PENTABORANE DISPOSAL: TAMING THE DRAGON Jeffrey W. Gold Integrated Environmental Services, Inc. Atlanta, Georgia Christopher Militscher United States Environmental Protection Agency Atlanta, Georgia D. Douglas Slauson Environmental Office US Army Missile Command Huntsville, Alabama
INTRODUCTION The ball of fire that shot from the first empty cylinder, disintegrating part of our secondary containment wall while slightly grazing my legs in its passage, was my first real experience with a chemical known as pentaborane. I had handled and safely disposed of small quantities of this chemical on other projects but the large eight-hundred pound cylinders we were dealing with now contained enough residual product to warrant serious respect. Its unique characteristic of burning with a deep green flame along with the potentially lethal consequences of waking it from its slumber to confront head-on has made this a dangerous chemical “dragon.” Appreciating specific characteristics of chemicals from simply reading a Material Safety Data Sheet is often difficult. Information such as a chemical’s Threshold Limit Value (TLV) and flammability is easy to understand academically but often more difficult to comprehend from a practical standpoint. When reading about pentaborane, a TLV of .005 ppm and indications that it is pyrophoric over a wide range of oxygen concentrations meant this was toxic material prone to ignite very easily. What is difficult to comprehend from these numbers and quantitative data is that pentaborane is as toxic as some of our Nation’s nerve warfare agents and will spontaneously burst into persistent flame in nearly oxygen deficient atmospheres. This type of knowledge is especially memorable when gained through direct experience.
results and proved extraordinarily hazardous to manufacture and use. The class of chemicals to which pentaborane belongs, the boron hydrides, faded from prominence in the early 1960's with its use relegated to small-scale research projects at universities and military weapons stations. Stocks of high-grade pentaborane in excess of 200,000 pounds, produced to power top-secret weapons were quickly forgotten and “moth-balled” on military reservations across the nation. It is the legacy of this once classified, Cold War weapon’s development program that remains with us today. After sleeping nearly forty years, the aging cylinders and their toxic contents pose a greater hazard now than anytime in the past. In October 1995, the Department of Defense and Environmental Protection Agency formed a National Pentaborane Task Force to formulate strategy and disposal options to rid the country to this potentially lethal “dragon.” Recent advances in disposing of pentaborane have yielded a process by which the shortcomings of past disposal methodologies are overcome in a low cost, reliable system. Known as “enhanced hydrolysis,” this system uses water with specific additives to achieve a rapid yet controlled hydrolysis reaction that transforms this lethal chemical into harmless borax and hydrogen gas. CHARACTERISTICS
Originally thought of as a “super-fuel” due to its ability to develop very high specific impulse values when combined with various oxidizers, development of pentaborane and its many relatives was pursued vigorously during the Cold War to gain a competitive advantage in powering faster airplanes and missiles. Early promise led to later disappointment after ten years of research and testing failed to produce the desired
Pentaborane (with the chemical formula B5H9 or its unstable form, B5H11) is a colorless liquid with a low vapor pressure (66 mm Hg @ 0oC) and a density of 0.618 g/ml at 25oC (lighter than water). It ignites spontaneously in air over a very wide range of concentrations and has a listed Lower Explosive Limit of 0.42% and an unknown Upper Explosive Limit. It is listed as an “extremely hazardous substance” under
Section 302 of Superfund Amendments and Reauthorization Act (SARA). A Threshold Limit Value of 0.005 ppm has been established for pentaborane by the American Conference of Governmental Industrial Hygienists. This low TLV coupled with the chemical’s high odor threshold of 0.8 ppm suggests that odor is not a good detection criterion and exposure to even sub-detectable levels can be debilitating or fatal. Pentaborane is toxic by inhalation, ingestion, and dermal contact. It attacks the central nervous system, liver, eyes, and circulatory system. Blood acidosis and lowering of blood pH are early symptoms of high exposure along with fatigue, convulsions, memory loss, coma, and death. BACKGROUND As early as 1933, pentaborane was isolated in experiments where acid was added to magnesium boride (1). Shortly after World War II, both the US Army and Navy undertook research to determine if boron hydrides could be used as fuel additives to help power a new aeronautical development, the jet engine. Initial work was done at universities but in 1946 the US Army contracted with General Electric Company to explore fuel use of boron hydrides under the classified code name, “Project Hermes.” Concurrent with the army’s Project Hermes, the US Navy initiated its own classified development program to fund work on boron fuels entitled “Project Zip.” (Because of the potentially high speed characteristics of boron hydrides as fuels, they were nicknamed “zip” fuel). Simple in structure, the boron hydrides possess a great deal of chemical energy within their bonds, burning with very high heats of combustion. Consequently, they were attractive candidates as fuel and fuel additives. If this energy can be captured and harnessed, it can propel jet aircraft at speeds exceeding 2,000 mph and provide the thrust needed to launch missiles and rockets. Pentaborane is one of a family of chemicals known as boron hydrides that includes three primary members; diborane, pentaborane, and decaborane, although a broad array of variants has been formulated. Diborane is the basic building block for the “higher” boranes and exists as a gas that is used today primarily in the semi-conductor industry. When heated without air (pyrolyzed), diborane releases hydrogen and forms pentaborane as a liquid. Further pyrolysis leads to formation of decaborane that exists as a solid. In the late 1940's and early 1950's, General Electric Company (GE) built a small pilot plant at its research and development facility at Malta in upstate New York. The pilot plant at the Malta Test Facility was operated by GE for several years during which the plant produced diborane, pentaborane, and small amounts of decaborane before being abandoned by GE. Apparently, so many problems were encountered in producing and handling the boron hydrides that GE elected to terminate its contract with the Army. In addition, several
