Mining and Refining: Sulfur | hackaday

Sulfur scaled

When you think of the periodic table, some elements just have an atmosphere that is completely unscientific, but nonetheless undeniable. Precious metals such as gold and silver are obvious examples, as they have always been associated with the wealth of kings. Copper and iron are sturdy, working-class metals, each worthy of having entire centuries of human industry named after them, with silicon now forming the backbone of our current information age. Carbon builds the chemistry of life itself and fuels nearly all human endeavors, and none of us would get very far without oxygen.

But what about sulfur? Nobody seems to think much about poor sulfur, and when they do, it’s usually derogatory. Sulfur brings the stench to rotten eggs, threatens us when it spews from the mouths of volcanoes, and can become a deadly threat when used to make gunpowder. Sulfur seems more associated with the damaging processes and dismal factories of the early industrial revolution, not a part of our modern, high-tech world.

And yet, despite its malodorous and low-tech reputation, there are actually very few industrial processes that are somehow not dependent on massive amounts of sulfur. Sulfur is a crucial ingredient in processes that form the basis of almost all industry, so its production is usually a matter of national and economic security, which is strange considering that almost all the sulfur we use is recovered from the waste of other industrial processes.

It’s always oil

Sulfur is one of those elements that is remarkably abundant in the universe and although it exists in its elemental state, it is more typically found as a compound of something else. This is due to sulfur’s ability to form more than 30 allotropes, or different forms in the same physical state, and to the wide range of chemical reactions it participates in – there is a sulfide or sulfate of almost every other element on it. periodic table, except for those cocky noble gases.

On Earth, sulfur is most commonly found in sulfide minerals, where an atom with a positive charge bonds to one or more negatively charged sulfur atoms. Examples are chalcocite (copper sulfide), galena (lead sulfide), cinnabar (mercury sulfide) and pyrite (iron sulfide). Sulfates, where sulfur and oxygen bond with a cation, are also common; for example, the gypsum used to make drywall and PVC pipes is calcium sulfate.

The abundance of sulfide and sulfate minerals, and the fact that, in general, anything to which the sulfur is bound in these minerals is valuable in its own right, means that sulfur can be recovered as a by-product of smelting activities, particularly from the smelting of lead, copper, and zinc ores. We’ve covered copper smelting to some degree; the basic process is the same for most sulfide mineral melts and uses heat to expel the sulfides. In less environmentally conscious times, and when there were other, cheaper sulfur sources, the sulfur-laden flue gases were simply vented, triggering a series of reactions in the atmosphere that culminated in sulfuric acid falling from the sky — acid rain.

However, the recovery of sulfur from smelter flue gas is only a small fraction of current sulfur production – currently only about 7% in the US. The majority of sulfur production worldwide comes from petroleum refining or natural gas production, where sulfides are contaminants that must be removed. Clearing sulfides from “acidic” gases — so called because they are both sour and foul-smelling thanks to hydrogen sulfide (H2S) — is the job of an amine treater or sweetener. Amine treaters are used in a variety of industrial processes; we came across them when we were discussing how helium is refined from natural gas. Amine treatment relies on the ability of amine solutions, such as monoethanolamine (MEA) and diethanolamine (DEA) to react with the acid gases, such as H2S and CO2, and make them more soluble in the scrubbing solution than in the process gas. The sulfide-rich amine solution is then boiled to remove the sulfides and regenerate the amine for reuse. The process cleans the incoming sour gas enough to be released into the atmosphere, as well as a supply of H2S, which can then be processed into elemental sulfur.

From sour to sweet

The hydrogen sulfide extracted from the acid gas is highly toxic to humans and particularly dangerous because at sufficiently high concentrations it paralyzes the olfactory nerves; people who are exposed to it for only a few minutes think that the gas has disappeared and that the danger has passed because they can no longer smell the rotten egg smell. Although it has some industrial uses, most hydrogen sulfide is converted to elemental sulfur, which is much easier to store and transport. The main process used to convert H2S into elemental sulfur is the Claus process, named after German chemist Carl Friedrich Claus, who invented it in 1883.

