EVALUATION OF THE HEATING OF THE IRON ORE USING
THE MICROWAVE ENERGY1
2
3
4
LEONARDO MARTINS DA SILVA , DIMITRY BUBNOFF , MARISA NASCIMENTO , JOSÉ ADILSOM
5.
DE CASTRO
ABSTRACT
The phosphorus in steels when found in high levels becomes harmful to steel quality. In
Brazil and in several locations around the world are discovered large quantities of iron
ore deposits with the phosphorus content above acceptable levels for direct use thus
causing the depreciation of the ore and its subsequent treatment as waste from mining.
A viable alternative for reducing the phosphorus content is to use the acid leaching
process which is considered even more economical process for the desphosphorization
of the iron ore, however depending on the compound containing the phosphorus
element in the iron ore will demand additional energy for its liberation. The objective of
the present paper is to study the effective heating rate of iron ore using microwave
energy following the subsequent cooling effect in a short period. Through the X-ray
diffraction technique using the Rietiveld refinement method was possible to demonstrate
the mineralogical composition of the iron ore sample and using heating and cooling
charts was possible to estimate the variation of the effective heating time. By using
scanning electron microscopy (SEM) it was analyzed the structure of fissures formed
during rapid temperature changes which is expected to enhance the efficiency of the
phosphorous removal.
KEYWORDS: Reduction of phosphorus ; iron ore ; leaching; microwave.
th
1. 9 japan-brazil symposium on dust processing - energy - environment in metallurgical industries
2. M.Sc in chemical engineering and metallurgical materials. Student of the Universidade Federal
Fluminense, RJ Volta Redonda.
3. M.Sc in metallurgical materials. Student of the Universidade Federal Fluminense, RJ Volta
Redonda.
4. Ph.D in Engineering with emphasis on process simulation. Associate professor of the
Universidade Federal Fluminense, RJ Volta Redonda,
5. M.Sc. And D.Sc. In Experimental and Applied Mineralogy / igc / USP; Researcher CETEM / MCT
1 INTRODUCTION
The phosphorus element when found in the steel in concentrations above
0.04% becomes harmful to the quality of steel. In Brazil and in several places in
the world are found large quantities of iron ore deposits at levels above 0.1%
thus occurring the devaluation of the ore by the high cost due to the reduction
processing of phosphorus content during the production of steel. One
alternative for reducing the phosphorus content in iron ore is the use of acid
leaching process where it is considered more economical process for the
dephosphorization reaction of iron ore, however, depending on the manner in
which phosphorus is element contained in the iron ore will depend on the
addition of additional energy for its release. Usually deposits primary phosphate
mineral is the apatite and rare earth phosphates (monazite and rhabdofânio).
Through the process of migration occurs weathering of phosphorus element
causing the formation of other phosphate materials as phosphates lateritic of
series of crandallite being easily removed through the process of chemical
leaching in acid medium. However, the element phosphorus in the iron ore can
also be attached to the molecule goethite, FeO(OH) in the form of solid
solution11. The distribution of phosphorus in the crystals of goethite prevents the
effective use of physical separation techniques, and requires the use of
chemical separation techniques after the heat increment.
The present work aims to study the effective heating time of iron ore utilizing
microwave energy to heat the iron.
2. MATERIALS AND METHODS
2.1 Equipment, Materials and Methods.
2.1.1 Sample Analysis
Samples of iron ore used for analysis are originating in the Iron Quadrangle
region on Minas Gerais, Brazil. All samples were crushed and milled, resulting
in average particle size less than 200 micrometers.
Chemical analysis was performed using the method of optical emission
spectrometry with inductively coupled plasma located in Research Center of
Mineral Technology, Brazil, which indicates where the phosphorus content in
the ore sample is 346mg/kg which corresponds to 0.0346 %. The pattern of Xray diffraction of the ore sample is shown in Figure 1, and indicates that the ore
is mainly composed of kaolinite 4.64%, gibbsite 3.18%, goethite 7.25%,
hematite 72.89 %, and quartz 12.05% as shown in Table 1.
