Dental Scrap - Not Dissolving Fully

[Digitaria]

Hi all,

I have just found this forum and wanted to share my experiences as a newby. I would welcome any support or warning, or overall benefit of your experience, as I have no scientific friends, to call upon for help.

So I'll start with a brief lesson, in what not to do.. But I'm learning. I have started my entrance into aqua regia gold refining. This is what happened, I measured out the ar carefully. When mixed, I added to the dental gold scrap, this was all fine until the heat from the small plate burner, caused a slight overflow and white fumes billowed into the house, through the open door which was ajar because of the hot plate lead. Mmm - I know this was incompetent and I had to clear the house for half an hour and check on the neighbours... So fume cabinet, is on the list of essential items, I have my eye on one locally, but at the time, I thought I might be able to get away with this expense, and I concentrated all my efforts in buying acids and eqt and containers and a few basic safety items. But following my eye opening experience, I will have purchased an acid fume gas mask, which really was not that expensive, which is yet to arrive and I will defo get a fume cabinet. I just can't be worrying about the wind and the neighbours. And I was counting myself lucky, that ar fumes did not affect my eyes, because I sure felt a slight burn in the throat. But I did act quickly, but what an idiot, or at least that's how I felt. I will learn though..

My thinking was that I should just be able to process small amounts outside but no, even outside, you never can tell which way the wind will blow. I also know, that generally people learn by trial and error and from the experience and guidance of others, but I was basing everything I know on hints and papers online, so what I thought I knew, was clearly, not very much at all. But I'm optimistic, I've already invested too much to let this venture fail. But I was mostly busy buying raw materials, rather that thinking about 'fume cupboards.' Anyway, I've practically learned that adding ar bit, by bit, prevents overflow and prevents large masses of fumes, where controlled bouts of fumes, quickly dissipates. I wonder why - I never heard that adding ar slowly from the beginning of the process is advisable? Anyway, the reason for joining is I was hoping members might join in with this thread, and give me some advice in fully dissolving the dental gold scrap which seems not to be dissolving. I have been following a dental gold paper - from Stefan / I think he is quite well known in gold refining? Anyway his preparation was with fluorine, and well that stuff scares the hell out of me, so I thought I would just crush up the material the best I can to remove the ceramic, but I suppose there is plenty still there, but the fluid should, I thought, just dissolve around it and within the crowns?

My mixture was 1,262 g dental gold scrap alloy (ceramic fused to metal) crowns and bridges; 261 g of copper metal; initially 750 ml hcl; 250 ml nitric (adding 10ml every 3 hrs) but as I said earlier - I had an overflow and ended up adding more hcl. Basically the copper was heavy gauge and is still dissolving. So - I read another paper -instructions for large scale gold refining by the aqua regia method, and it said to filter and make fresh ar when no more chemical reaction, so I did but the dental alloy is still not dissolving properly, can anyone help please?

Thanks for listening - to my short version of war and peace. lol - I never do anything by halves.. ;-)
Lisa

 

[FrugalRefiner]

Lisa.

Welcome to the forum. I noticed that you downloaded a copy of Hoke's book, so you're off to a good start here.

As you learned, you made a lot of mistakes on your first attempt at refining. NEVER refine in your home. Always keep your recation vessel inside a second container in case of boil overs or a broken reaction vessel. A fume hood only draws the fumes away from you. It still blows them outside, so your neighbors are still at risk unless you scrub your fumes. Most respirators do NOT protect against nitric acid fumes. Instead of premixing aqua regia, it's best to start with the calculated amount of HCl, and add the nitric in increments. This is definitely not something you want to learn by trial and error. Always start witrh small batches, and if everything goes well, you can scale up.

You're right. You got very lucky. My recommendation is to put the chemicals away in a safe place and read until you understand what you're trying to do. Here are a few more links to help you in your journey.

Before you jump into any of the processes you see discussed here, be sure you understand the potential hazards and do everything you can to minimize the risks. Read EVERYTHING in the Safety section of the forum, especially the Dealing with Waste topic. No amount of precious metal is worth jeopardizing your health or the health of those around you.

