EFFECT OF CACO3 ADDITION TO ALUMINUM AND SILICON
DISSOLUTION WITHIN SODIUM ALUMINATE SOLUTION
Pengaruh Penambahan CaCO3 terhadap Pelarutan Aluminium dan Silikon
dalam Larutan Natrium Aluminat
Dessy Amalia, Tatang Wahyudi, dan Husaini
Puslitbang Teknologi Mineral dan Batubara
Jalan Jenderal Sudirman No. 623, Bandung
Email : dessy@tekmira.esdm.go.id
Abstract
Reactive silica within lateritic bauxite dissolves in NaOH solution, reacted with sodium aluminate and
precipitated forms desilication product (DSP) such as sodalite. The ability of calcium carbonate (CaCO 3) as
desilication agent has been used in this experiments.The -60 mesh washed bauxite is reacted with 129 g/L
NaOH solution (1.5 stoichiometry) and CaCO3-containing Whitton 800 addition based on the ratio of reactive
silica to CaCO3 namely, 1:1; 1:1.5 and 1:2. Digestion process is performed in autoclave at certain
temperatures (140, 150 and 160C) for 15, 30, 45 and 60 minutes. The experiment result shows that the SiO2
concentration in sodium aluminate solution can be reduced to 0.01 g/L SiO2 by adding whitton 800 1.5 times the
SiO2 content in -60-mesh washed bauxite. Such a result provides less SiO2 content compared to that conducted
by Noworyta. A 7.1-g CaO of Noworyta experiment presents 0.02 g/L SiO 2. The research showed that Whitton
800 has ability as desilicating agent that can be reduce dissolve silica to 99.27%. However, the performance
only got 91.28% of extracted Al. Based on such a fact, it is still required several experiments using various feed
size to get the optimum condition.
Keywords: CaCO3, SiO2 concentration, Al percentage.
Sari
Silika reaktive yang terkandung dalam bauksit lateritik larut dalam larutan NaOH, bereaksi dengan
sodium aluminat dan mengendap membentuk produk desilikasi (DSP) seperti sodalite. Kemampuan kalsium
karbonat (CaCO3) sebagai reagen awasilika digunakan pada percobaan ini. Bauksit tercuci berukuran 60
mesh direaksikan dengan larutan NaOH 129 g/L (1,5 stoikiometri) dan Whitton 800 yang mengandung CaCO3
berdasarkan perbandingan jumlah mol silika reaktif terhadap CaCo3, yaitu 1:1; 1:1,5 dan 1:2. Proses
digestion dilakukan dalam autoclave dengan variasi suhu (140; 150 dan 160 C) selama 15; 30; 45 dan 60
menit. Hasil percobaan menunjukkan bahwa konsentrasi SiO2 dalam larutan natrium aluminat dapat diperkecil
sampai 0,01 g/L SiO2 dengan menambahkan whitton 800 sebanyak 1,5 kali dari kandungan SiO2 dalam bauksit
tercuci. Jumlah siO2 terlarut tersebut lebih sedikit dibanding dengan yang diperoleh oleh Noworyta.
Penambahan 7,1 gram CaO yang Noworyta lakukan menghasilkan SiO2 terlarut sebesar 0,2 g/L. Hasil
percobaan menunjukkan whitton 800 dapat menjadi bahan desilikasidan dapat mengurangi silika terlarut
sampai 99,27% . Namun, persen Al terekstraksi pada suhu reaksi 150 C sebesar 91,28%. Oleh sebab itu, perlu
dilakukan penelitian lanjutan dengan variasi ukuran partikel.
Kata kunci: CaCO3, konsentrasi SiO2, prosentase Al
INTRODUCTION
Commercially, aluminium extaction from bauxite into sodium aluminate is proceed
through digestion in Bayer Process untill now. Several efforts has been done to get higher
extraction percentage of sodium aluminate. Some efforts are dealing with reduction of NaOH
consumption and dissolution of silicate.