2.2 Experimental Procedure.
Original sample of iron ore powder was comminuted in a mill bars generating
particle of size less than 200 micrometers.
2.2.1 Determination of Microwave Power.
For testing of heating samples of iron ore was used a microwave oven domestic
brand philco, pms24 model with a capacity of 24 L, maximum power of 1200W
and operates on a frequency of 2450 MHz.
The determination of the power of the microwave was performed by indirect
measure of the temperature rise of the water by heating for a set time.
However the amount of heat Q (energy) exchanged for a substance with mass
m when its temperature varies (θf - θi) can be calculated by the following
expression:
Q = m.c.(θf – θi)
(01)
Where:
m: mass of the substance (kg).
c: specific heat of the substance (J/kg.K).
θf - θi: difference between the final temperature, θf, and the initial temperature
θi, of the substance.
Being 1 cal equal to 4180 joules and assuming that all the electricity consumed
by the furnace is converted into microwaves and transferred to the substance in
the form of heat we have: E = Q
However, the time variation of the oven during the experiment, ∆t (s), and the
electrical power P(W), developed by the unit of time can be calculated by the
following equation:
(02)
May also be given by the following equation:
(03)
Where: Cp = heat capacity of water (75.312 J K-1 mol-1), n = moles of water
used (moles), T = final temperature - initial temperature t = heating time (s)
2.2.2 Heating Theoretical Sample Iron Ore Using Euler´s Method.
The equation of Euler´s method to E.D.O. first degree of y = y(x) has the form:
(04)
However, taking as a basis the assay, was established that in five interval of
time of 360 second the temperature reached was 428OC where the initial
temperature was of 26OC generating a heating rate of 670C/min yielding the
equation y = 67x. Thus the equation 9 takes the following form:
(05)
2.2.3 Calculation of Energy of microwave Absorbed by Mass Sample Iron
Ore
When a system absorbs heat may or may not occur variation temperature
depending on the nature of the process. If the system experiences a change in
temperature θf and θi during the transfer of Q units of heat, the heat capacity is
defined as the ratio:
(06)
If both Q and (θf - θi) are small this ratio tends to the instantaneous value of the
heat capacity C:
(07)
The heat capacity may become negative, zero, or positive infinity, depending of
the process to try the system during heat transfer. Under conditions where the
pressure remains constant C we has a heat capacity at constant pressure
represented by Cp.
Being:
(08)
The heat capacity is a function of variables, however, within a small range of
variation of these coordinate the heat capacity may become almost constant.
Often the heat capacity can match up to another without much error, so the heat
capacity of a magnetic solid is sometimes equated the calorific capacity at
constant pressure of conventional solids.
However, where relates to the calorific capacity as a function of time and the
solid mass has:
(09)
Using equation 09 have:
(10)
Varying the energy in function of time have:
(11)
Using Equation 12 it is possible to calculate the amount of energy absorbed by
a given mass of iron ore during the contact time with the energy of microwave.
(12)
3. RESULTS
In Figure 1 and Table 1 are shows the spectrum of x-ray diffraction and results
of the sample of iron ore.
Figure 1. Spectrum of x-ray diffraction of the sample of iron ore.
Tabe l. Chemical composition 0f the sample of iron ore (% by mass fractions)
MINERAL
MASS FRACTIONS, %
HEMATITE (Fe2O3) (1)
72,89
KAOLINITE [Al2Si2O5(OH)4] (2)
4,64
QUARTZ (SiO2) (3)
12,05
GIBBSITE [Al(OH)3] (4)
3,18
GOETHITE [FeO(OH)]
(5)
7,25
In Figures 2a and 2b are showed the heating profiles and absorption of
electromagnetic waves occurring in the sample of 50g of water.
A
B
Figure 2a. Heating profile of sample of 50g of water. Figure 2b. Profile absorption of sample of
50g of water.
