Patent Application
20240279772
When exposed to the atmosphere, molten aluminum easily oxidizes and forms inclusions, such as large amounts of oxides. Examples of such inclusions may include oxides such as Al2O3, MgO, MgAl2O4, SiO2, silicates, Al, Si, O, FeO, and Fe2O3, carbides (Al4C3, Al4O4C, graphite carbon), borides (AlB2, AlB12, TiB2, VB2), Al3Ti, Al3Zr, CaSO4, AlN, and various halides. Since the free energy of formation of magnesium oxide is lower than the free energy of formation of aluminum oxide, magnesium that is included in molten aluminum baths, such as during recycling and remelt operations, is preferentially oxidized and remains suspended in the molten metal as inclusions. These inclusions can impair the final quality of products formed from the molten metal. Therefore, while processing molten aluminum, fluxes are used to separate oxides from the molten bath, in particular during recycling and remelting operations for removal to improve the quality of the molten aluminum and subsequently formed products.
Use of molten salt fluxes in the secondary aluminum industry is known to improve direct recovery of aluminum in remelting processes. Remelting of the aluminum in a furnace is carried out in the presence of molten salts to prevent oxidation of the aluminum in the furnace atmosphere and to promote coalescence of the molten aluminum so as to maximize recovery of aluminum. During processing, an oxide film tends to form on the surface of molten aluminum droplets. The oxide film inhibits coalescence of the molten aluminum, causing smaller particles to be lost in the process thereby reducing the amount of aluminum recovered. The unrecoverable aluminum droplets having the oxide film are sometimes referred to as dross.
Use of a salt flux in the furnace helps to strip away and suspend the oxide film so that coalescence of the droplets increases and dross formation decreases. The salt flux wets the oxide film and initiates disintegration of the film, stripping it from the surface of the molten aluminum droplets. Fragments of the oxide film stripped from the aluminum remain suspended in the flux. The aluminum droplets, which have a density greater than the flux, then form a continuous molten pad beneath the flux layer. The flux also prevents further oxide formation by keeping the metal protected from the atmosphere of the furnace.
In some examples, a salt flux that includes magnesium chloride and sodium chloride is used to treat molten aluminum, however these salt fluxes are not effective at reducing magnesium in the molten bath. For examples, a pure commercial aluminum may typically have a magnesium level of less than 250 parts per million (ppm). Many uses and implementations of aluminum are driving commercial aluminum production to have magnesium in a level below 30 ppm. Accordingly, the traditional reducing salt fluxes, including Potassium Aluminum Fluoride (PAF), AlF3-Cryolite, and other such salt fluxes are used however the furnaces, refractory linings, and other equipment, must be compatible with aluminum fluoride (AlF3) such that they don't react and break down during processing of the aluminum.
Magnesium is introduced as an alloy element to produce aluminum cans and some other products, however most aluminum alloys do not contain magnesium as an alloying element. Consequently, removal of magnesium may be necessary if different types of scrap are mixed during the recycling process. One such technique commonly employed to reduce magnesium involves chlorine injection into the gas.
With increasing awareness and demand for recycled and recyclable products, processing of aluminum, a highly recyclable material, is especially in demand. Accordingly there is a need for techniques and components for processing recycled aluminum to improve the quality of recycled content.
The detailed description is set forth below with reference to the accompanying figures. In the figures, the left-most digit(s) of a reference number identifies the figure in which the reference number first appears. The use of the same reference numbers in different figures indicates similar or identical items. The systems depicted in the accompanying figures are not to scale and components within the figures may be depicted not to scale with each other.
Systems and methods described herein are related to aluminum metallurgy, and in particular to refining fluxes used to remove and/or reduce magnesium and other residues from molten aluminum. Though described herein with respect to treatment of aluminum and aluminum alloys, the salt fluxes described herein may be used for treatment of other non-ferrous metals such as zinc or lithium with their respective alloys.
Salt flux can be distributed into or on top of molten metal baths using a variety of techniques. The phrase “metal bath” is understood to mean any melt of a metal in which a major portion of the metal is present in liquid form, and only a small part is present in solid form. Liquid metal streams are also referred to as metal baths.