accidents with one resulting in a death convinced GE that boron fuels were just too risky. Olin Mathieson (Niagara Falls, NY) took over the Malta pilot plant while Callery Chemical Company (Callery, PA) built pilot scale facilities on its property in Pennsylvania. Project Hermes and Project Zip provided funding for boron fuels development from 1946 to 1959 in the amount of approximately $250 million. Most of this money was to develop a fuel for use in the B-70 bomber and more specifically, the J93-5 jet engine. When built, this engine with afterburners on would propel the four-engine B-70 bomber at speeds up to 2,000 mph to evade enemy pursuers or to advance on a target by surprise. (Using the afterburners, each of the four engines would consume 20 tons of fuel per hour at a cost of about $5 per pound!) Two prototype B-70 bombers were built and tested but a fleet of such bombers powered by boron fuels was not to be. The boron fuels were to be used with conventional fuels and power the bomber only for short durations. Several fundamental problems appeared shortly after testing began that doomed the fuel’s future use as a jet propellant. When burned in the jet engine, the boron-enhanced fuels left deposits of boron oxide that blocked fuel injection ports and eroded the engine’s precision parts. In addition, boron-laden exhaust from the engines blanketed areas beneath the flight path with a toxic residue. At the same time, General Electric Company, the vanguard of boron fuels development, had refined another jet engine, the J93-3. Performance of this engine reached a point where it was nearly as efficient as the boron-boosted engine and could meet many established performance standards using conventional jet fuel. Before funding for Project Hermes and Project Zip finally ended in 1959, the projects resulted in construction of eight pilot and production plants, employed more than 2,000 people, and produced an array of borane derived energetic products. Table 1, on the following page, lists the plants by location, year built, and operator. Besides the fundamental building blocks of diborane, pentaborane, decaborane, these plants produced a variety of derivatives designed to enhance or alter certain characteristics of the fuels. Both Olin Mathieson and Callery Chemical Company pursued a path where the basic components were alkylated thus changing the precursor’s properties. Examples of this include Olin’s High Energy Fuel 2, HEF-2 (propylpentaborane), HEF-3 (ethyldecaborane), HEF-4 (methyldecaborane), and Callery’s Hi-Cal 1 (ethyldiborane), Hi-Cal 2 (ethyldiborane/-ethylpentaborane/ethyldecaborane), and Hi-Cal 3 and 4. Most of these compounds produced fuels that were less pyrophoric, less toxic, but also less powerful than unadulterated parent material.
Table 1 Boron Fuels Manufacturing Plants
INCIDENTS
Accidents related to pentaborane fall roughly into two categories, production, and disposal-related. Known FACILITY LOCATION BUILT OPERATOR production accidents occurred from the time the first pilot Malta Pilot Plant Malta, NY 1950 Olin Mathieson plants were built up to when the Muskogee facility furnished its product to Air Force. Disposal accidents date from the early Callery Chemical Callery, PA 1952 Callery Chemical 1980's to early 1996 when an explosion destroyed processing equipment and ripped through an adjoining building wall at the Plant Callery Chemical Company facility in Callery Pennsylvania. Pad 70N Pilot Plants Niagara Falls, 1955 Olin Mathieson NY The first recorded accident attributed to production of pentaborane occurred in the early 1950's. Details surrounding COP SemiNiagara Falls, 1956 Olin Mathieson this event are incomplete but it appears that a General Electric commercial Plant NY Company employee was killed while vacuum-distilling Lawrence Plant Lawrence, KS 1957 Callery Chemical pentaborane at the Malta Test Facility. In part because of this accident and other difficulties encountered handling the boron Navy Plant Model City, 1957 Olin Mathieson hydrides at the facility, GE turned the Malta pilot plant over to Olin Mathieson. NY Muskogee Plant
Muskogee, OK 1957
Callery Chemical
Air Force Plant
Model City, NY
Olin Mathieson
1957
Each of the alkylated materials was produced in varying quantities, tested, and evaluated. The larger production scale plants built during the later program stages focused on producing HEF-2, HEF-3, and Hi-Cal 3. Nearly all production of the Nations boron fuel plants was used by the Air Force and Navy in testing applications and very little remained after the programs were canceled. All production facilities were shut down by 1960 except the Callery Muskogee plant that was placed in a standby mode to enable production to resume should the need arise. This plant would later become the source of most bulk pentaborane found in this country today. The need for storable space propellant revived interest in the boron hydrides, particularly pentaborane and diborane, in the early 1960's. Both the Air Force and the newly formed National Aeronautics and Space Administration (NASA) grew interested in the boron fuels as possible propellants for ICBM’s, low-orbit rockets, and spy aircraft, such as the U-2 and Blackbird. Under contract to the Air Force, the Muskogee plant was brought out of retirement to produce and package a large quantity of pentaborane for use at Edwards Air Force Base as a missile propellant. Little if any of this production run was ever used and most pentaborane produced under the shortlived contract currently remains at Edwards AFB. Small quantities of this shipment were parceled out for related testing work at many facilities, both Air Force and Navy, and evidence indicates that it has found its way into some bunkers and forgotten storage igloos across the country.
