# Question concerning cyanide leaching



## yvonbug (Jul 24, 2009)

I'm thinking of taking some of my stuff to a refiner who does cyanide leaching. What exactly is cyanide leaching? What is the step by step process? What should I look out for? Can I be cheated easily with this process? And should I make sure that there is no encapsulated gold in my material? Does this leaching only affect the gold? Do they collect the liquid and then drop the gold out with some kind of precipitant? I don't know a thing about this method and so I'm not doing it until I know enough to cover my ***. I'm sure the method is mentioned somewhere in this forum, but when I punched in "cyanide leaching method" I couldn't find anything to help me learn what exactly the method is. Thanks for anyone's help, Yvonne


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## goldsilverpro (Jul 24, 2009)

yvonbug said:


> I'm thinking of taking some of my stuff to a refiner who does cyanide leaching. What exactly is cyanide leaching? What is the step by step process? What should I look out for? Can I be cheated easily with this process? And should I make sure that there is no encapsulated gold in my material? Does this leaching only affect the gold? Do they collect the liquid and then drop the gold out with some kind of precipitant? I don't know a thing about this method and so I'm not doing it until I know enough to cover my ***. I'm sure the method is mentioned somewhere in this forum, but when I punched in "cyanide leaching method" I couldn't find anything to help me learn what exactly the method is.



A cyanide stripper, as used by refiners, usually consists of a sodium cyanide solution containing an oxidizer, such as hydrogen peroxide or meta-nitrobenzoate. Additional additives can be used to prevent the attack on other cyanide soluble metals, such as copper. The parts are either dipped in the solution or run in a tumbler. I used to use a cement mixer to run the material in small batches. I then combined all the solutions and rinses and precipitated the gold. The gold dissolves in a matter of a few minutes. It is best used on parts that are mostly gold plated, such as pins, fingers, lead frames, etc. Basically, cyanide will only strip the gold plating that is visible to the eye. Anything encapsulated won't be touched. It will most probably not dissolve gold alloys, such as the gold/tin braze that is used on the lids of some CPU packages. The gold is usually precipitated from the solution with powdered zinc, but other chemicals can be used.

Although I haven't seen or heard of this done, I suppose it is possible that some refiners could grind the material and heap leach it with cyanide solution. That would take days.

The refiner can run your material a couple of different ways, batch or continuous. If you have a lot of material, say 1000#, he may run a separate batch for your stuff. With lesser material, he may run your stuff in a solution that already has been used to strip other people's stuff. In this case, he should sample before and after running your stuff.

If the refiner won't let you witness the entire process, pull the required samples, and measure the solution volume, it is very possible that you could be cheated. Also, I would make sure this can be done in one day - you have no idea what happens after the doors are locked. For some types of solution, it is possible to form gold bearing solids that must be redissolved by heating before sampling and dropping the gold. Even when doing all of this, I can think of ways that he can cheat you. If a refiner wants to cheat you, he will. If you do this, you will basically be operating under the faith that he won't cheat you.


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## semi-lucid (Jul 24, 2009)

goldsilverpro said:


> Additional additives can be used to prevent the attack on other cyanide soluble metals, such as copper.


Can you elaborate on what those additives are?


goldsilverpro said:


> Although I haven't seen or heard of this done, I suppose it is possible that some refiners could grind the material and heap leach it with cyanide solution. That would take days.



Or instead of heap leaching it, wouldn't it be possible to vat leach it?


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## yvonbug (Sep 25, 2009)

Thank-you for that info. I'm trying to think of what the best thing would be for how to get my stuff done. I've got way more than I could ever process myself, and I'd like to get my money's worth out of it for all the hours I put into cleaning it and/or storeing it. I've got to move again, and I don't even know if I'll be able to do ANY refining in my next home. I'm bummed. I don't want to move. This place I'm at is perfect for my hobby. I simply can't keep all this stuff with me anymore.


