Purpose of removing magnetics?

[autumnwillow]

When a smelt is to be done instead of wet chemistry is it necessary to remove magnetics? If so, why? I know this is mentioned somewhere in Hoke's book, I've skimmed the book and have seem not to find it. This is after processing wastes. Whether sweeps, low grade, high grade etc.

The reason I ask is because I have forgotten the reason why I remove the magnetics. (After about a year of refining!)
I think some nickel white gold becomes a little bit magnetic when they are fine and are mixed with iron.

 

[kjavanb123]

I think ferrous metals in a smelt takes the oxygen from the smelt and all other base metal oxides are converted to metallic form and report to dore instead of being oxidized and report to slags.

By removing ferrous the task of oxidizing is enforced during smelting.

 

[g_axelsson]

Usually the magnetic fraction contains very little values. To put it into a melt only dilutes the values and make it harder to recover the precious metals. In a melt metallic iron works like a reducing agent for more noble metals, for example copper oxide + iron turns into copper and iron oxide.

If I remember correctly, Hoke talked about refining waste from jewellery manufacturing and in that material the magnetic fraction consists of broken files, iron wires used to hold pieces together while soldering, springs, iron filings, saw blades... and so on. Alloys used in jewellery is usually (at least I don't know of any) non magnetic so removing magnetics will concentrate the gold.

There might be some precious metal grains mixed in with the iron so I think Hoke recommended to use the magnetic pieces in the stock pot.

Göran

 

[autumnwillow]

I did an experiment not to remove magnetics on a small lot of jewelers bench dust.

When the smelt was attempted it could not complete due to the magnetics like saw blades not even melting, they were just sticking out. I thought of reducing the temperature by adding silver so I did, no result, next I added lead, no result. More flux, nothing. I ended up using an old iron tong to remove the magnetics.

The torch used was Propane/Oxy. I think Acetylene/Oxy could have done it. But I wonder what will happen after inquarting then proceeding with wet chemistry?

 

[g_axelsson]

liquid silver and iron doesn't mix if I remember correctly. ;-)

If you have larger pieces and want to inquart then removing iron is a good thing to do. Iron have so much higher melting temperature than gold, copper and silver. The more reactive iron easily oxidizes on the surface so the pieces doesn't mix, even if molten. A proper flux makes it easier for the molten droplets to coalesce into a larger melt.

The higher the temperature of the melt the higher the heat loss will be, to go from melting gold to melting iron is quite a big step. Material where iron has been removed will just be easier to work with.

Btw, I've never gone so far as melting iron, any iron in my scrap comes out as recognizable pieces. (Iron springs, pins and other small parts in jewellery not easily removed.) I've never worried so much about it, even if I have a solid piece of iron, the metal will dissolve in aqua regia.

Göran

 

[anachronism]

Ive never had iron contamination in gold dropped from AR that had iron dissolved in it.

 

[Topher_osAUrus]

liquid silver and iron doesn't mix if I remember correctly. ;-) .

Göran

I cant remember which, but a refinery mentioned in an old book used to use that principle in a brilliant way.

They would use an iron scoop, or hook, to remove/scrape slag off of gold smelts. There would be a decent amount of adherent gold, after so many smelts. So once the utensils were at the end if their useful life, they would stick them in a crucible with some silver. The silver would soak up and alloy with the gold, and the iron tools would then be cleaned.

After that, they would treat the inquarted gold in the usual way, with nitric (actually, I think they used concentrated sulfuric for their parting method), and the gold would be left on the bottom of their vessel.

It was an elegant fix.

 

[autumnwillow]

Okay I don't think those saw blades were plain iron to begin with. I think they were some special type of hardened steel.

Now let me take back the acetylene melt, as the melting point of steel is about 1400degC. Nickel is about the same and I do make my own gold solders with nickel alloyed with zinc and they melt well using a primitive torch. No Oxy/propane torch needed.

I guess steel wont alloy with precious metals, maybe if I used zinc it would alloy with steel then it would report off at the slag during the smelt?
Or maybe there's a different flux mixture I need to use to melt it.

If there's a way to remove the steel during the smelt I would love to learn it rather than having to remove the magnetics in the mechanical process.
The purpose are to minimize the gold losses and save time as when we do remove the magnetics fine gold powders usually get dragged with the magnetics.

I have also found out that after the smelt it is best to let it cool down in the crucible and just break it to retrieve the PMs rather than pouring in a mold. If we poured in a mold prills of gold would report in the slag, if we let it cool in the crucible prills of gold does not report in the slag. We use cheap clay crucibles for smelting.

 

[Eu_citzen]

Doesn't zink and silver have an affinity for each other? (parkes process?)

In theory you could deal with small amounts of iron with addition of silica and sodium carbonate.

