Numerical and Experimental Analysis of Dross Generation in Continuous Galvanizing Operations

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Ajersch, F.; Koutsaris, C.; Ilinca, Florin; Goodwin, F. E.

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Numerical and Experimental Analysis of Dross Generation in Continuous

Galvanizing Operations

F. Ajersch, C. Koutsaris École Polytechnique de Montréal

F. Ilinca

National Research Council, Industrial Materials Institute F.E. Goodwin

Numerical and Experimental Analysis of Dross Generation in Continuous

Galvanizing Operations

F. Ajersch, C. Koutsaris, F. Ilinca, F.E. Goodwin

ABSTRACT

Numerical analyses of the aspects of the generation of dross formation in a continuous galvanizing bath are presented. Previous studies have shown that the rate of dross formation within the bath depends on bath composition, temperature and flow dynamics of the liquid zinc. Experimental observation however, has shown that the top dross skimmed from the bath surface consists of a mixture of intermetallic dross particles, oxide particles and zinc forming a stable layer at the surface of the bath. These oxides are produced by the reaction of the air impacting on the strip and on to the surface of the bath generated by the flow from the air knives. Numerical simulations were carried out to determine the air/nitrogen velocity in the vicinity of the steel strip and at the bath surface in order to determine the propensity of oxidation and the formation of dross. Bench scale simulations were also carried out to simulate dross formation. The experimental system consists of a graphite crucible containing zinc at 460˚C which is stirred using a six-bladed impeller. A jet of either nitrogen or air was directed on to the bath surface simulating the flow from the air knives. The rate of skimmings produced was monitored for variable conditions of bath compositions and stirring rates. A numerical simulation of the liquid flow in the crucible was carried out to evaluate the surface area of the liquid surface exposed to the gas stream. The results of these tests showed that the rate of skimmings generated increased with the stirring rate. An air jet at the surface produced much higher rates than for the case of nitrogen. When comparing the rate of generation per unit area, the experimentally determined rates were found to be comparable to the rates obtained in industrial tests. It was also observed that the morphology and the composition of the skimmings from the two sources were very similar.

Numerical and Experimental Analysis of Dross Generation in Continuous

Galvanizing Operations

F. Ajersch, C. Koutsaris, F. Ilinca, F.E. Goodwin

INTRODUCTION

The minimization of dross formation continues to be of major concern in the efficient operation of continuous galvanizing lines. New steel compositions, high line speeds, variable bath composition and pot configurations all contribute to the factors that influence the nature of the operation and the resulting coated product quality. The generation of top dross particles (Fe2Al5Znx) in galvanize (GI) operations and of bottom dross particles (FeZn10Al) in gavanneal

(GA) operations within the liquid bath volume has been previously reported [1]. This detailed numerical analysis of the non steady state operation illustrated the variation of the local Al and Fe content of the bath following a sequence of two ingot additions. The analysis consists of computational fluid dynamic calculations to determine the local velocity, temperature and compositional variations of Al and Fe in the flow field. By assuming that these constituents attain their solubility limit at the local bath temperature according to the Zn-Al- Fe phase diagram, any excess Al from ingot addition or of Fe from strip dissolution will generate these inter-metallic dross particles. The calculated amount of internally generated dross particles was found to be relatively low, in the range of 3-5 kg for a two hour period including two ingot additions in a 250 ton pot during a GI operation. The rate increased to about 20 kg for the case of a GA operation at a nominal Al bath content of 0.10 % which would theoretically only generate bottom dross. These values still represent a very small portion of the mass that is normally skimmed from the surface of a galvanizing operation. These skimmings, generally called dross, can be of variable composition depending on the operating line.

In a further study sponsored by ILZRO [2] pot skimmings samples were taken from two operating lines (ArcelorMittal - Cleveland and US Steel - Hamilton) and analyzed for Al and Fe content and examined by optical and electronic microscopy in order to identify the nature of the material that is skimmed periodically during operation. The average values of the skimmings compositions are given in Table 1.

