Things which may be of interest to members

[Deano]

The only reference I can find to leaching of PGMs with thiocyanate says that only palladium is leached by thiocyanate at room temperature. No reference is made to leach conditions apart from the temperature.

According to the literature nickel thiocyanate can only be formed as a reaction between salts in solution. If this is correct then there should be no attack on the nickel layer by a thiocyanate leach assuming that the reagents in the leach apart from the thiocyanate do not strongly attack the nickel. I cannot find any data for solubility of nickel thiocyanate in water.

0.1M ammonium thiocyanate solution is right on the cusp of silver solubility. Think of it as around 7.6 g/l solution.

If you were to dilute it to 5g/l the solution will not dissolve silver but will just take up gold.

If you want to dissolve silver then you will want to raise the level of the ammonium thiocyanate so that you always have 10 g/l ammonium in solution after dissolving the silver.

You can increase the ammonium thiocyanate level in solution by boiling off some of the water from your solution. Keep in mind that ammonium thiocyanate is only stable up to 170C, so keep your heat source on low or use a water bath and accept the extra time involved.

Deano

 

[sprigsie]

Hullo there Deano,

Thank-you for your very interesting information.

Do you know if the thiocyanate leach is as selective as the thiourea leach as I have ore that is 30% Antimony (as stibnite), chalcopyrite, arsenopyrite and better than 5oz gold and 10oz silver per ton.

My idea is to crush the ore using a jaw crusher and ball mill and then float off the stibnite using flotation tanks and then put the remains left in the bottom of the tanks over a vibration table to separate and treat the free gold and silver and then roast the sulphides to release their values using a suitable leach.

I understand that the antimony will interfere and destroy most luxivants, but thiourea is not effected by this effect.

I was planning to use thiourea but I also understand that there is problems with finding a precipitant that is reasonably well priced and easily available.

Cheers Mate,
Allan.

 

[Deano]

Generally you are going to float all the sulfides at once along with any free gold, you might want to reconsider your flowsheet.

Thiourea is not used much because gold thiourea complexes are not easily concentrated on an adsorbent medium such as activated carbon. This means that you are restricted to either direct electrowinning from the leach solution and the related destruction of the thiourea or chemical precipitation from the leach solution.

Both of the above involve a filtration stage.

Personally I don't use thiourea because it is a known carcinogen.

That said, if the thiourea leach is totally kept away from UV then it is a very good leach for use with stibnite.

Personally I would look at doing a bulk float and selling the float cons or having them toll treated at a smelter. You can then test the float tails to see if they are worth further treatment.

I have no information on the use of thiocyanate with stibnite.

Deano

 

[Eamonn]

Just saw this thread.
Brilliant info thanks Deano.

sprigsie
It was interesting to me that you mention thiourea and that it was carcinogenic because it is in Goodards Silver dip available in the UK. It was the only thing that removed the silver nitrate stains from my worktop which had been there for about six months when I started silver refining. I tried everthing else I had, to remove those stains. Thought they would be there for ever.

 

[sprigsie]

Hullo there Eamonn,
Yes, that's correct that theourea is a known carcinogen, but I don't know just at what concentrations that it is actually carcinogenic.

Cheers Mate,
Allan.

 

[Deano]

Further to the previous thiocyanate testwork I was interested in the recovery of gold thiocyanate from leach solution.

There is a lot of literature which says that gold thiocyanate will load onto activated carbon from a pulp. There is not a lot of literature which compares the loading rates and levels of gold thiocyanate onto carbon with gold cyanide or gold chloride.

I ran three liquors with the same gold tenor of 1ppm gold as a chloride at pH 2 (pH adjusted with HCl), gold as a cyanide at pH 11 (500 ppm cyanide in solution), gold as a thiocyanate at pH 2 (5 grams per litre thiocyanate and 5 grams per litre iron as ferric chloride).

Each liquor was contacted with activated carbon at the same rate of 5 grams per litre carbon in liquor.

After 1 hour the gold levels in the liquors were chloride 0.21ppm, cyanide 0.32ppm, thiocyanate 0.06ppm.

