Sphalerite generally does not float well with xanthate collectors especially short xanthate collectors. So to be able to float sphalerite, it should be activated by a metal at a certain PH. Some of the metals that can do the job are copper and lead. At a pH between 8-9 lead can activate sphalerite to cause it flotation. So the reason is during the flotation of the two there may be lead re dissolution from galena which may activate sphalerite to cause it flotation. It may best if you are doing bulk flotation of the two.

The following are the few reason why galena is floated prior than sphalerite.

•Usually Galena is having coarse liberation size.

•Galena can't be depressed successfully.

•Galena floats at lower pH (7) and then sphalerite (9-10).

•Some of Pb-Zn beneficiation plants are having minerals in combination of Galena, sphalerite, Pyrite, Pyrotite, silica gangue, graphitic carbon, mica schist and traces of sulphide forms of silver, cadmium, bismuth and etc.

•While floating galena the other minerals are depressed with Znso4, NaCN, Na sio3, and etc in combination.

•The depressed sphalerite in galena floatation is activated by Cuso4 solution in zinc conditioner while other minerals like pyrite, pyrohotite , silica are depressed with NaCN, Nasio3, lime for increasing pH.

•Galena floats with PEX and sphalerite floats with SIPX as collectors and MIBC as common frother.

•The cleaning stages are also more for sphalerite then Galena.

•Agreed that bulk floatation will increase the recovery but the need of galena and sphalerite as separate mineral concentrate by smelters will not leave any other option for concentrators.

Short story (digest of above comments):

Sphalerite usually requires activation with copper sulphate to interact with the sulphide collectors (xanthates, dithiophosphates, etc).

To enhance selectivity, a combination of de-activator (usually sodium cyanide) and depressant (usually zinc sulphate) is used. The de-activator "leaches" the activating ions from the surface while the depressant essentially inhibits the activation reaction (Cu(II) + ZnS =>CuS + Zn(II)).

Yes your question is good. Why to float Galena first. It is the requirement of smelters. You can float both as bulk and feed for pyrometallurgical operations. But for hydrometallurgy sphalerite is roasted and leached. Hence separate floatation. It is like taking starters before drink.

Why not gravity separation first to get +70% galena concentrate? This will save lots of chemicals, and with a small machine, you can get a capacity of 8-12t/h.

Here I have a video of galena separation, if anyone interested.

While so-called natural flotation does occur to some extent for most sulphides, by and large, there will be no significant flotation unless you add a collector (molybdenite the classic exception) and for a select group of sulphide minerals (sphalerite/marmatite the best known but also includes antimonite, pyrite, pyrrhotite and even pentlandite) flotation is significantly enhanced after activation.

The inability of sphalerite to readily float without activation is an important separation tool in the selective recovery of sulphide minerals and has been routinely exploited industrially since selective flotation was developed in the early 20th century (Broken Hill).

In passing the comments about gravity recovery techniques are worth pursing as well as noting that galena is over ground in milling circuits (SG effect in the classifiers - a marketing guy once told me that lead flotation concentrates were not coarser than 25 microns) and the use of screens is also worth examining.

It is easier (proven) to selectively float galena by depressing sphalerite than the other way around if you are trying to make two concentrates. Bulk (or combined Pb/Zn) concentrate is the quickest way into bankruptcy because of heavy penalties imposed by smelters which becomes a real issue if your concentrate happens to contain Gold and Silver. As he says this flotation regime is probably the best understood of all because it was the origin of flotation.

Another example of why innovation is so slow in mineral processing.

Please permit me to add another example on your above last line.

Really we (Mineral processing community as a whole) took too long time to explore the principle of fastest settling rate of particle falls in between 5 to 7 % solids slurry density. This was materialised in High rate thickeners (HRT) by providing auto dilutionin feed well design very lately.

The conventional approach for Pb-Zn sulphide flotation arises from the chemistry of the mineral system: general chemistry, electrochemistry, and surface chemistry. What has been done over the years (since its discovery around a century ago) is the refinement of the approach mostly in terms of costs. The day that a multi-billion dollar Pb-Zn sulphide resource is discovered and found not amenable to the conventional approach is the day on which focused and intensive research for an innovative approach will be initiated.

The need for more efficient thickening schemes, aiming at decreasing the specific thickening area (m2 per tonne per hour), has arisen from the drive to reduce costs and stay in business. As labour is a component of the operating costs of a mine-mill facility, the obvious efficiency driven solution has been to increase throughput via larger equipment along with inclusion of instrumentation and process control to reduce the intensity of human intervention per tonne per hour processed. Since the capital costs of a thickener (and cost of the building surrounding it or roof above it) are essentially dictated by the settling rate of the material, the obvious efficiency driven solutions have been to use chemical principles giving rise to the use of flocculants (along with pH control) and physical principles giving rise to the high efficiency thickener design which dilutes the feed material to the pulp density yielding the highest settling rate.

In short - innovation in mineral processing has arisen from challenges encountered and needs related to the development of mineral resources. In many cases, innovative solutions developed for a specific mine-mill site were largely ignored by the industry as a whole until driven to react to changing market conditions (e.g. column flotation).

Your remarks about the perception that innovation is so slow in mineral processing were valuable ones and deserved to be written and read. I believe that we need to go further than just mention the slowness - we need to find a convincing way for mine executives to allow innovation take flight when it is appropriate.

