Chemicals I Have Known (and Made) – Hydrogen Cyanide

hcn

As I look back on my career in industry, I realize that I became inured to the chemicals I dealt with and produced. I will be posting occasionally on some of the materials I worked with and made during the first part of my career. The first chemical I worked with was hydrocyanic acid – a simple molecule consisting of a hydrogen atom, a carbon atom, and a nitrogen atom (HCN). This molecule is so simple that there are molecular clouds in space where HCN is found, released from stars that have synthesized carbon and nitrogen in their core. But HCN has a well-known reputation as a poison, one that prevents oxygenated blood from being able to deliver their life-giving load to cells. Once oxygen transport ceases, energy production in a cell stops, and the cell and the organism that contains the cell dies.

 

So at the chemical plant I worked at, one of the requirements to work in the cyanide area was to ensure that I could detect cyanide leaks so I would not wander into an area with a fatal concentration. This was done by means of a sniff test. Three beakers of water were set on a tray. Two were plain water, and the third had a concentration of cyanide in it that resulted in small amounts of cyanide vapor in the air above the beaker. To pass the test, you had to tell which beaker held the cyanide. The first time I took the test, I was guessing somewhat. None of this “bitter almonds” smell, just something that was a little off. By the last time I took the test, almost 10 years later, I picked up the beaker with the cyanide and before it made it halfway to my nose, I put it back down on the tray and said “That’s the one.” What was originally too faint for me to be certain had become so overwhelmingly repugnant over the course of a decade that it gagged me.

 

Cyanide. What’s it good for? Hydrogen cyanide is used in quite a few chemical processes as a feed stock. One of the chemical processes is used to make another chemical called methyl methacrylate (MMA), used in acrylic paints and in plastics like Plexiglass. My chemical plant made MMA as well, but that’s a story for another day. The other main use of cyanide was to make sodium cyanide, which is used in the mining of precious metals. Sodium cyanide solutions are able to leach small concentrations of gold, silver, and other precious metals out of ore, allowing it to be concentrated and extracted into product. Our plant produced sodium cyanide as well as HCN. Some HCN is shipped to other locations for use. When it was shipped, the tank cars that contained it were painted in a distinctive manner. They had red stripes on them – one that circled the car lengthwise, and one that circled the circumference of the car, forming a cross on both sides of the car where the stripes collided. These cars were called candy stripers in the trade.

 

Hydrogen cyanide is produced when ammonia, natural gas, and air are heated and passed over a platinum – rhodium gauze mesh. The off-gases are then absorbed, and the cyanide produced is concentrated and purified. At our plant, HCN was stored in tanks surrounded by dikes. One of our safety features was flare guns mounted on posts throughout the tank farm. If the worst happened, and liquid cyanide were to leak out onto the surface of the dike, folks were instructed to fire a flare gun and set the liquid on fire. HCN is volatile (78ºF boiling point), but the vapor will not explode. Instead, it will undergo a deflagration where the combustion wave front is slower than the speed of sound. Other gases like methane will explode, where the combustion wave front is faster than the speed of sound, which causes the pressure wave that creates damage in an explosion. So for HCN, it is much better to let it burn and eliminate the toxic vapors evaporating from the liquid surface.

 

One day in 1979, I was out at the plant on a Saturday. I remember that Dr. Jenks was there on that day as well, and he invited me into his office. Dr. Jenks was one of those older generation chemists who knew everything about the chemistry and processes. He had a wooden box in his office, about 18″ on the narrow sides, and about 10′ long. In that box was the replacement platinum/ rhodium gauze for the catalyst change. At that time, when precious metal prices were at a 30 year high, his office held about 2 million dollars in platinum and rhodium. I was impressed.

 

My main job in manufacturing support was in the waste treatment process. As you can imagine, the waste water from these cyanide processes needed special treatment and segregation from other waste water. The “state of the art” water collection system consisted of cypress lined trenches, with cypress boards covering the top. This ran downhill to the bottom of the plant, where we had the Trade Waste water treatment facility. Waste water came into a collection point, where sodium hydroxide was added to make sure that the water was basic. If cyanide ions were in an acidic solution, cyanide vapor would be released above the solution, and that is not a good thing. So once the pH was adjusted to make the waste basic, then it would be mixed with liquid chlorine. Our plant produced sodium metal and liquid chlorine, so we had only to send the chlorine down a pipe to the water treatment plant, and mix it in with the waste water. When the chlorine hit the basic water, it produced chlorine bleach solution (sodium hypochlorite). Bleach attacks the cyanide and converts it to a non-toxic degradation product. To ensure that the reaction took place, after treatment the water was diverted into what were called 8-hour ponds. These ponds were on either side of the treatment building, and were unlined ponds where the water was held until the reaction was complete. Then the water was released into a baffled chamber called the one hour pond where it was analyzed to make sure that all of the cyanide was destroyed, and after the last test, the water was combined with the other sewer waste and went into the City of Memphis sewage treatment system. Unfortunately at the time, our interceptor sewer did not hook up to the sewage treatment system, and the water along with all of the domestic wastewater was discharged directly into the Mississippi River. Environmental protection has definitely improved in the 40 years since I was working in this process.

 

I would imagine that the staffing situation for the Trade Waste process has also improved. Back when I worked at the plant, there was a single operator who was stationed at the treatment plant. This individual sat in a central control room, and on either side of the control room were the chlorine injectors with the liquid chlorine flowing through them. Now, I don’t know about you, but I would be hesitant to work by myself, with cyanide-laden waters and liquid chlorine surrounding my office, but back in the late ’70’s, I didn’t think as much about the implications of what could go wrong. The plant had a safety procedure where the person working in a remote location had to check in with the main control room at least once per hour. Believe me, at that time, so much could have gone wrong in an hour’s time that the operator could have been dead for 59 minutes. But it never did at that facility during at least the first 30+ years of operation. Looking now at the facility on Google earth, it is obvious that they have made significant changes and improved the safety of the treatment operation. But some of the facilities look similar to what I worked with 40 years ago.

 

There’s much more I could go into. Cyanide has some amazing chemistry, and the waste treatment is almost an art unto itself. I did some large-scale testing there where we added a hydrogen peroxide waste stream that was what I considered to be fun chemistry. But it was definitely a good process to work on for my first real production support for making a nasty chemical.

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