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Hypothetically, I have a chemical tank that was brought out of service and the solid deposits at the bottom of the tank primarily contain aluminum chloride. The whole system never sees water, and the aluminum chloride is generated upstream in the process as a by-product (anhydrous under tank conditions).

It's time to clean this solid out, and water flushing seems to be an easy solution. However, previous experience tells us, extensive heat and HCl (aq) and HCl (g) generation during the flooding process, corrodes the tank material and the welded joints, inner pressure rings, etc.

I want to use an alternative solvent to dissolve the aluminum chloride without generating HCl. Ideally, I would like to clean out the tank afterward with water, to get rid of the solvent and remaining solids. My first initial screen takes me toward polar aprotic solvents with high vapor pressure.

What does this community think?

Edit: After some comments, I think I should clarify. There are some other factors, such as solvent toxicity, carcinogenicity, $$ cost (tank is quite large), safe disposal of solvent at that scale, getting rid of the solvent completely from the tank, etc. might reduce the chances for some candidates.

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  • $\begingroup$ Carbon tetrachloride $\ce{CCl4}$ and also (to a smaller degree) toluene $\ce{C6H5CH3}$ are solvents of $\ce{AlCl3}$. Toluene can be used to produce metallic aluminum by electrolysis, although the yield is low. $\endgroup$
    – Maurice
    Commented Sep 21, 2022 at 20:53
  • $\begingroup$ Hypothetically, the OP should not use $\ce{CCl4}$ because it is a carcinogen, and it sounds like the hypothetical tank is quite big. $\endgroup$
    – Karsten ♦
    Commented Sep 21, 2022 at 21:47
  • $\begingroup$ Yes, that is correct. Both CCl4 and Toluene are carcinogens. Toluene also has low electrical conductivity, so develops static charge. This provides potential ignition sources for flammable residual chemicals in the AlCl3 solids. $\endgroup$
    – Sam
    Commented Sep 21, 2022 at 21:49
  • $\begingroup$ Indeed the tank is quite large, and the cost of solvent, disposal, removing all solvents from the tank, etc. becomes limiting. $\endgroup$
    – Sam
    Commented Sep 21, 2022 at 21:51

2 Answers 2

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Several common organic solvents will dissolve this compound which has a lot of covalent character. From Wikipedia:

Soluble in hydrogen chloride, ethanol, chloroform, carbon tetrachloride Slightly soluble in benzene

Comments identify carbon tetrachloride as a carcinogen, ethanolhas nucleophilic oxygen and thus may react, and benzene is also carcinogenic. Chloroform might work.

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    $\begingroup$ Of course, chloroform also is toxic, so perhaps ethanol, despite chance of reaction, might be best. Perhaps a first cleaning with 99% C2H5OH, cycling the ethanol through a filter filled with an alkali, such as CaOH, to remove any evolved acid? Final rinse with water/ethanol (or cheap wine ;-) to remove remainder? $\endgroup$
    – DrMoishe Pippik
    Commented Sep 21, 2022 at 22:30
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    $\begingroup$ @DrMoishePippik This paper might be relevant – doi.org/10.1016/S0141-3910(96)00201-7 – which details the reaction of Al(Cl)3 with EtOH, and reports the formation of AlCl(OEt)2 and similar compounds but also (toxic) ethyl chloride, predominantly so if the temperature rises. There are lots of bad examples in chemical engineering of bad things happening when the temperature rises. I would suggest getting some of the suggested solvents and running a series of lab tests and in particular paying attention to the enthalpy changes going on... $\endgroup$
    – Landak
    Commented Sep 22, 2022 at 9:38
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If there is an industrial application, costs including disposal of solvent will be important.

A possible solution, if the material cannot be scraped or shoveled out, might be to dissolve it with slower reacting alcohol, rather than water.

A strong nitrogen purge into the tank and out through through an acid neutralizing trap should take care of the HCl vapor and reduce flammability hazards from solvent fumes.

Treatment of the material once out of the tank could include recovery of the alcohol through vacuum distillation.

It would be recommended to try this out on a small scale first. IPA might be slower reacting (better) solvent than ethanol.

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    $\begingroup$ In some circles your last sentence suggests using Indian pale ale 🍺. $\endgroup$
    – Oscar Lanzi
    Commented Sep 22, 2022 at 9:45

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