Red-mud treatment using oxalic acid by UV irradiation assistance
YU Zhang-long1, SHI Zhi-xia2, CHEN Yong-mei1, NIU Yin-jian1,3, WANG Yong-xia1, WAN Ping-yu1
1. School of Science, Beijing University of Chemical and Technology, Beijing 100029, PR China
2. Department of Mineral Resource and Metallurgical Materials, Beijing General Research Institute for Non-Ferrous Metals,
Beijing 100088, PR China
3. The Nonferrous Metal Society of China, Beijing 100814, PR China;
Received date: accepted date:
Abstract: Red-mud is the residue from the Bayer Process, in which the iron minerals should be removed before red-mud is used to
produce refractory materials. In this study, the iron minerals in red-mud were extracted by oxalic acid solution. The content of Fe
(calculated in Fe2O3) in red-mud was reduced from 17.6% to less than 1% after being treated by 1 mol/L oxalic acid solution at
75 ℃ for 2 h. The Fe(III) oxalate solution obtained was then irradiated by UV light, resulting in the precipitation of Fe(II) oxalate.
Under UV photocatalysis, more than 90% of Fe(III) oxalate in the extracted solution was transformed into the precipitation of Fe(II)
oxalate crystallite (β-FeC2O4·2H2O), which was identified by XRD, FT-IR and SEM. The filtrate from the Fe(II) oxalate precipitate
filtration could be reused in the next cycle. The mechanism of UV photocatalysis precipitation is also discussed in this paper.
Key words：red-mud; iron minerals; oxalic acid; UV irradiation
The annual output of alumina is about 80 million weakness to produce refractory materials. Many
tons in the world, while 100 million tons of residues researchers have tried to remove iron from red-mud
(red-mud) are simultaneously produced per year [1, or other ores.[9, 10] The Canada patent (CA1234478)
2]. Therefore, the storage of red-mud becomes a disclosed a method to remove iron minerals from clay
serious problem for many alumina refineries [3, 4]. as follows: sintering clay in a reductive atmosphere of
Moreover, red-mud causes pollution to soil and harm CO or C, and then adding Cl2 to transfer ferric oxide
to the living organism due to its strong caustic nature into evaporable ferrous chloride . Ambikadevi
[3-5]. Transforming red-mud into an useful stuff is and Lalithambika  tried many organic acids to
thought to be an important way to solve these extract iron minerals from red-mud and found oxalic
problems. acid was better than other organic acids (such as
The main components of red-mud are SiO2 and Al2O3 acetic acid, formic acid and citric acid) because
which are similar to the raw materials to produce oxalate group has a strong acidity and the chelating
cement and brick [6-8]. However, low fire resistance ability to iron ions.
due to high content of Fe2O3 in red-mud is the fatal Nowadays, oxalic acid can be produced through
Foundation item: Project (2010AA101703) supported by the National Hi-tech Research and Development Program of China (863
Corresponding author: CHEN Yong-mei, WAN Ping-yu; Tel and Fax: +86-10-64435452; E-mail: firstname.lastname@example.org;
biofermentation of glucose，which makes oxalic acid decomposition ratio was defined as the fraction of
cheap enough for large-scale production. This study decreased oxalate content to the initial content. Every
focuses on how to leach iron minerals in red-mud experiment was repeated for 3 times.
with oxalic acid and how to treat the leachate to make
oxalic acid recyclable. It is concluded in this study 2.2 UV-irradiation for leachate treatment
that ferric oxalate could be precipitated as ferrous The filtrate was irradiated by using a UV light
oxalate by UV-irradiation assistance and then it is source (355 nm/150 W, GGY400-125, GuangDong,
possible to recycle the remaining oxalic acid for China) at the room temperature, and yellow-orange
further reducing the cost. crystal powder was precipitated gradually from the
solution. 20 ml of the supernatant was sampled every
2 Experimental 10 min for detecting the content of iron and oxalate
groups. The irradiation was stopped when the iron
2.1 Iron removing by oxalic acid solution
content was not further decreased. The precipitated
The red-mud sample was collected from Henan
powder was filtered, washed and dried at 50 ℃ for
branch of CHALCO and was air-dried without
further XRD、FT-IR、SEM tests.
washing. 1 mol/L oxalic acid solution was prepared
with analytical grade reagent of oxalic acid and
deioned water. 10 g of the crushed red-mud (about 3 Result and discussion
mesh 200) and 150 ml 1 mol/L oxalic acid solution
3.1 Red-mud components analysis
were added to a 250 ml three-neck flask. The reaction
XRD pattern of the dried red-mud is shown in
mixture was stirred continuously for 0.5~2 h in a
Fig.1. Red-mud is the residue of bauxite after being
water bath at the certain temperatures.
digested by alkali in Bayer process, the main
Fe content in the filtrate was analyzed by the
components in which are calcium aluminosilicate
spectrophotometric method using phenanthroline.
hydrate, sodium aluminosilicate, iron oxide, calcium
The leaching efficiency was calculated as the fraction
titanate and other silica minerals.
of iron content in leachate to that in red-mud, and the
change of oxalate group content in the leachate was
also measured through titration by standard
potassium permanganate solution. The oxalate
Fe2O3 conditions should be optimized on the basis of both
3CaO Al2O3 SiO2 4H2O
Na2O Al2O3 1.7SiO2 2H2O
the iron leaching efficiency and oxalate
decomposition ratio. The results are shown in Fig.2
and Fig. 3.
