Page 6 - Viscosity measurement and prediction of gasified and synthesized coal slag melts

Arman et al. / Fuel 200 (2017) 521–528 525
Fig. 4(b) shows the temperature dependences of viscosity in a The alkaline earth oxides (RO), where R = Ca, Mg, tend to break
series of (50-x)RO–xA 2 O 3 –50SiO 2 and (40-x)RO–xA 2 O 3 –60SiO 2 slag the Si-O bonds to de-polymerize the SiO 4 network by forming non-
melts (A = Al or Fe) with various Al 2 O 3 and Fe 2 O 3 contents. The vis- bridging oxygens and contribute to the decrease of viscosity [27].A
cosity increases when Al 2 O 3 is added to binary slag melts named previous study reported that among the alkaline earth oxides (RO),
CA00.50 (50CaO–50SiO 2 ) and CA00.60 (40CaO–60SiO 2 ) at constant BaO most contributes to the reduce of viscosity, followed by Sr, Ca
SiO 2 contents of 50 and 60 mol%. On the contrary, it decreases with and Mg oxides [22] from the point of cation-oxide ion interaction
[29]. However, the other researchers have studied that the viscosi-
FeO and Fe 2 O 3 additions. In the series of (40-x)RO–xA 2 O 3 –60SiO 2
slag melts, the viscosity decreases as FeO and Fe 2 O 3 contents ties of CaO–Al 2 O 3 –SiO 2 melts are larger than MgO–Al 2 O 3 –SiO 2
increase in the range of 0 and 14.1 mol%. The viscosity of melts at fixed 50, 67, and 75 mol% SiO 2 contents [30]. Our result
CA10.60 (30CaO–10Al 2 O 3 –60SiO 2 ) is higher than that of MA10.60 shown in Fig. 4(b) is in good agreement that the viscosity of
(30MgO–10Al 2 O 3 –60SiO 2 ) slag melt due to the substitution of CA10.60 is larger than that of MA10.60 at a fixed SiO 2 content of
CaO for MgO. 60 mol%. Further study is needed to understand the effect of CaO
In summary, the viscosities of CAS and CFS synthesized slag and MgO on viscosity and structure of coal slag melts.
melts show different trends with respect to increase in contents
of Al 2 O 3 and Fe 2 O 3 as amphoteric oxides. Namely, the viscosity of 3.3.2. Behavior of Al 2 O 3 on viscosity
synthesized slag melts increases monotonically with increasing The study on SiO 2 –Al 2 O 3 binary slag melts indicates the struc-
Al 2 O 3 content and decreases with increasing Fe 2 O 3 content.
tural roles of Al 2 O 3 vary from AlO 4 to AlO 6 polyhedra with increas-
ing Al 2 O 3 content [31]. In the case of the molar ratio [Al 2 O 3 ]/[RO]
3.3. The behavior of slag melt components <1, Al 2 O 3 mainly behaves NWF as described in previous studies
of alumino-slicate melts and glasses [32,33]. Several authors
A previous study reported that the definition of acid-based the- reported that viscosity increased with increasing the polymeriza-
ory is an acid as an oxide ion (O 2 ) acceptor and a base as an oxide tion degree of network structure by Al 2 O 3 addition [34]. In other
ion donor [25] by the scheme of reaction: words, Al 2 O 3 ideally plays the role of NWF for AlO 4 formation in
order to link and share the corner between AlO 4 tetrahedra and
Acid þ O 2 ¼ Base ð2Þ
with SiO 4 tetrahedra when the charge compensating cation such
According to this definition, the oxides to form acids and bases as CaO co-exists enough for Al at [Al 2 O 3 ]/[CaO] 1 [31,33]. The
in slag melts have reactivity reflecting their acidic, basic, and addition of Al 2 O 3 to CaO-SiO 2 synthesized slag melts due to the for-
amphoteric characteristics. Fig. 5 shows the simplified drawing of mation of AlO 4 tetrahedra [32] corresponds to the increase of vis-
slag melt structure. SiO 2 as an acidic oxide consists of three- cosity [34], as shown in Fig. 4(a).
