Cementing PGM's from AR using Copper Bus bar

[RickRag]

Hi Guys,

Per Frugal Refiners suggestion I droopped all of the gold from an AR solution and transferred the solution to a larger vesssel to cement all the PGMs remaining with a copper bus bar using an inexpensive air pump intended to aerate aquariums. The copper plate/bus bar is 1/4" thick by six inches by 8 nches. It's been bubbling now for about a week and I still get a positive reaction with SnCl. See picture enclosed. Could the bus bar too encrusted with precipitated metals to continue to dissolve? I've tried to scrape the thin coating of black off to wxpose fresh Cu but but it is tenacious and won't wipe or rub off, at least not without some mechanical abrasion which I haven't tried. Is the test in the picture still positive for values or could it be a false positive due to the SMB used to drop the gold? Suggestions please!!

 

[anachronism]

It's not a false positive from SMB, certainly not after a week anyway.

How much base material did you have, and what exactly was it? So how much and what types of PGMS do you believe you have?

Also let's see a pic of the copper bar.

Jon

 

[RickRag]

This was all from dental scrap that wasn't inquarted, just directly dissolved in AR. There is expected to be some copper, zinc, nickel and a fair amount of palladium and possibly some platinum, although there is not much Pt used in dental gold. There is no silver in the filtrate as it was precipitated and filtered from the AR after all the gold dissolved. All nitric was killed with either the dissolution process itself or what little remained was neutralized with sulfamic acid. It did not require much as I was careful with the nitric additions. After filtration the solution was diluted three times in preparation for dropping the gold. The filtrate from the gold drop was crystal clear but had a greeish blue tint to it and it tested positive for values, but not any dark purple or black indicating the presence of gold, just blue-green and reds mixed. After dropping gold, it was decanted off, allowed to settle overnight again and just to be sure I had dropped all the gold, I added another teaspoonful of SMB to the depleted AR just to see if any more gold dropped. Tested again and got nothing. I got the slightest trace of precipitant and concluded that the gold had indeed been dropped. The filtrate was transfered to a larger vessel to accommodate the large bus bar and cementation was begun with the aerator. A pic of the bus bar is included. The greenish stuff was black when the plate was removed from solution. This pic was taken approximatey an hour after it had been exposed to air. The entire solution containd appproximatley 3-5 oz. of gold although I haven't weighed it. Because it dissolved directly in AR it likely had 15-30% base metals as most quality dental gold is 14-18 karat. I got maybe 10 - 15-grams AgCl. Total solution volume after dilution and dropping was approximatley 8 liters. I'm guessing that this is likely Pd remaining in solution but I can't seem to figure out why it hasn't been cemented out as yet. Should I try concentrating the solution and abrading the bus bar to expose fresh Cu? Maybe switch to Cu powder as Dave recommends? Is the solution acidic enough after the dilution process to enable good Cu dissolution for cementation?

 

[Lino1406]

Stannous test is sensitive to traces of pm's. But so high copper concentration seems to interfere somehow, as we see copper color in your test

 

[RickRag]

I found this link to a previous discussion on this topic. Does anybody have anything to add to maybe clarify the solution? No pun intended.

https://goldrefiningforum.com/phpBB3/viewtopic.php?f=49&t=14482

Another search revealed the following:

Cementation of metals from acid solutions
United States Patent 3902896

Abstract:
Discloses the use of soluble thiosulfate in aqueous acidic solutions to aid the cementation of metals from solution. The invention is particularly useful in the recovery of copper, silver, gold and platinum group metals from aqueous acidic solutions.


Description:
The present invention relates to the recovery of copper, silver, gold and platinum group metals from solution, and more particularly to the cementation from acid solutions.

