Production
of Amorphous Silica from Geothermal Sludge of Dieng Indonesia.
Srie Muljani*,.Heru Setyawan** and Ketut Sumada
Department of Chemical Engineering,
Faculty of Industrial Technology
UPN
Veteran Jawa Timur, Surabaya
60111
**Department of Chemical Engineering,
Faculty of Industrial Technology
Tehnology
Institute of Surabaya
*Corresponding Author’s E-mail: eboy_tk@yahoo.com
Abstract
A
method to produce amorphous silica from Dieng geothermal sludge was proposed. It
consists of leaching followed by drying. The X-Ray Fluorescence analysis showed
that the silica concentration increased from 39.55 wt% to 90.3 wt% by HCl
treatment and increased further up to 98.8 wt% by drying at
temperature 7500C for 30 minute. For H2O treatment the
silica concentration increased from 39.55 wt% to 86.6 wt% and increased further
up to 97.6 wt% by drying at temperature 7500C for 30 minute. This
result indicated that the drying process can reduce the metal concentration and
confirmed that the silica was in the range of standard purity of precipitated
amorphous silica (>95%SiO2). The XRD analysis also confirmed that
the silica resulted in this experiment was amorphous.
Keywords: geothermal
sludge; metals removal; drying temperature, amorphous silica
1.
Introduction
Geothermal energy is a renewable
energy resource. However, large scale production of energy using geothermal
sources produces considerable amounts of waste in the form geothermal brine and
geothermal sludge. The geothermal sludge contains solids which precipitate out
in the waste water treatment plant during the power generation process and the
sludge can be highly concentrated in a variety of metal salts, many of them
were heavy metals such as iron, titanium, manganese, zinc, arsenic, boron,
cadmium, lead, nickel, and copper. The sludge also contains a large proportion
of silica (Gallup at al, 2003).
Precipitated silica can act as a seed
for silica crystallization and scale inhibition. In contrast, there is no large
quantity of natural precipitates that remove silica that may potentially
deposit as scale from fluids with low salinity, so that some chemicals have to
be added to these fluids to remove the excess silica (Kato
at al, 2003). Many types of chemical addition methods have been examined so far
(Jost et al., 1983; Heinen,
et al., 1987; Nakazawa et al., 1994; Vitolo
and Cialdella, 1995; Sugita et al., 1998; Ueda et al., 2000; Bourcier et al, 2009). Quicklime is a very useful reagent
to remove excess silica in geothermal brines (Kato et al, 2000, 2002; Gallup
at al, 2003; Pascua at al, 2007). The product from these methods is a
calcium-silica sludge that containing heavy metals. Although the cleaned brine
can be resulted, the sludge would require toxic waste disposal. Precipitated
silica (geothermal sludge) from Dieng geothermal brine was collected from
natural precipitation and discarded as a landfill.
Some methods
have been developed to produce amorphous silica and silicate from precipitated
silica. The silica product was dried at a temperature from about 250C
to about 3000C without burning process. It was done to prevent the
transformation of metal to metal oxide. The morphology of the product can also
be varied through the drying temperature of the product in this range. (e.g. Bagshaw et al, 2001; Premurzic et al, 2003; Hirowatari et al, 2004; Johnston, et al, 2004; Nishi, et
al, 2007; Southam, et al, 2008).
2. Experimental
Geothermal solids waste collected
from a landfill in the Dieng field, whose initial silica concentrations was 39.55
wt% (table 1). The first step in this experiment was to ground the geothermal
solids waste into fine powders with particle size distribution of 50-70 mesh. A
small example of the fine powder was analyzed for their elements by X-ray
fluorescence, X-ray diffraction and
SEM-EDAX. X-ray diffraction patterns were obtained on a diffractometer by using
Cu Kαradiation (l=
0.154 nm, 40kV per 30
mA) and a sweeping velocity of 2 degree min-1 (units of 2θ). The amount of components
in the silica was determined by X-ray fluorescence analysis. Scanning electron microscopy (SEM) and X-ray
emission analyses (EDAX), samples were fixed onto a double faced tape
adhered to an aluminum support and coated with a layer of gold (ca. 15
nm) by a Sputter Coater apparatus. The scanning electron microscopy analysis
(SEM) was carried out using low vacuum microscopy operating at an accelerating
voltage of 25 kV. Images were obtained by using secondary electrons. X-ray
emission spectroscopy (EDAX) was used for elemental mapping with a Noram
Voyager instrument. The micrographs and the elemental maps were obtained for
all contents of silica.
