experiments with naocl/hocl leach

[Traveller11]

Following information unearthed from several sources, I set out today to replicate some of the procedures I found. I performed two experiments; one using hydrochloric acid and the other acetic acid.

First, I poured 250 ml of 6% sodium hypochlorite bleach into a large Pyrex measuring cup. The tested pH of this NaOCl was 12.3. I then added 15 ml of acetic acid, bringing the pH to 10.8. Another 15 ml of acetic acid brought the pH to 9.9. At this point, I began pouring in small trickles of acetic acid until the pH was at 7.3. I could have done this in a plastic jar as there was no exothermic reaction at all.

Next, I poured another 250 ml of NaOCl into another Pyrex measuring cup. Using a plastic straw as a pipette, I added HCl a dribble at a time. There was localized fizzing where the HCl entered the NaOCl but this lasted only a couple of seconds. I continued adding HCl until I brought the pH down to 7.3. There was the slightest rise in temperature I felt I could detect through the Pyrex.

To each cup, I now added NaCl (salt) to the equivalent of 400 ml/litre and stirred it in. The solution became a cloudy white but eventually cleared to a point. From here, I poured the contents of each into two plastic jars, already a third filled with a black sand (haematite, magnetite, garnets, blonde sand) concentrate collected by a friend of mine. Reputedly, this concentrate contained a certain amount of very fine gold but there is no way to guarantee this. I stirred the contents together and put the lids on these jars, gave them a shake and left them to soak. I noticed the solution colour was slightly darker after stirring.

While I saw no fuming coming from this solution and there was no great exothermic reaction, every once in a while I caught a whiff of this solution in my nose. It had the oddest smell, not chlorine and not bleach but very familiar. I was glad I was working outside and standing upwind of it as I instinctually realized it was not something you would want to get great lungfuls of.

From what I have read, the cup where I used acetic acid would have combined the sodium hydroxide in the bleach with the acetic acid to make the salt sodium acetate. The other would have combined sodium hydroxide and hydrochloric acid to make sodium chloride salt. Essentially, I believe the two proceduress have made almost the same solution, except for the acid/base salts created. Of course, acetic acid is far safer though more of it is required.

It will be interesting to test each jar with stannous chloride tomorrow to see if I have managed to put gold into solution.

As this black sand we are dealing with can be unpredictable and sometimes has no gold at all in it, and all the gold that is present there is free gold, could a person cheat and put a 14K gold chain in the leach solution to test the solution? Or would the unoxidized impurities in the 14K gold go into solution and leave the gold behind?

 

[butcher]

Without knowing the gold content of the sands before and after it will be hard to tell how effective the leach is besides maybe an indication with stannous chloride that something went into solution, you really need to have an idea of gold content before and after of the sands, and of solutions.

I think running test with a known gold content with and without black sands involved may also help to see if the leach is effective for free gold and if the black sands somehow effect how well the leach works, then some tests on some samples of assayed black sands, samples of roasted and not roasted samples with known gold content, the more you try different approaches with known contents the better you can judge what is working the best, and so on.

Maybe using just gold before adding other variables, to get a base idea how it reacts with gold alone, what conditions it best dissolves the gold, then begin adding the other variables, with added salt and without, with sand and without, with roasted sand, and with none roasted sand, with heat and without...

 

[Traveller11]

Interesting development. Overnight, the solution pH of both containers dropped from the original 7.3 to 5.1 and there was unmistakable chlorine gas coming from each. On the third day, the pH of each seemed to be stabilized at 4.8 and did not drop any more.

From what I have been able to gather from the members of a chemistry forum I frequent, the hypochlorous acid (HOCl) I made by lowering the pH of sodium hypochlorite (NaClO) by the addition of acids is very unstable and likes to decompose to hydrochloric acid and oxygen (HCl and O2) in the following formula:

2 HOCl = 2 HCl + O2

This, of course, would tend to lower the pH of the solution and increase the proportion of HOCl to hypochlorite (OCl) until, reaching pH 5.0, all of the free chlorine exists as HOCl and also, due to acidity, begins to be released as chlorine gas (Cl2). Ultimately, the solution stabilized at pH 4.8, simply because the original bleach I used only had so much hypochlorite available in it. A stronger bleach solution likely would have stabilized at a lower pH because it would have turned a larger volume of HOCl into HCl and O2.

According to some people I consulted with at the chemistry forum, the stabilized solution now should be made up of water, HCl, NaCl and possibly a minute amount of chloric acid (HClO3) although they felt even the chloric acid would decompose to HCl.

So, with this in mind, I have inquired as to what would happen to the unstable HOCl I made if it were immediately placed in a pressure vessel and air was pumped into the vessel at 60 psi and a valve to the vessel closed; sealing in and maintaining the 60 psi pressure. Would this ****** the release of O2 from the HOCl and stabilize it somewhat? I asked this because the one leaching method from the 1890's I read involved piping Cl2 gas into a cylinder into a body of ore and water and, following the dissolving of Cl2 in water and the subsequent production of HOCl (plus HCl), air was pumped in at 60 psi and maintained at that pressure. A similar method was used in the 1890's process where sodium hypochlorite solution (and ultimately HOCl) was made by electrolysis of saltwater. This solution was also piped into an airtight cylinder but, in this case, a valve was fitted at the top of the cylinder and solution pumped in until the solution overflowed out the top of the valve. When the valves were closed on this cylinder, any air had effectively been excluded from the cylinder leaving nothing but liquid inside. I do not think it a coincidence that both methods of making HOCl would also go to such great effort to stop the release of O2 from solution; one by pumping in air at a pressure great enough to overcome gassing off of O2 and the other by excluding air altogether and allowing the gassing off of O2 from HOCl to quickly create its own back pressure to overcome gassing off of O2 from HOCl.

I believe this can be the only explanation possible as Cl2 dissolves very easily in water even at atmospheric pressure. Its solubility does not seem to be increased by elevated pressures although elevating the temperature of the water does increase solubility. My experience in the water treatment industry also tells me that chlorine added to water becomes HOCl and this HOCl is quite stable in drinking water as long as the water is in pressurized pipelines. Once the water comes out of the tap into an open glass or pitcher, the HOCl will decompose, leaving behind HCl. However, I am patiently awaiting an answer from some of the greater minds at the chemistry forum on this matter to confirm or dismiss my theories before I proceed further.

What all this seems to tell me at this point, though, is that leaching with chlorine solutions at a pH close to neutral will require the leaching to be done in a sealed vessel where either all of the air is excluded or air is maintained at a high enough pressure to stabilize the HOCl and prevent its decomposition to HCl and O2 from lowering its pH to undesirable acidic ranges. Plus, because HOCl will react with steel and other base metals, this vessel must either be made from non-metallic materials or lined with something non-reactive to HOCl. In the 1890's, steel cylinders were lined with lead as lead did not react. This complicates things quite a bit and I can see now why leaching with cyanide salts caught on so quickly in the 1890's.

 

[Traveller11]

Just thought of a good way to test out the theory in my last post. I have some 1.5" PVC pipe and numerous fittings for it. If I cut a 12" piece of this pipe and glue a cap on one end and a PVC ball valve on the other, I should have a leaching chamber equal to the ones from the 1890's I described in my last post. First, ore will be added through the ball valve, followed by a pH 7-7.5 solution of HOCl + NaCl, gently stirring ore and solution to release trapped air. Once the level of solution reaches the end of the ball valve, closing the ball valve should give me only solution, with no trapped air, inside the pipe.

I'll let you know how things go.