In Figure 3 is shown the theoretical temperature of 25g sample of iron ore of
average particle size less than 200 micrometer placed in contact with
microwave energy for the different periods of time.
Figure 3. Theoretical temperature of 25g sample of iron ore of average particle
size less than 200 micrometer.
In Figure 4a is shown the heating the iron ore sample per second. In Figure 4b
is shown the profile of variation in the power absorbed by the sample of the iron
ore per minute.
A
B
Figure 4a heating of sample of 25g of ore given every second. Figure 4b variation in the
power absorbed per second by 25g sample of iron ore.
In Figure 5a is shown the profile of variation the power absorbed within a time
corresponding to 10 seconds. In figure 5b is shown the profile of variation of
power absorbed per minute through the sample of iron ore thus allowing a
maximum value of absorbed power within the time interval 360 seconds.
A
B
Figure 5a. variation in the power absorbed through of 25g of sample within a time interval of
10 seconds.Figure 5b. variation in the power absorbed per minute through 25g sample of
iron ore.
In Figure 6 is show the theoretical temperature of a sample of iron ore with
average particle size less than 200 micrometros and 50g in weight.
Figure 6. heating theoretic of sample of 50g of iron ore of average particle size less
than 200 micrometer.
In the Figure 7a is shown the heating the sample of 50g of iron ore having an
average particle size less 200 microns. In the Figure 7b is shown the profile of
variation of power absorbed by the sample of 50g of iron ore having an average
particle size less to 200 microns.
A
B
Figure 7a. heating of ore sample of mass 50g every second. Figure 7b. variation in the
power absorbed per second through of the sample of 50g of iron ore.
Is shown in Figure 8a the profile of variation of power absorbed within of a time
corresponding to 10 seconds. In the Figure 8b is shown the profile of variation
the power absorbed through the sample of iron ore per minute thus is shown a
maximum value of absorbed power within of time interval of 360 seconds.
A
B
Figure 8a. variation in the power absorbed by the sample of 50g in a time interval of 10
seconds.Figure 8b. variation in the power absorbed per minute through of the sample of 50g
of iron ore.
4. DISCUSSION
4.1 Determination of Oven Power of Microwave.
According to Figure 2 the greater amount of microwave absorption by water
sample was in 8 seconds reaching the value of 970W of power absorbed
thereby generating a temperature of about 44.40C with a variation the
temperature above 4,60C.
In Figure 2a and 2b is seen temperature evolution as a function of time and the
absorption of electromagnetic waves. However as seen in Figure 2b the high
variation of electromagnetic wave absorption through water sample given in
inside the conventional microwave oven. These machines do not have a
uniform distribution of microwave radiation due is not designed for this purpose.
They produce interference between microwave and with it, some parts of the
oven are given greater incidence in relative other parts. According to Rosini et
al 2011 An important attribute for microwave heating is the direct absorption of
energy by the material to be heated, unlike what happens when the heating is
carried out by convection, where the energy which is transferred slowly to the
reaction. Thus electromagnetic wave absorption is highly dependent on the
contact of the sample with the electromagnetic wave thus depends also on the
position where the sample is located in the applicator. However, the result of the
maximum absorption corresponding to the location where the sample was
placed in the applicator was of (970W) despite being within the range of power
typical of ovens household is considerably less than that stated by the
manufacturer (1200 W).
4.2 Interaction of Energy of Microwave In Samples of Iron Ore of Average
Particle Size Less Than 200 Micrometers
It is observed in Figure 4a the great warming potential given in a small time
interval thus reaching the value of 4280C at 360 seconds. In Figure 4b is
observed the absorption profile of electromagnetic being waves observed
larger values of absorption of microwave energy on given values of the time.
