# HCL/CL Spent Acid, after the leach?



## awbrew (Nov 1, 2011)

Hi All, I have a question. I hope its not stupid... In Steves PGM Video We use the HCL/CL leach method. after recovering the spent acid(droping the metals with Zinc) the acid stilll seems to have a little life left in it. Would It be possible to reuse it to Boil the small beads out of a DP Converters. Steves video starts out this way but with clean acid. Is there still enough strenght in the used leach HCL, to remix it with a little Nitric. That way it would be cutting down on the waste factor of the cold leaching process(the one using the buckets with no heat over time). Again I hope this isn't a stupid question.


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## butcher (Nov 2, 2011)

zink is very high in the series of metals, the zink chloride solution would not displace other metals lower in the series,and you would be adding zink to the materials your trying to leach, could make it harder for you to leach them. so I do not see any use for using it in the fashion your asking about.


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## awbrew (Nov 2, 2011)

Thanks butcher for the reply, I was wondering just how much zinc was left in the solution and other things for that matter. It just looked so clean (crystal clear again) after the 2 stage filter process. I went for it... LOL


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## Geo (Nov 2, 2011)

after hcl/cl leach the liquid can be clear and still contain metal.i always condense mine by evaporation as much as 3/4 and then cement with copper. i have a little over an ounce of mystery metal that wont dissolve hcl/cl. i haven't tried it in nitric acid yet as im just saving it up till i have enough to process.


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## dtectr (Nov 2, 2011)

Someone (maybe Ammen? :?: ) indicated that metallic ions have unique colors in a leach/solution - for example, small amounts of silver, nickel, & copper could all be dissolved in a solution, but only the copper & nickel ions would color it - silver ions are clear, or colorless.

remember that with ionic exchange precipitation, which zinc precipitation is, I believe, ( I've never actually messed with PGM's), for every ion of recovered target metal, you put an ion of the other metal into solution, regardless of its color/clarity.


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## awbrew (Nov 2, 2011)

Thanks Geo & Dtectr, I think my orginal hope was I could have condensed Spent HCL & at the same time I was boiling the convertor beads and leaching them. Butcher commented that the zinc may even cause problems leaching. I remember In steves video, he said that if its longer than a hour on the filtering process that you need to do another stanious test to make sure that some of the some PGMs haven't been reasorbed. It would have been nice to have used up most of the spent acid by boiling the beads with a little nitric. Geo I seen where you was talking about cementing out(I'm assuming the zinc) the with copper after reduction, but then I wouldn't know what to do with the copper. Are you reuseing your acids? or just reducing down the problem until time to send them off. Thanks again for all the help..


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## Geo (Nov 2, 2011)

awbrew said:


> Thanks Geo & Dtectr, I think my orginal hope was I could have condensed Spent HCL & at the same time I was boiling the convertor beads and leaching them. Butcher commented that the zinc may even cause problems leaching. I remember In steves video, he said that if its longer than a hour on the filtering process that you need to do another stanious test to make sure that some of the some PGMs haven't been reasorbed. It would have been nice to have used up most of the spent acid by boiling the beads with a little nitric. Geo I seen where you was talking about cementing out(I'm assuming the zinc) the with copper after reduction, but then I wouldn't know what to do with the copper. Are you reuseing your acids? or just reducing down the problem until time to send them off. Thanks again for all the help..



after cementing with copper any values will be in a fine black powder in the bottom of your vessel.if you want to reclaim the copper then just cement the copper out using a piece of light steel not harden steel as you would find in tool making or high carbon steel.i use scrap steel from computer cases. after you have depleted the solution of copper its safe to neutralize the solution and discard.