Table I: Chemical analysis of skimmings

Operation GA GI Wiping gas

Al Fe Al Fe

AM - Cleveland 0.195 0.065 0.716 0.566 N2

USS - Hamilton 0.789 0.625 N2

These compositions indicate that the skimmings are enriched in Fe and in Al with respect to the bath composition due to the formation of intermetallic particles as well as to the formation of oxides of Al, Fe and Zn. Microstructures of typical samples of skimmings are shown in Figures 1 and 2. As can be observed from these microstructures, the bulk of the material consists of a heterogeneous mixture of zinc of the same composition as the bath with a dispersion of particles of inter-metallics and oxides. It appears that these skimmings form a stable porous layer that remains on the surface of the bath throughout the galvanizing operation [3]. It was shown previously [2] that the rate of formation of these skimmings varies with line speed, GA or GI operation, air or N2 wiping pressure, and with the type of gas used in the knives.

(a) Secondary electron image (b) Backscatter electron image (c) Oxygen distribution

(d) Aluminum distribution (e) Iron distribution (f) Zinc distribution

Figure 1: Micrographs of bottom dross (FeZn10) section sampled from ArcelorMittal GA process using nitrogen wiping [2].

(a) Secondary electron image (b) Backscatter electron image (c) Oxygen distribution

(d) Aluminum distribution (e) Iron distribution (f) Zinc distribution

Figure 2: Micrographs of dross section with oxide films sampled from ArcelorMittal GA process using nitrogen wiping [2].

The objective of this paper is to analyze the conditions of dross (skimmings) generation in bench scale simulation experiments and to compare the results with the rate of skimmings generation observed in industrial operation. In addition, numerical simulations are performed to investigate the flow dynamics of the simulated experimental configuration in order to obtain the necessary parameters for the gas –liquid contact surface as a function of mixing speed.

METHODOLOGY Bench scale experiments

The experimental apparatus consists of a graphite crucible containing liquid zinc at 460°C which is stirred using a six bladed steel impeller shown in Figure 3. The crucible has a diameter of 16.5cm and a height of 23cm. The maximum load of the crucible never exceeded 20kg in order the avoid spilling at high mixing rates. Furthermore, the steel impeller served as a continuous source of iron to the system simulating the contact with steel coils in the continuous galvanizing process. An alumina lance, 5mm in diameter, delivered a jet of either nitrogen or air onto the liquid free surface at a flow rate of 10 SLPM. The lance was held approximately 5cm from the free liquid surface. The gas jet was used to simulate the effects of the gas wiping system in the industrial process. The rate of skimmings generated for variable rotational speeds was measured for zinc compositions corresponding to GA and GI operations. Moreover, the temperature of the bath and the oxygen content above the free liquid surface were monitored using a thermocouple and an oxygen probe. A second set of experiments was conducted under the same conditions using a shroud which prevented oxygen infiltration to the liquid free surface when a nitrogen jet was used.

Figure 3: Open (left) and shrouded (right) experimental skimmings generation apparatus

Numerical simulation

Numerical simulation of the bench scale experiment was carried out using a three-dimensional finite element method [4]. The free surface fluid flow is described by the Reynolds Averaged Navier-Stokes equations. The turbulent viscosity is computed using the two-equation k- model. The position of the free surface is tracked using a level-set method: a transport equation is solved for a scalar function whose values are positive inside the liquid region and negative outside of the liquid. Because the continuous rotation of the propeller blades implies changes in the geometry and in the corresponding mesh we solved the problem in a reference frame tied to the

blades. Therefore the model includes centrifugal and Coriolis forces introduced by the change of reference frame.

RESULTS Bench scale results

A series of bench scale experiments were conducted for GA compositions ranging between 0.11 and 0.14wt%Al. Two cases were examined: a shrouded crucible using both air and N2 impinging

jets and an open crucible using both air and N2 impinging jets. Table II summarizes the results.

