Purging a Ballmill with inert gas?

Question

I wish to grind Sulfur in my ballmill, can I purge my ballmill with an inert gas to reduce any chance of explosion? The mill is a pressure vessel designed to do batches of 100kgs. I am experimenting with fertilizer manufacturing as a hobby. As a cheap alternative, can I use Carbon Monoxide? If as it will be suggested I am manufacturing explosives, I would not need to grind any of the ingredients as they can be purchased freely on Ebay Australia already as fine powders.

Answer 1

I think the risk of the sulfur exploding is low enough that the use of carbon monoxide is not justified.  Carbon monoxide is also hard to obtain, so nitrogen or CO2 would be much easier.  If the ball mill is mostly full, the risk of explosion is very low.  Bear in mind though, my experience is mostly with small batches, and different hazards may exist for larger mills.  I would suggest that you look at procedures in the mining industry: sulfide ores are commonly ball milled before concentration, and as these powders can also burn in air, you could look to see if refiners bother to fill the mills with inert gas.

If you mill the sulfur together with something nonflammable and non-oxidizing (kaolin or potash should be fine), you will further reduce the risk of fire.  Do not mill sulfur with an oxidizer such as potassium nitrate or ammonium nitrate unless using non-sparking media (lead is most common, and is obviously not something you'd want in fertilizer).

I assume you are using steel media, else you wouldn't need to worry much about sparks.  The addition of water will speed up the corrosion of the steel, so if you mill wet with this media, you will probably need to replace/clean it more often.

Since you plan to make fertilizer, this will not be of concern to you, but when milling aluminum powder for pyrotechnics, it is actually necessary to air out the container every hour or so!  As aluminum particles get pounded into flakes, their protective oxide layer gets stretched out.  When the container is opened to fresh air, the oxide layer forms.  This produces heat, and so if the aluminum has been allowed to go for too long without air, the sudden oxidation upon opening will sometimes ignite the aluminum powder!

Answer 2

Whoa, I think I'd do a little more research on that program.  I wouldn't use any gas containing oxygen or anything that might react with the material in the ball mill.  CO2 and CO could potentially react with some metals, I don't know about sulfur.

I've operated lab sized ball mills to grind pigments for coatings and we ran those wet to keep the dust down.  That might be a better option.