Hoke and hard rock mining

[Greenhorn]

I have to say first off, that you all offer so much good information, and I appreciate all the members here and all of the help they offer.

I purchased a claim in central Idaho that has been active on and off over the last 70 years that I'm aware of, I've been surveying and pulling samples, but as my login name implies, I'm a green horn. As I've read many different articles here, I saw that the book by C.M.Hoke is a book that I should have on hand, I just ordered a soft bound copy.

My main concern though is from it's title, is it geared towards E waste and jewelry recovery? Is there a better book that helps address hard rock gold recovery? I did a google search that pulled a lot of books from the turn of the last century, but no reviews of them.

I know the more I read on your forum, the less I really know about how to proceed, and a lot of caution should be used.

A second question I had was about a comment left by Deano when addressing "smelting Shaker Table Cons(June, 2015)" and at the end of that thread, Deano said "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."

The man I purchased my claim from told me that they would grind the ore down to 100 mesh, run it across a shaker table to get the freemill gold and then do a cyanide leach to extract the remaining "micro"gold. I'm under the impression that he may have been sending it to someone to have the leaching done, but from what Deano had said, should the freemill gold be extract out?, or just leave the ore ("head ore"?) as one body after crushing it and do a leach for all of the ore?

Thanks,
Greg

 

[FrugalRefiner]

I can't answer most of your questions, but Hoke's book deals primarily with jewelers scrap. She wrote the book in 1940, and e-waste didn't exist at that time.

Dave

 

[Deano]

In what these days is a less common ore, a sulphide free quartz ore, it virtually does not matter much how you treat the ore. It is the now common sulfide ores which give you problems.

If your ore contains only fine gold, say mainly minus 100 microns, then you will get all of this gold in a cyanide leach and the gravity step is redundant.

The only reason for doing a gravity concentrate would be for security concerns or if a toll leaching company was suspected of not properly reporting the gold recovery at the leaching stage.

Deano

 

[Greenhorn]

Thanks for the reply's, I had assumed that it would be better to remove the light material with gravity separation of some kind, so that the chemicals used to strip the PM's from the ore, (and yes it it an iron sulfide ore,) would be more effective, but if I understand you correctly, I should keep all of the ore from the ore seem together, crush it, and have it heap leached together?
I've only seen chemicals being used (both AR and Cyanide) when it comes to E waste or jewelry, but nothing when it comes to hard rock vein material.
thanks again. Greg

 

[Lino1406]

Seems no concern is given to activating the ore by roasting or hydrogen peroxide or else to free gold (and other precious metals) from sulfide hold.

 

[Greenhorn]

I kind of wonder if I should have done a thread directly relating to hard rock processing.

I know from reading different threads that a lot of respect is given to many of you and Deano is no exception, so when I read his comment, I thought I should try and clarify as I know I've never heard anyone say that before.

From what I've read about heap leaching, they always show the ore going directly into the leach, I wondered if this was done as it requires less equipment and requires more time when your dealing with large tonnage amounts.

As a small miner, I've been trying to figure out if crushing, gravity separation, and maybe some kind of roast of the cons before using the cyanide leach would require more equipment, but would cut out a lot of waste to clean once the ore was removed from the rock and yield the same results.

Then I read Deano saying it was more effective to put all of the head ore in as one body to process, and I would like to be efficient in my efforts, I was hoping to get clarification on this.
If your dealing with maybe 2 ounces of Gold to the ton, roasting a ton of ore seems like a big task, but will it help in the Gold recovery?

I'm hoping that someone knows of a good book when it comes to this technique (leaching), I was hoping the C.M. Hoke book would shed some light on the subject, but until it arrives, I'm not sure if it will help. Does anyone have any good suggestions?

I appreciate all of the good information that is available here on this forum, and will continue to read and learn from your experiences, but I do say that I find more questions when I read your answers, lol.

To clarify it's my understanding the ore is tied up in sulfides, and only a small amount can be extracted as visible Gold, most of it is micro Gold.

Greg

 

[FrugalRefiner]

Hoke's book does not cover the refining of ore.

Dave

 

[Deano]

If your ore is a sulfide type and little gold is released as free gold on milling then the obvious question is why would you put in a gravity circuit for little return.

Each step you perform will have costs associated with it, there is no sense in performing processing which not profitable.

