This is from the "Journal of Metals"
http://www.tms.org/pubs/journals/JOM/0005/Poulsen-0005.htmlThis article is probably not "hot air" by amateurs discussing metallurgy.
INTRODUCTION
The titanium industry dates back to the turn of the century, although commercial production of the metal actually started in about 1950. By the end of 1999, the industry was producing more than 100 million pounds per year. Early on, safety problems arose from a lack of knowledge regarding furnace design and related explosions. The knowledge at the time was based on steel technology, and the hydrogen explosions were a completely new problem. When molten titanium reacts with water, the titanium metal breaks down the water, absorbing the oxygen and liberating the hydrogen, which results in a major explosion. During the first five years of the industry, furnace explosions killed six employees.
The next problems that plagued the industry were fire and explosions from sponge and fines fire, which killed four employees. The third problem was confined space entry. Five fatalities have resulted from argon, nitrogen, and other inert gases.
METAL FIRES AND EXPLOSIONS OCCURRING IN THE HANDLING OF FINES, SPONGE, AND TURNINGS
The most prevalent accident occurring in the industry during the past 50 years has been metal fires. The major cause is poor housekeeping and inexperienced operators. Using the fire triangle as a base, there is always fuel for the fire when working with finely divided metals. When handling the material in the atmosphere, there is always oxygen available. Thus, the controllable leg of the triangle is the ignition source, even though the extent of the loss can be controlled by design and housekeeping.
In the past year, the titanium industry has experienced five major fires at a cost of well over $1 million. All were preventable. We can do better.
* A magnesium fire at Timet Henderson was caused by charging molten magnesium into a wet mold at a cost of $200,000 and two injuries.
* A dust-collector fines fire at the Albany Casting Plant was caused by housekeeping and a bad dust collector at a cost of $250,000. No injuries.
* A dust-collector fire at THT was caused by mechanical problems at a cost of $150,000. No injuries.
* A magnesium fire at Timet Henderson was caused by 1,000 pounds of spilled molten magnesium metal that reacted with water from melted cooling lines at a cost of $300,000 and months of lost production. No injuries.
* A fines fire at Gemeni, a powder pressing plant in Albany, Oregon, was caused by static electricity from a bad electrical motor. There were excessive amounts of powder stored in the area. The cost was $450,000; the company went out of business. No injuries.
n 1974, an employee was working as a press and weld operator. He was running titanium sponge through a 24 to 1 splitter to produce containers of sponge for feed to the press. The material was a blend of Kroll sponge and Hunter process sponge. (The latter material was very fine due to the sodium-reduction manufacturing route.) With 30% of the material split, the material began catching in the tote bin. The operator went to the top of the splitter, which was 30 ft. above ground. He used a metal bar to rap the tote bin to get the metal to flow out of the bin. Evidently, he created a spark that ignited a dust cloud of titanium fines in the area. In an instant, the fire flashed to the ground level and back up. He was completely engulfed in flames and died from his injuries. The ensuing fire is depicted on the cover of this issue of JOM.
The investigation of this accident revealed several shortcomings of the system. First, the dust-collecting system was not adequate to handle the fines content of this material mix. Second, the use of the bar was an ignition source. Third, employees should not be allowed to work in dangerous areas.
Following this accident, the dust-collector system was replaced. The bar was taken out of the area, and procedures were established to keep operators from going to the top of the splitter when it was in operation.
Three lessons were learned from this accident.
* A cloud of metal powder can ignite and burn with explosive force.
* A mechanical spark is enough to ignite a dust cloud.
* Operators must be kept out of harm’s way.
There have been other significant metal fires, related fires, and explosions experienced in the industry. When it is realized that even wheat flower in a dust cloud can cause a catastrophic fire, all efforts must be taken to eliminate the dust cloud and the ignition source.
Chemically, titanium has an enormous affinity for oxygen. This results in a thin film of titanium oxide being produced almost instantaneously on the surface of the titanium when exposed to the atmosphere. The titanium-oxide film is inert and protects the underlying metal from further attack.
When a titanium-powder particle is heated to a certain temperature (known as the ignition point), the mass of the particle is so small that the entire particle may oxidize almost instantly. Thus, a pile of such particles will burn. Since sponge particles are much smaller in mass than atomized or granular particles, they will ignite more readily and burn faster than the coarser types of powder.
Fine particles of titanium powder, like some organic powders, such as flour, starch, and coal dust, are easily dispersed in air, where their light mass allows them to remain suspended. Like particles in a pile, they will burn when the ignition temperature is reached; but when dispersed in the air (mixed with oxygen) in a certain proportion, the burning extends from one particle to another with such rapidity (pressure rise in excess of 20,000 psi/s) that a violent explosion results.
Laboratory tests by the US Bureau of Mines and others have established the proportions required for an explosion. They extend throughout a wide range, and very little titanium powder is needed. Very small amounts of energy are required to ignite certain mixtures of titanium powder and air. In some cases, energy as low 25 millijoules may cause ignition.
Previous Topic
Index
