Improve safety and avoid hazards in your aluminum melting practice.
The melt department is the site of some of the highest accident risk operations in foundries and diecast plants. Nevertheless, it frequently receives minimal attention in terms of operator training and safety considerations. Because workers deal with melting hazards on a day-today basis, they may develop a complacency that can lead to accidents, possibly resulting in serious injury or death.
Unfortunately, in many melting departments, an operator's "training" consists of a few cautions passed on by coworkers about the addition of wet material to a molten bath, often without any explanation of what constitutes or causes "wet materials."
In recent years the labels and warnings on melt stocks that stipulate a need for preheat before charging have become more evident, primarily to reduce the product liability of the furnace and charge manufacturers. Often, the labels are printed in more than one language, in an effort to reach all workers. However, daily association with the charging activity soon reduces the warning to "background noise."
The creation and maintenance of a safe workplace is the best method of ensuring that company profits and resources are not distributed to hospitals and doctors as a result of industrial accidents. An active and continuing management program is required to achieve a safe working environment, and all levels of any organization dealing with molten metal should make an ongoing effort to establish and maintain such a program.
MOISTURE AND MOLTEN METAL
Anyone who has experience with molten metal has some knowledge of the potential hazard of mixing water and molten metal. What they may not realize is the magnitude of the forces generated by the explosive conversion of water to steam when it contacts molten aluminum. For example, the volume of water vapor at 1263F (684C) is approximately 1900 times that of its liquid water source. The near instantaneous conversion of the liquid to vapor has the potential to throw the light molten metal over large distances. Further, because of its high fusion heat, a unit volume of molten aluminum alloy will give off almost twice as much heat in freezing as a copper alloy and, thus, cause more serious burns.
Sources of Moisture
Beyond the obvious sources of moisture (in-house charge materials contaminated by machining fluids, green sand residues, etc.), there are a number of possible sources on purchased materials. While the materials may start out as clean and dry at the manufacturer, operations involving storage, transportation, delivery and even charging may add dangerous levels of moisture to the material and create an explosion hazard. Moisture may be conveyed with purchased charge materials as a result of surface oxides, internal shrinkage cavities and condensation problems. In addition, wet tools and equipment may cause an explosion in the furnace.
All aluminum materials have a layer of oxide upon their surfaces, but the nature, thickness and chemical composition of this layer can change dramatically, depending upon the exposure of the material to various environments. Aluminum alloys also are more sensitive to the formation of complex oxides than pure aluminum because of their composition, which may include magnesium (Mg), silicon (Si) and other trace elements. The surface oxide color may range from bright aluminum through gray to a chalky white. In their worst form, these oxides have picked up chemically combined water, which cannot be driven off with a mere preheat above the boiling point of water. Temperatures in excess of 600F (316C) are required to remove this moisture, eliminating it as a possible hazard. Normally, these temperatures are not achieved unless the material is charged upon a dry hearth in the furnace.
This does not mean that material with a gray or white coating should automatically be rejected at the receiving dock, but, rather, that it be treated with more than the usual amount of "respect." Many of the possibly suspect materials may be used without incident, but some spitting and popping can certainly be expected with the more extreme examples of oxidation. In any event, more careful charging, possibly using smaller quantities, should be mandated. Higher melt losses to dross also are typically experienced with oxidized material. Many companies are not prepared to accept these penalties to their operation and will automatically reject off-color material.
All aluminum ingots and sows are merely unrisered castings and, as such, will contain a probable zone of sponge shrink or even a shrinkage cavity that may contain moisture. The size and distribution of the shrinkage zone will vary according to the alloy composition, the ingot/sow dimensions, and the cooling practices during casting, but they always will exist in some form. Typically, the volume of shrinkage will vary from approximately 4-8% in the transition from liquid to solid and, at a minimum, will produce a surface sink on the ingot or sow.
In recent years, there have been a number of attempts to minimize the shrinkage cavity in sows through control of directional cooling and design changes to the sow mold. The results of these developments have been reasonably successful and are gradually finding their way into wider use in the industry. With the industry increasing its use of sows in high-volume melting operations, these designs offer real promise for reducing the risks associated with melting the large blocks.
