Further things which may be of interest to members

[Deano]

It should be noted that with a ferro cyanide leach the uv conversion to cyanide is, assuming enough uv is present as sunlight etc, pretty well completely to the cyanide side of the equilibrium.

In the case of the ferri cyanide leach under the same uv conditions the conversion to cyanide is minimal.

However this is an equilibrium condition and any effect which removes cyanide from the solution will cause some more of the ferri cyanide to convert to cyanide.

Locking up some of the cyanide as gold cyanide will effectively remove that cyanide from the equilibrium and some more of the ferri cyanide will convert to cyanide in order to maintain the equilibrium.

Thus a solution of ferri cyanide will have most of the ferri cyanide as ferri cyanide with only a small amount converted to cyanide.

This means that there is a reserve of ferri cyanide in solution available for the production of cyanide if any of the already converted cyanide should be locked up as gold cyanide.

If you have a lot of time available and wish to be using the safest possible cyanide type leach then use the ferri cyanide leach.

If you are willing to trade off some of the safety buffer for time then use the ferro cyanide leach.

Deano

 

[Deano]

I have never made any secret of my preference for leaching of ore in a vat rather than agitated tanks.

One further point in the favour of running a vat leach is the treatment options available for cyaniding sulfide ores.

Most easily treated oxide ores have been found and mined, what is still available are sulfide ores.

Many sulfide ore deposits have been not worked because the deposits are not large enough to cover the very large costs of CIP circuits in the cases where they can be successfully leached in cyanide CIP.

If it is decided that the ore should be concentrated into the sulfide fraction and then to treat that fraction there are losses and extra costs you will run into.

Unless you are extremely lucky you will be unable to get high recoveries of the sulfides by gravity separation.

This leaves flotation as the best option but will require a float plant and suitable milling.

Even an extremely well run float circuit will have sulfide losses at a level which hurts a small operator.

Big operators can use economies of scale to offset these losses, by definition small operators cannot.

A further problem is that most sulfide concentrates do not respond well to cyaniding without some form of pre-treatment such as roasting or bacterial oxidation.

This requires further plant expenditure and operator expertise.

If the head ore is run as a vat leach there are unexpected benefits.

There are no concentration losses as there is no concentration.

Milling does not need to be tightly controlled, usually a jaw crusher followed by an impact mill of some form will do the job nicely. This is as cheap a milling circuit as you can get both in capital and operating costs.

It also changes the mining cycle requirements in that you no longer are attempting to have a steady supply of blended feed to the mill in order to maintain consistent pulp density and gold tenor through the CIP tanks.

Instead you can stockpile ore on the ROM pad in dry weather periods ready for milling before wet weather.

This often means that you can have the stockpiled ore toll milled on site by a large trailer mounted jaw/impact mill combo on an as needed basis, you do not have to purchase the milling circuit upfront.

Once the ore has been milled and blended it can be left in a holding dam until required to fill a new or rotated vat.

If the head ore does not respond well to straight cyaniding then a high pH caustic cyanide leach can be used to break down the sulfides and allow leaching of the gold.

Because this breakdown is slow, requiring weeks to months, it cannot be employed in a tank type circuit.

As you are going to be running the vat for several months anyhow the sulfide degradation time overlaps with the normal leaching cycle.

Because you are running a vat leach there is no loss of liquor from leakage in the tailings dam as you do not have a tailings dam as such.

The liquor is recycled through the vats as required.

The main extra expense with a vat leach is the cost of fitting rain shedding covers over the vats to prevent flooding of the vats in wet seasons.


Deano



Cyanide recovery is often acceptable without further treatment of the ore

 

[Deano]

One other thing I forgot in the vat leach post is regarding the pumping system.

Regardless of what type of pump is used there will at some stage be entrained air passing through the carbon column.

This means that you need to have a top screen to retain the carbon in the column.

Usually a 1mm hole diameter punched 316 SS screen will do the job well.

Deano

 

[Deano]

A further omission in the vat leaching post concerns the pumps used.

Many operators use self-priming centrifugal pumps, these work well but are not designed for extended use if a suction blockage occurs.

I have always favoured a diaphragm pump because they are almost bullet - proof.

However if using a diaphragm pump the liquor is presented to the carbon column in a pulsing flow pattern, this tends to attrition the carbon unnecessarily.

The flow pattern can be smoothed out by placing a vertical air vessel in the discharge line next to the pump outlet.

Generally the air vessel has a height 5 X the outlet pipe diameter and a width 3 X the outlet pipe diameter.

So if the pump discharge is, say, 50mm diameter then the height of the air vessel will be 250mm and the diameter will be 150 mm.

These dimensions are approximate but work well if the vessel is made from a piece of 150 mm diameter pipe.

If the pulsations in the liquor are still greater than you would like you can place a second air vessel in the line about a metre from the first vessel.


Deano

 

[4metals]

I agree that a diaphragm pump is bullet proof and can run dry without any issues. Try that with an impeller pump! I have had issues with some particulates fouling the diaphragms and stopping the pumps, always solved with appropriate in-line strainers. But I do most of my work with recycled values not natural ores so I have more control over what crud may be sucked up. You have to deal with whatever nature throws at you in large outdoor pits.

So your chamber is a pressure chamber which will pressurize as the trapped air is compressed. How do you accommodate air in the feed? There will come a point where the "pulse tank" will have too much air in it. Do you allow for pressure relief, either automatic or manual?

 

[Deano]

I really did not phrase the location of the air vessel very well.

For a horizontal pump discharge line the air vessel is fitted vertically upwards from a 90 degree tee piece in the discharge line.

This means that the air in the vessel is permanently retained in the vessel.

