Things which may be of interest to members

[Deano]

It appears, based on the advice posted on this forum, that members are burning filter papers so that they can recover gold from solution or suspension which has been adsorbed onto these papers.

It has been fairly common knowledge that burning filter papers will lead to metal losses both from dissolved and particulate gold.

Out of curiosity I have run a series of tests to quantify the extent of these losses.

I loaded gold chloride as a solution onto Whatman 90mm diameter No 1 and No 42 ashless papers.
The loadings were calculated at 10 micrograms and 50 micrograms on each paper. The papers were then dried at 80C.

A straight digest of these papers in aqua regia with DIBK/aliquat 336 extraction from the liquor and flame AAS analysis gave recoveries of 9.6 and 48.5 micrograms for both paper types and gold levels.

When the filter papers were burnt with flame the recoveries were 3 and 21 micrograms respectively for the #1 papers and 2.9 and 5.2 micrograms for the #42 papers.

A repeat of the above but with ashing in an electric furnace for 2 hours at 380C and no flame gave recoveries of 3.7 and 24 micrograms for the #1 papers and 8.8 and 9.6 micrograms for the #42 papers.

I then crumpled the papers into a tight ball before placing the gold solution onto them and drying as before.

Recoveries for the # 1 papers burnt with flame were 3.7 and 22.9 micrograms and for the #42 papers 4.4 and 16.1 micrograms.

The repeat of the crumpled papers ashed in the furnace gave 8.0 and 43 micrograms for the #1 papers and 9.5 and 42.6 micrograms for the #42 papers.

A further set of tests were conducted where the papers were not pre-dried bur were put wet into the furnace.

The #1 papers gave 8.1 and 45.6 micrograms, the # 42 papers gave 8.8 and 45.6 micrograms.

The crumpled wet #1 papers gave 9.7 and 46.3 micrograms, the crumpled wet #42 papers gave 9.8 and 44.9 micrograms.

The results indicate that if you want to minimise gold losses when ashing filter papers you should conduct the ashing in an electric furnce at 380C with the filter papers tightly crumpled.
Having the papers wet going into the furnace may slightly improve the recovery further again.

All ashings were conducted in 100 mm diameter porcelain dishes.

Deano

 

[butcher]

I am not surprised at all by your experiment, gold and a chloride salt will put gold into a vapor form the finer the gold I suspect more of the loss involved, the faster the temperature is raised and not controlled the more gold I would expect to vapor off.

Even table salt NaCl and Gold will vapor off the gold, when hot enough, it is also hard to get to a temperature that the chloride would vapor off without taking gold with it, as the boiling point of the salt is so close to the vapor point of the gold chloride.

As an mass of different base metal chloride powders and gold are heated, it is my belief that as one base metal chloride gives up its chloride ion as gas, another base metal in the mass, or even a more noble metal can take up the newly formed chlorine gas, and if the temperature was hot enough, and raised fast enough there your gold goes up in smoke.

It would be extremely hard to control the temperature to drive off chlorides and not have the gold (or silver) follow the (chloride vapors, chlorine or HCl fumes).
I think it may be possible in a controlled lab experiment, if the powders could be heated to just the right temperature and held there long enough, where just the chloride would vapor off without taking the gold with it, but in reality we do not have this kind of control of temperature with what we do.

Harold speaks of a purple trailer parked where his exhaust fumes had colored it in his early years of learning recovery and refining. He also spoke of the gold mine from his fume hoods as a nice little piggy bank once cleaned out.

Gold can be dissolved in chlorine gas, and carried along with fumes (normally around 800 degrees).
Vaporization of gold chloride, from your filters is no surprise, like silver chloride which is also volatile at higher temperatures.

Gold chloride I would expect high lose to vaporization of the gold, the more control of the temperature rise in the electric furnace may help somewhat, but I do not believe it will solve the problem.