Table II: Rates of skimmings generation under air/N2 and open/shrouded conditions

Shrouded crucible Open crucible

GA-N2 GA-Air GA-N2 GA-Air

T bath (°C) 461 458 458 467

[O2] above bath (mol%) 0.00 20.91 9.76 20.90

Generation rate (g/s) 100 rpm - - 0.0248 0.0359 200 rpm 0.0653 0.0914 0.0539 0.125 250 rpm 0.0812 0.141 - - 300 rpm 0.0771 0.303 0.291 0.805 Specific generation rate (g/s/m2) 100 rpm - - 1.15 1.66 200 rpm 2.30 3.21 - - 250 rpm 2.56 4.46 1,89 4.41 300 rpm 2.21 8.69 8.34 23.1

The average skimmings generation rate is the ratio of mass of skimmings produced over the duration of the experimental run. The average specific skimmings generation rate is the ratio of average skimmings generation rate to the free liquid surface area [5] for a particular rotation rate. Clearly, the use of an air jet significantly contributed to the formation of skimmings especially at high impeller rotation rates. Moreover, it is evident that shrouding the bath over a N2 jet inhibited

the formation of skimmings to a greater degree than all the other cases. Figure 4 below is a plot of average specific skimmings generation rate as a function of impeller rotation rate for GA and GI compositions ranging between 0.11-0.14wt%Al and 0.16-0.19 wt%Al respectively. The data collected would suggest that at low rotation rates, the specific rates of both GA and GI bath compositions produce similar amounts of skimmings but that is no longer evident at higher rotation rates. Furthermore, the specific rate of skimmings generation for GA remains relatively constant over the range of rotation rates used.

Figure 4: Specific rate of skimmings generation vs. impeller rotation rate for both GA and GI Figure 5 below compares the limiting cases of a shrouded crucible using nitrogen and an open crucible using air for a GI composition ranging between 0.16wt%Al and 0.19wt%Al. The data depicts the contribution of air to the amount of skimmings formed at high rotation rates.

Figure 5: Effect of air on the specific rate of skimmings formation for GI

Numerical simulation

The goal of the numerical simulation is to determine a correlation between the rotation speed of the blades and the shape and velocity of the zinc free surface, which then in turn can be correlated to the quantity of dross (skimmings) and oxides generated [6]. Simulations were therefore carried out for three values of the rotation speed: 100, 200 and 300 rpm. The initial level of the liquid zinc was set at either 12 or 14 cm, thus setting the total quantity of zinc in the crucible. The flow solutions for the three rotation speeds and the 14 cm initial height are shown in Figure 6. The colour on the free surface, bottom wall of the crucible and blades is from the

0 1 2 3 4 5 6 150 200 250 300 350 S p e ci fi c sk im m in g s rate (g/s/ m 2) RPM Shrouded-N₂: GA vs. GI GI GA 0 5 10 15 20 25 50 150 250 350 S p e ci fi c sk im m in g s rate (g/s/ m 2) RPM

GI: Shrouded-N2 vs. Open-Air

Shrouded-N2 Open-Air

velocity magnitude. As can be seen, at faster blade rotation speed the velocity of the zinc and the deformation of the free surface are larger. For the higher velocity the free surface gets very close to the blades. It is also expected that the turbulent oscillations in the flow near the surface to be more important as the rotation speed increases. However, the numerical model solves only for the mean flow. The turbulent flow is characterized by the turbulent kinetic energy which is proportional to the square of the velocity. The distribution of the velocity at the free surface is shown in Figure 7 using the same scale for the three solutions. It is clearly seen that the zinc velocity increases when increasing the rotation speed. As expected, higher velocities are observed near the outer boundary (crucible wall).