It is frequently desired to remove copper and other valuable metals from process solutions to produce a metal concentrate for further treatment and/or to purify the process solution. Copper ores are, for example, sometimes heap leached with sulfuric acid to provide a comparatively dilute pregnant solution, and copper is advantageously recovered from such solutions by cementation. As another example, nickel process solutions that contain undesirable amounts of copper can be treated with particulate metallic nickel to cement the copper from solution. Another solution might be to add nitric to etch the copper plate during the cementation process per

Cementation is, to a large extent, a surface phenomenon in which copper or other valuable metals in solution are, by a metathesis reaction, deposited on a particulate base metal higher in the electromotive series. As cementation proceeds, the particulate base metal is coated with the valuable metal and the rate of cementation subsides. If and when the entire base metal is coated with copper or other metal the cementation reaction comes to a halt. In order to minimize the slowing down or the complete termination of the cementation reaction, the suspension of particulate base metal in the pregnant solution is agitated to remove the copper coating continuously from the particulate base metal thereby exposing fresh surfaces upon which the reaction can proceed.

Full utilization of the particulate base metal is important from the standpoints of base metal consumption and subsequent concentrate treatment. If the cementation reaction comes to a halt before substantially all the base metal is consumed, additional base metal must be added to the solution increasing the overall costs of the process. When the base metal is incompletely utilized the cement copper or other metal must eventually be separated from the base metal adding to the cost of recovery.

It has now been discovered that cementation of metal values dissolved in acidic aqueous solutions on particulate base metals can be improved by incorporating small but effective amounts of a special additive to the acidic aqueous solution to minimize termination of the cementation reaction before the particulate base metal is consumed by the reaction.

Generally speaking, the present invention involves an improved process for recovering metal values dissolved in acidic aqueous solutions by cementation on a particulate base metal higher in the electromotive series than is the dissolved metal or metals. The improvement comprises adding a water soluble thiosulfate to the acidic aqueous solution in small but effective amounts to produce an exfoliative metal deposit on the particulate base metal so that the metal deposit readily flakes off the particulate base metal exposing fresh surfaces for the cementation reaction to continue.

Copper-containing aqueous solutions derived from any source can be treated by the process in accordance with the present invention. For example, pregnant solutions produced by leaching cupriferous ores can be treated to recover copper by the improved cementation process of the present invention. Other process solutions, such as nickel-containing electrolytes for electrowinning or electrorefining nickel, can be treated for copper removal by the technique present invention by cementation on nickel.

The process in accordance with the present invention can be employed to treat a wide range of copper-containing process solutions and the invention will be described in conjunction with the treatment of nickel process solutions.

Nickel process solutions that can be treated by the process in accordance with the present invention contain nickel in amounts of up to about 100 grams per liter (gpl) or more, up to about 10 gpl copper, up to about 230 gpl sulfate ions, up to about 100 gpl chloride ions, up to about 100 gpl sodium and up to about 30 gpl boric acid. The pH value of the nickel process solution can vary over wide limits but the pH value of the solution is advantageously maintained between about 2 and 5.

The conditions of cementation are not critical. The temperature of the process solution can vary from ambient temperatures to above the solution's boiling point at corresponding pressures. Although the kinetics of the cementation reaction are improved by increasing temperatures, the advantages gained by the improved kinetics are soon outweighed by the additional costs required for pressure equipment once the boiling point of the solution is exceeded. The advantages of improved kinetics without additional capital costs are best realized by conducting the process at temperatures between about 25°C. and 100°C.

Atmosphere control is not essential but the atmosphere should not be so oxidizing that the cemented copper is oxidized and redissolved. Under ordinary conditions special precautions need not be taken; however, if excessive agitation is employed the cementation process should be conducted under a neutral or slightly reducing atmosphere. Maintenance of a neutral to slightly reducing atmosphere can be achieved by conducting the cementation process in a closed vessel and by excluding air or other free-oxygen-containing gases from the vessel or introducing neutral gases such as nitrogen or reducing gases, such as hydrogen or carbon monoxide, into the vessel.

Cementation is conducted with sufficient agitation to maintain the particulate base metal in suspension. Agitation such as prevailing in a mixer tank insures good liquid-solid contact so that the cementation reaction between the dissolved copper values and the particulate base metal can proceed at commercially attractive rates. Not only does the agitation insure good liquid-solid contact between the copper-containing solution and the particulate base metal but such agitation provides interparticle collisions which provide effective abrading action, particularly for the exfoliative copper deposit produced in accordance with the present invention. The cementation reaction may also be carried out in a liquid fluid bed reactor, pachuca tank, packed column or other type of contactor. The type of reactor is not critical to this invention.

