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A washing tower that rains water down through a polluted gas will take up the particulates in the gas. They do this through adhesion, I'm pretty sure.

Now, there's apparently 5 types of adhesion:

Mechanical

Chemical

Dispersive

Electrostatic

Diffusive

In my case, the particulates are mostly coal, some ilmenite and probably some bentonite. As far as I know, these are not very conductive materials, which probably rules out electrostatic adhesion. Since mechanical adhesion relies on large bodies connecting via interlocking through many pores and grooves, it probably isn't the case here either.

In my uneducated opinion, that makes the chemical, dispersive and diffusive types of adhesion left. I think diffusive adhesion works best with substances that are more spacious on a molecular level, and I'm not sure how the aforementioned dust particulates are in relation to that.

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  • $\begingroup$ Coal (graphite) is not a bad conductor of electricity. $\endgroup$
    – Gert
    Mar 19, 2021 at 21:11
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    $\begingroup$ Why do you think that being not very conductive materials might rule out electrostatic adhesion? Isn't it charges that are involved in electrostatic adhesion, not the flow of charges? $\endgroup$ Mar 19, 2021 at 21:11

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The answer here is that the water wets the dust grains. The wetting process involves weak chemical bonding between the water droplet surface and the dust grain surface. In the ideal case, the droplet "swallows" the dust grains it touches and because the droplet is far far bigger than the typical dust grain, gravity pulls the droplet down through the air and it (and the dust grains it has collected) falls into the collection sump at the bottom.

A little bit of detergent added to the water greatly improves its ability to "wash" the air like this if the particulates it is intended to trap have any poorly-wetting hydrocarbons adsorbed into their surfaces.

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  • $\begingroup$ So it is a chemical adhesion, mixed with a kind of absorption, since the water droplets absorb the particulates? $\endgroup$
    – A. Kvåle
    Mar 20, 2021 at 10:49
  • $\begingroup$ Yes, most likely "just det". $\endgroup$ Mar 20, 2021 at 18:06
  • $\begingroup$ "Just det", what does that mean? $\endgroup$
    – A. Kvåle
    Mar 22, 2021 at 7:26
  • $\begingroup$ I'm sorry, I assumed you were Swedish. I was taught that "just det" was a Swedish expression meaning "exactly that". Sorry if I was wrong! $\endgroup$ Mar 22, 2021 at 7:40
  • $\begingroup$ Well, I'm Norwegian, and "det" does mean "that" in Norwegian. However, "just" isn't a Norwegian word, and one would more likely write "akkurat slik" or something like that. But I did suspect you meant that when you said it, just wasn't sure :) $\endgroup$
    – A. Kvåle
    Mar 22, 2021 at 12:51

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