Dross Generation Research Articles

Thermodynamic Modeling of Solid Flux Interactions with Molten Aluminum

Oxide and dross formation during aluminum melt processing results in a considerable amount of loss of metallic aluminum. The total melt loss generated during melt processing can be greatly reduced by efficient use of flux, particularly for melting aluminum scrap or secondary alloys. Effective use of cover fluxes can significantly reduce dross generation by creating a barrier between the metallic aluminum and the atmosphere. The amount of metallic aluminum trapped within the dross layer can also be reduced by up to 50% using drossing fluxes. However, reducing the industrial average melt loss has remained difficult. To evaluate flux ingredients used in the casting industry, computational thermodynamic software ThermoCalc was used to calculate the driving force for reactions between twenty-one flux ingredients and eighteen common alloying and impurity elements in foundry alloys. The thermodynamic calculations, combined with other properties, were used to provide a desirable list of cover and drossing ingredients, which are being experimentally validated.

Sustainable recycling of aerospace-grade ultra-clean 7050 aluminum alloy melts through argon refining without secondary aluminum dross generation

The refining of aluminum scraps commonly employs nitrogen-fluxes to eliminate impurities. However, the aluminum nitride (AlN) generated by the reaction between nitrogen and the aluminum melt, along with residual fluxes, contribute to low cleanliness of the recycled aluminum melt and pose difficulties for the disposal of secondary aluminum dross (SAD). Our innovative research has resulted in the recycling of aerospace-grade clean aluminum alloy melts. The results indicate that the argon bubble floating process adsorbs inclusions and hydrogen, achieving an aerospace-grade ultra-clean melt. The solid-liquid interface between the tube wall and the melt provides a diffusion channel for hydrogen atoms, and the bubbles rising up along the wall lead to a higher hydrogen content in the melt. The argon refining dross is primarily composed of Al and Al2O3, which can be recycled as a raw material for aluminum electrolysis. Argon refining can decrease the hydrogen content and the number of inclusions with particle size ≥40 μm in the recycled aluminum melt to the level of aerospace aluminum alloy melt. Nonetheless, the particle size ≥20 μm remains 1.5–2.5 times that of primary aluminum.

Thermodynamics-CFD-DPM modeling of dross formation and diffusion in a Zn-0.3Al galvanizing bath

In order to further reduce the production cost and enhance product of steel strip marketing competitiveness, industrial enterprises will choose to increase production rate, leading to the more dross generation. This paper proposes a numerical simulation based on thermodynamics, computational fluid dynamics (CFD) and discrete particle method (DPM) to simulate the η-Fe2Al5 dross formation distribution, and to track the trajectories of different sized dross particles in a Zn-0.3Al galvanizing bath. An optimized modeling of interfacial reactions involving the instantaneous rate of iron dissolution and aluminum uptake is firstly set up and validated with the previous experimental results and theoretical calculation. On the basis of the analysis on interfacial reaction, a potential influence factor (strip velocity) is discussed. The results show that higher strip velocity will result in much more η-Fe2Al5 dross around the strip entry side and the average dross formation within the V-typed region increases with the increase of the strip velocity due to the higher dissolved Fe concentration in molten zinc. Additionally, the trajectories of dross particles are discussed by injecting particles in the specific generation area. It can be concluded that the η-Fe2Al5 is likely to adhere to the steel-strip and roll surface. Furthermore, the trajectories of the dross particles are affected significantly by the flow of zinc and the particle size, thus the trajectories of particles resemble the streamlines. The average residence time of the suspended dross particles in the outer area of steel strip exit is longest. It is illustrative that the dross build-up region is the front wall area of the zinc pot.

Sampling Procedure, Characterization, and Quantitative Analyses of Industrial Aluminum White Dross

Up to 10% of all metallic aluminum (Al) produced is lost due to dross generation during production. It also causes an environmental problem due to hazardous waste generation following treatment, as well as toxic gas generation when in contact with water/humidity. As a result, it is desirable to identify the parameters contributing to the generation of the Al white dross mixture. In view of this, a sampling procedure for collecting representative dross samples directly from the casthouse holding furnace has been developed together with a methodology for analyzing and evaluating the results. XRD was used for phase/chemical composition analyses of sieved dross samples, and both ring milling and cryomilling were used during sample preparation. Cryomilling proved to be the superior method allowing dross fractions < 1.25 and 1.25–4.5 mm to be pulverized. The fractions were analyzed and revealed that the sampling location inside the furnace plays a vital role as the injection of primary-produced Al into the furnace influences the dross characteristics. From Location 1 (closest to the injection point) to Location 4 (furthest away from the injection point), the metallic Al content in the dross decreased simultaneously as the oxide content increased. The results also confirmed that the larger size fraction had a higher metallic Al concentration, which correlates well with literature findings. By adopting a methodical sampling procedure followed by consistent routines for sample preparation, characterization, and analyses, process operations can be studied and thereby potentially used to minimize the loss of Al due to dross formation.Graphical

