How much iron ore is needed to make one ton of DRI?
Direct Reduction: Technology Issues
Extracted from “Direct From Midrex” Article
An appreciable degree of confusion has arisen concerning the efficiency of direct reduction plants in converting iron ore (in the form of pellets or lump ore) to iron, specifically, regarding the quantity of ore required to produce one ton of DRI; the oxide to product ratio. This point is particularly important in light of today’s remarkably high iron ore costs.
Mostly the confusion derives from plant suppliers stating the efficiency of their plants while using differing ‘end points’ within the ore/product handling stream. Various options for boundary limits are shown in Figure 1. For instance, if one were to describe the system as beginning upstream of an oxide screen and another were to begin their system downstream of the oxide screen, the latter might appear to have better efficiency when stating the ore-to-product yield, as the number would be smaller. Unless the boundaries of the systems were clearly stated, and understood, the latter could boast of having superior technology even if that is nott he case.
There are three boundary limits of interest when analyzing iron oxide requirements in DR processes: across the DR furnace only, from up stream of the iron oxide screen through the product screen, and from upstream of the iron oxide screen through feeding of DRI to the EAF. These three cases are shown in Figure 1 and Table I describes the significant sources of yield loss through the EAF.
Boundary Limits 1: Across The Shaft Furnace Only
The factors that determine the amount of oxide required to produce one ton of DRI include the loss of oxygen, addition of carbon, oxide fines losses from screening, processing in the reduction furnace, and screening of DRI product. What is the theoretical value for direct reduction measured across the shaft furnace? This is represented by the smallest (red dashed) box in Figure 1 were there no fines or dust losses. Figure 2 shows how the oxide/DRI ratio varies with DRI metallization and carbon content for a good quality DR pellet. Assuming 94 percent metallization and 2.5 percent carbon, the the oreticaloxide/DRI ratio(or yield) is 1.35.The theoretical yield is the same for any type of shaft furnace DR process because it is only dependent on the chemistry of the iron oxide and the DRI.
The theoretical yield is unrealistic in practice because it does not take into account fines losses. A full scale, month long, commercial test was conducted some years back in order to demonstrate the MIDREX® Technology to a customer. This test showed that the MIDREX® Direct Reduction Furnace operating day after day with good quality pellets had fines losses of only 1.3 percent above the theoretical yield. Theoretical yield being the point at which only the mass changes caused by chemistry (reduction, carbonization and variances in gangue/total iron) are calculated with no adjustment for fines or dust losses, whatsoever.
Some claim that oxide consumption can be decreased by 2 percent to 4 percent by operating at higher gas pressure within the reduction furnace. Obviously, it is physically impossible to decrease the oxide consumption by this amount because it would be below the theoretical value.
Typical fines losses from the reduction furnace to the offgases in either the Midrex or a competitor’s process are one percent to one and half percent.
Boundary Limits 2: Upstream Of The Iron Oxide Screen Through The Product Screen
These limits are indicated by the purple box in Figure 1. Inmost cases when discussing the oxide/DRI ratio, these boundary limits are the ones of interest, with the figure generally ranging from 1.42-1.50. Most DR plants screen the iron oxide feed to reduce losses to the offgas and minimize flow problems in the shaft furnace. MIDREX Plants generally screen the oxide at 6 mm and 3 mm and feed the minus 6 mm/plus 3 mm material to the furnace at a controlled rate. This facilitates better material flow through the shaft furnace. Other plants often screen at 3 mm, resulting in the same net fines losses of two to four percent, but the lack of controlled feeding of the minus 6 mm/plus 3 mm fraction can result in more erratic furnace flow and lower process efficiency.
Many plants making cold DRI (CDRI) do not screen the DRI product for feeding to the EAF and screening is not necessary for plants making hot DRI. For the CDRI plants that do screen product, fines losses are typically two to four percent.
Summarizing the losses from oxide screening (2 to 4 percent), to the shaft furnace offgas (1 to 1-1/2 percent), and in product screening (2 to 4 percent), the typical value for either a MIDREX or other technology plant is 1.45 t of oxide per ton of DRI with 94% metallization and 2.5% carbon.
The best way to compare the actual oxide-to-product efficiency of various designs of direct reduction technologies is to evaluate the experiences of companies that operate plants from multiple suppliers. There are four companies that own both a MIDREX® Direct Reduction Plant and a plant of a competing technology. (These processes represent over 98 percent of all currently operating DR capacity that is fueled by natural gas.) Responses from Midrex questionnaires regarding comparison of MIDREX Plant’s oxide-to-product ratio to that of competitor plants show that the Midrex plant is better than the competitor, with an advantage of as much as 2.8 percent when the comparison includes DRI product handling through the product screen.
