash (MFA) samples were artificially composed by mixing silica, sodium chloride, and activated carbon, and doping this matrix with five selected catalytic metal chlorides administered as CuCl 2 •2H 2 O, CrCl 3 •6H 2 O, FeCl 3 (anhydrous), ZnCl 2 and anhydrous CaCl 2 . Without additives, these samples were first finely milled for 0 (blank), 1 and 8 h, and the effect on the formation of PCDD/F was investigated during de novo tests. These simulate the conditions prevailing in deposits in the heat-recovery zone of an incinerator, where dioxins are formed and destroyed, and the tests were conducted at the same temperature, reaction time and air flow rate. Metal chlorides produce specific and distinct homologue and isomer patterns. The isomer signatures of 2,3,7,8-PCDD/F, precursor route, and other congeners were recognised and are studied in some detail. Principal component analysis (PCA) was applied to the 2,3,7,8-PCDD/F-congeners, indicating CuCl 2 as major contributor of WHO2005-TEQ values. Surprisingly, for CuCl 2 •2H 2 O the total yield of dioxins reduced drastically with milling. Since this study revealed various unexpected results, as well as experimental limitations, some suggestions for further ongoing work are formulated.

Mechanical Treatment of MSWI Fly Ash: A Way Forward to Inhibit PCB Reformation

“…The physical concentration of the sum of dl-PCB for gas and solid phase was 98.9 and 104 ng g -1 in original sample, respectively, and this value dropped with increasing milling time, down to 31.3 ng g -1 (+ 68%) in gas and to 36.7 ng g -1 (+ 65%) in solid phase after 1 h of milling. It demonstrates that 1 h of milling largely reduced MFA potential for de novo formation of toxic PCB, similar results were obtained with 2,3,7,8-PCDD/F and dl-PCB output during de novo tests of MFA in our previous work (Mubeen et al, 2017). However, further milling of MFA for 2 h slightly raised sum dl-PCB concentration to 35.7 ng g -1 (-14.1%) in gas phase and to 37.1 ng g -1 (-1.09%) in solid phase.…”

“…In our previous work on MFA milling, reformation in terms of total output of PCDD/F and dl-PCB, as well as erratic reformation of certain isomers (e.g., penta-CB #123) after 8 h milling without any additive have been observed in gas phase (Mubeen et al, 2017). Similarly, unexpected results were obtained by Cagnetta et al (2016): after 4 h milling of secondary copper smelting fly ash they got 76% destruction of pollutants, but in experiment of 6 h, with CaO:SiO 2 (4:1) as co-milling reagent at 275 rpm speed, PCDD and PCDF reformation was observed.…”

“…Model fly ash (MFA) sample preparation and ball milling procedures have been described in detail in our previous work (Mubeen et al, 2017). Briefly, required materials were grinded together in a mortar for about 10 min in the following weight proportions: activated carbon (AC; 2.5 wt%), sodium chloride (NaCl; 10 wt% Cl), 0.2 wt% copper chloride (CuCl 2 ) was added and silica (SiO 2 ) formed the balance.…”

“…Previous work showed that some undesired results have been obtained via MC treatment (Cagnetta et al, 2016a;Mubeen et al, 2017) due to presence of metal chlorides and oxides. These catalysts can induce PCB and PCDD/F regeneration via de novo synthesis under milling treatment conditions.…”

Suppression of PCDD/Fs by Raw Meal in Cement Kilns

“…The share of PCDFs in the total PCDD/Fs was 79%, thus signifying the dominance of the de novo synthesis over the precursor formation pathways (Conesa et al, 2002). The formation mechanism was consistent with the model fly ash at a single temperature, gas flow rate and reaction time (Mubeen et al, 2017). OCDD accounts for 66% of the total mass of PCDDs, whereas 2,3,4,7,8-PeCDF, 1,2,3,4,6,7,8-HpCDF, and OCDF contributed most to the PCDFs.…”