I strongly encourage all new members to follow the Guided Tour created by LazerSteve. It will provide an introduction to the forum and numerous valuable links including the General Reactions List. Be sure to follow the link to his web site as he has many outstanding videos, a collection of great reference documents, and he sells a lot of the supplies needed to get started including detailed instructional DVDs. Samuel-a also has a lot of videos, guides and tutorials at his web site Gold-N-Scrap.

You'll also find a tremendous amount of information in the two Forum Handbooks compiled by aflacglobal, Forum Handbook Vol 1 and Forum Handbook Vol 2.


There is one last post that I think every member should read. ms32462 shared this experience with us : http://goldrefiningforum.com/phpBB3/viewtopic.php?f=47&t=11957

Once you understand the basics you can start to try some small scale experiments. If there's something you don't understand or you get stuck, there will be someone here to help you.

Good luck,
Dave

 

[butcher]

Digitaria,
Along with the good advice you have gotten from Dave, I would like to welcome you also, and give you some tips that I think will help, yes put away the chemicals, And begin this journey from a proper beginning, up until this point you have done everything wrong, and from your post you have been reading missing and misinformation, you have a lot of work to do studying, and spending the time needed you will become successful, jumping in before, is just pushing for more problems, from your posts it does not sound like you have learned to prepare your scrap, or learned to recover values from the scrap, it is not ready for aqua regia and neither are you yet, after you study for a while you will gain the understanding you need. it sounds like you have a nice lot to recover the values from, these values can wait until you are ready, and this way you will not lose them in a process or boil-over, or broken glass vessel...

Hoke's book, the forum,and Harold has also posted some great information for recovery and refining dental scrap.

You need to first learn the basics, like learning to fly a jet you must learn some on the ground, spending time studying, before you are ready to fly solo, take the time needed to study so you will be a good pilot, do not just jump in and crash.

 

[nickvc]

Welcome to the forum Lisa, it's good to see another Brit and a lady joining us here.
Take the guys advice they will point you in the right direction. I have to say you have picked a difficult material to try to refine for your first attempt. Dental gold alloys frequently contain Pd and Pt and in some cases high percentages of these metals which are not the easiest to refine. I'm not sure why you are refining as here in the UK costs for assays is low and returns on melted bars is pretty high unlike many other countries. If your doing this for the experience then fine but find some yellow gold jewellery scrap to start with, avoid white gold as again they can contain Pd or have Pt settings.
As the guys have pointed out even a fume hood will only move the fumes away from the area but will not scrub the dangerous gases and acids from it so I fear your neighbours may get a little upset with you if your processing for any length of time, perhaps look to running a sealed set up with one vessel dedicated to scrubbing the fumes before venting them off. Be warned these acids will eat all metals and the fumes are just as corrosive so cars are at risk as are all metal items they come into contact with hence the warning about your neighbours especially if you live in close proximity to others, those same fumes are dangerous to us and our pets, do some reading, lots of reading and avoid mistakes and upsetting your neighbours.
Good luck and safe processing.

Ps the solutions your creating need treating before you can safely dispose of them, it's all covered here on the forum.

 

[butcher]

Digitaria,

You have a solution now that may have some values in it, although it may not as the base metals may have already cemented them back out of solution, testing would be the only way to know for sure at this point.

It sounds like you were having trouble getting the metal to dissolve, so I am assuming most of your metal is still intact.

Just in case some valuable metals are in solution I would add a large piece of clean copper metal to the solution, do this outside, let any acid dissolve on the copper (if there are any valuable metal ions in solution the copper will give them electrons as the copper atoms dissolve into solution as copper ions) if the copper gets a black coating those are valuable metals cementing from solution, let the copper sit in solution and stir it every now and then, so ions in solution comes into contact with the copper atoms on the surface of the copper bus bar, you can let this work a day or so, when no more metals cement from solution, I would take an old tooth brush and brush any of the black powder stuck to the copper back into the solution (cleaning off the copper bus bar) rinse the copper bar and let it dry, and store it in a plastic bag for this purpose.