Evaluation of Aluminum Dross Generation Rate During Mechanical Stirring of Aluminum Through Model Experiment and Numerical Simulation

The present study proposed, for the first time, an evaluation method to predict the generation rate of aluminum dross in melting or holding furnaces during mechanical stirring of molten aluminum through water model experiments and numerical simulation. In the experiments, a gas–liquid interface reaction between CO2 gas and NaOH solution was exploited as a model reaction between the aluminum melt and air moisture, and perlite particles were used to model dross layer on the free surface of liquid aluminum. The results reveal that the reaction rate is increased with the impeller rotation speed. Besides, the reaction rate depends on the impeller rotation direction, but this dependence is different for the cases of presence and absence of particles on the liquid surface. In the first case, the reaction rate at the counter-clock rotation was higher than that at the clock-wise rotation. On the other hand, in the second case, the clock-wise impeller rotation yielded a higher reaction rate compared to the counter-clock rotation case. These phenomena can be explained by the mass transfer across the gas–liquid interface at the particle-free area.

Modelling Approach and Challenges in Simulating Dross Formation in Ductile Iron Castings

Dross is one of the most challenging quality issues in high integrity ductile iron castings. The formation of dross is linked to the applied metallurgy, metal treatment, process control and gating/rigging design. One of the primary concerns in this regard is entrainment of air into the melt during filling of the casting. Today, avoiding and controlling dross relies on the practical experience and process know-how of foundry experts. A quantitative understanding of the formation mechanisms or the prediction of final amounts and locations of dross in castings is not available. In this paper a computational model is developed for predicting the formation, motion and final location of dross inclusions during pouring of ductile iron castings. The focus is on the important mechanism of dross generation due to air entrainment during filling of the mold. The model predicts the local air entrainment rate as a function of the turbulent kinetic energy of the liquid metal and the normal velocity gradient of the metal at the liquid metal-air interface. The dross inclusions resulting from the exposure to the entrained air are transported with the melt flow under the combined influences of drag and buoyancy, and captured by the solidifying casting structure. The model is implemented in a commercial casting simulation software code. Ductile iron casting experiments are conducted to validate the model using castings having a variety of gating systems, section thicknesses, and surface orientations. Dross is measured by serial sectioning of the solidified castings. Good agreement between measured and predicted dross amounts and locations is obtained. The results reveal how gating system design affects dross formation. Clean ductile iron castings can be obtained by minimizing air entrainment during pouring. The present model allows foundries to evaluate different gating designs and pouring parameters before the first casting is poured. The paper also discusses open issues and practical challenges in quantifying the amounts of dross as a function of the entire processing route of a casting.

Solution Strengthened Ferritic Ductile Cast Iron Properties, Production and Application

The ductile cast iron standard in Europe also considers new ductile cast iron grades with a unique combination of tensile strength with high elongation (e.g. Rm 72520 psi with A514%) achieved by the solution strengthening of the ferritic matrix by silicon with contents between 3% and 4.3%. The new grades tolerate a higher amount of pearlite and carbide stabilizing elements without the generation of embrittling pearlite or carbides in the microstructure. The presence of Cr, Mn, and V do not influence the elongation significantly. An advantage of the new cast irons is the cost efficient machining due to lower tool wear. The tool life with the machining of the fully ferritic grades GJS-500-14 and GJS-600-10 is about 50–60% longer in comparison to the ferritic-pearlitic grades. The investigated grades show very good cyclic mechanical properties but lower dynamic properties. The pre-condition to achieve optimized properties by well-shaped graphite nodules, achievable with special inoculation techniques, adjusted to the high silicon content and the solidification rate, otherwise threaten graphite degenerations. There is no increased danger of dross generation or of a worsening flowability (investigated with special test equipment). Also the supposed inclination to the generation of porosities with the higher Si content was not confirmed.