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## butcher (Nov 2, 2011)

There are rules in chemistry how these things work, cementing a metal from a solution of metal ions is a replacement reaction, and we are replacing the metals dissolved in solution, study the reactivity series of metals, you will see where copper metal very low in the series but above the valuable metals will cement our values from solution, and we can see if we had copper ions dissolved in solution that zinc metal would precipitate out copper powder as the zinc dissolved into solution,

But we also see that if we had a zinc metal dissolved in solution and added a copper buss bar we would have no chemical reaction

http://www.google.com/search?sclient=psy-ab&hl=en&site=&source=hp&q=reactivity+series

http://en.wikipedia.org/wiki/Reactivity_series


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## skippy (Nov 2, 2011)

Ok, so what you want to do is to reuse the acid post cementing for dissolving pgms from catalyst again? If you've adjusted the pH down a lot, there's not enough acid to worry about reusing - if you adjust from a pH of 1 to a pH of 3 you have one hundreth of your starting acid.
It is possible to cement from concentrated HCl though, but you end up using more zinc or iron in the cementing out of the precious metals. 
I don't think iron or zinc in solution will affect your leaching, but the chlorine generation reaction and the cementation makes the acid weaker than it was on the first round. 
It might make a good base for an initial extraction solution for your catalyst, or use it for dissolving base metals from other scrap/material.

Please elaborate more about how you envision your whole process working and how your reuse/condensing idea fits into it, I am unsure about what you are proposing.
You can distill hydrochloric acid but it will boil off mostly water until the concentration of acid reaches 20% where it will boil off a fairly concentrated acid.


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## awbrew (Nov 2, 2011)

skippy I'm useing HCL/CL to leach cats cold in a bucket over time (steves video) after the leach. Droping all metals with Zinc and filtering that acid twice a course and then fine filter. After that the acid came out really clear and I though that I could restrenghten it with nitric and Boil the little convertor beads with it(Beads are from a big flat gm convertor) . Butcher has already told me that it wouldn't probably work because of the zinc still in solution. It would keep the new cats from being leached properly. I'm not to cheap to buy the acids, I was just trying to reduce the wast HCL from the cold leach that was done in buckets. Geo had pointed out that I can Reduce and Concentrate my acids(which I will be doing). After that I will call for disposal latter on. Not sure what size container I should have Ex. 40 gal barrel, as far cost goes to remove. I am new to all this and just trying to be conscience of environment and have a good plan...


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## dtectr (Nov 3, 2011)

my friend
Did you follow the reactivity links?
Once you get below iron, all that will be left upon neutralization is salt water. It would be a greater impact on earth to wash your driveway!
read, then read some more.
Keep us posted on your progress!
Dtectr


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## Palladium (Nov 3, 2011)

The whole point in refining/recovery is to start with dirty materials and remove impurities in steps until you arrive at a near pure state. The metals are hard enough to segregate as it is. If you start with a solution of acid contaminates already and introduce this into material that you are trying to recover then you have already set yourself back further than if you had just not done anything at all.


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## awbrew (Nov 3, 2011)

Thanks Guys, I got a Long way to go I'd Be the first ont to admit. I'm reading, I'm around a third of the way threw hokes book. I Do appreciate all the help...


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## skippy (Nov 3, 2011)

Catalyst is a rather clean as a metal on ceramic rather than metal on or in metal. And how much zinc is there actually going to be in this spent acid? It's dissolved and its had it's reduction power killed. Feel free to correct me but it doesn't seem like a big deal. 

This thread is really confusing me, I feel like I am missing something, but I don't see why you couldn't try to do an extraction with the acid you have used before, it wont be as strong as fresh acid, so it might be questionable, but then agian if might work out ok. Another thing that has been suggested is using the same leach solution to process multiple amounts of ceramic. IE leach catalyst, remove the catalyst and use the same leach to dissolve more metals. Not really all that different if you ask me. 

If there is free acid it could also be distilled out. Weak HCl will boil off mostly water until it reaches a concentration of 20% and then it will boil off more concentrated vapours.
A lot of work for something that is so cheap though.


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## skippy (Nov 3, 2011)

Palladium said:


> The whole point in refining/recovery is to start with dirty materials and remove impurities in steps until you arrive at a near pure state. The metals are hard enough to segregate as it is. If you start with a solution of acid contaminates already and introduce this into material that you are trying to recover then you have already set yourself back further than if you had just not done anything at all.



