Gravity Separation & Concentration Methods

Gravity Separation & Concentration Methods 2017-04-04T06:57:23+00:00
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Gold lost in ball mill (8 replies and 4 comments)

explorer64
10 months ago
explorer64 10 months ago

We sent a substantial amount of quartz gold ore to a mill for processing. After a couple hundred tests on the material an average grade was calculated. After milling for several days the mill says there is way less gold in the ore than what had been estimated.

Once milling began samples were taken once per shift, ball mill discharge, cyclone overflow, concentrate, and tails. Samples were sent to a lab for assay. With these samples the Lab has determined that there is at least 1 gpt missing that did not report to the concentrate or the tails. Theory from the lab is gold is stuck in the grinding circuit. 

This is mainly free gold, although it is very fine gold, as small as 10 micron size and the largest pieces being in the neighborhood of 500 micron.  Also the ore has very little sulfides.

I have a couple questions that maybe someone could help me with.

  1. Can you get an accurate head grade without sampling the feed to the ball mill?
  2. Can gold get stuck in the ball mill and not get to the cyclone?

 

Sugar Watkins
10 months ago
Sugar Watkins 10 months ago

Which gold recovery process are you using explorer64? Gravity/Leach/Float or combos?

explorer64
10 months ago

They are using flotation, metallurgy testing showed a decent recovery with gravity but a better recovery with flotation. Although in reality recovery was horrible below 60%
This is mainly free gold, although it is very fine gold, as small as 10 micron size and the largest pieces being in the neighborhood of 500 micron. Also the ore has very little sulfides

AJNeale
10 months ago
AJNeale 10 months ago
1 like by explorer64

If the cyclone underflow gold grade was higher than the cyclone overflow grade then you have free gold and you should look at putting a centrifugal concentrator in the grinding circuit.  For low tonnage operations you can feed 100% of the ball mill discharge to the concentrator (this requires an extra pump, but is worth the effort).  Modern units can recover free gold down to the 10 micron size range or finer.

In regards to the met balance, normal practice is to take the tails grade, feed tons, and gold produced and back calculate the head grade.  Sampling the feed for head grade is always difficult.  As a side comment, I'm not sure how your assay would be any more accurate than what is calculated in the met balance, but that is always the case to be argued between the miner and the mill guy.  You can also compare the mill feed grade to the cyclone overflow grade to see if there are any losses in the grinding circuit.

You also need to check all the gold traps in the grinding circuit, i.e. cyclone feed pump box, cyclone underflow launders etc.  They should be clean when you start your run, and cleaned out after you finish the run, including a full grind-out of the mill.

Free gold can also accumulate in the mill, either in the liners (at the Echo Bay Lupin operation when they took out the steel liners during a liner change they would put them in a secure area and grind off the surface to recover a lot of gold), or behind the liners (just ask any mill mechanic that does liner changes).

If you are having your ore processed by a toll processor you should insist on a gravity recovery unit in the grinding circuit to minimize gold losses within the grinding circuit. 

I hope this helps.

Best Regards

Andrew 

explorer64
10 months ago

Thank you Andrew this is helpful, big problem with this material is the nugget affect when it comes to sampling. I was allowed to take 1 set of samples when I visited the mill, I took cyclone overflow and underflow, the difference was about 1.8 -1 and this is not a surprise as we know there is free gold.

Sugar Watkins
10 months ago
Sugar Watkins 10 months ago

Is this in a custom/contract milling plant or your plant?  What tonnage? Where?

explorer64
10 months ago

This is a custom mill, I don't want to get to much into details as I don't want to smear anyone's name just yet. I'm just trying to understand whats happening and what can happen as for my one question to do with the head grade being way lower than expected grades.

max skinner
10 months ago
max skinner 10 months ago
1 like by explorer64

Absolutely you can loose gold in the mill grinding circuit, in the classifier, in the bottom of the sand pump, behind the liners etc. particularly steel liners. When starting a new mill it will take some time before you can come up with a proper metallurgical balance, every crook and cranny, needs to to be filled before you can come up with what is in the head shows up in either the concentrate or the tailings. I have taken 40 ounces out of a small, 4' x 4' mill when I removed the liners, and any time you tear down a gold mill you have a pretty good pay day from trapped gold. Also since I am somewhat pessimistic, be sure there are no hidden traps, there on purpose, for the mill owner to make a little extra money, I have run into this, gold doe's funny things to people. My experience with custom mills is that you will likely never agree, best thing to do is come up with a good sampling system where you both can agree on a assay, sample for you, one for them, one for an umpire assay and then sell them the ore. Once you sell the ore to them you don't care what they do with it even if they want to make road gravel out of it.

 

David
10 months ago
David 10 months ago
1 like by explorer64

Hi, great answers guys. 

explorer64 about mill liners, see https://www.911metallurgist.com/grinding/gold-in-grinding-circuit-liners/ 

Lock up of gold on mill liners, sumps, gold traps etc. This is common. Mill upsets and coarse grinds floods and plant disruptions can occur, which may not be picked up with infrequent sampling.

Gold settling in tanks, gold room sumps or locked up in roasters This can be difficult to detect unless one goes looking for it or there is a policy of regular clean up. Similar problems occur in hydrometallurgical plants.

Ref: https://www.linkedin.com/pulse/metallurgical-accounting-damian-connelly has more for you.

https://www.youtube.com/watch?v=-zF8apw5jPc

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lostsierra
10 months ago
lostsierra 10 months ago
1 like by explorer64

#2 - easy to get coarse gold stuck in a ball mill. They are great gold traps. 

explorer64
10 months ago
explorer64 10 months ago

Thank you all for you great answers, and David thanks for the links.

 

max skinner
10 months ago

You may want to hire an experienced extractive metallurgist to give you a hand. This is the kind of thing I have done for over 50 years and helped many people with this kind of problems, it would likely be well worth the money.

David
10 months ago
David 10 months ago

Funny how Treve here posted photos of a Mozley Multi Gravity Separator to recover gold ... do you see the ball mill parallel?

 

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