dimensional interconnected network of SiO 4 tetrahedra with The viscosity of coal slag melts increases with increasing total
bridging oxygens and plays a role of network former (NWF) [26] contents of SiO 2 and Al 2 O 3 at all temperatures. The Al 2 O 3 probably
in the slag melts. The basic oxides such as CaO and MgO break behaves as NWF in the present coal slag melts. The polymerization
the silicate network to form non-bridging oxygens as network degree of network structure increases with the formation of both
modifiers (NWM) [27]. On the other hand, Al 2 O 3 and Fe 2 O 3 as SiO 4 and AlO 4 tetrahedra in the gasified coal slag melts.
amphoteric oxides play dual roles as NWF and NWM depending
on slag composition [28]. NWF contributes to increase the viscos- 3.3.3. Behavior of iron oxide on viscosity
ity, on the contrary, NWM affects to decrease the viscosity. The viscosity decreases with increasing FeO and Fe 2 O 3 contents
at a fixed SiO 2 content of 60 mol% (Fig. 4(b)). The present study
3.3.1. Behavior of SiO 2 , CaO, and MgO on viscosity has a good agreement with the previous results that the viscosity
The viscosities of CAS and CFS slag melts were compared to decreases with increasing iron oxides in CaO–FeO–Fe 2 O 3 –SiO 2
evaluate the effect of SiO 2 addition. Fig. 4(b) exhibits that SiO 2 based slag melts [35–37]. The viscosity decrease by the addition
of iron oxides indicates that not only RO (CaO and MgO) but also
addition contributes to viscosity increase for binary CaO-SiO 2
(CA00.50 and CA00.60) slag melts and ternary CAS and CFS synthe- FeO and Fe 2 O 3 reduce the polymerization degree of the silicate net-
2+
sized slag melts. In the slag melts, the increase of SiO 2 content work. Iron oxide plays roles of both NWM (Fe ) and amphoteric
3+
enhances the degree of polymerization due to the formation of oxide (Fe ) and the behavior depends on temperature, composi-
SiO 4 tetrahedral network. SiO 4 in tetrahedral site is connected in tion, pressure, and atmosphere of slag melts [38]. In this present
three-dimensional polymerized structure with bridging oxygens study, roles of Fe 2 O 3 on viscosity based on temperature and com-
[26]. The viscosity also was found to decrease in the order of position were mainly investigated using coal and synthesized slag
CV > TH > MA > AD coal slag melts with decreasing SiO 2 content. melts. One of previous studies reported that the oxygen coordina-
tion number of Fe 3+ ions can be determined by the ratio [RO]/
[Fe 2 O 3 ] [32,36]. In the similar compositions of the CFS quenched
slag samples, Fe 3+ as NWF and NWM are favored at 10 mol%
and at 10–20 mol% Fe 2 O 3 , respectively [28]. The increase of Fe 2 O 3
content decreases the viscosity of gasified and synthesized coal
slag melts, as shown in Figs. 3 and 4.
A previous study [28] also suggested that in relatively-acidic sil-
icate melts of CaO–FeO– Fe 2 O 3 –SiO 2 with a CaO/SiO 2 molar ratio of
0.5, Fe 3+ ions have mainly octahedral oxide coordination. In the
acidic silicate melts, Fe 2 O 3 acts as a basic oxide with partial disso-
ciation [28] by the following Eq. (3):

þ
Fe 2 O 3 ! 2FeO þ O 2 ð3Þ
Therefore, it is estimated that Fe 2 O 3 would behave as a NWM in
Fig. 5. Simplified drawing of slag melt structure. The presence of NWF contributes
to increase viscosity, on the contrary, NWM affects to decrease viscosity in the slag the acidic synthesized and gasified coal slag melts mainly with
melts. (CaO + MgO + FeO)/(Al 2 O 3 + SiO 2 ) molar ratios of 1 as a basicity