(I can not think of where I would incinerate filters that were saturated with gold chloride solution, as these are normally rinsed with water, so the soluble gold chloride passes on through the filter material), although you can have elemental gold powders with a base metal chloride salt, (or silver chloride), in the filter, which if incinerated would lead to the gold becoming volatile in the chloride fumes at a high temperature...

I normally wash my filters, or impure gold and base metal chlorides powders, in a sodium hydroxide solution, before incineration, in the hopes of rinsing out the NaCl (that forms from the sodium hydroxide and the involved chloride metals), with the following water washes, converting many of the base metal chlorides, and silver chlorides to oxides, (and some possible hydroxides depending on base metal involved), to remove as much of the chloride ions as possible before incinerating.


(Precipitated or powdered gold is elemental metal and not a chloride), so removing the salt or chloride base metals, that may be mixed with the fine gold.
It is my thought, or theory, I would have less lose of gold because of chloride salts, because I remove them as much as possible with pretreatment.
I also raise heat slowly at the beginning portion of the incineration process, normally with the hot plate prior to using the torch to finish the incineration process, to drive off any chloride the previous treatment of hydroxide and washing may have missed, heating the powders to drive off water and fumes of the acids, before raising the temperature of the crushed powders to the red hot and oxidizing them further with air or oxygen.

I have not done any controlled experiment's to verify this theory that the hydroxide wash will do as much as I believe it to in this regard.

But would be interested in the results of a controlled lab test.

I

 

[justinhcase]

Thank you Deano for taking the time to help improve our recovery process.
I have a good sized bin bag of filter papers to process and have held off until I built a flue baffle to catch P.M.'s in the fumes.
I will try to work your findings into my practices' had been salting my wet papers with ferrous sulphate crystals to minimise loss through heating gold chloride.
you and the more experienced members of the forum continue to amaze me with the amount of time and effort you put into improving the craft for others.
Much thanks
J

 

[g_axelsson]

Interesting topic! Again! 8)

I have always thought about the volatility of gold chloride but never very hard. Now I have been thinking for a while...

If we lose gold while incinerating and it isn't as dust then it has to be in the form of a vapor, a chloride vapor. But gold(III) chloride breaks down from heat into gold(I) chloride and chlorine gas at a quite modest temperature of 160 °C. Gold(I) chloride in it's turn breaks down into gold and chlorine gas at 198 °C. But if the temperature is raised above 420 °C then gold(I) chloride transforms into gold and gold(III) chloride which is stable at this elevated temperature. The equilibrium is controlled by the partial pressure of chlorine gas so we want to get rid of it.

What if we raise the temperature of the filters to about 200 °C and keep it there for a longer time and slowly exchange the air in the chamber. For the hill billy refiner it can be done with an electrical stove out in the yard. The chlorine gas should be removed over time and we end up with metallic gold in the filters and no gold chloride that can evaporate.

One problem I see with this way to process the filters is that there may be other chlorides in the filter papers that doesn't break down and can emit chlorine that attacks the gold at higher temperatures, like when gold ore were roasted with sodium chloride to make it easier to dissolve the gold. (A procedure used before cyanide leaching was invented.)

Göran

 

[Deano]

Goran

I actually ran a much longer series of tests trying to find out what was the minimum temperature that the filter papers would ash at without flaming. I started at 200C and went up in 20C increments holding each temperature for 2 hours. They required a temperature of 380C to completely ash both types of paper.

The test series were also extended to temperatures up to 660C. There were no differences in results between the 380C ashings and the 660C ashings.
All the losses had occurred at the 380C level.

There was no difference in recoveries if the furnace and contained paper was slowly brought up to temperature over a 2 hour period prior to the temperature being held at 380C for a further 2 hours or if the papers were placed straight into a furnace at 380C.

