1、G ar 1 1 Guidelines for Handling Molten Aluminum Third Edition Editor: Seymour G. Epstein Technical Consultant Editorial Board Michael K. Anderson Christian M. Bickert J. Martin Ekenes Seymour G. Epstein E Robert Hubbard John E. Jacoby Charles D. Johnson Vincent J. Newberry Jake J. Niedling Warren S
2、. Peterson Donald C. Pierce Greg Rawls Ray T. Richter Clark Weaver Wagstaff, Inc. Pechiney, Inc . Hydro Al Dross processing operations and equipment are not discussed. Emphasis is placed on large-scale melting and casting practices for the production of process ingot. Slabs or billets are subsequent
3、ly mechanically worked into forms such as sheet, plate, foil, forgings, extrusions, or T-bars, sow and Remelt Scrap Ingot (RSI). These are then remelted at another site. More attention is also given to scrap melting operations. Terminology used in these guidelines reflects that employed in the indus
4、try. 2.1 .I: Aluminum-Lithium Alloys These Guidefinesdo not cover procedures for the additional hazards involved in melting and casting aluminum-lithium alloys. For information, see Aluminum Association Pub- lication T4, Safe also hydrated lime. Some appreciation of the magnitude of the release of e
5、nergy when aluminum is converted to its oxide can be realized by noting that the energy release in this reaction per pound of aluminum is about three times that from a pound of trinitrotoluene (TNT). Section 6 Suggested Purchase Specifications for Charge Materiais Purchase orders offer the first opp
6、ortunity to control the presence of harmful contaminants in materials to be added to melting furnaces. It is suggested that purchase orders specify in writing that aluminum, aluminum scrap, alloying materials, and fluxes be relatively free of water and contain no volatile materials, or other oxidizi
7、ng agents which can cause an explosion when charged into a melting furnace. An exception is the purchase of aluminum scrap in the form ofbofigs, *gS, Saw Chips, fines, Pit cledgs, etc. Which can contain oil and water. Extreme care must be taken in processing these forms of scrap. Section 7 heiving,
8、hspection, Storage, Drying of Scrap and AU Other Components of the Furnace Charge As in the case of the proposed purchase specifications noted in Section 6, the goal in Section 7 is to provide guidance for each facility to devise and implement a system that covers all plant functions and cast shop r
9、elated activities to anticipate and prevent water and other hazardous contaminants being present on and in materials added to the melting furnaces. Recommended actions are given in Sections 12-1 9. For more detailed descriptions, refer to Aluminum Association Publication GSR, Guidelines for AZuminum
10、 Scrap Receiving and Inspection, Second Edition (2002). 12 Section 8 Melting, Melt ktment and Tiruisfer, and Casting Processes This section provides general information on these processes for those readers not closely associated with or directly involved in handling molten aluminum. More detailed in
11、formation and recommendations on melting and casting operations are given in Parts V and VI. Since the initial fiidefines were issued, a large number of improvements have been made in controlling and automating melting and casting operations. Also, new systems have been devised for removing dissolve
12、d gas (hydrogen) and non-metallic particles from the liquid metal. In general, these new controls, systems, and equipment are proprietary; details are available from the manufacturers and, frequently, in the open literature such as the LightMeta2.s volumes published by The Minerals, Metals and Mater
13、ials Society (TMS). 8.1: Melting Large scale melting of aluminum is usually done in reverbatory or “open hearth” refractory-lined furnaces, with capacities that in some cases exceed 200,000 pounds (100 Metric Tonnes (MT). Some furnaces are topcharged, in which case the charge falls directly into mol
14、ten metal that may be in the furnace or onto an unmelted charge of metal. Some furnaces are charged from floor level, through doors or into a side well adjoining the hearth. In the latter case, the charge does not fall into the main body of molten metal but into a small well or connecting pool of me
15、tal. In some installations, the aluminum is melted in one furnace (melting furnace), and transferred to a second furnace (holding furnace) for further processing such as composition adjustments, fluxing and close control of temperature prior to casting. In the primary aluminum industry, it is usual
16、to find melting holding furnaces where metal from the potrooms (or cell lines) is transferred into the furnace together with process metal scrap. These furnaces are frequently of the tilting variety to provide good control over temperature and flow of metal to the casting machine and to permit rapid
17、 and complete draining. In the secondary aluminum industry, rotary salt furnaces, side well furnaces and induction furnaces are used to melt lighter gauge secondary scrap. Drosses are typically processed in a rotary salt furnace where salt is used to separate metallic oxides from the molten metal. I