After 2hours the gold levels in the liquors were chloride 0.11ppm, cyanide 0.16ppm, thiocyanate 0.01ppm.

After 3 hours the gold levels in the liquors were chloride 0.07ppm, cyanide 0.12ppm, thiocyanate 0.00ppm.

After 3 hours the carbons were removed from the liquors and dried at 90C overnight.

The dried carbons were then ashed in open top ceramic dishes in an electric furnace at 600C.

The ash was digested in aqua regia and the recovered gold was calculated from AAS analysis.

For gold chloride 93 micrograms of gold loaded onto activated carbon returned 91.3 micrograms

For gold cyanide 88 micrograms of gold loaded onto activated carbon returned 86.5 micrograms.

For gold thiocyanate 100 micrograms of gold loaded onto activated carbon returned 98.6 micrograms.

Effectively the three types of gold complexes recovered similar similar percentages of gold from carbon within the accuracy of the analyses.

It should be noted that the gold thiocyanate loaded faster and more completely than the gold chloride or gold cyanide under the conditions described.

The above indicates that activated carbon is a suitable adsorbent for gold thiocyanate complexes at pH 2 where iron is used as an oxidant.

Deano

 

[Lou]

Always appreciate such a study, and I compliment you on you your work--always good stuff from you. Seems like the recoveries by AA might mean even better news. Sometimes AA runs low.

You should test a poly acrylic ester resin for gold sorptiin. XAD-7 (Dow) is supposed to be good. Gold loves esters/ethers/ketones. Imagine if it took gold out of reverse aqua regia. I theorize it should. Reckon it elutes with water. Should be oxidatively stable.

 

[Deano]

Testwork carried out on the ashing of carbon with low levels of adsorbed gold

Over the years I have carried out many ashings of activated carbon which had been used to adsorb gold from a liquor.

Early tests I did many years ago indicated that when using an electric furnace with the door just cracked for air access a temperature of 600C was suitable.

The carbon was always ashed in porcelain dishes 100mm diameter and 40mm deep, claimed capacity of 150 ml.

I thought that it was time to run a series of tests and define the best conditions for ashing carbon in these crucibles.

I loaded 10 micrograms of gold as gold chloride onto replicate batches of dust free virgin activated carbon of various brands.

The batches varied in quantity from 1 to 20 grams in weight with the 20 gram samples having a bed depth of 15mm.

Due to the difficulty of getting complete ashing using carbon which had been used to adsorbed gold from solutions containing large quantities of base metals I have always used a maximum of 20g of carbon giving the bed depth of 15mm.

If larger quantities of carbon are used in these dishes then several episodes of rabbling the carbon during the ashing will be needed to achieve full ashing.

I ran the batches of carbon for 8 hours at temperatures of 400C, 500C, 600C, 700C and 800C.

All of the 400C batches failed to ash completely.

The 500C batches containing less than 5 grams of carbon ashed completely but the 10, 15 and 20g batches failed to ash completely unless rabbled several times.

All of the 600C, 700C and 800C batches ashed completely without rabbling.

All of the 600C batches returned the full 10 micrograms of gold.

All of the 700C batches returned 9 micrograms of gold.

All of the 800C batches returned between 6 and 7 micrograms of gold.

The above results indicate that although the temperatures in the furnace did not reach even the temperature at which gold melts there is still substantial volatilization of the finely divided gold at 800C.

Overall ashing of carbon should be conducted at 600C to minimize gold losses but still be done in an 8 hour time frame.


Deano

 

[g_axelsson]

You didn't say which form of gold chloride you used to load the samples. I assume you used HAuCl₄ as that would be what we get when leaching gold.

Then I suggest that the reactions while ashing the carbon would be...

1. When heated (haven't found the temperature yet) HCl is removed.

2 HAuCl₄ → Au₂Cl₆ + 2 HCl

2. Further heating breaks down the gold tri chloride above 160 °C and takes away 2/3 of the remaining chloride.

AuCl₃ → AuCl + Cl₂ (>160 °C)

3. At yet higher temperatures (>420 °C) the remaining gold mono chloride reacts and creates metallic gold and gold tri chloride, which probably goes off as a gas.