Quantitative analysis of top dross formation mechanism in metal coating baths

Simulation experiments were conducted to identify the dominant factors that affect top dross generation in metal coating baths. From the experiments and the microstructural analyses of the top dross, it is proposed that top dross is generated through bath metal entrainment by the oxide film and subsequent drainage during holding. A new concept, taking into consideration the fresh area creation during the wiping process, is introduced and a mathematical model is developed to quantify dross generation in the metal coating process. From the proposed mechanism, a new metric is developed to quantify top dross generation in the coating bath. The experimental results agree well with both theoretical results and production data of galvanising (GI) baths based on this metric.

Factors Affecting the Metal Recovery Yield during Induction Melting of Aluminium Swarf

Machining operations of cast parts usually generate considerable amounts of waste in the form of chips (usually 3–5% of the casting weight). Traditionally, swarf is sold to scrapers and remelters, but this option is quite expensive because the selling price is roughly 30% of the acquisition price of the commercial 2nd melt raw material. For most aluminium foundries that incorporate machining operations in their products, reusing aluminium chips as raw material for the melting stocks is perhaps the best option as waste management policy in what concerns to economical and technical aspects. Nevertheless, aluminium swarf is a low density product (0.25 kg/dm3) and is usually covered by a thin film of aluminium oxide and machining fluid. Melting such a product without suitable previous preparation leads to very low metal recovery rates, high energy consumption, gases and smoke generation and very low quality of the final product. During the last years, the authors have developed a high efficient and environmentally friend aluminium swarf recycling technique, using direct incorporation in aluminium melts. The influence of processing parameters, namely melt temperature and holding time, melting atmosphere, swarf briquetting pressure and melting charge composition in the metal recovery yield and dross generation was studied and characterized, and the optimal processing parameters were established. The microstructure of the final product obtained in those conditions was evaluated and is also presented. It is shown that the recycling efficiency depends on the swarf conditioning, the melting technique and the melt treatment methodology. Swarf moisture reduction, induction melting under protective atmosphere and a specially developed degassing technique were found the most important factors influencing the recycling process. By using the developed technique, cast ingots with microstructure and sanity similar to commercially available AlSi12Cu1 2nd melt raw material were successfully obtained with minimal dross formation and metal recovery rates around 90%, without using traditional salts and fluxes.

Plasma arc cutting technology: simulation and experiments

Transferred arc plasma torches are widely used in industrial processes for cutting of metallic materials because of their ability to cut a wide range of metals with very high productivity. The process is characterized by a transferred electric arc established between an electrode inside the torch (the cathode) and another electrode, the metallic workpiece to be cut (the anode). In order to obtain a high quality cut and a high productivity, the plasma jet must be as collimated as possible and must have the higher achievable power density. Plasma modelling and numerical simulation can be very useful tools for the designing and optimizing these devices, but research is still in the making for finding a link between simulation of the plasma arc and a consistent prevision of cut quality. Numerical modelling of the behaviour of different types of transferred arc dual gas plasma torches can give an insight on the physical reasons for the industrial success of various design and process solutions that have appeared over the last years. Diagnostics based on high speed imaging and Schlieren photography can play an important role for investigating piercing, dross generation, pilot arcing and anode attachment location. Also, the behaviour of hafnium cathodes at high current levels at the beginning of their service life can been experimentally investigated, with the final aim of understanding the phenomena that take place during those initial piercing and cutting phases and optimizing the initial shape of the surface of the emissive insert exposed to plasma atmosphere.

Furnace Operations to Reduce Dross Generation

While it is generally acknowledged that dross generation should be kept to a minimum, too often, the importance of maximising the aluminium content of the dross is overlooked. Some mistakenly believe that a low metal content is a good thing and that the aluminium is being kept in the furnace. In reality, this metal is most likely being lost due to insufficient cooling and thermiting. Much can be gleaned from looking at the dross that is generated in a casthouse; in fact, the quality of dross can provide a good indication of the overall efficiency of the operation. Even with the very low aluminium prices of today, of circa. US$1400 per tonne, a reduction in dross generation within the furnace can provide huge savings per year. Effective dross management also results in better metal quality, improved fuel efficiency, prolonged refractory life and improved yield in the entire facility. This paper will look at how dross is generated within the furnace in the first place, followed by ways to minimize the dross generation within the furnace using continuous and sub-surface circulation which can also provide significant energy and CO2 reductions. A separate paper will discuss dross processing options and possibilities. In summary, by careful attention to the equipment and process techniques around the furnace and the follow-on dross management significant cost savings and environmental benefits can be realized by cast house operations.

