1. Introduction
The process of mini steel mills using oxidized pellets as a feedstock includes production of metallized pellets, which are used further for the production of direct reduction iron (DRI), production of steel in electric arc furnaces (EAF) and its continuous casting and rolling. Such companies are importing annually large amounts of the iron ore pellets for DRI production and during the transportation, stockpiling, charging of the pellets to the metallization reactors and discharging of the metallized pellets dozens of thousand tons of fine iron-containing materials are being generated. Pellets fines and DRI sludge are usually dumped in piles and EAF dust and mill scale are sold to a third-party. Results of the preliminary analysis indicate that the recycling of such materials in the form of briquettes would help to produce additional quantities of steel and release significant area occupied by dumped wastes. Utilization of these wastes would generate additional revenues, surpassing revenue from direct sales of wastes.

Previously we have studied the possibility to apply Stiff Vacuum Extrusion (SVE) for agglomeration of the natural and anthropogenic fine materials generated during ironmaking and Ferro Alloys production [1,2,3]. The main aims of the present study are: (a) to develop cost-effective procedure for recycling of the oxide materials, wastes of DRI production and EAF dusts, in order to improve the environmental situation in the enterprise; (b) to choose the efficient binder which will allow to achieve the highest degrees of briquettes metallization.

The present paper considers the possibility to use SVE to agglomerate the above-mentioned materials with the metallurgical quality suitable for use as a charge component in the DRI reactors.

2. Materials and Methods
Full-scale testing of the experimental Extruded Briquettes (brex) behavior took place in the industrial Midrex reactor. Experimental brex were placed inside deformable steel packages permeable for the reducing gas (Figure 1). Such input method allows to adequately simulate brex behavior in real conditions of the reactor, including mechanical pressure of the surrounding traditional charge. At the end of the process, these packages were drawn out of the reactor allowing to visually determine the condition of the reduced brex and to explore their chemical composition and properties.

Figure 1. Deformable steel packages for feeding of Extruded Briquettes (brex) into Midrex reactor.

2.1. Raw Materials
Prepared mix of pellets fines (52.6%); metallized sludge of DRI production (26.4%), mill scale (15.8%) and EAF dust (5.2%) has been used. Particle size analyses were performed using wet screens from mesh 4 (4.75 mm) to mesh 325 (45 μm). Particle size distribution of the mix before and after its grinding by the lab-scale roll-crusher is given in Figure 2.

Figure 2. Particle size distribution of the briquetted charge. Lower—original mix; upper—after grinding.

Table 1. Chemical composition of the mix components.

2.2. Experimental Brex Production
All test samples were extruded using a 25 mm round pelletizing die. A Hobart laboratory mixer was used to simulate the mixing with water and pugging of the ground feed material in the open tub of the pug sealer. The laboratory extruder simulates the processing of the material through the sealing auger and die, into the vacuum chamber, and then final extrusion.

The laboratory extruder consists of two chambers with a sealing die between. The rear chamber is fitted with a 3-inch diameter sealing auger that pushes material through the sealing die. The second chamber can be subjected to vacuum. It is also fitted with a 3-inch diameter auger that extrudes material through a pelletizing die. A PC-based data system monitors and records extrusion data. All mixes were extruded immediately after mixing. Three types of the experimental brex were produced with the compositions given in Table 2.

Table 2. Experimental brex compositions, %.

Magnesium sulfate-based binder has been composed based on the heptahydrate of the magnesium sulphate (MgSO4·7H2O). Binding properties of the magnesium sulphate were first described by Zhuravlev et al. [4]. The change in form that occurs when a dehydrated inorganic salt is converted to the hydrated crystal form is the basis for the recommendation that a partially dehydrated magnesium sulfate be used industrially as a binding material. Setting begins in 3 min and is complete in 6 min. Chemical composition of the brex is given in Table 3.

Table 3. Chemical composition of experimental brex before reduction, %.

2.3. Testing of the Brex Physical Properties
A calibrated electronic scale with density measuring attachment was used to determine brex density. A compression tester was used for sample strength measurements, tensile splitting strength has been measured in accordance with ASTM (American Society for Testing and Materials) C1006-07, Paragraph 7.1. Porosity has been measured in accordance with DIN (Deutsches Institut für Normung) 51056. Moisture content was measured using a moisture balance. The physical–mechanical properties of the raw brex are given in Table 4.

Table 4. Physical–mechanical properties of raw brex.