At today’s iron ore prices, this amounts to more than $10 million for each one million tons of DRI produced.
Boundary limits 3: Upstream of the Iron Oxide screen to EAF feed point
This is the relevant system for a DR plant discharging hot DRI (HDRI) and transporting it to the EAF in either a continuous or batch transport system and is shown as the green box in Figure 1. Therefore Midrex’s techniques for conveying DRI to the EAF were designed to be gentle in order to create as few fines as possible. There are three types of hot DRI (HDRI) handling designed by Midrex, according to the distance from the reduction furnace to the steel making furnace. If the reduction furnace can be placed immediately alongside the steel shop, Midrex can design the reduction furnace to direct feed the shop using a predominately gravity system. This is known as HOTLINK® . If the reduction furnace is within 200 meters of the steel shop, the MIDREX® Hot Transport Conveyor system is recommended. For longer distances, up to 40 km, MIDREX® Hot Transport Containers, analogous to the torpedo cars that are used for hot metal from blast furnaces,may be used.
A “real world” comparison can be made between two alternative continuous systems, the MIDREX® Hot Transport Conveyor and a pneumatic technology from a competitor, since both are in operation. Since the MIDREX Hot Transport Conveyor uses buckets to carry the HDRI, fines generation is minimal. In contrast, pneumatic systems use gas to blow DRI at high velocity through pipes to carry it to the EAF or to a product cooler. This method results in significant fines generation, as much as 8-1o percent. These fines may never make it into the steelmaking bath; instead they may be trapped in the slag layer or simply carried out in the EAF exhaust system.
Minimizing Oxide Handling Losses
To minimize loses, it is important to provide gentle handling for both oxide and DRI product at all steps throughout the materials handling system.An approximation of a typical oxide handling procedure is shown in Table II. (This is not to be construed as a best practices concept, but merely as a typical example.)
A primary guideline for any handling system is that drop distances are always to be kept to a minimum. If this is not possible, cushioning of the drop location should be provided by a system that is easily reparable. (The continuous stream of falling material will be damaging to almost any cushioning material, thus thought should be put into how to quickly and inexpensively repair the cushion.)
Normally the greatest drop and thus the most damaging step that oxide experiences is the loading of the ship as ore is dispatched from the mining company. For this reason, some buyers require sampling and surveying of cargoes at vessel discharge.
The next most damaging step is usually the unloading of the vessel, and third is likely tobe stacking of ore for storage after unloading. In each of these steps the rule to minimize the drop should be obeyed as much as practical. For unloading, which is almost
always performed with a clamshell bucket, one helpful technique is to employ a “soft unloading” whereby the bucket carrying ore from the ship is lowered onto the surface of the receiving pile of ore (whether in a hopper or alongside on the quay) before opening the bucket rather than allowing the bucket to open and drop the ore onto the pile. Similarly, a guideline for stacking is to keep the stream spilling onto the storage stack as short as possible. One technique is to always maintain the stacker close to the top of an existing pile, frequently moving the stacker along as it fills the space below itself, and with each movement of the stacker, only moving along a short distance of a meter, or so, always staying close to the peak of the storage pile.
For design of conveyor and transfer systems, again, drops must always be kept to a reasonably practical minimum.
Minimizing DRI Handling Losses
Table III shows the components of a typical DRI handling system.
Great care must be taken with DRI since its strength (crush strength)is less than half that of oxide. The superiority of Midrex’s handling systems is especially evident in the handling of hot DRI.
To summarize, confusion has been introduced into the marketplace for DR plants concerning the oxide/product efficiency (yield) of competing DR technologies. When doing this analysis, it is essential to carefully define the boundary limits of the System and it is recommended the customer for a new plant carefully educate himself regarding this subject. A typical theoretical minimum oxide/DRI ratio across the DR furnace is 1.35 at 94% metallization and 2.5% carbon assuming use of good quality DR pellets. More commonly, the oxide/DRI ratio is specified from the inlet of the iron oxide screen through the discharge of the product screen and on this basis the actual yield for a MIDREX Plant is typically 1.45 including fines losses. The best way to compare the performance of a MIDREX Plant with the competition is to discuss the subject with companies who own each of the competing technologies. These comparisons indicate that the MIDREX oxide consumption is equivalent to or lower than the competition.