Leach tanks -

• measure volume of slurry in tank.
• sample 1l slurry. Measure the wet weight of the sample. Filter the entire sample collecting a solution sample and solid cake sample.
• dry solids cake.
• weight solids cake and calculate the % solids in the tank.
• assay the solid and solution.
• calculate the gold attributes to the solids in the tank and the gold in the solution of the tank based on the density and tank volume.
• do this for each leach tank and sum the total gold in the leach circuit.

Absorption Circuit

• same as leach tanks but need to also account for gold on carbon.
• collect a 2l slurry sample and pass through a 850um screen to collect the carbon.
• wash and dry the carbon.
• ash and assay the carbon.
• given the tank volume and the mass of carbon per 2l you can work out your total carbon tonnage in the tank.
• multiple out your assay value to your carbon tonnage to get the total gold in carbon. Sum with the gold in solution and in solids to get the total gold content for that tank.
• do the same for each tank and sum the total gold in every tank.

Elution circuit.

• Work-out where the carbon is in your circuit. I.e. work out the tonnes of carbon in your acid wash column, elution column regen hopper basically wherever you store carbon at that point in time.
• Assay each of the carbons.
• Multiple your assay by your tonnage. I.e. if you loaded carbon is 9 tonnes and your assay is 3500ppm then 9 X 3500 will give your gold in that hopper.
• sum the total gold attributed to carbon out of circuit for each of the hoppers/columns.

If there is gold on your wool, for each strip on the wool subtract your barren assay from your loaded assay and multiple by your column size.

You may also want to work out your volume of your eluate and assay that too for accuracy but this provides very little gold to the inventory.

Sum the total amount of gold in your leach tanks, absorption tanks, carbon out of circuit and gold on wool to provide a total gold in circuit.

If there is dore in the safe this will also need to be included.

Conduct another gold in circuit at the end of the week/month/reporting period to calculated production. Opening inventory - closing inventory + shipped gold.

Best practise is to clean your wool and pour it before a gold in circuit so that you only end up with barren strips and dore in the gold room as this the most accurate due to dore being "tangible".

Is the gravity gold and material entering the mill necessary?

I have few doubts for tank leaching with NaCN.

Do you agree that air is injected in tanks for better reaction? Is it due to O2 in air causing the effect? If so then why not use Air Spargers which give micro bubbles and more air distribution, with more surface area of reaction

I you agree for this above statement --then why not we directly inject O2 in to the tank with the help of gas spargers which can create micro bubbles for more surface area of reaction.

Get yourself a professional there to look at your plant, get the best from what you have and train you in G.I.C’s, write up procedures so all your people are on the same page, every plant is a little different, ores vary as well.

Don’t include material in the mill because it is too difficult to sample and estimate volume and the residence time is SFA. If you have a gravity circuit you will definitely need to include this. If you table your gravity just weigh it and assign a fineness to it. This needs to be included in your full inventory. For a leach reactor it will be a case of cleaning your wool and pouring.

Clean out all gold room gold (electro-winning cells and cathodes and any gravity gold) and smelt it all before the GiC determination sampling either on the last day of the month or the first day of the next month but ensuring it is covered as production of the month (or period) in question. It is often hard to convince accountants and auditors that this is the case as it is often hard to convince them that the last day does not end at midnight if you work 12 hour shifts. Converting all gold in the goldroom to bullion eliminates all the issues of trying to back calculate gold on wool from solution assays etc. believe me it is much simpler. Gold in bullion is much more easily and accurately accounted for than any gold that you try to calculate from assays of solutions etc.

Its true and am supporting Peter in the sense that, seriousness should be made during sampling. Some of the operators are not smart enough so attention should be made to shift supervisors. This depends on commitment. All the solids and solutions sample from Leach tanks, Add tanks together with carbon from Ads should be taken .Then solid samples pulverised and you can weigh about 25 g each and add with Nitric acid followed with HCL and keep all in industrial or lab cooker for a couple of minutes before dilution .Finally reading the conc on AA machine.