For me the interesting part of the results was the high recoveries for the crumpled papers (around 93%) and the low recoveries for the uncrumpled papers (around 50%). It appears that the gold is lost in proportion to the surface area of the papers during the ashing step.
If the crumpling could be better performed to give, say, a hard paper pellet I would expect the losses to be even less than I recorded.

Butcher

I have always found that a boiling solution of gold chloride will have some carry over of gold into the vapour phase.
Tests I carried out many years ago indicated that the low levels of gold chloride in an aqua regia digest of an ore sample had such a low carry over level that losses were difficult to quantify.
However when working with concentrates there was a substantial carry over gold loss.

This is a separate issue to the ashing losses.

You are correct that you would not normally ash filter papers with high gold levels in the papers but the results indicate that you may be able to be less diligent than usual in the rinsing stage if you can minimise the surface area with an ashing of 380C.

I will run a caustic test in the following manner.

Put the gold chloride onto the paper as per previous tests.
Add caustic solution to the paper such that the paper is saturated but not losing liquid to the dish.
Use caustic levels of 1 and 10% solutions.
Dry the papers after a 30 minute contact with the caustic solution.
Ash at 380C with both preheated furnace and a slowly ramped furnace with papers inside.

If there are any steps I have missed out let me know and I will run further tests to incorporate them.
Let me know if I have totally misunderstood what you were interested in.

Deano

 

[butcher]

If the test was to reduce the gold, and to wash out the chloride salts, And I was going to try and do it with hydroxide, I would go for a stronger caustic solution, and more solution, letting the solution settle with the paper, decant and filter the gold and paper and any settled powder through another filter, rinsing it with water to wash out as much of the soluble sodium chloride as possible to remove the sodium chloride from the gold powders and the filter papers...

Again I cannot think of where I would be incinerating a paper soaked with gold chloride solution.
If for some odd reason I had some to do, I would most likely soak them in a minimum amount of liquid of concentrated solution of SMB, or sodium sulfite before hand and filter the clumps of filters.

SMB or some sodium sulfite, would reduce the gold chloride in the paper, working better than a hydroxide solution...

The hydroxide wash would be better where chloride base metal salts where involved in the filtered powders...

I do not know how to explain my thinking here.
Carbon in the paper may hold some of the answer here.

The paper forming carbon (slowly heating the waded paper which will naturally have less exposure to the air inside the tight clump of the paper, the paper can form carbon better), the carbon which may help to capture the gold, of the gold chloride fumes, the carbon capturing gold in the papers carbon, giving the chloride anion a chance to fume off leaving the gold Cation in the carbon, before the paper incinerates to ashes, giving the chloride ion time to form vapor leaving more of the reduced gold in the carbonized paper. (something like using carbon in a gold leach to capture gold).

This I believe would work better with slowly bringing up the heat, in a controlled manner (reducing the oxygen at least for the first of the process, to carbonize the paper to carbon).

And not just flashing the paper to fumes with the hot flame of the torch, where incinerating the paper almost before the fumes of the chloride have a chance to be driven off from the gold, and to carry the gold with them in the fumes.

The tightly waded paper would not only heat up more slowly, but would act more as pyrolysis of the paper to carbon easier, (where the carbon can act to absorb the gold fumes), than say it could with a faster burning of the open paper exposed to plenty of air, or the filter open (not into a wad), where the paper in the open air with the carbon of the paper possibly incinerating or burning off the carbon almost faster than the gold chloride has a chance to vapor off in the fumes.



It is with base metals and chloride salts that I am more concerned with lose of fine gold or silver powders in an incineration processes. Where I would be more interested in doing controlled experiments.

It has been some time since I have had time to do any work out in the lab, with this job, and my back and hip giving me so much trouble, this is one project that I think would be fun and interesting to work on.

I have been doing some work on paper, getting ready to try some experiments whenever I get a chance to get back out to my little lab, which is making nitric acid out of some sodium nitrite, and doing some work with the little bit of silver I have been collecting.

