Rice Husk Ash to Stabilize Heavy Metals Contained in Municipal Solid Waste Incineration Fly Ash: First Results by Applying New Pre-treatment Technology

Abstract

A new technology was recently developed for municipal solid waste incineration (MSWI) fly ash stabilization, based on the employment of all waste and byproduct materials. In particular, the proposed method is based on the use of amorphous silica contained in rice husk ash (RHA), an agricultural byproduct material (COSMOS-RICE project). The obtained final inert can be applied in several applications to produce "green composites". In this work, for the first time, a process for pre-treatment of rice husk, before its use in the stabilization of heavy metals, based on the employment of Instant Pressure Drop technology (DIC) was tested. The aim of this work is to verify the influence of the pre-treatment on the efficiency on heavy metals stabilization in the COSMOS-RICE technology. DIC technique is based on a thermomechanical effect induced by an abrupt transition from high steam pressure to a vacuum, to produce changes in the material. Two different DIC pre-treatments were selected and thermal annealing at different temperatures were performed on rice husk. The resulting RHAs were employed to obtain COSMOS-RICE samples, and the stabilization procedure was tested on the MSWI fly ash. In the frame of this work, some thermal treatments were also realized in O2-limiting conditions, to test the effect of charcoal obtained from RHA on the stabilization procedure. The results of this work show that the application of DIC technology into existing treatment cycles of some waste materials should be investigated in more details to offer the possibility to stabilize and reuse waste.

Keywords: COSMOS-RICE; DIC; fly ash; heavy metals stabilization; rice husk ash.

Figure 1

Picture of the employed experimental DIC instrument.

Figure 2

X-ray diffraction analysis of TQ rice husk ashes treated at different temperatures (see Table 1).

Figure 3

Concentration of Zn (a) and Pb (b) in the leaching solutions of the COSMOS-RICE samples obtained by using the whole RHA samples (see Table 1).

Figure 4

(a) SEM images of internal and external surfaces of untreated rice husk (TQ) and pre-treated by T1 and T2 DIC treatments. (b) SEM images of internal surfaces of corresponding RHA shown in Figure 4a.

Figure 5

XRD patterns collected on COSMOS-RICE samples, obtained by using TQ RHA (see Table 1). ✻: Calcite (CaCO₃); Σ: Halite (NaCl); Δ: Gypsum [CaSO₄·2(H₂O)]; θ: Thaumasite [Ca₃(SO₄)[Si(OH)₆](CO₃)·12(H₂O)]; ▪: Quartz (SiO₂); ᴥ Potassium chloride (KCl).

Figure 6

Pictures of several “green composites” realized by using the material produced with the COSMOS-RICE technology as a filler. It was inserted in polyethylene (30% in weight) and polyethylene foils (5% in weight). In addition, it was also employed to obtain tiles, with different colors (from 30% to 50% in weight).