According Rosini et al 2011 large value of electric dipole in material, stronger
must be the molecular orientation under the action of the electric field. If a
material has a higher value of dielectric constant, in principle, more energy can
be stored. In a field alternated phase, as in the case of an electromagnetic
wave, the molecular orientation varies cyclically, and in microwave with a
frequency of 2450 MHz which is the frequency used in domestic oven and
laboratory oven, where occur 109 positions per second of molecules in
materials. However, the microwave absorption in same materials can be highly
non-uniform, depending on the size and dielectric properties of the substance2;
3;5;6;7;8
, which could cause due the different rates of absorption, large increases
in the rate of heating of the material as shown in Figures 4a and 4b. Studies
microwave heating were investigated by many researchers. Accord
ing to Datta et al 2007 was observed the occurrence of reduction of power
absorbed in a liquid sample due to a reduction in the dielectric properties of the
sample. According with elevation temperature in the water layer occurs a
decrease in dielectric properties thus the rate of heat generation is increased
due resistance for orientation between molecules and subsequently the
absorbed energy is converted into thermal energy increasing thus the
dimension of the material. Such occurrence is accordance with information seen
in Figure 4a. However, in Figure 5b is observed the absorption of microwave
energy by the sample of iron ore in a given time thus demonstrating the
occurrence of an accumulation of 7000W of power in 360 second.
It is observed in Figure 7a an increase in temperature of heating in a smaller
time interval. However in Figure 7b was observed increased rates of absorption
of microwave energy per unit time. These fact can be explained by the increase
in mass of the sample. According Farag et al 2011, the predominant
mechanism of heating induced microwave frequency of 2.45 GHz involve
molecular dipoles due to the presence of an oscillating electric field. Such an
explanation for the high heating of certain materials including the minerals
magnetite and hematite is due to the phenomenon of ferromagnetism. However
according to Ferreira et al 2011 the phenomenon of ferromagnetism is caused
by a strong interaction between electrons belonging to an incomplete layer of an
atom or between the electrons of neighbor atoms. This interaction, called
exchange interaction, makes a pair of electrons have a lower energy if the spins
are pointing in the same direction in relation to that is pointing in opposite
directions. however In substances ferromagnetic, the high degree of alignment
of magnetic moments continues even after the external magnetic field is
removed due the influence the magnetic moments of neighboring in the time,
making all times a small region of space are aligned same having absence of
an external field13;14. However the existence of material is material which is not
orientation of magnetic dipoles, classified as diamagnetic materials. However as
shown in Figures 4a and 7a with an increase in mass of the sample of iron ore
containing a large proportion the mineral hematite in a given small particle size
generates a large increase in absorption of microwave energy reaching the
emission limit of equipment can also reach values of higher power and values of
higher temperature in small time intervals.
5 CONCLUSIONS
Aiming to evaluate the absorption profile of microwave energy through the iron
ore, the following conclusions were developed:
• The Rapid heating of iron ore, with average particle size less than 200
microns, occurs due to the increased mass of iron ore in contact with
microwaves fact explained by increasing the concentration of hematite which
follows with increasing mass .
• With increasing temperature the sample of iron ore occurs a decrease of the
dielectric properties of the sample causing a decrease of the microwave power
absorbed by the sample.
• The process of heating and rapid cooling of the sample of iron ore led to the
phenomenon of fracturing processes due to volumetric expansion and
differences in the extent of absorption of microwave energy given by the mineral
sample.
• The process of removing phosphorus from iron ore containing goethite
complexed with the element phosphorus is highly favored by microwave contact
due to heating of the iron ore where cause a rearrange in the complex formed
between the goethite and the element phosphorus thus releasing for leaching
solution where the same due to fractures generated have a significant increase
in the contact surface between the mineral and the leaching solution.
6. THANKS
The authors thank the organs funders: Coordination of Improvement of Higher
Education Personnel, Ministry of science and technology. They also thank the
staff of the Centre for Mineral Technology by the development of tests and
chemical analysis and program graduate in Metallurgical Engineering by
Universidade Federal Fluminense by the technical and scientific support.
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