Greenhouse Emissions in Primary Aluminium Smelter Cast Houses - A Life Cycle Analysis

The aluminium industry is reducing its carbon dioxide emissions and environmental footprint. In order to identify and prioritise areas in the cast house where greenhouse gas emissions can be reduced it is necessary to quantify CO2e (CO2 equivalent tonnes) emissions for the various cast house operations. In this study two typical cast house layouts are examined. In one case, 22kg 99.85% aluminium remelt ingots are produced using chain conveyor ingot casting machines. In the second case, wrought alloy extrusion and rolling slab direct chill cast products are made. Both plants are sized at 500ktpa. The various process inputs in terms of energy and materials were identified and typical usage rates assigned. The results show that general electricity consumption, dross generation and furnace energy consumption are the three biggest areas of CO2e and should be targeted for improvement. Magnesium consumption also has a large effect in the case of the

Launder System for Aluminium Casting

In 2000 Aluar installed its first in-house designed launder system for liquid metal transfer from the furnaces to their continuous casting machines. Since then, this type of launder system has been successfully installed in 9 casting lines, both batch and continuous, with flow rates ranging from 2 to 44 tonne/h, for conventional open mould casting, Wagstaff vertical DC billet casting , and Properzi rod casting. The Aluar system is designed to ensure an effective metal temperature control as well as a constant steady “under skin” metal flow, so avoiding turbulence and oxide generation. The highly adaptable system consists of a metal structure, which houses the pre-moulded high durability refractory launders of standardized shapes and dimensions required for the geometry of the desired lay-out: Y shaped, V-shaped, straight, elbowed launders, etc. In this way, the launders enable the connection of the furnaces to the casting machine optimizing the new or existing lay-out, and can be fully integrated to the degasser unit and the filter box without dross generation. The pre-moulded refractory pieces can be easily and quickly removed when their life time is over, thus reducing maintenance costs. Each module is insulated by a thick layer of material to reduce temperature loss (approx. 1°C per m in continuous casting). Another important feature of the system is the thermally insulated heated lids that can be commanded by a PLC to ensure excellent metal temperature control. Efficient launder level control systems ensure an effective achievement of consistent process control parameters. Control flow devices with laser commanded pins allow handling metal height differences up to a range of 200mm, without turbulence. This paper describes the main features of the launder design and presents some case studies in continuous and batch casting lines to show their performance in aluminium alloyed and unalloyed products. Maintenance performance indicators showing typical refractory life time are also included. INFA, an Engineering company of the Aluar group, with a vast experience in engineering works for the aluminium industry, was responsible for the installation and commissioning of these systems and is planning to market this product worldwide in the near future.

Dross Processing Technology

While it is generally acknowledged that dross generation should be kept to a minimum, too often the importance of maximizing the aluminium content of the dross is overlooked. Some mistakenly believe that a low metal content is a good thing and that the aluminium is being kept in the furnace. In reality, this metal is most likely being lost due to insufficient cooling and thermiting. Much can be gleaned from looking at the dross that is generated in a casthouse; in fact, the quality of dross can provide a good indication of the overall efficiency of the operation. Even with the very low aluminium prices of today of about US$1400 per tonne, a recovery improvement of just 3% for a facility producing 500t of dross per month can provide savings in excess of $250.000 per year. Effective dross management also results in better metal quality, improved fuel efficiency, prolonged refractory life and improved profitability in the entire facility. Over the years, as facilities have focused on better dross cooling and handling techniques, dross recoveries have improved. Today, dross recoveries should be in the range of 60 – 70%. These numbers will raise debate but 30 years of experience give us deep insight into these results. The paper looks at the different techniques of handling the dross that is produced within the melting/casting operation with the objective of maximizing aluminium recovery. This paper will consider both the initial dross handling within the cast house but then also how secondary processors should be evaluated to maximize the value of the dross being processed. A company can lose as much dross recovery opportunity here as in their own facility. . In summary, by careful attention to the equipment and process techniques around the furnace and the follow-on dross management, significant cost savings and environmental benefits can be realized by cast house operations.

E19 Effect of Nozzle Shape on Dross Generation in Fine Cutting of Thin Metal Plate by Pulsed Nd:YAG Laser(Laser processing)

The nozzle shape was investigated in order to develop a high performance nozzle for pulsed Nd:YAG laser cutting, and its effect on fine cutting of thin metal plate was also discussed. In the case of 0.5mm throat diameter Laval throat nozzle with an initial expansion zone, the pressure on workpiece increased, and the straightness of gas flow was improved compared to a traditional straight throat nozzle, and the observation using high-speed video camera could reveal the behavior of removing molten metal downward. The experimental results proved that Laval throat nozzle with an initial expansion zone made it possible to reduce the dross height and the kerf width.

