Phosphate Rocks Chapter 39: Recycle

John had always known that women were the stronger sex – look at what they put up with to propagate the human race. He had come across a handful of female engineers in his time. All terrifying. Fishy Olga from the Russian factory boat, hard as the iron spikes she hammered into the ice of the Bering Strait to free a trapped a Royal Navy Frigate. Massachusetts Meg who fixed instruments on the aircraft carrier. And Fiona, the cause of so much new plumbing. 

Not to put too fine a point on it, Fiona was not in the same league as Olga and Meg, just a pain in the neck. Confident without skill, impetuous without experience. Nothing but trouble. 

The new granulation plant at SAI had pipe reactors; the sort of equipment chemical engineers get excited about. Normal reactors are just scaled-up test tubes. You add the ingredients, liquids from a pipette, solids with a spatula, one then another, they react and then you tip the product out. Or scrape it out. Or break the test tube to get it out.  

It’s called batch production because you produce one batch at a time, then start all over again. That’s a chemist’s idea of process plant design. 

In pipe reactors, you get the sort of plant design a practical engineer prefers. You add reactants at one end and the product comes out the other end, pushed out by more stuff being fed in all the time. It’s continuous production. When everything is balanced, it can be much more efficient, but more temperamental to start up. Harder to get right, and there’s another tricky aspect.  

Ammonium nitrate is also used as an explosive. 

Pure ammonium nitrate is pretty stable unless you detonate it or set it on fire. In Oppau, Germany in 1921, attempts to unblock an ammonium nitrate silo using dynamite led to a much larger explosion and the deaths of more than 560 people. Twenty-six years later, in Texas City on Galveston Bay in 1947, a crowd gathered on the dockside to watch a spectacular fire aboard SS Grandcamp. Unfortunately, the ship was loaded with ammonium nitrate for French farmers, along with ammunition and bales of sisal twine. The cargo exploded and killed the crew, all but one of the town’s fire brigade, and many of the spectators who came to watch the fire. Over 580 people died. 

More recently, in 2013 in West Texas, USA a fire at a warehouse caused about 240 tonnes of ammonium nitrate to explode, killing fifteen people. In 2020, 2,750 tonnes of ammonium nitrate exploded in the port of Beirut, Lebanon, killing at least 220 people, injuring more than 5,000 and leaving 300,000 homeless. 

So if you are going to avoid turning a pipe reactor into a pipe bomb, you need a way to remove heat. And the most efficient way to remove heat is with water. The clever French designers added a quench. If the temperature or pressure rose faster than expected, if there was so much as a hint that the reaction was running away, a deluge of water put a stop to all that. The watery mess was diverted to a slop tank. The slops could go one of two ways: back into the granulator or out to Gypsum Island. 

Now, although the sea is full of sodium fluoride and calcium sulphate, it is not that full of ammonium nitrate. It’s just another salt, but not one found so commonly in the sea. You might expect some fertiliser run-off from Brodie’s farm, the fields stretch right down to the beautiful beach near Tantallon castle, but Brodie is a careful farmer and always waits for dry weather before spreading. 

Fiona was on night shift when the pipe reactor hiccupped, and the quench went off. Everything happened as designed, and the team got ready to try again. Unfortunately, the slop tank was full. Previous quenches from previous hiccups on previous shifts had been allowed to accumulate. The high-level interlock system prevented restart until the liquid in the slop tank was removed. 

Fiona had a choice. Two options. 

Sending the quench into the Firth of Forth is what every other shift would do. Except they hadn’t, and now the tank was full. 

The lead operator’s instruction was clear. Out to sea. What were the chances of an Environment Agency inspector taking any notice at one o’clock in the morning? 

Fiona, being young and foolish, disagreed. She had been put on the planet to save it, her reasons for studying chemical engineering were partly altruistic. Here was the first chance to do something useful. She spoke persuasively in favour of recycling. The lead hand disagreed vehemently. He knew it was wrong, though he couldn’t explain exactly why. Wrong because it was different. He made the mistake of adding some rude words about the competence of women in general and Fiona in particular. 

Fiona ordered him to recycle it back to the granulator as punishment. Unfortunately, it being one o’clock in the morning, she failed to do any of the calculations she had spent four years at university being trained to do. Nor did she refresh her memory on the design criteria, which could have formed the basis for detailed instructions. All commissioning documents were locked up in the office block half a mile away and Fiona was in a hurry. She had to be at a training course at 9am the following day. In order to get a few hours’ sleep, she needed to leave the site by 2am. She knew that as soon as she left, the contents would be dumped to sea, so she waited until her instructions were obeyed. 

What should have been dribbled in to the pre-granulator over days was sent in one, bad-tempered, fell swoop. 

The pre-granulator was a cylinder that rotated. Shorter than the main granulator and only a man and a half in diameter, it rotated to mix the discharge from the pipe reactors. Fixed wheels with thick rubber tyres pressed against steel belts that wrapped the girth of the cylinder. As the liquid from the slops tank gushed in, the solids that rose and fell in a helical dance along the length of the pre-granulator became soft and squishy, and then joined together into lumps before puddling into pancakes and then finally melting into a single liquid wave that rose and splashed back. 

The lead operator watched the temperature dropping and the pre-granulator filling and the motor current rising with growing satisfaction before calling John. John ordered him to increase the feeds to get more heat in, but to no avail. By the time John had arrived in the control room, the motor current had increased until it could drive the cylinder no longer. The pre-granulator stopped. The liquid inside solidified. There was only one thing for it. 

Dig it out.