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## Juan Manuel Arcos Frank (Sep 25, 2009)

GSP,my Dear Friend:

Finally,defeating my own fears and thanks to your valuable knowledge you have posted,I started the cyanide process for electronic gold plated scrap.It works lovely!!!!!,but I have some troubles...here they are:

At the begining,when you mix the tap hot water,the scrap and the cyanide..what pH must be?..I am using pH of 9..is it correct?.

Is there any difference between using sodium or potassium cyanide?Here in Mexico sodium cyanide costs 3 USD/kg,but it is hard to find,funny but potassium cyanide is free availabe at 6 USD/kg

Sometimes,when almost all the scrap is cleaned suddenly it becomes gold plated again.... What am I doing wrong?...I am recovering gold from pins,military scrap and gold plated circuits,all mixed togehter.

I precipitate gold with zinc dust,but the gold when I melt it is not pure due to zinc or other metals residues...What is the best way to treat the gold/zinc mixure to eliminate zinc? Once washed many times,I add sulphuric acid to the gold/zinc mixure (of course,using a good hood) and boil until stop bubbling. 

I treat the waste solutions,collect all of them,add sodium hydroxide,wait for 15 minutes then add sodium hypochlorite (2 galons 15% for each 2 pounds of cyanide used),wait 15 minutes and then the cyanide is destroyed,but, How can I be sure that there is not any cyanide in solution?By the moment I storage all the waste solutions in plastic drums.

One last question,does really hydrocyanic acid smell like bitter almonds?

Chris,I do appreciate all what you have done for me.

Have a nice day.

Your friend:
Manuel


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## 4metals (Sep 25, 2009)

Juan,

The pH of 9 should be good enough, and while I always have preferred potassium cyanide the literature suggests sodium cyanide will work as well, what oxidizer are you using? 

Sometimes when the free cyanide level is too high the gold will immersion plate out of solution. Especially when the oxidizer is running down. Generally KCN lower than .001percent stop stripping and solutions over .25 percent are too high.

The acid boil should eliminate the zinc so your procedure sounds good to clean up the gold / zinc mixture. 

I seriously doubt that you are effectively destroying all of the cyanide using 2 gallons of Clorox for 2 pounds of Cyanide, plus you are not totally reducing the cyanide cyanate complex. The method to completely decompose the cyanide follows. An ORP meter can be had relatively cheap and is your friend when dealing with cyanide. 

Cyanide waste treatment is best accomplished as a 2 stage treatment process. To monitor the process it is necessary to have an ORP probe, sort of like a pH meter but it measures Oxidation - Reduction Potential. 

The first stage converts cyanide to cyanate, pH should be over 10, best at 11 to 11.5 Add hypochlorite bleach or granulated pool chlorine until you get an ORP reading of +325 at minimum, +400 is best. Good mixing is a must. Let the solution mix for at least 15 minutes, an hour is best.

There is a test paper called Potassium Iodide - Starch test paper. this paper indicates an excess of chlorine - indicative of a complete reaction. An excess of chlorine after an hour at +400 mv ORP means it's safe to proceed to the next step.

Now the pH has to be lowered to 8.5, this means adding acid, dilute is best, to a cyanate solution. This is why an ORP probe is really needed, and the test papers make certain. If you properly converted the cyanide to cyanate the dilute acid will not generate cyanide gas. 

Now raise the ORP to +600 minimum or +800 best. Again using hypochlorite or pool chlorine. The mixer has been on all this time, agitation is a must. After at least 30 minutes and better after 2 hours the cyanate has been broken down.

Now you have destroyed all of the cyanide. If the waste contains any base metals they have to be treated by caustic precipitation at pH 10, don't go too high here because the zinc will redissolve. 

Finally, there has been a rumor in the electroplating industry for years that hydrocyanic acid does smell like bitter almonds. There is also a rumor that if you smell that bitter almond smell you're a dead man! So I don't personally know of anyone who can confirm or deny the rumor.


