My suggestion is using mixture of Dextrin and Sodium bisulfite.

The problem of separating lead and zinc from pyrite has been around for a long time and there are many methods available to achieve an improved separation. Perhaps one of the suggestions you receive in this discussion will be successful, but the chance of success will improve if you undertake a systematic program to identify why you are getting poor separation and then investigating methods of improving the separation. Although there will be a cost, I suggest you work with a consultant who has experience of this problem.

I've worked at a zinc and lead flotation plant. Pyrite was originally depressed using cyanide, but discontinued due to similar concerns you've mentioned. We've successfully depressed pyrite using lime (increasing pH of the feed to the zinc roughers to about 10.5). Hope this helps.

Few scientists tried High Intensity Magnet Separator. HGMS of SLON, LONGI, METSO. You can try bulk flotation of Pyrite and Zinc and then go for magnet separation. This will give two products, Sulphur (Pyrite) and Zinc con. May be Pyrite con may contain both Lead and Zinc in traces recovered in Smelter during Sulphur recovery.

http://is.gd/EQrl7o using http://is.gd/gxRurB

Try an aeration step followed by sodium sulphite and DETA. After this step utilize dithiocarbamates for a collector in lieu of xanthate to achieve better selectivity.

We are a mineral processing company mainly do EPC service. We usually use CaO as the depressant for pyrite. My suggestion is same with earlier comment; this is the normal and economic method.

I agreed Cao used as depressant to depress pyrite, there are several points that can be given Cao during this process, especially in ball mill and mixer before entering the flotation cell-tank will be very effective at all and is very safe to use, may be useful.

Are you talking about a sequential flotation where you depress pyrite + sphalerite first to recover galena? Or are you talking about bulk flotation where you depress pyrite only to recover galena & sphalerite? Can you please be a bit more specific about the circuit arrangement? From what I've experienced, aeration + sodium sulphite can also have a deleterious effect on sphalerite recovery.

Also I think sphalerite is depressed with cyanide as well. If we use a sequential flotation circuit (galena then sphalerite), what would you use to depress pyrite in the flotation of sphalerite? 

Adjust cell pH to around 11 using a basic reagent and the job is done.

Have you tried depressing with Lime?? It has been proven to help.

It is frequent to use CaO in the crusher and to make aeration (approximately 20 minutes) in the cell while keeping the PH to 10.5. It would be interesting to know if you float the lead or the zinc in first. For the zinc it is possible to rise to 12.00 in pH

Quicklime is the most economical reagent for fresh mineral can to aggregate together in grinding , it serves to change the viscosity of the pulp , thus the rheology of the same and is the most effective to depresor pyrite working with pH 10 , 8 to 11.5. Now it depends on the quality and reactivity of the lime with their effect a is constant and pH not reach the plateau where it begins to exhibit high consumption.

Slaked lime solution is used in Rougher and more in cleaner, along with NaCN dosage, to depress PYRITE. It is a very common process in the world. New methods are many. Need to be studied for your ore.

Copper activates not only sphalerite but also pyrite. Optimise copper sulphate doses. There are reagents that can partly replace copper sulphate (for example Nickel sulphate). Some R&D effort is required here to select the suitable pre activator. Cyanide is very effective but use starvation amounts. Cyanide is poisonous but it decomposes very fast. Cyanide depression of pyrite is enhanced in alkaline pH. Milk of lime may be used to adjust the pH.

Remember AM, Gaudin, “Flotation reagents are effective at starvation doses”!

I like your reference to the Great Gaudin; can you please summarise now whether one should go to cyanide or copper? Can we use combination of some of theseto completely depress pyrite?

Cyanide is used in the lead circuit to depress both sphalerite and pyrite. From the lead tail we have to recover sphalerite depressing pyrite. Now we use copper sulphate to activate cyanide depressed sphalerite while doing so we must ensure that pyrite is not activated. Copper added to lead tails has to do two functions:

To remove cyanide/zinc cyanide from the sphalerite surface
Form copper layer or patches on sphalerite surface to facilitate xanthate adsorption.

