Carbon dioxide

[tomferry]

I am after some advice regarding carbon dioxide in iron mines, me and a few mates have been exploring these mines for a number of years now. We own a 5x and have a good general knowledge, but some things still don’t seem to add up…

I cannot not name the mines or where about”s in the uk they are but I will describe them.I will stick with a single mine to make it more simple for anyone aiming to help us out.

So what we no for sure is.

changes in atmospheric pressure we understand this.

Co2 is heavier than air, this means any outside co2 will naturally settle inside the mine,A lot of our issues are due to only having 1 or 2 points of entrance, so atmospheric pressure is creating a vacuum and you get co2 blowing out the entrance , outside the 5x often goes of reading up to 7% co2

I believe the lack of travel inside the mines also plays a part in this creating co2 pools ?? I no
The co2 often finds the deepest place in the mine and pools . With no workers there now and only a few visitors does this make it worse ?

The oxidation of the iron rich sandstone itself, is this the only thing going on ?? “Continue to next paragraph “

So let’s compare our mine to 1 we all no well —Browns folly.

We have 30-45miles of pillar and stall workings in iron rich sandstone, lots of nice space to walk around in .. similar entrance to browns folly couple ways in . Why does browns have no co2 at all ?? It’s lower down than the outside ? It has no ventilation now ? I understand “our” mine is iron but this is the only Difference ?? Is this purely the oxidation of iron making this issue ??


1 other thing also .

I have done the very very long trip to the end of westbury Brook iron mine. This is 4 hours each way at a good pace . Why is there no co2 issues in here ? This has just 1 entrance ? Does the difference in type of iron itself as a mineral play a part in this ?

Please note, I am not saying iron mine exploring itself is safe by any means, as it is not ! Stick with other stuff ..

I’ll wait your reply lez

@BikinGlynn
@ChrisJC

if either of you have anything to add please do !

 

[BikinGlynn]

No idea of the science of it but think the small entrances into said mines compared to the vastness of 40+ miles of tunnels must be a issue.

I know there are "a few" air holes but they are all at on end (eastern side in this instance) so there will be hardly any through draught between these holes & any air flow wont go further in than the first link tunnel between holes (path of least resistance)
Essentially then its only atmospheric pressure that can draw anything in or out of the any part of the mine beyond this.
Combined with the fact we know the mine levels go down the further in we go I can see why C02 would accumulate.

Re the actually question of why this is not occurring in other places I have no idea, maybe the sandstone of browns doesn't release C02 as these purer ironstone ones do, & Browns does have some pretty large entrance holes.

I honestly believe if there was just one air hole right at opp end of mine this would flush out a large proportion of bad gas, but thats just my uneducated oppinion.

 

[tomferry]

Just to add after further thinking .

Westbury Brook iron mine : I have not done much research into this, it is part cave with natural voids , the deposit itself I would assume is hematite found in these extensive rift like workings . Still 4 hours going inbye into an iron mine with no other adit ?

Is it to do with the cubic meter void space, left inside and the percentage& purity of iron ore oxidising ? Some super formula??

Or does geology play a part on when it deposits the type of ore ?

 

[AR]

Is there a significant amount of carbonate mineral like siderite present in the rock of the blackdamp-prone mine? If there is, then it's entirely possible that it's getting altered to an oxide over time, releasing carbon dioxide as part of the reaction.

 

[royfellows]

I dont want to pour petrol on the flames but ironstone mines are classed alongside thus "coal, stratified ironstone, shale or fireclay" in the Mines and Quarries Act 1954. This explains why the Coal Authority have taken responsibility for securing ironstone mines in south Wales near Pontneddfechan.

 

[Cantclimbtom]

Iron ores can "rust" and slowly remove oxygen given that most of air is Nitrogen and Oxygen - give or take a bit, the O2 removed by "rusting" is displaced by nitrogen, so from just rusting you'd expect reduced oxygen, (and consequently increased Nitrogen) but not greatly increased CO2 just from iron oxidation. So the source of CO2 in your mine isn't a simple result of iron rusting, but rather something that releases CO2 And... that depends on the type of the ore!

https://www.igg.org.uk/gansg/12-linind/m-met.htm "..The most common ore type in England is a dark ore commonly called Spathic, the technical name is Siderite and chemically the material is iron carbonate. Siderite is often found mixed with either clay or coal deposits, mixed with clay it is called ironstone, when mixed with a coal deposit it is called blackband ironstone. Siderite is a poor quality ore, in Britain typical specimens contain just over thirty percent iron.."