Silicate materials in lateritic bauxite occur as kaolinite and halloysite (Bardossy,
1990). The former will dissolve in NaOH solution as digestion process that based on reaction
as follow:
3 Al2Si2O5(OH)4 + 18 NaOH  6 Na2SiO3 + 6 NaAl(OH)4 + 3H2O
Dissolved silica is then precipitated to form desilication product (DSP) such as sodalite
through a following reaction :
6 Na2SiO3 + 6 NaAl(OH)4 + Na2X  Na6[Al6Si6O24].Na2X +12 NaOH + 6H2O
The X represents various anorganic anions such as sulfate, carbonate, chloride,
aluminate and hydroxide (Smith, 2009). The DSP causes scaling to the operation unit usually
to the heat exchanger tube (Jamialahmadi dan Müller-Steinhagen, 1998). Such a material
owns structure similar to zeolite, however, it depends on its dissolution temperature and
dissolved Al and Si concentrations that will result in various Al2O3.SiO2 ratios. Each Al3+
replaces Si4+ in its crystal lattice, where the single ion Na+ is required to neutralize the
charges (Habashi, F, 1997). DSP formation can reduce Na+ that should be reacted with Al3+
within the gibbsite.
To prevent Na+ reaction to Al3+ in DSP formation, addition of additive material as a
binder to reactive silica can be conducted to form quite small solubility compound such as
calcium aluminosilicate. Some additive materials to be used for this purpose include lime
compounds such as calcium oxide (CaO) and calcium hydroxide (Ca(OH)2). Noworyta
(1981) applied 7.1 gram/L CaO and is able to reduce dissolved SiO2 until < 0.02 g/L. Lime
derivatives compound with high desilication capacity are 4CaO·A12O3·CO2·11H2O and
tricalcium hydroaluminate (C3AH6: 3CaO·A12O3·6H2O). The former is made from Ca(OH)2
while the later from CaO. Another compound that can be used for desilication process is
Friedel's salt (3CaO·A12O3·CaCl2·10H2O). It has ion exchange capacity to silicate in
synthetic sodium aluminate and can reduce more than 95% of dissolved silica. The Friedel’s
salt is more effective than CaO due to its hydrated crystal structure. Silica will attached with
FS-forming DSP of calcium alumina-silicate chloride known as chabazite dan wadalite (Ma
et al, 2009).
CaSO4·2H2O is another alternative material for desilication process that is cheap and
abundant. It is proved that the material retains high desilication capacity in synthetic sodium
aluminate. Desilication process is held in 90C within 300g/L NaOH. Initial concentration of
Al and Si are 100 g/L and 6 g/L respectively and the CaSO4·2H2O dosage is 40 g/L. Such
condition can reduce dissolved silica more than 95% (Ma et al., 2011). The DSP form is
identified as alumina-silicate-sulfate and called hauyne and lazurite.
A preliminary desilication research used Whiton 800 – a reagent brand name
containing CaCO3. The research was conducted by Amalia and Aziz (2011). Desilication
occurs along with digestion process in 129 g/L NaOH and 140C for 2 hours. CaCO3 addition
was two times larger than that of stoichiometry amounts of reactive silica content in bauxite.
The produced sodium aluminate solution contains dissolved silica concentration much less
than the solution produced without CaCO3 addition. The dissolved silica decreased until
93.97%. The result shows that CaCO3 is a potential desilication agent.
Indirect CaCO3 consumption as desilication agent has been studied by Amer (2013).
The experiment applies Na2CO3 as reactant and Ca(OH)2 as desilication material. Chemical
reactions that occur in the process are illustrated as follow:
Na2CO3+ Ca(OH)2 → 2NaOH+ CaCO3
Al2O3.3H2O + 2NaOH → 2NaAlO2 + 4H2O
SiO2 + 2NaOH → Na2SiO3 + H2O
Na2SiO3 + CaCO3 → CaSiO3 + Na2CO3
Last reaction shows that CaCO3 reacts with sodium silicate to produce calcium silicate. This
means that CaCO3 is the real desilication agent. Optimum concentration of Na2CO3 for 0.3 M
provides the optimum result.