 

[justinhcase]

Would Hoke's recommendation of using an excelerant or liquid fuel to incinerate paper's make a little more sense when look at with this new data.
I know it is no longer recommended but a flame burning on a wick like structure like filter paper would keep the main body of the material relatively cool.
Then it would increase in temperature as the fuel runs out and the ends of the "wick" turns to carbon before being fully consumed .
It would be interesting to compare results from the traditional method and the latest advice.
Regards
Justin

 

[Deano]

Butcher

I will try three test methods.

The first will be as described previously but using 20% NaOH solution. As all liquids will be retained in the paper it will show any losses by ashing gold metal in the presence of chlorides.

Secondly I will add an excess of the 20% NaOH solution to the filter paper, again allow to contact it for 30 minutes and then filter with five rinse cycles of water using 200 ml per cycle.
This will show any losses from ashing in the absence of chlorides.

Thirdly I will contact the paper with a saturated solution of sodium sulfite and filter and five times rinse with water.
This will show any losses from ashing in the absence of chlorides but comparing NaOH with sodium sulfite for gold chloride reduction to gold metal.

I expect that there will be some losses of gold through the filter papers during the filtration steps, all of the solutions will be analysed for gold values.

Let me know if I have missed anything.

I can have a look at base metals and gold in chloride matrices for you if you can lay out what you want done.

Deano

 

[g_axelsson]

Goran

I actually ran a much longer series of tests trying to find out what was the minimum temperature that the filter papers would ash at without flaming. I started at 200C and went up in 20C increments holding each temperature for 2 hours. They required a temperature of 380C to completely ash both types of paper.

The test series were also extended to temperatures up to 660C. There were no differences in results between the 380C ashings and the 660C ashings.
All the losses had occurred at the 380C level.

Hi Deano and thanks for sharing your results.

Göran

 

[Deano]

Finally completed the filter paper ashing tests.

The standard addition of gold chloride for each test was 47.0 micrograms.

Straight aqua regia digest of the filter paper returned 47.1 micrograms.

NaOH tests using 20% NaOH solution

Whatman #1 papers

Ashed flat using flame returned 38.8 micrograms, ashed crumpled using flame returned 40.0 micrograms.

Ashed flat in furnace at 380C returned 43.8 micrograms, ashed crumpled in furnace returned 45 micrograms

Pre processed using 5 cycles of rinse water after NaOH contact.

Ashed flat using flame returned 29.2 micrograms with 0.8 micrograms in the rinse water. Ashed crumpled using flame returned 32.5 micrograms with 1.0 micrograms in the rinse water.

Ashed flat in furnace returned 35.8 micrograms with 0.7 micrograms in the rinse water. Ashed crumpled in furnace returned 39.2 micrograms with 0.7 micrograms in the rinse water.


Whatman #42 papers

Ashed flat using flame returned 34.6 micrograms, ashed crumpled using flame returned 40.4 micrograms.

Ashed flat in furnace returned 42.9 micrograms, ashed crumpled in furnace returned 42.9 micrograms.

Pre processed using 5 cycles of rinse water after NaOH contact.

Ashed flat using flame returned 34.6 micrograms with 0.5 micrograms in the rinse water. Ashed crumpled with flame returned 38.8 micrograms with 0.8 micrograms in the rinse water.

Ashed flat in furnace returned 36.7 micrograms with 1.0 micrograms in the rinse water. Ashed crumpled in the furnace returned 38.8 micrograms with 0.8 micrograms in the rinse water.

It was very noticeable that the NaOH formed what could best be described as a skin over the filter papers which appeared to offer some protection to the gold during the ashing processes. NaOH is difficult to wash out and even after 5 rinse cycles there was still some caustic skin on the papers during ashing.

Repeat of the above tests using saturated sodium sulfite solution instead of NaOH solution.

Whatman #1 papers

Ashed flat using flame returned 30.2 micrograms, ashed crumpled returned 32.2 micrograms.