So one iron ore mine may have little CO2 (but reduced oxygen!) and another iron ore mine may have reduced oxygen and increased CO2. It depends on the local ore

 

[mikem]

Browns Folly is limestone. Worsening CO2 in caves seems to have more to do with what's being spread on fields above

 

[langcliffe]

Some shales also oxidise to form C02, as we found to our cost once. We once dug through a blockage in Hebden Horse Level which dropped a sump by about three sumps. The rock is slabby limestone beds split by carbonaceous shales. When we went back ten years later, we found a big build up of C02 which forced us to retreat.

 

[tomferry]

Thanks for the helpful response .

Serious amounts of clay “many various types “ ganister in decent size random deposits, a thin bed of limestone . I believe that’s it ..

Broadly speaking 3% co2 inside the mine is commonly 18%o2 .

 

[ChrisJC]

This is in the Northampton Sand formation

Northampton Sand Formation - Wikipedia

en.wikipedia.org

Chris.

 

[Bob Mehew]

Co2 is heavier than air, this means any outside co2 will naturally settle inside the mine

If that were true, then the earth would have a layer of CO2 in which we would suffocate. Try reading "The Legend of Carbon Dioxide Heaviness" by Giovanni Badino at https://caves.org/journal-of-cave-a...icles/the-legend-of-carbon-dioxide-heaviness/ . The topic is complex in caves and even more so in mines due to as mentioned other chemical reactions which can take place.

 

[tomferry]

Thanks for the reply bob . I shall give the section a read. It’s going to take some convincing though, my x colliery books which are about ventilation say it is heavier than air.

Thanks for the interesting link though !

 

[Bob Mehew]

Sorry to upset your understanding but if what you had read was correct, then not only would the CO2 have overwhelmed us but also the argon in air which is present at 1%, twice as much as CO2. Fortunately for all of us, the molecules in air are in random motion due to their temperature, so diffusion occurs spreading gases away from a source and reducing concentrations to average values. Technically, CO2 is more dense, i.e. weighs more per litre than N2 or O2 but that ignores diffusion due to the random motion of the molecules in air.

The main reason for 'pools' of CO2 is due to the simple fact that the CO2 is being liberated from water into the atmosphere and takes some while to disperse / diffuse to normal concentration levels. The CO2 in water arises from the water having passed through decomposing earth where rotting organic material creates a high concentration of CO2 which dissolves into the water. When the water reaches a location where the air concentration of CO2 is less, then CO2 is released. No where near as spectacular as opening a bottle of champaign (but that also has pressure on its side). However once the pressure is relieved, it is simply differences in concentration which drive the transfer of CO2.
.

 

[tomferry]

So I have tried very hard to read that, it’s rather complicated I must say . It seems to me that in a perfectly still shaft with no movement. Air eventually diffuses leaving co2 behind ?

If the host rock is creating co2 “iron, coal or shale this can obviously speed it up .

Also water can contain co2 .

Are these facts correct bob ?

We are talking 20+ miles of passage way, with 2-4% co2. then behind the furthest collapses extremely dangerous levels .

Some of the mines are mainly dry with no ingress, so it’s back to just iron oxidising .

 

[Bob Mehew]

Sorry, it is wrong to say there is no air movement. Crudely, whilst the movement of bulk air may cease so it feels as if the air is still, the molecules of various chemicals such as CO2, O2, N2 and others which make up air are still moving. It is this movement of individual molecules which cause diffusion. (You may not realise it but you do feel these individual molecules, not individually but in bulk as pressure on your skin.)

Please think of air as a vast number of molecules which are moving randomly in all directions. Now take a container with an internal dividing wall. On one side we have molecules of O2 & N2 (that is air) and on the other side we have molecules of CO2. If we remove the wall (without disturbing things), those molecules of CO2 which were moving toward the now removed wall will continue onwards into the other half of the container. And like wise, those molecules of N2 & O2 moving towards the wall in the other half will pass into the first half. I hope you can accept that eventually the molecules will mix and the concentrations of CO2, O2 & N2 will be the same across the whole of the container. It is (almost) impossible to reverse this process. So one does not get CO2 concentrating out of air, rather one gets CO2 diluting into air.