Sodium silicate produced from kaolinite and NaOH reaction will react with sodium
aluminate in conventional Bayer process to form sodium silicon aluminum hydrate
hydroxysodalite or HS (3[Na2O·Al2O3·2SiO2] ·2NaOH.aH2O) – the a depends on the amount
of water crystal available within material pores. If the reactive CaO is added, the Cacontaining
product
will
develop
into
cancrinite
or
Ca-CAN
[3(Na2O·Al2O3·1.8SiO2)·1.42CaCO3·2.4H2O]
or
hydrogamet
known
as
HG
[(Ca3Al2(SiO4)n(OH)(12-4n)] where n < 0.8. The HG formation can reduce dissolved silica
content in sodium aluminate solution. Transformation of HS or Ca-CAN to HG because the
presence of CO32- ions in solution (Pan et al., 2012). Therefore, CaCO3 can be effective as
desilication agent. Possible reaction in desilication using CaCO3 is as follows:
NaAl(OH)4 + Na2SiO3 + CaCO3
[Na2O·Al2O3·1.8SiO2]·1.42CaCO3·2.4H2O+NaAl(OH)4
Desilication can also be carried out in different process steps. Those are before or
after digestion. Figure 1 is a flow chart for desilication before digestion while Figure 2 refers
to desilication process after digestion. Both steps require additional equipment in alumina
plant that will increase capital and operational costs. The objective of study is investigating
effect of some variables on Al and Si dissolution by simultaneously conducting both
desilication and digestion in terms of reducing the operational costs .
Figure 1. Bayer process for desilication before digestion process (Smith, 2009)
Figure 2. Bayer process for desilikation after digestion process (Ma et al., 2009)
METHOD
The experiment employs West Kalimantan washed bauxite. Preparation and
characterization for the experiment is exactly the same as previous experiments by Amalia et
al. in 2014 (paper in progress). The washed bauxite is prepared through sampling and
comminuting procedures to get -60#-mesh size. The material is then reacted with 129 g/L
NaOH solution (1.5 stoichiometry) and CaCO3-containing Whitton 800 as desilication agent.
The amount of Whitton 800 is based on the ratio of reactive silica to CaCO3 namely, 1:1;
1:1.5 and 1:2. Digestion process is performed in autoclave at certain temperatures (140, 150
and 160C) for 15, 30, 45 and 60 minutes. The process yields sodium aluminate solution and
red mud. The solution is then analyzed through Atomic Absorption Spectrophotometer
(AAS) to identify the concentrations of aluminum and silicon. The results are also compared
to previous digestion process that is conducted without desilication (Amalia et al., 2014).
Figure 3 shows the step in desilication process.
sampling
sieving
milling
crushing
-60#
NaOH
sodium aluminate solution
Bauxite
CaCO
3
Red mud
filtering
digestion
measuring
Figure 3. Desilication conducted with digestion process
RESULTS AND DISCUSSION
The best result of Al extraction perecentage in sodium aluminate from previous
experiments occurs for -100 mesh feed at 160C (Amalia et al., 2014; paper in progress).
Though the concentration of Si2+ has already determined to < 0.6 mg/L (commercial
standard), the dissolution of Al3+ still decreases because of DSP formation during the time.
Nevertheless, the milling process to get the mentioned size requires high energy. As a result
desilication agent should be added to reduce the dissolved concentration of Si2+ in sodium
aluminate as well as applying rougher size (-60 mesh) to reduce energy consumption of
milling.
Desilication occurs along with digestion at certain SiO2 and CaCO3 ratios at 150C as
seen in Figure 4. Generally, CaCO3 addition increases Al concentration in sodium aluminate
solution. The amount of Al extraction is better than that of without any addition of CaCO3.
