Mei C., Zhou J., Peng X. Simulation and Optimization of Furnaces and Kilns for Nonferrous Metallurgical Engineering
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7 Coupling Simulation of Four-field in Flame Furnace
Fig.7.4 shows that the uniformity of temperature distribution in the optimal
combustion chamber has been greatly improved. The values of several parameters
before and after optimization are listed in Table 7.3. From this table, it can be found
that the optimization of the chamber structure not only improves the yield of
refined zinc, but also prolongs the life of the trays. If the entire tower-type zinc
distillation furnace is analyzed and simulated, further improvements can be
achieved.
Table 7.3 Calculated parameters before and after optimization
Parameter name Before After Difference
Average temperature outside the wall of tray/ć 1156.7 1191.9 +35.2
Temperature standard deviation outside the wall of tray/ć 136.4 104.5
−31.9
Heat transferred from chamber to tray/W 7.141×10
5
7.563×10
5
+4.22×10
4
Maximum temperature of gas phase/ć 1785.6
7
34.6
−51.0
7.3
Four-field Coupling Simulation and Intensification of
Smelting in Reaction Shaft of Flash Furnace
Flash smelting is one of the major modern copper pyrometallurgical techniques.
Compared with traditional smelting techniques, flash smelting carry many
advantages, including thorough use of the specific surface area of the powder
concentrate, which greatly reduces energy consumption, promotes sulfur
utilization and is friendly to the environment. Since the birth of the first Finland
Outokumpu flash furnace in 1949 and operation of the Canada INCO flash
furnace in 1953, the flash furnace has been rapidly developed, and the
production of flash-smelted copper now accounts for nearly 50% in
pyrometallurgy of the world. The flash furnace is not only an important
smelting equipment, but is also replacing traditional P-S converter as a
continuous blow-smelting equipment.
There are two basic types of flash furnace: The Outokumpu type and the INCO
type. For the former, concentrate is fed vertically into the furnace from the roof of
the reaction shaft (refer to Fig.7.5) (Ren, 2001), and for the latter, concentrate is
fed horizontally from the side of the furnace. In this book, flash smelting furnace
refers to the Outokumpu type.
In flash smelting powder concentrates, which has been deeply dehydrated
(water content less than 0.3%), and oxygen-enriched air are rapidly jetted into the
reaction shaft through a concentrate burner and mixed in the shaft. At high
Ping Zhou, Zhuo Chen and Kai Xie
temperatures, the suspended particles complete oxidation, desulfurization,
melting and slagging reactions in a very short time. The melt then deposits on to
the settler where further slagging occurs, the matte settled and separated from the
slag.
Fig. 7.5
The sketch of Outokumpu flash furnace
In order to increase the output, reduce energy consumption and improve
economic benefits, the “Four Highs” techniqueühigh concentrate loading,
high matte grade, high oxygen enrichment of the process air, and high
thermal intensityühas been broadly put into practice. However, to adopt
the “Four Highs” technique, it is essential to ensure uniform mixing
between the concentrate and oxygen, and an effective protection of furnace
lining for high-load production. Major requirements ofĀFour Highsāare as
follows:
a) Metallurgical chemical reactions are fully completed and no raw concentrate
exists.
b) No erosion on the linings of the reaction shaft occurs.
c) The limited space in the reaction shaft is fully used.
Therefore, research in some fundamental are as of the flash furnace is
necessary in order to understand the smelting process and develop measures to
improve copper recovery in the intensified smelting. Meanwhile, appropriate
mathematical models are also needed to optimize both the operation and structure
of the furnace in order to meet the requirement of intensive smelting process in
the flash furnace (Chen et al., 2004; Li et al., 2003).
7 Coupling Simulation of Four-field in Flame Furnace
7.3.1 Mechanism of flash smelting process
ü
particle fluctuating
collision model
To understand the mechanism of the flash smelting process, particle samples are
taken from a reaction shaft in operation and are analyzed. The following
conclusions are obtained:
a) The particle distribution in the reaction shaft is quite uneven, with most
particles concentrated in the center of the reaction shaft.
b) Particles are oxidized to different extents in the reaction shaft. Some react
rapidly, and are over-oxidized where by magnetic iron oxide and metal copper or
copper oxides were formed; some are deficiently oxidized or even nonreactive.
The over-oxidized particles mostly appear in the outer region of the particle flow
and consum excessive oxygen, while the under-oxidized and nonreactive particles
are found in the center of the particle flow.
c) Many free silica are found in the reaction shaft indicating that the slagging
reactions are not fully completed and will continue in the settler.
d) In microscopic observation of the slag, particles in the shape of strawberry
(or honeycomb) are found. Some of the particles are found with pores, and some
with oxidized layers. These information of the particles’ microstructure somewhat
reveal that the particles split during the flash smelting process.
e) The average diameter of particles increase approximately seven times as
they flow along the reaction shaft, indicating that the particles collide and
aggregate in the flash smelting process.