The
results are shown in Figure 1 that corresponds to Table 1
Table 1. SEM analysis for element
|
In the laboratory, 100 gram waste
powder was leached in different volume (500 ml,1000 ml and 1500 ml) and
different solvents ( H2O and HCl 0.6% ) with stirring in the jar test at 100 rpm for
0.5h. The experiment divided into two categories of sludge after reacted with
solvent for 0.5h, as follow: 1) the mixture was filtered and all of sludge
(code : FsI) from filtration process was dried 2) the mixture divided into two
layers as bottom layer and upper layer. The bottom layer was discarded and the
upper layer (code: FsII) were filtered through Whatman paper (2.5μ) and dried. For
the acid treatment the filtrate was discarded and the solids were washed with
distilled water (DW) until the filtrate wash water pH decreased to below 8.5
The sludge after neutralization were dried
in an oven at temperature 2000C for 15h and then dried in the
furnace from temperature 5000C for 1.5h to 8500C for 15
minutes. The silica product was analyzed by X-ray fluorescence (XRF) for
elemental analysis; XRD for amorphicity and crystalline analysis; SEM-EDAX for
element and photomicrograph analysis.
3. Results and discussion
3.1. Effect of volume solvent and
sludge feedstock
Table 3 that corresponds to Figure 2
shows the effect of solvent volume on concentration of silica. It can be seen that the silica concentration
produced from FsII was higher than that FsI. For HCl treatment on FSII the concentration of
silica increases from 39.55 % to 90.3 % and for H2O treatment the concentration
of silica increases from 39.55 % to 86.6%.
Table 3. XRF analyze after
silica sludge dried at 2000C.
|
Figure.2 Effect of volume solvent on concentration of silic
Table 4 that corresponds to
Figure 3 shows the concentration of silica and Fe after dried at a temperature
between 5000C-8500C. The silica concentration increases
with the increase of temperature and the other hand Fe concentration decreases.
The calcinations process tends to increase silica concentration after HCl treatment from 90.3% to 98.8 % at
7500C for 30 minute and after H2O treatment increase from 86.6% to 97.6% at the
same temperature. These results
indicate that more materials in silica sludge can be removed by leaching process
but not sufficient to achieve the standard grade. The result suggested that the
drying at high temperature
as one of alternative method to increase the silica concentration after
leached.
The effects of solvent volume were
also investigated and it was found that the purity of the silica increased as
the volume increased. The results
for the acid treatments were not different significantly between 1000 ml
and 1500 ml solvent volume. In contrast, the results of treatment with water
there was different sufficiently
for materials removal between 1000ml and 1500ml.
Figure.3 Effect
of calcinations temperature on concentration
of silica
Table 4. XRF analyze silica
resulting from calcinations at 5000C-8500C.
Solvent
|
Volume
(ml)
|
SiO2
(wt%)
|
||||
200
0C
|
500
0C
|
750
0C
|
850
0C
|
|||
500
|
71.7
|
78.6
|
84.7
|
86.5
|
||
H2O
|
1000
|
77.5
|
83.5
|
89.7
|
91.2
|
|
1500
|
86.6
|
90.8
|
97.6
|
97.2
|
||
500
|
87.4
|
90.9
|
97.7
|
98.5
|
||
HCl
0,6%
|
1000
|
89.9
|
92.9
|
98.3
|
98.5
|
|
1500
|
90.3
|
95.6
|
98.8
|
98.8
|
||
The
silica products produced in the laboratory via two different feedstock and
different solvent differ in physical properties. The morphology of the product
can also be varied through the drying temperature in the range about 2000C
-8500C for over time periods of up to 15 min to18 h. SEM
photomicrographs of the two different solvent from the second feedstock as
shown in Figure 4. Figur4a shows silica from geothermal sludge before
treatment. Figure 4b shows silica resulting from water solvent treatment.
Figure 4c shows silica resulting from acid solvent treatment. Figure 4d shows
silica resulting from acid treatment followed by calcination at 7000C.
