Further things which may be of interest to members

[Deano]

I was recently doing a drop of gold from a high grade liquor containing very high levels of copper along with high levels of iron and other base metals.

The aqua regia digest was carried out with a finish of boiling so that the fumes emitted were pure white HCl.

The Eh of the filtered solution was 680 mv before addition of metabisulfite and 390 mv when the reaction had been carried out to completion.

The accompanying photos demonstrate the effect of re-dissolution of gold by base metals in a high chloride solution where the Eh is not generally thought to be high enough to dissolve gold.

The rate and level of the re-dissolution are enhanced by the ultra fine sizing of the precipitated gold as well as the heat of the reduction reaction.

The photos start with the addition of metabisulfite sprinkled onto the surface of the liquor, a gold precipitate forms , this precipitate then re-dissolves.

I also added a solid lump of metabisulfite to the beaker, this lump sank to the bottom of the beaker and a bubble trail of precipitated gold can be seen rising upwards until it is redissolved. These photos are in the second batch.

 

[Deano]

Continuation of photos from above post.








Deano

 

[samuel-a]

Seems like Copper (1) is formed and redissolved rather than metallic gold.

 

[anachronism]

Seems like Copper (1) is formed and redissolved rather than metallic gold.

If that's the case what is the copper redissolved by because it wouldn't be the HCl- and why would SMB drop copper in preference to gold?

I've seen base metal heavy solutions redissolve gold and racked my brains as to why when I know for a fact there's no Nitric around. Hoke alludes to this process in her stock pot chapter too when she mentions coming back the next day and finding your gold redissolved.

 

[Quwannar]

Dear Deano;
the accompanying photos above your post is a marvelous demonstrative examples of re-dissolution of gold by base metals in high chloride liqour. Even if Eh is below 400 mV in Aqua Regia you told us that this phonemenon can occur. what is electrochemically mechanism of that. is that replacement reactions depending on metal reactivity series? how can we hinder this phonemenon?
Could you advice us the cementing procedures with copper bus bar in pregnant solution of saline/hypochlorite which contain high lead, copper and zinc sulfides?
İ am working on copper lead and zinc of bulk flotation concentrates bearing high gold content. My aim is to leach mentioned conc with saline/hypochlorite in a small vat to get the gold. İ observed this phonemenon on Aqua Regia of flotation conc. with SnCl2 test before. in fact, you warned me, due to high base metal content in Aqua Regia of flotation conc.'s., that re-dissolution can take place by other base metal.
also how can İ set up vat leaching unite for saline/hypochlorite leaching?
any valuable comments would be appreciated.
erdem

 

[Deano]

Many people look at a Pourbaix diagram and think that the lines thereon represent a hard cut line where a reaction will suddenly change from one form to an other depending on the change in conditions on each side of the line.

This approach is incorrect, the lines represent equilibrium phases where the reactions represented each side of the line are more likely to occur.

This does not mean that these are the sole reactions which can and will occur, just that these are the predominant reactions.

The further you move the conditions from the lines the greater the likelihood that the represented conditions will be the dominant ones but there will still be some of the alternative conditions present.

Think of it as at the lines the species on each side of the line are in equilibrium.

As you move away from the line so there will be more of the dominant species for that side of the line and less of the species from the other side of the line. You do not, however, get to a point where there will be only one species present to the total exclusion of the other side of the line species.

The other side of the line species may only be present in very minute amounts but it will still be present.

In the case of gold chloride the line for equilibrium reaction of solubilising of gold is 0.994 volts for gold 3 chloride.

Above 1.4 volts you need massive levels of agitation to prevent the formation of gold oxide layers which inhibit the formation of gold chloride.

As you move the voltage down from 0.994 volts towards zero volts so you will change the ratio of gold metal to gold chloride, this means that you will have less gold in solution and more gold in suspension.

It is only when you reach zero volts that all of the gold is in suspension with no gold in solution.