Ashed flat in furnace returned 33 micrograms, ashed crumpled in furnace returned 41.3 micrograms.

Whatman #42 papers.

Ashed flat using flame returned 18.6 micrograms, ashed crumpled using flame returned 24.3 micrograms.

Ashed flat in furnace returned 26 micrograms, ashed crumpled in furnace returned 40.3 micrograms.


Pre-processed using 5 cycles of rinse water after sodium sulfite contact.

Whatman #1 papers

Ashed flat using flame returned 26.1 micrograms with 0.8 micrograms in rinse water, ashed crumpled using flame returned 31.1 micrograms with 0.7 micrograms in the rinse water.

Ashed flat using furnace returned 33.9 micrograms with 0.6 micrograms in the rinse water, ashed crumpled in furnace returned 38.2 micrograms with 0.9 micrograms in the rinse water.

Whatman #42 papers.

Ashed flat using flame returned 27.1 micrograms with 0.8 micrograms in rinse water, ashed crumpled using flame returned 33.1 micrograms with 0.8 micrograms in the rinse water.

Ashed flat using furnace returned 32.2 micrograms with 0.7 micrograms in the rinse water, ashed crumpled in furnace returned 36.4 micrograms with 0.6 micrograms in the rinse water.


In summary the NaOH appeared to have a protective effect on the gold even after the rinse cycles.

The sodium sulfite treated papers gave less losses of gold than the untreated papers but still allowed substantial losses during ashing.

Low levels of gold were rinsed out of the papers for both reagent types and paper types.

The lowest level of losses were from the caustic only contacted papers even though these papers were the most difficult to maintain in a crumpled shape during ashing, the crumpling appeared to have a minimal effect for these papers when furnaced.

The use of sodium sulfite improved the gold recovery from the ashing process but the results were inferior to the use of NaOH.

The rinsing out of chlorides in the sodium sulfite tests improved the gold recovery in the poorer performing test modes but had a lesser effect in the better performing test modes.

The rinsing out of chlorides had a negative effect on gold recovery for the NaOH tests, this was attributed to loss of a protective NaOH layer on the papers.


Deano

 

[g_axelsson]

Then the question is... does the missing gold go off as metallic gold or as gold chloride? Where does it go?

Göran

 

[Deano]

Goran

As part of the test program I fitted a stainless steel condenser plate, water cooled, inside the electric furnace just under the exhaust hole.
This condenser can be quickly removed and placed in a cyanide bath to check on the level of adhering gold, if any.

I still have most of the cyanide strip solutions to analyse but those which I have looked at show that nearly all of the missing gold is recovered on the condenser.
Recoveries are better than 90% of the missing gold.

Judging from the prior test results I consider that the sulfite treated chloride rinsed free filter papers had the gold volatilise from particulate metal.
The greater volatilisation levels from the untreated papers indicates that the gold chloride form of the gold is, as expected, more readily volatilised than the particulate metal.
The median gold volatilisation levels from the sulfite treated but not chloride rinsed papers indicates that the presence of chlorides adjacent to particulate gold enhances the volatilisation of the gold but not to the level of the untreated gold chlorides.


Deano

 

[butcher]

Nice work, on the experiments.
Thank you for taking the time and going through the trouble do this work to help us better understand
what may be happening.

I would still find it interesting the results of incineration or of melting impure gold powder, brought to red heat to drive off chlorides, or melted with base metal chlorides.
And how well using NaOH to convert the base metal chlorides into NaCl, and base metal oxide hydroxides, (then rinsing out the salt water) will work to keep the volatility of the gold to a minimum.
But I believe your experiment's results help to show it should be at least somewhat helpful.

Good work thanks.

