2.2.2. Charging of Scrap Baskets

Several aspects that affect the scrap charging either directly or indirectly must be highlighted. The various steel grades have pre-specified recipes, i.e., charge types, which are used to determine how much of each scrap type that will be charged into the baskets. This, in its turn, is determined by the selection of primary scrap and alternative scrap. Primary scrap is always used if it is available in the scrap yard. Alternative scrap is used if the primary scrap is not available. Each primary scrap type has a matching alternative scrap type. Hence, the recipe is a representation of an ideal charged basket, but not necessarily representative of the actual charged basket.

Available scrap types on the market also affect what charging strategies are possible. The available scrap types in the market set the limit to what performance can be expected with regards to the EE consumption. If a major scrap type that requires less EE to melt is not available, then the EE of the next heats could be expected to increase, if a scrap type with higher apparent density is used instead. Since the scrap turn-over rate can be as low as one week for several scrap types, varying available scrap types are thus to be expected. Customer demand also influences the scrap requirements since scrap with higher amount of tramp elements cannot be used for steel types with high performance

requirements. The market factors and customer demand variations will not be accounted for in the numerical experiments. However, it is important to keep these factors in mind since they indirectly influence the EE consumption.

Some charging strategies are subsequently described. The first basket is usually filled with scrap to 75–100% of the total volume of the basket. The total volume of a scrap basket is 65 m3. The second scrap basket is filled to reach the pre-specified amount of molten steel, which is approximately 100 tonnes. A third scrap basket is used if the amount of required scrap was not satisfied by the first two baskets.

Each scrap basket can be made up of a total of 10 layers of different scrap types. The scrap types in each layer are pre-specified by the scrap recipe. Combined, the layers will consist of a blend of various scrap types. Bulky scrap with high apparent density (1.4 ≥ tons/m3) such as internal casting residuals are put in the bottom layers of the basket. This ensures that the bulkier scrap gets a longer exposure to the steel melt and thus reduces the risk of solid scrap pieces in the latter stages of the EAF process. Consequently, scrap with lower apparent density (0.4–1.0 tons/m3) such as sheet and turnings are put in the intermediate and upper layers. These scrap types do not require as long exposure to the steel melt compared to other scrap types. Furthermore, the main source of heat for melting these scrap types, most likely, comes from the radiative heat transfer from the electrode arcs.

Some effect of compaction of scrap types with lower apparent densities is expected due to the weight imposed by the upper layers of scrap. Hence, the apparent density of these scrap types will be higher in a fully charged basket compared to when they are solely present in the basket or in the scrap yard. The apparent density of the scrap in the charged furnace can be assumed to have a reasonable agreement with the apparent density of the scrap in the scrap bucket post-charging.

#### 2.2.3. Parameters Governing the EE Consumption

Several previous articles have presented estimates of the energy sources and energy sinks during the EAF process [14–18]. These reported values have been compiled in Table 1 and presents a guidance regarding which input variables that need to be considered when predicting the EE consumption. The reason behind the large percentage differences in energy sources and energy sinks is because one of the studies compiled information from 16 EAF [14], some of which used up to 90% Direct Reduced Iron (DRI) and 10% scrap as raw materials. Due to the gangue content of the DRI, mainly *SiO*2, more slagformers must be added which increases the energy consumption. Furthermore, DRI contains some remaining iron oxides that will be reduced by carbon in the melt. This also requires additional energy. In addition, the amount of injected oxygen per ton charged raw material ranged from 5 m3/t to 40 m3/t, which contributes to the large difference in oxidation of alloying elements [14]. The other articles provided data on one EAF each, three of which used 100% scrap [15,17,18] and one that used an unspecified mix of scrap and DRI [16].

A further discussion about the choice of input variables related to the energy balance equation, but not related to scrap charging, has been published previously [1].


**Table 1.** Synthesized values of energy sources and energy sinks reported in [14–18].

In addition, the EE consumption is partly governed by the selected scrap mix in the steel plant. This has been schematically illustrated in Figure 2. The available scrap in the scrap yard sets the limit to the charged scrap mixture in each basket. The charging strategy, comprising of both the scrap types and the layering of the basket, determines the exposed surface area of each scrap piece to the hot heel and the furnace arcs during the process. The burners and oxygen lancing facilitate the melting of the scrap. These factors in combination determine the aggregated melting time of the baskets combined. An increased melting time contributes to a longer TTT, which increases the total heat losses during the process. The increased heat loss has to be counteracted by increased amount of energy sources, which is a combination of EE consumption, burner fuel, and oxidation by alloying elements in the scrap (Table 1).

**Figure 2.** The relationship between the available scrap and the scrap contribution to the Electrical Energy (EE) consumption.