Let the solution sit overnight for the powders to settle well, carefully decant liquid without disturbing settled powders (carefully pour off, remove with suction tool or siphon off), rinse with water, again letting rinse water sit so fine powders settle, decant wash, dry and save these powders and any parts and pieces of your dental scrap (I would use a jar with a lid).

In the safety section, study dealing with waste (topic flagged), so you can safely deal with the toxic waste solution you have now.

Dental scrap can be made out of several precious metals, and some metals that are not precious, much of the white metals may not be very valuable, I would separate material (yellow gold metals and white metals) (testing the white metals), beside metal you can have other materials that need separation from the metal (tooth) wires plastic...

Dental materials can have mercury involved, which can be very dangerous in some of the steps you will need to do to prepare your scrap like burning off organics (be careful here and study this well you may need a retort as mercury fumes are extremely toxic and will contaminate an area if worked with improperly),

Dental materials can be some nasty stuff, biological waste (and possible disease) care should also be taken here take precautions when handling, gloves, bleach wash, rinsing and incineration (take special note of mercury caution above, before burning anything) are some good tools here, a hammer and anvil are handy to separate metal from tooth or plastic pieces.

I would in-quarter the metal with silver and use nitric acid to dissolve the silver and base metals, leaving the more noble metals ready to process in aqua regia...

You will learn this and much more of how to prepare your dental scrap, and how to work with this material (that can have its own dangers), by reading Hoke's book, and studying the forum, and reading Harold's posts on dental gold (Harold Posts will be a good resource in your study in this area)...

I like processing dental scrap, but like with anything valuable you have to put in some work, and study to get to the values.

Be careful and work safely, study well, so you will stay healthy.

 

[nickvc]

Butcher if she inquarts with silver some if not all the PGMs will dissolve with the silver and will need separating either by cell or by forming silver chloride and filtering the remaining solution off and then cementing the PGMs with copper.

 

[butcher]

Nick you are very correct.

Adding extra silver is also a useful way to get a lot of the platinum group metals out later, in the silver cell.

Copper can also be used to in-quarter the metal, before the nitric treatment to separate values, but for me nitric is just too expensive, and hard to get, to use up my nitric acid to just to dissolve copper and base metals.

I would rather use the nitric acid to dissolve sterling silver (making the acid go further) and recover most of the PGM's later in the silver cell.

I personally like to mix my karat gold, my placer mined gold and my yellow dental gold, and in-quarter these with sterling silver (adding an extra amount of silver) with the purpose of getting most of PGM's back in the silver cell later, and saving on nitric acid consumption.

 

[Digitaria]

Thank you all, for such a warm welcome. I'm off reading 'Hoke' at the moment and will concentrate on all your comments a little later, but for now - I have put the acid away, I will strain and try and process the scrap a bit more and come back to things later, I've already invested in this project and I really do want to understand more, so thanks for the advice.

I went off to sleep last night, after reading loads and loads and woke this morning in what seemed like a lightbulb moment....lol
Perhaps - I can simply make an water absorbing fume cupboard, a bit like a sand blasting cabinet with arm holes and rubber gauntlets, with water water within to absorb the fumes? In the book from 'Hoke' she uses an up-turned cloche to catch the fumes, over the reaction vessel, which she says the fumes are then absorbed by the water. The difference between the cloche, and this is I could make a large acrylic cabinet with arm hole's and small door, with room to work, and with corked hole at the top for topping up the water within the stoneware. The resovoir could even be heated from below with a spirit burner. I think this could work, and be in-expensive? This seems to make sense then for me, rather than investing in a fume cabinet, which will only eject the fumes, albeit with a fan, out and into the back yard anyway? What, with teh fumes being heavier than air. Or is this type of water enclosure, simply what you already mean, when you refer to 'washing the fumes'?