With these results, it can be concluded that the splitting and aggregation
of particles occur simultaneously in the smelting process in the reaction
shaft. In fact, the splitting and aggregation of particles are not mutually
exclusive; they are two phenomena occurring in different stages of the
smelting process. Particle splitting is an inevitable result of quick ignition
and combustion reactions of copper concentrate; it is also the prelude to
particle aggregation. Particle aggregation then ascertains the continuation of
oxidization and desulfurization reactions, and is an important process in
ensuring oxygen be transferred between the solid and liquid phases.
In gas-particle two-phase jet flow with intensive turbulence and high
concentration of particles, violent reactions occur between the concentrate
particles and oxygen, releasing large amount of gas. The interaction of particles
and gas then induces particle collision and aggregation. When settling along the
Ping Zhou, Zhuo Chen and Kai Xie
reaction shaft, particles may exhibit five collision modes:
a) Collision resulted from the velocity differences of particles.
b) Collision induced by the intersections of gas-flow.
c) Collision caused by the horizontal random motions of particles under the
actions of Magnus force, Saffman force and counterforce induced by the gas
released from chemical reactions.
d) Mutual collision of particles and gas caused by fluctuations of gas flow and
particles.
e) Collision of particles, resulting in splitting and aggregation of particles.
Here, the particle fluctuation collision (PFC) model is proposed with the
following assumptions:
a) The splitting of concentrate particles and collisions within the particle
groups exist simultaneously at different heights of the reaction shaft.
b) The fluctuation of concentrate is the main reason for the particles’ collision
and aggregation.
c) As oxidation progresses, molten sulfides and SO
2
bubbles are formed in the
center of a particle, while porous iron-oxides are formed as an outside cover of
the particle. The gas phase forming in the molten core then causes the particles to
split and collide. The higher the oxygen concentration and local temperature, the
more intensive the process.
d) Due to the different sizes, compositions, local oxygen concentration and
local temperature, concentrate particles are oxidized to different extents.
e) Along the reaction shaft, over-oxidized particles collide with each other or
with under-oxidized particles, then aggregate to form larger particles.
Meanwhile, the over-oxidized particles are reduced by the under-oxidized
particles.
7.3.2
Physical model
A model of a practical flash furnace including the reaction shaft and a portion of
the settler (only the gas-phase region) is simulated for this research. The inner
space of the reaction shaft is a cylinder with a diameter of 5m and a height of
6.640m. The settler is a rectangle with a length of 18.500m, a width of 6.700m,
and height 1.590m (refer to Fig.7.6). The concentrate burner has the same
structure as shown in Fig.7.7, and consists of four parts. The area of the inner ring
is 0.0274922m
2
, the outer ring 0.0414478m
2
, the distributor 0.0029452m
2
, and
the central oxygen pipe 0.0037285m
2
.
7 Coupling Simulation of Four-field in Flame Furnace
Fig. 7.6 Computational grid of the reaction shaft
Fig. 7.7 The structure sketch and grid of the concentrate burner
7.3.3 Mathematical model
ü
coupling computation of particle
and gas phases
The existence and motion of particles have influences on the flow field of the gas
phase, while gas flow also affects the motion and the track of particles. Therefore,
coupled computation of gas flow and particles is necessary to correctly describe
their motion and behaviors. Here, gas phase is described by Euler’s equations and
particle phase by Lagrange’s equations. The coupling between the gas and
Ping Zhou, Zhuo Chen and Kai Xie
particle phases is achieved by introducing the source terms of particle phase in
each computational cells (PSIC, particle source in cell).
When Lagrangian method is applied to predict the particles tracks, each track
represents the behaviors of a particle group in the same diameter. If
i denotes a
particle group of diameter
d
pi
, then the mass of particles moving along the track j
is given by:
pp
000ij j i
M=MXY
˄
7.3
˅
where
p
0
M is the global mass flow rate of particles at the inlet; X
j0
is the mass
fraction of the track
j at the inlet ; Y
i0
is the mass fraction of particle group i at the
inlet. The number of particle group
i moving along the track j is:
p
p
ij
ij
ij
M
n=
m
˄
7.4
˅
where
p
ij
m is the mass of a single particle.