18、 figurn 3: Charping fumacehm 7ianshrCtucib/e 8.2: Transfer of Molten Aluminum For direct chill (DC) casting, molten aluminum is usually transferred by gravity from the melting hace to the holding furnace and to the mold. In stationary rnaces, the metal flow rate from a tap hole in the furnace is con
19、trolled by tapered plugs, whereas with a tilting furnace the flow rate is a function of tilt rate. After leaving the furnace, the metal flows through slightly sloping refractory lined troughs, or launders, toward the casting * As indicated previously, process ingot is a casting which is subsequently
20、 mechanically worked into shapes such as sheet, forgings, extrusions, wire, etc. 13 machine. The system usually includes one or more flow rate or molten metal level control devices. In some instances, special pumps may be used to move the liquid metal. To process the metal, the melt may be treated i
21、n the furnace with a fluxing agent. Processing may also be accomplished by “in line” systems as the metal flows from the hace to the casting station. Examples of metal processing include removal of hydrogen, trace alkali removal, inclusion removal and alloying. A typical melting, holding and casting
22、 process is shown schematically on the flowsheet in Figure 4. Molten metal may also be moved from one funiace to another or to a casting station by means of crucibles and II Emergency Water I Charging 3Lg n I Melting Furnace k - Liquid Metal I Holding Vertical D.C. Caster II II 1 Hydraulics I -. -ig
23、ure 4: Typical Melting, Holding, Casting Process Flow Sheet 14 ladles. When the distance is such that excessive cooling may take place, the metal is moved in insulated containers that can be carried by trucks through the plant and over public roads. In the case of rotary salt furnaces, the metal is
24、usually tapped directly into crucibles or sow molds located beneath the furnace, although some salt furnaces have intermediate a base plate. As molten aluminum flows into the mold and as the mold fills up, the starting block is lowered at a controlled rate, as required for the size and alloy of the
25、ingot being cast. Metal flow is adjusted to keep the mold filled. A relatively thin shell of solidified metal is initially formed by cooling through the mold wall. Additional water cooling is provided fromthe water jacket surrounding the mold or from another source, which flows against the hot ingot
26、 shell as it launders. In these cases the flow rate is controlled by the tap hole position (furnace rotation). Some rotary furnaces tilt and pour the molten metal out the front opening. 8.3: Casting Molten aluminum is cast into process ingot in semi-continuous vertical DC and horizontal DC casting m
27、achines, and in various types of proprietary continuous casting machines. Other forms of aluminum and its alloys are cast in open, cast iron molds or sand, plaster, or steel molds. Details of sand casting, permanent mold and die casting operations are not covered in these GuideZines. 8.3.1: Vertical
28、 DC (Direct Chill) Casting Process The most common method of casting process ingots is by the vertical DC casting process. In the conventional system, the molten metal flows from the furnace into a transfer trough, n YY Cu. _- t B - 3 , c through a filter, through a downspout, a level control device
29、, and a distributor into a water cooled mold. Several ingots are usually cast at the same time. At the start of casting, the lower opening of the mold is closed by a starting block (also referred to bottom block, starting head, stool cap, dummy block) typically attached to is formed, providing the “
30、direct chill” cooling necessary to achieve complete metal solidification. Once the ingot has reached the desired length, the flow of metal into the mold is terminated. The downwardmovement of the ram is stopped to allow the ingot head to solidi the aluminum is contained by the electromagnetic forces
31、 while simultaneously being water- cooled and solidified into an ingot. Metal level control in the mold is critical to prevent bleed-outs. More details of the EMC system are given in Section 24. 8.3.4: Horizontal DC Casting Process In this process, the solidified ingot is withdrawn from the mold in
32、a horizontal direction. At the start of casting, liquid metal enters the mold through a tundish and starts to solidi. In some systems?, the metal solidifies around clinches (bolts fastened to the starting block); in so doing, the metal becomes attached to the starting block. At this moment, the star
33、ting block is pulled horizontally, slowly at first, then increasing to a faster and steady rate. As in the vertical process, metal flow is adjusted to suit the casting rate required. As the ingot forms behind the moving starting block, it is cooled by water. When the ingot has reached the desired le