 

[kurtak]

This is awesome :!:

Thank you Deano for taking your time & trouble in posting not just the filter/incineration experiments but the whole thread in general 8)

Kurt

 

[colchis]

Deano - Thanks for a informative process practice. There's one problem about the pH - eH balancing for best leaching- The description states: "What you end up doing with the leach is a continual sequence of adding HCl to adjust the pH and then adding hypochlorite to adjust the Eh, hence the need for the meters." The problem is this: In using Clorox, it contains sodium hydroxide, and will tend to increase the pH. Does this mean that it is necessary to use expensive technical grade sodium hypochlorite? About Clorox- according to the MSDS, the "germicidal" branded clorox contains only 2 ingredients- hypochlorite and hydroxide- the other types of clorox contain a long list of other chemicals in addition. What to do?

 

[Deano]

Butcher

I can fairly readily do some furnacing tests using impure gold electrowinning cons containing around 10% silver and 10% copper.
I can add copper chloride at various rates to see what effect this has on gold volatilisation levels at various temperatures.
I can also add NaOH solution to the above blend and run the rinsed and unrinsed products at the various temperatures.
Gold volatilisation would be measured by the levels recovered from the condensing plate in the furnace.
Keep in mind that the cons have the gold in a fairly finely divided form, it is not like a sheet or as grains.

Let me know what copper chloride rates and temperature levels you would like looked at.

Deano

 

[Deano]

Colchis

It is not necessary to use the expensive tech grade hypochlorite, I use pool granular hypochlorite if I am performing the leach in larger quantities.

The liquid hypochlorite is easier to use but it degrades fairly quickly and if it has been sitting on a supermarket shelf for a while it may be fairly weak. Usually the supermarket varieties will have at least one which is fairly pure hypochlorite with caustic present as a stabiliser, this will usually be the no name house brand which is also usually the cheapest.

I would never use hypochlorite which has other chemicals added, you will get enough garbage in solution from the leach without starting off with something which you have no idea what effect it will have on the leach.

The amount of HCl required to keep the pH in the best zone is fairly minimal, for leaches using just a couple of litres of solution I use a plastic 3ml pipette for the acid additions and a separate pipette for the hypochlorite additions. pH 3 is a fairly weak acid level, you do not need much HCl to get there.

Once you have done a couple of leaches you get to know fairly well the times and quantities for the reagent additions.

Deano

 

[Deano]

I was interested to read about leaching gold from stainless steel using 97% of 50% HNO3 with 3% HCl as a leach.

I do not have access to a source of this type of material but I was interested in what effect this leach may have on an ore.

I have access to samples of a gold ore of grade 1.15 ppm gold by fire assay and by aqua regia digest. Cyanide recovery was 1.0 ppm gold.
The above grades were from 7 replicates with a confidence interval greater than 95%.

This ore contains around 70% quartz and 30% iron as haematite.
The sample was milled to 100% minus 150 microns and 25 gram splits were riffled out.

Leaches were carried out in beakers using 200 ml liquor to the 25 gram ore samples. All leaches were carried out for 24 hours without agitation.

Baseline leaches using the starting leach formula above gave a recovered gold grade of 0.75 ppm gold.
The gold could not be read directly or by organic extraction from the leach liquor by AAS.
The liquor had to be converted to full strength aqua regia before the readings could be reasonably accurately done.

This effect sparked my curiosity and I spiked blank leaches with gold chloride to see what level of hydrochloric acid was required in the leach before it could be reasonably accurately read on AAS.
It appears that any HCl level less than 50% at the spiking stage causes problems at the AAS analysis stage.
I then contacted the spiked liquors with activated carbon, dried and ashed the carbon and analysed the residues.
The results were identical to the AAS readings for the spiked liquors.

Overall the analyses of the spiked liquors showed only 30% of the added gold.
When the spiked liquors were converted into full strength aqua regia I could read 75% of the added gold.

It appears that using this leach type formulation leads to the formation of some gold complexes which neither report to an organic extractant (1% aliquat 336 in DIBK) or are adsorbed onto activated carbon. Why this is I do not know.