The question is how do you make the above work for you in a gold based process.

In the following it is assumed that all conditions off temperature, agitation, Eh and chloride levels remain unchanged.

If you can remove the dissolved gold from the liquor then some of the metallic gold will dissolve to maintain the ratio of dissolved to undissolved gold.

The best easy way to remove the dissolved gold is by addition of activated carbon, gold chloride is a very fast loader onto this material.

Note that a lot of the gold chloride will reduce back to gold metal on the carbon and will thus be available for re-leaching in the equilibrium reaction.

If you are using a canister of carbon as an adsorbent for gold chloride, it is necessary to assay both the inflow and outflow line gold levels to see when the adsorption rate is matched by the desorption rate. This is the stage where you send the carbon for stripping and replace it with fresh carbon so that the process of gold recovery can continue.

If the dissolved gold tenor is high enough then the dissolved gold can be removed from the solution by electrowinning, usually enough over voltage is applied that the electrowon gold is cathodically protected from redissolving.

There are few gold processes now where the dissolved gold is not continuously removed from the leach liquor during the processing, zincing is the obvious candidate.

There are ways of increasing the ratio of dissolved gold in a process.

By raising the temperature you increase the energy available to dissolve the gold metal, a rule of thumb is that the reaction rate doubles for every 10C increase in temperature.

By increasing the chloride level in the solution you increase both the stability of the gold chloride complexes and the ease of forming them, both effects will increase the level of dissolved gold.

Gold chloride complexes are notorious for adsorbing onto ore particles, any increase in temperature of the pulp will desorb some of this adsorbed gold.

Having high levels of especially divalent metals in solution will increase the solution energy available to dissolve gold, these metals will be oxidised to the higher valence state by dissolved oxygen in the liquor and then reduced to the lower valence state when they react with particulate gold.

However by far the greatest effect of increasing the dissolved gold ratio is by having the gold in a form where the surface area is greatest.

This is best achieved by having the gold as a precipitate from a solution, dilute is best but even from a concentrated solution the gold particle size is still such that the surface area is high.

If you are leaching gold from a float concentrate in a chloride matrix you are making things really difficult for yourself.

The float con will have high levels of divalent base metals which will consume high levels of hypochlorite.

You will have the effect of contact passivation which will require all of the base metals contacting gold to be dissolved before the gold can dissolve.

It becomes a very expensive exercise but will certainly enrich the suppliers of hypochlorite.

My advice is to sell the cons to some one who will treat them in large scale, this may be a specialist treater or a larger mine who already has a treatment circuit in operation. You may a be able to find someone who is sending their cons to a smelter nearby or overseas and have your cons shipped with theirs.

The aim of treatment is to maximise profit, you will find that in general the cost of you leaching the cons is greater than than the costs of a third party treatment.

Deano

 

[Quwannar]

A cyanide leach is the simplest and easiest to run in a vat format so if you are time poor you do this first.

The hypochlorite leach is more efficient than the cyanide leach at allowing the leach access to the gold through the coatings on the gold particles. It does , however, require more supervision than the cyanide leach as the hypochlorite will react with more metals than will a cyanide leach.

Usually there is a lot of iron in the slag from the electrowinning stage of processing. This iron will not interfere with a cyanide leach but will consume highish levels of hypochlorite.

By removing what gold grades you can by using a cyanide leach you minimise the contact time required for the hypochlorite leach and thus lessen the supervision time needed.

Assaying the residual slag is difficult, it is not fire assay's finest moment.

Aqua regia digest will often (read usually depending on the flux formulation) form a gel and be unreliable in the values.

The best method I have found is to cover the slag sample with 10% HCl and let it sit for several months. Keep topping the liquid level up with water.

After leaving the brew to stew for as long as your impatience will let you, you can filter off the residual acid solution and do any of the standard assays.

This generally shows a gold level in the residual slag of 100 - 400 ppm.

