Hydrometallurgy: Leaching in Heap, Vat, CIL, CIP, Merrill–Crowe, SX Solvent Extraction

Hydrometallurgy: Leaching in Heap, Vat, CIL, CIP, Merrill–Crowe, SX Solvent Extraction 2017-03-23T09:50:58+00:00
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Effect of Lead Nitrate on Gold Leach Kinetics (15 replies and 1 comment)

Helena Russell
1 year ago
Helena Russell 1 year ago

Lead Nitrate addition in CIL operations and how the dosage of Pb Lead nitrate affects gold leaching kinetic rates. The main aim for adding lead nitrate is to help in Gold dissolution in CIL plants. Should the gold solid tails be the main aim for dosing lead nitrate in the treatment of slightly complex ores? Not forgetting the environmental impacts of lead. Do more Pb nitrate always result if faster and speedier gold leach time?

Alan Carter
1 year ago
Alan Carter 1 year ago
1 like by joanne

Two years ago, I did my experiment using Lead nitrate and it increases the dissolution of gold above 85%. Lead nitrate prevents layer formation on the surface of gold by sulphide minerals and hence accelerate the dissolution of gold.

joanne
6 months ago

l want to do a project on the effect of lead nitrate on gold dissoluton. the mine where l am attached processes a dump with a head grade of 4.32g/t, tails 2g/t and a cyanide consumption of 4kg/t. l think reason for the high tails is because of the sulphides in the dump. l would like your help on how to go about this experiement.

Helena Russell
1 year ago
Helena Russell 1 year ago

Surely it does because we have had significant gold dissolution by using lead nitrate. I just wanted to know the consequence of its over usage.

John Koenig
1 year ago
John Koenig 1 year ago

I would suspect that the overuse of Pb2+ would show up in the dore (dilution) as well as the operating cost (increased). Otherwise no obvious impact on gold recovery in an ideal system.

Helena Russell
1 year ago
Helena Russell 1 year ago

The impact on cost is obvious and we are hand tied too. Often, high solid tails leads to increase addition to reduce to a barely minimum. Is that okay to practice ?

JohnnyD
1 year ago
JohnnyD 1 year ago

Being proactive rather than reactive may circumvent some of the above issues...i.e. your lab gets geology samples from the pit...you should have mineral (S2-, tot S, mineralogy..) assays way before the ore is delivered to the mill stockpile area; am assuming the lab also gets daily SAG feed composite samples including 12-hr shift composites for CIL feed & tails that should be assayed for the same. If you haven't already done so, suggest you collate historical data & determine the effect of sulfide minerals on your Au tails w/varying lead nitrate addition. In other words, you might have a leg up before hi-S minerals show up into the circuit, adjusting or shutting down Pb+2 addition as the situation warrants.

Helena Russell
1 year ago
Helena Russell 1 year ago

The daily S2- and tot. S are being analysed for the sake of this problem. Am taking your advice and relaying down to my people. Thanks a lot. Always helpful.

David
1 year ago
David 1 year ago

Good read Lead nitrate is added to the leach circuit and has the effect of accelerating the gold dissolution rate. 

Reagent consumption is minimised by maximising slurry density, since optimal concentrations can be achieved at lower dosages, due to the smaller volume of solution per unit mass of material.

Marshal Meru
1 year ago
Marshal Meru 1 year ago

Lead nitrate is added to passivate sulphide mineral surfaces, particularly pyrite, so the addition rate should be related to the sulphide mineral content (ignoring pyrrhotite - which requires pre-aeration to diminish the oxygen demand) and specifically the slurry oxygen demand (OD).

For a desired dissolved oxygen (DO) level, the amount of air/oxygen being added to a leaching tank is an indirect measure of the OD. The lead nitrate addition rate could be based on this measure if it is available.

Sudhirkumar
1 year ago
Sudhirkumar 1 year ago

But should its addition rate be related to solid gold tails? Or only the un-passivated pyrite should call for more lead nitrate addition?

Raje Singh
1 year ago
Raje Singh 1 year ago

As noted, Pb+2 will react w/soluble sulfides & ppt out as insoluble PbS; if you have hi-Au tails as a consequence of SCN- (CN- deprivation) formation and/or O2 scavenging, Pb addition should help; FYI, Pb+2 addition in small amounts electro-chemically helps in the Merrill-Crowe Zn pptn RXN.

Standartenfurer
1 year ago
Standartenfurer 1 year ago

Lead nitrate is a bit of a tricky one as anyone who has used it can attest; and it is essential that you do the theory analysis as well as do lab test work to see if it offers a financial benefit under plant conditions.