 

[rickbb]

Washing or scrubbing the fumes means to remove the toxic acid based gases, and any metals that may be present in the vapors, from them, converting them back to a liquid/solid and collecting them, then venting then now clean, (at least not toxic), air outside.

I also noticed that you have purchased a acid/gas mask, please don't rely on that. It will not save your life or protect your health. Those types of masks are only to supplement an active fume venting system and in case of the need for any emergency evacuation of the work area. They will only handle a very small amount of the types of gases in this type of work and then become useless.

The type of "water absorbing" containment hood you propose is a bit off the mark. You want to vent those fumes to outside air away from any window or door or neighbors. The fumes from AR digesting process are quite toxic so any attempt to "contain" them is not a good idea. You want to neutralize them, (scrubbing system) and vent them to outside air to be disbursed.

 

[butcher]

From the, nitric acid and aqua regia process, you will be generating several different types of toxic fumes, some of these fumes are easily soluble in water, some are not soluble in water unless oxidized further like nitric oxide.

A series of small scrubbers can be added to the reaction vessel, this will normally consist of a lab glass heated vessel, with a refluxing condenser, then a series of 3 or more small glass vessels with solutions to oxidize and scrub the fumes (making the solutions in these) acids or salts from the fumes bubbling through the small scrubber setup before the air is exited out the fume hood.

Another type of scrubber system scrubs the exhaust of the fume hood air, these normally have to handle very large volumes of air mixed with toxic fumes, and not back up the air flow of the fume hood, so this type of scrubber has to be very large to handle the volume of air and gases.

The up-turned jar to catch the fumes over the dish of water as pictured in Hokes book, with the vessel of the nitric acid reacting with metal, will capture most of the deadly red gas of NO2 from the nitric acid absorbing it into water, but this NO2 is not the only NOx gas produced from reacting nitric acid with a metal, actually there are many compounds of dangerous oxidized nitrogen compounds produced from HNO3 reacting with metal, we can see the red NO2 gas and it is is water soluble, some of these nitrogen compound gases are invisible and not water soluble but can be just as dangerous.

 

[Digitaria]

Thanks, Dave - I will not refine in my home, or near the back door...
I did contain the reaction vessel inside another container and for this reason, I never lost a great deal of acids; I had considered a fume cabinet, but in-light of what you have advised, it seems pointless to just move the fumes, not sure what scrubbing the fumes are yet but will continue reading… I did not make aqua regia and then add it mixed, but I measured out separately, one of my mistakes was adding the measured portion of nitric to the reaction vessl in one go, planning only top ups every few hours. I'm working my way, through a shed load of material, and slowly, slowly, through the 'guided tour.'

I found a really good value gas mask which protects against nitric fumes and its good value as well - for those occasions outdoors to protect against, changes in wind direction and for mixing and filtering, I will think about some kind of wind break, shelter also - but for now, I will generally try and prevent unwanted violent reactions: http://www.ebay.co.uk/itm/MOLDEX-5984-FFABEK1-P3-RD-Disposable-Half-Mask-Respirator-Dust-Gas-Mask-/161026260864?pt=UK_BOI_ProtectiveGear_RL&hash=item257de9bf80

 

[Digitaria]

Butcher - thank you for the sound advice. With regard to processing my scrap, I only saw on youtube the instuction to process the scrap was to hit it hard, with a hammer, however I have found that many pieces are so very tough, that no noticeable cracks or damage occurred - really tough stuff..

I am comfortable with hydrochloric, nitric and sulphuric acid but hydrofluoric, for me, just seemed like a complex and dangerous step, which I would rather avoid, I was thinking that perhaps a dental lab, would consider removing the ceramic for me? Although - I would appreciate any suggestions.

The dental material - I have purchased does not contain fillings and if I do come across any, I will just set them aside for now, can't be doing with low quality, or the mercury fumes. I do use gloves, mainly because the material looks a icky, I do tell myself that the material is not from real human teeth, but fabricated. It's so macabre looking also, so will defo use gloves.