Any base metal that stands higher in the electromotive series than does copper usually can be used to cement copper from solution. Examples of base metals that can be employed include aluminum, zinc, cobalt, iron and nickel. When treating nickel process solutions it is particularly advantageous to use particulate nickel. For example, nickel oxide sinter that has been reduced substantially to metal can be employed to cement copper from nickel process solutions. Advantageously, the nickel oxide sinter is reduced at low temperature, e.g., below about 600°C. in order to provide a kinetically active metal. The nickel oxide does not have to be completely reduced to the metallic state, and, in most instances, nickel oxide that is about 90 percent metallized can be used.

As those skilled in the art will appreciate, the particulate metal used to cement another metal out of aqueous solution should be compatible with the anions in solution. For example, even though lead is above both copper and silver in the electromotive series (for example, as set forth on page 1740 of Handbook of Chemistry and Physics, 44th Edition Chemical Rubber Publishing Company), lead would be a poor choice for cementing copper out of a sulfate solution because of the formation of sparsely soluble lead sulfate and again a poor choice for the cementation of silver out of a fluoride containing solution owing to the formation of sparsely soluble lead fluoride. Zinc, on the other hand woudl be a good choice in both instances. As another example, difficulties may arise when using iron to cement copper out of solution if the pH of the copper solution is too high. Hydrolytic precipitation of the iron from the aqueous medium can occur under those conditions.

Another factor which those skilled in the art will appreciate is that the metal added for cementation purposes should not be too active. For example, most of the metals above about aluminum in the electromotive series will react not only to cement metal from aqueous solution but also with water to form gaseous hydrogen. In the usual case, this production of hydrogen constitutes a waste of cementing metal and thus an increase in reagent cost. Hydrogen can also be produced in substantial quantities using zinc, aluminum, iron etc., as the cementing reagent if the aqueous solution is too acidic. In order to minimize hydrogen production when using zinc, aluminum, iron etc., as the cementing reagent, the pH of the aqueous solution should be controlled so that it is no less than about 1.

Another factor that is important in controlling the cementation reaction is the particle size of the base metal. The rate of cementation is to a great extent determined by the surface area of the base metal with the rate of cementation being a direct function of surface area. Smaller particle size not only increases the rate of cementation but the problems associated with maintaining the base metal in suspension are also minimized. From the standpoint of cementation kinetics, a particle size distribution of 100 percent minus 20 mesh, Tyler Screen Size, is advantageously employed. Most advantageously, a particle size distribution between about 35 percent minus 200 mesh and about 10 percent plus 60 mesh are employed to maximize the cementation reaction and to avoid the problems associated with the handling of finely divided materials.

Some base metals for use in cementation may become slightly oxidized prior to use and do not react rapidly or completely during cementation, particularly when cementing from solutions having pH between about 3.5 and 7. In this instance an acid wash of the base metal prior to use may be advantageous.

An important aspect of the present invention is the production of an exfoliative cement metal deposit on the particulate base metal so that the deposit is easily dislodged from the base metal exposing fresh base metal surfaces on which the cementation reaction can continue. Production of an exfoliative cement metal deposit is obtained by incorporating at least one water soluble thiosulfate in small but effective amounts in solution. Examples of water soluble thiosulfate are sodium thiosulfate, ammonium thiosulfate, magnesium thiosulfate, potassium thiosulfate and lithium thiosulfate. Sodium thiosulfate is advantageously employed because of its ready availability and low cost.