0 10 20 30 40 50 60 70
Leaching efficiency / %
Decomposition ratio / %
Fig.1 XRD pattern of the red-mud
(●: 3CaO·Al2O3·SiO2·4H2O; ◆: Fe2O3 ;
□: Na2O·Al2O3·1.7SiO2·2H2O; ◇: SiO2; △: CaTi2O4)
The contents of these components measured by 20
20 40 60 80 100
Temperature / ℃
chemical analysis are shown in Tab.1. The Fe2O3
content is as high as 17.6%. According to GB/T Fig.2 the effect of temperature on the leaching efficiency of
3995-1983, the Fe2O3 content in the refractory iron in red mud and the decomposition ratio of C2O42-(a:
material should be less than 2%, so most of iron has leaching efficiency of iron; b: decomposition ratio of C2O42-)
to be removed before red-mud being reused to
produce refractory materials. As shown in Fig.2, the iron leaching efficiency
rises as reaction temperature increases. Leaching
Table 1 Chemical Analyses of Red Mud (mass fraction, %) efficiency reaches 95% at 90 ℃. On the other hand, it
Material SiO2 CaO Al2O3 Fe2O3 Na2O TiO2 could be found out that some oxalate groups
Content 16.8 23.4 22.3 17.6 9.3 5.3 decomposed at the temperature above 60 ℃. The
decomposition becomes more severe at higher
temperature and the decomposition ratio reaches
3.2 Optimization of acid leaching conditions
The removal efficiency of iron minerals from 4.5% at 95 ℃. Therefore, the optimum temperature
red-mud by oxalic acid solution depends on its should be chosen as 75 ℃.
acidity and the chelating ability to iron ions. Leaching
efficiency increases with increasing temperature and
reaction time, but oxalate could be decomposed into
CO2 at too high temperature. Therefore, the leaching
100 7 the criteria for producing refractory materials.
Decomposition ratio / %
Leaching efficiency / %
3.3 Reductive reaction of ferric oxalate under UV
88 When the resulted leachate was settled in the
indoor environment, some yellow-orange
0 2 4 6 8 10
Reaction time / h
precipitation is started to form after 10 hours, while
Fig.3 The effect of reaction time on the leaching efficiency of
the precipitate will form much more rapidly if the
iron in red mud and the decomposition ratio of C2O42-(a:
leachate was irradiated by UV light or sunlight. Fig.4
leaching efficiency of iron; b: decomposition ratio of C2O4 )
shows the concentration change of Fe (calculated in
Fe2O3, same below) and oxalate groups in the
The extraction process is a process involving a
leachate during UV irradiation.
solid-liquid heterogeneous system, in which reaction 1.0
rate is limited by mass transferring and diffusion. 0.9
Conc. of C2O4 / (mol·L )
Conc. of Fe / (g·L )
Therefore, extending reaction time is beneficial to 0.8
improving the iron leaching efficiency. It is shown in
Fig.3 that the iron leaching efficiency is 85% for the
first 0.5 hour at 75 ℃, and it increases to 96% after 2
0 50 100 150 200 250
hours. However, for further extending time, the Irradiation time / min
leaching efficiency is only a little increased, e.g. only Fig.4 The concentrations change of Fe (calculated in Fe2O3)
about 98% after 9 hours. On the contrary, the and C2O42- in the leachate solution during UV irradiation (a:
decomposition ratio of oxalate groups increases concentrations change of Fe; b: concentrations change of
almost linearly with the time increasing. The C2O42-)
decomposition ratio is 3.5% after 2 hours, while up to
6.5% after 9 hours. Therefore, the optimized leaching The initial concentration of Fe and oxalate
time is selected as 2 hours in consideration of both groups was 10.2 g/L and 0.95 mol/L, respectively. As
the economic benefit and the leaching efficiency. As a shown in Fig.4, the concentration of Fe in the
result of the leaching, the Fe2O3 content in the leachate reduces from 10 g/L to 1 g/L in the first 60
leached red-mud was reduced to 0.7%, which meets min, which indicates that almost 90% of Fe in the
leachate is precipitated in only 1 hour. Meanwhile, The 2 angle and intensities of the peaks well match
the concentration of oxalate groups also decreases those of the standard pattern (JCPDS 22-0635) of
from 0.95 mol/L to 0.65 mol/L during 1 hour β-FeC2O4·2H2O. For the FT-IR spectrum of in Fig.5
irradiation. The mole ratio of the lost oxalate groups (B), the strong peak at 1700 cm-1 could be assigned to
to the removed iron is about 3:1 (oxalate:Fe). After vibration of carboxylic group C=O. The two medium
being irradiated for longer time, the Fe content hardly strong peaks in the ranges of 1500 and 1000 cm-1 are
changes while the oxalate group content still attributed to the vibrations of C-O and O-H groups in
decreases slowly. the oxalic acid. The two strong sharp peaks below
1000 cm-1 are attributed to the presence of Fe(II)-O
3.4 Characterization of the precipitation group . The SEM image of ferrous oxalate
crystallite (Fig.5C) shows that it has regular
The precipitate formed during the irradiation rod-shape with the lengths of 5-10 μm and width of 1
was collected for characterization by XRD, SEM and μm.