3 AuCl → AuCl₃ + 2 Au (>420 °C)

This could explain why you see about 2/3 of the gold remaining in the ash and 1/3 going off as gas.

Ref:
Wikipedia : Chloroauric acid
Wikipedia : Gold(III) chloride

Göran

 

[Deano]

Gold was in the form of tetrachloroauric acid standard from Merck, Germany.

The standard interpretation of reactions during the ashing of the carbon is as you have shown and should lead to gold losses at all of the temperatures used in the test regimes.

The reason I posted the results is that there were no losses at all until the temperature reached 700C.

The indication is that the conventional expectations for gold loss do not occur in the expected manner and that further testwork is required to ascertain what is actually happening.

This is a secondary result separate from the fact that 600C is probably near optimal for carbon ashing.


Deano

 

[Deano]

Further on the ashing of carbon.

I re-ran the ashing tests under both cold start furnace and steady temperature furnace conditions for all tests.

All results were identical with the previously reported numbers.

This indicates that furnace temperature ramp up conditions play no part in the gold losses.

I then took dishes containing carbon which had been ashed at 600C and subjected half of them to furnacing at 700C and the other half at 800C for 8 hours.

I also subjected some of these 600C ashings to aqua regia digest to confirm that the gold levels were indeed the same as previously reported, they were, no losses.

The ashings subjected to the higher temperature furnacings returned identical results to the original higher temperature ashings.

This indicates that the losses are solely due to the finely divided gold being kept in the higher temperature furnacing conditions.

They are not due to volatilisation of gold chlorides as such.


Deano

 

[Deano]

By accident I placed some activated carbon blanks in a furnace which has become contaminated with gold from smelts.

Normally this furnace is now only used for smelts, certainly not for precise analytical work.

To my surprise the carbon blanks all registered high levels of gold.

Repeating this test at various temperatures from 600C to 900C showed gold from all samples, the lowest temperature samples registering the highest gold values.

I repeated the tests but only placed empty dishes in the furnace, no carbon.

There were no gold values registered from the empty dishes at any temperatures.

I took dishes containing ash from carbon blanks which had been ashed in a clean furnace and placed half of them in the contaminated furnace at the various temperatures.

All of the samples registered no gold, thus the ash plays no part in the adsorption of gold in the contaminated furnace.

In summary, a dish containing blank carbon ashed in the contaminated furnace registered gold values.

An empty dish placed in the contaminated furnace registered no gold values.

A dish containing ash from a previous ashing in a clean furnace registered no gold values after being subject to the same temperature regimes in the contaminated furnace.

All furnacings were carried out for 8 hours.

Unless activated carbon is present in the dishes in the contaminated furnace there are no gold values registered from these dishes.

It appears that the activated carbon is capable of adsorbing volatilised gold values from the furnace atmosphere until such time as the activated carbon is completely ashed.

This most likely is the mechanism by which gold losses as gold chloride during carbon ashing are eliminated.

It appears that any gold volatilised as gold chloride is adsorbed on the carbon, this effect is repeated until the furnace temperature is high enough to convert the gold chloride to metallic gold.

At this stage there is still enough un-ashed carbon present to stop any volatilisation losses.

Once the gold chloride has been converted to gold particulates then the volatilisation of these particulates will continue in the closed cycle until all of the carbon present has been ashed.

At this stage any volatilisation of the particulate gold will lead to gold losses.

As the 800C ashing is completed in a much shorter time than the 700C ashing there is a longer time for volatilisation to occur when all ashings are conducted for 8 hours.

The 600C ashing is completed just short of the 8 hours and so very little time is available for free volatilisation of the particulate gold.

All of the above fits with there being no gold losses for the 600C ashings, a small loss for the 700C ashings and a larger loss for the 800C ashings.