Further testing of the ore showed that the level of iron dissolved in the leach was dependent on the temperature of the leach and the HCl %.

If the leach was run at 25C with 3% HCl then very little iron was solubilised and very low levels of colour were seen in the leach liquor.
Running the leach at 50C, 75C and 97C gave greatly increasing levels of iron in solution even with leach times of less than 1 hour.

Lowering the HCL level to 1% in the leach lessened the iron dissolution level but had no effect on the gold level leached from the ore or the difficulty with AAS analysis.

Raising the HCl level in the leach to 5, 10, 20 and 30% greatly increased the iron dissolution levels but had no effect on the AAS analytical difficulties or gold analysed as dissolved from the ore samples.


It appears that the use of 3% HCl in the leach at room temperature is about the optimum condition for gold leaching with minimal iron dissolution.

The fact that there is little iron dissolved in the above leach in the presence of stainless steel is probably less due to any passivating effect of the stainless steel and more due to the low levels of iron dissolution under the conditions of the leach.

It should be noted that there are gold losses in solution associated with the use of such a leach, I do not know if these losses are low level losses associated with low level spiked solutions or if they are proportional through all gold levels.


Deano

 

[Lou]

Certainly not proportional. One can have very high gold loadings with this leach but it is in appropriate to ore due to cost. Your use in the lab for high Fe matrices is interesting. It is a passivation phenomena--even low chloride content will corrode Fe, but less so for its oxides.

Using this leach proficiently is difficult both in achieving optimum leach conditions and most critically, maintaining proper ORP and ligand content to prevent loss of passivation, a wholesale nightmare.


FYI, these solutions analyze fine for Au w/ Flame AAS using standard addition with acceptable recoveries.

 

[Deano]

Alternative gold leach

There is little information on Thiocyanate leaching of gold which would be useful to a small scale user.

Here is what I have found to be a realistic method of using thiocyanate for gold leaching.

First note that thiocyanate complexes of silver have only low solubility so thiocyanate leaching of a mixed gold/silver piece may run into passivation problems.

The most important part of thiocyanate leaching is that thiocyanate leaches are not considered to be toxic, the Australian MSDS for them has no restrictions on transport or storage.

Sodium thiocyanate is preferable to ammonium thiocyanate purely because the ammonium thiocyanate is deliquescent and will turn into a solution if exposed to moisture in the air.

Apart from the toxicity aspect thiocyanate has another advantage over cyanide and that is a much cheaper price.


For a thiocyanate leach to dissolve gold it requires an oxidant to be present in solution.

The standard oxidant used is ferric ions in acid solution. This has the downside of the ferric ions attacking and solubilising many base metals.

Other oxidants which can be used are hypochlorite and chromic salts.

Hypochlorite is tricky to use as it requires constant monitoring to maintain the free chlorine levels below the point where the chlorine attacks the thiocyanate, not recommended.

Chromic salts are a health hazard and are not recommended for use outside a specialist processing facility.


However sulfuric acid can be used as an oxidant for thiocyanate solution. The sulfuric acid levels referred to are dilutions of concentrated sulfuric acid 98%.

Battery acid of 35% or so can be used with the dilutions adjusted for the lower starting strength.

A useful starting level is 5 grams of sodium thiocyanate in a litre of 5% sulfuric acid, mix by adding the acid to the water and then adding the thiocyanate.

The sulfuric acid level will start gold leaching at 1% sulfuric solution, the more sulfuric added the faster the leach rate.

The ability to trade off gold leaching rate against sulfuric level allows the user to have a leach which they are comfortable with from a user safety aspect. This does not mean that all usual safety precautions are not needed. Eye protection is a must, gloves can be of a lower acid rating than for nitric acid etc.

The trade off above also allows the user to refine the process in that the sulfuric acid levels can be adjusted so the the acid attack on any base metals present is minimised.


Deano