The flux is introduced to the metal bath to physically and chemically interacts and refine the metal. The most effective treatment methods for inclusion and alkali removal are generally those employing some type of sub-surface distribution technique (e.g., inert gas injection) and stirring, such as by means of mechanical rotors or electromagnetic pumps/inductors.
The salt flux discussed herein provides multiple metal treatment functions. For instance, the salt flux discussed herein provides for removal of non-metallic inclusions and impurities, reduction or removal of alkali and alkali earth elements (e.g., magnesium, beryllium, sodium, calcium, lithium, etc.), and to dry layers of dross that form on the surface of the metal bath.
The salt flux includes a mixture of alkali chloride salts, such as sodium chloride and potassium chloride as well as a fluoride source such as aluminum fluoride. During recycling (e.g., remelting, processing, treating), the alkali chloride salts liquefy and the salt “bubbles” are less dense than the metal bath and therefore slowly migrate to the surface of the metal bath due to the difference in density. Typical salt fluxes that include magnesium chloride and sodium chloride will not reduce the magnesium content of the molten bath, as required for many commercial implementations. Reduction of magnesium is typically accomplished using chlorine or other gases. These processes can include potentially dangerous exposure for workers to the chlorine gases during the magnesium reduction process. Therefore, the salt flux described herein provides for magnesium reduction i) without an additional processing step and ii) without exposure to potentially dangerous gases in the process.
In some examples, the salt fluxes may include cryolite (Na3AlF6). The cryolite may form an active interface around the “bubble” of salt and provides contact with the metal bath. The cryolite particles act, during the movement of the “bubbles” of the salt flux as collectors of oxides (e.g., less than 20 micron oxides) that are suspended within the metal bath. The small oxides may be suspended due to a small density difference between the oxides and the metal bath. Accordingly, over a period of time for casting or solidification of the metal bath through one or more processes (e.g., commercially typically in a range of about 30 minutes to 1 hour), these oxides may remain in the metal bath rather than collecting at the surface for removal (e.g. skimming).
The suspended oxides may be removed and/or separated from the metal bath through a filter such as a molten metal-oxide filter. However, in the case of recycled material or scrap material, the quantity of the suspended oxides may be in a range of one thousand to ten thousand oxides (e.g., in a size of approximately 20 micron) per kilogram of liquid. Accordingly, the oxides collect and prematurely block the molten metal flow through the filter. Such plugging of the filter causes production stops that may interrupt a production flow and may destroy value or delay production for large suppliers who typically produce forty to one hundred tons of solidified aluminum over a relatively short casting period such as a window of less than two hours. Accordingly, delays for plugged filters can delay and cause problems that interrupt the casting process. The salt flux described herein provides for treatment of the metal bath to reduce or prevent the likelihood of such blockage at the filter by cleaning the metal bath at the furnace or other early processing stage.
One of the components of the salt flux can be an alkali chloride salt, such as sodium (Na) chloride and/or potassium (K) chloride and/or lithium (Li) chloride. The alkali chloride salt can fuse with aluminum oxides at relatively low temperatures, enabling removal through a chemical reaction or by means of physical separation. Similarly, after forming liquids in the melt, these salts also help by removing inclusions and supplement hydrogen removal through flotation. In addition, the alkali chloride salt can provide a covering effect on the melt to prevent additional oxidation of the molten metal. The alkali chloride salt can also react at the interface of the aluminum and the dross, penetrating the oxide skin that contains trapped liquid aluminum. Alkali chloride salt can also clean the furnace environment when injected sub-surface.
A second component of the salt flux can include a fluoride containing particle, such as aluminum fluoride (AIF3). The fluoride containing particle collects at the outer surface of the liquid salt and collects oxides from the metal bath as the bubbles of liquid salt travel through the molten bath towards the surface, thereby collecting the suspended oxides within the metal bath. The use of aluminum fluoride, as shown in testing, was effective in reducing the total oxide count in the molten aluminum fifty to seventy-five percent more than a typical approach that used cryolite.
