Jouko J. Härkki

Optimisation of steel plant recycling in Finland: dusts, scales and sludge

Abstract The quantities of the dusts, scales and sludge arising in the steel plants of Rautaruukki Group (two in Finland, one in Sweden and one in Norway) and Imatra Steel (in Finland) have been determined. Not all materials can be recycled, a material may contain some harmful component(s) like Na, K, Zn, Pb, Cd, S, cyanide, oil or humus. It may also be too fine-grained and a small iron content may prevent utilisation or it may be too difficult to process the sludge. The total amount of the unutilised materials is 92 140 t/a, that is about 32% of the fine-grained remains. The recycling capabilities of the sintering plant, the blast furnace (BF) and the basic oxygen furnace (BOF) have been evaluated. Usually, the fine-grained remains disturb the primary production processes. Additionally, most of the non-recycled remains contain harmful components and cannot be utilised in the sintering process or in the BF. In the BOF, it is possible to use the iron oxide remains, but the maximum amount would be only about 10 000 t/a, so the recycling to the BOF can be only a partial solution. It is possible to recycle small amounts of appropriate dusts, scales and sludge to the sinter plant, to the BF or to the BOF, but this cannot be an overall solution of the waste problem. Big amounts of fine-grained remains must be treated separately. The costs of the separate process can be covered due to the savings caused by the more smooth-running primary processes. Moreover, it is possible to choose a separate unit process, the product of which can be fed back to the production chain in the optimal stage. The separate processing of the dusts, scales and sludge may also decrease the emissions and energy consumption of the steel plant.

Splashing mechanism in combined blowing

Abstract In combined BOF blowing, lance parameters and the combination of bottom or side wall tuyeres have an influence on splashing behaviour. The aim of this study was to clarify the interaction of the lance jet cavity with the bottom blowing plume and the side wall blowing jet and to determine its effects on splashing. According to the water model tests, three basic axioms existed in the combined blowing. First, when the bottom tuyere (or side wall tuyere) was located exactly beneath the lance jet, the lowered cavity turned the direction of splashes to lower trajectories. Second, the total amount of splashing was constant and the splashing peak was generated on the wall above the bottom plume. Third, both the plume and the side wall jet formed a so called protected zone beyond it. The model experiments showed clearly that the combination of bottom tuyeres and interaction of cavities and plumes play a very important role in splash generation in real converters.

Vibration of argon-oxygen decarburisation vessel during gas injection

Abstract Vibration of the molten bath surface in steelmaking vessels can enhance wear of the refractory lining, cause problems with the vessel support system and complicate control of the refining processes. The occurrence of vibration of the vessel during gas injection in argon–oxygen decarburisation (AOD) has been studied. The frequency and intensity of vibrations were measured using an accelerometer. According to a model and industrial tests, the intensity of vibration depends on the gas flowrate, gas composition, carbon removal rate, wear of the refractory lining and penetration of the gas jets. In AOD, the intensity of vibration is dominated by the blowing procedure, not by the carbon removal rate. Also, according to both industrial and model tests, there was a clear link between intensity of vibration and penetration of the gas jets in the liquid bath.

Behaviour of Coal associated Minerals during Coking and Blast Furnace Processes – a Review

Metallurgical coke is made from a mix of several types of coal that contain various minerals, including quartz, carbonates, feldspars, sulphides and a number of phyllosilicates, represented by clay minerals and micas. During coking and coke consumption processes the minerals undergo various physical and chemical changes, which in the case of coke oven batteries include desulphurization, decarbonation, dehydration, dehydroxylation, polymorph transformations, melting with the formation of an alkali and silica-rich liquid, and transformation to another crystalline phase. The typical modifications of mineral phases in a blast furnace (BF) coke are polymorph transformations, melting, and the formation of another crystalline phase. These events are also preceded by notable changes in the shape of crystalline aggregates. All these changes affect the coke properties and the processes occurring in a BF. Mineralogical data can help us to better understand the processes taking place in coke batteries and in a BF, and to improve the quality of a coke by more careful selection of coal blends and setting optimal temperatures for its pre-heating. The degradation of a BF coke by alkalis can be reduced by the addition of certain minerals to the feed coke before charging into the BF.

Experimental simulations of primary slag formation in blast furnace

Abstract Primary slag formation of the blast furnace ferrous burden was experimentally simulated using synthetic MgO-Al2O3-CaO-SiO2 slags with FeO, Na2O, or FeS additions. The combined effect of FeO and Na2O or FeS was also examined. The melting behaviour and viscosity of five different base slags (sinter, pellet, or lump ore) were investigated using optical dilatometry, thermogravimetric-differential thermal analysis (TGA-DTA), and viscometric analysis. The results indicate the importance of FeO (wüstite) in the formation and nature of liquid primary slags. Solidus temperature, fusion temperature, solidus-fusion interval, and viscosity were all significantly affected by FeO. A clear, but not simple or linear, tendency showed a lowering of the solidus and fusion temperatures and a concomitant decrease of the viscosity with increased FeO addition. The presence of Na2O or FeS in the slag system, alone or combined with the FeO addition, created an initial melting at lower temperatures, but the liquid volume produced was limited. The effect of the added components was distinctly different on different base slag systems. It is proposed that the effect is fundamentally dependent on the chemical or, more exactly, on the mineralogical composition of the base slag systems at the initial stage of the melting. The results of the experiments are considered to represent reasonable simulations of the melting behaviour of the corresponding primary slags of the blast furnace ferrous burden in the cohesive zone.

Fe-Si droplets associated with graphite on blast furnace coke

Fe-Si droplets on the surface of blast furnace (BF) coke from 25 to 50 cm at the tuyere level are mostly composed of Fe3Si, which has various shapes (round, elongated, and irregular) and penetration degrees into the BF coke matrix. The shapes and penetration degrees may depend on the saturation of molten iron by silicon during interaction with the coke matrix. The droplets are covered by a tiny shell of carbon. Graphite observed inside the droplets can be divided into two categories: well-formed tabular crystals with relatively large size and flakes with structures similar as those in cast iron. The textures of the droplets reflect composition, interaction with the coke matrix, and cooling conditions.

Carbon Tubular Morphologies in Blast Furnace Coke

The paper reports on the first occurrence of microscale carbon tubular morphologies (CMTs) in a blast furnace (BF) coke. The CMTs were probably formed as a result of the conversion of solid disordered carbon via liquid phase metal particles involving a gas phase containing a substantial amount of N2 and O2. The presence of CMTs may lie behind the generation of the smallest fraction of fines in BF exhaust dust. If the amount of CMTs present in the BF exhausts gases at any particular metallurgical site proves to be substantial, it could become a subject of environmental concern.

Gas flow model in the pusher-type slab reheating furnace and flow pattern improvement by computational fluid dynamics

Although a lot of numerical simulations have been developed for the pusher-type reheating furnace, most of those focused on the heat transfer between gas phase and slabs. The flow pattern in the furnace has been seldom reported before. This paper describes the CFD simulation about a pusher-type reheating furnace combined with fluid dynamics, combustion and radiation models. Calculation results show a detailed flow distribution in the reheating furnace and indicate that reverse flow under the slab near the lower burner is the main reason for scales accumulation in the front part of the heating zone. A block wall was added ahead of the lower burners in the heating zone in this CFD simulation in order to study its influence on decreasing such kind of reverse flow. The average oxygen content was also calculated in the present model. The oxygen distributes evenly inside most parts of the furnace except the areas near the burners.