It all comes down top how much slag you can get hold of and how long you can leave it in the HCl soak as to what level of recovery you get.

You will certainly recover more gold than the plant operators who just throw the slag in the mill expecting total recovery.

Deano

Dear Sir Deano;
İn your method you developed "covering the slag 10 % HCl and let it sit for several months" can we get reaction time shorter for shaking table concentrate of slags by using ferric chloride.?
Due to highly strong metallic bond with iron you tought us that there will be especially difficulty in assaying and recovering the gold
from slag.
Any comment would be appreciated.
Erdem

 

[Deano]

The only way to shorten the slag dissolution time in HCl is to apply heat to the process. Usually the cost of the heat is relatively large compared with the value of the time saved so heating is used only when the heat is free or near free.

Think heat sources where heat is vented to the atmosphere such as boilers etc.

Heating the process will require you to keep a closer watch on the liquid levels and will also require a more robust form of venting of the fumes from the process.

There are generally three sizes of gold particles contained in slag.

There are the coarse blobs and pellets which are released from the slag by fine jaw crushing and screening.

There are the fine gold particles, some of which are released from the slag by jaw crushing.

Both of these liberated fractions of gold will report to the concentrates when tabled carefully.

If the tabling is really done carefully or a re-run of the cons is performed, then the final con should be clean enough that a fluxless smelt can be performed.

If flux is needed to bring the gold particles together then the outside of the gold particles is dirty with base metals.

You will get gold losses into the slag from such a smelt, this can be avoided by performing a HCl leach on the gold particles before smelting.

The third size of gold particles in the slag are those which are not released from the slag by jaw crushing and remain scattered through the slag like plums in a pudding, albeit very small plums.

These gold pieces can only be recovered by performing a leach on them.

In order for the leach to be effective you need two things to happen.

Firstly you need the gold particles to be accessed by the leach liquor, this means dissolving the slag and contained metals.

Secondly you need the leaching conditions to be such that the dissolved slag does not precipitate out of solution and recoat the gold particles.

Always keep in mind that I am talking here about slag from a commercial gold mine containing a large range and quantity of metals and metallic complexes.

This type of slag will present you with many more headaches than a slag coming from a relatively clean source material such as electronic scrap.

The best method of dissolving both the slag and contained metals is by using a long term leach of HCl.

Other acids will dissolve only some of the slag components and contained metals such as iron.

Even aqua regia is not effective, you can prove this to yourself by running aqua regia digests on splits of milled slag. The recoveries will always be substantially less than the recoveries gotten by gravity systems.

The HCl leach is maintained until there is only fine sludge remaining.

You now have an acid chloride pulp which you wish to keep acid to prevent any of the dissolved parts precipitating from solution.

The obvious gold solvents for this situation are thiocyanate and chlorine, thiocyanate being preferred for ease of use. Chlorine will require the use of a sealed system to avoid health concerns.

An alternative method of recovering the gold from the undissolved slag is fine milling with gold leaching in the mill.

This method is not as efficient as the HCl digest method but is substantially more effective than just performing a leach on unmilled or undigested slag.

If the mill is fully plastic lined or constructed, then the most efficient leach system is neutral pH hypochlorite/saline.

The hypochlorite will digest the metals adhering onto the gold particles and then dissolve the gold particles themselves.

Unfortunately the slag, under these conditions, will go into a rolling circuit of dissolving and precipitating so that in order to keep the gold clean it is necessary to run an overload of ceramic balls in the mill.

If the mill is not fully plastic wetted then it is possible to use cyanide to dissolve the gold but a fully graded overload of steel balls is required.

In both of the above mill examples the pulp after leaching will need filtering before zincing or carbon contact for cyanide, or carbon contact or dechlorination and zincing for the hypochlorite.

