THE OBTAINING OF IRON RED PIGMENT FROM SLUDGE
RESULTED BY SPENT ACID NEUTRALIZATION
L. Lupa, P. Negrea, A. Iovi, M. Ciopec, C Muntean
Faculty of Industrial Chemistry and Environmental Engineering, Victoria square, no. 2, Timisoara, 300006,
Romania ([email protected])
Key words: ammonium extraction, sludge, iron recycling
ABSTRACT
Hot-dip galvanization is one of the oldest and reliable commercial methods for protection of steel
against corrosion.
To achieve a homogeneous surface reaction, and hence a uniform zinc alloy coating, the surface of the
work must be chemically prepared by a series of treatments before dipping in zinc. These usually include
degreasing, to remove oil or grease from the surface of the work, acid pickling to remove iron oxides such as rust
or scale, and fluxing to activate the surface and promote reaction with zinc.
With a past reputation as a “dirty industry’ galvanizers have abated environmental impact and currently
recycle the majority of solid waste zinc products. However, by far the greatest quantity of waste produced by
galvanizers is the sludge resulted from the neutralization of the spent acid.
Pickling is carried out in hydrochloric acid. Spent pickling acid is a solution containing various
concentrations of HCl, FeCl2 and ZnCl2. The residual acid is neutralized with lime, so the residual waters can be
discharged to the sewerage and the solid precipitates are sent in the most of the cases to landfill. This treatment
process results in all potentially valuable iron and zinc being lost.
In this paper we tried to put in good use the valuable metals from the sludge by recovering the iron ions
under iron red pigment form, which can be used at the construction materials obtaining. The zinc ions from the
sludge were removed by extraction in ammonium solution and the residual sludge was incinerated and the
resulted iron oxide was submit to thermogravimetric and X-ray diffraction analysis to establish if this can be
used as iron red pigment.
1. INTRODUCTION
Galvanizing is a coating process applied in order to prevent iron-based materials from corroding. Hotdip galvanizing is one of the most common and economical coating methods and its particularly used to coat
steel [1]. Before the hot-dip galvanizing the pieces are submit of a preparing process which consist in degreasing,
chemical cleaning, washing, fluidizer treatment and pre warming. After these operations result waste waters
which are neutralised with lime. This method can efficiently remove heavy metals from wastewaters, but it
generates a lot of heavy metal sludge that is classified as hazardous industrial wastes and causes disposal
problems [2]. For this reasons and from the economically point of view can be find a way to extract the valuable
metals from the sludge, so that to be reused [3-6].
In this paper we tried to put in good use the valuable metals from the sludge by recovering the iron ions
under iron red pigment form, which can be used at the construction materials obtaining. The zinc ions from the
sludge were removed by extraction in ammonium solution and the residual sludge was incinerated and the
resulted iron oxide was submit to thermogravimetric and X-ray diffraction analysis to establish if this can be
used as iron red pigment.
2. EXPERIMENTAL
The sludge resulted from the residual acid neutralization with lime, was dried and than was separated in
three fraction (1<0.32 mm, 2= 0.32÷1 mm, 3=1÷2 mm). The sludge was analyzed in the view of metals ions
composition establishment.
To put in good use the waste sludge, the zinc ions were extracted from sludge in ammonium solution. In
this way the zinc ions can be reused under microelements form in the fertiliser obtaining process with the zinc
microelement. The resulted sludge was submitted to a thermo gravimetric and differential thermal analysis.
Than was incinerated so that the iron oxide to be obtained. The resulted iron oxide was submitted to X-ray
diffraction to establish if this can be used as iron red pigment.
The metals ions concentrations were determinate using an atomic absorption spectrophotometer
VARIAN SpectrAA 110.
The thermogravimetric analyse of the obtained product was made using the thermogravimetric balance
Netzsch TG 209.
The X-ray powder diffraction patterns were registered with a Bruker D8 Advance diffractometer (Mo
Kα radiation).
To establish the optimum condition of zinc ions extraction from sludge was study the dependence of the
zinc ions extraction degree versus the ammonium solution concentration, sludge fraction, stirring time and ratio
S:L.
3. RESULTS AND DISCUSSIONS
3.1 Sludge analyse
The experimental data regarding the metals ions content from sludge are presented in table 1.
Table 1. The sludge analyse
Fraction
1
2
3
Metals ions concentration, %
Zn
Fe
Pb
Ca
1.61
47.56
0.11
0.92
1.48
32.85
0.14
0.97
1.42
30.23
0.15
1.30
From the experimental data can be observed that the sludge has almost the same composition of the
heavy metals.
3.2 Extraction of zinc from sludge in ammonium solution
Because the sludge contains a lot of quantity of iron ions, this can be used as iron oxide. From this
reasons we tried to extract the zinc ions from the sludge so that the obtained product to be as pure as possible.
The experimental data regarding the dependence of the zinc ions extraction degree versus the sludge
fraction, stirring time and ammonium solution concentration are presented in figures 1-3.
