Dross Particles Research Articles

Simulation of special flow affecting dross formation on steel strip in galvanising bath

The zinc flow in a galvanising bath is numerically simulated for three cases, and the flow field is analysed using three-dimensional views. It is found that the flow near the zinc surface directs from the strip to the ingot side when inductors are equipped and whether ingots are melting or not, and the flow direction is opposite to that near the zinc surface for the case without inductor and ingot, which does not exist during the bath operation process. Whirlpools are found to be formed between the snout and the bath side walls, which will increase the chance of dross particles touching the strip surface. Some effective measures are proposed to stop more dross from adhering to strip. This work will lay foundation for the optimisation of the galvanising process to improve the coating quality of steel strip.

Physical and Mechanical Properties of Aluminum Dross

The study on the physio-mechanical behavior of aluminum dross has been carried out. The amount of aluminum dross used varied between 50 and 90 wt %, while bentonite added to the dross varied from 10- 50 wt % with a fixed amount of water. Using dross particle sizes of 106 µm and 184 µm, 10 samples are produced from each particle size. The bricks are dried in still air for 24hrs at 31oC, oven drying at 110ºC for 24hrs and sintered at 450ºC for 8hrs. The bricks characteristics in terms of volume shrinkage, apparent porosity, bulk density, cold crush strengths and permeability are then evaluated. The results show that the 106µm particle size dross brick has the highest volume shrinkage of 24%, apparent porosity of 15% and peak bulk density of 1.9g/c.c. However, the dross brick exhibits relatively low cold crush strength of 940KN/m². The 106µm size bricks demonstrate a minimum of 85% permeability compared with 70 wt% of 184 µm bricks which may be due to variation in dross particles agglomeration. Given these results, the 106µm particle size brick can serve as acid refractory because its properties compared well with medium-alumina fireclay.

The Influence of Aluminium Dross on Cement Paste's Porosity

In this study aluminium dross, an industrial waste product from the aluminum casting industry which contains mostly aluminum, aluminium oxide, and volatile gases such as ammonia (NH3), methane (CH4) and hydrogen (H2). In this study it is used as an admixture in cement paste to observe its effects on the porosity and bulk density of the material. The parameters used in the study include; the effects when different quantities of aluminium dross are added to portland cement (portland cement:aluminium dross by weight; 1: 0.05, 1: 0.10, and 1: 0.20) and the effect of different sized particles of aluminium dross (average diameter of aluminium dross particles; <45, 60, and 90 μm) on porosity and bulk density of cement paste. According to the study, as the amount of aluminium dross content in cement paste increases, the area fraction of porosity in the cement paste increases; yet the bulk density of cement paste decreases. In addition, when the particle size of aluminium dross decreases both of the pores size in the cement paste and bulk density of cement paste decrease.

Using arc voltage to locate the anode attachment in plasma arc cutting

Plasma arc cutting is a widely used industrial process in which an electric arc in the form of a high velocity plasma jet is used to melt and blow away metal. The arc attaches inside the resulting cut slot, or kerf, where it both provides a large heat flux and determines the flow dynamics of the plasma. Knowledge of the position of the arc attachment is essential for understanding the phenomena present at the work piece. This work presents a new method of measuring the location of the arc attachment in which the arc voltage is measured during the cutting of a range of work piece thicknesses. The attachment location is then interpreted from the voltages. To support the validity of this method, the kerf shape, dross particle size and dross adhesion to the work piece are also observed. While these do not conclusively give an attachment location, they show patterns which are consistent with the attachment location found from the voltage measurements. The method is demonstrated on the cutting of mild steel, where the arc attachment is found to be stationary in the upper portion of the cut slot and in reasonable agreement with existing published findings. For a process optimized for the cutting of 12.7 mm mild steel, the attachment is found at a depth of 1.5–3.4 mm. For a slower process optimized for the cutting of 25.4 mm mild steel, the attachment is found at a depth of 3.4–4.8 mm, which enhances heat transfer further down in the kerf, allowing cutting of the thicker work piece. The use of arc voltage to locate the position of the arc attachment is unique when compared with existing methods because it is entirely independent of the heat distribution and visualization techniques.

Numerical Simulation of Flow and Dross Particle Transfer in a 55% Al-Zn Pot

Computational fluid dynamics (CFD) is nowadays a powerful and reliable tool for simulating different flow processes and temperature. CFD is used to analyze the various pot geometries and operative variables in 55% Al-Zn pot of CGL. In this research, different strip velocities were assumed and then shown the flow pattern in the pot that was similar in the different strip velocities. Temperature distribution in the pot depended on inductors and inlet strip temperature at the steady condition. Generation of dross particles and transport models were considered to describe dross particles evolution inside the pot. In order to observe dross influence by scrap location, dross particles were generated upon the sink roll. Floating time of dross particles is different by scraper locations above the sink roll.

