About 78,000 pounds of truck and hazardous phosphoric acid met a shredded, discarded tire in the slow lane of the Capital Beltway yesterday morning, and for hundreds of commuters and holiday travelers, it was a close encounter of the worst possible kind.

"We fine but this rig ain't," said Owen Black, 37, of Mobile, Ala., who was behind the wheel of the tanker truck when it overturned and ruptured shortly after 3 a.m., spilling a small portion of its 20 tons of corrosive acid onto the outer loop of the highway.

"It was so quick when that liquid shifted," said Black, as he stood beside his battered rig, surrounded by a special squad of Montgomery County firefighters who were still mopping up the last of the spill nine hours after the crash.

Neither Black nor Eddie Young, 41, also of Mobile, who was in the truck's sleeping compartment at the time of the crash, was injured.

The accident backed up traffic for about three miles and forced Maryland State Police to shut down three lanes of the beltway -- two fast lanes on the outer loop and one on the inner loop -- near the Montgomery-Prince George's County line. They sealed off ramps linking New Hampshire Avenue and the beltway and directed traffic on I-95 away from the crash.

A state police spokesman said it was "unlikely" that Black would be cited as responsible for the crash, which delayed scores of Friday morning commuters and Labor Day beach-goers.

The accident, which occurred as the number of such beltway accidents appears to be falling slightly, also set in motion a coordinated but slow-moving cleanup that involved more than 20 firefighters, a 10-man wrecking crew and a fleet of rescue vehicles, pump trucks and cranes.

"If the material had been something much more hazardous, jeez, it could have been worse and we'd shut the whole beltway down," said Sgt. Jeffrey Anderson, a Montgomery County firefighter from the nearby Hillandale station.

As it was, a cleanup crew equipped with gas masks had to laboriously transfer 3,500 gallons of acid from Black's ruptured truck to another tanker. Employes of a Silver Spring wrecker company then used inflatable rubber bladders and trucks with hydraulic cranes to right the overturned tanker.

The procedure, which lasted several hours, highlighted the persistent problems of interstate transport of hazardous materials and heavy-truck traffic on the beltway, several officials said.

The American Automobile Association yesterday issued new figures showing that tractor-trailer trucks, including big tankers, were involved in 156 accidents on the beltway in the first six months of this year. Of that total, 114 occurred on the Maryland side and 42 in Virginia, AAA spokeswoman Mary Anne Reynolds said.

Reynolds said the statistics were an improvement over the same period in 1983 when there were 208 such accidents: 147 in Maryland and 61 in Virginia.

However, a spokesman for the American Trucking Association said the association had received 890 complaints about tractor-trailers on the beltway since it started a special telephone hotline in mid-February.

Black, an employe of Miller Transporters Inc., a Jackson, Miss., firm, picked up his cargo of acid in Bayonne, N.J., and was bound for a Monsanto chemical plant in Augusta, Ga.

When his truck struck the discarded tire, its left front tire came off its rim, sending the tanker into a slow roll that ended in a collision with the concrete barrier that separates the inner and outer loops of the beltway.

"It was a fairly simple accident," said state police 1st Sgt. Ed Johnson. "No injuries, just a long cleanup."