Optimal hot metal desulphurisation slag considering iron loss and sulphur removal capacity part I: fundamentals

Abstract

In hot metal desulphurisation (HMD) the slag will hold the removed sulphur. However, the iron that is lost when the slag is skimmed off, accounts for the highest costs of the HMD process. These iron losses are lower when the slag has a lower viscosity, which can be achieved by changing the slag composition. A lower slag basicity decreases the viscosity of the slag, but also lowers its sulphur removal capacity, therefore optimisation is necessary. In this study, the optimal HMD slag composition is investigated, considering both the sulphur removal capacity and the iron losses. In part I the theory is discussed and in part II the optimal slag is validated with plant data, laboratory experiments and a thermodynamic analysis.

Desulfurization of High‐Sulfur HIsarna Hot Metal

“…A slag with a low liquid fraction has a high apparent viscosity, which leads to high iron losses. [6,[26][27][28] Furthermore, a high solid fraction decreases the reaction surface and therefore the reactivity, of the slag, hampering the reaction between MgS and CaO (Reaction 2). As MgS, in contact with oxygen from the air, thermodynamically favors the formation of MgO, the sulfur would return to the hot metal, a phenomenon called resulfurization.…”

“…Also, the addition of small quantities of alkali metal oxides, like Na 2 O and K 2 O, could lower the slag's melting point and viscosity. [26] It would require specific equipment to add the synthetic slag, but such a procedure would lead to lower iron losses and less slag that needs to be treated after the HMD process, eventually leading to lower costs. Alternatively, BF slag (if available) could be used instead of synthetic slag, which would lead to roughly the same slag composition as is common today at the HMD.…”

“…This will, despite slag modifiers, lead to higher iron losses. [26,27] Apart from the iron losses, the slag skimming time will increase when there is more slag to remove, leading to a longer total process time and higher temperature losses.…”

“…Also, the addition of small quantities of alkali metal oxides, like Na 2 O and K 2 O, could lower the slag's melting point and viscosity. [ 26 ]…”

“…It is not certain that a slag modifier could completely compensate for this, as its use is limited by slag basicity (a too acidic slag would not desulfurize) and alkali metal oxide concentration (slag that is recycled at the BF cannot contain too much alkali metal oxides, because these will build up inside a BF). [ 26 ] Furthermore, the total slag volume will increase, because more reagents are added to remove the higher amount of sulfur in HIsarna hot metal. This will, despite slag modifiers, lead to higher iron losses.…”