If you really want to have every government agency give you a lot of attention then go ahead and roast the ore. The cost of setting up a non-polluting roasting circuit is extremely high and is not viable for a small miner.

Some sulfide ores are gold leachable with cyanide as is, some are not.

Testwork needs to be done before any decisions are made regarding processing options.

The first thing to be done is to get a representative bulk sample of your ore, at least 50 kg is needed, preferably 100 kg.

Get someone who is experienced in mining to do this, many failures in mining are due to poor sampling.

Splits of the minus 25mm jaw crushed bulk sample are passed through various milling regimes, usually rod and ball milling in lab scale so that you can be sure that the results can be applied to a larger plant. A proper sample splitter is needed, accept no substitute method apart from cone and quartering.

The first testing is running cyanide bottle rolls with an aqua regia digest on the bottle roll tailings. This will give you a % recovery of the gold by standard leaching in agitated tanks as well as an average leachable head grade.

Often you will perform fire assays on the bottle roll tailings as well as the aqua regia assays, this gives you the total gold, any gold which does not report to the cyanide and aqua regia leaches is locked in quartz and may indicate the need for finer milling but this may not be viable depending on the degree of this gold.

These bottle rolls are run on several sized millings to see which milling regime gives you the best results. The initial milling sizes are usually 75, 100 and 150 microns, you use the results to see if any expansion of the size range is needed.

The above are tests which any competent lab can run for you, they are probably running similar tests on a daily basis.

You are probably thinking that the testing will be expensive, I can absolutely guarantee that not doing the test work will be many times more expensive.

If you have a friend who can do the above test work for you then this is OK, just remember that it will be your money at risk so you need the test work to be properly done.

If the % recovery by cyanide is poor then any attempt to improve the recovery by pretreatments with various oxidising agents will not be viable, look at what the large miners do, any attempt by them to carry out pre-leach oxidation is done in very expensive autoclaves at very large scale.

If the test work indicates that straight milling and cyaniding will not be viable then you have three alternatives.

The first is to have your ore toll treated by someone who has a bi-ox circuit set up, if your grade is actually around 50ppm gold then you can cart the ore a long way and still be viable.

A subset of the above is to make a sulfide con and take the con to the bi-ox operator.

The second is to have your ore or cons toll treated by or sold to a smelter.

The third is to run a heap leach on the jaw crushed, not milled, ore as in my thread "Further things which may be of interest to Members".

If you are interested in going this route then some test work involving column leaches and various jaw crushing sizes is needed.

The good thing about running such a leach is that in most cases the sulphides will oxidise naturally during the leach cycles, keep in mind that the run time for such leaches is months if not years.

Deano

 

[Greenhorn]

Wow, a lot of very usual information, thanks Deano.

This mining location was one of those that was operated last in the 80's, he was able to support his family from mining year round, but then the price of Gold dropped to around $300 USD and he had to find another means of supporting his family and left the mine. It was a 80-90 foot shaft that they then would tunnel out and then stope up. When he left, he simply put up chicken wire around the exterior to keep hikers from falling into it. The BLM wasn't happy about that and they filled it back in for safety reasons. I've spent the last year looking into the best way to open the operation back up and hope to 'dig in' in the new year, with the BLM's blessing of course.

I guess I'm not sure why he (the previous owner) was running the milled ore across the shaker table to get the free mill Gold, I'm sure they had their reasons, and maybe there was a lot of free mill Gold in the high grades that simply made more sense to them. Unfortunately, he a little to old to show me around, and his memory seems to have some gaps in it. He said he wished he could help, but suggested I find someone who knew this stuff and get their help. I believe this ore will work with cyanide leach, as this was the method used when the mine was active. I just assumed that maybe there were some new processes that had replaced the way they did it 30-40 years ago. Hats off to those old timers who figured this stuff out!

In the mean time I've been trying to get some equipment (looking into used to save start up costs) but have been concerned if I'm moving in the right direction.

You gave me a lot of information to chew on, as my claim has several feet of fresh snow on it and I'll have to wait until a spring thaw to get the ball rolling.

I'm glad I read your comment on that thread, and then getting the clarification that I needed, so that I can start moving in the right direction with a little more confidence.