Deano

 

[Quwannar]

Dear Sir Deano;
I have a 500 kg of ancient silver mine slag
sample in which densely contain gray color of metallic prills. Thats gray colured prills is so fragile that when I crush them in my impact crusher they can easily scatter into fine particle.
when I screneed them below 2 mm in kitchen sieve and than I tabled 40 kg of sampşe at my shaking table in which I built. I got 750 grams of concentrates. They can be easily seen the semi-yellowish phase zone in my table while tabling is under way. Here is a scene of phase zone in which magnified 100 times. When I looked at them, all of particles in concentration zone was stanied with shining gold. They can easily be seen gold particles adhered to black color of fayalite parts. As you geniously proposed, I mean that yes Sir' slags greatly responds easy gravity separation.
After the a100 mesh sieve of fine milling I will do 10% HCL leaching with heating on 750 grams of concentrates. What do you think about adding H2O2 to speed up reaction in order destroy fayelite parts. Can it directly be run the saline/hypochlorite leach at neutral conditions and or thiocyanate leach with sulfuric acid You proposed.
Thank you very much indeed again.
Erdem

 

[Deano]

A lot depends on your definition of "ancient" slag.

Old european silver slags typically have low or no borax levels so you need to see if the slag sample you have will dissolve in HCl.

The HCl dissolution of borax type slags is only successful if run as a long term leach, trying to speed up the process by adding a strong oxidant usually just starts the cycle of dissolution/precipitation which is what you do not want.

I suggest that you try a HCl leach at room temperature and just let it run for several weeks at room temperature to see if dissolution does occur.

If it does dissolve the slag then you will have to run a chlorine leach, around pH3 with around 20% sodium chloride added to solubilise the high silver level and allow the leach access to the gold.

Always keep in mind that much of the gold will be suspended in the slag dissolving leach liquor so you cannot filter and then come in with an alternate gold leach onto the solids, much of your gold will be lost in the filtrate.

Also remember that thiocyanate is not a leach for silver, the silver thiocyanate formed is insoluble and will block access to the gold.

If you try a neutral pH hypochlorite leach the dissolved slag will precipitate out and cause you much grief.

Deano

 

[Quwannar]

Dear Sir Deano;
I live in Blacksea Region of Anatolia. The other name is Minor Asia. There are many historic mining place in our mountains at the Black Sea region. there are lots of tonnes of slag here... Our mining history goes back Rome and Byzantium era and maybe early ages. Actually I dont think so that borax in smelting of ores was used as flux in slag in that historic period.
Most of slags seems to be ancient.
Best regards
Erdem

 

[Deano]

If the slag is soluble in HCl and has no borax present then I suggest that you do a milling and gravity separation step followed by the HCl dissolution step on the table tails.

I would expect that the bulk of the silver/gold will be in the gravity cons but there will be substantial levels of precious metals in the table tails.

The cons are best treated with a cyanide leach but the tails must be treated with the HCl leach.

Deano

 

[Deano]

One area of recovery which is not widely recognised is that activated carbon will extract metals from fume cupboard or similar extraction air flows.

Any form of processing where metal containing solutions are heated will produce vapours containing those metals.

The temperatures do not have to be at, or even near, the boiling temperature of the liquid phase.

Apart from the obvious application of precious metals, this method will also recover base metals from heated solutions.

Particularly in the case of copper recovery, the copper can be recovered from the carbon by running a cold cyanide wash followed by electrowinning from the eluate.

Deano

 

[Quwannar]

Dear Sir Deano;
First of all I submit my great respect to you. I am on the right way on recoverying gold and silver from the old slags in which has no borax. I feel that you are pointing us a new perspective of a some kind of revolution on processing of slag in which has high gold content.
An attached picture below is the wet assay results (Aqua Regia digest and AAS readings) of silver slags and tabling concentrate and tailings readings respectively without HCL leaching.
According your evaluations and your instructions, If a slag is run for without HCL leaching, There will always be inconsistent readings due to occurring the slica gel phase in aqua regia digesting and not accessing of aqua regia solution into slag metallic pool crust in which save gold. Gold value in every reading is fluctuating.
So I built a small leaching column with a small peristaltic pump that is enabling the mixing. perhaps it may be prototype of a hypochlrite and cyanide leaching column. Meanwhile, I am running for 10 % HCL solution of leaching with a heat source. Temperature is about 35 C. I will wait for several weeks until slags dissolve in sludge form. and then I am gonna send them in private laboratory for assaying. let's see that how will the assay reading change?
Silver content is so high in readings. So Can I recover the silver and gold with a saline/hypochlorite leaching at pH 3.
Best regards
ErdemView attachment 1