If you wish to minimise any losses you should keep a close watch on any ashings and turn the heat off as soon as ashing is complete.

Deano

 

[Deano]

There has previously been substantial posting regarding hypochlorite leaching.

This post is an attempt to pull all of the previous work into a single post and also to incorporate further work not mentioned in the earlier threads.

Hypochlorite in solution in its generic sense exists in three forms depending on solution pH.

Below pH 3.5 it exists as chlorine gas.

Between pH 3.5 and pH 7.5 it exists as hypochlorous acid.

Above pH 7.5 it exists as the hypochlorite ion.

At all pH levels there will be some of all of the above forms present.

Thus at pH 10 there will not only be hypochlorite ions present but there will be some hypochlorous acid and some gaseous chlorine present.

In the above example the main form present will be the hypochlorite ion with minor levels of the other forms. You can smell the free chlorine gas present when you open a bottle of bleach even though the pH is above 10.

The pH only controls what dominant species will be present, it does not allow only that species to be present.


If leaching is carried out as a chlorine leach at a pH below 3.5 the chlorine will offgas and be lost to the process unless the reaction is carried out in a sealed vessel.

It is only when the sealed vessel is pressurised that enough of the chlorine will stay in solution and be available for leaching with.

If leaching is carried out as a hypochlorous acid leach it is most efficiently carried out at a pH between 6 and 7.

This is because you are operating in the part of the hypochlorous acid leaching zone which is furtherest away from the free chlorine gas zone and so there is less loss of free chlorine gas.

If leaching is carried out as a hypochlorite leach it is best carries out at pH near 8.

This is because it is operating close to the hypochlorous acid leach zone and will have a fairly large component of hypochlorous acid present.


How can you improve the operation of a hypochlorite system.

Very simply, add salt.

Gold chloride is very enthusiastic about adsorbing onto quartz and sulfides as well as other ore components.

This adsorption can be minimised by adding salt, sodium chloride, to the leach.

About 20% by weight is a good working level of salt addition.

Technically, apart from minimising adsorption of gold chloride complexes onto ore components the presence of the salt stabilises the gold chloride complexes and allows leaching to occur at higher pH levels and lower Eh levels.

Generally pH levels should be kept under 9 even with the salt additions.

The maximum salt addition level theoretically is around 36%, practically it is around 30%.

Care must be taken that evaporation does not cause the salt level to rise to the level where salt starts to crystallise in the leach solution, apart from the operational problems this can cause there will be gold losses from co-precipitation.


What are the benefits of leaching with hypochlorite.

Near neutral pH conditions.

Very little attack on iron complexes at these pH's.

Rapid adsorption of gold complexes onto activated carbon.

Rapid leaching.

Will attack sulfides thus allowing leaching of gold values from sulfides.


Disadvantages of leaching with hypochlorite

Health and safety, chlorine is a cumulative poison and should only be used where all safety gear is being used.

If used around pH 7 the offgassing of chlorine is minimised but is not stopped.

Always keep in mind that chlorine is effective enough as a chemical warfare agent to have been used as such in world war 1.

Plant must be constructed of non-metallic parts, a slow attack on iron complexes does not mean no attack.

A close check must be kept on carbon loadings, if loadings are allowed to get too high there will be loss of gold by attrition from the carbon.

This loss also indicates that the gold should be stripped in the loading canister rather than be transferred to a specialist stripping column, there will be losses during the transfer.

It also indicates that gold loadings during CIP operations should be closely monitored so as to not overload the carbon.

Attack of sulfides by the leach will cause a drop in pH, this needs to be closely monitored during the leach cycle.


Best conditions for running a hypochlorite leach.

My preference is for pH 7, 200 grams per litre plain salt, calcium hypochlorite starting at 1 gram per litre.

The lower you can keep the hypochlorite levels the less attack on metals apart from gold.

This also makes a good silver leach if the solution is circulated through a cannister of activated carbon so that the silver level in solution is always kept low.

The silver will load on the carbon to the point where it can actually be burnished, similarly to gold.

Recover the precious metals by ashing the carbon after drying.