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## goldsilverpro (Sep 26, 2009)

There are several ways to cyanide strip gold plated materials. You can use strong cyanide fast or you can use a very weak cyanide, slowly. A definite low CN concentration will supposedly not attack the copper nearly as much, if any. However, I prefer the strong cyanide because of it's greater speed and the lower solution volume. NaCN was cheap and I was able to easily destroy large amounts of it, at least to a much less toxic cyanate. I will concentrate on the High-CN methods.

*Commercial Strippers* - these are widely used to strip faulty gold plating, with minimum attach on the base metals, so that the parts, themselves, can be salvaged and replated.

The most common way is to purchase commercial gold strippers, which are usually a mixed powder that you dissolve in water. The simplest and cheapest one consists of about equal parts of sodium cyanide and m-NBSS (many different names - meta nitrobenzoic acid sodium sulfonate is one - meta nitrobenzoate is another). The m-NBSS is the oxidizing agent. It is made up to a 2-4 oz/gal NaCN concentration and is used warm - maybe 130F. It will slightly attack copper. The Au is easily dropped out with zinc dust.

To avoid this copper eating problem, a small amount of a lead compound (lead acetate, I believe) is added to the above formula, which provides a super thin protective coating of Pb on the Cu. One product of this type is Technistrip Au. The lead is reduced onto the Cu by another chemical that is added. The gold can't be dropped out of this solution efficiently with zinc. A complicated, dangerous method using large amounts of sodium hydrosulfite must be used. Also, the gold drops as a compound which must be further treated to obtain gold metal. Due to these difficulties, most users ship the solution to a refiner and let them count their money for them.

*Homemade strippers*

*(1)* At one place, we used a 4 oz/gal NaCN solution containing half as much m-NBSS. We added a small bit of 35% H2O2, to perk up the reaction. The pH was about 10. I don't think we had to heat the solution. The parts were placed in a large (4' long) barrel plater (tumbler) and rotated in a large tank of solution. Several batches could be run in the same solution. The gold was then easily zinced out when the gold concentration reached about 1.0 oz/gal, or more.

*(2)* I have stripped many tons of material in a portable cement mixer. I used 2 or 3 gallons of hot tap water and added about 4-6 oz of granular Dupont NaCN, along with the parts. When the NaCN was mostly dissolved, I added about 50 mls of 35% H2O2 - this heated the solution considerably. With a decent nickel layer (which is usually the case), the gold stripped in less than 1 minute. Occasionally, with thin nickel, the CN tended to get to the Cu and greatly slow down the Au stripping. More H2O2 could be added but the H2O2, at the same time, destroys the NaCN and this produces a precarious balance. Often, thin nickel material had to be run a second time. In this case, you only could run one batch and then use a fresh solution. However, you could, depending on the bulk density of the parts, run as much as 30-40# of material in 2-3 gallons of solution. The parts don't have to be completely covered with solution. When tumbling, everything will get wet. The gold was then simply zinced out.

For both of these, the idea is to overpower the gold and do it as fast as possible, in order to minimize attack on the copper, prevent cementation of the gold, and maximize production. High NaCN and H2O2 (along with the heat produced by the H2O2) both speed up the reaction. Adequate H2O2 also makes the solution concentrate more on the Au than on the Cu and prevents cementation. 

You must watch the parts closely to see how they are stripping and get them out at the right time. The CN/m-NBSS and/or H2O2 will dissolve Ni slowly and the abrasion from the tumbling will rub off nickel. Both will expose the copper. This is another reason for getting the parts out ASAP. I occasionally had to add a few more mls of H2O2. If you destroy a lot of NaCN with too much H2O2 or use too little H2O2, the dissolved gold can cement onto the copper. If the parts are coated with a black smut (better seen when dry), the black smut is gold. We used to pay copper scrap price for black-smutted parts that were stripped by other refiners or scrap buyers and make a tidy profit on the gold. Right after stripping, the copper must be pink. When dried and oxidized, it should be a green or blue color with no black.