 

[Deano]

When you dissolve the table tailings slag in HCl there will be a lot of the precious metal values suspended in the leach liquor.

If you filter the sludge from the liquor after your HCl leach, the solids will be OK to run aqua regia digest on.

The filtrate will contain particulate precious metals in suspension, if you try to dry out this liquor you will recoat the metal particles with problem metals and complexes.

The filtrate must be leached untouched, no drying.

If the silver levels in the filtrate are low then a thiocyanate leach can be used, the silver thiocyanate complex is insoluble but the gold values will dissolve.

If the silver levels in the filtrate are high then adding salt and leaching with hypochlorite at pH 3 is preferred. Have good ventilation, low levels of free chlorine are not the same as no levels.

Always work with known weights of material so that you can calculate recoveries. You show results demonstrating concentrating of gold values in concentrates but without weights of the three component phases, heads, cons and tails, you do not know what actual grades referred to the head are.

Deano

 

[Quwannar]

Dear Deano;
Thank you very much again. HCl leaching is run for seven day at 37 C°.All silver as free fine particle in filtrate is floating inside liqour. When I stop the mixing, some fine particles is also floating at surface of liquor. HCl leaching is still under way by topping 10 % HCL solution to keep volumetric level. What is expreminental solid-acid liquid ratio in HCL leaching. If I take a sample from liquor and solids precipiant, Can AAS analysis be done for silver and gold either liqour without hypochlorite leaching or with running AR digesting of solids sludge. My aim is to determine the effectivetness of HCL leaching dissolution rate.
I am trying to set up closed hypochlorite leaching vessel against chlorine gas escape made by UPVC pipe. To simultanously monitor both pH and Eh parameters in leach solution, I am planning to put a mini plastic valve under fume hood
to rubber pipe of peristaltic pump to take sample from leach solution. What should be the best design of a hypochlorite leaching vessel at pH of 3 to monitor reaction parameter.? When leaching goes on with a peristaltic pump, with the help of another low capacity of peristaltic pump at the same time, Can the gold and silver be loaded and or adsorbed onto active carbon surface in its column?
Any consideration would be appreciated.
Best regards erdem.

 

[Deano]

HCl leaches are usually run at 10 -20% solids.

There will usually be enough base metals dissolved inn the leach to dissolve some of the precious metals, but the bulk of the precious metals will not be dissolved.

Usually I just carefully pan the slag each week or so and estimate the amount of precious metals in the pan concentrate, all parts of the panning method are returned to the leaching container for further leaching.

AAS requires a digest to be performed on the HCl leach tails, the real problem with doing this is that the sample is now not available for further HCl leaching.

Do your aqua regia digest after you have decided that the HCl leach has stopped showing any more precious metal values. This will then give you the level of precious metals available by using this method.

Always keep in mind that there will be large variations in precious metal values between slag samples, you may need to run several samples and average the values to get a reasonable average value for a large heap.

pH 3 leaching is usually done in a polypropylene vessel of square section. Agitation is by a down thrust polypropylene impellor belt driven from a remote motor. Sealed bearings are used throughout.

Dip samples can be taken for measurements or probes can be lowered into the pulp when required for readings.

The leach will generally ruin any probes left continuously in the pulp.

Pulp density of the pH 3 leach varies from 20m - 50% solids.

