Smelting Shaker Table Concentrates

[smelterville]

Hi All-

I have been a long time reader on the forum and I appreciate all your posts and knowledge. I thought I would test the waters with some questions and see if you guys can give me a hand with my current smelting project.

I have a shaker table and I am concentrating out a lot of free gold from my hard rock ore. I have gathered the necessary things to smelt (furnace, flux, crucibles, etc.) and I have tried my first few pours with disappointing results.

Here are the some of the details.

I have took about 1 lb of my high grade shaker table con and dried it out. There was about 1/2 oz of free gold in the one pound of dry cons. My cons carry some nasty things for smelting such as: some copper, tellurium, and bismuth. From what I have read these things are really difficult in the assay process and special care needs to be taken.

One problem is I can't find anything detailing how to deal with them in the smelting process so here is what I did. I purchased some refining flux from Legend inc in NV. Here is what they say about it on their website "Legend's premixed REFINING Flux is suitable for a wide variety of melting jobs. End users can adjust the components to adapt this flux to their specific needs. Typically used for smelting, refining, and melting concentrates."

It contains the following: 20% potassium nitrate, 8.75% soda ash, 62.5% borax, 8.75% silica.

My thought was that I would need an oxidizer for the tellurium and bismuth. I was trying to get away from roasting, too and I have read in "The Chemistry of Gold Extraction" By John Marsden, Iain House, that by using an oxidizing agent in the flux the roasting step can be avoided, however there may not be quite as high a recovery as if the roasting step is undertaken.

For my first smelt and pour I did the following ratios: 1 part concentrates, 1/2 part silica sand (I added this as the refining flux seemed to be quite low on silica compared to most other fluxes), 2.5 parts Legend flux. I brought it up to 2200 degrees and I stirred it 3 separate times (once every 1/2 hour) and then after 2 hours I poured the material into my conical mold. The result was a black slag (good? or should it be more clear/greensih like from an optimum assay slag?) but the top and sides of the slag had a sliver metallic like residue with some interlaced stripes of slag and sliver metallic material. I did get a button of metal at the bottom of the cone, it was about 1/2 oz +/-, but it was coated in what looked like the silver metallic substance on the outside of the slag. The silver coating (matte?) on the button was very brittle and once chipped off the button it revealed a copper colored button. See pics below.

Being disappointed I didn't get a shiny gold button I crushed the slag, flattened out the copper colored button (very malleable, not brittle at all) for lots of surface area, and mixed in 2 more parts flux (2 parts to the original cons charge) and re-ran the smelt again. This time after doing some more reading on the subject it was suggested to add some pieces of iron to the mix when using an oxidizing flux so I put in a few strips of mild steel strips to the crucible.

This time the shiny metallic residue from the earlier smelt was very minimal (almost none on the slag block) and the slag was a jet black color again. The problem now is I have no button at all. There was nothing at the bottom of my cone mold, except black slag. I am not sure where my metals went?

I am sure I am forgetting some important information, but I think I will sign off now and let this steep with you guys for a while. Any questions/suggestions are most welcome. My last question is: Is there any way for me to get my gold back???

Thanks,

 

[butcher]

Ore can be a complicated mix, metals or metal salts, and minerals, acidic or sometimes basic.
The ore needs to be roasted to remove the acid or base component, not often, but sometimes an additive is added to the roast, (oxidizer or reducer) to improve the ore for smelting, depending on the composition of the ore. 

A general flux will not work for all ore. 

The flux for each ore should be determined for that ore.

Some ore may need an oxidizer or reducing agent in the smelt, or even a collector metal in the smelt, again depending on the ore composition.

Tests need to be done on the ore, similar to how they do tests prior to an assay on ore, you need to determine the type of flux you to get the best chemical reaction in the melt to give you a button of metal, or if you need a collector metal...

The flux your bought is a highly oxidizing flux, the high nitrate, makes it an oxidizing flux, intended to oxidize base metals.

You may find you will need the base metals to collect values, and thus a reducing flux, or an added collector metal with a reducing flux to collect the values.

Other times you may need an oxidizing flux to help oxidize base metals to get to the values.

Did you do any assay of your ore to find out about it, does it have the values you believe it to have, did you find out what it is composed of (acids or salts of metals and acids?), what minerals are you working with and how will they chemically react with metals or flux in your melting process?