I thought I was in-quartering the dental scrap by adding copper scrap to the aqua regia, but I don't think that I used enough copper, so I have some copper powder, on its way for this purpose. I don't have a silver cell and didn't plan on recovering PGM's that way, as I want to obtain the values from the aqua regia, using a long cooking and reagent. I also have every intention of recovering the copper also, following each step to recover the different metals and to have a greater understanding of the processes.

Butcher - you said 'copper can also be used to in-quarter the metal, well I'm glad about that because, I've already started on that route and for now, have plenty of nitric. Could you please clarify, what you mean when you say, 'copper can be used to in-quarter the metal, 'before' the nitric treatment to recover the values…? ' Its just you mention in-quartering 'before' the nitric treatment, which has me confused?

I wanted to in-quarter in solution because, I do not have a furnace. Basically, I was thinking of adding 70% of the dental alloys weight in copper, to allow the values to dissolve, but of course I was trying to this in aqua-regia, whereas on reading 'Hoke' it certainly does seem an excellent method of separating values with nitric.

 

[Digitaria]

nickvc; rickbb and butcher - thanks for the info about fume scrubbing but I fear, this info has, for now, gone straight over my head. Its strange how some processes seem so complex, and others which other people might consider complex, easy.

Anyway - I have enough to be getting on with, so I'm going back to studying now.
Have a nice weekend, guys. 8)

 

[Digitaria]

Butcher, just one thing before I go, when you suggested cementing out the values onto a thick piece of copper, of a couple of days, is this the same method I have heard of called 'single displacement? Also will there be valuable platinum in the mixture, which will not be attracted to the copper, I would not want to displose of the mixture if it has PGM's in it, still?

 

[sharkhook]

There is a good description of inquarting in the following post. Hope it helps.

http://goldrefiningforum.com/~goldrefi/phpBB3/viewtopic.php?f=39&t=355

 

[Digitaria]

One last thing, Nickvc - thanks for your post and welcome. Are there many UK enthusiasts on this forum then? I have not had a good look, as there is so much else to do at the moment.

I decided to choose dental scrap - as this will provide me with the highest return on any investment I make, and I fully intend, eventually to collect its full values. I knew dental gold would be a tough but when it comes to gold, it seems that glitters, attracts very high prices. But you can still find a bargain on other types, including coins if you look about.

As for assay costs and smelting costs, I've learnt that using these services can cause a loss of value on the platinum present, and well, I can do this myself, or at least I will be able to soon.

 

[Digitaria]

Thanks for link - Sharkhook. You guys are ace. 8)

 

[FrugalRefiner]

I'll just deal with the issues mention in this post and let other members discuss replies you've made to them. If you're serious about refining, you need a fume hood, but a fume hood alone is not enough. If you just run reactions in the hood, you protect yourself by keeping the fumes away from yourself, but you blow them, untreated, somewhere else.

"Scrubbing" fumes simply means that the fumes are treated to remove any toxins they may contain. Different processes create different types of fumes, so the type of treatment needed is tailored to the process in use.

Fume hoods move a lot of air, so properly treating them requires a large scrubber. But there is a better approach known as a closed system. It can be as simple as a few mason jars with lids, each connected to the next with some plastic tubing. The closed system is put into your fume hood. The first jar is the reaction chamber where you put your scrap and add acids. As soon as you add the acid to the scrap, you put the lid on the jar. The fumes from this jar flow through the tubing into the next jar, which contains chemicals that begin to treat the toxins to neutralize them. Fumes from this jar flow to the next jar, and so on down the line. At the end, the fumes have been scrubbed of most of the toxins and any remaining fumes are blown out by the fume hood. The fume hood protects you while you're adding acids and starting reactions and from any residual fumes left after passing through the scrubber system. You scrub the vast majority of the fumes, and done right, thee air coming out of your fume hood is much safer than running an open reaction.

I would never rely on a respirator/gas mask. As someone else mentioned, it's fine to have around in case you suddenly lose power to your hood, or have an accident that produces fumes outside your hood and you have to evacuate the area, but always strive to treat and remove fumes before relying on personal protective equipment. It is intended to be your last resort.