The position of the free surface as a function of the blade rotation speed is shown in Figure 8 for the two values of the initial zinc level in the crucible. The vertical position of the free surface is shown with respect to the initial level. Hence, a non perturbed surface would have been located at y=0. The solutions indicate that the free surface displacement is not affected by the initial zinc level for the range of values considered. Note also that the displacement of the free surface increases rapidly when increasing the rotation speed. This translates into a sharp increase of the size of the free surface as can be seen in Figure 9(a). The graphs in Figure 9 show the dependence with respect of the rotation velocity of the size of the free surface and of the zinc velocity. The size of the free surface is expected to affect directly the quantity of dross and oxides formed. The zinc velocity is also an important factor, as higher velocity translates into a higher level of turbulence at the free surface which in turn may determine increased oxides formation. Higher velocity and turbulence of the liquid zinc may determine a faster regeneration of the zinc at the free surface and thus increase the possibility of oxidation.

(a) 100 rpm (b) 200 rpm (c) 300 rpm

(a) 100 rpm (b) 200 rpm (c) 300 rpm

Figure 7: Velocity of liquid zinc at the free surface (view from above the crucible)

(a) Initial zinc level = 12cm (b) Initial zinc level = 14cm Figure 8: Position of the free surface for different initial zinc levels

(a) Size of the free surface (b) Mean velocity at the free surface

Comparison of results from Bench Scale and Industrial Operation

A comparison of the rate of skimmings generation from the the ArcelorMittal–Cleveland operation [2] and the bench scale simulation is presented in Figure 10. The industrial data represents average values taken for a four month period for a range of line speeds for both GA and GI operations. Results are averaged over this period for GA (0.11-0.14% Al) and GI (0.18-0.20% Al) compositions for both air and N2 wiping. The skimmings generation rate was

determined from the material collected by the robotic skimmer and was calculated in units of grams per second per square meter of strip below the air knives. The previous results [2] have shown that coating weight and air knife pressure have little effect on the skimmings rate. The most significant factor detected was the effect of line speed. An average value of strip width of 1.30 m and an air knife position of 0.50 m above the bath was used to calculate the rates of generation. The results clearly showed the variation of the rate as a function of line speed in the range of 1.50 to 2.50 m per second.

Figure 10: Rate of skimming generation as a function line speed

A similar value for the skimmings generated from the bench scale experiments are also shown in Figure 10. The numerically simulated value of the free surface of the stirred crucible as a function of rotational speed was used in the calculation as well as the average surface velocity of the free surface.

It can be seen that the values of the industrially generated skimmings are considerably higher than those of the bench scale experiments. However, at high rotational speeds representing an average surface velocity of 0.82 m per second and when using an air stream at the surface, the rate approaches the value of the industrial rate at a line speed of 1.52 m per second. It was not possible to increase the stirring rate above 300 rpm without causing severe splashing and metal expulsion from the crucible.

Even though the actual flow conditions for the industrial operation is very different from the bench scale experiments, the parameters of line speed and surface velocity in the crucible are realistic values that can be compared, which can represent the liquid zinc velocity in contact with the air or N2 stream. It is highly probable that the free liquid surface area for both the exiting

strip in the industrial case and that of the stirred crucible is larger than the values used in the calculations due to the increasing turbulence at the surface with increasing velocity. The data presented in the Figure 10 indicates this tendency in that the skimmings rate appears to increase almost exponentially as the line speed increases. It is well known that very high line speeds can cause splashing and liquid surface instability leading to excessive skimmings generation.

Discussion

The overall results indicate that the rate of skimmings generation and the physical and chemical characteristics of the skimmings from the two sources are very similar. The morphology is the same consisting of a dispersion of fine particles of oxides of Fe and Al together with intermetallic particles of either or both of top and bottom dross in a porous matrix of zinc. The fact that the generation rates can be favorably compared as a function of liquid zinc velocity validated the use of the stirred crucible experiments in simulating the conditions of skimmings formation in industrial operations. It is very difficult to accurately determine the effective liquid zinc surface area that is in contact with the surrounding gas for the case of the air knives or for the case of a gas jet impinging on the surface of a stirred crucible. This interfacial area is the reaction site for the formation of oxides which eventually end up in the skimmings. In terms of the reaction mechanism it is assumed that the rate of the reactions is controlled by the liquid phase mass transfer of the constituents in the liquid phase that react at the gas-liquid interface. This is due to the fact that liquid phase diffusion coefficients are about 5 orders of magnitude smaller than the gas interdiffusion coefficients, thus resulting in liquid phase mass transfer coefficients that are much smaller than the gas phase transfer coefficients. Increasing the turbulence of the liquid zinc flow also decreases the liquid phase boundary layer for mass transfer for Al and Fe from the solution to the gas-liquid interface where the following reactions occur:

2Al(l) + 3/2 O2(g) --- Al2O3(s)

2Fe(l) + 3/2O2(g)---Fe2O3(s)

These finely dispersed oxides have been identified in the microstructures of the skimmings generated in both the industrial and bench scale samples.

A more precise determination of the mass transfer rates of these constituents can be obtained by the design of a more rigorous experimental system where the interfacial area and the relative liquid zinc velocity can be accurately determined. This approach has now been undertaken by the authors in order to generate more precise mass transfer data.

Conclusions

Results are first shown for a bench scale experiment trying to reproduce the actual process tacking place at the surface of a continuous galvanizing bath. Simulations indicate that an

increase in the rotation speed of the immersed blades has several consequences which impact in the production of dross. The size of the free surface increases at higher rotation speed. The velocity of the liquid zinc and therefore the turbulence level near the free surface increases also when increasing the rotation speed. These remarks are in good agreement with experimental observations which indicate an important increase in dross formation at higher rotation speeds. The analysis of the industrial data showed that line speed and wiping gas pressure were the main variables influencing skimmings formation during nitrogen wiping. Moreover, air wiping significantly contributes to the formation of skimmings even at low line speeds.

The analysis of the experimental data showed that the skimmings produced at the bench scale were similar in composition and morphology to those of the ArcelorMittal industrial galvanizing line. The samples were heterogeneous mixtures of bath solution, intermetallic particles of top and bottom dross and zinc, aluminum and iron oxide films.

Furthermore, the experiments also showed that skimmings formation increased with mixing rate. GI alloys also produced more skimmings then GA alloys for all mixing rates. Finally, a shrouded crucible using a nitrogen jet exhibited the least amount of skimmings formed.

Acknowledgments

The authors want to acknowledge all the operating staff of ArcelorMittal and US Steel Canada who participated in coordinating and carrying out the sampling tests. The financial support of ILZRO is also greatly appreciated.

References

[1] F. Ajersch, F. Ilinca, J.-F. Hétu and F.E. Goodwin, “Numerical Simulation of the Rate of Dross Formation in Continuous Galvanizing Baths”, Iron & Steel Technology, pp. 93-101, 2006. [2] F. Ajersch, C. Koutsaris, S.G.Fountoulakis, M.A. Miller, F.D. Barrado and F.E. Goodwin, “Characterizing Top Dross Sampled from Galvanizing Lines Using Nitrogen and Air Wiping Systems”, Galvanizers Association Proc., 101st Meeting, Louisville, Kentucky, 2009.

[3] R. Thiourm, R. Zanfack, H. Saint-Raymond and P. Durighello, “Understanding Hot-dip Galvanizing Skimmings”, Galvanizers Association Proc., 100st

Meeting, Baltimore, 2008.

[4] M. Bright, F. Ilinca, J.-F. Hétu, F. Ajersch, C. Saliba and C. Vild, “Fluid Modeling of the Flow and Free Surface Parameters in the Metaullics LOTUSS System”, Proc. TMS Conf., 2009. [5] C. Koutsaris, F. Ajersch, F. Ilinca and F.E. Goodwin, “Experimental Simulation of Dross Generation for GA and GI operations”, Proc. Galvatech 2011.

[6] F. Ajersch, F. Ilinca and F.E. Goodwin, “Numerical Investigation of Dross Formation and Minimization in Continuous Galvanizing Baths”, Proc. Galvatech 2011.