Deano

 

[g_axelsson]

Good write up. I hope I will get an opportunity to test this leach some day.

Is temperature affecting the leach in any major way? I guess higher temperature gives higher leach rates but also increases evaporation.

What is typical leach times? I understand that it depends a lot of ore chemistry and how fine the ore has been milled, but are we talking hours, days or weeks here?

I has probably been mentioned before but what is the maximum loading of gold on carbon?

Göran

 

[Deano]

The hypochlorite leach rate is between cyanide and aqua regia.

For an ore comparison, generally hypochlorite is around 3 times as fast as cyanide but can be sped up by having more hypochlorite present.

This usually means leach times in hours rather than days.

The leach rate is controlled by the rate at which the oxidant can access the gold so some form of agitation is usual.

The agitation can be applied by stirring or by pumping the liquor, the former for ores and the latter for component leaching.

As you stated, the leach rate increases with temperature as does the evaporation rate.

The maximum loading of gold onto carbon is dependent on both the form of the gold complexes and the tenor of the gold liquor( think concentration of gold in the liquor).

The higher the gold tenor the greater the loading possible.

Gold chloride will load to a higher level and do so faster than gold cyanide.

The gold cyanide loading is an adsorption system whereas the gold chloride is mainly a reduction mechanism.

This means that gold chloride will reduce to gold metal on activated carbon and will do so to levels where the gold particles will fall off the carbon. Great care must be taken with carbon loaded from gold chloride not to get to this stage.

The maximum gold loading levels onto carbon are also very dependent on the levels of other metal species present in the leach liquors as these other metals will take up sites on the carbon and prevent gold from loading on these sites.

Temperature and pH are also among the major loading controllers.

In CIP circuits the gold loadings are anywhere from 2,000 to 20,000 ppm on the carbon depending on the mine conditions.

In artificial laboratory conditions under acid pH and low temperature gold loadings over 200,000 ppm have been achieved.

Note that gold cyanide complexes are stable under acid conditions, it is only when the acid conditions are strongly oxidising that the cyanide complex is degraded.

Deano

 

[solar_plasma]

While reading discussions about recovering gold from carbon, I often wondered, if nickel will be of greater concern, since finely devided nickel reacts with CO at 50-100 °C to nickel tetracarbonyl ("liquid death" or "Nickel carbonyl is one of the most toxic substances encountered in industrial processes"), - though it will be decomposed at 180-200°C.

At 100°C or below the reaction of carbon to CO is minimal, even though it exists. So, is it correct to assume, that we are save (regarding nickel tetracarbonyl), when heating let say 100g nickel contaminated carbon to 600°C in a muffel furnace?

 

[Deano]

Regarding the ashing of carbon containing nickel I ran a quick series of ashing tests using carbon loaded with nickel chloride, carbon loaded with nickel cyanide and carbon loaded with nickel chloride reduced to nickel metal with hydrazine.

A fourth dish had only the reduced nickel present, no carbon.

After ashing the ash was digested in aqua regia and the nickel losses were calculated.

There were no nickel losses for any of the dishes where carbon was present.

There was a loss of 20 micrograms of nickel from 100 micrograms from the dish where no carbon was present.

The above indicates that the carbon acts as an adsorbent for any nickel volatilised during the ashing stage, similarly to how it acted for gold.

It is only when all of the carbon has been ashed that volatilisation of the nickel and associated losses occurs.

It appears that the nickel carbonyl does not present a threat during the ashing of carbon.


Deano

 

[FrugalRefiner]

The moderators would like to thank all of the members who contributed to the original thread that this post was based on, as well as those who asked questions showing what was missing. Because of the extra length and interest in this thread we have created the above, consolidated version making for an easier read. We encourage all members to read, comment, and ask questions in the original thread, Things which may be of interest to members.

The moderators have edited some posts in this thread that made reference to posts that were deleted. We strive to make these Library threads as concise as possible by deleting posts that do not add information to the original topic. All of the original posts are still preserved in the thread listed above.

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