IR. Fig.5 (A) shows XRD patterns of the precipitate.
A 200 B
β-FeC2O4·H2O - C-O Fe-O
10 20 30 40 50 60 70 80 90 4000 3500 3000 2500 2000 1500 1000 500
2 Wave length / (cm)-1
Fig.5 XRD pattern (A), IR spectra (B) and SEM photo (C) of the UV- irradiated precipitation
3.5 Mechanism of UV- photocatalysis reductive
Once Fe(II) oxalate is formed and getting more and
reaction of ferric oxalate to ferrous oxalate
more concentrated, the precipitation of
Several researchers were interested in the
β-FeC2O4·2H2O would form due to the equilibrium
dissolution of Fe2O3 by oxalic acid solution and some
existing between Fe(C2O4)22- and FeC2O4. The main
of them had observed the precipitation process with
reaction equations can be described as the following:
or without irradiation[14-17]. One kind of mechanism
Fe2O3 + 6H+ + 6C2O42- = 2Fe(C2O4)33- + 3H2O Eq.1
was described as that the reduction reaction from
2Fe(C2O4)33- = 2Fe(C2O4)22- + C2O42- + 2CO2 (slow) Eq.2
Fe(III) to Fe(II) due to oxalate groups had occurred
during the leaching process. So the predominant 2Fe(C2O4)33- + h = 2Fe2+ + 5C2O42- + 2CO2 (fast) Eq.3
species is Fe(II) oxalate in the leachate and the Fe2+ + 2C2O42- Fe(C2O4)22- FeC2O4(s) Eq.4
precipitation of Fe(II) oxalate occurred when the pH The remaining Fe in the solution after UV-irradiation
of the solution changed into lower value because the was about 1 g/L, which accords with the solubility of
solubility was limited. The another mechanism β-FeC2O4·2H2O in water (0.97 g/L calculated in
considered that Fe(III) oxalate was transformed into Fe2O3). It should be considered as indirect evidence
Fe(II) oxalate under irradiation, in which it underwent to support the above mechanism.
the charge transfer from ligand (ox2- ) to metal (Fe3+) In the case of this study, about 35% of oxalate groups
(LMCT) [13-15, 18]. in the original solution were lost in the whole process,
It is found in our study that the precipitation of Fe(II) in which about 4% of oxalate groups decomposed
oxalate could form in indoor environment but it spent into CO2 during leaching process. And the other 31%
more than 10 hours, while about 90% of Fe could be were lost during irradiation, in which 11.3% of
precipitated in only 1 hour with UV-irradiation. Since oxalate groups were precipitated together with Fe(II),
the standard redox potential of while another 5.7% were stoichiometrically oxidized
Fe(C2O4)33-/Fe(C2O4)22- is -0.158 V (comparing to to CO2 in accordance with the Eq.3, and the other
0.77 V of Fe3+/Fe2+), it is difficult for Fe(C2O4)33- to 14% were probably oxidized by O2 in the air during
be reduced into Fe(C2O4)22- by oxalate even if in the UV-irradiation. The filtrate after ferrous oxalate being
concentrated oxalic acid solution like that in our filtrated could be reused as the extractant in the next
study (about 1 mol/L). However, UV-assisted LMCT cycle with replenishing certain amount of oxalic acid.
leads the reduction reaction to be accelerated greatly.
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余章龙 1，石志霞 2，陈咏梅 1，钮因健 1,3，王永霞 1，万平玉 1
（1 北京化工大学理学院，北京 100029；
2 北京有色金属研究总院矿物资源与冶金材料研究所，北京 100088；
3 中国有色金属学会，北京 100814）
赤泥在 1 mol/L 的草酸溶液中浸出 2 h，氧化铁的浸出率可达到 96%，浸出后赤泥中氧化铁含量由 17.6%降低至
小于 1%。在紫外光照催化作用下，1 h 内浸出液中 90%以上的草酸铁转变成草酸亚铁，实现剩余草酸循环再利
用。产物的 XRD、FT-IR 和 SEM 显示，该草酸亚铁为 β-FeC2O4·2H2O，长度约为 5 μm 左右的棒状结晶。最后根
据测试结果对 UV 催化沉淀的机理进行讨论分析。