Palladium, usually recovery from catalyst you are cementing out, the washing with hcl and what not before redissolving and precipitating, refining etc your values.
What's a bit more dissolved base metals and alumina going to do anyway when you already have to go through all these steps anyhow?


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## Palladium (Nov 3, 2011)

Let me see if I can explain. As Dtectr pointed out look at the reactivity series. Think of your acid as a battery. That battery only has a certain amount of energy stored up and can only do X amount of work before the energy is depleted. In order to get your metals (precious) out with zinc you have to add a metal below them in the series (Zinc) to the acid. This is a cementation process. The zinc being lower on the series will take the place of the metals above it and reduce those metals from solution taking their place. Energy spent! But before it will take their place it will use up any free acid in the solution and saturate the solution with zinc then start cementing. Energy spent! So once the solution is saturated and your metals are dropped there is no energy left in the acid to do any work. Now if you have something below zinc like manganese in the series that you are trying to dissolve then the lower metal will go into solution and the zinc will drop leaving room for it to take the zinc’s place. Reusing acid before cementation is a different story than using after. The thread started with reusing spent acid. If there is no energy left in the solution then why would you want to add all those metals back into the equation just by adding acid to it and reintroducing it to the situation? Why not just use new acid?


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## Geo (Nov 3, 2011)

when your spending relatively small amounts on chemicals for a large amount in return then why start with a dirty solution?im sure when your processing metal with acids that you would use the acid without stopping until the acid is spent anyway (i always do) so the question of starting with used solution shouldn't even be something anyone would want to do unless the solution is made to be reused.i understand the need to conserve and reuse when and where you can as far as the economical and environmental point of view is concerned but that has to be weighed against viability of the process that your doing.AP should be reused as well as sulfuric from a stripping cell as well as certain chemical byproducts from other processes but chemicals used when finishing refining processes probably should not be reused.


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## awbrew (Nov 3, 2011)

Palladium, Thank you so much. Excellent Answer. Now I understand.


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## skippy (Nov 3, 2011)

Ok but the HCl doesn't drive oxidation. It's not an oxidizing acid. And the zinc doesn't have to neutralize the acid , it just has to reduce the oxidant and the precious metals. And you are unlikely to get your leach solution for catalyst pregnant with so much values that you would saturate the solution while cementing with zinc. 

I would hazard that the acid might become saturated with aluminum ions though, from the alumina wash coat or the beads. At which point the solution still has a very low pH, but little free acid. Then the solution will no longer dissolve alumina, and may not dissolve your metal values well either, due to the lack of free acid and common ions reducing the solubility. I think you could distil off hydrochloric acid from this solution, leaving aluminum hydroxide, and by this recover some of the acid. 

Interesting question Awbrew! Good discussion. Lou or Freechemist, care to comment?




Palladium said:


> Let me see if I can explain. As Dtectr pointed out look at the reactivity series. Think of your acid as a battery. That battery only has a certain amount of energy stored up and can only do X amount of work before the energy is depleted. In order to get your metals (precious) out with zinc you have to add a metal below them in the series (Zinc) to the acid. This is a cementation process. The zinc being lower on the series will take the place of the metals above it and reduce those metals from solution taking their place. Energy spent! But before it will take their place it will use up any free acid in the solution and saturate the solution with zinc then start cementing. Energy spent! So once the solution is saturated and your metals are dropped there is no energy left in the acid to do any work. Now if you have something below zinc like manganese in the series that you are trying to dissolve then the lower metal will go into solution and the zinc will drop leaving room for it to take the zinc’s place. Reusing acid before cementation is a different story than using after. The thread started with reusing spent acid. If there is no energy left in the solution then why would you want to add all those metals back into the equation just by adding acid to it and reintroducing it to the situation? Why not just use new acid?


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## awbrew (Nov 3, 2011)

Thanks Skippy... One small Question ? If I dropped the Zinc, With say aluminum would the zinc still be pure? ( I would think not.) Seems what I learned so far from Cementing is it will bring metals down, but usually isnt that pure. Is that correct?


