Seetharaman S. Fundamentals of metallurgy
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366 Fundamentals of metallurgy
Part II
Improving process and product quality
9.1 Introduction
The design of a process requires teamwork, since it must take many major
factors into consideration, such as the aims of the process ± the product(s), the
choice and availability of the raw materials, the environmental constraints, the
energy consumption and the cost of the production.
Any analysis of the process route should begin with the identification of the
aim or aims of the process, namely the products in the present discussion. This
should be followed by the examination of the potential raw materials. Only after
knowing the endpoint (the products) and the possible starting point(s), can one
start looking for the process route. There will be a number of processing
alternatives as shown schematically in Fig. 9.1.
Environmental regulations would rule out several possible rates. The
availability of the raw materials would further reject a couple of options. The
remaining production routes, however, are usually still multiple, and choosing
the production route is, in fact, always an optimization process. It depends on the
choice of the raw materials, energy consumption, the quality required of the final
products, the extent of pollution and the cost. In the optimization process, the
potential processing route has very often to be modified. Therefore, the final
choice of the process route and, consequently, the design of the process have
usually to go through a number of planning cycles. The present chapter does not
intend to discuss the optimization process. Instead, it focuses on the fundamentals
of the technical design or development of the individual route. Hence, we shall
confine ourselves only to the chemical engineer ing aspects in this chapter , while
leaving the economical and environmental concerns to the readers' further study.
9.2 Overview of process design
9.2.1 Whole process
A production line, viz. the whole process is usually composed of a number of
sub-processes. The design of a production line requires a comprehensi ve
9
Improving process design in steelmaking
D S I C H E N , Royal Institute of Technology, Sweden
investigation and optimization to make the final choice of the process route.
While this choice would pu t specific demands on the design of the sub-
processes, the design of the individual sub-processes might, in turn, lead to
reconsideration and modification of the preliminary design of the production
line. We shall discuss the steelmaking process as an example.
9.1 Alternative processing routes.
9.2 Production line(s) for steelmaking.
370 Fundamentals of metallurgy
Figure 9.2 presents schematically the production line(s) for steelmaking. The
whole process can be divided into three major steps before metal forming.
Step 1 Crude steel production
There are two major process routes for making crude steel today. The choice of
the production route presents a very good example of how the raw materials
influence the process design. The use of iron ore as the main starting material is
usually associated with the route of the basic oxygen converter. On the other
hand, the electric arc furnace would be the alternative when the steel plan
operates mainly on recycled steel scraps. The former requires a charge of molten
iron, which is produced in blast furnaces.
The raw materials for producing molten iron are iron ore, coking coal and
fluxes ± mainly limestone. Preparation of the charge materials plays an essential
role to ensure the gas flow through the burden column. Iron ores are charged in
the form of lump ores, sinters and pellets. The use of pellets and sinters is a
typical example of rationally utilizing natural resources. The prepa ration and
enrichment processes in the iron ore mines result in very fine-grained ores. An
optimized process design will naturally include the sub-process to agglomerate
the accumulated fine-grained ores by means of pelletizing or sintering.
Pelletizing involves the forming of ore fines (pellet feed) and concentrates
with grain sizes of well under 1 mm into pellets of around 10 to 15 mm in
diameter by addition of a binding agent. The pellets are first formed in rotating
drums or on rotary discs and thereafter dried and indurated at temperatures
above 1273K.
Sintering is made by charging a mixture of ore fines together with coke
breeze, fluxes, in-plant returns and return fines onto a circulating grate, or sinter
strand, and igniting the coke bree ze contents in the surface. A flame front passes
through the layer of the mixture over the strand length and agglomerates the
mixture into coarse lumps of ore.
With the increasing demand from environmental regulation, there has been a
trend of replacing sinters by pellets, since the sintering process leads to more
serious pollution. The trend of replacing sintered ore by pellets illustrates how
the environmental issue affects the design of a whole process. Similarly, to
reduce the release of CO
2
, SO
2
and other polluting gases in coke production,
much effort has been made to increase the amount of coal injection into the blast
furnace. Unfortunatel y, many existing blast furnaces have not been designed to
facilitate this operation. This example indicates that a process design might need
modification with the development of environmental regulations, availability of
natural resources and so on. The facilities for future modification should always
be an important aspect in designing a process.