The smaller ingot molds used by foundry alloy producers do not offer the potential for cooling optimization and are more dependent upon process cooling variables. On that basis, they can be expected to have a cavity of some size and should always be treated as if they contained some moisture.
One of the least recognized sources of moisture in charge materials is condensation. Condensation occurs when the substrate temperature is below the dew point of the surrounding moist atmosphere. Common examples are the fogging experienced on cool bathroom mirrors during a hot shower or on eyeglasses when coming into a warm room after being out in the cold.
All charge materials, whether ingot, sows or even home scrap, are potentially susceptible to the formation of condensation on the surface. Once formed, that condensation can pool and enter cavities through capillary action, or it may be aspirated/pumped into voids with pressure changes induced by atmospheric conditions [ILLUSTRATION FOR FIGURE 1 OMITTED].
Condensation problems are a lot more prevalent in the winter months, especially for operations in the northern part of the U.S. In one instance, an ingot manufacturer had to adjust shipping practices to ensure that ingot arriving at one customer's plant would not be too cold when unloaded. An in-furnace explosion of charge material was traced to shipping practices in which material was preloaded on Friday and allowed to cool to sub-freezing temperatures before delivery late Monday. When the cold material was brought into the customer's humid plant environment, condensation occurred on the ingot, and the moist product was charged in the furnace without adequate preheating, resulting in a serious explosion. To prevent additional problems, the manufacturer ensured that a prompt transfer was made from the ingot warehouse to the customer's plant.
Other common condensation problems occur when cold ingots or sows are exposed to a furnace environment containing a high percentage of water as one of the products of combustion. For example, when cold ingot has been placed on an exterior furnace ledge with the charging door left slightly open to facilitate preheat, the rate of condensation may be rapid enough to allow water to drip onto the floor and puddle on the ingot surface. Plants preheating ingot in this manner usually stipulate that the ingot be placed on the ledge "skim side down" to avoid condensation puddling in the shrinkage cavity [ILLUSTRATION FOR FIGURE 2 OMITTED]. Notwithstanding, the risk is still high, and operators must pay careful attention to ensure that partially preheated (and potentially wet) ingot are not charged into the furnace bath.
A similar problem can occur when sows or ingot stacks are placed upon an internal charging ledge for preheat before being pushed into the furnace. Beyond any moisture that may be present in internal voids and shrink cavities, moisture from condensation also may form and contribute to an explosion hazard when the material is submerged in molten aluminum. Remember that internal cavities can potentially contain appreciable volumes of water that will not necessarily be eliminated as soon as the sow reaches a temperature above the boiling point of water. Many operations that charge material in this manner stipulate that the material must reach a "mushy" stage before introduction into the molten pool. If outside preheaters are used, typical requirements stipulate that the load must be heated to 400F (204c) for a minimum of 4-6 hr before the material is charged into a molten bath.
Tool and Equipment Risks
Tools and equipment subject to careless maintenance practices also have caused explosions in aluminum melt areas. One particular hazard lies in the application of protective washes to skimmers and ladles. Good practice requires that the coatings be applied to preheated tools, and that subsequent immersions in molten metal be done with great care. Newly coated ladles have been known to cause serious "pops" when immersed too quickly in metal - the problem being that moisture entered a faulty weld between the bowl and handle shank. Other problems have been experienced with encrusted flux-plunging bells that had picked up moisture hygroscopically and exploded upon immersion.
One area of a melt operation that is frequently left to individual operator preference is the timing and method of introducing charge materials to the melting furnaces. Because this is a particularly high-risk operation, management should take a close look at how materials are charged, and standard procedures should be documented.
Reducing Charging Risks
It is best to move charge materials from the dry storage area to the furnace area well before charging. It is preferable to store materials in a warm area for as long as possible before charging.