The salt flux discussed herein is used in a method for removing magnesium by adding a flux including 90% or more, by weight of NaCl and KCl with 10% or less addition of AlF3 to a molten bath of aluminum. The salt flux is added beneath the surface as discussed herein, for example through the use of a submergence impeller in a side well of a furnace. The salt flux is added to the molten bath at less than 5% by weight of the molten bath. In some examples the salt flux is added such that the salt flux is 1%, by weight, of the molten bath. In some examples, the salt flux may be added in a range of greater than 0% to 5% by weight of the molten bath.
The salt flux and method of use herein provides for a flux including at least 90% by weight of metal chlorides (such as NaCl and KCl) to selectively remove the magnesium in the molten bath. However, metal chlorides such as NaCl and KCl have low reactivity with magnesium, resulting in poor removal efficiency. In addition, since the typical salt fluxes had to be introduced in such large amounts in the melting furnace, the salt flux may cause corrosion of the melting furnace. The present salt flux is added at a rate of less than 5% by weight of the molten bath and may be added at a rate of 1% or less by weight of the molten bath.
The flux for removing magnesium impurities in the molten aluminum or aluminum alloy according to the present invention contains 10% or less by weight of metal fluoride. Metal fluorides have much higher reactivity with magnesium than metal chlorides such as NaCl and KCl, so magnesium removal efficiency is high. Therefore, magnesium in the molten metal can be effectively removed even in a relatively small amount of the flux mainly composed of metal chlorides, resulting in high economic efficiency. Accordingly, the present salt flux and method of use provides for reduced wear and tear on equipment such as furnaces and also provides for increased economic efficiency as it requires less salt flux to recycle the content of the molten bath and reduce the magnesium content of the molten bath.
In an example, the salt flux includes a balanced mixture of sodium chloride and potassium chloride in an amount of ninety percent or more by weight. The balanced mixture may include equal parts of sodium chloride and potassium chloride, such as forty-five percent by weight or more of each of the sodium chloride and the potassium chloride. The balance of the salt flux is formed of aluminum fluoride, in an amount of ten percent or less by weight.
In some examples, the salt flux may be formed into a solid flux by melting or fusing the sodium chloride, potassium chloride, and aluminum fluoride together and mixing before solidifying. The solid flux may be crushed and added to a metal bath or may be broken into granules or powders for adding to the metal bath while preserving the composition of the salt flux and providing a homogenous flux. In some embodiments, the chemistry of each granule is at least a substantially even distribution of the raw material components. Similarly, a granule-to-granule comparison of a flux batch will yield a substantially insignificant variation in raw material concentration (e.g. less than about 5% variation).
The flux composition may be provided in the form of granules, wherein each granule comprises at least three separate solid components (i.e. the listed components of the sodium chloride, potassium chloride, and aluminum fluoride). “Separate solid components” is to be understood to mean that the components are not chemically combined to a significant extent, but rather the different salts are physically pressed or otherwise combined into a single granule. The components can react in the bath to the desired extent while still in the solid state, allowing the components to be separated in the melt and distributed accordingly. Moreover, some of the salt will liquefy at molten metal temperatures.
The present flux material can have a density between about 1.5 and 2.0 g/cm3. The grain sizes of the granulated material can vary within a certain band width. An exemplary range is between greater than 0 mm and 6 mm, particularly between 0.5 mm and 4 mm, or between 0.8 and 3 mm. In this connection, it is understood that grain sizes are generally present in distributions, for example in Gaussian distributions.
The granulate can have sufficient inherent stability to be easily supplied to a metal bath. Likewise, storage over an extended period is easily possible, since granulates are relatively chemically stable and can easily be protected against outside influences. Furthermore, the subject granulates are sufficiently durable to at least substantially retain their structure during shipping so long as kept dry.
The granulated particulate flux of the present disclosure can be formed for example by blending the desired constituents in powder form. The blended powder is then compacted under high pressure (e.g. roll compacting) to form either briquettes or rolled ribbons. The briquette/ribbon form is then granulated using milling techniques (e.g. crushing) and sieved to a desired particle size distribution.
Alternatively, all the components individually can be granulated jointly by mixing the salts in an anhydrous solid phase in a furnace. The temperature of the oven is increased to achieve a fused compound in liquid form. The liquid can be cooled, ground and sieved to obtain a desired granulometry.