The hot metal from the blast furnace contains impurities such as carbon,
silicon, sulphur and phosphorus. These are removed in a converter. While a
Improving process design in steelmaking 371
number of types of converters are available nowadays, converters having
combined blowing are mor e commonly adopted. In such operations, pure
oxygen at high pressure is blown into the vessel through a water-cooled lance
above the molten metal; at the same time inert stirring gases and/or oxygen are
additionally injected through the converter bottom. The bottom blowing was
beneficially introdu ced in view of its high efficiency in enhancing the mass
transfer process in the molten steel. It is a typical example of the improvement
of a process design by considering the mas s transfer aspect. Converter process is
still a necessary step before secondary steelmak ing, though studies are being
carried out to look into the possibility of combining the converter process with
the secondary steelmaking process.
Increasing importance is being attached to scrap recycling for reasons of
optimum raw materials utilization and environmental protection. Electric arc
furnaces functioning as the melting unit have been more widely employed
recently, since this enables the transformation of electrical energy into melting
heat with high efficiency and high energy density. Another impo rtant advantage
of an arc furnace is that it can produce any steel grade, regardless of the charge
(scrap, DRI, hot metal, as well as any combinations). The need for recycling and
the environmental advantage of using electrical energy are among the main
reasons for the increasing role of EAFs in steelmaking.
Step 2 Secondary steelmaking
In the case of both the basic ox ygen process route and the electric arc furnace
route, the molten steels, after bein g tapped (poured) from the furnace, undergo a
further stage of processing before casting. This step is usually called secondary
steelmaking. A number of routes are available. In general, the process should
facilitate operations such as stirring with argon and induction, adding alloys,
vacuum de-gassing and arc heating. The ladle furn ace is one of the attractive
alternatives. The objective in all case s is to fine-tune the chemical composition
of the steel, to improve the homogenization of temperature, to remove impurities
and to reduce the number of inclusions.
The design of this step depends greatly on the design of step 1. For example,
to determine the flow rate of argon gas and the stirring time, one must take into
account the nitroge n and hydrogen levels in the crude molten steel coming from
either the converter or the electric arc furnace. Similarly, the deoxidation
practice must be designed on the basis of the initial oxygen level in the crude
molten metal.
As the standard of steel cleanliness is continuously increasing with time and
technological improvements, the control of impurity level and non-metallic
inclusion population has become a serious concern of the steel producers.
Consequently, secondary steelmak ing has become increasingly important in all
modern steel industries. It has become one of the crucial aspects in designing the
372 Fundamentals of metallurgy
steelmaking process. In order to make the secondary steelmaking process more
efficient, even some modifications are made in the production line. For example,
many steel plants have introduced a sub-process of desulphurization using
calcium carbide in a torpedo furnace before sending the hot metal to the
converter. This practice lightens the load of the desulphurization in secondary
steelmaking. Introducing the torpedo furnace into the process line is an example
whereby an individual sub-process (secondary steelmaking in the present case),
in turn, leads to the modification of the initial design of the production line.
Step 3 Casting
After the secondary steelmaking, the steel is cast. Nowadays, the continuous
casting process is widely used throughout the world, although the ingot route is
retained for certain applications, e.g. high alloying tool steels. The use of ingot
casting is sometimes necessary to avoid chemical segregation in the case of high
alloy steels. It is also needed when the amount of produc tion is not so high as to
meet the requirements of continuous casting. The choice of the casting process is
a good example of how the process design is governed by the final product.
The routes for producing iron and steel, as well as product developments have
reached a very advanced state. Still, the steel industry continues to face
challenges with regard to innovations in plant and process engineering, product
development, and product application. The present section has only discussed
briefly the steelmaking process. New thinking and new designs of the
steelmaking processes will always be needed with the advance of technology.
Althoug h diffe ren t metallurgical processes require different designs, the
discussions in general would be similar. Since the purpose of this chapter is
not to introduce the different metallurgical processes, but only the principal
factors influencing process design, it will leave the particular processes to
readers' further study.
9.2.2 Sub-process
As discussed in the previous section, a production line generally consists of a
number of sub-processes. Each sub-process fulfils a number of specific tasks.
While the present section will introduce the principal factors that need careful
consideration in the design of a sub-process, the consideration of these factors
will be further elaborated in later discussions.
We could first take ladle treatme nt as an example to illustrate the key factors
for process design. In the modern steel industry, a ladle no longer functions only
as a container for transportation, but also as a very important reactor for refining.
An advanced ladle would facilitate the removal of impurities, such as oxygen,
sulphur, hydrogen and nitrogen. It would also enable the final adjustment of the
alloying elements to meet the target of the steel compositions. The design of the
Improving process design in steelmaking 373
ladle treatment must take the following points into consideration to fulfil the
above-mentioned tasks.