While being operated by Olin Mathieson, the Malta pilot plant experienced several near-misses involving detonation of material in lines, failure of processing equipment, and incidental vapor exposure, the symptoms of which workers referred to as “the goodies.” Although workers at the plant were equipped with gas masks and on occasion, constant-flow air hoods, exposure to the toxic vapors occurred during normal equipment maintenance and operation. On June 21, 1955, the Malta pilot plant exploded. The blast killed two Olin Mathieson employees working inside the facility and resulted in total destruction of the physical plant. The explosion was thought to have been caused by using carbon tetrachloride (CCl4) as a cleaning solvent in some process vessels. Subsequent research showed that combination of the two chemicals forms a highly shock-sensitive compound. A second accident occurred at a relocated Olin Mathieson pilot plant in Niagara Falls, NY on December 5, 1956. In this incident, air inadvertently entering a diborane process vessel caused an explosion and fire that killed one and injured four others. Unlike the earlier explosion, this one was less devastating to the plant and operations continued. In early 1959, another fatal accident occurred at the Olin Mathieson plant when a process vent line thought to contain residual amounts of penta- and decaborane exploded. The shock wave from the explosion passed down the pipe into an acetone storage tank that exploded, dumping its contents on a plant worker. A flash fire followed that engulfed that area of the plant, killing the worker. Records reveal several worker compensation claims having been filed by Olin Mathieson plant workers related to boron hydride exposure problems. The victims in all these cases experienced symptoms of nervous system distress but all of them were said to have fully recovered.
Records of accidents at the Callery Chemical facilities during the superfuels era are limited except for a single accident that killed three and injured another three people. Although this accident did not occur as part of the production or packaging process, it was directly caused by boron hydrides. The three people killed were Callery workers who took it on themselves to construct an experimental rocket using one of the Hi-Cal products as fuel along with an unknown oxidizer. While loading the homemade rocket (a copper tube), an explosion occurred with devastating results. After the closeout of the boron jet fuel program, Callery continued to produce pentaborane. The largest production facility (Muskogee) employed approximately 150 engineers and technicians. A study over three years beginning in 1961 of personnel involved in production of pentaborane indicated 21 cases of serious exposure and 46 cases of minor toxicological exposure(2). No fatalities were attributed to these exposures although they do illustrate the range of symptoms that can accompany exposure to varying concentrations of pentaborane. Symptoms of affected personnel included myoclonic spasms, tremors, loss of memory, anxiety, depression, fatigue, and abnormal electroencephalograms. Independent of degree of exposure, all affected workers returned to work and there was no long-term follow-up with these victims. A striking conclusion drawn from the study was that there was no treatment or antidote for pentaborane exposure short of administration of barbiturates to control convulsions. Very few, if any, documented accidents involving pentaborane occurred throughout the remainder of the 1960's and 1970's. This stands to reason since the material was neither being produced nor used to an appreciable extent except small, isolated research projects during this period. Not until 1982 did pentaborane strike with deadly force after its brief slumber. In Hanover County, Virginia on February 25, 1982, a cylinder decommissioning operation involving twenty-one cylinders excavated from a nearby construction site was interrupted when one worker opened a small pipe-like device used to open old cylinders to determine if the target cylinder was empty. Pentaborane immediately spilled out onto the worker’s bare hands. Four minutes after the dermal exposure, this worker went into convulsions and four minutes later became limp. This person died eight days after exposure with acute liver, kidney, and brain deterioration. A second worker nearby went to help the first worker and inhaled large amounts of pentaborane vapor. This worker remained conscious at the site but started convulsing en-route to the hospital and experienced a full cardiac arrest upon arrival at the hospital. This worker recovered consciousness but experienced significant brain damage and became quadrapeligic, mentally debilitated, and required institutionalization. A third worker also inhaled pentaborane vapors and experienced seizures and convulsions en route to the hospital. This worker was eventually released with no apparent physical problems.(3)