In my first gold mine we noted that the solution plus solids gold leaving the leach circuit was 2/3rds of what was entering. This soon resulted in suggestions of massive theft by us, the mill crew. The first time that we had to drain a tank I went to the bottom with a camera, a sample bag, and a measuring tape. The settled solids formed a hemisphere for a tank bottom and graded hundreds of grams per tonne. Part of the problem was that the geologists in charge of exploration claimed that there was no gravity gold, which was patently false, and it was settling in leaching before it could be dissolved. We developed a "paper gold" inventory of hundreds of kilograms.

Years later I learned that management gave my report to a bank to secure a loan, and it was accepted. A lesson is that gravity gold should be measured during a feasibility study, not decided by opinion because it was not wanted. Fortunately, the 43-101 laws in Canada and their equivalents elsewhere require proper due diligence today.

I have sometimes requested that a new plant run low grade until the corners of the tanks are filled, or even that they actually be cemented in after start-up. The mass that will settle in the corner is a function of the energy intensity so I cannot give hard rules about how much to expect, but you do not want the situation where coarse gold is accumulating in a live bed there.

Depending on their size of the mine you need at a minimum a month end inventory of the entire plant at the same moment; not necessarily exactly at month end but perhaps the end of the second last shift, or noon on the last day. Besides the tank inventories mentioned by others, sample and estimate tonnes for any spills in the plant. A classmate told me once of 60 kg of missing gold in a short length of floor gutter due to a shutdown of a pump and the need to use the gutter for transport. I myself once recovered about 27 kg from the gutters of a grinding circuit that had not been operating properly, allowing gold to accumulate.

A perhaps more controversial note, never allow "paper gold" to accumulate. By this I mean inaccurate accounting. Be sure that the belt feed moisture is measured accurately then add up to 0.5% consistently to account for evaporation losses during handling. If you end the year with 500 ounces to the good no one will ever say thanks, or question your honesty, but if your paper gold accounting results in a deficit of 10 ounces at year end every bean counter will wonder what you spent it on. For a 100,000 oz/y plant 1,000 oz is only 1%, and not much of the equipment in a plant is that accurate. Be sure that the belt scale(s) are frequently calibrated. See if you can measure tailings flow as a cross check. Remember that the official recovery of a gold plant is based on the product gold and tailings, not the feed measurement. Consider the month to date feed as a check on the product and tails, wit h no one day being important.

I once had to explain why, despite rigorous calibration of belt scales, our mine was always short on inventory of a bulk product when the storage barn was emptied. Keeping inventory was complex, with dust from loading rail cars being returned to the plant. A cumulative discrepancy of around 4% was known, so I decided to deduct that from every batch of product sent to the barn on top of all other corrections used. Months later the Plant Super was furious when he found out what I'd done, but saw why. A few months later the inventory showed about 500 tonnes to the good instead of the usual 20,000 tonnes to be written off. The super became a hero and HO was ecstatic. My point here is that instruments are not perfect and sometimes a "fudge factor" must be included. Sadly, a loss/discrepancy can be accepted for a base metal or an industrial mineral but accountants go berserk if the subject is a precious metal.

I disagree with your comment regarding operators not being smart enough. Operators are only as good as how they are trained and how well things are explained to them. So if you are having problems with operators incorrectly sampling I suggest spending time showing them how/why it is so important to sample correctly and also why they are doing the task in the first place. Most operators once they explain that this can 1 determine their bonus and 2 gets reported to shareholders are more than willing to do the right thing. If that doesn’t work collect the samples yourself.

Also aqua-regia while cheap and cheerful is not accurate enough for all gold plants

Any fool can show a person how to take a sample etc. The difference is helping the person to understand why the sample is taken and for what purpose it is taken and how they can improve the results from those samples by the way they run their circuit.

I will like to ask some questions for further discussion.

1. Do we have a software for GIC?
2. Is it always advisable to relay on only one laboratory for solution and solid gold reading? (Remember the world laboratory competition)
3. Is it not advisable for the Mineral/Process/Metallurgical Engineers to further courses in Finance, Accounting and Auditing in other to overcome most of the obstacles and queries?
4. How can AMIRA Metal Accounting and Reconciliation be introduced to fresh graduate and how is the AMIRA Metal Accounting and Reconciliation helpful currently?