Effects of Laval Nozzle on Precision Micro Cutting of Thin Metal Plate by Pulsed YAG Laser

The stagnation pressure, which is the sum of static and dynamic pressures, has a great influence on machining performance in laser cutting. Therefore, increasing the velocity of assist gas is effective in reducing the height of dross. In this paper, Laval nozzle was newly designed, because it can increase the velocity of assist gas flow spouted from nozzle tip. Effects of Laval nozzle on assist gas flow and the machining results in precision cutting of thin metal plate by a pulsed YAG laser were experimentally investigated. Assist gas flow from Laval nozzle does not have Mach Shock Disk and goes more straight than a traditional convergent nozzle. Therefore, the pressure on a workpiece increases, since assist gas flow from Laval nozzle can utilize energy more efficiently. However, caution is necessary in setting the cylinder gas pressure, since Laval nozzle has many unstable regions, where the pressure on the workpiece changes periodically. Using Laval nozzle makes it possible to reduce the height of dross even under low cylinder gas pressure condition compared to convergent nozzle. Thus the consumption of flow quantity of assist gas can be reduced, when the same height of dross is desired. Moreover, it was pointed out that the convexed inner wall of nozzle leads to the reduction of dross generation and unstable region.

317 ラバルノズルを用いた YAG レーザによる薄板の精密切断加工

Effects of Laval nozzle on assist gas flow and the machined results in precision cutting of thin plate by pulsed YAG laser were experimentally investigated. Assist gas flow from Laval nozzle does not have Mach Shock Disk, and goes more straight than a traditional convergent nozzle. Therefore, the pressure on workpeice increases, since assist gas flow from Laval nozzle can utilize energy more efficiency. However, the caution is necessary in setting of cylinder gas pressure, since Laval nozzle has many unstable regions in which the pressure on workpiece changes periodically. Using Laval nozzle makes it possible to reduce the height of dross even under low cylinder gas pressure condition. Thus the consumption of flow quantity can be reduced. Moreover, it was pointed out that a convex curve on inner wall of nozzle leads to reduction of dross generation

ＹＡＧレーザを用いた精密切断加工におけるアシストガスの供給法に関する研究

YAG laser has widely been used for precision micro machining in many fields. However, adhesions of dross and spatter to base material due to high energy beam machining lead to the deterioration of surface integrity. It is important to understand the assist gas flow from the tip of a convergent nozzle in order to improve the machining accuracy, since most materials are removed away by gas flow. In this paper, the effects of assist gas flow on machined results in micro machining with pulsed YAG laser are investigated using Schlieren method. Experimental analysis made it clear that there was a region in which the pressure on workpiece didn't increase linearly with an increase of cylinder gas pressure. Mach shock disk repeatedly appeared and disappeared under this condition, and the pressure on workpiece changed periodically. The fluctuation of assist gas flow led to the irregular dross generation. On the other hand, there was no unstable region under small gap and/or high pressure conditions, in which the assist gas flow was stable and the height of dross became smaller. Consequently, it is recommended to use the assist gas under higher pressure condition more than 300kPa on workpiece for fine precision machining.

ＹＡＧレーザを用いた微細加工におけるドロス生成に関する研究

YAG laser has actively been used for precision micro machining, since it has become possible to get stable and high quality beam and the spot diameter can be focused less than 100 micrometers. High energy beam machining with YAG laser or CO2 laser is heat energy machinig accompanied with heating, fusion, evaporation processes, therefore the adhesions of spatter and dross to base material lead to the deterioration of machining integrity. In this paper, the dross generation mechanism in precision micro cutting of thin plate with pulsed YAG laser is experimentally investigated.Main conclusions obtained are as follows:(1) In a single pulse machining, the spatter scatters on the base material surface at irradiation side and the dross sticks around the hole at exit side.(2) Oxygen assist gas makes the mobility of melting material increase, which leads to the decrease of the adhesion of dross.(3) The height of dross is greatly affected by the beam overlap rate, and it takes maximum when the beam overlap rate is about 93%.(4) The dropout phenomenon of dross is occurred when the beam overlap rate is more than 96%.(5) The generation of dross is restrained by shortening the gap distance between the nozzle of assist gas and the workpiece.