Besides the three workers directly involved with the incident, fifteen other people, including twelve rescue workers, two bystanders, and one environmental protection worker were exposed to pentaborane and required treatment. Because of the exposure, ten of the rescue workers were admitted to the hospital one day after the exposure with complaints of tremors, hallucinations, and mental confusion. (The effects of pentaborane exposure are often delayed up to twenty-four hours after exposure). Subsequent follow-up with all exposed individuals revealed that although the amount of vapor inhaled was very small, 50% of the patients experienced Post Traumatic Stress Syndrome and several others became clinically depressed (4). Protracted symptoms of exposure included lethargy, insomnia, nightmares, and poor emotional control. Follow-up studies with these patients revealed that the organic brain insult and resulting psychological trauma from pentaborane exposure have far longer lasting effects than previously thought. During clean-out operations of approximately twenty large pentaborane cylinders at the Callery Chemical Company site in Callery Pennsylvania conducted by an outside contractor, a bolt of fire erupted from one of the first “empty” target cylinders and shot upwards into the sky. Witnesses stated that the ball of fire was “the size of a Volkswagen.” This incident took place in July 1993 and the worker closest to the cylinder at the time of the incident experienced burns on his neck and face that required a brief hospitalization. Remaining cylinders in this group were decommissioned using different procedures and Callery personnel. In June 1994, eight large pentaborane containers were scheduled for decommissioning at the Divex Explosives Superfund site. These cylinders were part of a larger inventory of cylinders stored at a site that included a variety of other boron hydrides and smaller containers of pentaborane. Despite rigorous air quality monitoring and use of gas-tight protective ensembles during decommissioning, two workers were affected by pentaborane vapors, one almost fatally. During cylinder rinse-out, one worker became exposed to pentaborane vapors (cause unknown), lapsed into a coma, and was not expected to live. The affected worker made a miraculous full recovery. Significant amounts of specific preplanning along with emergency medical personnel administering oxygen to this victim may have played an important role in his recovery. The second worker became exposed over time due to a nearly undetectable fault in his breathing gear. This worker’s exposure symptoms included uncontrollable shaking, loss of memory, and loss of fine neurological motor control. Several months following cessation of exposure, this worker likewise made a full recovery. During operations at the Divex site, several other incidents occurred which underscored the difficulty of working with this product. The fire ball described in the opening paragraph was
the first incident to occur. Several smaller fires broke out in an inerted secondary containment chamber during processing operations due primarily to material failure or undetected plugging of vent and exhaust lines. No injuries resulted from these fires, but a strong respect developed for the unpredictability, toxicity, and pyrophoric properties of this boron-based adversary. The latest incidents on record involving pentaborane occurred in 1995 and 1996 at the Callery Chemical Company facility near Pittsburgh, Pennsylvania. Conversion of approximately twenty-five cylinders of pentaborane into elemental boron and hydrogen through pyrolysis had produced several medium sized cylinders of viscous still bottoms. While treating this material, several incidents occurred, one with injuries to contractor personnel. The first notable incident involved an explosion in a gas scrubber used by the contractor to neutralize vapors released during processing. In December 1995, an explosion in this scrubber ruptured the scrubber holding tank that resulted in a release of several hundred gallons of caustic scrubber solution on Callery grounds. During processing operations in December, workers reported experiencing symptoms consistent with pentaborane exposure and several required time off from work. However, it was not until January 1996 that an explosion at the processing site so powerful it shook nearby windows and was heard more than six miles away ended pentaborane treatment at the Callery site. The explosion destroyed the contractor’s equipment and containment system, blowing a ten-foot diameter hole in a building adjoining the operation site. Physically, the site appeared as though taking a direct hit from a howitzer shell or bomb. Fortunately, none of the workers at the site were seriously injured. A blast door between the system operator and the explosion collapsed on the operator causing injuries to his knees but little other damage. Support personnel were far enough away from the blast to avoid serious injury. STORAGE LOCATIONS Today, most remnants of the abandoned boron fuels development program are found at Edwards Air Force base in California. The last large production run of pentaborane was done under an Air Force contract and delivered to Edwards AFB sometime in the early nineteen-sixties. It is interesting to note that although material produced under this contract was shipped to the Air Force, it was packaged in containers stamped with US Navy markings. It is not known how many cylinders were shipped to Edwards under this contract but the current inventory stands at approximately 400 full cylinders. The large 800-lb Edwards AFB pentaborane cylinders are distinctive in that they are all painted with a light lime-green paint. Stenciled in black on the sides of each is information about the lot number, contract number, etc. In addition, most cylinders possess a slightly domed lid that fits over and onto the