At one mine site we then tested it at AMMTEC and it worked great. We tested in the lab and nothing happened. After study we found the sulphate content in the water was very high and the lead precipitated as PbSO4 when we mixed it. We may not have picked that up in a plant trial except that it did not work.

Remember what you are trying to do incrementally recover more gold than the cost of the reagent. Test work in the lab under plant conditions should give you some indication of whether a plant trial is warranted.

Sudhirkumar
1 year ago
Sudhirkumar 1 year ago

Can the accumulation of the ppt have detrimental effects on the process stream with the CIL inclusive hence the high un-dissolution of Au? Also per your experience how often should it be added when treating complex ores?

But the ore we treat contains traces of sulphides like pyrite and arsenic-pyrite. This therefore presented us the reason to passivate the sulphides in order not to have high effects on gold dissolution. It has been used for a while now but i want to know if its usage with time may pose another problem like eventually having high solid tails because there might be the accumulation of the side reaction products and the products as well. Like said, the ppt may be formed. It is insoluble though but can it hinder the leaching process?

Raje Singh
1 year ago
Raje Singh 1 year ago

The precipitated PbS (Ksp -10^-28) likely gets partially oxidized to a slightly more soluble PbSO4 (Ksp- 10^-8) form downstream of the process; trace amounts of cations such as Pb+2 & Hg+2 actually assist in the Au dissolution rate via a surface depolarization mechanism; insoluble are retained in tailings. I would strongly recommend following through Andrew & Mark's suggestions in terms of simulating the set-up on a bench scale & starting w/low dosages of Pb(NO3)2 i.e. 100- mg/L Pb; an IC would help in determining SCN-, SO42-, CN- etc., a DO meter for soluble O2 levels etc.

Sudhirkumar
1 year ago
Sudhirkumar 1 year ago

That’s a great insight into the chemistry of the entire process. I have read similar stuff on it. Thanks a lot. We have passed the bench scale and are now using it on the plant for some time now. It gives a great benefits with regards to the gold dissolution probably because of the ore we treat contains traces of appreciable sulphides which by analysis of extended leach tests shows. But as it will go into the tailings stream then my objective is met. The benefits are ascertained and appreciated with the ore types. I believe you would appreciate it more if your plant treats our ore type.

Maya Rothman
1 year ago
Maya Rothman 1 year ago

Cyanidation tests have shown that when orpiment and stibnite are present in synthetic gold ores best extractions are obtained at pH 10. These extractions, however, were considerably lower than those of the control tests where orpiment and stibnite were absent. Tests were run in which the effects of lead nitrate at pH 10 were investigated on the cyanidation of synthetic gold ores containing these minerals. The usual procedure was followed. The pregnant solutions were analysed for total sulphide sulphur, total sulphur, thiocyanate, and arsenic or antimony. Cyanide and lime consumptions per ton of ore were determined.

It may be noted that the addition of 0.30 lb. of Pb(N03)2 per ton of ore failed to accelerate the dissolution of gold in the presence of orpiment at pH 10; the pregnant solutions still contained appreciable amounts of total soluble sulphide. However, when the Pb(N03)2 was increased to 1.5 lb./ton, soluble sulphide disappeared and gold extractions were high, corresponding closely to the control tests. In addition, the pregnant solutions from these tests contained large amounts of thiocyanate; of the total NaCN consumed about 80 per cent could be attributed to thiocyanate formation, and practically all the sulphur in solution reported as NaCNS.

The lead nitrate performs at least two functions. First, to precipitate soluble sulphide, and secondly, to decompose thioarsenate. The precipitated lead sulphide is then oxidized to thiocyanate

The various lead compounds which might be formed, such as lead hydroxide, lead cyanide, calcium plumbite, or basic lead cyanide Pb(CN)2 . Pb(OH)2, are slightly soluble in alkaline cyanide solutions and are available for further decomposition of alkaline sulphide or thioarsenite. In those tests where lead nitrate was used and where satisfactory gold extractions were obtained the pregnant solutions contained appreciable amounts of arsenic. These solutions carried no sulphide sulphur, therefore the arsenic could not have been present as thioarsenite. Whatever its form, whether arsenite. arsenate, or some other complex salt or mixture of salts, it apparently had no retarding effect on gold dissolution.

Stibnite behaved in a manner similar to orpiment with the exception that a smaller amount of lead nitrate was required to correct the retarding effects of its decomposition products on gold dissolution. This apparently was due to the slower decomposition of stibnite and the correspondingly smaller amounts of sulphide compounds formed.


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