However, more slag also equals loss in gold. Small amounts typically can stay in the slag.
So there's a balance, I think.

 

[Lino1406]

One way for purifying silver from iron: Sulfur will convert iron to sulfide which is easy to get rid of.
One effect of magnetics: gold .9999 = magnetic (most probably). .99999 - dia magnetic. What is a basic test used to asses gold purity

 

[autumnwillow]

Yesterday I forgot to remove the magnetics again.
It took me more than 2 hours just to melt a stone setter wastes. It was only 125g after the smelt but I was not satisfied with the smelt result so I repeated.
Tried various methods such as addition of copper, silver, potassium nitrate, borax, boric acid and soda ash. It seemed to remove the metallics little by little. I guess it would require about 1:20 ratio of flux for it to slag off.

So after I few tries, I gave up and decided to inquart. When mixing the molten metal I could still feel some of the magnetics not thoroughly melted. I tried to remove them but gold was sticking to it. So I let it stay in the melt. Poured the lot, the magnetic filings did not alloy and were left on the end of the pour. I didn't pour what was left and let it be as is. After cooling I took the button with magnetics and cutted them with a bolt cutter to small pieces. Finally I digested in nitric. The magnetic stirrer did the job at removing the magnetics. It was iron that was last to dissolve (figured this out as they were the only one's sticked to the magnetic stir bar and they were the only one's reacting) so I turned the heat off and poured everything out, washed well then removed the magnetics that was with the stir bar. Proceeded with another nitric digest then AR. Everything went well after this.

Could there be a flux that will oxidize steel/iron in a smelt?

I think some of the magnetics scratched my pyrex reaction vessel as I was using a stir bar.

 

[nickvc]

To remove magnetics from bench sweeps or floor or polishing sweeps incinerate as usual then use a magnet in a plastic bag to pass over and through the powders, any material removed should be separated and the same pass with the magnet so it leaves any values to be treated, the magnetics should be used in your stock pot as some values will make it through and if you have Pt casting filings or dust in the mix it’s also magnetic.

 

[nickton]

I wonder if those magnetics in a stock pot would cement out some copper or other metals, and then turn to rust in the diluted mix over time.

 

[snoman701]

This thread goes back and forth and I'm having trouble following it, too early in the morning still.

In hydrometallurgical processing, the magnetics contain considerably less precious content, and consume considerably more acid and take time.

In pyrometallurgical processing, the magnetics require oxidation to the slags, which also takes time. So, you pay out on the concentrates, and the rest of it you batch up until you have enough to send off to someone who specializes in smelting.

 

[Geo]

When smelting, you must think of the mixture as a solution. That is what it is but only at higher temps. When when dissolve metals into solution, sometimes we want to remove the higher reactive metal and leave the lesser reactive metals behind. Smelting is this process. You want to dissolve the magnetic parts and put them into solution (slag). Some of the Fe material will go into solution willingly but not all. You still need to add an oxidizer in the mix to oxidize the Fe and put it into solution.

 

[4metals]

A big reason for removing magnetics is to have less undesirable metals that we have to deal with in the smelt. Often it is unavoidable but if you think about all of the methods available for separation of different undesirable materials, what is easier than magnetic separation? It is easier to pass a magnet over a pile of scrap to remove the magnetics than it is to deal with using chemical means to eliminate it from a molten mass.

Simple trumps complicated every time. Just because I can spell big words and balance equations doesn't mean I want to go out of my way to complicate things!

 

[Kkugler]

I recently took two samples of placer gold to a gold dealer in my area. The first sample was clean with a little hematite gravel in it. I figure the rock would melt out during smelting. The gold was pretty clean other than that. I had another sample on hand that was heavy with black sand and likely magnetite. I know little about gold refining but I figured it all separated in a fire assay. Before sending the samples to the smelter the dealer who was handeling this for me mixed the two samples together. The clean sample contained at least 10oz of placer gold. One week later I was notified that the refinery had rejected my gold. I was sent back a 9oz Dore Bar that was magnetic along with A very small portion of material that had not been melted. I can not find anyone that can tell me that the magnetite can be removed from the already melted material. As I have studied this online, magnetite melts at about 3000 degrees F while it’s usually smelted at under 2000 degrees. Can anyone suggest anything I can do or do I now have a 9oz paperweight?

 

[jobinyt]

This will be interesting. Personally, I think 'samples' should be assayed - not smelted/refined for recovery - after which you send a 'sample' to a refiner to test his recovery ability/cost before submitting greater/production quantities for refining . I think your 2 samples should have been assayed - separately. It seems there was some mis-understanding/mis-communication between you and the agent that sent the material for refining.

 

[Ayham Hafez]

Most of gold plated connectors in E-waste are magnetic, is that mean its not effective to smelt gold connectors?