cylinder collar that rises above and protects each of two valves found on the cylinder top. Some Edwards AFB stock was sent to other testing facilities in small shipments; mostly one and two cylinders at a time. Cylinders with markings showing they originally came from Edwards AFB under the pentaborane contract have been found at the following locations: 1) Reno, Nevada. Buried at an abandoned rocket engine test site, forty-eight cylinders had been shot with rifles from long distance and buried in the desert although they still contained residual amounts of pentaborane. These cylinders were decommissioned in 1993. 2) Malta, New York. A single empty cylinder was discovered buried at the old Malta Test Facility. This cylinder was decommissioned in 1994. 3) Columbia, South Carolina. Disguised by silver paint and labeled as propane tanks, eight cylinders containing residual amounts of pentaborane were discovered at the Divex Explosives Superfund site. These cylinders were decommissioned in 1994. 4) Huntsville, Alabama. Four large pentaborane cylinders are currently at the Redstone Arsenal. Three of these cylinders are full and weigh approximately 800 pounds while the fourth cylinder is approximately one-half full. Pentaborane at Redstone Arsenal was apparently used in studies conducted in the late 1960's to determine viability for use in the Viper Missile System that ended unsuccessfully and the cylinders were set aside for “future study.” The first large cylinders of pentaborane were “discovered” in September 1992 during a RCRA site audit. Soon after, an installation-wide query was made to learn if any other pentaborane was stored on site. From this second search, two additional 800 lb. cylinders were found and in 1995 a nondestructive evaluation was done which determined the four large cylinders were in good structural condition. The cylinders are currently stored in an isolated ammunition bunker along with several other smaller cylinders, also containing pentaborane. A smaller twenty-pound cylinder owned by the US Navy and stored at the University of Florida was recently discovered through joint EPA/DOD Pentaborane Task Force initiatives and is being relocated to Redstone Arsenal for interim storage. Shipping records from Edwards AFB show small quantities of pentaborane were sent to: 1) 2) 3) 4) 5)
Ohio State University (1981) University of Pennsylvania (1982, 1988) University of Virginia (1984, 1989, 1992) Syracuse University (1988) Rockwell Sciences (1993)
6)
Northwestern University (1994)
Twenty-one cylinders from the Edwards AFB contract were returned to Callery Chemical Company in 1985 for recovery of elemental boron. The contents of each cylinder were processed through a distillation unit followed by injection into a hightemperature Inconel pyrolysis reactor where the pentaborane dissociated into boron and hydrogen. Boron from this operation was collected and staged on site. The empty cylinders were similarly staged and later decommissioned. Smaller quantities of pentaborane are in storage (and use) throughout the United States at military propulsion and weapons testing facilities, commercial rocket development sites, and university laboratories. These cylinders are usually found in small, blue-colored containers holding approximately one pint of liquid pentaborane. One of these cylinders was decommissioned as late as June 1996 at a Naval facility near Washington, D.C. Other smaller sized cylinders may be found that share the appearance of a household grill propane container. These mid-sized cylinders are typically painted deep blue and have a large valve for liquid removal and a smaller valve for introducing inert gas to force liquid from the cylinder out its dip tube. Project Hermes and Project Zip relics may still be found in dusty storage sheds at testing facilities around the country. As an example, in the fall of 1995, a container of Hi-Cal 3 with a barely discernable label was discovered at a West Coast research facility. This cylinder was disposed of commercially due to the small amount of material known from its weight to be present inside. Recent audits and inventories over the last several years have identified several military locations where large cylinders of pentaborane are currently stored. These locations are primarily arsenals and depots but in all cases, disposing of the material is complicated by the following issues: 1)
2)
3)
4)
Edwards AFB cannot accept any pentaborane as returns or “new” stock due to regulatory, political, and technical restrictions. Commercial disposal facilities are unwilling to accept pentaborane, with one exception where the facility accepts lecture bottle sizes of pentaborane on a caseby-case basis. No intermediate staging areas have been identified or sanctioned by the EPA or DOD for storage of remnant stocks of pentaborane. Although the overall condition of most pentaborane cylinders has been fair to good, the catastrophic consequences of an in-transit accident involving pentaborane make shipment of the material politically very difficult, very costly, and almost completely unfeasible.