Air in the feed simply bypasses the air chamber as the chamber is already full of air.

If the feed has a lot of air slugs in it these can be gotten rid of by running the discharge line in a single upwards peaking wave shape and fitting an air release valve at the top of the wave.

The horizontal and vertical components of the wave are designed on the flow rate of the pump.

This will not get rid of all the air but will get rid of most and minimises its effect on the carbon.

Deano

 

[justinhcase]

Absolutely top notch.so much useful information in so few post's.
Thank you for introducing us to such a practical and fascinating's process as vat leaching .
Could you recommend some further reading? such as environmental impact reports or any well put together study.
Have you any published paper's your self?
As this is a relatively unknown process in my part of the world having some case study's to use in discussion would be of help.
The councillors in Devon are all in there sixties and need information in triplicate from several different sources before you can make any headway with them in discussion.
Much Thanks
J

 

[4metals]

So kind of like this;




This will also allow a diaphragm pump to supply a constant flow rate at a constant pressure if you use a pressure switch in line. I have used systems like this in aquaculture setups to keep the column of packing where the denitrifying bacteria live aerated (by a constant flow of fresh water, not necessarily with air) and freely moving in the column. Never did this with carbon but it makes perfect sense.

Thanks Deano, we truly appreciate your perspective from the mining world. You don't stumble over facts and tips like these every day, even when you do this for a living.

 

[kurtak]

Thanks Deano, we truly appreciate your perspective from the mining world. You don't stumble over facts and tips like these every day, even when you do this for a living.

Very well said 4metal & I absolutely agree :!:

Deano - your posts are absolutely :G

Thank you Sooooo much for taking you time to share your VAST knowledge with us :!:

It is much appreciated 8) :!:

Kurt

 

[anachronism]

I have recently had the extremely humbling experience of working with Dean for ten days.

Three days in a cyanide carbon stripping plant and a week at his own lab. Whilst it was a long way to go from England, it was worth every dollar. The man's a genius.

I could never thank him enough for the opportunity.

Jon

 

[kurtak]

I have recently had the extremely humbling experience of working with Dean for ten days.

Three days in a cyanide carbon stripping plant and a week at his own lab. Whilst it was a long way to go from England, it was worth every dollar. The man's a genius.

I could never thank him enough for the opportunity.

Jon

This reminds me of another friend I have --- he lives in southern California - he calls me up in the middle of the winter when its like 20 below zero with wind chills of 35 - 45 below zero here in Wisconsin & tells me how he is out riding his Harley in a Tee shirt :lol: :lol: :lol:

I am JEALOUS as hell mate :!: 8) :lol: :mrgreen:

Kurt

 

[Deano]

I have always tried to keep pressure switches etc. out of my pump circuits.

They always give trouble at the most inconvenient times.

You can usually design a circuit which is totally basic, little or no electronics involved.

It may not look as pretty as the fancy circuits but the reliability is far superior.

Deano

 

[Deano]

There must be some papers around on vat leaching but I have never chased them as the method or running a vat I practice is in many aspects different to standard vat leaching practice.

That is the reason I put out these posts on vat leaching is to spread the knowledge.

Usually people running vat leaches are either too busy or not inclined or not capable of putting together a study.

Deano

 

[justinhcase]

There must be some papers around on vat leaching but I have never chased them as the method or running a vat I practice is in many aspects different to standard vat leaching practice.

That is the reason I put out these posts on vat leaching is to spread the knowledge.

Usually people running vat leaches are either too busy or not inclined or not capable of putting together a study.

Deano

O well,If you are trying to put a proposal together it is always of help to have some one go before.
Trying to communicate the details of the process and answer proactively environmental concerns with out a successful model to work from will be interesting.
The first step in Europe would be to employ a specialist to conduct an environmental impact study and hope to high heaven that no one else funded a counter argument..it is very important to get the right specialist.Do you not have to produce such ground work in Australia.
Much thanks
Hope all is well in the land down under.
Regards
J

 

[Platdigger]

anachronism, that must have been soooo good! Do you think you learned anything?
I bet you are wishing you filmed or somehow recorded every minute.

 

[anachronism]

anachronism, that must have been soooo good! Do you think you learned anything?
I bet you are wishing you filmed or somehow recorded every minute.

I learned an absolute boatload, starting with the fact that I actually didn't know very much :idea:

The joys of modern tech mean that I was able to take a lot of pictures!

 

[Deano]

Vat leaching has been practiced in Australia in large scale for many decades.

The main requirements are vat design particularly in relation to liner type and properties and wall construction.

Other requirements are similar for vat or tank leaches.

Deano

 

[4metals]

I understand that the quantities of material processed by vat leaching can greatly exceed the quantity processed in an agitated vat but how does one ascertain that all of the contents of the vat are effectively leached. Is it possible that a design can eliminate the channeling effects often seen when flowing a leach over a granulated pile of scrap material?

 

[Deano]

Channeling effects in a vat are eliminated mainly by the hydraulic lifting approach to flooding the vat.

If you have been diligent in blending the feed then your main concern is to wet all ore particles, hydraulic lifting does that for you.

When the vat is being rested the ore is fluffed up with an excavator, this removes any problem areas which have formed during the leaching cycle.

The cycle of leaching and resting a vat is maintained until there is not enough gold recovered from a leach cycle to maintain viability.

Testwork carried out on exhausted vats register residual leachable gold levels of well less than 0.1ppm.

Deano

 

[4metals]

So it is a multiple leach process, flood, drain, fluff, test and flood again if necessary. I'll bet the job of driving an excavator through a damp pit of cyanide soaked ore isn't one people are lining up to do. No sipping coffee on that job!