The first step is pre activation and the second step is activation. If copper is replaced by other reagents to carry out pre activation then copper consumption is reduced and hence any inadvertent activation of pyrite is eliminated leading to improved separation efficiency. That is less Fe in Zinc concentrate. Lime/ Cyanidemay be added in the Zinc cleaners to prevent pyrite flotation.

Nice to get such detailed explanation; I am sure many operating engineers of complex sulphide plants would benefit from this. It also shows how challenging and the fundamental knowledge required operating a flotation plant.

Innovative technologies:

Since specific gravity of pyrite, Zinc, Lead are different we can try Innovative newly developed Spirals with wide trough and low angle to process even up to 45 micron.
HGMS can develop 20000 gauss strength in the process zone. Since Pyrite is feeble magnetic, we may try in lab model.
Today looking to global market we need to process tailing to recover 40%S pyrite to extract S in smelter and also associated Silver, Zinc, Lead. This gives added revenue and reduces smelting costing.

I have published a paper in RRL Bhubaneswar on this "pilot plant for extraction of pyrite from plant tailing.”

Bulk flotation is more economical with Column flotation technology.
Please discuss with Smelter Engineers and decide for a pilot project.
It is possible to reduce costing of Mining, Processing and Smelting if we spend more time in R&D. It is a Key to open treasure Box. 

I wonder whether we can think of spirals for the sizes we treat as flotation feed; my general observation is that we should not think of gravity/centrifugal separators for such finer sizes of feed, particularly when the ore is complex one because we will have equal settling particles and there would be misplacements.

We call this process as pre concentration. In HZL Pb, Zn ores 80% waste is dolomite. In Rampuraagucha we do two stage grinding. 1st in Rod/SAG mill. Then regrind ball mill. Before RGBM if we can separate at least 40% of ore as gangue rest can be sent to RGBM. Today we have spirals which can handle up to 45 micron particle sizes.

PYRITE DEPRESSION:

I agree with you. But as per my experience pH play a major role in flotation. Different minerals behave differently at different pH. Lead is floated at low pH. 8 or 9. In cleaner cells pH >9. Cyanide consumption will be less if pH is controlled using instruments and PLC. Pyrite floats at very low pH or gets entrapped during flotation. Column flotation will give good results for such problems. It has forth washing system where entrapped minerals are separated, to get very clean concentrate. 4 years of R&D in RDM proved to reduce silica from 7 to 3% in Zinc cleaning in column with very low pyrite.

Pyrite if found interlocked 10, 20, 30, 40, 50% then it behaves as a different mineral and pH will not work efficiently. You need to grind such particles in RGBM with lime to make them free.

Our experience:

RGBM in closed ckt will not give good results, due to more slime generation. RGBM discharge should be directly fed to roughers. 100% good results with high grade and recover. Pyrite at this stage gets depressedat >9 pH in cleaner cells.

I am happy that liberation factor is brought into discussion; theory is O.K. Let us first know the degree of liberation at each stage of comminution circuit(s). Depending on the percent liberation of the minerals under consideration, One should do cost benefit analysis if recovering liberated particles by a process i.e. flotation in this case or some other process. Even flotation of cyclone underflow by unit cells were thought of at one stage but discarded because of reasons not known to me. All this makes mineral processing very interesting and exciting; at each stage we have to keep economics also in our mind some may be technically feasiblebut is it economically viable?

Cost benefit analysis for liberating interlocked pyrite:

Yes it was economical; by consuming few KW we could upgrade ore by additional 2%. From 50 to 52%Zn, and recovery by 1% which is highly economical and cost benefiting process!

Underflow in unit cell:

Yes we did both lab and commercial scale testing. During that period we were not aware of controlling instruments. Today we can try again. Problem was underflow was getting jammed frequently. Otherwise it gave very good results. The name of the cell was called as FLASH FLOTATION.
Today mineral engineers left company and very few are left out. New engineers require few more years to get experience for R&D. There is lot of scope. From 1998 to 2003 HZL had good team of mineral engineers. We could develop many new and innovative technologies to operate plant without shut down and give more recovery and grade.
Today due to instrumentation and more facilities R&D is easy and successful.
We can take few projects for R&D with HZL and give support from outside to HZL engineers.
We can make a team and discuss with HZL.