DISPOSAL OPTIONS A number of disposal technologies have been used with varying degrees of success to destroy or neutralize pentaborane. These approaches have included: 1) 2) 3) 4) 5) 6) 7)
Flaring/Incineration Pyrolysis Alcoholysis Amination Ammoniation Open air oxidation Hydrolysis
Large-scale disposal of pentaborane has only been attempted using flaring, pyrolysis, oxidation, and to a lesser extent, hydrolysis. Chemical treatment of pentaborane has been conducted on a bench scale but never used to neutralize significant quantities of the chemical. Flaring/Incineration Flaring of vapors from pentaborane synthesis and handling operations was the standard procedure during the boron fuels development era. Flares were used in combination with largely ineffective water scrubbers to burn off vapors vented from tanks, reactors, and process lines. Product information bulletins published by both Callery Chemical Company and Olin Mathieson illustrated flare configurations to use when venting excess pressure from cylinders or connection lines. These flares consisted of kerosene-soaked rags positioned at the end of a vent pipe that was poised over a shallow pond or burn pit. When operating, flares burning pentaborane vapors burn with distinctive green streaks in the flame. Flares work well with small concentrations of pentaborane vapors, generating small quantities of boron oxides according to the formula, 2B5H9 + 12O2 6 5B2 O3 + 9H2 O. When faced with larger pentaborane loadings, flares do not perform to a standard where all the feedstock is oxidized. This can lead to release of non-oxidized pentaborane and formation of partially oxidized material as B4H12O and B2H2O3. Consequently, flaring has not been used extensively to reduce this country’s existing stockpile of pentaborane. Pyrolysis Although not truly used as a disposal procedure, pyrolysis was used very successfully by Callery Chemical Company to convert approximately twenty-one 800-pound containers of pentaborane into elemental boron and hydrogen. The process entails injecting purified pentaborane vapor into a hightemperature reactor in the absence of air. The chemical immediately dissociates into its component parts with boron powder leaving the system as a solid and hydrogen gas expelled through a filtered vent.
The actual pyrolytic process as configured by Callery works well if incoming pentaborane is very pure. If any impurities enter the small orifice injection nozzle, plugging is a likely consequence. The system must then be dismantled and cleaned, making it unsuitable for processing large volumes of material in a cost-effective manner. The process of purifying incoming pentaborane involves a distillation step that leads to high purity output but results in a viscous “still bottom” material that possesses most toxic, reactive, and pyrophoric characteristics of the incoming feed stock. Disposal of these still bottoms can be very problematic and it was treatment of this material that led to the devastating explosion at the Callery plant in January 1996. Alcoholysis Pentaborane reacts with methanol to form methyl borate and hydrogen gas according to the following equation: B5H9+15CH3OH 6 5B(OCH3)3 + 12H2 The reaction is rapid, generally running to completion in less than 24 hours (5). Alcohol can also be used as a mutually miscible solvent to increase the rate of hydrolysis since water and pentaborane do not readily mix. Alcohol has not been used extensively for disposal of pentaborane because of formation of methyl borate (a toxic byproduct), flammability hazards associated with handling alcohol, and a hazardous waste stream that still requires disposal at completion of the process. By contrast, alcohol has been used successfully in cylinder and process pipe decontamination work where small quantities of pentaborane must be removed from an enclosed system.
away from the ignition site and toxic residue from the operation as boron oxide and boric acid remains behind. This method may be appropriate for use in areas where population density is very low and surficial contamination of the ground is of little concern. In open air oxidation, pentaborane reacts rapidly and pyrophorically with oxygen in air to form boron oxides. This reaction is exothermic and simultaneously releases large amounts of hydrogen that in turn, tends to ignite violently. When hydrogen combines with oxygen in the air, water vapor is formed and presents no toxic or physical hazard to personnel nearby. The boron oxides, however, are toxic particularly to plant and aquatic life. Based on this knowledge, open air oxidation is best suited to dry, barren environments such as deserts. Both known examples of large scale use of open air oxidation have been or are being done in desert environments away from population centers, waterways, and large flora concentrations. As noted, the forty-eight cylinders unearthed in the desert near Reno, Nevada had been perforated with bullets before burial in an effort to release pentaborane to the air, allowing it to ignite. Photographs of this operation in progress show brilliant orange and green flames in a large pit after the cylinder has been breached. Personnel shooting the cylinders were located a distance away on a hill overlooking a small valley where the cylinders were placed. Unreacted pentaborane released from perforated cylinders would likely travel down the valley away from the shooters and leave them unaffected.
Ammoniation Ammonia has been used to some degree for surface decontamination of pentaborane. When combined, ammonia forms a stable complex with pentaborane (B5H9.4NH3) (1) which is suitable for handling low concentrations and small quantities. Like the amines, ammonia has never been used for large scale disposal operations involving pentaborane.
An adaptation of open air oxidation was adopted by Phillips Laboratories under contract to the Air Force at Edwards AFB for pentaborane stockpile disposal. Over the past several years almost ten pentaborane cylinders have been fitted with shaped charge explosives and detonated in the desert at a remote part of the base. Although incomplete, air quality monitoring conducted downwind of the detonation site has not shown the presence of pentaborane vapor escaping from the area. On several occasions, however, monitoring equipment was either damaged or rendered nonfunctional during testing intervals, primarily from the large concussion caused by shock waves. Additional damage to monitoring equipment (mostly cameras set up to monitor operations visually) has occurred due to release of large amounts of heat from igniting pentaborane. In several cases, heat was reported to have literally melted monitoring cameras. No reports of boron residue monitoring in the desert around the test area have been made.