When particle group
i moves through a computational cell along the track j,
mass source term of the particle can be expressed as :
ppp
cell out in cell
[Δ ] [() ()]
ij k ij ij ij k
M=nm m−e
˄
7.5
˅
The subscripts
Āinā and Āoutā represent the time that the particle enters
and leaves the computational cells, respectively. Total mass source term of
particle in kth computational cell can be written as:
cell
cell
()
p
⎡⎤
ΣΣΔ
⎢⎥
=
⎢⎥
Δ
⎣⎦
p
ij
ij
mk
k
M
S
V
˄
7.6
˅
where VΔ refers to the volume of kth computational cell. Similarly, the
momentum source term of particles in
kth computational cell can be written by:
pp pp
out in
p
u cell
cell
[( ) ( ) ]
()
⎡⎤
ΣΣ × −
⎢⎥
=
⎢⎥
Δ
⎣⎦
ij ij ij ij ij
ij
k
k
num um
S
V
˄
7.7
˅
pp pp
out in
p
v cell
cell
[( ) ( ) ]
()
⎡⎤
ΣΣ × −
⎢⎥
=
⎢⎥
Δ
⎣⎦
ij ij ij ij ij
ij
k
k
nvm vm
S
V
˄
7.8
˅
pp pp
out in
p
w cell
cell
[( ) ( ) ]
()
⎡⎤
ΣΣ × −
⎢⎥
=
⎢⎥
Δ
⎣⎦
ij ij ij ij ij
ij
k
k
nwm wm
S
V
˄
7.9
˅
Energy source term of particles includes the released or absorbed heat from
their reactions, as well as the heat transferred by convection and radiation
between the particle and gas phase. Because radiant heat is related to the time
t
when particles pass through a computational cell, energy source term can be
written as:
7 Coupling Simulation of Four-field in Flame Furnace
pp pp
cp rp out cp rp in
p
h cell
cell
[( ) ( ) ]
()
⎡⎤
ΣΣ × − − − − −
⎢⎥
=
⎢⎥
Δ
⎣⎦
ij ij ij ij ij
ij
k
k
nhmQtQ hmQtQ
S
V
˄
7.10
˅
It should be noted that the source term of the turbulence kinetic energy
()
cell
p
k
k
S
and that of the turbulence dissipation rate
()
cell
p
k
ε
S are often assumed to be zero.
7.3.4
Simulation results and discussion
For the reaction shaft modeled in the simulation, the concentrate is fed at the rate
of 93t/h, and heavy oil is supplied at the rate of 390kg/h. The major operating
parameters of the reaction shaft are listed in Table 7.4.
Table 7.4 The major operating parameters in reaction shaft
Item
Flow rate
/m
3
gh
−1
Temperature
/K
Mass concentration of
oxygen /%
Velocity of gas
flow /mgs
−1
Gas flow through inner ring 15,100 298 61.6 166.4
Gas flow through outer ring 3,233 298 61.6 23.6
The central oxygen 733 298 97.1 56.9
Gas flow for dispersion 1,233 313 23.3 180.2
Gas flow for combustion 3,066 298 39.2 8.8
Results of coupled stimulation of the reaction shaft are shown in Fig.7.8 and
Fig. 7. 9.
The streamlines of the gas phase are shown in Fig.7.8, from which the
following conclusions can be drawn:
a) Once jetted from the concentrate burner into the reaction shaft, the high-
speed gas flows directly down and concentrates in the central region of the shaft.
It then scatters upon reaching the slag surface in the settler, and rebounds when it
hits the south, north and east walls of the settler, forming a series of small eddies.
With the gas flowing to the uptake shaft, the energy of the eddies is enhanced and
their sizes increase. As a result, two large eddies are ultimately formed. One is
located in the east side of the reaction shaft, filling nearly all the eastern space.
This large eddy then causes a portion of the flue to rise to the top of the reaction
shaft, where it joins the high-speed gas jetted from the burner. The other large
eddy is formed in the settler’s upper space, and is the confluence of small eddies
from upstream of the settler. As it moves toward the uptake shaft, this eddy
expands in size, keeping its direction unchanged.
Ping Zhou, Zhuo Chen and Kai Xie
Fig. 7.8
Gas streamline
b) The streamlines cluster mainly in the central region of reaction shaft,
indicating that the gas flow has very high speed. The streamlines in the west
side of the reaction shaft is quite sparse, indicating a cower-speed gas flow in
this region, called the “stagnant region”.
Fig. 7.9 Particle traces
Fig. 7.9 shows the particle traces and illustrates that the flow field of the gas
phase plays a key role in the particle motion, especially the two eddies of in the
gas phase mentioned above. Some particles move to the bottom of the reaction
shaft, then rise to the top through the east space. The eddy downstream of the
settler benefits from the separation of the particles, causing bigger particles to
settle and smaller ones to enter the rising flue.
When settling along the reaction shaft, the average speed of the particles is up to
25~30m/s, and up to 106m/s at the exit of concentrate burner. When approaching
the molten slag surface, the particle speed rapidly declines to approximately 3m/s.
