Things which may be of interest to members

[Deano]

One of the more enduring myths in gold processing is that any gold reporting to the slag in a gold smelt of the cathode material from electrowinning the gold strip solution of the carbon used in a CIP circuit can be successfully processed by throwing the slag into the mill and leaching the resulting product in the cyanide circuit.

If the slag is passed through a hammer mill and the crushed slag is then passed over a wilfley table a disconcertingly large amount of fine gold particles report to the table concentrate.

If this concentrate is leached in cyanide or aqua regia only a fraction of the gold dissolves. This does not fit in with the reprocessing of slag approach used by the gold industry.

It appears that the surface passivation on the gold particles not only prevents leaching but also is the reason for these particles not entering the main body of metal in the smelt.

The only way in which these gold particles may be successfully processed is to use these particles as the feed in the previous post " Smelting of gold concentrates".

This was the method I developed for recovery of the slagged gold, the fact that it improved the gold purity was a bonus.

Deano

 

[Lou]

What do you mean surface passivisation of gold? Under what conditions???


Regarding the cathode from cyanide plating--can usually get down to 10-15 ppm Au with steel wool cathode, stainless anode, pH 12+; run until much O2 fizzing.
Incinerate the cathode (it will be brown like gold coming out) and redissolve in aqua regia; filter to remove AgCl; precipitate, wash gold, 9995%+

Lou

 

[Deano]

Lou

A lot of the material I work with is the product of mine CIP carbon stripping circuits.

Basically the electrowin is very similar to what you wrote but we always have some copper and lead on the cathode wool. Depending on the mine there may be up to 20 kg of gold per strip cycle. Not surprisingly the mine management want to get this fairly portable gold on wool product into a single less portable bar as quickly as possible. As soon as it is dried after aciding out the residual steel wool it is smelted.

All gold mined in Australia is sold to refineries such as the Perth Mint. There will be a refining charge for each bar sent to the refiner along with various other batch and sales charges.

There is the same charge per ounce for refining if the precious metal levels are 80% or if they are 9999. No mine is going to attempt higher purity in the bars when it will cost more and there is no discount on the refining charge.

The surface passivation on the gold beads is caused by a coating of lead minerals. This coating is only a few hundred angstroms thick and is not detectable by optical methods, I had to use XRF.

The coating is not soluble in any solvent in the classical chemistry line and believe me I tried them all. You get a little depressed when you boil the treated beads in aqua regia or cyanide and nothing happens. A few hundred times.

The only way I found to recover the gold from these beads is with the borax smelt in clay crucible trick.

Deano

 

[Deano]

Lou

I was rushed on the last reply and left out a broader description of carbon stripping, my apologies.

Most mines in Australia use a hot caustic cyanide strip on their carbon from the CIP/CIL circuit. If a slight overpressure is used in the strip columns with a pressure drop chamber at the entrance to the EW box then the strip time can be substantially reduced as the temperature of the leach can be run slightly above 100⁰C. The change can be as much as going from a 24 hour strip to a 6 hour strip. This effect is used to increase the capacity of a strip circuit without having to install greater power rectifiers and more EW cells etc.

We usually do a HCl contact of the carbon before the cyanide strip, this gets rid of a lot of the base metals and improves the gold recovery from the carbon.

In a lot of mines we have difficulty getting the gold on carbon level below 100 ppm. This is usually due to high levels of base metals in the ore.

Usually we are trying to get the gold on carbon level down to less than 50 ppm, preferably less than 30ppm.

In most stripping circuits the gold on carbon levels have been well established as a function of stripping current, there is a definite drop in current as the gold level drops.

There is not a great emphasis on getting all of the gold from the strip liquor as the used liquor is later used as make up liquor for the leach circuit. Provided the gold on carbon level is about right and the strip liquor tenor is less than 20ppm gold then most mines are happy.

If you put the recovery into context, you are getting say $500,000 of gold from a strip. Actually carrying out this strip might cost you $ 1,000. There is not a great incentive to be more efficient in the strip process.

Usually a fair amount of the gold will fall off the cathode and form a sludge layer on the bottom of the cell. This gold has to be drained from the cell, filtered and dried before being added to the other gold before smelting.

The steel wool is acided out from the gold cons with HCl. An incredible amount of gold is dissolved in this step, the acid solution and rinse solutions are usually poured through a plastic drum full of carbon for recovery of this gold.