Open Air Oxidation Open air oxidation has been used for many years as a means to dispose of pentaborane. It is the least complex, least expensive, and most rapid means of eliminating the chemical. The process involves allowing pentaborane to contact air rapidly, thereby permitting it to react pyrophorically. The downside of employing this method is that a large amount of pentaborane does not ignite pyrophorically and simply drifts
Most recently, open air oxidation at Edwards AFB has involved opening two cylinders simultaneously using shaped charges. This was part of a larger plan that would eventually involve opening up to four cylinders at a time. During the first trial using two cylinders, the shaped charges performed as expected and opened the cylinders in quarters along their longitudinal axes. In one report, very high heat was reported that melted one monitoring camera and part of the “burn box”
Amination Pentaborane reacts with a range of amines to form unstable and occasionally reactive complexes. The most water reactive amination product is made by reacting pentaborane with isopropyl amine which forms B5H9.4NH2(C3H7) as an oil(6). Amines have not been used outside the laboratory for disposal of pentaborane.
equipment designed to contain target cylinders. Following this initial trial using multiple cylinders, plans are being revised for continuing the operation. No reports of boron residue sampling or monitoring in the desert surrounding the test area have yet been made available to the scientific community or general public. An additional concern with open air oxidation at Edwards AFB was that other contaminants, such as lead, would be released to the environment during testing. Lead, it was thought, could come from cylinder welds, tubing, and other cylinder components. Preliminary sampling results performed during one trial burn showed some indication that lead compounds may pose a problem during more extensive cylinder disposal operations. The greatest disadvantage to this disposal technique, however, remains that vast, unpopulated areas are required for the this “uncontrolled” method. Hydrolysis Reaction of water with pentaborane (hydrolysis) was first documented in 1954(7). In early studies, it was noted that although a reaction did occur between water and pentaborane, the reaction was quite slow. A phase separation between pentaborane and water was noted, even after shaking samples. The phase interface was marked by “white solids” and bubbles, indicating the active hydrolytic site. Shaking was observed to increase the rate of hydrolysis but had to be maintained to keep the reaction going. By-products of pentaborane hydrolysis are hydrogen gas and boric acid according to the formula: B5H9 + 15H2O 6 5B(OH)3 + 12H2 The reaction is exothermic, liberating approximately 275 kcal/mol of pentaborane reacted. Addition of a mutually miscible solvent, such as dioxane, greatly increased the rate of reaction between water and pentaborane. Reactions between the two chemicals using this strategy proceed very rapidly and exothermically while liberating large amounts of hydrogen gas. ( However, dioxane itself is highly toxic and dangerous to use and store). Increasing temperature also plays a role in determining reaction rates with higher temperatures encouraging the rate of reaction. Hydrolysis has been used in pentaborane scrubbing processes beginning with work done under the Project Hermes and Zip programs. Very little quantitative data was developed to support this practice but it was believed that light chemical loadings into large, countercurrent, packed bed water scrubbers could be effectively neutralized. Downstream flares connected to scrubber exhaust ports to burn hydrogen given off in the hydrolysis reaction were often observed to contain green streaks, providing evidence that hydrolysis using this system was incomplete at best. More recently, hydrolysis variants have been used to convert pentaborane into a fairly nontoxic boric acid by-product
and gaseous hydrogen. This approach was used at the Divex Explosives site, the Malta Test Facility, and several sites where very small quantities of pentaborane were found. In these approaches, hydrolysis reactions were improved by addition of sodium hydroxide and in some cases, alcohol. Alcohol speeds hydrolysis due to its mutual miscibility with water and pentaborane but also reacted on its own through alcoholysis. Sodium hydroxide in the reagent solution reacted with boric acid formed during hydrolysis to create nontoxic borax. Although hydrolysis of pentaborane in small quantities is possible using sodium hydroxide and alcohol as reagent additives, the rate and efficiency of hydrolysis is insufficient for use on the larger quantities known currently to exist in storage. As a technique for rendering pentaborane nontoxic while at the same time addressing its pyrophoric nature, hydrolysis continues to be a preferred option with the lowest potential for negative environmental and human health and safety consequences. Open air oxidation can be successfully employed in very remote regions where environmental impact is negligible (ie. desert environments) but for safely and responsibly processing pentaborane in more temperate climates, hydrolysis appears to be the most viable option. Enhanced Hydrolysis To address needs for a high-capacity pentaborane processing system that can be employed at a variety of sites, Integrated Environmental Services, Inc. (IES) has developed an enhanced hydrolysis processing system. This system incorporates a mobile design, allowing it to be used across the country where