The silica from geothermal sludge after direct contacting with acid depigmented
agent by Premuzic et al (2003) is more agglomerate than the product shown in
Figure 4d.
a b
c d
Figure 4.SEM photomicrograph of the two different
solvent (a) silica before treatment (b)
silica resulting from H2O treatment (c) silica resulting from HCl treatment (d)
silica resulting from HCl treatment + calcination at 7500C
3.2. Effect of temperature on removal metals and amorphicity
For identifying the effect of drying
temperature to removal metals from silica, silica samples were dried at
different temperatures ranging between 5000C and 8500C.
XRF identification was
conducted for several temperatures (Table 5) as shown in Figure
5 and Figure 6,
The
significant reduction for Ti, Cu, Fe, As, Eu and Pb was observed. Figure 6
explained that burning temperature acts to reduce Fe for all feedstock and
kinds of solvent and it can be known that second feed stock treatment was
better than the first feedstock. XRF analysis showed that the drying at the
temperature range from 2000C to 8500C
did affect the metals reduction. The higher metals reduction achieved when the
silica dried at a temperature 7500C. This was likely attributed to
the fact that a drying temperature of 7500C or higher is required
for reduction of heavy metals. However, a burning temperature higher than 10000C
was not recommended in order to prevent silica being melted.
Figure 5. Effect of burning temperature on Figure 6. Effect of drying temperature on Fe
|
|
The XRD pattern (Figure 7) shown the silica before treatment (Figure 7a) compared
with the silica product from FSII (Figure 7b) and the silica product from FSI
after treatment and drying at 7500C (Figure 7c). The result showed
that HCl treatment that followed by drying on geothermal silica
Figure 7. XRD-patern for (a) silica
before treatment (b) silica products from FSII (c) silica product on drying
temperature 7500C
The effect of drying temperature
on the color of silica products shows in Table 7. The pink colors of product
were indicated that more materials remained in silica product. The iron (Fe)
was the higher component in geothermal solid waste as shown in Table 2. This
iron as pigmentation in silica generally due to different chemical compounds of
iron such as ferric/ferro oxides (black), hydrous ferric oxides (brown), alpha
Fe2O3 (red brown), Fe(III) Chloride-8H2O
(colorless), and Fe(III) Chloride-6H2O (yellow) (Premuzic et al,
2003). It can be explained that the leaching treatment was important process to
removal the pigment compounds in feedstock.
Table 7. The colors
of silica product with water solvent
Temperature
0C
|
First feedstock (FsI)
|
|
Silica
product
concentration
|
Color of product
|
|
200
|
61.92
|
white
|
500
|
72.23
|
soft pink
|
750
|
81.55
|
dark pink
|
Second feedstock (FsII)
|
||
200
|
86,60
|
white
|
500
|
92.06
|
white
|
750
|
98.54
|
white
|
3.3 Pore diameter
and particle size
BET
analysis shows the pore area and specific surface areas increased up to the
temperature range 200–750°C which caused a reduction of specific surface area
and total pore volume as well as a shift of pore size distributions to larger
size
Amorphous to crystalline phase transformation
temperature in different types of silica source was dependent on the amount of
impurity and residual hydroxyl group content (Nayak et al. 2009).
Table 8. Surface area
amorphous silica
Drying
Temperature
(0C)
|
Surface area
BET (m2/g)
|
Cummulative Pore
Area
(m2/g)
|
200
|
8.20
|
12.05
|
500
|
12.11
|
17.33
|
750
|
13.56
|
17.97
|
850
|
9.31
|
12.86
|
In this experiment the drying temperature
up to 7500C did not affect the amorphicity of silica. The XRD pattern of dried sample FSII (Figure
8) indicates that the silica is totally amorphous and contains very little
silica impurities at temperature 7500C.
 |
Figure 8. XRD pattern of
silica sample FSII
at 7500C
4. Conclusion
The greatest
amount of metal salts or impurities in geothermal sludge was removal by HCl
0.6% (wt/vol) treatment and drying
temperature about 700-7500C. The product is the amorphous silica, above
98.8%wt of purity, white in color. The experiment proofed that the drying temperature up to 7500C
for 15 minute as one of the alternative choices to increase the silica
concentration after leached and to increase the porosity without destroyed
their morphology.
Acknowledgements
The authors would like to thank to Ermawan, Ir, P.T
Geodipa Energi for providing the geothermal sludge at PLTPB Dieng, Wonosobo.
This paper includes a part of the results of the National
Strategies Research from DP2M-DIKTI -2010
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