The granulate particulate flux can also be obtained from liquid solutions, for example by cultivating crystals or by recrystallization.
The flux described herein advantageously provide efficient alkali and inclusion removal; efficient dross drying action for reduced metal content in dross and improved melt recovery; standard grain size for injection through rotary and lance flux injection systems; are more effective than equivalent powder blends, requiring lower application rates and; improved furnace cleanliness and reproducibility between furnace batches.
The present flux material advantageously has been found to be a eutectic, allowing certain of its ingredients to be melted into the molten metal at a lower temperature than if introduced separately. The present flux material can have a melting point between about 400° and 800° C., and can be less than 700° C. For certain applications, it may be desirable to have a flux melting point between 400° and 600° C. Moreover, a rapidly melting flux can be beneficial to overall performance.
In its broadest aspect, the salt flux composition of the invention comprises standard purity NaCl and KCl, and a fluoride source, specifically Aluminum fluoride. The amounts of fluoride, along with the standard purity NaCl and/or KCl, are effective for improving coalescence and reducing aluminum loss in the recovery of aluminum from molten scrap aluminum, as well as for reducing oxide suspension and magnesium levels in the molten aluminum. Generally, in this aspect, the salt flux composition comprises at least about 90 weight percent and preferably from about 90 to about 99 weight percent NaCl and KCl and at least about 2.5 weight percent to 10 weight percent aluminum fluoride, all based upon the weight of the salt flux composition. In a particular example, as used herein with respect to at least some of the examples, the salt flux includes an equal balance (by weight percentage) of NaCl and KCl making up 95 percent of the salt flux with the remaining 5 percent made up of aluminum fluoride. In a second example as used herein with respect to at least some of the examples, the salt flux includes an equal balance (by weight percentage) of NaCl and KCl making up 97.5 percent of the salt flux with the remaining 2.5 percent made up of aluminum fluoride. In a third example as used herein with respect to at least some of the examples, the salt flux includes an equal balance (by weight percentage) of NaCl and KCl making up 90 percent of the salt flux with the remaining 10 percent made up of aluminum fluoride. In a fourth example as used herein with respect to at least some of the examples, the salt flux includes an equal balance (by weight percentage) of NaCl and KCl making up 99 percent of the salt flux with the remaining 1 percent made up of aluminum fluoride. Other examples may include mixtures within the ranges described herein, with an alkali salt mixture of NaCl and KCl forming at least 90 percent by weight and a balance thereof being formed of aluminum fluoride.
In some examples, when a mixture of standard purity salts is used, the ratio of NaCl to KCl is from about 30:70 to 70:30. In some examples, the ratio of NaCl to KCl is 50:50. In some examples, an essentially equimolar mixture of standard purity NaCl and KCl is used to provide a lower melting temperature for the alkali salt composition, as well as to lower the cost of the salt flux. More particularly, it may be desirable to provide a mixture of standard purity salts having a composition at or near the eutectic point of the NaCl and KCl blend so as to minimize melting temperature. It is possible, however, to use only NaCl or KCl with similar recovery results.
In an example, the salt flux described herein is used to treat a bath of molten aluminum at an additive weight of 1% (1% g salt/g Al), with 1 percent salt flux added. The half life of the first order reaction representing the reduction of magnesium is illustrated with respect to
After the initial time period illustrated in
Further,
Table 4, below, summarizes final cleanliness results for aluminum baths treated with the various fluxes mentioned above, including a Potassium Aluminum Fluoride (PAF) typically used in industry. As shown in Table 4, the different metal baths were tested through a pressure filtration test for inclusion level in the final metal bath after treatment and reaction time with the various flux salt compounds. In the pressure filtration test (e.g., Prefil), a measure of the cleanliness of an aluminum melt is determined by filtration of the molten aluminum sample through a fine porous ceramic filter. Pressure is applied to provide a force to drive the liquid metal through the porous filter. The pressure filtration test provides a measure of resistance to metal flow due to build up of inclusions, the greater the level of inclusions, the greater the deviation in the metal flow. In an example, a pressure filtration test uses a model to predict metal flow through a fixed filter in series with an ever-increasing inclusion buildup from which metal cleanliness can be determined. The metal cleanliness is a parameter that is proportional to the volume fraction of inclusions and inversely proportional to the permeability of the inclusion buildup. Changes in flow may result from the number and/or type of inclusions. The data in Table 4 presents metal cleanliness results from a pressure filtration treated using different fluxes. The metal cleanliness (results of the test) may be generally categorized along a scale with a value in a range of: 0 to 1 indicative of inclusion levels so low that the inclusions are almost undetectable; 1 to 5 indicative of clean metal; 5 to 30 indicative of moderately clean metal; greater than 30 indicates a high level of inclusions; and greater than 100 indicates an excess of very fine particles. Therefore, as illustrated, the fluxes tested, when compared against the salt flux described herein, are not as effective at reducing the level of fine inclusions. The salt flux described herein provides for metal cleanliness that is unsurpassed and results in removal of fine inclusion particles that other fluxes may leave behind, at varying levels.