The water soluble thiosulfate is added to the solutions in small but effective amounts to produce an exfoliative metal deposit. The amount of water soluble thiosulfate added to the metal-containing process solutions can vary over wide limits but, in most instances, it is advantageous to add the thiosulfate to the process solution in amounts up to about 10 parts of sulfur equivalent thiosulfate (i.e., sulfur present in thiosulfate form) to 100 parts of metal to be cemented from solutions. Ordinarily, a minimum of about 0.1 part by weight of thiosulfate sulfur is used per 100 parts by weight of cementable metal and advantageously in amounts between about 0.2 part and 3.0 parts sulfur equivalent thiosulfate are used. Thiosulfate additions within the foregoing ranges insure exfoliative metal deposits on the particulate base metal while minimizing the costs associated with the reagent.

Another important aspect of the present invention is the concurrent effective removal of trace elements, such as lead, arsenic, antimony, bismuth and selenium, when treating nickel process solutions with nickel and thiosulfate to remove copper. Trace amounts of soluble precious metals are also effectively recovered with the precipitated copper.

The discussion above encompasses methods to recover copper and some of the difficulties in the process. These appear to be analagous to the problems encountered here. Although a thiosulfate solution is not going to be introduced here, the physical method of keeping the cemented metals from occluding the base metal, in this case copper, may be incorporated by other means. Per 4 metals post previously, powdered Cu is likely going to be a good answer. Or Kurt's suggestion of adding nitric so the copper plate is constantly be attacked to "freshen" it's surface area.

Thoughts anyone??

 

[Platdigger]

Well, at this point you could use iron, zinc or a strong reducer, and basically just start over.

 

[RickRag]

I appreciate your suggestion Platdigger, but that isn't an option. Iron or zinc would drop other base metals, including a lot of copper that is now in solution, that I am attempting to exclude. I do intend to re-refine both the gold and the PGM's a second time for greater purity. I just want to eliminate as much trash as possible the first time around.

This is a solvable problem and others on this forum have success cementing out values from gold free chloride solutions. I am having difficulty and don't know why and would like some help. I suspect the solutions are too dilute, thus depositing a very fine tenaciously adherant particulate on the copper plate and thus occluding access to fresh metal. It does not flake off and settle as it did in a previous attempt that used a more concentrated solution and a much thinner copper plate. I would like some input from others that either have had this problem and what they did to solve it or from those that dont seem to have this problem and what they do differently.

Based on my research, there seem to be three easy alternatives that make sense. I'm sure there are others, I just don't know where to look or haven't enough expreience yet.

1. Concentrate the solution - Someone, I think it was Frugal Refiner or 4 Metals, posted that dilute solutions drop a finer particulate than concentrated solutions and from a physical perspective this makes sense as a means to reduce or eliminate the aggregation of occluding layers on the substrate.

2. Use a finer particle size for cementation, such as 300 mesh copper powder, also suggested elsewhere by either Frugal Refiner or 4 Metals, as a way of maximizing the surface area for ionization/electron transfer. Thinner/smaller substrates would react both faster and more completely. The potential problem here is the introduction of too much copper powder, which may not be a problem if I re-refine the metals dropped. Keep in mind that I'm trying to eliminate base metals, not reintroduce them.

3. Introduce nitric acid to aid in the digestion of the copper. I'm not sure this is a viable or suitable alternative but it does make some sense. I have not come accross but one post where this was used as a remedy. I'm reluctant to ry it without further input from others that have more experience.

If you have a suggestion on solving the current problem, other than going higher on the electromotive scale, I would be very interested in learning more as I am a problem solver and don't like to give in prematurely.

 

[Platdigger]

Ok, but you are missing something here. If you were to reduce all the metals out of solute with say (and I hesitate a bit to suggest) hydrazine, it would be a simple way to concentrate your values. I mean rather than trying to evaporate down the whole batch.
And then at that point a simple digest in dilute nitric will leave behind your pt. Any pd could be recovered from the solution a number of ways.
And this time don't use so much acid.
Only enough to get things in solution.

 

[Geo]

Aluminum is much cheaper and easier to source. Add small amounts slowly because it cause an exothermic reaction but it will cement any noble metals quickly. Since the majority of the metal cemented will be copper, you can melt and part the cemented metal or simply redissolve it in the smallest amount of acid needed for a concentrated solution. From this solution, it will be much easier to remove the less reactive metal salts using copper.