34、ngth, the ingot may be sawed or the supply of molten metal may be shut off. Horizontal DC casting of aluminum is shown schematically in Figure 6. 8.3.5: Continuous Casting Processes Vertical DC and some horizontal DC casting processes are classified as batch or semi-continuous systems since ingots a
35、re cast to a predetermined length. In a truly continuous casting system, the product form is continuously ?pulled? fi-om the mold and cut to lengh ?on the fly? and coiled in _ II 7 ,?,?,.?/,1?,. . i ._. ? . 2 A - otherwise, metal spills on the floor may surround conduit and piping and burn wiring. T
36、his may make equipment inoperative and hinder the clean up of spills. If ground level service trenches are required in areas of potential metal spills, they should be backfilled with sand or some other insulating material to prevent molten metal penetration. Ground level openings should be protected
37、 with a curb to prevent molten metal penetration. Screws, bolts, clamps, brackets, and other hardware re- quiring operator use should be designed to be handled with “fmgerless hot gloves” and should be placed in a position Warning devices should be employed to alert personnel to conditions which cou
38、ld adversely affect safe operations, ag. water flow, casting speed, metal level, loss of air, loss of water. Equipment should be designed to be “fail safe” so that loss of utilities, such as air pressure, electricity, hydraulic pressure or even human energy, will permit safe termination of casting.
39、Hydraulic systems should be designed with remote shut off protected from molten metal spills. All units should have manual shut-Offs clearly marked to be used in case of emer- gencies. These systems should be shielded to prevent high pressure leaks from spraying hydraulic fluid onto hot sur- faces,
40、such as molten metal or furnaces. The use of fire resistant hydraulic fluid is recommended. Some plants use waterglycol mixtures, water-soluble oil mixtures (10% oil), or phosphate esters. Fire resistant synthetic hydraulic fluids can also be used. I Electrical systems should be designed and located
41、 to pro- tect them from heat, water, metal splash or spills from fur- naces or metal treatment boxes and fumes which could adversely affect them. Air systems should also be designed 20 away from metal splash. Solidified metal splash encases parts and pre- vents movement of components. No floor drain
42、s with traps should be lo- cated in areas where molten alumi- num can accumulate in the event of a furnace or trough leak. Whenever possible, process controls should be automated for critical functions such as metal temperature, casting speed, cooling water flow rate, trough metal level, mold metal
43、level and process gas flow rates in order to ensure consistent safe starts and to minimize the opportunity for operator error. In designing such systems, great care must be taken to ensure process control logic and interlock devices function properly and safely. Fault tree analysis or similar method
44、ology should be deployed to discover possible errors in process logic, especially under emergency conditions. Dry runs are recommended to test controls and alarms prior to actual casts with molten metal. In 1986, a catastrophic explosion occurred in a plant casting aluminum process ingot that was at
45、tributed to a lightning strike, by the company. Based on this incident, the company recommended that plants check their lightning protection against applicable building codes. One organization has recommended that casting plant stacks should be connected directly to ground with two driven rods, and
46、then interconnected to the building counterpoise system. If necessary, jumpers should be installed across any stack joints to ensure electrical continuity. However, each facility should consider its own protective system. 9.3.2: Provisions for Drying/Preheat- ing of Furnace Charge The provisions for
47、 drying and preheating must reflect the nature, size, etc., of the charge material. The preferred means of preheating sow, ingot, or T-ingot, which may be wet and which may contain voids, is to provide a separate preheating furnace, as shown in Figure 10. In one organization where gas-fred ovens are
48、 used for this purpose, material to be dried is held in the oven for four hours at an air temperature of 750F (400C). However, each organization should develop its own drying and preheating practices reflecting the material being charged and equipment employed. Adherence to defined practices and suf
49、ficient data collection are necessary to assure that the organizations requirements are followed. It is recommended that dryingpreheating ovens be surveyed at regular intervals to insure proper dry-out of the charge material. 9.3.3: Melting and Molten Metal Trans- fer Facilities Plants should establish an emergency plan to contain and deal with furnace run-outs. Some companies design reservoirs to receive inadvertent run-outs of molten metal Concrete can spall and “explode” on contact with molten aluminum. Areas subject to frequent minor spills should be protected with mate
