2. Materials and methods
Rice husks were collected from suburb areas of Thrace
(Pazardjik, Bulgaria). It was sun dried and the moisture content
reduced to less than 10 mass%. These raw rice husks were used
without any pretreatments.
2.1. Preparation of rice husks ash
On the basis of the results obtained from the studies carried out
with the laboratory equipment [10], a pilot plant for pyrolysis of
raw rice husks with productivity of 100 kg/h rice husks was build
(Fig. 1).
The fluidized bed technology is selected for the production of
amorphous silica from rice husks. From both economic and
technologic point of view, it is most favorable to carry out the
pyrolysis process in a fluidized-bed reactor [29,30]. By pyrolysis,
destructive and recombination processes occur in the biomass to
produce moisture and volatile components and substantial
changes are observed in the structure, morphology and chemical
activity of the end product [28,31,32].
2.2. SEM analyses
The SEM analyses were performed on a scanning electron
microscope Tesla BS under regime of secondary electrons at
acceleration of 20 kV. The samples were metallized with aluminium
in Edwards vacuum camera. The average thickness of the
coating was measured to be 20 nm.
2.3. X-ray diffraction
The diffraction patterns of the samples studied were recorded
in a symmetrical reflection mode using URD-6 Seifert diffractometer
and a copper target X-ray tube (l = 0.154 nm) operated at
40 kV and 30 mA. Cu Ka radiation was monochromized with a
graphite monochromizer and a Ni filter. The XRD patterns were
recorded in the 2u range 4–608 at 0.18 steps. The samples for the
experiments were powdered and pressed into a sample holder.
Samples with the radius of 2 cm and the thickness of 1 mm thick
were prepared.
Fig.