49.5
Fraction 1
Fraction 2
Fraction 3
49.0
Zn extraction degree, %
48.5
48.0
47.5
47.0
46.5
46.0
45.5
45.0
0
10
20
30
40
50
60
Stirring time, min
Figure 1. The dependence of Zn extraction degree versus the stirring time and sludge fraction in the case of use
for extraction a 15% ammonium solution
52
Zn extraction degree, %
Fraction 1
Fraction 2
Fraction 3
50
48
46
0
10
20
30
40
50
60
Stirring time, min
Figure 2. The dependence of Zn extraction degree versus the stirring time and sludge fraction in the case of use
for extraction a 20% ammonium solution
68.5
Fraction 1
Fraction 2
Fraction 3
Zn extraction degree, %
68.0
67.5
67.0
66.5
66.0
65.5
0
10
20
30
40
50
60
Stirring time, min
Figure 3. The dependence of Zn extraction degree versus the stirring time and sludge fraction in the case of use
for extraction a 25% ammonium solution
From the experimental data can be observed that the Zn extraction degree increase with the increasing
of the ammonium solution concentration. The Zn extraction degree present a slow decreasing with the sludge
fraction increasing, but this influence is not so meaningful and also with the increasing of the stirring time, this
could happened because of the evaporation of the ammonium solution.
The experimental data regarding the dependence of the zinc ions extraction degree versus the ratio S:L
(1=1:3, 2=1:5, 3=1:6, 4=1:8, 5=1:10) and versus the ammonium solution concentration when is used the first
fraction of the sludge at a stirring time of 5 minutes are presented in figure 4.
68
Zn extraction degree, %
66
64
62
NH3, 15%
NH3, 20%
NH3, 25%
60
58
56
54
52
50
1
2
3
4
5
Ratio S:L
Figure 4. The dependence of Zn extraction degree versus the ratio S:L
From the experimental data can be observed that the zinc extraction degree don’t depend in a big
measure by the ratio S:L, so is not necessary to use in excess the ammonium solution. Can be observed that the
zinc extraction degree increase with the increasing of the ammonium solution concentration. The optimum
conditions of zinc ions extraction are: fraction 1 of the sludge, time of 5 minutes, ammonium concentration 25%
and ratio S:L=1:3.
3.3 The analyse of the obtained product
The product resulted after the extraction of zinc ions was submit to thermogravimetric analysis in order
to find the temperature at which the product must be burned to obtain the iron oxide. The thermal behavior of
the product is illustrated in Figure 5. When the product is non-isothermally heated with a constant heating rate
of 5°C/min up to 1000°C, in the sample take place two processes with mass loss. The first one occurs in the
range of temperature 30-290°C in two steps: I. between 20-130°C, with maximum rate at 75°C and a mass loss
of 3.64% (which can be due to the loss of humidity); II. between 130-290°C, with maximum rate at 215°C and
9.24% mass loss. In the range of temperature 290-730°C the sample loses slowly 5.59%. The second process
with mass loss begins at 730°C and takes place in two steps: in the range of temperature 730-910°C, with
maximum rate at 840°C, the sample loses 7.67%; the second step begins at 910°C and still continues at 1000°C.
At 985°C the residue is of 70.26%.
Because in the range of temperature 290-730°C the mass loss of the sample is very slow, we consider
that the product is decayed in iron oxide until is reached the temperature of 300°C. So we burned the sludge
resulted after the zinc ions extraction at the temperature of 400 °C and the resulted product was submitted to
X-ray diffraction analyze.
The X-ray diffractometry of the decayed product at 400°C is presented in figure 6.
100
730°C
95
0
290°C
910°C
90
%Mass
-0.2
75°C
85
840°C
DTG
80
-0.3
-0.4
75
215°C
TG
70
0
200
400
600
800
dm%/dt, %/min
-0.1
130°C
-0.5
-0.6
1000
Temperature, °C
Fe2O3(214,300) Fe3O4(440)
Fe2O3(116) Fe3O4(422)
Fe3O4(511)
1000
Fe2O3(024)
2000
Fe2O3(012)
Lin (Counts)
3000
Fe2O3(113)
Fe3O4 (400)
4000
Fe3O4(220)
Fe2O3(104)
Fe2O3(110) Fe3O4(311)
Figure 5. The thermogravimetric diagram of the product
0
10
20
30
40
50
2-Theta Scale (degree)
Figure 6. The X-ray diffractometry of the decayed product
From the X-ray diffractometry can be observed that the product contains two crystalline phases: Fe2O3
(rhombohedral) [7] and Fe3O4 (cubic) [8].
4. CONCLUSIONS
From the experimental data can be observed that the sludge resulted after the spent acid neutralisation,
contain iron as a major element, so we tried to put in good use the iron ions under iron red pigment.
In this way we extracted the zinc ions from sludge with ammonium solution. The optimum conditions
of zinc ions extraction are: sludge fraction <0.32 mm, stirring time of 5 minutes, ammonium solution
concentration 25% and ratio S:L = 1:3. The resulted solution can be used for example at the obtaining of
fertilisers with the microelement zinc.
From the X-ray diffractometry of the resulted product after the zinc ions extraction and after burn at the
temperature of 400°C can be observed that is obtain iron oxide which can be used under iron red pigment at the
construction materials obtaining.
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8.
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