Removal of Fe from fly ash by carbon thermal reduction

Accepted Manuscript Removal of Fe from fly ash by carbon thermal reduction Minghua Wang, Hui Zhao, Yan Liu, Chuiyu Kong, Amin Yang, Jingyu Li PII:

S1387-1811(17)30130-0

DOI:

10.1016/j.micromeso.2017.02.068

Reference:

MICMAT 8178

To appear in:

Microporous and Mesoporous Materials

Received Date: 2 November 2016 Revised Date:

8 February 2017

Accepted Date: 21 February 2017

Please cite this article as: M. Wang, H. Zhao, Y. Liu, C. Kong, A. Yang, J. Li, Removal of Fe from fly ash by carbon thermal reduction, Microporous and Mesoporous Materials (2017), doi: 10.1016/ j.micromeso.2017.02.068. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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Fig. 3 XRD characterization of samples at different reduction temperatures

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Removal of Fe from fly ash by carbon thermal reduction

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Minghua Wang1,2,*Huizhao1, Yan Liu1,2, Chuiyu Kong1, Amin Yang1, Jingyu Li1

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(1 School of Metallurgy , Northeastern University, Shenyang, 110819, China ) (2 LiaoNing Key Laboratory for Metallurgical Sensor and Technology, Northeastern University, Shenyang

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110819, China )

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Abstract: Main chemical compositions of fly ash is SiO2 and Al2O3, which are also main chemical

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components of zeolite. Therefore, fly ash can be used to prepare zeolite after removing impurities such as

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Fe. Previous studies have only used the methods of magnetic separation and acid leaching to remove

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impurities, which can not wipe out the impurities thoroughly, especially Fe impurities. In the paper, carbon

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thermal method is used to reduce hematite, and then low valence Fe is removed by magnetic separation

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and acid leaching. The experimental results derived from XRF, XRD, SEM/EDS analysis show that the

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total iron content of the fly ash is reduced to be 0.49% from 4.62%, the CaO content is reduced to be

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2.08% from 9.61% after 1 pretreatment cycle. The raw material requirements for the preparation of zeolite

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are preliminarily achieved.

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Key words: Fly ash; carbon thermal reduction; zeolite; Fe removal; Ca removal

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1. Introduction

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0.12 billion tons of fly ash is produced yearly since China is a big coal-producing country. The

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country has accumulated billions of tons of fly ash. The large amount of fly ash, if not to be used, not

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only occupies land resources, but also pollutes the environment. The utilization rate of fly ash in

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China is only 40-50%, which is far lower than that of Japan, also lower than that of the Western

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European countries of 70-80%. Fly ash is mainly used for building materials, such as heat

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preservation board, cement and fly ash brick and other building materials[1]. Although the use

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amount is large, but high added value has not been got. The 70 mass percent of fly ash are silica and

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alumina, the two ingredients are also main components of zeolite. Synthesis of zeolite using fly ash as

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raw material can reduce the cost, is an important way to increase the profit since zeolite is expensive

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[2-3].

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Zeolite has the advantages of uniform pore structure and great specific surface area (usually up to 500

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m2/g), the aperture size of zeolite is usually between 3-10 Å like a molecular size. Its complex internal

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*Corresponding author, Minghua Wang, Associate professor, [email protected], Tel: 024-83687731 -1-

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cavities and pore structure enable zeolite to have special properties of adsorbent. Zeolite can be used for

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the separation and purification of gas and liquid, selective catalytic dehydration, in petrochemical, fine

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chemical industry, agriculture, environmental protection and other fields.

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Fly ash was firstly used for synthesis of zeolite by holler and Wrisching since 1985[4]. In recent years,

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all

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synthesize zeolite is widely carried out. More and more zeolites have been developed, and their use is also

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being studied. Fly ash after all is not pure aluminum silicate, contains considerable iron, calcium and other

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impurities, in addition to containing silicon aluminum. The impurities block channels of zeolite, resulting

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in impure phases in zeolite, lowered exchange capacity and cycle performance. Therefore, pretreatment for

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fly ash to remove iron impurities before producing zeolite is necessary.