(i) Thermodynamic constraints
A process will always proceed towards but not beyond the thermodynamic
equilibrium. For example, in the case of most of the steel grades, one would like
to reduce the hydrogen content to a minimum level in the ladle. On the other
hand, the minimum hydrogen concentration (or precisely, the hydrogen activity)
is directly related to the partial pressure of hydrogen in the system. The
dehydrogenation reaction is described as:
2
Hmetal H
2
gas 9:1
G
o
1
ÿ2 36480 30:46 T J 9:2
K
1
exp
236480 30:46 T
R T
9:3
K
1
is the equilibrium constant of reaction 9.1. Assuming the hydrogen activity
equals its concentration in a dilute solution, K
1
can be expressed as:
K
1
P
H
2
mass%H
2
P
total
X
H
2
mass%H
2
9:4
In the above equation, P
H
2
is the partia l pressure of hydrogen, P
total
is the total
pressure and X
H
2
is the mole fraction of hydrogen gas. Hence, a low hydrogen
con cen tration in the liquid would need vacuum treatmen t to meet the
thermodynamic constrain.
(ii) Mass balance
In the design of a process, mass balance is a crucial issue. As mentioned earlier,
deoxidation is one of the most important tasks in ladle treatment. Usually,
aluminium or/and ferrous silic on alloy are employed for this purpose. In the case
of aluminium deoxidation, the dissolved oxygen content in the liquid steel
would depend on the aluminium activity in the steel, which in turn is related to
the alumina activity in the slag.
The addition of aluminium will change the final equilibrium. The final
aluminium activity in the metal and the alumina activity in the slag will be reached
by the transfer of the aluminium element from the metal to the slag. In this
transferring process, mass balance must be maintained, viz. the total amount of Al
loss in the metal phase must equal to the total amount of Al coming to the slag.
Hence, the amount of Al addition must be calculated on the base of the mass
balance. Since in many cases the reactions cannot reach thermodynamic equili-
brium, the calculated amounts of the addition should be considered as minimum.
374 Fundamentals of metallurgy
(iii) Reaction rate s
Like most of the metallurgical processes, ladle process is also heterogeneous in
nature. A heterogeneous reaction always involves a number of reaction steps.
For slag±metal reactions in a ladle treatment, the reaction rates would depend on
the mass transfers in both liquid metal and liquid slag, the kinetic condition of
the interface, the kinetics of the chem istry and the heat transfer. A
comprehensive analysis of all these steps would be a precondition for a reliable
estimation of the reaction rate, which is one of the key factors of process design.
While the above discussion shows the importance of thermodynamic
constraints, mass balances and reaction rates in the design of ladle process,
these factors, in general, should always be taken into account in the design of
most of the processes. We could take the smelting of lead in a rotary furnace as
another example.
There are two major smelting practices in secondary lead production, namely
the silicate system and soda±iron system. The soda±iron system is generally
adopted using short rotary furnace. In the furnace, lead sulphate is firstly
converted to lead sulphide by the reaction,
PbSO
4
2C PbS 2CO
2
(gas) (9.5)
The sulphide is then reduced by iron through the reaction,
PbS Fe FeS Pb (9.6)
The battery paste contains also certain amounts of PbO and PbO
2
. The lead
oxides are reduced by carbon forming CO and CO
2
gases. To form slag, soda
ash (Na
2
CO
3
) is added. The slag can generally be expressed by the formula
FeSNa
2
S(O).
Both reactions 9.5 and 9.6 are heterogeneous and endothermic in nature. The
preconditions for the reactions to take place and proceed are: (i) a reducing
atmosphere is maintained in the reactor, so that thermodynamic constraints are
met; (ii) correct quantities of the raw materials are adopted, so that correct mass
balance is obtained; (iii) a high enough furnace temperature is maintained, so
that the reactions can proceed at a reasonable rate; (iv) the furnace charge is well
mixed, so that the solid reactants have good contact with each other; (v) heat
must be supplied to the reaction sites, so that the temperature is not decreased by
the endothermic reactions. Failure in meeting any of these preconditions results
in the failure of the process.
9.3 Thermodynamics and mass balance
9.3.1 Thermodynamic description
Most of the metallurgical processes are heterogeneous in nature, involving a
number of phases, such as liquid slag, liquid metal, solid refractory and gas. Of
Improving process design in steelmaking 375