The gold cons after aciding are dried, chopped up finely with a steel scoop and are ready for smelting.

There are as many methods of actually doing the smelt as there are mines.
Some mines heat the crucible to temperature before adding the flux, they get the flux up to temperature and then add the gold.
Other mines will heat the crucible to temperature and then add the flux and gold as a single charge.
Still others will put the flux and gold charge in the crucible before starting heating.
Any variation of the above is being used somewhere.

Similarly every mine has a pet flux formula, usually set by the first gold room superintendent and not changed as an article of faith ever since.
If the gold charge is split into two before smelting there is usually no difference in gold recovery between the formula flux and straight borax. Takes a bit of the mystique out of it all.

After the carbon has been stripped and a sample retained for analysis the carbon is usually run through the regeneration kiln to restore adsorption efficiency. A lot of times it doesn't need the full regeneration but is improved by a quick clean in the kiln.

Deano

 

[g_axelsson]

The steel wool is acided out from the gold cons with HCl. An incredible amount of gold is dissolved in this step, the acid solution and rinse solutions are usually poured through a plastic drum full of carbon for recovery of this gold.

Have you ever tried to precipitate this gold with SO₂ (or sodium meta bisulfite)? Afterwards the really low level gold bearing fluid could be treated in a "stock pot" as being described in many places on this forum or in Hokes book, then discarded as waste (properly).

As I'm not familiar with the running of a CIP facility, I suspect there is a (unknown to me) reason to do it your way.

Göran

 

[Deano]

Goran

Think of processing in a gold room as industrial scale but carried out by people with little understanding of the processes involved. Would you really want someone like this carrying out a precipitation and filtration stage with the tying up of the equipment when the liquor can just be poured into a drum of carbon.

This carbon is stripped with the other CIP circuit carbon when the gold level is high enough, if you have gold metal showing on the carbon you have left it too long before stripping.

The liquor from the carbon adsorption drum has effectively no gold in it and is disposed of in the tailings dam, 100 litres in 10,000 tons of water does not constitute a hazard.

The major problems in the gold room relate to processing and theft.

The processing problems are overcome by following a standard set of simple procedures, no intellectual input required.

The theft problems are minimised by having the gold in a portable form for the shortest time possible, having the gold on carbon does not rate as portable gold.

Always remember that high purity of the bullion is nice to achieve but not an economic requirement at this scale.

Deano

 

[g_axelsson]

Ahhh... the people factor. :mrgreen:

Even Hoke has a section on losses due to dishonesty.

Göran

 

[Deano]

Water sourcing

Many mines use water pumped from an underground aquifer for make-up water. This water is usually stored in a dam until required. There are often two problems with doing this.

One is the growth of algae in the dam water to the point where it interferes with the use of the water.
The algae may be removed by two methods.
The first method is to stock the dam with carp at a rate of one fish to about 200 tons of water, this rate is dependent on the nutrient load in the water, temperature and surface to volume relationship. Make sure that only one sex of fish is used if more than one fish is required.
Many authorities will not allow dams to be stocked with carp so the second method has to be used.

This involves spraying the surface of the dam with a ferric salt solution, usually ferric chloride. This must be done using all plastic equipment including pumps and spray nozzles. You are aiming to cover the surface of the water only, not turn the dam into an acid bath.
When the ferric chloride turns into ferric hydroxide in the dam water there are short lived hydroxyl radicals formed.
These radicals are toxic to algae, in fact to most small organisms including bacteria.
The spraying must be done in daylight so that the algae are present in the upper layers of the water.

The second problem with the dam water is that many aquifers have substantial levels of ferrous salts present. When the water is pumped into the dam a slow reaction with dissolved oxygen will convert the ferrous salts into ferric salts.
These salts will precipitate out slowly forming a coarse sand. Unfortunately the precipitation occurs not only in the dam itself but also in the pipework from the dam to the mine plant.
This causes blockages in the pipework and valves, a surprising amount of this sandy material is formed.

To stop the sand formation the aquifer water should be sprayed into the dam in a fan jet so that the water is saturated with oxygen from the air. This causes the ferrous to ferric reaction to happen much faster and cuts out the slow sand formation.

The above is applicable to farm situations as well as mines.