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the world pays more attention to the environmental protection work, the study on using fly ash to

So far, the literature on removing iron in fly ash is mainly magnetic separation and acid leaching for

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removal of iron. Only some representative literature are given here for examples. The invention

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CN101786041A uses vertical ring magnetic separator [5], while invention CN201848307U uses new fly

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ash magnet for iron removal[6]. The iron exists in fly ash mainly as trivalent iron with weak ferromagnetic

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property, so the iron can not be separated completely. The patented CN103131860A [7]calcines fly ash

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with alkali, and then acidifies the produced mixture to remove iron, which consumes a large amount of

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acid. Patent CN103833061A only removes iron out of leaching solution, but solid iron did not completely

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transfer to the liquid phase and solid is used for making zeolite, it is not divisible for all iron [8]. The

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researcher Lujia[9] also confirmed that only acid leaching can not remove all the iron in fly ash. Most of

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the iron is in the form of ferric iron, which is difficult reacted with acid. What is more, the ferric iron,

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mullite, gypsum, anorthite and quartz are mixed together in fly ash. Some ferric iron must be wrapped by

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other phases, can not contact acid in direct leaching process. Therefore, their impurity removing effect is

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not ideal, only to reach 70%. At present, people are trying to improve the efficiency of iron removal in the

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process of fly ash pretreatment.

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In view of the shortage of iron removal efficiency, this paper puts forward the method of reducing ferric

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iron into low valence iron and destroying mullite phase by adding carbon powder at high temperature, then

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increasing the rate of iron removal by magnetic separation and hydrochloric acid leaching.

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2 Experiment -2-

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The fly ash used in this experiment is from a power plant in Shandong Province, and the main chemical composition is shown in Table 1.

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Table 1 Chemical composition of fly ash

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2.1 Chemical composition of fly ash

Ingredients

SiO2

Al2O3

CaO

CaSO4

MgO

Na2O

TFe

other

Mass fraction/%

42.91

22.40

3.62

14.55

1.15

0.26

4.62

9.49

Appendix: TFe means total Fe representing mass sum of Fe , FeO and Fe2O3.

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2.2 Process of carbon thermal reduction

Firstly, standard sieve machine was used to separate the fly ash into different mesh size. TFe content in

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the fly ash was determined by XRF analysis. The sieved fly ash and needed carbon powder of the purity of

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99.9% were mixed by grind in a mortar, both of the reactants are in about 48µ m, was added the

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appropriate binder and put in a special mold under the pressure of 100 kgf/cm2 to press into a block of 5 g.

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The block was then dried in an oven at 105

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stove before Ar gas flowed into the sealed stove. Temperature of the stove was regulated as 800, 900, 950,

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1000, 1050 and 1100

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reaction time was set as 0.5, 1, 1.5, 2, 2.5, 3 and 4 h. Ar gas was shut off after temperature dropped below

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200℃. The XRF instrument is made by Japanese company, and the model is ZSX100e. XRD

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characterization was done by Rigaku MiniFlex 600 made in Japan. SEM was checked by TM3030 made in

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Japan.

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2.3 Magnetic selection and acid leaching

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for 24h. The dried material was taken into a tube furnace

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by temperature control procedure during the reduction reaction process and the

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A certain amount of reduced fly ash was weighed, grinded fully in a mortar, and mixed with water for

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magnetic separator to remove magnetic Fe by XCRS400300 magnetic separator made in China. After that,

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the residue was mixed with 4mol/L HCl solution, and the mixture was heated at 90℃ for 2h to provide

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enough reaction time. The mixture was filtrated after the reaction. The solid was washed firstly by dilute

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acid, then distilled water, finally dried for analysis.

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3 Results and discussion -3-

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3.1 Effect of added carbon amount on final Fe content

1.48

1.40

1.36

1.32

0

1

2

3

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nc / nO

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W (TFe) / %

1.44

Fly ash size: 48µ m; Reaction temperature:900 ; Reaction time:1h

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Fig.1 Effect of nC/nO on final Fe content

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During the process of carbon reduction, the amount of C is beneficial to the reaction. In order to enable

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carbon to contact the iron fully in fly ash and improve reduction rate, the excess C of 100~200% of the

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theoretical value is often added. The theoretical value is calculated by the mole ratio of carbon to oxygen

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of Fe2O3 to produce CO2. Fig.1 gives effect of nc/no on Fe content in ultimate product after reduction,

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magnetic selection and acid leaching. With the increase of carbon amount, the concentration of CO

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increases, and reduction of iron oxide can be promoted, and the occurrence of SiO2-FeO, Al2O3-2FeO and

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so on can be reduced. However, we can not blindly increase the carbon amount though the higher carbon

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content is conducive to reducing reaction, but also can inhibit the side reaction, but the effect is not very

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significant when the carbon oxygen ratio is larger than 2. Therefore, reasonable carbon oxygen ratio is 2

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considering the cost at the same time.