What tests of the ore have been doing to find its composition.
What tests have you done on the flux composition needed to collect the values.

Smelting much like the other chemistry we do, cannot be done by just throwing some chemicals in a pot and expecting gold to come out.

The flux and the ore react chemically in the melt, using the wrong chemicals in a flux, will not produce a desired result, without knowing the chemistry of the ore you have no clue of the chemical composition of the flux to use.

You need an assay on your material, and get an understanding of its composition, and how it will react chemically in the smelt with different fluxes, tests will also need done to determine the best flux, and much more study on smelting processes, study of how ore is assayed will also be helpful as the techniques are very similar.

Basically I suggest you do a lot more study. and tests to determine what you have, and what you need to to get the desired results.

 

[smelterville]

Hi Butcher-

Thanks for the response. I will go into some more detail here as to the ore and my background with the mine.

Our mine has been producing about 300-1000 tons of ore since 1992 when we started mining again. There were a few years in the late 90's when we quit due to low metal prices. We were sending our ore to Asarco in MT (smelter) until they closed and then we have been dealing with Kinross's mill in Republic WA (Cyanide Leach) since. We have done extensive ore chemistry with both mills and have lots of old reports which show basically the same thing.

The mine is a high grade quartz vein with the following mineralogy abundance: quartz, pyrrhotite (massive in places), pyrite, chalcopyrite, tellurobismuthite, gold. Here is a wikipedia link to tellurobismuthite (https://en.wikipedia.org/wiki/Tellurobismuthite). Our grade runs between 1/2 oz per ton and 1 1/2 oz per ton gold with very little silver (approx 92% gold 8% silver). The smelter loved our ore due to the high quartz content. Kinross is getting >92% recovery on our ore by cyanide leach. The issue is the Kinross mill is about to shut down and their mill contracts are getting worse and worse (take more gold % and higher costs overall). It has been quite easy to make a gravity con from our ore with really good recoveries, now I am looking to smelt out the values and have a salable product all in house.

The gold and tellurobismuthite are very closely associated with each other in the vein, and the testing shows there is actually some minor calaverite as well. Also the tellurobismuthite is dense (~8 g/cc) and follows the gold in the gravity circuit. So what we end up with is a high grade gold/tellurobismuthite con with some FexSx minerals too.

So here have been my thoughts on the fluxes for my first couple of pours (obviously didn't turn out right):

I chose an oxidizing flux so I could eliminate the roasting step. Roasting is just a way to oxidize the metals by heating in an oxygen rich environment without using a flux (correct?). Most of the time the oxidizing flux will do a pretty good job of oxidization if roasting did not occur previous to the smelt (summarizing from "The Chemistry of Gold Extraction"). Tellurium does need to be oxidized (in Colorado in the "old days" they did this through roasting most of the time) to be removed from the mix in the form of TeOx (correct?).

I have looked at many different types of fluxes:

Chapman flux (also available at Legend inc): 40% borax, 20% soda ash, 20% MnO2, 20% silica
"Legend refining flux": 62.5% borax, 8.75% soda ash,20% potassium nitrate, 8.75% silica.
Flux from GPEX forum: 5 parts borax, 10 part soda, 20 part sodium nitrate, 40 part silica
Flux recommended in "The Chemistry of Gold Extraction" for concentrates with no roasting pre-treatment: 10% borax, 40% silica, 50% sodium nitrate (highly, highly, oxidizing flux)

For highly oxidizing flux it is recommended to add some scrap iron (both Action Mining and "The Chemistry of Gold Extraction") to bring the slag into a neutral state at the end of the smelt (correct?). If your iron is all used up then more is needed to ensure the flux has oxidized to completion.

Some sources say the gold content needs to be 70% (action mining), or 30%, or 5% (article about smelting gold cons in Ghana needing 5% gold content http://nora.nerc.ac.uk/9991/1/IJEP_0011_Amankwah_et_al_.pdf). My material was about 5% gold in the concentrate by weight.

So there is some of the background into my smelting research. I am now wondering where I go from here. I do understand the need to experiment and test, which is what I am doing now, but I am not sure how to proceed with experimenting. Should I get another flux (chapman maybe?) and try another pour? Any insight here as to a good next step would be very helpful so I'm not a blind mouse looking for a nut

I don't understand why the oxidizing agent isn't working in the flux and why I had no metal button in the second pour? I know there is gold in there and I know about the amount, but why didn't it end up at the bottom of the cone. Any thought appreciated.

