Abstract

     DEM computer simulation was implemented to reveal the effects of different parameters on the efficiency of the roller screen. The numerical simulations were conducted under different device settings. In addition to the overall efficiency, further index was defined to determine the portion of cooperation of the rollers with different locations along the length of the device. It was revealed that the increase in the deck angle and rollers diameter reduced the overall efficiency while there was an optimum speed delivering the highest efficiency. It was concluded that the local cooperation share also depends on the introduced factors. In addition to the efficiency as an operational aspect, mechanical strength criteria were also considered to avoid probable device failure. Discussions were drawn concerning the obtained results and probable physical reasons of the obtained dependencies. The results can assist pelletizing factories to reduce their overall cost owing to increasing the process efficiency.

Introduction

     Iron and its alloys are the most important engineering materials utilized in industries. After the Second World War, the demand for all engineering materials was increased rapidly, with iron still ranking first. There are different approaches to obtaining metallic iron from iron ore concentrate including blast furnace and direct reduction. In the blast furnace technology, powdered iron ore concentrate with a size of tens of microns is charged into the furnace. On the other hand, in the reduction process the charge into the reactor is in the form of pellet. Via chemical reactions between the charged natural gas and the iron pellet in the reactor, oxygen is eliminated from either Hematite (

) or Magnetite (

) to obtain pure iron (

). Such process results in iron sponges suitable for subsequent melting and casting process. The diameter of the pellets should be in a certain range, preferably 8-16 mm, to have a successful reaction in the direct reduction process. Therefore, iron pellet is the main requirement for making sponge iron, and is made via the agglomeration of iron ore small particles in a rotating drum or rotating disk (flange on the rim). The agglomeration occurs due to different reasons including the surface tension of the water around the particles, adhesion of the binder, wan der Waals forces, magnetic force associated with the paramagnetic property of the particles and physical interlocking of the particles. The green pellets produced in the balling drum/disk are sent into the induration furnace to be heated up to 1300 °C and then cooled down to the ambient temperature. The objective of the present research is studying the screening process of the green pellets prior to feeding the induration furnace; thus, for more illustrations about other processes, interested readers can refer to relevant references such as Tupkary and Tupkary (2013).

Numerical results and discussion

     As depicted in Fig. 7 the pellets can be categorized into four groups including: ‘Much undersized’ that are much smaller than the gap between the rollers and creates the least trouble for passing through the gap and therefore this group takes critical degree of 1. The pellets that are much larger than the gap, named ‘Much oversized’, stay on top of the batch and do not remain inside the trap between the rollers and they can travel to the end of the screen with minor trouble and hence are categorized as critical degree of 2. The pellets that are slightly smaller than the gap named ‘Near undersized’ have a high probability of coming inside the trap between the rollers. They can pass through the gap although not easily and hence take critical degree of 3. The pellets a little larger than the gap named “Near oversized”, and create the most serious problem for the screen as they remain inside the trap between the rollers similar to a wedge causing blinding the gap. Therefore, the highest critical degree, 4, is assigned to this group of pellets.