2. The Thomas - Gilchrist Process
|Many iron ores contain some phosphorous and so does the pig iron smelted from them. Blowing the oxygen contained in air through liquid pig iron in the Bessemer process obviously did not remove the phosphorous. It neither bubbled out as a component of a gas like the carbon in carbon monoxide (CO), nor did it it become part of slag or dross swimming on top. But you must catch the phosphorous in some slag in order to remove it. The question to consider is simple: Why was phosphorous not incorporated as some kind of oxide in the slag formed during the blowing? With hindsight, a different but closely related question is: why did the phosphorous problem not come up during puddling?
|We have an exceedingly difficult question here. This becomes obvious
as soon as one checks the Net for answers. There aren't any - just a lot of confusion; I'll get to that.
If we look at "the question" in a broader context, it becomes clear that we ask for the equilibrium between a lot of chemical processes that can happen in liquid iron at very high temperatures. At the minimum iron, oxygen, nitrogen, carbon, silicon, phosphorous, sulfur and manganese are involved, plus calcium, magnesium and aluminum if we add the oxides of these elements as flux or as part of the lining.
The reaction products could be gaseous, liquid or solid. Some of the solids might dissolve in the essentially liquid iron, some might not.
|The naive approach we took so far (without being aware of that!) was to assume that carbon, silicon, and so on oxidize during the air blast independently of each other according to
|Not completely naive, actually. All these reactions do happen, and they can go in both directions as the double arrow (Û) signifies. They are, however, not independent and there are far more reactions that must also be taken into account.
|The first thing to realize is that all the reactions above release energy at high
temperatures - but in different specific amounts. I have actually given you a diagram for this already - in this link. Running through numbers makes clear that most of the energy released during an air blast
comes from silicon and phosphorous. That explains the old and puzzling observation to the practitioners that in the acidic
Bessemer process you need to have about 2% silicon in the pig iron, while the basic
Thomas process depends on 1,5 % -2 % phosphorus in the mix. Otherwise your converter "blows cold", not a good
And now I have mentioned the magic words: acidic and basic that invariably turn up when steel making with the Bessemer or Thomas process is discussed. At the same time a problem turns up, even for the small minority of elite people who are not chemically challenged:
|I'm not sure I know the answer. Just like (hopefully) almost everybody else, I
know that acids and bases in common chemistry relate to the concentrations of H+ (hydrogen) or OH-
(hydroxide) ions in some (watery) solution. If H+ dominates, like when you pour some hydrochloric acid
into water, you call that an acid. The H+ concentration goes up according to HCl ®
H+ + Cl-. Likewise, sodium hydroxide raises the OH- concentration according to NaOH ® Na+ + OH-; it is a base. React an acid
and a base and you get a salt: HCl + NaOH ® NaCl + H2O
I do know a bit more about the issue - but not enough. Most likely the whole notion of acidic - basic bricks goes back to chemical concepts predating the modern definition hinted at above (and going back to Svante Arrhenius; Nobel price in 1903). There are, however, about as many concepts for acidic - basic (Wikipedia mentions 8) as there are sexy chemists, and most likely the so-called LuxFlood definition is at the root of things here. What is that? To quote Wikipedia:
This acidbase theory was a revival of oxygen theory of acids and bases, proposed by German chemist Hermann Lux in 1939, further improved by Håkon Flood circa 1947. It is still used in modern geochemistry and electrochemistry of molten salts. This definition describes
MgO (base ) + CO 2 (acid) ® MgCO 3
CaO (base) + SiO 2 (acid) ® CaSiO3
That might be interpreted simply as: a (solid oxide) acid and (solid oxide) base can react and form a salt.
|Aha! The phosphorous pentoxide P2O5 formed according to
our first naive view then is also an acid. It can form a salt
with a base like CaO ( (Ca3(PO 4)2) according to that - just like SiO2. Same
thing for the wüstite, FeO, that also is a base. If we now realize that the components of slag are
salts like fayalite (Fe2SiO4),
the usual question concerning competing chemical reactions comes up: who wins? The answer
is simple: silicon! Phosphorous always looses to silicon and that means that the P2O5 formed will
not make it into the slag but decomposes again (with the help of the carbon monoxide produced), and elemental phosphorous
remains in the iron - provided it is still liquid after everything has reacted and the air blast is stopped. That will be
so as long as there is still some unreacted silica around, and that will be so as long as the silica-containing lining of
the converter is in place.
As a corollary it becomes clear why phosphorous is not a big problem at puddling. The temperatures are too low for the reduction, gases do not bubble through the stuff but are formed a the surface, and chemistry is different anyway when solid iron / steel is present.
It is far more complicated, of course. But I will spare you more details (mostly because I don't know them myself).
|As a final conclusion a recipe emerges: Use basic fluxes like quicklime (CaO)
and basic bricks for the lining of the converter. As long as you have acidic silica
bricks, you have a source of silicon / silica and phosphorous looses. Adding basic flux to a converter with acidic bricks
simply encourages salt formation between the two, and that means your bricks dissolve and your lining crumbles.
You can't afford that. Relining a big converter is time consuming and expensive and the whole point of the exercise, after all, is to boost production.
|It was thus quite remarkable that already in 1857 a German chemist (Grüner)
pointed out that Bessemer's silica-based bricks could not possibly "absorb" phosphorous. Sidney
Gilchrist Thomas became obsessed with the topic, and with the help of his cousin Percy Carlyle Gilchrist got the "basic" process going. I have
recounted that in the main part and won't repeat it here.
Let's instead look at what happens in a Thomas converter a little bit more closely:
|The phosphorous concentration only goes down after the carbon concentration is almost zero. Manganese behaves a bit oddly, and nitrogen positively strange. Try to calculate these curves and you will appreciate that there are a lot of interconnected reactions at work.
|We see once more that having a good idea is one thing, making it work another. The Thomas process worked and has changed the world quite a bit before it succumbed to the even better Siemens-Martin process and its followers.
|There is still a lot of Thomas steel out there. A lot of the steel towers or high-voltage cables in Germany were made from Thomas steel - and there might be a problem now. Thomas steel (and Bessemer steel) contains small but appreciable amounts of nitrogen, and in time this may lead to nitrogen embrittlement. Consequences might be severe:
|This may or may not have been due to nitrogen embrittlement; I haven't heard the final word yet. Black-outs by the way, are very rare in Germay but this one lasted 5 days!
Books and Other Major Sources
Early Metal Technology - 2. Silver and Lead
Leda and the Swan
Antique Texts Concerning Iron
Swords and Symbols
Steel Revolution. 1. The Kelly - Bessemer Process
10.5.3 Making Steel after 1870
Some Old Names Around Steel and Iron
Smelting Science - 1. Furnaces
Antique Texts Concerning Crucible Steel
Medieval and Modern Texts Concerning Crucible Steel
Smelting Science - 2. Charcoal Technology
Early Metal Technology - 1. Gold
The Cyprus Copper and Bronze Industry
Free Enthalpy of Reduction or Oxidation Processes
© H. Föll (Iron, Steel and Swords script)