Also attached are a few pics of some free gold and tellurobismuthite. Thanks again.

 

[butcher]

Roasting process cannot be accomplished in the smelt (using an oxidizer), these are two different processes doing similar things, but in totally different ways, and for different reasons.

Roasting gets rid of the acid portion of the ore (like sulfides as SOx gases), and oxidizes the base metals preparing the ore for the next processes (leaching or smelting...), roasting can be a very important step to change the chemistry of the ore, to set it up in a better condition to extract values, with latter chemistry (leaching or smelting...).

You do not have to buy a store made flux (or a general or house flux) you can make up your own with different chemicals and ingredients, in fact it may be better as you have better control of each of the components added.

After roasting test your ore in a melt, study assaying, to see if the ore itself is oxidizing or reducing, using litharge in the test, this will help you determine the flux needed (an oxidizing agent in the flux or a reducing agent in your flux), this will help you get an idea of the type of basic flux you may need (added oxidizer like NaNO3 or a reducing agent like flour or carbon...), you can also determining if you need an added metal as a collector, like lead silver copper,
I would not use iron as a collector it oxidizes too easily and could cause more problems than benefit.
Iron added to the smelt as a reducing agent to compensate, for a poorly chosen oxidizing flux is also a bad idea, sounds like adding one problem to fix another problem.

Using iron to reduce the oxidizing power of the flux (as you did) in my opinion was a very bad idea, changing the flux would have been better choice.

Each ingredient in the flux serves a purpose in the melt, chemically reacting with the ore at the high temperatures, some are to oxidize metals to make them form glass with the slag like silica metal oxides, or to reduce the metals to metal where they can also collect the values, some help the melt to become fluid enough for the reduced metals to collect and settle as a button and the oxides to flow into the slag glass above, learning what each ingredient does in the flux will help you to determine the best flux composition for the job at hand.

At this point I would forget about buying a company’s general house fluxes, and study the art of smelting, and assay, learning how to prepare the ore (roasting) and how to find the right chemical makeup of the flux through testing, and how to makeup a flux that will get the job done.

Like most of recovery and refining there is a lot more to this than we think when we first begin, we have to study a lot to learn how much we do not know, and then study more so that we can know what to do.
Assay and smelting is an skill art and craft, we need to learn the chemistry as well as many aspects of the art to be successful, not something where we just melt a bunch of chemicals in the pot with our ore and expect it to chemically react in the melt to make gold slugs of metal.

I personally have almost no experience in this field, although I have studied it a lot and done a few small experiments (enough to know how much, I do not know).

 

[Deano]

Re: Smelting Shaker Table Concentrates

At 5% gold in cons you have only a leaching proposition, not a smelting proposition.

Base metals will drag a lot, if not all, of your gold values into the slag irrespective of the form of the base metals.

You have two options for commercial operation.

One is to do your own leaching with thiocyanate leach, easy to do and no use of restricted chemicals needed.
My work with thiocyanate shows recoveries equivalent to cyanide for a large range of ore types.

Two is to leach the base metals out of the concentrates before smelting.
This is best done by leaching in HCl for an extended time in a warm environment.
Depending on the base metals present the HCl concentration required can vary from 5 to 50%.
Temperatures range from 20 to 30C.
There must be liquid flow through the con bed, doesn't need to be a high flow but must be consistent.

You will always be trading off time against acid strength and temperature.

It is usually very visually obvious when the base metals have been removed.
Depending on the acid strength and temperature leaching time can vary from one to sixty days.

If your smelting has poor results after the acid leach then either you have a problem with the rock particles present in the smelt or your flux is wrongly composed.

Generally if the base metals have been properly removed then the flux can be just borax.
Adding iron or other base metals will just get you back to where you were before the acid leach.

At 5% gold in the cons you do not have a lot of room to be using any chemistry which is not absolutely required for your processing.

Always keep in mind that gold which is easily leached or even smelted in the head ore will be much more difficult to process in a concentrate, this is the great truth in mineral processing which is not always remembered even by experienced operators.

Deano

 

[Diehardmining44]

I would add some soda ashe and iron. I use it in pretty much all of my smelts and it is good for getting rid of the sulfer and mat.