Deano

 

[Deano]

Thiourea leaching

Many people try different methods of leaching gold out of curiosity and general interest. These efforts are to be applauded, even failures add to the sum of knowledge.

Effectively there are two types of gold leachants, organic and non-organic.
The non-organic leachants comprise the halide complexes and some of the more esoteric leachants such as selenic acid.

These non-organic leachants all suffer from one or more of the following defects.
1 They must be run under acid conditions.
2 They have serious health and safety issues.
3 They are expensive to purchase let alone use.
4 Complex chemistry.

The organic leachants suffer from one or more of the following defects.
1 Toxicity.
2 Reagent degradation.
3 Complex chemistry.

When thiourea became of greater interest as a precious metal leachant back in the 1980s a lot of work was done by many groups to optimise operating conditions.
Unfortunately all of these groups failed to recognise that the major cause of leach difficulty and high reagent consumption was reagent degradation.

Thiourea or its oxidised form called formamadine disulfide, when in soultion, is attacked by ultraviolet light and degrades to elemental sulfur.
The degradation rate increases as the solution Eh decreases and/or the Ph increases.

Thiourea as such will not complex with gold, it requires an oxidant to form formamadine disulfide (FDS). This is what actually complexes with the gold.
FDS is formed when an oxidant with an Eh of greater than 300mv, preferably greater than 350mv, is added to a solution of thiourea at levels so that the Eh is maintained in the solution.

Most testwork carried out with thiourea has used ferric salts under acid conditions as the oxidising agents. All sorts of clever Eh balancing acts have been used to minimise the losses of thiourea in the leach solution, what no one has done is to carry out the experiments in the absence of uv light.

Thiourea can be used under alkaline leach conditions with oxidisers such as hypochlorite, however the thiourea is very sensitive to uv degradation at these higher Ph ranges.
If you want to run thiourea under alkaline conditions you must do so in the absence of uv.

Keep in mind that thiourea is rated as a carcinogen and as such is not recommended for general use outside of specialist labs.
The gold thiourea complex also suffers from poor loading levels on carbon and resin, it needs to be electrowon or zinced out of solution.

Thiosulfate and other organic form leaches can be made more robust and employ simpler oxidants if used under low or no uv conditions, remember that they also suffer from extraction difficulties.

Deano

 

[Lou]

Thiourea cannot be used under basic conditions at all.

I used to elute PGM and Au thiourea complexes off of a macrocylic PS resin functionalized with thiouronium groups (Purolite S920 IIRC).

The options at that point were to use borohyride and save your thiourea, or if really concentrated, you could bring up the pH and boil and destroy your thiourea. Precious metal sulfides precipitate in quantitative yield and may then be worked up in aqua regia.

 

[Deano]

Lou

The standard elution for gold from strong base resins is thiourea in sulfuric acid solution where the sulfuric acid supplies the Eh required to complex the gold from the cyanide or chloride form into the gold thiourea complex. This is usually carried out as either a multi-stage series of contacts or as an extended flow through contact. Either way you get a strong acid solution of gold thiourea which, as you pointed out, can have the gold extracted in a multitude of ways.

If you raise the pH you destroy the Eh as the sulfuric acid is neutralised. At this stage the thiourea complex is very sensitive to degradation by uv and will precipitate elemental sulfur in minutes. If the solution is not accessed by uv there is no precipitation of sulfur.

An alkaline solution of thiourea in alkaline hypochlorite solution and exposed to uv is stable only for a few minutes but will leach gold in that time. If the uv is not allowed to contact the solution then the solution is stable as a gold leachant for a long time.

I have kept some of these solutions in a uv free area for over a year, they are crystal clear when they are then exposed to uv and last only minutes before elemental sulfur is precipitated.

None of the above means that I use thiourea as a leachant for gold, the health concerns and difficulty in recovering the gold values as a concentrate on an adsorbent see to that.

Deano

 

[Deano]

Gold complex adsorbents

In industry there are two adsorbents used for collecting and concentrating dissolved gold values.
These are activated carbon and ion exchange resin.