pentaborane is believed to be stored. In addition, to reduce hazards to operating personnel (where most exposures have occurred in the past), the process is completely remote controlled and operated from a safe distance. Worker interface with target cylinders is limited to placement of cylinders into an airtight chamber and connection of process piping to each cylinder valve. Key factors in achieving an enhanced hydrolysis on a continuous flow basis are rapid and extensive physical mixing augmented by addition of several key chemicals. Additions of sodium hydroxide (to neutralize boric acid and form borax, which is later precipitated out of the waste stream), hydrogen peroxide, and a reaction catalyst are the key elements in achieving hydrolysis. Water and sodium hydroxide are consumed in the reaction and are replenished as needed. Continuous process monitoring through various pH and ORP sensors gives operators information needed to maintain correct chemical balance throughout the reaction process. Deleting alcohol from the reaction process is important from two standpoints; alcohol is a flammable liquid that creates a hazard by virtue of simply being part of the process, and, when added to an aqueous process stream, creates an organically-contaminated media. Once in the process stream, alcohol not consumed in the process must either be distilled out
or disposed of as costly hazardous waste. Enhanced hydrolysis eliminates alcohol from the process and by that avoids the cost and system complexity needed to remove it on the downstream side. Briefly, the system works in the following manner: 1)
Target cylinder placed into airtight chamber and connected to processing piping. 2) Air in system removed and replaced with nitrogen. 3) Actuated valves open to allow pentaborane to flow from cylinder into process piping. 4) Metering pumps feed pentaborane into vacuum line at approximately 10 lbs/hr. 5) Pentaborane contacts reagent in jet pump chamber and travels to high-shear mixer. 6) Process liquor passes through high-shear mixer and enters extended torturous path. 7) Heat given off during hydrolysis is removed by liquidcooled jacket around piping. 8) Process liquor enters settling reactor. 9) Hydrogen and any residual vapors are vented from reactor to dry scrubber and flare. 10) Vapors analyzed for chemical constituents. 11) Process liquor reagent concentrations measured and adjusted. 12) Process reagent re-circulated to jet pump. Once the target cylinder is empty, it is inverted mechanically and residual liquid is removed through the vapor phase valve. Final clean-out of an empty cylinder is accomplished using an organic solvent mix that simultaneously dissolves and reacts with residual solids that may be found inside the cylinder. The final clean-out system is a separate circuit from the main processing system and will not contaminate the aqueous reagent stream. Prolonged storage, especially under high heat conditions, can lead to formation of light “sludges” or solids inside pentaborane cylinders. These solids can present problems during hydrolysis and other neutralization operations because they are not readily dissolved. This problem only adds to an already very dangerous situation when handling pentaborane. Use of alcohol in a kerosene matrix is effective at dissolving accumulated solids while simultaneously neutralizing them. Following final rinse-out, the cylinder is de-valved inside its chamber, inspected to ensure it is completely empty, and removed for decommissioning by cutting in half. CONCLUSION From an understanding of the development history and use of pentaborane in this country, it is likely that cylinders of this chemical will continue to be “discovered” at various research facilities around the country. It is essential, based on the advanced age of these cylinders and their contents that a usable
plan be put in place to effectively neutralize hazards posed by continued storage or transport of this material. Despite the difficulties and extreme hazards associated with handling pentaborane, enhanced hydrolysis offers a viable disposal alternative to the many other methods used in the past. Its primary advantages are that it is relatively simple in operation, does not generate toxic or costly by-products, can be configured as a mobile system, and can be operated at low cost with a reasonably high throughput. The joint EPA/DOD Pentaborane Task Force is supportive of the enhanced hydrolysis approach for pentaborane cylinders found in populated areas or where other limitations preclude alternate disposal methods. Based on experiences of others dating to the 1950's and on those acquired during recent pentaborane neutralization operations, enhanced hydrolysis represents a cost efficient, reliable, and safe method to “battle” the dragon known as pentaborane and win. REFERENCES 1) Stock, A. E., Hydrides of Boron and Silicon, Cornell University Press, Ithaca, New York. 1933. 2) Mindrum, G., Pentaborane Intoxication, Archives of Internal Medicine, 114, 364-374, 1964. 3) Yarbrough, B.E., et al, Severe Cental Nervous System Damage and Profound Acidosis in Persons Exposed to Pentaborane, JAMA 23 (7&8), 519-536, 1985. 4) Silverman, J.J., et al, Posttraumatic Stress Disorder From Pentaborane Intoxication, JAMA 254 (18), 26032608, 1985. 5) Schecter, W. H., Jackson, C. B. and Adams, R.M., Boron Hydrides and Related Compounds, 2nd Edition, Callery Chemical Company, May, 1954. 6) Schlesinger, H.I., University of Chicago, Navy Contract N173S-9820, Final Report (1945-1946). 7) Shapiro, I. And Weiss, H.G., Hydrolysis of Pentaborane, JACS 76, 6020-21, 1954.