If you can find the equation, the *destruction of cyanide*, using bleach, also requires some NaOH - if I remember right. It takes a huge amount of bleach and NaOH to kill a pound of cyanide. For this reason, I used an electrolytic method to decompose most of the cyanide, and then polished it off with bleach/NaOH. I simply hung two fairly heavy sheets of steel in the cyanide, hooked them up to a power supply, outdoors, and ran it at about 6V. I gave it a stir, with a plunger type stirrer, every hour or so. A lot of the dissolved base metals also plated out, freeing the cyanide tied up with them for destruction. I monitored the the CN level with a quick titration using a .1N AgNO3 titrant and a KI indicator. After a couple of days, the free cyanide level was down to about 1 gm/liter, or less, a level that could be taken care of economically with bleach/NaOH.

You also must consider that the parts, even after rinsing a couple of times, still are coated with dangerous amounts of cyanide solution. After pouring off the pregnant solution and rinsing, I tumbled the parts with a bleach/NaOH solution to kill the CN and then rinsed.

I must admit that, although I owned an ORP meter, I didn't feel that I absolutely needed to use it. Everything went quite smooth without it and I got essentially 99.5% of the exposed gold. Were I to do it again, I would definitely use an ORP meter and get some real numbers. Thanks for the education, 4metals!

Chris


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## silversaddle1 (Sep 26, 2009)

Chris,

Interesting post to me, as I have a huge amount of gold pinned connectors to have stripped. We are talking close to 1000# I'm guessing. I can't do them myself, so what do you you recomend?

thanks,

Scott


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## Irons (Sep 26, 2009)

> "Finally, there has been a rumor in the electroplating industry for years that hydrocyanic acid does smell like bitter almonds. There is also a rumor that if you smell that bitter almond smell you're a dead man! So I don't personally know of anyone who can confirm or deny the rumor."



It does smell like bitter almonds. Not everyone can detect it by smell, so it's not a good idea depend on your nose.

The good part about Cyanide, is that most non-lethal exposures result in no ill effects. 

People who smoke inhale Cyanide every time they light-up.


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## 4metals (Sep 26, 2009)

Determination of cyanide by titration is fine if you know how to do it, for a non technical refiner using cyanide the ORP probe is easier. The last thing we should do is encourage people to use cyanide and not treat the hazard which remains after the gold is stripped properly. 

Since GSP used a concentrated cyanide to speed up the stripping the electrolytic destruction followed by caustic and bleach when the cyanide was low enough was a cost effective choice. However having a buret and knowledge of chemistry made this a feasible option. Without detailed instructions I doubt all others could follow suit.

Since we've turned down this road maybe GSP can post his analytical method for cyanide determination so we can give our members a fighting chance to destroy their wastes. If I don't see a method posted in a few days I'll take a stab at teaching our non chemists how to titrate a solution. 

Cyanide is a compound which can do wonders for refiners who take the time to do it right and who respect the cyanide for what it is and for what harm it can do. I can't over stress the importance of doing this right. Since we've gone this far maybe it's time to clear the air so no one gets hurt, and the environment doesn't suffer either!


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## goldsilverpro (Sep 27, 2009)

This is word for word from the 1968 Metal Finishing Guidebook.

It only measures free cyanide. It does not measure any of the cyanide tied up with the metal complexes. The method is not that sensitive for measuring very low cyanide levels. For these low levels, there are better methods. 

*Analysis for free Cyanide:*

(1) Pipet a 10 ml sample into a 250 ml erlenmeyer flask.
(2) Add 90 ml of distilled water and 5 ml of a 10% KI (potassium iodide) solution
(3) Titrate with standard 0.1 N AgNO3 (silver nitrate) solution until a faint yellowish turbidity persists.
The standard AgNO3 solution is prepared by weighing out exactly 17.00 grams of the reagent grade salt and dissolving it in distilled water, after which it is diluted to exactly 1 liter in a volumetric flask.
(4) The Sodium Cyanide (NaCN) in gm/liter is equal to: .9825 x ml of 0.1 N AgNO3 used.

