Removal of Iron from Red-mud using oxalic acid as the extractant

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Removal of Iron from Red-mud using oxalic acid as the extractant Powered By Docstoc
					           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

1 Introduction

    The annual output of alumina is about 80 million                  weakness to produce refractory materials[9]. 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 [11]. Ambikadevi

[3-5]. Transforming red-mud into an useful stuff is                   and Lalithambika [12] 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:;
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
                                                               CaTi2O4
                                                                                                                           decomposition ratio. The results are shown in Fig.2
                                              

                                                                                          
                                                                                                                           and Fig. 3.
                                                                                                  
                                                          
                                                                                                               
                                                                
                                                                                    
                                                                                                       
                                                                            
                                                                                                                 
                                                                                                                                          100                                                              5

     0         10            20                30       40                           50                60            70

                                                                                                                                 Leaching efficiency / %
                                                                                                                                                                         a
                                                                                                                                                      80                                                   4

                                                                                                                                                                                                               Decomposition ratio / %
                         Fig.1 XRD pattern of the red-mud
                   (●: 3CaO·Al2O3·SiO2·4H2O; ◆: Fe2O3 ;
                                                                                                                                                      60                                                   3
      □: 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
                                                    b                                            irradiation
                       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

                                                                                                                            4                 b
Fig.3 that the iron leaching efficiency is 85% for the
                                                                                                                            2                                                  0.6

first 0.5 hour at 75 ℃, and it increases to 96% after 2
                                                                                                                            0                                                  0.5
                                                                                                                                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 [13]. 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
                                                                      Intensity (a.u.)


                                                        sample                                                                           Fe-O
                                                  β-FeC2O4·H2O                                     -                              C-O         Fe-O
                                                                                               OH                            C=O
                                                  JCPDS: 22-0635
         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[17]. 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)


    进行紫外光催化照射,将草酸铁还原成草酸亚铁沉淀,实现了草酸溶液循环再利用。实验结果表明,75 ℃下,

    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。


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