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3.2 Effect of reducing temperature on the content of Fe

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1.2 1.0 0.8 0.6 0.4 800

850

900

950

1000

1050

1100

Temperature / oC

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w (TFe) / %

1.4

Fig. 2 Effect of reducing temperature

Elevated temperature can accelerate the reduction of the reaction, but the higher temperature puts

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forward higher requirements for the equipment. It is bound to increase the cost. So, the effect of

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temperature on reduction was also carried out. According to thermodynamic calculation results, the

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reduction temperature was selected as 800, 1000, 1100

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ratio of carbon to oxygen is 2. The reaction time is 1h. Fig.2 shows the effect of temperature on the Fe

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content of fly ash after reduction, magnetic separation and acid leaching. From Fig. 2 we can see that with

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the increase of the reaction temperature, the content of TFe decreased at first, then increased, and reached

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the minimum at 1000 . In order to obtain more accurate reduction temperature, two temperatures of 950

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and 1050

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1000 . Therefore, the experimental results show that the optimum temperature for the reduction of carbon

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is 1000

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respectively. The size of fly ash was 48µ m. Mole

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were also selected as experimental temperature, and the results of the experiment were still

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because too high temperature promotes oxidation both of carbon and Fe.

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Fig. 3 XRD characterization of samples at different reduction temperatures Figure 3 shows that the crystal phase composition are mainly gypsum, mullite, quartz, and hematite. At

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the same time, there is a steamed bread peak between 5~30 degrees, which shows that the fly ash contains

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the glass phase. By XRD diagram, the peak of the gypsum in the fly ash almost disappeared because

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CaSO4 reacted with C to produce CaS and CO2, the CaS was removed by subsequent acid leaching. There

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is gypsum in fly ash owing to previous SO2 capture by using Ca(OH)2 in carbon combustion tail gas in

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power plant. It is satisfying that the peak of hematite also disappeared, showing that hematite was

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successfully reduced into iron or FeO. The gray sample color changed into pea green after reduction,

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verifying the reduction of hematite. It was found that the reaction of Al2O3 with SiO2 and FeO produced

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fayalite (2FeO· SiO2) FeO· Al2O3 FeO· SiO2 at less than 1400℃. The fayalite is neither easily to be reduced

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nor to be acidified. Therefore, higher temperature than 1000℃ is adverse for Fe removal.

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3.3 Reduction time on Fe content

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In the actual production, in addition to reducing efficiency, temperature, reducing time is also an

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important factor to consider, in a certain period of time in order to achieve productivity optimization, to

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produce the highest economic benefits, but also to avoid the waste of energy.

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So in order to achieve the above objectives, the impact of reducing time on the reaction was studied.

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Among them, the size of fly ash was 48 µ m, the mole ratio of carbon to oxygen was 2, the reduction

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temperature was 1000℃, and the reduction time was between 0.5h and 4h. Fig.4 shows the effect of

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reduction time on Fe content in final product.

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w (TFe) / %

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1.0

0.8

0.6

0.4 0

1

2

3

Time / h

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Fig. 4 Effect of reduction time -6-

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ACCEPTED MANUSCRIPT By Figure 4, the reduction reaction in the initial stage, with the extension of the reduction time, the Fe

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content in final product decreased significantly, and reached minimum in 1h. This is due to in the early

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reaction, the reduction agent content is higher, and the fly ash is in good contact, so the reaction rate is

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high, the reduction reaction is fierce. When the reaction proceeds for a certain time, the reducing agent is

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consumed, and the iron oxide is also reduced, and the contact between carbon and fly ash will become

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poor; meanwhile, the gasification rate of carbon is reduced, that is, the CO concentration decreases,

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resulting in the rate of other reactions is also slow. Therefore, adding reduction time to increase the

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reduction rate is not much help, but will cause energy waste, but also affects the production efficiency.

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3.4 Comparison of morphology and composition of fly ash before and after pretreatment

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1,4 Hematite; 2 Mullite; 3 Quartz

a: SEM of fly ash ;

b: magnified SEM of fly ash Fig.5

SEM diagram of fly ash

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By Fig.5a, the morphology of the fly ash is not regular, as the spherical micro beads, and some loose

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particles and crystal particles. The morphology of fly ash with high magnification, can be seen from the

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Fig.5b. Spherical particles surface is uneven, there are obvious different mineral particles, and adhesion -7-

ACCEPTED MANUSCRIPT with smaller particle size of fly ash. Energy spectrum analysis results show that regions 5b.1 and 5b.4 are

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mainly iron oxide present in the glass phase, there may also be a small amount of mullite. The 5b.2 area is

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main mullite in the glass phase, there may also be quartz. Region 3 is mainly quartz, there may also be

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some mullite. Visible hematite phase and the main mineral phase mullite doped into each other. Some

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hematite may be wrapped by mullite, only to destroy the mullite phase can let hematite and acid leaching

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solution direct contact and react.