If KCN (potassium cyanide) is being used, the KCN in gm/liter = 1.306 x ml of AgNO3 used.


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## Juan Manuel Arcos Frank (Sep 27, 2009)

4metals:

Thank you,very much!!!!!!!.....I have read, word by word ,your wonderful post and learned a lot.

I am using oxygen peroxide (50%) as oxidant,it works nice but is expensive.

I now using potassium cyanide and works pretty good.My gold contamination is dued to silver so I am dissolving the zinc/gold mixure in weak sulphuric acid first(using a good hood,of course),boling until no bubbling,pour off,wash a little and then I add nitric acid to dissolve silver and I get yellow shininig gold.Some of the military e scrap musts contain silver.

I am looking for an ORP probe and I will get it soon because I do not have plenty of space to storage more cyanided solutions.

Is there any way to stop hydrocyanic acid emission once it starts when ,accidentally, an acid reacts with cyanide?

Thanks again.

Have a nice day.

Manuel


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## goldsilverpro (Sep 27, 2009)

In the Air Force, I taught a 40 hour class each month on Chemical, Biological, and Nuclear Warfare. In Tech school, they had "sniff kits", large test tubes that contained small amounts of all of the various war gases. Each had a distinctive odor and we had to sniff each one so that we could recognize them. HCN was included. The few times that I have been exposed to HCN, I experienced a bitter almond taste in my month. I'm not sure if I can explain the smell. I just know that, when you smell it once, you'll never forget it. 

I've always thought that, if you don't get enough to kill you, you're probably OK. The Catch 22 is that it sure doesn't take much to kill you.


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## Juan Manuel Arcos Frank (Sep 27, 2009)

GSP,my Dear Friend:

I do appreciate all your teachings,thanks a lot.

How does electrolytic cyanide destruction work?..Is it true that cyanide can be regenerated?.

Best regards.

Your friend.

Manuel


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## goldsilverpro (Sep 27, 2009)

Juan,



> I precipitate gold with zinc dust,but the gold when I melt it is not pure due to zinc or other metals residues...What is the best way to treat the gold/zinc mixure to eliminate zinc? Once washed many times,I add sulphuric acid to the gold/zinc mixure (of course,using a good hood) and boil until stop bubbling.



I prefer using nitric to leach out the base metals, instead of sulfuric. Nitric the the more universal solvent. It will dissolve essentially everything but gold that is likely to be in the sludge (Ag, Cu, Ni, Zn), rapidly. There could be lots of copper, as metal, mixed with the gold, in that sludge. The copper is much easier dissolved by the nitric, than by the sulfuric. The silver won't be eaten by the sulfuric. About the only thing that sulfuric is good for is the zinc. The final brown gold powder will be fairly pure when you use nitric. You will still have to AR refine the gold, however. The easiest way to dissolve gold in AR is while it is a powder.

I transfer the rinsed sludge, filter paper and all, to a bucket. I cover the sludge with water and stir it up. I then add nitric in small increments and stir gently and carefully in between additions. When the reaction stops and another small addition produces no further reaction, stop adding nitric.



> Sometimes,when almost all the scrap is cleaned suddenly it becomes gold plated again.... What am I doing wrong?...I am recovering gold from pins,military scrap and gold plated circuits,all mixed togehter.



The idea of doing this hi-cyanide thing is like opening a door with a bomb. In fact, it is nearly identical to the "bombing solutions" used by jewelers. The H2O2 is the all important ingredient. It does 3 things. 
(1) It oxidizes the gold, copper, etc., so that these metals can be dissolved as cyanide complexes. 
(2) It destroys cyanide, over time. In fact, the Dupont cyanide destruct method primarily uses peroxide.
(3) The CN destruction mechanism gets the solution very, very hot. This is good, since it oxidizes the gold faster. This is bad since it destroys the cyanide.

