Smelt explosions

Mixtures of the nitrate with powdered aluminium or its oxide (the latter seems unlikely) were reported to be explosive [1], and the performance characteristics of flares containing compressed mixtures of the metal and nitrate have been evaluated [2]. A violent explosion in a copper smelting works was caused mainly by reaction of aluminium with sodium nitrate [3],... 

The editor has been told that air can be explosive in its own right in a eucalyptus wood on a hot day, and, having smelt one, does not find this absolutely incredible. Explosive air is sometimes also found in caves and mines when decaying vegetable matter is present. 

See MOLTEN METAL EXPLOSIONS, SMELT, SUPERHEATED LIQUIDS, VAPOUR EXPLOSIONS... 

This comprehensive survey of the title topic is in three parts, the first dealing with the theoretical background and laboratory studies, with 29 references. The second part, with 21 references deals with case histories and experimental studies of industrial vapour explosions. These involved the systems molten titanium-water, molten copper-water, molten aluminium-water, smelt-water, water-various cryogenic liquids, molten salt-water and molten uranium dioxide-liquid sodium. In the third part (with a further 26 references) is discussion of the various theories which abound, and the general conclusion that superheated liquids most likely play a major role in all these phenomena [1]. A further related publication covers BLEVEs and pressure let-down explosions [2],... 

E. Statistical Survey of Smelt-Water Boiler Explosions. 148... 

Whereas one might classify the LNG-water studies as a response to a concern that industrially sized operations might result in a large-scale spill on water with subsequent RPTs, studies of molten salt-water explosions were carried out because industrial accidents had taken place. Emphasis has been placed on events occurring in the paper industry where molten smelt is produced in recovery boilers. This smelt is primarily a mixture of sodium chloride, sodium carbonate, and sodium sulfide. In normal operations, the molten smelt is tapped from the furnace, quenched, treated, and recycled to the wood digestors. Accidents have taken place, however, when water inadvertently contacted molten smelt with severe explosions resulting. The smelt temperature is much higher than the critical point of water 1100 K compared to 647 K (see Section IV). 

Laboratory investigations into the mechanism of smelt-water explosive boiling events have been primarily of value in delineating the effect of smelt composition on the sensitivity of the salt in producing RPTs. For example, pure molten sodium carbonate has never led to explosive boiling. Addition of either (or both) sodium chloride or sodium sulfide lead to smelts which are more prone to explosive boiling. Investigators experimented with many additives both to the smelt and to the water in an attempt to obtain less sensitivity. Most had little or no effect. 

The superheated-liquid model introduced earlier to explain LNG-water RPTs was not considered applicable for smelt-water explosions since the very large temperature difference between the smelt and water would, it... 

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-... 

For small-scale experiments, the LNG and liquid refrigerant cases are analyzed using a model which assumes that the volatile liquid (or some part of it) is superheated to a temperature at which homogeneous nuclea-tion occurs. Such nucleation is very rapid and the event resembles an explosion. An attempt is made to employ this same model to explain R s in the water-smelt, water-aluminum, and water-reactive metal cases, but data to make definitive conclusions are lacking. 

Chemical reactions were shown to play a minor role in a smelt-water explosion, although gas samples from kraft smell-water incidents showed that hydrogen evolution could be correlated with Na2S content. There was general agreement that the explosion mechanism was physical in nature. 

Apart from the laboratory studies, statistical surveys of actual recovery bmler explosions have shown that such incidents are relatively rare and, in the United States occur, on the average, about once every 100 years of boiler operation. All explosions have been traced to events which allowed water to enter the furnace and contact the smelt, e.g., broken water tubes or dilute liquor feed. A listing of the presumed causes of all known smelt-water explosions is given in Table XII. 

Few in-depth studies have been made of actual furnace smelt-water explosions and, therefore, it is difficult to delineate expected overpressures and impulses. One case history is presented to indicate in a qualitative fashion the type of damage in a large explosion. 

As noted earlier, no viable theories of the smelt-water explosion had been widely accepted during the early period of investigation. Nelson in... 

The smelt-water explosion problem is of primary interest in kraft recovery furnaces where, from operational error or an equipment failure. 

Molten salt (smelt) tapped from black liquor boilers is quenched and dissolved to form green liquor in tanks near the boiler. On occasion, explosions have resulted which severely damaged the dissolver tank. (These events are different from the smelt-water boiler explosions described later.)... 

