Removal of carbon constituents from hospital solid waste incinerator fly ash by column flotation

Abstract

Hospital solid waste incinerator (HSWI) fly ash contains a large number of carbon constituents including powder activated carbon and unburned carbon, which are the major source of polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs) in fly ash. Therefore, the removal of carbon constituents could reduce PCDD/Fs in fly ash greatly. In this study, the effects of the main flotation parameters on the removal of carbon constituents were investigated, and the characteristics of the final product were evaluated. The results showed that loss on ignition (LOI) of fly ash increased from 11.1% to 31.6% during conditioning process. By optimizing the flotation parameters at slurry concentration 0.05 kg/l, kerosene dosage 12 kg/t, frother dosage 3 kg/t and air flow rate 0.06 m(3)/h, 92.7% of the carbon constituents were removed from the raw fly ash. Under these conditions, the froth product has LOI of 56.35% and calorific values of 12.5 MJ/kg, LOI in the tailings was below 5%, and the total toxic equivalent (TEQ) of PCDD/Fs decreased from 5.61 ng-TEQ/g in the raw fly ash to 1.47 ng-TEQ/g in the tailings. The results show that column flotation is a potential technology for simultaneous separation of carbon constituents and PCDD/Fs from HSWI fly ash.

Reburning Treatment of the Froths Obtained after the Flotation of Incinerator Fly Ash

“…2(b)] with simultaneous weight loss over a wide temperature range (300°C-600°C); the total weight loss was approximately 61.5%. These results are consistent with the high LOI findings for the froth products (Liu et al, 2013). Because the carbon content of the froths is higher than that of the fly ash, the heat release peak was relatively high.…”

“…The hydrophobicity and lipophilicity nature of PCDD/Fs and other organic contaminants such as PAHs, CBs, CPs and PCBs makes them easy to float into the froths after flotation (Cheruiyot et al, 2015;Lee et al, 2016). The previous report indicated that the flotation process could be successfully applied to substantially decrease the amount of carbon constituents and PCDD/Fs in HSWI fly ash (Liu et al, 2013). In that study, the removal efficiency was more than 96% for PCDD/Fs and 92% for carbon constituents.…”

“…By contrast, SiO 2 and CaO emerged as major components in the froth products, which had an LOI of 56.1%. The calorific value of the froths was 12.3 MJ kg -1 and exceeded a low heat value of 10.5 MJ kg -1 , which is typical of hospital solid waste (Liu et al, 2013). Thus, reburning treatment of the froths in the combustion chamber of incinerators may achieve energy recovery of the carbon constituents.…”

“…Fly ash was treated with column flotation, and the experimental flotation apparatus and operating conditions were the same as those in our previous study (Liu et al, 2013). After flotation, the froth products were carefully vacuum-filtered, dried.…”

“…After flotation treatment, the chlorine content in the froths is obviously lower than that in the fly ash because most of the inorganic compounds, such as sodium chloride (NaCl), in the fly ash have been dissolved and removed, thus reducing the undesirable effects. Furthermore, the froths have approximately 56% loss on ignition (LOI), which might make them suitable for reburning as an additional fuel for the incinerator (Liu et al, 2013).The altered characteristics of fly ash after flotation might influence the decomposition effect of the PCDD/Fs. However, no study has compared the destruction efficiency of PCDD/Fs between fly ash and froth products.…”