If you are using lime or caustic to adjust the pH, the obvious answer is to add less of it. You could use automatic feeders to add caustic if the pH drifts down toward 10.3 and stops adding when the pH rises to say 10.7.

The mill should have a ratio control for lime addition. This can be adjusted down in consumption and the pH adjusted in the leach tanks using milk of lime addition. Does the ore body have wide variation in lime consumption?

We no longer use lime since our pH leaching is more than 11. So the idea is to reduce the pH by adding acid! What do you think? We will maintain pH between 10,3 - 10,8 to get optimum gold recovery and to avoid excessive of dangerous cyanide gas formation. But I still wonder what kind of acid that I can use.

I think the best set of actions to reduce pH:

Update leaching solutions with the addition of fresh water (if possible)
A decrease in supply alkali
An increase in supply of NaCN

You need automatic control pH when performing this action.

I've come across this before, and it is a pretty vexatious issue to address, because the addition of an acid to your leach feed introduces a number of both safeties and cost issues. Getting acid to site is a painful exercise if you are remote, and you will need to spend capital on storage etc. Is the increase in pH as a result of ore changing, or are the mining chaps being a bit reckless with their concreting underground? If the latter, you might have a case to discuss with the mining manager/supervisor, it should save him a bit of money too if he can get concreting costs down.

That said, I found that there are two routes to follow - pH adjustment with CO2 or H2SO4. I prefer to use carbon dioxide for pH control in this environment, for a number of reasons - control is slower, you are far less likely to find yourself going below the pH setpoint and because with any luck you can make the entire reagent storage and vaporizer part of a vendor package, and not have to pay the upfront capital. However - you will need to test this to get a really good handle on where your slurry buffers, and what the longevity of the pH adjustment is. You might even see an improvement in viscosity using CO2.

The other alternative is H2SO4 - this shouldn't impact your leach negatively, and a little generally goes a long way. However - you can end up spending a LOT of money to get the site storage, and hauling trucks of acid around the countryside is not always attractive.

I would suggest setting up some testing using these two (or any alternatives you may think of) side by side, get a handle on the reagent consumption and the impact of leach recoveries, then armed with that data design the two systems, go out to vendors and get budget quotations for the equipment and/or rental costs, and do a side by side comparison. Then you can look at them on a combined capital/operating cost versus improvement in recovery basis, and get a good handle of what way to go. I would consider pH adjustment in a pipeline rather than having a tank dedicated to it, to minimize costs. Depending on your flow sheet this might be thickener underflow, or you might need to use a small intermediate hopper between screen undersize and the first leach tank.

Do you know what mineral is present that is causing the high pH?

If you do intend to use acid addition then you will need very good automatic pH control as said earlier. Sulphuric acid is the cheapest option but you would need to be very careful.

Can you test this on a small scale first? If there is a lot of calcium in your ore, sulphuric acid may lead to gypsum scaling problems.

First of all are you sure that the pH is the one affecting your recoveries? Most pH control to minimize cost of reagents can be done by constructing a lime tank where its agitated continuously 20% and pumped to the mill or to first pre oxidation tank at a control rate, by controlling the valve. I think it’s very important to ascertain that lime is the cause of low recoveries, because in principle pH is to keep cyanide in solution, although if they are precipitation at higher pH encapsulating the Gold and making it inaccessible to the leaching agent.

This is a very interesting and important discussion. We have come across a similar situation where a pH above 10.5 negatively affected gold dissolution with cyanide. This was in a semi-refractory ore body which contained banded iron stone (BIF) and also pockets of siderite which is an alkaline iron carbonate. When the pH went above 10.5 the cyanide consumption went up and gold leaching reduced. With the higher demand for cyanide the pH went up even further although no lime was added - this was as a result of the caustic soda in the cyanide (added to stabilize the dissolved cyanide) - in the process the gold recoveries dropped by 10-20%. Once the pH was controlled in a very narrow band 9.8 to 10.2 (three stage controllers) the cyanide consumption reduced and dissolution went back to normal. At times no lime was added when siderite areas were mined - the blending of the ore to ensure better pH control in gold leaching was however the cheapest and best solution in this case.

What I heard from experienced people:

When pH is increased to 11 and above, gold tends to co-precipitate with hydroxides of base metals. Then, to hold gold in the solution, one needs to increase cyanide dosage. Thus, concentration of free cyanide must be considered together with pH.

pH can be reduced by different chemical means besides acid. It may be bicarbonates, salts of base metals forming insoluble hydroxides, etc.

It all depends with the mineralogy of the ore; if there is precipitation of the hydroxides it occludes the gold particle making it inaccessible to cyanide. I have run a plant with pH up to 11.5 and had no problems except the cost of lime. I suggest he runs a bottle roll test at different pH and see if there is any variation in recoveries. Before he can attribute low recoveries to pH.

Cut the addition rate of lime to the process. Let the ore consume the lime to adjust the pH lower. Lime is the lowest cost pH modifier. Others have presented very good reasons for not adding acid. The plant chemistry is a slow moving system. Trying to rapidly change conditions will result in excessive costs and increase the potential of safety issues.

I have not seen any gold recovery reduction while we are running leach tank at pH around 9.8 - 10.2 otherwise the recovery was increased.

A pH > 11 does not necessarily interfere with gold leaching. I worked on many projects with the pH above 11, and it was not detrimental on gold leaching. As mentioned above, some minerals may precipitate gold or reduce its dissolution when the pH is too high.

It's kind of complete explanation. I am interested in using CO2. I actually have no experience of using CO2 as pH modifier. Can you please give me more explanation about, technically? Our site is not so far from city. Regarding to mobilisation, I think it won't be problem (if we decide to use acid). And the concentration acid used must be low. And now, I'm doing some lab test to know the effect of H2SO4 on whole process.

Mineralogy of the ore is the crucial factor to affect the pH and lime consumption. Several times I came across the same controversial effects in plants: cyanide addition made recovery worse.

CO2 is a good solution with mild effect. We used SO2 gas to control pH. It is safer than acid to transport and easier to control. It has stronger and quicker effect than CO2. You need to test both to make sure you have the right solution. As said it may be a difficult to have necessary separate acid/SO2 storage from CN.

You may need a basic modal mineralogy on the plant feed or make a ore variability study and geometallurgical model/plan and prove that it is carbonates that make the effect. It may be the opposite way – some acid-producing minerals can affect the pH.

After mineralogical and variability study, in-line x-ray analytical system may give you quick and certain indication to control pH in the needed range. The investment quickly pays off with higher and stable recovery.

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