A Kinetic Study Investigating the Carbothermic Recovery of Chromium from a Stainless-Steel Slag
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TION
THE production of stainless steel goes hand in hand with the generation of slags that are currently landfilled for the most part. In 2017, 48.1 million tons of stainless steel were produced.[1] Considering the specific amount of 270 to 335 kg slag per ton of steel, approximately 13.0 to 16.1 million tons was produced during this year.[2] They mainly consist of the typical slag-building compounds CaO, SiO2, Al2O3 and MgO with minor concentrations of FeO, Cr2O3, NiO, MoO3, MnO and others. The high ratio between CaO to SiO2 leads to disintegration caused by hydration of free CaO and MgO, preventing any application in the construction industry.[3] Mechanical treatment processes were developed to separate metallic particles from the oxidic matrix, recovering only metal inclusions leading to low recovery rates and producing a fine-grained residue that must be dumped. The studies of various authors demonstrate that a carbothermic reduction treatment can not only recover chromium but can also improve the properties of the slag by decreasing the basicity to
MANUEL LEUCHTENMU¨LLER, JU¨RGEN ANTREKOWITSCH, and STEFAN STEINLECHNER are with the Chair of Nonferrous Metallurgy, Montanuniversitaet Leoben, 8700, Leoben, Austria. Contact e-mail: [email protected] Manuscript submitted April 14, 2019.
METALLURGICAL AND MATERIALS TRANSACTIONS B
stabilize it and prevent disintegration.[4–10] However, no literature is available that deals with the kinetics of such a treatment process, although this knowledge is of major importance for the planning and engineering of a treatment plant and additionally to evaluate its economics. For this reason, the present article focuses on the development of an appropriate kinetic model that can be applied in future implementations of the carbothermic melting-reduction technique.
II.
THEORETICAL BACKGROUND
A reaction takes place when two requirements are fulfilled: Two substances must collide, and the collision must overcome the activation energy. The collision rate is proportional to the concentrations of the reactants, and the reaction rate is described as the fraction F of collisions that overcome the necessary activation energy as outlined in Eqs. [1] and [2]. Then, the product between the collision constant C and the fraction of successful collisions F can be combined to the theoretical rate constant ktheoretical as shown in Eq. [3].[11] Collision rate / C ½A ½B
½1
Reaction rate ¼ F C ½A ½B
½2
Reaction rate ¼ v ¼ ktheoretical ½A ½B
during the chromium oxide reduction dissolves in the liquid phase. Then, the exact reaction steps can be ignored, leading to the reaction shown in Eq. [7].
½3
Many reactions are found to follow a reaction model in the form of Eq. [5]. It describes the generalized form of a simple order model for the reaction in Eq. [4].[12]
v¼
½A þ ½B ! ½C
½4
dA ¼ kðTÞ ½Aa ½Bb dt
½5
ðCr2 O3 Þ þ 3½C / 2½Cr þ 3fCOg
Assuming a constant carbon concentration and low concentrations of chromium in the liquid metal phase