 

[kurtak]

Here is a discussion about cementing PGMs

It started out with a "problem" I was having

4metals got me on the right track

http://goldrefiningforum.com/phpBB3/viewtopic.php?f=49&t=21218&hilit=cementing#p219231

Kurt

 

[Lino1406]

Wants to answer stannous chloride revealing copper question?
use the link in signature

 

[Platdigger]

Could not read it Lino.
I suppose it's because I do not have facebook, and don't want it.

 

[Geo]

Sulfur dioxide is a reducer of metal salts. The brown, false positive from stannous chloride is the reduction of copper by tin(II) chloride using the sulfur dioxide as a form of catalyst. It is not truly a catalyst because the sulfur dioxide is consumed in the reaction. Normally, stannous chloride will not react with copper chloride but with the excess sulfur dioxide present in the solution after adding SMB, the tin reduces the copper back to copper metal creating the brown stain. If you heat the solution after precipitating gold long enough to expel the excess sulfur dioxide gas, the false positive will not occur.

 

[Lino1406]

Former post edited in favor of those who don't use Facebook.
N.B. Reduction by SO2. We know, from SMB dropping of gold that excess SO2 will reduce Cu++ in the presence of chlorine ions to CuCl
H2SO3 + 2Cu++ + 2Cl- + H2O = H2SO4 + 2CuCl + 2H+
although thermodynamically the reduction to Cu is quite differentially more favorable:
H2SO3 + Cu++ + H2O = H2SO4 + Cu +2H+

 

[anachronism]

Sulfur dioxide is a reducer of metal salts. The brown, false positive from stannous chloride is the reduction of copper by tin(II) chloride using the sulfur dioxide as a form of catalyst. It is not truly a catalyst because the sulfur dioxide is consumed in the reaction. Normally, stannous chloride will not react with copper chloride but with the excess sulfur dioxide present in the solution after adding SMB, the tin reduces the copper back to copper metal creating the brown stain. If you heat the solution after precipitating gold long enough to expel the excess sulfur dioxide gas, the false positive will not occur.

Honest question. If the result is brown and the result for gold is deep purple/black then how is it defined as a false positive? The colours are completely different. Surely a true false positive would by definition be a false purple/black that mimics the result for gold.

This is a different result.

On a side note thanks for showing the workings behind this Jeff, I never understood it and when I've maintained that I have never had a false positive I've been using the logic/reasoning that my stain is brown not the colour for gold if that makes sense?

Jon

 

[Geo]

The term "false positive" is for beginners to understand that not all dark reactions with stannous is gold. Sometimes there is a little gold in solution but the brown covers it up. It's just a darker brown. It's an easy fix but it takes someone to explain it sometimes.

 

[Shark]

Thanks Geo, I had always been curious about that also.

 

[Lino1406]

Given the information above, we may see that obtaining Cu metal by stannous chloride reduction, though unfavorable, is a borderline case. Only 2.16 kcal/mol separating it from being favorable, a gap that normally can be bridged by slight heating.
We then render to potentials:
Anode: Sn++ + 2e- = Sn++++ ΔGa = 11.17 –(-6.55) = 17.72 kcal/mol
Ea = 17.72x4.18/96.5x2 = 0.38v on hydrogen scale
Cathode: Cu++ + 2e- = Cu ΔGc = 15.56 kcal/mol
Ec = 0.34v
That is Ec – Ea = - 0.04v only needed to turn the process favorable, namely to turn Ec – Ea positive
One way of doing it is dealing with the additional item in Ec – Ea, namely:
- 0.059/2 log□((⁡(Sn++++))/((Sn++)(Cu++)))
1. By augmenting ( extremely) Sn++, Cu++ concentrations, or reducing Sn++++ such that the logarithmic item passes – 0.04
2. By driving the process towards SnO2, e.g. by higher temperature. This makes the process more favorable by 22 kcal/mol
Notice that SnO2 appearance also minimizes Sn++++

 

[anachronism]

And your meaning? 8) 8)

 

[Lino1406]

The answer to: Under which conditions stannous chloride reduces copper ions to copper metal