Preparation of the Iron Oxide Red from the Converter Dust by the Magnetic Separation and Roasting Process

Preparation of iron oxide red (α-Fe2O3) from the converter dust by the superconductivity high gradient magnetic separation (S-HGMS) and roasting process was investigated in the paper. The basic properties of the dust were studied by the X Ray Fluorescence, the chemical analysis and the X Ray Diffraction methods. The results showed that the raw dust mainly contained elements of Fe‵O‵Si‵Ca, the iron content of the raw dust was 61.80%, and there were ferrous phases of Fe3O4, α-Fe2O3, Fe2(SiO4) and CaFe(Si2O6) in the raw dust. Under the optimum conditions of magnetic field intensity of 1.8T, the dispersion agent of 30mg/L and velocity of 500mL/min, the powders absorbed by the magnetic medium mainly contained Fe3O4 and α-Fe2O3, and the iron content of powders absorbed was up to 65.90%. The Fe2+ content of the powders absorbed under the optimum magnetic conditions dropped to 0.25% from 19.10% after roasting of fifty minutes, and the iron content of powders absorbed under the optimum magnetic conditions fell to 64% due to oxidation, and the Fe3O4 was removed. Finally the α-Fe2O3 content was up to 91.07% in the iron oxide red.


Introduction
In recent years, the converter dust mushroom with the rapid development of the iron and steel industry in china. Due to the high iron content, the dust is usually used as coolant in the converter by making the heated pressed block [1]- [2], and the dust also can be used as sintering burdening [3]. But the K, Na, Zn, S in the dust are bad for the ironmaking and steelmaking process or the steel quality. The high-iron-content dust can also be made into the high purity α-Fe 2 O 3 . The α-Fe 2 O 3 has excellent light fastness, high temperature, alkali resistance and corrosion resistance. It can be made into the high temperature resistant coatings [4], the anticorrosion coating [5], strong acid resistance floor coating [6], ceramic colorant [7] and nano iron oxide material [8]. The dust contains magnetite and weakly magnetic hematite, and more weakly magnetic minerals. The hematite only is absorbed by high magnetic field. The experiment magnetic equipment in the study has the maximum magnetic intensity of 5T.

The raw material
The chemical compositions and ferrous phases of the converter dust were studied by the XRF-1800 X Ray Fluorescence, the chemical analysis and the D8 ADVANCE Cu X Ray Diffraction. The results were showed as the table 1 and the figure 1.According to the figure 1, Fe is in form of Fe 3

The process route
The magnetic intensity is the decisive fact, and the particle dispersion and velocity of flow are the secondary facts. The optimum conditions which Fe 3 O 4 and α-Fe 2 O 3 were absorbed mostly, and Fe 2 (SiO 4 ) and CaFe(Si 2 O 6 ) were not absorbed as far as possible by the magnetic medium was found in the experiment. The experiment was based on the other certain empirical facts, such as the size and concentration of particles, and the magnetic medium filling ratio. The powders absorbed were roasted in the temperature of 250℃, because Fe 3 O 4 turned into α-Fe 2 O 3 [9] under 250℃. The process route was showed as the figure 2.
According to the formula 2-1, the magnetic susceptibility K, the particle volume V and the magnetic gradient dH/dx are certain, thus F m is proportional to the magnetic intensity H. Fe 3 O 4 is totally absorbed under the low H , and when the intensity is under 1.8T, the more higher the H is, the more α-Fe 2 O 3 are adsorbed, when the intensity is out of 1.8T, the more higher the H is, the more particles which the H is less than α-Fe 2 O 3 are adsorbed by steel wool, so the total content of Fe 2 (SiO 4 ) and CaFe(Si 2 O 6 ) increases.   According to the figure 4, in a certain range, the iron content of powders absorbed increased with the increasing of the dispersion agent. And when the dispersion agent concentration reached 30mg/L, the iron content reached the maximum of 65.90%, then the iron content declined with the rise of the dispersion agent. Na 6 (PO 3 ) 6 ionizes out the PO 3 -1 , the PO 3 -1 is absorbed by the particles surface, which is called the surface ion. Ions with the opposite charge in the medium are called the antiparticle. Some antiparticles are absorbed by the static electricity, which forms the close layer with negative charge. The antiparticles around the particles form the diffused layer. So the particles can repel each other by the electric double layer. But if the agent exceeds 30mg/L, more antiparticles will be absorbed by the close layer, the potential declines until the electric double layer disappears.

.The influence of velocity of flow on the magnetic separation
According to the formula 2-2, the kinematic viscosity u, the particle relative speed v and the particles radius b is certain. The F n is proportional to v, so the iron content declines with the increasing of velocity of flow.

The effect of roasting time on Fe 2+ content of powders absorbed under the optimum magnetic conditions
The powders absorbed under the optimum magnetic conditions were roasted in the air under 250℃ with the different time.  The According to the figure 6, the Fe 2+ content declined with the increasing of the roasting time. The Fe 2+ content reached 0.25% after the roasting of fifty minutes. Then the Fe 2+ content was almost unchanged after more roasting time, so Fe 3 O 4 was removed. The iron content of powders absorbed under the optimum magnetic conditions fell to 64% due to oxidation.The particle morphology of iron oxide red was showed as the figure 7.