From my understanding of the depression mechanism of NaHS it is a problem predominately for sulphide minerals (in mixed floats) not sulphurdized particles. I may be wrong here (and I'm happy to be corrected). Knowing your deposits (at least the ones named after navigators) well I'm pretty confident you are not going to have that problem.

Excessive addition of sulphide salts has been shown to depress both galena and similarly sulphidizedcerussite. The depression response is strongly affected by the sulphide dosage, and when aqueous residual sulphide concentration becomes significant, pulp

potential (Eh) will drop and both cerussite and galena will be depressed. The flotation of cerussite is best performed following sufficient preconditioning to precipitate dissolved lead as lead sulphide and to sulphidize the mineral surface prior to collector addition. However, over sulphidizedcerussite will significantly reduce flotation response. Dosage levels of NaHS for galena flotation are significantly lower.

Thanks for your response. Yes the deposits contain mainly cerussite with minor lead sulphides. I wasn't sure if excess NaHS actually depresses cerussite but we have noticed in the past that when there was too much, the con grade dropped off. At the moment we can only roughly tell based on a visuals and an ORP reading - I don't know if there is a better way of measuring residual NaHS.

Don’t know what pH you are operating at but your nose is a pretty good indicator of too much NaSH. We have touched on this before. Eh is a good indicator for NaSH addition and is relatively simple to measure but other factors affect Eh. Es is a better indicator for NaSH addition but much more difficult to measure accurately on line.

Depression of sulphide surfaces (natural or sulphidised) by either sulphide or hydrosulphide ions is due to displacement of the collector from the mineral surfaces (this was well known in the 1950's - refer to Sutherland and Wark's classic book "Principles of Floatation", page 137 onwards, Figure IX-14).

To that end, during sulphidisation the addition of the sulphidiser is carefully controlled (e.g. Controlled Potential Sulphidisation which employs an ion selective electrode, namely the sulphide ion probe [Es]) based on test work where the range of Es values has been determined and correlated to ORP values (more practical way of measuring sulphide/hydrosulphide ion concentrations in the plant).

Additionally, strong collectors are used for sulphidised minerals such as PAX.

Sulphidiser, predominantly sulphide ions at the pH used (circa 12), is routinely used to separate chalcopyrite frommolybdenite; it is quite difficult to depress most base metal sulphides once they have contacted collector and chalcopyrite is probably the hardest. Fortunately high quantities of sulphidiser at a high pHs works quite well.

ORP measurements are used to control the addition rate and thus set point of this separation.

It is interesting to hear that NaSH is required to float galena; if the surfaces are oxidised only a very small amount would be required and the role of the sulphidiser would be to the clean the 'oxides' from the surfaces rather than actually sulphidise.

In summary, sulphidisers have three roles in flotation, which are totally dependent upon the concentration; in low concentrations they clean 'sulphide surfaces' [species charge effect], in moderate to high concentrations they sulphidise surface oxidised sulphide minerals and so called 'oxide' base metal minerals [chemical conversions of 'oxides species' (e.g. oxide, carbonates, sulphates, phosphates, some silicates, hydrated variations, etc.) to sulphide] and in high to very high concentrations they depress basemetal sulphide minerals [collector competition].

Thanks, beautiful and comprehensive answer.

A question that arises in my mind is about this part of your comment "It is interesting to hear that NaSH is required to float galena; if the surfaces are oxidized only a very small amount would be required and the role of the sulphidiser would be to the clean the 'oxides' from the surfaces rather than actually sulphidise"

As you suggested before to me about this Sulphidising reagents like NaHS to removing complexes from pulp in milling stage, we do it by addition 50gr/tons sodium sulphide in pump to circulate in milling. But again the amount of zinc in lead didn't reduce and again we have 20% sphalerite in galena concentrate.

NaHS converts oxide minerals to sulfides, but can also depress sulfides. Thus, NaHS additions have to be controlled. Jim Woodcock and Mike Jones of CSIRO in Australia devised a system called "Controlled Potential Sulfidisation (CPS) in which the sulfide concentration in solution is controlled with an electrode inserted in the pulp. This system was marketed by AMDEL in Australia.

Just as a passing comment, I presume that the sphalerite is liberated. If this is the case, some form of activation must be occurring, although the addition of NaSH to the milling should precipitate the potentially activating base metal ions.

Which does seem to suggest that it is a composite problem? Otherwise there is something we have overlooked.