Metastable Phases of Dross Particles Formed in a Molten Zinc Bath and Prediction of Soluble Aluminum During Galvannealing Processes

The morphology, chemistry, and crystallographic characteristics of metastable dross particles were identified. These particles are formed during the initial stage of precipitation. The particles had aluminum concentrations of 15 to 80 mass pct, with values that decreased gradually as particle size increased. These metastable dross particles were a mixture of the crystalline phase of FeZn10, which is called the “delta phase,” and the high-aluminum amorphous phase, which covered the surface of the crystalline phase. The new “meta Q” concept was proposed to predict the amount of soluble aluminum in the zinc bath by considering nucleation kinetics and particle growth. The results calculated using the “meta Q” concept were compared with the values measured by the aluminum sensor, which were taken during the same period at the commercial galvanizing line. The mean of the absolute values of the differences between the calculated and measured values was 9.7 ppm.

Influence of Aluminum on the Formation Behavior of Zn-Al-Fe Intermetallic Particles in a Zinc Bath

The shape, size, and composition of dross particles as a function of aluminum content at a fixed temperature were investigated for aluminum added to the premelted Zn-Fe melt simulating the hot-dip galvanizing bath by a sampling methodology. In the early stage, less than 30 minutes after Al addition, local supersaturation and depletion of the aluminum concentration occurred simultaneously in the bath, resulting in the nucleation and growth of both Fe2Al5Zn x and FeZn13. However, the aluminum was homogenized continuously as the reaction proceeded, and fine and stable FeZn10Al x formed after 30 minutes. An Al-depleted zone (ADZ) mechanism was newly proposed for the “η→η+ζ→δ” phase transformations. The ζ phase bottom dross partly survived for a relatively long period, i.e., 2 hours in this work, whereas the η phase disappeared after 30 minutes. In the early stage of dross formation, both Al-free large particles as well as high-Al tiny particles were formed. The dross particle size decreased slightly with increased reaction time before reaching a plateau. The opposite tendency was observed when the Al content was 0.130 mass pct; with a relatively high Al content, the nucleation of tiny η phase dross was significantly enhanced because of the high degree of supersaturation. This unstable η phase dissolved continuously and underwent simple transformation to the stable δ phase. The relationship between nucleation potential and supersaturation ratio of species is discussed based on the thermodynamics of classical nucleation theory.

Separation behaviour of zinc dross from hot dip galvanising melts of different aluminium concentrations by alternating magnetic field

Dross in zinc pots is the most important factor that impairs the surface quality of hot dip galvanising automobile steel sheet. Separation behaviour of zinc dross from galvanising melts of different Al concentrations (0·12, 4·5 and 55·0 wt-% by an alternating magnetic field has been investigated in order to reduce the dross defects on steel sheets. The properties of different zinc drosses were investigated by means of EDAX and the quantitative metallographic method and the conductivities were measured by physics property measurements system. The experimental results show that the zinc dross in different hot dip galvanising melts has different morphologies, compositions and structures, and the average particle size of the dross increases with the Al concentration. In the case of extra iron (above the solubility limit) in the zinc melt, zinc dross quantity also increases with the Al concentration. All of the three types of zinc dross particles >5 μm can be successfully separated when the magnetic frequency is 17·5 kHz, effective magnetic flux intensity is 0·05 T, imposed time is 15 s and the cross-section of the ceramic square pipe is 5 × 5 mm.

Microstructural study of galvanized coatings formed in pure as well as commercial grade zinc baths

This investigation is an effort to have a better understanding of the growth kinetics and morphology of the coating formed during the galvanizing process in pure as well as commercial grade zinc baths. The protective coating that is formed during hot dip galvanizing, normally between 450 and 480°C, consists of a series of Fe-Zn intermetallic layers, which have been identified as gamma (Γ), delta (δ), zeta (ξ) and an outer eta (η) layer, highly rich in zinc. There is apparently no delay in the formation of ξ or δ phases in both pure as well as the commercial grade zinc baths. The gamma (Γ) phase is formed after an incubation time of about 30 s at a bath temperature of 470°C in the pure zinc bath. Its formation is further delayed in the commercial grade zinc bath. The last morphological feature is the formation of a second ξ layer at the ξ/δ interface in the pure zinc bath. In the commercial grade zinc bath two different morphologies of ξ phase are seen starting from the lowest dipping time, and also the overall coating is considerably thicker due to formation of several iron-zinc intermetallics which degrade its ductility and outward appearance. Commercial grade zinc also enhances the dross formation in the bath and deteriorates the quality of the coating. Presence of transverse cracks as well as entrapment of dross particles in the coating is attributed to the less compact coating that is formed in the commercial grade zinc bath.

