Molten metal-water explosions

As with the smelt-water case, if an RPT did take place, the event was localized and rarely was dam e severe far from the site of contact. Modeling molten aluminum-water incidents (and, in fact, other molten metal-water explosions such as in the steel industry) has not been partic-... 

In Table XV, some data are given for molten metal-water explosions not involving aluminum. In most of these cases, the quantity of metal was large. Because only the more serious explosions are reported, these tend to illustrate the most damaging type of event. Again, it is interesting to note that usually only small quantities of water were involved. 

Very few molten metal-water explosions are well documented essentially no data are available to estimate the overpressures or force-time relationships. The few incidents which have been described in any detail suggest that a two (or more)-step sequence is involved. First, contact is made between water and molten metal. Second, the mix is tamped or a shock wave occurs near the mix. The resulting explosion is sharp and has an associated blast wave. 

The lead in studying both the cause and prevention of molten metal-water thermal explosions has been taken by Alcoa. The Aluminum Association also sponsored several research programs both at Alcoa and at Battelle-Columbus. The Ai onne Natio Laboratory has carried out experiments with molten aluminum and water. 

In this article, we suggest that a modified superheated-liquid model could explain many facts, but the basic premise of the model has never been established in clearly delineated experiments. The simple superheated-liquid model, developed for LNG and water explosions (see Section III), assumes the cold liquid is prevented from boiling on the hot liquid surface and may heat to its limit-of-superheat temperature. At this temperature, homogeneous nucleation results with significant local vaporization in a few microseconds. Such a mechanism has been rejected for molten metal-water interactions since the temperatures of most molten metals studied are above the critical point of water. In such cases, it would be expected that a steam film would encapsulate the water to... 

Besides the aluminum industry, the nuclear power industry has been interested in molten aluminum-water explosions due to the presence of aluminum metal in some boding water reactors. Certain accident scenarios lead to a meltdown of the reactor core with concomitant contact of molten aluminum and water. 

An aluminum ingot which had just been cast was submerged below water. The skin covering the top apparently broke, thereby exposing molten metal. An explosion resulted... 

It is possible that this theory can be adapted to explain molten metal-water thermal explosions although many needed data are still unavailable. One might presume that, at the molten metal-wet surface interface, there is some chemical reaction. Possibly that of the metal plus water or metal plus surface to lead to localized formation of salt solutions. These may then superheat until homogeneous nucleation occurs. The local temperature and pressure would then be predicted to be far in excess of the critical point of pure water (220 bar, 647 K) and a sharp, local explosion could then result. Fragmentation or subsequent other superheat explosions would then lead to the full-scale event. 

The examples quoted above involve very low-speed waves but higher velocity evaporation waves may also exist. Molten metal-water interactions, often known as steam explosions, can result in quite violent interactions involving transient evaporation waves with velocities of up to 500 m/s. While the evidence for such high-speed waves is limited, numerous industrial accidents and large-scale experiments (see the references in Frost et al, [4] of this symposium) have demonstrated the destructive nature of these events. 

Explosions have occurred in aluminum and magnesium plants as a result of molten metals coming into contact with water (134). 

Contact of water with molten metals or salts or hot oil (above 100°C at atmospheric pressure) can result in a steam explosion , or a boil-over , with ejection of process materials. Similar effects occur with other volatile liquids. 

Steam explosion rapid vaporization of water within molten metal, molten salts or hot oil or through them eontaeting surfaee or adsorbed moisture (refer to page 47). 

Accidents due to thermcil shock mixtures of cryogenic liquids with water at ambient temperature. There will be an explosion if the difference in temperature of the mixture of molten metals with water is greater than 90°C. 

Mixtures of potassium and solid carbon dioxide are shock-sensitive and explode violently on impact, and carbon monoxide readily reacts to form explosive carbonylpotassium (potassium benzenehexoxide) [1]. Dichlorine oxide explodes on contact with potassium [2], Potassium ignites in dinitrogen tetraoxide or dinitrogen pentaoxide at ambient temperature and incandesces when warmed with nitrogen oxide or phosphorus(V) oxide [3], At — 50°C, potassium and carbon monoxide react to give dicarbonylpotassium, which explodes in contact with air or water, or at 100°C. At 150°C, the product is a trimer of this, potassium benzenehexoxide. The just-molten metal ignites in sulfur dioxide [4],... 

The earlier references, which state that this powerful oxidant is stable when pure, but explosive when formed as a layer on metallic potassium [1,2], are not wholly correct [3], because the superoxide is manufactured uneventfully by spraying the molten metal into air to effect oxidation [4], Previous incidents appear to have involved the explosive oxidation of unsuspected traces of mineral oil or solvents [3]. However, mixtures of the superoxide with liquid or solid potassium-sodimn alloys will ignite spontaneously after an induction period of 18 min, but combustion while violent is not explosive [3], The additional presence of water (which reduces the induction period) or hydrocarbon contaminant did produce explosion hazards under various circumstances [5], Contact of liquid potassium with the superoxide gives no obvious reaction below 117°C and a controlled reaction between 117 and 177°C, but an explosive reaction occurs above 177°C. Heating at 100°C/min from IT caused explosion at 208°C [6],... 

Although carbon dioxide reacts slowly with lithium at ambient temperature, the molten metal will bum vigorously in the gas, which cannot be used as an extinguisher on lithium fires. Carbon monoxide reacts in liquid ammonia to give the carbonyl which reacts explosively with water or air. Lithium rapidly attacks silica or glass at 250°C. 

Molten Metals and Water Explosions, HSE Rept., London, HMSO, 1979... 

Events of this nature have been described by various terms, e.g., rapid phase transitions (RPTs), vapor explosions, explosive boiling, thermal explosions, and fuel-coolant interactions (FCIs). They have been reported in a number of industrial operations, e.g., when water contacts molten metal, molten salts, or cryogenic liquids such as liquefied natural gas (LNG). In the first two examples noted above, water is the more volatile liquid and explosively boils whereas, in the last example, the cryogenic liquid plays the role of the volatile boiling liquid and water is then the hot fluid. 

The aluminum spill tests in water led to effective plant preventative schemes wherein organic coatings were placed on all solid surfaces which, in the event of an accident, might be in contact with molten metal and water. Use of such coatings has greatly diminished the frequency of thermal explosions. 

This model may possibly be adapted to metal-water thermal explosions if one assumes that there are reactions between the molten metal and water (and substrate) that form a soluble salt bridge across the interface between the two liquids. This salt solution would then be the material which could superheat and, when finally nucleated, would initiate the thermal explosion. As noted, the model rests on the premise that there are chemical reactions which occur very quickly between metal and water to form soluble products. There is experimental evidence of some reactions taking place, but the exact nature of these is not known. Perhaps, in the case of aluminum, the hydroxide or hydrated oxides form. With substrates covered by rust or an inorganic salt [e.g., Ca(OH)2], these too could play an important role in forming a salt solution. 

The spout on a holding furnace broke during casting and molten aluminum fell into a pool of water. An explosion resulted An aluminum-manganese alloy was being melted in a refractory crucible. Due to an accident to the crucible, molten metal spilled into the furnace pit which contained water. An explosion resulted... 

In casting or processing reactive metals such as titanium, tungsten, and molybdenum, incidents have occurred in which molten metal contacted water. In some instances, explosions have resulted. There have been no published papers describing such accidents. ... 

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