As for Caribou - it is a decent deposit. I believe a technological alternative to ultrafine grinding for lead zinc flotation and separation was investigated decades ago when Brunswick Mining & Smelting and Heath Steele Mines were struggling with flotation selectivity at regrind P80s of 35 um. I don't recall all the details but it was hydrometallurgically based and had been developed at the Research & Productivity Council in New Brunswick. Whether it would be cost effective (capital and operating) is a question not easily answered. I believe the Imperial Smelting Process can take a bulk Pb-Zn concentrate. However, facilities using this process were far away from New Brunswick and shipping costs become significant. At the end of the day, and no matter how old or new a technology is, costs remain the determining factor.

In response to the comment about the lack of adoption of new technologies by our industry, it is fair to say that this lies mainly at the feet of the bankers and financiers. Unfortunately for the technologists, mining projects are principally about making money, and since the company proposing the project generally doesn't have the money, or chooses to share the 'risk' by involving other investors, risk comes into play.

Bankers and investors are not willing to lose money, indeed they expect to make money, so during the inevitable due diligence, the nature of risk is a major issue and how it may be mitigated is a major theme.

Unless an 'innovative' flow sheet has been proven on the bench scale, pilot plant scale (continuous, with recycle streams and intermediate product management as well as reliable measure of metal recoveries, product grades as a function of feed grade are shown) and then on a larger continuous scale (demonstration plant where other issues such as engineering, design, materials of construction, safety and environmental) are convincingly demonstrated then it is unlikely that money from these sources will be attracted.

This is particularly the case for 'novel' hydrometallurgical flow sheets, where each scale of test work can introduce some real surprises that need to be resolved such as the formation of intermediate species, precipitates, etc. since our knowledge of the chemistry, kinetics and thermodynamics of these systems is so limited outside the copper and gold systems.

One of the best examples of the development of 'innovative' hydrometallurgical was the 'borate' leaching/EW flow sheet undertaken by Doe Run to replace lead and zinc smelting requiring a significant amount of research, testing, time (many years) and money (deep pockets).

So bankers and financiers prefer to see the words 'conventional' or 'proven' and become very nervous when the words 'innovative' or 'novel' are used.

Well said about technological risks with innovative processes.

Some other risk factors about innovative processes are the incidental higher inaccuracy on capital and operating costs estimates, schedule (time to build, commission and reach name plate throughput), design criteria on piping and material selection for long life, and scale-up parameters from laboratory/pilot plant to full plant. Also, where one can find experienced operators with in depth knowledge of the process it-self and the equipment used?

Incidentally, a likely consequence of the poor track record of hydrometallurgical processes build in the last decade to reach their business objectives (at the costs and within the schedule initially estimated) will be much higher hurdles for any innovative process in the mining and metallurgical industry to be financed by banks and investors.

What if the content of sphalerite is much higher than galena like say ore with 2% galena and 30% sphalerite? Should we still have to depress sphalerite first? What do you think is the best way to treat this ore? It also contains Cu 0.5%, Au 9.8 ppm, Ag 17 ppm.

The onward technology for desulphurisation or oxidation of spharalite concentrate was with fluidised bed calciner at 950-1100 Degree Celsius. The increase in galena content in zinc concentrate and in combination of spharalite, pyrite and pyrohotite (Pb-Zn-Fe) results in the formation of hard nodule which were un-reactive in the above temperature range and certainly these modules can’t be in fluidised state because of higher size and higher density.

In floatation Silver recovery used to reduce with increase in Na CN and to some extent can be taken care with the reduction in slurry density.

I see that there is no need to reiterate the mineralogical, metallurgical or surface chemistry underpinning differential flotation of galena and sphalerite, but you may be interested to know that the traditional galena float with sphalerite depression followed by sphalerite activation and flotation is not the only approach in use. The Zinkgruvan mine in Sweden uses bulk flotation of galena and sphalerite followed by depression of the sphalerite to produce separate galena and sphalerite concentrates; however, it is still a variation on the traditional approach in that after the bulk float, it is still sphalerite that is depressed first.

The relative galena and sphalerite contents do not change the surface chemistry and tend only to have an impact in cases where the galena content is higher than the sphalerite content, e.g. at Cannington where if the lead feed grade relative to zinc is too high, then too much of the faster floating zinc with lower iron content is recovered to the lead concentrate, making zinc target concentrate grade difficult to achieve.

In the case of your project, differential flotation to produce separate copper, lead and zinc concentrates would be the likely first choice of flow sheet. Provided that you can make saleable grades in all three products at reasonable recoveries, payment terms and markets are better for individual concentrates than bulk concentrates, payable gold and silver in copper concentrate is better than the other concentrates and payable silver in lead concentrate is better than in zinc concentrate.

Your suggestions have merit.

If investigating flash flotation routine for copper, you should play close attention to the REDOX at which the collector is added. Even with a thionocarbamate collector, there is a threshold REDOX below which it will not interact with the copper minerals. This will be important for scalability from the laboratory to commercial plant.

With respect to Pb-Zn selectivity, the froth washing technology, whether with flotation columns, or as a retrofit to mechanical cells, will likely be required.

With respect to floating sphalerite first than galena, success will likely be dependent on the pulp chemistry and the susceptibility of the galena to oxidation. The more susceptible to oxidation the galena in the feed is, the more difficult it will be to recover it as the last valuable mineral.

Finally, not much is said about the iron sulphide gangue in your query. Whether it is pyrite, pyrrhotite, or a combinationof both will impact the path to selective flotation of the values.

Your quoting Zinc gruvan, Sweden process, bulk floatation followed by galena - sphalerite conventional floatation was good example of simple and lucid mineral processing practise. A different approach, interesting!

Please let me know the leverages or advantages achieved in reduction in OPEX ( reagents and power consumptions) as well as capex ( i mean reduction in cell volume or residence time, shed area if any) if possible.

I have been to Sala once and seen silver mines, but couldn't see zinc gruvan.