Therefore, the residence time of particles in the shaft is very short
üless thanls, on
average. Particles suspending in the large eddies have lower speed and thus longer
7 Coupling Simulation of Four-field in Flame Furnace
residence time.
In addition, the simulation results also include species distribution of the
particle phase, such as CuFeS
2
, Cu
5
FeS
4
, Cu
2
S, FeS
2
, FeS, FeO, Fe
3
O
4
, SiO
2
,
Fe
2
SiO
4
and C, and the gas phase, such as SO
2
, O
2
, CO
2
and H
2
O. The
results provide a wealth of micro-information and distributions of 18
parameters such as the flow field, temperature field, releasing heat field and
concentration field.
7.3.5
Enhancement of smelting intensity in flash furnace
As the flash smelting process becomes more and more intensive, technical
problems are confronted in practice. In particular, operation difficulties are
considered to be related to the performance of the concentrate burner. Simulation
is then carried out to study the influences of burner structure and operating
parameters on the smelting process.
7.3.5.1
Problems of the concentrate burner faced in intensified smelting
The central jet distributor is a type of jet burner, where the central oxygen pipe,
concentrate loading tube, inner ring and outer ring are all coaxial. Characteristics
of the jet burner in a clude simple structure and small flow resistance. However,
since the concentrate and gas are injected in parallel, the mixing is slow and the
flame is long. Therefore, a generous space is needed to ensure complete reactions
of the concentrate. As a result, why the height of a typical flash furnace is
generally designed to be over 7m.
When the process air flow, loading amount, and, consequently, copper output
increase, the distribution air in the central. Jet distributor design is ineffective,
and the mixing between the concentrate and gas becomes move uneven. As the
process air increases, the residence time of concentrate particles in the reaction
shaft greatly decreases, which results in incomplete reaction and existence of raw
concentrate in the reaction shaft.
To solve the problem of uneven mixing between the concentrate and gas
in the central jet distributor, particulary in the burner design from Finland’s
Outokumpu
®
, a horizontal distributor was added to enhance the horizontal
movement of particles and their mixing with the gas. The effect of the
distribution air is characterized by the ratio of its horizontal momentum to
the vertical momentum of the mixture of the process air and concentrate
particles.
To increase copper output and intensify the flash smelting process, it is necessary
to improve the burner design: to achieve uniform mixing and complete reactions
between the concentrate and oxygen; to enhance collision among particles and
Ping Zhou, Zhuo Chen and Kai Xie
reduction reactions; and to constrain the shaft lining erosion caused by high
temperature particle jet-flow. Intensifying flash smelting requires that the
smelting reactions be carefully controlled in a limited space, that is, in the highly
efficient reaction region. This region has the characteristics of high temperature,
high oxygen concentration and high particle density, and is also referred to as the
“three centralizations” concept (Mei et al., 2003):
a) High temperature centralization. High temperature is the essential condition
for particle ignition, melting, collision and reduction reactions. Generally,
complete combustion reactions for concentrate particles should occur 0.5~3.0m
away from the burner outlet, where a high temperature centralization region is
formed. In this region, high temperature speeds up oxidation-reduction, melting
and slagging reactions. With further intensification of smelting, this highly
efficient region becomes more crucial, and is very helpful to achieve self-heated
smelting process.
b) Relative oxygen centralization. The controlling factors of the reaction rate of
particle at high temperatures are the oxygen concentration and the rate of oxygen
diffusion into the particle core. Increasing the oxygen concentration and relative
oxygen centralization are the pre-requisite to forming the high temperature center
in the reaction shaft.
c) Relative particle centralization. Relative particle centralization provides
higher probabilities for collision and reactions among particles. Moreover, it can
reduce shaft lining erosion caused by particles, thus prolonging the lining life.
The concept of relative partide centralization is consistent with that of relative
oxygen centralization.
7.3.5.2
Research of the concentrate burner in intensified smelting (Mei et
al., 2003)
Using the current flash furnace design as a physical model, the intensified
smelting processes are numerically simulated where the copper output is doulled.
The influences of the burner structure and operating parameters on the smelting
process are studied by simulating the particle motion, degree of mixing between
the gas and particle, and chemical reactions at different operating conditions
(such as the inner ring area, outer ring area, flow rate of central oxygen, flow rate
of distribution air, flow rate of process air, etc.). This research can provide a
helpful guidelive for the design of concentrate burner for intensified smelting.
The main conclusions are as follows:
a) The swirling process air can form swirling flow in the reaction shaft and
drive particles to revolve around the central region (Fig.7.10). The revolving
intensity has a close relationship with the initial velocity and direction of particles,
as well as the swirl number of the process air. The traces of particles from the
swirl burner are shown in Fig.7.11.