Understanding the solidification of stainless steel slag and dust mixtures

Abstract

The solidification of a multicomponent stainless steel slag and dust composite has been studied by thermodynamic calculations using Factsage and analyses of samples using EPMA and Rietveld refinement of synchrotron X-ray powder diffraction after various cooling rates. At an apparent cooling rate of 1 K/min, the content of spinel (10.6 wt%) was less than thermodynamically calculated (16.6 wt %), largely because of difficulties in the diffusion of depleted ions (including Cr, Mn, and Ni) in the liquid with very gradual compositional gradients. Melilite showed a uniform but distorted crystal structure of P2 1 2 1 2 and its content (48.9 wt%) was larger than the calculated result (32.3 wt%). At apparent cooling rates of 10 K/min and 50 K/min, a slight decrease in spinel and a significant decrease in melilite were observed, and the spinel was divided into two regions with an identical space group of Fd 3m but with distinguished composition and lattice parameters. However, the amorphous proportion consistently increased with the cooling rate from 29.4 wt% at 1 K/min to 69.6 and 92.9 wt% at 10 K/min and 50 K/min respectively. K E Y W O R D Shazardous waste, phase transformations, spinels, X-ray methods

Selective elemental concentration during the solidification of stainless steel slags for increased Cr recovery with MnO addition

“…The other relatively recent area of research is selective Cr concentration into a predefined primary crystal phase and the subsequent phase separation of the Cr‐containing slags through controlled cooling. This third approach is in line with sustainable process development and makes full use of the residual heat of metallurgical slags by modifying the slags at high temperatures 2,12‐14 . This treatment includes the selective nucleation and growth of Cr‐containing primary and secondary crystals and subsequent gravity separation or magnetic separation methods.…”

“…Approximately 15 million tons of Cr‐containing slags classified as environmentally hazardous byproducts are generated annually worldwide during the production of stainless steel, and most of the slags are directly discharged to the environment 1‐4 . Under typical weathering conditions, the direct discharge of Cr‐containing slags into the environment will eventually result in Cr dissolution into water and soil (4Cr 3+ + 10H 2 O + 3O 2 = 4CrO 4 2− + 20H + ), damaging the ecology, hindering animal and plant growth and causing various diseases as Cr 6+ enters the human body through the digestive tract and skin 5‐7 .…”

“…However, optimizing the selective crystallization and phase separation method requires a fundamental understanding of the thermophysical properties of slag melts at high temperatures and their subsequent crystallization behavior. Kim and Sohn 13 and Jung et al 2 observed that the metal cations Fe, Cr, and Mn could be locally concentrated during the controlled solidification of a molten stainless steel slag and dust mixture from high temperatures to accelerate the formation of the primary crystalline spinel phase ((Mn,Fe)(Cr,Fe) 2 O 4 ). Wu et al 15 and Li et al 16 also showed that a higher FeO content and lower basicity (mass ratio of CaO to SiO 2 ) increased the selective concentration of Cr 2 O 3 into the Mg(Al,Fe,Cr) 2 O 4 spinel solid solution.…”

Crystallization Behavior and Growth Mechanisms of Spinel Crystals in Vanadium-Containing Slags

“…In addition, many studies on the solidification of a multicomponent stainless steel slag have been studied to obtain a high yield ratio of valuable elements. 20,21) The effect of the slag composition on the formation of spinel-type inclusions has been investigated to improve the purity of liquid steel. [22][23][24][25] Formation of V-rich spinel phase in CaO-SiO 2 -MgO-FeO t -Al 2 O 3 -V 2 O 3 -P 2 O 5 slags at 1 573 K has been discussed to separate the vanadium-concentrating phase.…”

Immobilizing chromium in stainless steel slags with MnO addition

“…[10][11][12][13][14][15][16][17] The consensus of these researches is to selectively concentrate the potentially hazardous Cr in a primary crystal spinel phase by controlling the crystallization during the solidification of molten stainless steel slags, which generally exists as a complex spinel (Mg,Fe,Mn,Ni)(Cr,Fe,Al,Ni) 2 O 4 dependent on the type of elements in the stainless steel slags and is significantly diluted in Cr making it uneconomical for subsequent retrieval. [12][13][14][15][16] In addition to the relatively low content of Cr in the complex spinel, which is not conducive to selective enrichment and recovery of Cr, studies on the leaching behavior of modified slags are limited. Samada et al 18 suggested that some portion of Cr can remain in the unstable Ca 2 SiO 4 phase and be released during the elution of stainless steel slags with seawater.…”