At operation 602, the shredded and otherwise pre-processed metal is heated in a furnace. The furnace may include any suitable furnace type or configuration for heating metal such as aluminum to and beyond the melting point. The furnace may be at a temperature of at least 680 degrees Celsius to less than 950 degrees Celsius. The components of the salt flux composition of the invention are combined and used to produce the salt flux according to the process 700 described with respect to
The furnace may have as its melting zone a container that is relatively inert to the molten salt flux so that impurities are not introduced into the flux composition from the container. The temperature of the furnace may be held above about 750° C. The salt flux composition is melted at from about 740° C. to about 750° C. and may be held in the molten state for a period of time, which may range from a few minutes to several hours. In some examples, a reducing atmosphere is maintained in the furnace to increase the rate of removal of aluminum oxide.
The composition of the metal bath in the furnace may vary due to the nature of recycled content, but may include more than 20 ppm magnesium, such that the magnesium concentration is desirable to reduce for one or more implementations. The magnesium concentration may be greater than 300 ppm or more and may be processed according to the method 600 to reduce to less than 30 ppm magnesium, or less than 20 ppm magnesium in the final aluminum product.
The amount of flux composition used in the furnace is at least about 1 weight percent and, in an important aspect, is from about 1% to about 5%, based upon the weight of aluminum. In some examples, the amount of flux used in the process is from about 1% to about 2% based upon the weight of the aluminum. With each batch of aluminum processed, fragments of oxide film, particles of aluminum coated with the oxide film and other impurities become entrapped in the flux composition layer, causing it to become cloudier and more viscous. The salt flux composition may be re-used in the furnace until the flux composition becomes too viscous, which makes it difficult to remove purified aluminum. In some examples, the salt flux may be added and followed by a second flux addition to further treat or process the metal bath. For instance, the first flux addition may include a lance of solid flux (e.g., fused) and/or stirred in with an impeller (e.g., for a granule). In a second operation, the second flux addition may include addition of a bag or loose granule and/or powder of the salt flux at a surface of the metal bath for treatment of the metal bath. The second flux may be added and stirred in through a stirring system of the furnace. In some examples, the second flux may include a second type of flux with a different composition and/or added at a different weight percent of the metal bath to further reduce impurities within the metal bath. At operation 606, the waste material and aluminum may be separated, for disposal or processing of the waste such as the oxides while the aluminum may be used in various downstream processes.
While the foregoing invention is described with respect to the specific examples, it is to be understood that the scope of the invention is not limited to these specific examples. Since other modifications and changes varied to fit particular operating requirements and environments will be apparent to those skilled in the art, the invention is not considered limited to the example chosen for purposes of disclosure, and covers all changes and modifications which do not constitute departures from the true spirit and scope of this invention.
Although the application describes embodiments having specific structural features and/or methodological acts, it is to be understood that the claims are not necessarily limited to the specific features or acts described. Rather, the specific features and acts are merely illustrative some embodiments that fall within the scope of the claims.
This application claims priority to U.S. Provisional Application No. 63/446,762, filed on Feb. 17, 2023, the entire contents of which is incorporated herein by reference for all purposes.
| Number | Date | Country | |
|---|---|---|---|
| 63446762 | Feb 2023 | US |