Sallack (1955) was the first to publish a study of dissolver-tank explosions. He was motivated by incidents which occurred in a soda pulp operation with a dissolver tank 4.3 m in diameter and 3.7 m tall. Molten smelt entered the tank at the top and was to be broken up with a jet of recirculating green liquor. Agitation of the bulk liquid was also accomplished by air jets. Operation was normally smooth, but if a boiler upset led to a sharp increase in smelt flow, then the smelt-green liquor breakup operation was inefficient and unbroken slugs of smelt could enter the bulk green liquor in the tank. Explosions could then occur. 

The experimental procedure followed by Nelson and Kennedy was very similar to that used by Sallack, and their results agreed with Sal-lack s conclusions in regions of overlap. With kraft smelts an explosion was easier to achieve and the presence of Na2S was cited as a possible... 

In agreement with Sallack, if the water or green liquor temperature were high, the explosion probability decreased, but he cites one anomalous case a composition which gave an immediate violent surface explosion in cold water produced a terrificany violent deep explosion in hot quenching water. The blast was heard more than 1 mile away (100 g of smelt was used). 

There had been some suggestions that explosions were due to free metallic sodium in the smelt. This hypothesis was disproved by adding oxidizers (KNO3 or Na202) to the smelt with no significant changes noted in the explosion pattern. 

Explosions could not be obtained with smelt poured into transformer oil. 

Whereas these early experiments provided interesting data, no mechanism was developed to explain the explosion phenomenon. In fact, since the 1950s there has been little interest in conducting further studies in dissolver tanks because the addition of efficient steam-shatter jets at the smelt entrance has effectively eliminated explosions in this section of the process. Further studies were directed to the explosions which took place within the recovery boiler as a result of water contacting the smelt on the furnace floor. 

Rogers et al. confirmed that the presence of Na2S in Na2C03 could lead to an explosive smelt they also showed that the concentration of hydrogen in the vapor increased with the quantity of sodium sulfide originally present. It was suggested that Na2S reacted with water as follows ... 

To indicate the magnitude of the pressure pulse, their most violent explosion occurred with a smelt consisting of 20% Na2S, 75% Na2C03, and 5% Na2S04. Green liquor was injected. The pressure rose 27 kN/m (4 psi) and the exit gas contained about 8% hydrogen. 

The research results clearly indicated that the smelt-water explosion yielded a localized, high-energy shock wave which moved at velocities of over 700 m/sec. The maximum pressure rise was achieved in about 1 msec. These facts were related to the results found in real boiler explosions in descriptive terms as there were 3-4 inch depressions 3 to 6 feet... 

Lougher et al. (1968) reviewed the situation, presented some additional data, and developed recommendations for further work. They examined existing theories to explain smelt-water explosions and rejected all. They too noted that sodium aluminate in the smelt reduced the probability of an incident and also stated that CaCOj and Fe203 were effective. Most other additives (and over 90 were studied) either led to more violent explosions or were ineffective. No correlation of the smelt additive results was given. Also, water with various additives such as surfactants, starch, sucrose, glycerine, and hydroxymethyl cellulose still exploded when contacted with a sensitized smelt. 

Krause et al. (1973) carried out the last detailed U.S. experimental investigation of the smelt-water explosion phenomenon. A large number of experiments were conducted with variations in the smelt composition. The scale was quite small with 0.03-1 g quantities of water injected at high velocity (20-30 m/sec) onto the surface of the smelt. A few tests were also made with small drops (0.8-0.3 g) of water on the end of a ceramic tube that was dropped into the smelt. Some information concerning pres-... 

A detailed study of the physical properties of various smelts (viscosity, surface tension, density, and sonic velocity) led to the conclusion that there were no signihcant differences in these properties with explosive or nonexplosive smelts. 

Other studies showed that those smelts which could release CO2 at a high rate after contact with water were less explosive. This finding was related to the known fact that gas-evolving chemicals (CaC03, NaA102) were effective inhibitors. 

A model based on inertial restraints was developed, but it has not been widely accepted since it does not explain many of the experimental facts for smelt-water explosions. 

A final laboratory study was described by Shick and Grace (1982) as being carried out in Sweden (Bei an and Laufke, 1981). From 10 to 100 g of water were injected into 10-30 g of smelt. In some tests subsurface iiyection was used at pressures of 10 bar, whereas in others a ceramic capsule with water was burst under smelt with electric fuses. Smelts of a wide range of compositions could be exploded with the subsurface injection. Comparison of explosion intensities to those produced by TNT or black powder (as judged from pressure-time traces) suggested that 1 kg of water was equivalent to 0.03-0.2 kg TNT or 0.3-2 kg black powder. 

---