If you want to get a visual appreciation of diffusion, then watch

(though convection currents similar to 'wind' are also at work).

Yes some rocks can create CO2 under the right conditions (including limestone). And rocks like coal can have trapped CO2 from when the coal was formed from rotting vegetation (as well as gases like CO - carbon monoxide - white damp). Which is why disused coal mines are very dangerous / suicidal to enter. Also it is possible that rotting wood and other materials taken into a mine can create CO2.

CO2 dissolves in water (just like sugar). The more pressure of CO2 in the air above the water, the more CO2 dissolves into the water. I am going to ignore the complex chemical reactions which can take place. But I will point out that if the water is transported to a different place (i.e. through fissures in limestone rock into a cave passage with ordinary air), then because the pressure of CO2 above the water is lower, the CO2 will come back out. Hence the presence of higher levels of CO2. Normally the CO2 will diffuse into the air (or indeed be blown around such as by a strong stream) and not be noticeable. But some times it takes a while to diffuse and is noticed.

As has been mentioned one can also get oxygen depletion in mines. Usually CO2 levels do not significantly rise so what one suffers is lack of oxygen not elevated CO2. In fact the science of levels of O2, N2, CO2 and other gases such as CO and CH4 in mines is incredibly complex and beyond my level of competence. You also need to understand the instruments which measure gases in air as well. Some misleadingly just measure the oxygen level and for values below 21%, assume the difference is due to CO2 and report it as CO2, ignoring the potential for oxygen depletion.

By the way, so far I only know one person who died in a mine due to bad air (the generic term) so please don't become the second person. I won't say good luck, rather I will say get educated.

 

[langcliffe]

I can think of three deaths from bad air in the UK from recreational mine exploration: Peter Ireson, Will Smith, and Adam Perkins.

 

[mikem]

In, I believe, coal, slate & ironstone mines respectively.

 

[shotlighter]

Well, I've just read the article. Outside of academia, it seems to be a pedantic (in a caving/exploring context) scientific explanation of a common & widely observable phenomenon,. It does briefly note that the phenomena of gas layering exists but just not for the usually accepted reasons.

The title THE LEGEND OF CARBON DIOXIDE HEAVINESS & resulting posts here, may give the casual reader the impression that layering of gases doesn’t exist (or is very uncommon).

This could be a very dangerous misunderstanding. The accumulation of irrespirable air can & does occur near the floor of passages etc., both wet & dry. - The exact causes of which are in our context purely academic.

In a naturally vented/unventilated void this layering/ponding etc. is not uncommon. This is especially so in some types of mine. So if you enter somewhere that you feel may have poor air, just testing the general body of air is not good enough. If you trip, bend down etc., you may find your head in very O2 deficient air that only a couple of feet higher tests at 20.9%.

I’ve observed this “layering” even with forced ventilation of the general area, in dips in floor level, where there’s slight screening from the moving air current. Incidentally a maintenance worker in a rail tunnel local to me, was almost killed in just this type circumstance some 10yrs ago. Luckily his mate was able to drag him out by the feet & resuscitate him.

Quoting a scientific paper as a reason why you should be in rude health, is of dubious merit when inscribed on your tombstone.

 

[ChrisJC]

So we are saying that natural diffusion of gases ensures that O2, N and CO2 are all well mixed in a given void.

But there are sometimes localised pockets where this doesn't happen. I have seen this a couple of times recently, both times in ironstone after going over a collapse.

The 'open' passage had CO2 levels of about 2.5%, but in the pocket after the collapse, one had 4% CO2, and the was accelerating past 5% so I beat a hasty retreat!

Shafts are also well know to have CO2 collected in the bottom, although maybe this is because it is being generated in the bottom at least as fast as it can diffuse out (so it's not static).

We seem to have two theories about where it comes from - one being a chemical reaction from Iron Carbonate, and the other being oxidation of organic matter.

Chris.

 

[mikem]

In low energy environments (cold / no draft), it does take longer to diffuse.