If the melting equipment and facilities permit, all ingot should be preheated to a minimum of 400F (204C) by staging the material over open wells or on furnace ledges before introducing it to a molten bath. Even with that preheat, the operators should use caution when charging. If sows are used in the melting operation, some form of thorough preheat should be mandated before their addition to a molten bath. That preheat may be done in an external oven or a dry hearth ledge, but, without it, the risk of internal moisture in the cavities of cold sows is high enough to almost guarantee that an accident will result at some point.
Beyond the need to preheat charge materials, the method and rate of addition is equally important to ensure a safe operation [ILLUSTRATION FOR FIGURE 3 OMITTED]. Wherever possible, known dry home scrap and returns should be charged first (as a bed for new metal), and ingot should be added on top. This will minimize the direct immersion of new material, and reduce the risk of pops. While it is preferable to make smaller additions on a more frequent schedule, high-volume melting often will necessitate that bulk charges be made with dump hoppers, pay loaders or other machinery. Remember that the risk level escalates with the volume of material charged, and that any bulk additions should be metered into a molten bath as slowly as the equipment will permit.
In addition to the possible moisture problems, a rapid addition will splash metal, possibly affecting burner operation, and may even create a wave of molten metal that could wash out over the furnace walls.
Special Charging Equipment
Any equipment used for bulk charging of returns or home scrap should be dedicated to that function only. The equipment should be painted some distinctive color, marked accordingly, and should not be used as a repository for trash, sweepings or other contaminants that could create a hazard if introduced to molten metal. A solid steel container should have drain holes in its bottom to eliminate the possibility of pooled moisture under a scrap load being dumped into the furnace along with the charge.
Lift trucks used to introduce solid or bulk charges to a molten bath also should be dedicated solely to that function. Further, they should be equipped with a shield to offer the operator some protection from splashes or pops. Most shields used in the larger shops are made of multiple layers of glass and Lexan, with the outer layers containing reinforced glass. Exclusively plastic shields have been known to cause visibility problems after oils and a shop's environment contaminated their surfaces.
The first line of defense against accidents in the workplace is a well-engineered and safe working environment. However, the risk of burns and possible ignition of clothing is an additional hazard. Various regulatory agencies and advisory standards groups have issued a number of mandatory standards and guidelines that cover melting hazards and ways to reduce the risk of personal injury.
The hazards in each operation should receive careful analysis, and management should choose personal protective equipment (PPE) that fits the existing circumstances and operation. The employees who will be working in the melting area should help analyze the hazards and make these equipment choices, since, if they are not committed to wearing the PPE, any expense made in purchasing the equipment will be wasted.
All personnel working in the vicinity of molten aluminum should, at a minimum, wear a cap and clothing of flame-retardant material, as well as adequate eye protection. Molten aluminum splashes have the ability to melt and ignite many synthetic fabrics and to adhere to other fabrics while transferring their energy (heat) to the person wearing the clothing. The resultant burns are thus compounded in degree, and, in this way, a splash may be elevated from a minor incident to a major lost-time accident.
Personnel involved in charging a furnace or performing maintenance are exposed to a significantly higher level of risk and should be equipped with PPE that will provide a measure of safety against a volume of molten metal explosively conveyed to their area. At a minimum, that should include laceless safety toe boots (and/or spats); heat resistant and/or flame retardant gloves that minimize the opening at the wrist where molten metal might enter; a hard helmet and face shield worn over safety glasses; and appropriate primary protective clothing (worn over secondary protective clothing) to provide a layering effect for greater protection [ILLUSTRATION FOR FIGURE 4 OMITTED].
Finally, any manufacturer's claims for its product's protection level should be evaluated in the melting area where the equipment will be used. As noted before, molten aluminum will stick to many materials, burn through others instantly, and turn others into a blazing torch. The supplier of the equipment should have a thorough understanding of the intended application and should be prepared to demonstrate the adequacy of an offering with on-site splash tests.
Need for Recurrent Training
At a minimum, melt operators should be actively involved in safety committees and reminded of the hazards they see on a daily basis. Don't rely on word-of-mouth training. An annual review of PPE and safety practices should be necessary for experienced operators, and any new staff should receive a thorough indoctrination before accepting any duties in the melt areas.
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|Author:||Groteke, Daniel E.|
|Date:||Jun 1, 1998|
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