Semykina A. Improvement of recycling processes of metallurgical slag based on investigation of the formation of magnetic iron-containing compounds from oxide melts

The dissertation is devoted to problems of utilization/recycling of metallurgical slag based on studies of the formation of magnetic iron-containing phases from oxide melts. An approach to utilize steelmaking slag constituents based on a transformation of non-magnetic iron-based phases to magnetic iron-bearing compounds of magnetite and/or spinel-type ferrites (manganese ferrite) by an oxidation technique has been proposed by the current author. This approach is practically effective, ecological and economical as will be explained below. In this technique, air is used to produce an oxidizing atmosphere. This allows selective recovery of iron-bearing and non-iron-bearing slag constituents for the specific industrial purposes. From the technological point of view, a pre-treated slag is processed by applying a magnetic field where iron oxides, transformed to a magnetic form, are separated for a further utilization. Magnetic products may be used as components for a sintering mixture or for pelletizing iron ores. The remaining portion of the slag (non-magnetic) can be purposefully used for various applications such as a production of cement binder. During the study, the optimal conditions to facilitate the formations of magnetite and manganese ferrite were determined based on thermodynamic calculations performed by using FactSage 6.1 and experimental results from kinetic studies of FeO-oxidation in synthetic molten slags (FeO-CaO-SiO2-MnO and FeO-CaO-SiO2). The kinetic studies were conducted by combining advantages of Confocal Scanning Laser Microscopy (CSLM) and a Thermogravimetric technique. The present oxidation method can be practically applied to slags which are produced in the industry as well as to slags which are dumped in landfill. The utilization of metallurgical slags will allow saving natural resources, decrease the use of slag dump sites, and reduce contamination of environment. The current method of slag oxidation does not require a considerable capital investment as it can be integrated into the existing wastes utilization technologies. It is possible to extract approximately 0.2-0.25 t of magnetite from 1 t of slag, which is equivalent to about 0.5 t of medium grade iron ore. The expected total economic impact may reach 120 UAN/t of slag. Furthermore, it was experimentally shown the possibility of the formation of nanosized manganese ferrite in the spinel form from the MnO- and FеО-containing slags during oxidation in air. It was experimentally shown that obtained manganese ferrite from the oxidation of the slag with basicity equal to 2.0 was determined to be approximately 25.5 nm in size. In addition, a potential way of separating magnetite particles from liquid slags under electromagnetic buoyancy force in crossed electric and magnetic fields, was investigated by cold model studies.