Thanks again with the advise and knowledge,
Greg

 

[Greenhorn]

Well, first off, within the first few pages of Hoke's book, it says that it does not cover hard rock mining or anything related to it. It's an interesting read, but not helpful with my hard rock mine.

I've been studying the great info in the "Further thinks members might find interesting", as well as other threads. With the roll bottle test, you talk about using cyanide, but in different threads as well there is mention of Ferri- and Ferro- cyanides. With this bottle test, can you use Ferro-cyanide (it sounds like it's a safer form to use and for transportation when ordered) but can enough UV get through the canister used to release the cyanide from it's compound? Should I just use straight cyanide?

When using the bottle test or heap leaching (the previous owner of this mine was using cyanide heap leaching back in the 80's) what metals will cyanide be removing from the ore that I might need to be aware of or watch for? There's a lot of talk of using cyanide to get both Gold and Silver, and I think copper, but not much that I've seen so far in regards to other metals?

The use of activated charcoal in the bottle roll test, would I drain the fluids out and wash the material with clean water a couple times and put the charcoal in the drained fluid, or can I simply add it to top of the bottle after it's done rolling for 24-48 hours? In that post, Deano is talking about getting weights before and after sampling with someone asking for help, I'm assuming that getting weights of ore before and after the bottle roll test would be advisable, and the weight of gold recovered as well?

Thanks,
Greg

 

[Deano]

If you are carrying out bottle rolls then you weigh out, say, 200 grams of ore and put it into a bottle along with 400 ml water and 1 gram of sodium or calcium cyanide.

Always use the simplest form of chemistry and straight cyanide is best.

The above proportions will give a natural pH greater than 10.5 which is OK for a bottle roll, the cyanide level is higher than would be used for processing but you are looking at basic gold leaching and I have no idea what other cyanide consumers may be in the ore.

After a 24 hour roll you filter off some of the liquor and have it analysed by AAS or ICP. Basic maths will give you the gold grade recoverable by cyanide.

You do not use carbon with a bottle roll test.

Most ores will not be consumed under alkaline conditions so unless you have an unusual ore there is no need for weighing the bottle roll tailings.

The major cyanide consumer in an ore is usually copper, some ores may have mercury present but it is not usual for mercury levels in an ore to be of concern. If your analysis is done by ICP then you can get a readout of all metals in solution as well as the gold and silver.

Deano

 

[AJLnz]

Hi Gents
I'm used to using nitric and AR on scrap and e-waste.
BUT
I'm also very new and green to recovery from a sulphide ore with microscopic gold.
If I want to stay away from cyanide totally due to enviromental concerns and want to use chlorine, what steps would you gents suggest I follow.
My ore is hi in Fe, S, Al, K, Ca and Te.
On fire assay the cupel turns totally pink before it freezes

 

[Deano]

The first thing to do is to get fire assays done on a representative sample of your ore, this will tell you if it is worth going further with the project.

Most of your problems will be concerned with getting a representative sample, it is human nature to bias the sample with the better looking specimens.

Assuming that you have viable grades from your assays you have two possibilities.

The first is to leach your ore as a whole, the second is to get a float concentrate and work with that fraction.

Chlorine leaches will dissolve many ore components, this can make processing non-viable.

If you get a float con you will now be working with only a small percentage of the total ore.

You would expect a high percentage of your gold to be in the float cons, usually any residual gold values in the float tails are not viable to leach, especially with a chloride leach.

Most of your processing decisions will revolve around the results of your assays and float tests, you need to get these done before worrying about processing methods.

Pink colouring in the cupel can be caused by several metals, tellurium, lead and iron are the obvious candidates.

Deano

 

[AJLnz]

Hi Deano
Thanks for the info. My process so far.

Crushed ore to 200 mesh. Then roughly concentrated with a Miller table and ribbed carpet on top.
Then put it through a cube.

Roasted this at 450 degree C for 30 min, then at 700 degree C for 30min. Mixing constantly.
Then fluxed with litharge, soda ash etc and smelted below 1000 degree C. Then cupel and final result.
Just on the way it kept freezing with the pink layer and when it cools turns gold colour but it's only a colour not real.


Now want to use a chlorine leach and start scaling the process up.
But the HCl and sodium hypoclorite leach is more for gold alloy and not ore.
Trying to leach at a PH of 5 to 7 to try and leach gold and silver and not the rest