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1 Al2O3 and SiO2,

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Fig.6 SEM after mere acid reaction

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1 FeS, FeO,Al2O3 and SiO2; 2 mullite and SiO2 Fig.7 SEM after fly ash was reduced

Fig.6 shows the surface of the spherical particle is not as smooth as that without acid leaching treatment,

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there are small holes in the erosion. Some impurities were removed by acid leaching, but the glass phase is

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not completely destroyed. In the picture, there is no obvious sign of corrosion on the surface of the crystal,

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which can not be destroyed by the acid leaching process, meaning that Fe in crystal phase can not be

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removed completely by solo acid leaching.

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In comparison with Fig.6, the morphology of fly ash after reduction and calcination has changed a lot

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observed in Fig.7. Firstly, previous spherical particles almost disappeared, were replaced by irregular

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shaped particles; secondly, many small holes appear on particle rough surface after reduction. The energy

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spectrum analysis showed that the main chemical composition of the region 1 was FeS, FeO, and possibly

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some FeSO4, Al2O3 and SiO2, and FeS was the product of the reduction of sulfate minerals, which was

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consistent with the result of the acid leaching in which smell of bad egg like that of H2S spread. Energy

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spectrum analysis results of region 2 in Fig.7 show that, the chemical composition of the region is mullite

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and quartz, and small holes may occur due to gas escape at a high temperature and reducing atmosphere.

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The foregoing analysis shows that the reduction process has destructive effects on all glass phase and

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crystal phase besides direct reaction of carbon and hematite and gypsum, which are conducive to the -8-

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subsequent magnetic separation and acid leaching with larger reaction surface area, indicating that

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reduction process is beneficial to the whole of the experiment.

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The chemical composition of fly ash after carbon reduction - magnetic separation - acid leaching is

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shown in tab. 2. The result of carbon thermal reduction is derived under the optimized condition of

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temperature of 1000

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48µ m of fly ash.

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Tab. 2 Chemical composition of fly ash after treatment

SiO2

Al2O3

CaO

MgO

Mass fraction / %

66.52

20.02

2.08

0.76

Na2O

TFe

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Ingredients

0.21

0.49

other 9.92

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,reaction time of 1h, mole carbon oxygen ratio of 2 in argon atmosphere using

Carbon thermal reduction also facilitates the removal of Ca according to the following two chemical reactions:

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2C+CaSO4=CaS+2CO2 ↑

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CaS+2HCl=CaCl2+H2S↑

There is CaSO4 in the coal fly ash due to the use of CaO for SO2 capture in chimney. In comparison with

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Tab.1, Tab. 2 shows that the carbon thermal reduction, magnetic separation and acid leaching of fly ash

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can greatly reduce the content of CaO and TFe, which basically achieved the requirements of molecular

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sieve materials with 1 cycle. More cycle (carbon thermal reduction, magnetic separation, acid leaching )

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may lead to less content of CaO and TFe, seen in Tab.3.

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Tab. 3 Contents of CaO and TFe with cycle time

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Cycle time

CaO /%

TFe /%

2nd cycle

1.79

0.26

3rd cycle

0.82

0.23

4 Conclusions

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In this paper, the effect of carbon thermal reduction conditions on the iron removal rate of fly ash was

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studied. The experiment result shows that all kinds of mineral phases are in fly ash mutually doped.

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Hematite does not exist alone, but disperses in all kinds of mineral phases. Only after mullite phase, glass -9-

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can iron oxide in these phases react with acid. The optimized reduction conditions were: carbon oxygen

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ratio of 2, at 1000 , 1h. The TFe content of fly ash was reduced to 0.49%, CaO content decreased to

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2.08% under the optimized conditions in argon atmosphere using 48µ m of fly ash.

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Acknowledgement: The research work was supported by National Natural Scientific Foundation of

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China.(Grant No. 51574084)

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Wenhui.

Iron

magnetic

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separation

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China,

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combined

method

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iron

removal

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Carbon thermal reduction of fly ash to get metal Fe and FeO> magnetic separation to remove metal Fe>acid leaching to remove FeO Carbon thermal reduction of fly ash to get CaO from CaSO4> acid leaching to remove CaO