Activated carbon is the gold industry's adsorbent of choice in that it is cheap and easily handled through the loading and stripping cycles.
The downside of carbon is that the gold loadings from mine leach tanks are relatively low. This means that the carbon must be recycled through the load and strip cycles at a fair rate to recover all the gold available, you do not want to leave the carbon in the circuit for a long time in an attempt to raise the gold loading on the carbon as the attrition losses will increase.
Carbon will also adsorb most metals in the leach solution, especially copper and lead, these can give problems requiring extra steps in the stripping cycle as well as lowering the number of loading sites for gold in the adsorbence stages.

The elution of gold from carbon in a mine situation will always leave some low level gold irretrievably locked in the carbon, this gold can only be recovered by ashing, leaching the ash with cyanide and loading the gold onto fresh carbon.

Problems with using carbon relate to attrition of the carbon resulting in the generation of fines which carry gold values into the tailings dam, the time it takes to strip the carbon and the need for regeneration of the carbon in a kiln on a regular basis. Generally the industry uses a hot caustic cyanide strip solution for carbon but many variations are available.

Ion exchange resin is really only used widely in Russia and neighbouring countries.
The resin can load to higher gold levels than carbon and generally does so more quickly than carbon.
A lot of resin manufacturers have worked on providing a resin which is gold specific and thus will not load appreciable quantities of base metals. Generally they have been relatively unsuccessful, there appears that there is a trade off with ease and level of gold eluted in the strip cycle.

Most commercial resins used for gold cyanide recovery are functionalised with quaternary ammonium groups, these are called strong base resins and are commercially stripped with a sulfuric acid /thiourea solution.
Purolite have a gold resin which uses mixed non quaternary groups, this can be eluted with caustic cyanide solution (yay).

The pros of using resin are the faster and higher gold loadings obtained and the speed and ease of the strip cycle.

The cons of using resin are the high initial cost, the need for finer screens in the gold plant, the level of gold lockup in the resins and the effect of osmotic shock on the resins.
After a number of cycles the gold level locked up in the resin is so high that this inaccessible gold represents a substantial proportion of the gold inventory in the circuit.
This locked up gold also greatly lessens the amount of gold which can be adsorbed and desorbed per cycle as there are less adsorbance sites available.
Like carbon, this locked gold can only be recovered by ashing the resin and leaching the ash.

Generally the resin beads are less subject to wear than carbon particles. Unfortunately they suffer from osmotic shock which carbon does not suffer from.
Osmotic shock is the swelling and contraction which the beads go through when the liquor in which they are immersed changes from acid to alkali or reverse.
Going from strong alkali to very strong acid conditions and back again will ensure that the beads suffer osmotic shock and will start to physically fall apart into smaller particles.
This means that resin and thus entrapped gold losses are substantial even with extra screening.


Deano

 

[Deano]

Organics destruction

There are often requirements for the removal of organics from a water matrix. This can vary from cyanide destruction to dye oxidation and a general removal of a large number of of other organics which are wanted to be removed for health or other reasons.

The destruction of these organics can be achieved by reversing polarity of the electrowin cell detailed earlier. This means that the carbon felt will now be the anode and it will be generating very high levels of hydroxyl radicals. These radicals are short lived but very highly oxidising, the passing of the solution through the felt ensures that the organic particles will come into contact with these radicals.

There are two controls to the process, one is the flow rate of the liquor through the cell and the other is the rate of generation of the hydroxyl radicals.

There must be current flowing through the call for the radicals to be generated. This current can be carried by by acid or alkali in the liquid or by a neutral, non active salt such as sodium sulfate in which case the current is carried by water splitting at the electrodes.

Generally as long as the electrodes have gas evolution from them then there is enough current flowing to generate the hydroxyl radicals.

Due to the sheer number of unknowns in any solution such as organic type, concentration etc. I cannot give a general current level for any liquor.
I have always adjusted pH and/or salt in solution so that I had gas evolved at the electrodes, easiest seen at the now cathode in the centre of the cell.
I then adjusted the liquor flow rate through the cell so that all of the organics were destroyed in a single pass through the cell. You can use low gas evolution with slow flow rate or high gas evolution with faster flow rate.

Deano

 

[Deano]

Organics destruction

Once again some things I left out.

The felt cell run as per previous instructions will, by by removing organics from water, remove offensive odours.

It will also sterilise the water but the sterilising process is not an extended effect. The water is sterile only until it exits the cell. At this stage it can be recontaminated.

Deano

 

[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

 

[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 accelerant 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