The higher the % solids the faster the changes in pH/Eh of the leach and the more checks and adjustments required.

Gold and silver chlorides will load very quickly and completely onto carbon, unfortunately so will base metal chlorides.

A frequent check of precious metal tenors in the carbon column tailings stream is required to assure that the carbon has not been loaded with base metals.

At lab scale you cannot ash the carbon and then digest the ash residue as there will be heavy losses of the silver chloride and commitant losses of gold.

At full scale you would strip the precious metals from the carbon with a standard cyanide elution.

Deano

 

[Quwannar]

Dear Sir Deano;
Thank you very much indeed again.
While 10 % HCL leaching on slags is going on, all fine gray colour metal part of slags is disolving day by day. When I panned them, lots of free metal particle can easily be seen at the concentration zone of pan. Color of filtrate in beaker are between pale grayish and greenish color.
İt means that base and precious metal is transfering into filtrate liqour as an chloride salt form of metals. At this moment, Can I send a sample of filtrate liqour for AAS reading? Otherwise Should I wait for dissolving all metal in remaining sludge?
Moreover, you advised us that corbon loading of hypochlorite leach solution have disadvantage due to gold and silver loss in ashing stage of process in lab and full scale.
Well Could you please suggest us how we can precipitates the gold and silver together from hypochlorite pH 3 leach solution by means of zincing?
What is reaction conditions of zinc precipitation of hypochlorite ph 3 leaching.
Best regards
Erdem.

 

[Deano]

Always keep in mind that every mining operation will have different values and conditions.

If you are looking to treat slag from a range of mines then the only constant will be that you have stopped the HCl leach because no further dissolution of the slag is occurring.

It is only when you have finished the HCl leach stage that you start doing assays, doing these assays earlier will give you only partial values.

When you are sure that there is no more slag being dissolved you filter the pulp.

The filtrate has full strength aqua regia added 1:1 by volume and is then simmered until the fumes change from orange to white.

It is preferred that the then cooled liquor has the precious metals extracted with DIBK/1% Aliquat336 organic and run in the AAS against standards of the same matrix.

If you have a very large copper content in the slag then you may need to perform a second extraction as the high copper level can crowd out the precious metal values in the first extraction.

Alternatively you can just run the cooled solution through AAS with background correction on, this is not favoured due to enhanced precious metal readings caused by the presence of base metals, especially iron.

If you are feeling prosperous then you can get ICP analysis done on the liquor, usually very expensive for single samples.

The solids on the filter paper are weighed and then separately digested in either aqua regia or cyanide, preferably cyanide with aqua regia on the washed residue.

This tells you how much of the precious metals are present as fully liberated particles in the solids.

The values registered will tell you which parts of the processing are viable in larger scale and which leach products should be discarded without further processing.

You need to get to this stage before you start worrying how you are going to leach the precious metals as you do not yet know which of the leach products you are going to treat.

Important

Always weigh your slag sample before the HCl leach so that you can do calculations on the leach products.

DEano

 

[Quwannar]

Dear Sir Deano;
First of all I would like to thank you very much again. I wish you had happy new year Sir with your family and your friends.
İn my country, I learned that both commercial and universities laboratories have no knowledge
and skills about running of DIBK/Alıquat 336 organic exraction techniques to specify the precious metal content. If I can purchase DIBK and Aliqıat 336 , I can easily do either Aqua regia or organic extraction of filtrate and participitate with mentioned chemicals. So I will be prepared them for AAS reading of precious metal in a commercial lab. It will also cost to me cheaper.
Could you please teach us the organic extraction techniques and its standart and/or practical protocols with DIBK/Aliquat 336 in the high metallic salt solution (filtrate) and solid parts of sample.
How much DIBK and Aliquat 336 in wolume will We use for per volume Aqua Regia solution?
I am sory for my poor english I have difficulty in expressing and conveying my toughts to the forum.
Best regards
PS; I celebrate new years of all friends in our GFR community.