Identification of Fe–Al–Zn dross phase in galvanised zinc bath and its separation by method of alternating magnetic field

In this study, the separation of Fe–Al–Zn dross phase from hot dip galvanised zinc bath was conducted by using alternating magnetic field. The compositions of the zinc dross were detected by energy dispersive spectrum. The electrical conductivities were measured by physical properties measurement system. The separation efficiency of the zinc dross in different channels was examined by the quantitative metallographic method. Experimental results show that the zinc dross particles with diameter larger than 5 μm can be separated successfully when the processing parameters can selected appropriately.

Continuous separation of Fe–Al–Zn dross phase from hot dip galvanised melt using alternating magnetic field

Experiments to continuously separate Fe–Al–Zn dross phase from hot dip galvanising zinc melt were conducted on a laboratory scale apparatus by using high frequency alternating magnetic field. Effects of processing time (t) on separation efficiency were investigated. The experimental results show that using the electromagnetic repulsive force resulting from the electrical conductivity difference between zinc melt and Fe–Al–Zn dross phase, the deleterious zinc dross particles causing surface defects of galvanising steel sheets can be continuously separated from the zinc bath under alternating magnetic field, and the separation efficiency increases with the increase in processing time. When the magnetic frequency is 17·5 kHz, the effective magnetic flux intensity is 0·1 T, the cross-section of the ceramic square pipe is 10 × 10 mm, and the processing time is 0·6–2·5 s, the separation efficiency of zinc dross varies from 43·76 to 85·71%, and the experimental results are in reasonable agreement with the theoretical results.

Reaction of 316L stainless steel with a galvanizing bath

Type 316L stainless steel (316L SS), commonly used as pot roll material, was tested in an industrial Zn–Al galvanizing bath, with an effective Al content of 0.2 wt.%. Samples welded to the supporting roll arms and sides of the sink roll experienced heavy dross buildup. Samples placed at locations away from the incoming strip generally experienced much less dross buildup. Dross particles within the buildup on samples attached to supporting roll arms were small in size but numerous while particles on samples welded to the sides of the sink roll were much larger. SEM-EDS analyses indicated that the buildup consisted of two layers. The inner layer was the product of the corrosive reaction of 316L SS with the bath metal, and the outer layer was formed by dross particles built up on the inner reaction layer. The intermetallic phase, which formed at the reaction front of the samples was the inhibition compound Fe2Al5 containing some Mo and Cr. The formation of this intermetallic layer provided a thermodynamically favourable base for the attachment and further buildup of suspended dross particles on the sample surfaces.

Use of Flow Barriers to Eliminate Vortex in the Flow Field Generated in a Continuous Galvanizing Bath

The Navier-Stokes equation has been solved numerically along with the two equations eddy diffusivity k-e turbulence model in a continuous galvanizing bath of industrial size taking the details of the sink and the guide rolls into account where the strip speed and the width were kept constant. The main objective of the work was to eliminate the vortex formed between the strip and the sink roll, due to the movement of the strip over the sink and the guide rolls. This vortex feeds the dross particles back to the flow again so that the strip could pick up the dross over the time thus spoiling its own quality. Two alternative arrangements of placing a plate baffle (parallel and perpendicular) near the strip were tried, which could eliminate the vortex completely near the strip. The parallel plate baffle could eliminate the vortex as well as the perpendicular plate baffle but the flow field due to the perpendicular plate baffle seems to be more attractive than the one created due to the parallel plate baffle. So the choice of implementation remains with the plant personnel depending on the ease of fitting the plate to the galvanizing/zinc bath.

Simulation of flow in a continuous galvanizing bath: Part II. Transient aluminum distribution resulting from ingot addition

The coupled phenomena of momentum, heat, and mass transfer were simulated in order to predict and to better understand the generation and movement of intermetallic dross particles within certain regions of a typical galvanizing bath. Solutions for the temperature and aluminum concentration can be correlated with the solubility limits of aluminum (Al) and iron (Fe) to determine the amount of precipitated aluminum in the form of Fe2Al5 top dross. Software developed by the Industrial Materials Institute of the National Research Council of Canada (IMI-NRC), including k-ε turbulence modeling for heat and mass transfer, was adapted for the simulation of a sequence of operating parameters. Each case was modeled over a period of 1 hour, taking into account an ingot-melting period followed by a nonmelting period. The presence of an ingot significantly changes the temperature distribution and also results in important variations in the local aluminum concentration, since the makeup ingot has a higher aluminum concentration. The simulation showed that during the ingot melting, the total aluminum concentration is higher at the ingot side of the bath than at the strip exit side. The region below the ingot presents the highest aluminum concentration, whereas lower aluminum concentrations were found in the region above the sink roll, between the strip and the free surface. It was shown that precipitates form near the ingot surface because this region is surrounded by a solution at 420 °C, which is lower than the average bath temperature of 460 °C. When no ingot is present, the total aluminum concentration becomes much more uniform and decreases with time at a constant rate, depending on the coating thickness. This information is of major significance in the prediction of the formation of dross particles, which can cause defects on the coated product.

