Getting rid of excess Cl

[chemgal68]

Just wondering if anyone has used chemical means to eliminate the excess free chlorine from their leach solution and what the results were?

We are leaching ore with a HCl/hypochlorite solution and it has been suggested that heating the leachate will reduce the free chlorine which will pose problems on recovery steps. Wondering is ascorbic acid or sodium thiosulfate could be added to do the trick chemically since we are going to be dealing with approximately 1000 gallons at a time one we get to full operation. Heating this much leach or letting it stand with aeration for days is not really practical.

Thanks for any input someone might have.

 

[Lino1406]

Seems the cheapest way is NaOH
Of course, thiosulphate is no good in acidic environment
Ascorbic acid and urea of course neutralize Cl, among
many

 

[butcher]

NaOH, can actually help to keep chlorine in an acidic solution reforming sodium hypochlorite in solution.

Cl2 (g) + 2NaOH --> NaCl + NaClO + H2O

It is the acid in solution that generates chlorine gas in solution from the reaction of the acid and the sodium hypochlorite (bleach) solution.

2HCl + NaClO --> Cl2 + NaCl + H2O

Chlorine gas is kept in a solution of bleach by the addition of NaOH and keeping the solution on the basic side of the pH scale, if the bleach became acidic or more neutral in pH the chlorine gas would escape from the water.

chemgal68, I cannot see where heating if used properly could interfere with recovery stages, although you have a lot of solution it does not take that much heat to drive chlorine from solution (especially if most of the chlorines has already reacted with the metals in solution), unless the chlorines source was used in excess.

Actually heating can be very beneficial in the leaching stage helping to dissolve more metals than would in a cold solution and speeding the reactions, this heat can also help to lower un-reacted chlorine content for the recovery stages.

A test for chlorine, heat a sample of solution to be tested till fume's vapor off, hold a capfull of ammonia close to this vapor, the ammonia fumes and the chlorine fumes (if any) will mix to form a white smoke of ammonium chloride indicating chlorine is in vapors of the heated solution.

 

[chemgal68]

Thanks for the info on testing for the free chlorine. I had not thought of that one.

I did run a sample utilizing NaOH buffering last week and the following happened:

Adjusting pH up to around 5.6 yielded an opaque orangish precipitate. After separating out the top layer I then added yet more NaOH until pH of around 9.5 was reached with a white ppt being formed.

Since this is leached ore we know there are a lot of base metals present - my fear is that adjusting the pH with NaOH to eliminate the free chlorine will also ppt out the PGMs along with the base metals.

I think I confirmed this today by adding Sodium Borohydride to the remaining leachate and only obtaining a white ppt. Experience has shown that normally base metals and the PGMs will ppt as a grayish/black ppt with the addition of the above.

Another thought I had today was that if we aerated the leachate after filtration from the remaining ore we could possibly remove enough of the free chlorine. Since we are not doing an oxidative pressure leach, but rather an ambient temperature/pressure scenario I cannot think of a problem with aerating the leachate. Please let me know if I may be missing something here.

We are using an ionic resin bead to remove the PGMs but have been running into loading problems with freshly made leachate - possibly too much free chlorine locking the resin up so that the PGMs cannot load properly???

Anyway, any thoughts on this would be greatly appreciated.

We also tried doing this leach with some roasted ore yesterday and today and found that the amount of HCl and calcium hypochlorite needed to lower the pH and raise the ORP was much less than when non-roasted ore was used. Also, the degree of reaction between the bleaching compound, acid and ore in the form of foaming and chlorine gas production seemed to be also diminished.

I am greatly looking forward to the day, hopefully in the near future, when I can post a message stating we have successfully figured out this particular ore problem and have obtained sellable product.

 

[butcher]

Roasting of ore concentrate should alway's be done before leaching.

You are very correct that raising the pH will precipitate values along with base metals.

 

[chemgal68]

I wish we were only dealing with a concentrate from the ore. We are dealing with straight ore that has been crushed and that is all that has been done to it when it gets to me.

I am currently looking at a possible pre-leach process that would remove a large portion of the base metals from the ore, thus concentrating the values down and the volume of ore that needs to be processed. This would in turn also diminish the amount of HCl and OCL that needed to be used.

Maybe I need to start a new topic regarding base metal removal and surfactants???

 

[butcher]

This makes no sense to leach tons of ore to get a minute amount of value, the waste would cost more to dispose of than you would make.

I think you need to step back and make major changes to your operation, it sounds like you tried what will not work, now it is time to study and expieriment to find out what might work.

 

[chemgal68]

Actually this ore has great potential based upon assay numbers and leachate numbers already obtained. The tailings can easily be neutralized or utilized in various industries, we have already researched this aspect, so getting rid of the tailings is not an issue at this point.

The biggest hurdle I think we need to handle now is how to eliminate the majority of the base metals prior to leaching to help minimize the amount of ore that would need to move through the leaching and recovery process.

At least we are not in the position of some of the mines say in South Africa where we have been told they process over 10 tons of ore to recover 1 oz of metal - that by US standards would definitely be a losing proposition.

 

[butcher]

Concentrating can help to seperate the majority of values from the non values and the majority of base metals, this puts the valueable metals in a condition more easily recovered, this does not mean that the tailngs could not be gone through later for recovery, or that they may contain traces of value, in fact your waste solutions from the primary recovery process can be used to help remove base metals from the tailings.

Many times chasing traces of values can be like chasing your tail, also trying to get a small amount of valueable metals from a large volume of ore without concentrating well I think you would be chasing your tail.

Proper assaying and development of process for the specific type of ore is a must, also expieriments on small batchs can help to develop a process that works or works better without making a large mistake or a big mess (just little messes :lol: ).


If what your doing is not working, then doing the same thing probably will not work either, then maybe step back and look at doing things differently.