While there may be software available commercially, plants differ. If you cannot develop a spreadsheet for you own plant then the problem is yours.

Exploration regulations in the developed world require that every 20th sample is sent in duplicate to a second lab that for every 20th sample submitted to the "home" lab there is a blank, that every 20th sample to the home lab includes a known standard, and so on. In a producing plant the rules vary by country but a serious engineer will do the best CYA procedures. The cheapest is to randomly submit duplicate samples to the home lab as well as submitting blanks. It would be prudent to send material to an outside lab regularly. The 1 in 20 is a legal requirement for a PEA, as an operator you do not have that same pressure for accuracy. That said, you must be satisfied with your own lab. Duplicates of the most important samples, like a weekly composite, should be sent to a second lab. The seriousness varies with the mine owner (save money).

Engineering has traditionally included a definition of it as the ability to conduct a process at minimum cost to society. If your Alma Mater did not include economics, do not advise other potential students to apply there. We do for pennies what a chemist or physicist would expect to require dollars.

Make sure you sample the bullion bars prior to them leaving site and keep the samples in case of a need to go to them later. This can be done using sampling tubes or drilling of the bars. You will need to account for the samples in and out of your safe to ensure security protocols are followed.

Also get an umpire sample from the refiners checked each delivery so that there is a clear check on all the gold sent and the gold content.

The refiner should send back the left over samples or adjust your account for this gold. It’s not much but it should be recorded in your metallurgical accounting system.

When it comes to gold accounting and reporting clarity even for small amounts of metal is critical.

I would only make one extra comment re Peters comment on sampling the bullion, assuming it is obvious that all bullion leaving site must be weighed very accurately, to the gram or better, and recorded. If your bullion is reasonably pure, sampling shouldn't be an issue. If however there are high levels of residual Iron and or Copper, for example, correct fluxing and melt temperature (higher with impurities) is very important. Sampling is only correct / useful if the melt and final bullion bars are homogenous (and clean). Correct fluxing will ensure maximum impurities are removed and correct melting temperature will ensure the molten metal and the resultant solid bar is fully mixed and homogenous.

This topic is well discussed a lot of experienced mind has said many ways of accounting the GiC gold and the information here is worth keeping. I used to run a few gold plants and all of those issues discussed are correct.

What I wish to throw as a discussion item is a question like:

"Can you relate your experience of how gold metals are stolen and taken out from the gold plant without being accosted and noticed by securities?" Different methods and practices of stealing gold.

In small operations, some reports are showing that 20-40% of the gold production are lost be stealing by the operators, in connivance with security, etc.

In one of the gold mine/plant that I was assigned 30-40 years ago, I did a surveillance and emerge myself in the community where operators and people working in the mill and refinery were staying. Not many people know me, so I associate with them like a newcomer in the company. I try to go out in the evenings where people spend their time drinking and enjoying their weekend and observe their life styles. I noticed some of the people working in the plant to have spending habits that is so exorbitant and extraordinary. I took note of these people and when I reported to work after a week or so, I found that these people were working in the laboratory, some are in the refinery and others in the plant and others as security. It was a start of a close monitoring of their daily activities and found finally their strategies.

Can you provide us for experience on how people steal gold from the plant?

The answer to your question would fill a (large) book. There are so many ingenious ways people take gold all the way along the mining chain from the mine (underground and open pit) through to the processing of the ore and bullion delivery. As I lived and worked in the Goldfields of WA I found that the history of gold theft goes back to the commencement of mining there and by extension in all other mining operations.

There is a special policing squad in Kalgoorlie which was formed in the early 1900's that specifically looks at gold, diamond and other theft such as diesel fuel etc. from mines. One member of that squad once said to me that no matter how smart a person thinks he is by inventing a new way to steal gold he is not so smart because someone has already thought of it before.

What all responsible people who are in gold processing (and mining) management must do is to put systems into place that will minimise the opportunities to steal gold and those systems can be as extensive as the organisation want to put into place and are much more extensive than there is room enough to indicate here.