[Gary Park]

Hi - we regulate a company who use expanded polystyrene (EPS) as a sacrificial mould in metal casting.

The EPS is packed with casting sand and then molten iron introduced via pour points.

As I understand, with a molten metal above 1000°C and a good supply of oxygen - this should result in a complete (or something approaching it) destruction of the polystyrene to carbon dioxide and water.

However, the mould and sand are packed into steel boxes with an open top and I suspect that at the time of the pour there will not be sufficient oxygen available in the sand or to be drawn in from around the box.

For a cubic metre of EPS with a mass of 20kg/m3, this would contain about 18.4 kg of carbon and 1.6 kg of hydrogen.

This would require 49.2 kg oxygen to fully convert the carbon to carbon dioxide and a further 12.3 kg of oxygen to fully convert the hydrogen to water.

In a situation where that amount of oxygen isn't available and there is no other reactant, what is the likely fate of the polystyrene under these conditions.

I'm particularly interested as to whether it is more likely to deposit as a soot (or some solid) on the sand or as some gaseous emission ?

Many thanks

Gary

[wildfyr]

Significant soot seems very likely, even where oxygen is plentiful. Only the simplest alkanes have anything near perfect conversion to CO₂+H₂O.

An interesting phenomenon could occur if polystyrene is being heated above 400°C in the absence of oxygen, which is that it can depolymerize since it will be above its ceiling temperature. This will cause release of styrene monomer, which would be a gas at such temperatures (boils at 145°C at STP). If the gas hits an oxygen supply while still very hot (Temp=???) it will combust, otherwise its a VOC.

These guys would probably be royally pissed I am pointing this out. It would seem like a fairly simple measurement to check for styrene and other organics in the air around the process.

[pgk]

Furthermore, incomplete combustion can lead to the formation of carbon monoxide, which is odorless, colorless but highly poisonous and therefore, its emissions must be monitored with the appropriate sensor/detector during the industrial procedure.

[Enthalpy]

Welcome, Gary Park!

Could you detail some aspects?
- Are there voids in the steel box somewhere? I believe the box is full with sand and polystyrene foam.
- Do you remove the polystyrene somehow before casting the iron? I believe iron melts the polystyrene foam directly.

Unless you introduce significant oxygen, there will be too little present in the box. 60kg oxygen occupy 50m³ as pure gas or 200³ as air.

Now, if the polystyrene is pyrolysed by the molten iron, it will emit little gas: all the hydrogen evolves pure and as hydrocarbons like methane, ethylene, maybe some benzene. The 800 mol of H₂ from polystyrene would make in the order of 400 mol gases, which take 10m³ at 300K and 30m³ at 1000K, so large venting is needed.

I suppose that the rest of the carbon (=most) from polystyrene ends dissolved in the iron if you cast directly on the foam, and this can be a worry. For 1m³, 12kg C in 7800kg Fe make a mean 0.15%. This changes the properties of construction or allied steel (and iron?) and botches some steel compositions that guarantee <0.05% C. Worse: the added carbon must stay concentrated at the part's faces.

So: did I understand the process properly? It's the description I read long ago. Or do you remove the foam before casting the iron?
If you don't want to burn the polystyrene before casting, you could try to dissolve it. Acetone for instance does it. Just check that the polystyrene is really gone, since acetone first frees the foaming gas very quickly but leaves a compact residue.

[Gary Park]

Dear All,

Many thanks for the interesting and varied responses.

I'll try to address all the points raised so far.

The only soot is a rerlatively small wisp of it at the time the molten metal is first introduced.

I read that even benzene should be tolally "destroyed" above 732 deg C - although this is not the same as venting it into a thermal oxidiser.

The polystyrene is not being heated (per se) - the process is the introduction of molten iron (above 1200 deg celcius) which within a matter of seconds consumes the expanded polystyrene within an enclosed space.

The polystyrene is almost completely surrounded by casting sand with a pour point opening and possibly a few breather holes to allow the molten iron to compelety fill the area occupied by the polystyrene.

There are no voids in the compacted sand but there will be a certain amount of air within the voids between the grains (very limited) and in the expanded polystyrene itself.

I imagine that during the pour - all the pressure will be outwards through the breather holes and there is not much chance of air being drawn in.

The polystyrene is a sacrificial mould surrounded by the packed sand - the molten metal completely consumes the polystyrene during the pour.

I hadn't even considered the possibility of the polystyrene (or the hydrocarbons produced by the thermal breakdown) being dissolved into the iron. That would be an ideal fate for it - and preferable to toxic emissions. Although at the moment, anyone on the foundry floor involved at the time of the pour are in full face breathing apparatus.

For health and safety legislation, they are required to monitor dose levels on employees but I'm not sure what they monitor for. Without having some idea of the likely componds being produced - the list of possible things to monitor for could be very long.

The cast items are artist's sculptures so there are no issues of structural integrity being compromised.

The suggestion of dissolving the polystyrene with acetone is a novel one but the casts can be quite intricate and complex (ie probably not a route to all locations within the mould to dissolve it) and I'm not sure that the entire internal sand mould would remain intact without the support of the polystyrene.

Given that the virgin sand is becoming quickly blackened, It seems that at least some hydrocarbons are being deposited on this. I think the company may need to widen the chemicals being checked for in the dose analysis.

Once again - many thanks for your very helpful observations and insights.

Gary

[Enthalpy]

Then, any amount of air is negligible. Polystyrene is just pyrolysed by the liquid iron. Because (C₈H₈)_n has much carbon for little hydrogen, the pyrolysis produces little gas and much carbon or carbon-rich solids. These can impregnate the sand, but much must dissolve in the iron. If the target composition is like 3%C then mean 0.15%C more change little.

Any pyrolysis produces a vast and ill-defined spectrum of gases, liquids and solids, so protection is a good practice. Much evolve before reaching iron's 1200°C so they aren't destroyed. Expect polycyclic aromatics and others among the solids and vapours. Example of target of the dose analysis.

Polystyrene foam is made using a foaming agent. Pentane would add no new pyrolysis product to polystyrene, but other foaming agents would. Some are haloalkanes, which broadens at once the spectrum of the possible toxic pyrolysis products. This could make an important difference. Intuitively, I'd strongly prefer pentane in the pyrolysed polystyrene, as far as possible. Haloalkanes would be an important target in the dose analysis.

[wildfyr]

Enthalpy,
Where would the halogens come from?

[Enthalpy]

Bad surprise for me too: polystyrene foam can use hydrofluorocarbons or CFHC as a blowing agent.

This paper investigates blowing agents by Honeywell :
CFH=CHCF₃ and CClH=CHCF₃
https://www.honeywell-blowingagents.com/?document=processing-and-properties-of-polystyrene-foamed-using-hydrofluoroolefins-2012&download=1

This polystyrene foaming agent is R142B or CH₃CClF₂ plus R22 or CHClF₂
http://www.sanmeichem.com/product_detail_en/2.html

How much of C₃H₂ClF₃ remains in the closed pores after months isn't clear to me, but I suppose much is still present.

Fluorine and chlorine is what you dislike in a pyrolysis. The possible products are much worse than the hydrocarbon cocktail. Hence my suggestion to choose polystyrene blown by pentane.