The best result of Al dissolution (Figure 6) is reached for the ratio of 1:1.5 where Al
extraction increases in 45 minutes followed by the decrease of SiO2 as seen in Figure 5.
Smaller amount of CaCO3 (0.5 times of SiO2 amount) will lessen the dissolved Al after 15
minutes reaction because some CaCO3 have previously reacted to existed dissolved SiO2 to
form calcium silicate (Amer, 2013). Therefore, after 15 minutes reaction, dissolved SiO2
responds to less amount of CaCO3 and consumes the dissolved Al to form DSP. Therefore,
Al concentration decreases. Meanwhile, CaCO3 addition with ratio to SiO2 content in bauxite
more than 1 gives lower Al extraction because the developed calcium silicate has further
reaction with sodium aluminate. Such a reaction results sodium-calcium-aluminum silicate
complex (Amer, 2013).
95
Extracted Al (%)
90
85
80
Without CaCO3
1:0,5
1:1
1:1,5
1:2
75
0
10
20
30
40
50
60
70
t (minute)
Figure 4. Percent of extracted Al3+ using various ratios of SiO2 to CaCO3 compared to
experiments without CaCO3 addition
SiO2 concnetration (mg/L)
800
Without CaCO3
700
1:0.5
600
1:1
500
1:1.5
1:2
400
300
200
100
0
0
10
20
30
40
50
60
70
t (minute)
Figure 5. SiO2 concentration resulted from various ratios of SiO2 to CaCO3 compared to
experiments without CaCO3 addition
The concentration of Al and SiO2 in Figure 4 and 5 show almost same phenomena. Al
extraction raises followed by SiO2 due to the reactive silica performed by kaolinite has a
formula of Al2Si2O5(OH)4. This means that there is a chemical attachment between them.
Therefore, different behavior will reveal in different reaction temperatures. From the Gibbs
energy point of view, SiO2 dissolution is insignificant in higher temperature (Amalia et al.,
2014). Further reaction of sodium aluminate and sodium silicate will give less DSP formation
in higher temperature and raise Al dissolution as seen in Figure 6. However, the addition of
CaCO3 in higher temperature is not as good as the solubility of CaCO3 in the water (Cotoa et
al., 2012). The amount of Ca2+ species from CaCO3 decomposition lessens to attach to SiO32-.
Referring to such a condition (Figure 6 and 7), the optimum temperature for desilication
process using CaCO3 is 150C at which the SiO2 concentration in sodium aluminate is less
than 0.02 g/L, namely 0.01 g/L. This can only be achieved with 0.56 g/L CaCO3. The figure
is less than that of Noworyta (1981), namely by adding 7.1 g/L CaO and yields 0.02 g/L
SiO2.
Extracted Al (%)
100
95
90
85
140
150
160
80
0
10
20
30
40
50
60
70
t (minute)
Figure 6. Percent of extracted Al in different temperatures by adding CaCO3 1.5 times
relates to SiO2 content within bauxite
0.25
Dissolved SiO2 (g/L)
0.20
0.15
140°C
150°C
0.10
160°C
0.05
0.00
0
20
40
60
80
t (minute)
Figure 7. Concentration of SiO2 for different temperatures by adding CaCO3 1.5 times
relates to SiO2 content within bauxite
CONCLUSION
Calcium carbonate has the ability as desilication agent in digestion process. The SiO2
concentration in sodium aluminate solution can be reduced to < 0.02 g/L (99,27% dissolved
SiO2 reduced) showing percentage of extracted Al 91.28% at 150C by adding 0.56 g/L
CaCO3 or 1.5 times the SiO2 content in -60-mesh washed bauxite. The result provides lesser
SiO2 content. It is suggested to use finer particle size to get higher Al extraction with less
dissolve SiO2.
ACKNOWLEDGEMENT
Authors acknowledge the Ministry of Energy and Mineral Resources for the research funding.
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