The idea is to drive the reaction as hard as possible with the H2O2 and get it out ASAP. However, if you use too much H2O2, the solution can get so hot it will sort of explode a little out of the mixer. Wear a full face mask, gloves, and, maybe, rubber boots. For your 50% strength, I would start by adding about 35 ml to 2 or 3 gallons. If it's not stripped in a minute or two, add 18 ml more. Repeat maybe once or twice, at the most. If the gold is still not off, you may have to pour off the solution and add a fresh one. This, however, is rare. I have salvaged a strip by adding a little more cyanide.

If allowed to tumble for more than 5-10 minutes, most of the cyanide will be destroyed. This will allow the gold to cement out on the copper. This is why it's vital to get it in and get it out. This is controlled by the H2O2.

Strip it fast, pour off the solution and rinse twice (you can use the same two rinses for several batches). Combine all the solutions and rinses and zinc out the gold and silver and copper all together. With constant stirring, sprinkle on a little 325 mesh zinc dust. Estimate about 1 oz of Zn/oz of Au and weigh that out so you don't overadd. You may need a little more at the end. At some point, the color of the stirred up precipitate will appear almost all gray. Stop adding zinc, settle overnight, siphon, filter the sludge, rinse well, leach with nitric, add some water, settle, siphon, filter, treat powder with AR.


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## goldsilverpro (Sep 28, 2009)

Juan



> How does electrolytic cyanide destruction work?..Is it true that cyanide can be regenerated?.



I assume the cyanide decomposes by oxidation at the anode. I don't have the equation in front of me.

Could you be more explicit as to what cyanide solutions you wish to regenerate?


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## Gold (Sep 28, 2009)

Have you looked at using O3 as an oxidizer for cyanide leaching instead of H2O2 ?
I've read some interesting research on it.


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## Juan Manuel Arcos Frank (Sep 29, 2009)

Gold:

You are right,Ozone is a powerful oxidant but I think it is hard to handle,except you form it in situ.Besides,I think it is more expensive than hydrogen peroxide.

Regards

Manuel


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## Juan Manuel Arcos Frank (Sep 29, 2009)

GSP,my Dear Friend:

I have followed carefully all your instructions and my cyanide process has run beautifully...I have changed to nitric acid instead of sulphuric with better results.

About cyanide regeneration I wonder if waste cyanide solutions could be regenerated adding an alkali sulphide,i.e.,sodium or potassium sulphide if this reaction could takes place:

Metal cyanide complex+alkali sulphide==Metal sulphide+alkali cyanide

What do you think?.

I am very grateful to you for all your valuable help.

Have a nice day.

Your friend.

Manuel


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## goldsilverpro (Sep 29, 2009)

Juan,



> I have followed carefully all your instructions and my cyanide process has run beautifully...I have changed to nitric acid instead of sulphuric with better results.


Ain't it great?



> About cyanide regeneration I wonder if waste cyanide solutions could be regenerated adding an alkali sulphide,i.e.,sodium or potassium sulphide if this reaction could takes place:
> 
> Metal cyanide complex+alkali sulphide==Metal sulphide+alkali cyanide



Much of the cyanide will now be a cyanate. If the conversion from cyanate to cyanide is possible, you would need some sort of reducing agent, I would think.

There should be no metals in the solution, after zincing, except for zinc. I assume you are thinking about precipitating out metal sulfides. I would guess that the metal sulfides are soluble in cyanide.


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## Juan Manuel Arcos Frank (Sep 29, 2009)

GSP,my Dear Friend:

Yes,Sir...It is great!!!!!

I am talking about after I add the zinc,precipitate the zinc/precious metal powders and pouring off the liquid,this liquid(before adding sodium hypochlorite) is what I am trying to regenerate the cyanide by adition of sodium/potassium sulphide.

I will try it,let us see what happens.

Best regards.

Your friend.

Manuel


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## ritesh (Oct 20, 2012)

our cyanide solution has the ph of 11.after adding hypochlorite bleach the orp reading decreases to -75 what should we do to increase the orp reading.please help 4 metals!!!!!!!!!


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## Traveller11 (Oct 25, 2012)

Will sodium thiosulphate neutralize cyanide solutions?


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