The Claus process is a two-stage process: a thermal step, in which hydrogen sulfide is burned in an oxygen atmosphere, and a catalytic step, which increases the sulfur yield. The thermal step is extremely exothermic and takes place in a so-called Claus furnace, a strong chamber lined with refractory material to withstand temperatures above 1050°C, necessary to burn unwanted products that clog downstream. catalyst bed. The general reaction of the thermal step looks like this:

Due to the high temperatures in the Claus furnace, the sulfur produced by the thermal step is a vapor. The thermal step is responsible for most of the sulfur production, about 60-70%. To increase the yield, the sulfur-rich vapor from the thermal step is fed to a series of reheaters and catalysts. The reheaters are used to ensure that the sulfur vapor does not condense into a liquid, while the residual hydrogen sulfide and sulfur dioxide from the thermal step react on mixed beds of alumina and titanium oxide catalyst to produce more sulfur vapor along with water, in fact through the second repeat step of the above reaction to wring the last bit of sulfur from the feed. The exhaust gas from the sulfur recovery unit (SRU), as the Claus process equipment is collectively referred to, has yet to be washed before it is released, but generally about 95 to 99.9% of the sulfur in the feed is recovered as elemental sulfur.

Forbidden Lemon Drops

Until now, all processes used were at temperatures so high that the elemental sulfur was in the gas phase. But at this point condensing the vapor into a liquid makes it easier to handle. Sulfur is a viscous, dark red-orange liquid at 125°C, a temperature that is easy to reach and maintain industrially, meaning that liquid sulfur can be pumped into heated, insulated pipes around plants. Liquid sulfur can even be transported over short distances in insulated tankers, but in order to store and transport a lot of sulfur, it must be converted back to a solid.

Solid sulfur is quite easy to make. Hot liquid sulfur is pumped into a machine called a rotoformer, which is basically a large perforated cylinder. The liquid sulfur flows out of the holes as the cylinder rotates and is extruded as small liquid dots on a steel conveyor belt. Water sprayed on the underside of the belt cools the sulfur, which solidifies into small yellow pieces that resemble lemon drops. In fact, these little sulfur nuggets are called “pastilles,” a nod to their candy-like appearance. A rotoformer line can make many tons of pastilles a day, and the sulfur is piled up in the mountains before being loaded onto bulk freighters or trains for shipment.

The king of chemicals

But what’s the point of all this stuff? Elemental sulfur has many industrial uses – vulcanization of rubber for tires comes to mind – but most of the sulfur is converted into a single, immensely useful product: sulfuric acid. About 256 million tons of sulfuric acid were made in 2020; some estimates put future demand at 400 million tons per year. Most sulfuric acid goes directly to fertilizer production, where it is used to dissolve phosphate minerals into phosphoric acid, the raw material for phosphate fertilizers. Sulfuric acid is also used to make dyes, pharmaceuticals, plastics, inks, explosives and of course car batteries. It’s not known as “The King of Chemicals” for nothing.

Sulfuric acid is made in a process similar to a reverse version of the reactions used to remove it from natural gas and crude oil. There are two main processes, the contact process and the wet sulfuric acid process. Both are very similar and start by burning elemental sulfur in an oxygen atmosphere to create sulfur dioxide (SO2), then continue the oxidation of the products by passing it over a vanadium (V) oxide catalyst. This adds another oxygen and makes sulfur trioxide (SO3), which is then converted to sulfuric acid, or H2SO4:

Sulfur without carbon?

The royal status of sulfuric acid in the chemical world isn’t just an honorific – it really is an indication of a nation’s industrial might. Without sulfuric acid, most industrial processes in the world would quickly grind to a halt, leaving humanity hungry, naked, sick and without clean water. So a constant supply of it, and thus of sulfur, is critical for life as we have come to know it to run smoothly.

But because sulfur production has become so closely intertwined with fossil fuel production, we potentially face a future where sulfur becomes scarce thanks to decarbonization. There were methods of extracting sulfur before the oil industry essentially made sulfur a free by-product; the Frasch method used high-pressure steam injected into boreholes dug into natural formations, where ancient microbes reduced environmental sulfur, leaving behind huge deposits of elemental sulfur. But this method is much more expensive than current sulfur recovery methods and has a high environmental cost that is difficult to swallow.

However, one thing is certain: if modern industrial society is to survive, the sulfur must flow. How it is extracted safely and cheaply in a low-carbon world will be an interesting technical challenge.

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