Cold model experiment on dynamic behavior of dross in hot dip plating bath

The motions of top and bottom dross in a continuous hot dip plating bath were investigated using a transparent cold model vessel with a reduced scale of one-tenth. The flow field in the model bath was classified into three regions as usual; the entry region, the exit region, and the region enclosed with a belt. This belt was used as a model for a strip. Polystyrene particles were used as models both for the top and bottom dross while NaCl aqueous solutions of different densities were used as models for plating melts. The motions of model particles were observed by eye inspection and by using a high-speed video camera. Local particle frequency and particle holdup were measured with a newly developed sensor. Typical streak lines for the top and bottom dross particles were similar to main stream lines in the bath. Both the top and bottom dross particles were rich in the region enclosed with the belt. A lot of top dross particles floated on the bath surface near the side walls, while many bottom dross particles stayed on the bottom wall in the entry region.

Numerical Analysis of Fluid Flow and Heat Transfer in Molten Zinc Pot of Continuous Hot-dip Galvanizing Line.

A numerical model adopting a partially staggered grid system for the location of dependent variables has been developed to analyze the fluid flow and temperature distributions in a molten zinc pot of No. 2 CGL of POSCO Kwangyang strip mills. A control volume based finite difference procedure was employed to solve the conservation equations transformed by using the boundary-fitted-coordinate (BFC) system. The calculation results have shown that a change in the steel strip velocity has little influence on the overall flow pattern developed in the pot. The overall temperature distribution was rather uniform as predicted. However, charging cold ingots directly into the pot produced a non-uniform distribution of temperature. The local temperature fluctuations will promote the formation of intermetallic dross particles. It has been proposed that the non-uniform distribution of temperature could be reduced by selecting an appropriate channel inductor position as well as by optimizing the zinc ingot loading position.

The quality of aluminum dross particles and cost-effective reinforcement for structural aluminum-based composites

In this work, the potential of finely powdered aluminum dross by-product (d 50=67 μm) as a cost-effective reinforcement in discontinuously reinforced aluminum-matrix composites (A356) has been investigated on the basis of quality–cost modelling. By means of a standard rheocasting procedure, samples of composite material were obtained which consisted of 20 vol% of as-received and laboratory-processed grades of dross particles which differed mainly in particle size and the level of impurities. Inspection of the tensile properties of these different composite materials showed that a slight improvement in strength over the unreinforced matrix is achievable only by the introduction of fine dross particles with a particle size less than 10 μm, whereas in composites with larger dross particles this strengthening effect was not observed. By contrast, evaluation of the wear properties demonstrated that the introduction of coarse and even as-received dross particles into an aluminum matrix results in a significant improvement in the wear resistance of the composite material.

Aluminum Dross Particles as Cost Effective Reinforcement for Aluminum Composites

In this work, the potential of Finely powdered aluminum dross by-product (d50 = 67 (μm) as a cost effective reinforcement in discontinuously reinforced aluminum matrix composites (A356) was investigated on the basis of quality-cost modeling. Using a standard rheocasting procedure, samples of composite material were obtained consisting of 20 vol.% of as-received and laboratory processed grades of dross particles which differed mainly in particle size and level of impurities. Inspection of the tensile properties of these different composite materials showed that a slight improvement in strength over the unreinforced matrix is achievable only by the introduction of fine dross particles with an average particle size less than 10 μm. In composites with larger dross particles the strengthening effect was not observed. In contrast, evaluation of the wear properties demonstrated that the introduction of coarse and as-received dross particles in an aluminum matrix results in a significant improvement in the wear resistance of the composite material. However, in that case, there is a sacrifice in strength. Quality-cost modeling of these two grades of dross reinforced aluminum based composites indicates that both may be applicable for some less critical engineered and wear resistant components, with possible widespread application in the transportation industry.

Thermodynamic description of dross formation when galvanizing silicon steels in zinc-nickel baths

This paper proposes a thermodynamic description of liquid Fe-Ni-Zn alloys, which explains the dross formation observed when galvanizing iron alloys in a zinc bath with 0.1 wt.% Ni added. At 450 °C, dross particles are present at the ζ-FeZn13-liquid interface and are identified as the Γ2 2-Fe6Ni5Zn89 compound. At 480 °C, their shape and structure change drastically, and the Γ2-intermetallic compound is no longer observed. This behavior is related to the changes in the phase relationships in the Fe-Ni-Zn system between 450 and 480 °C.