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## butcher (Nov 4, 2011)

Just to make this a little more confusing: 

Say I use new acid HCl and add zinc.

Make notice of your reactivity series chart, zinc is above hydrogen in that list.

This means zinc will replace hydrogen from the acid and make a salt (acid and metal= salt of that metal) in a solution of water, now you no longer have an acid of hydrochloric acid but you have a salt of zinc chloride in solution.

Notice chloride ends in (ide), meaning a salt of (or made from) hydrochloric acid; this salt is not an acid.

Lets look at this experiment, mix two very strong chemicals, each one very dangerous, and can burn you, both would be very bad to taste, hydrochloric acid (HCL) and sodium hydroxide (NaOH), (caustic soda) or (lye), this will give us a salt of the metal sodium, 
HCl + NaOH --> NaCl + H2O
So we made table salt we could eat on our French fries,

Now we no longer have a strong acid or a strong caustic.


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## awbrew (Nov 4, 2011)

That is so cool, Butch your alwesome! I really love this forum! Theres so many of you guys on here, that would be dangerious if trapped in your kitchen or the bathroom. LOL


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## freechemist (Nov 4, 2011)

Palladium wrote:

_"Let me see if I can explain. As Dtectr pointed out look at the reactivity series. Think of your acid as a battery. That battery only has a certain amount of energy stored up and can only do X amount of work before the energy is depleted."_

This statement is only partly correct, because the whole system, the sum of dissolved metal-ions which can be reduced by metallic zinc and the total acid have to be regarded as a battery. In HCl it is the hydrogen-ion H(+), which is reduced by metallic Zn to elemental hydrogen:

2 H(+) in solution + Zn(metal) => Zn(2+) in solution + H2 gas
Cu(2+) in solution + Zn(metal) => Zn(2+) in solution + Cu(metal)
Pt(IV) in solution + 2 Zn(metal) => 2 Zn(2+) in solution + Pt(metal)

All reactions occur at the same time, but with different velocities, depending on individual concentrations and redox-potentials of reducible reactants. In addition, reaction products like metallic Cu formed primarily through reduction of Cu(2+) with Zn(metal) will reduce precious metals too. By far the fastest reaction at higher acid-concentration is the oxidation of metallic Zn by hydrogen ions to dissolved Zn(2+) and parallel generation of hydrogen-gas (H2). Hydrogen-gas itself is also able to reduce dissolved precious metals, itself being reoxidized to hydrogen-ions H(+).

freechemist


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## nōnāgintā trēs (Dec 10, 2012)

So long as there's no gold left, I'd use it to clean the toilet!!


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## NoIdea (Dec 10, 2012)

nōnāgintā trēs said:



> So long as there's no gold left, I'd use it to clean the toilet!!



Don't spill it on the seat as it will dissolve your jewels. :mrgreen:


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## gold4mike (Dec 10, 2012)

:shock: :shock:


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## NoIdea (Dec 10, 2012)

awbrew said:


> Thanks Skippy... One small Question ? If I dropped the Zinc, With say aluminum would the zinc still be pure? ( I would think not.) Seems what I learned so far from Cementing is it will bring metals down, but usually isnt that pure. Is that correct?



Not sure if your Aluminium Zinc question was actually answered, sooooo, .... zinc is way to reactive to be dropped by aluminium, the higher you go up the reactivity series the less likely they will be dropped as a metal, iron for example is not reduced to iron, butt the salts of iron in a higher oxidation can be reduced to a lower oxidation state (ferric to ferrous for example) or visa-versa. Dropping gold using ferrous sulphate changes the oxidation state of the iron to ferric sulphate. 

Just a case of moving those little electrons around.  

Hope that helps

Deano


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## butcher (Dec 11, 2012)

Aluminum forms a gel in HCl the aluminum really does not dissolve well and may not supply the zinc with electrons needed to reduce the zinc from solution to metal zinc, so you would probably end up with a zinc chloride solution full of aluminum gelatin in solution.


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