Keep studying. Everything I've said here is covered in MUCH more detail on the forum. You can learn all the details you'll need to succeed, but it takes a long time.

Dave

 

[bmgold2]

Here's 4metals' Makeshift scrubber.

 

[butcher]

Digitaria,

I am not sure but I think you misunderstand what we are talking about when we say in-quarter the gold with a metal, this is to melt the gold with a metal to mix the metals to lower the karat of the gold, (or gold content compared to other metals) to make the gold easier to remove base metals with a nitric acid treatment.

With karat jewelry you normally have silver, which will not dissolve in aqua regia, (the silver stops the gold from dissolving in aqua regia), gold will not dissolve in nitric acid, and a high gold content in the metal will not let nitric acid dissolve the silver or base metals, the jewelry also has other metals that we normally wish to remove before using aqua regia to dissolve the gold, we cannot use nitric to do this without in-quartering the metal with silver (silver normally, but copper can be used) to lower the karat of the gold to about 6 Karat, 6/24 parts, with 24K being pure gold here we have one part gold and four parts silver (or silver and base metal), (or copper) thus the term to in-quarter the gold.

By lowering the gold content we can now get past the gold to dissolve silver and base metals in the nitric acid leaving us with a more pure gold we will dissolve in aqua regia in a later process.

palladium will also dissolve in nitric with silver and base metals.

If we add extra silver in the melt, 10 times the amount of silver as we have platinum we can also get platinum to dissolve into nitric acid (at least a good portion of it), that will not normally be attacked by nitric acid.

Keep up with your study Hoke's book she can explain this better than I can.

In-quartering the metal this is not done in solution adding copper to the aqua regia the acids will attack copper (easier than the more noble metals) and copper or other base metals will cement any gold that may be trying to dissolve back out of solution (you can also have both base metals and gold in solution) but as long as there is elemental copper (or some other base metal they will displace or push your valuable metals out of solution.

Cementing the values out of solution with copper, there is a reactivity series of metals, where an elemental metal that is more reactive will displace a metal in solution that is less reactive.

If you dissolved gold into solution the gold is in solutions as gold ions (gold missing electrons which were take away by the acid, the gold Cations are combined with the chloride Anions from the acid, these gold ions in this case a gold chloride (gold ions need electrons) to change back to gold atoms to be able to come back out of solution as a metal (a metal powder in this case), copper is more reactive than gold in the reactivity list, so we can use copper metal, by putting the copper metal bar into our gold chloride solution, gold ions in solution at the surface of the copper metal, the copper metal will donate electrons to the gold ions, these gold ions receiving the full shells of electrons (3 electrons from the copper) become atoms of metallic gold, many of these tiny gold atoms combine to form the gold powder you will see cementing out of solution as powder in the bottom of the vessel or sticking to the surface of the clean copper bar of metal left.
the gold metal powder will normally look black as it is not very pure.

Note that when you look at the reactivity series of metal the valuable metals are below copper (less reactive than copper) so these metals will also cement from solution using the large clean piece of copper bus bar. (PGM metals will also cement out of solution with your gold).

The gold and other less reactive (more noble) metals cement from solution (or are reduced to metal) (reduction means to gain electrons), as the copper is dissolved into solution.

Now the copper metal atoms that gave up electrons to the more noble metals cementing these values out of solution, the copper giving up electrons becomes oxidized (oxidized means to lose electrons) the copper atom is oxidized into solution as ions (a dissolved salt of copper chloride in this case), (copper Cations and chloride anions) so as you cement the values out of the acid copper metal is dissolved into the acid.

Iron is more reactive than copper in the reactivity series, so we can add iron metal to our copper chloride solution and cement copper metal out of solution as we make a dissolved salt of iron chloride.

We can also use the copper bus bar to cement silver and other valuable metals from the solution we have from the nitric acid treatment where we made that made silver nitrate, palladium nitrate...

http://en.wikipedia.org/wiki/Reactivity_series
In the link on this same page note the single displacement reaction.