Froth Flotation (Sulphide & Oxide)

Froth Flotation (Sulphide & Oxide) 2017-04-04T06:57:31+00:00
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Carbon Dioxide for pH Control in Mineral Processing (12 replies)

Tony Verdeschi
1 year ago
Tony Verdeschi 1 year ago

I saw a really interesting presentation at the SME annual conference today (yes, I am a lead-zinc miner at heart so I am bias). To me it highlighted how really good science coupled with a bit of outside-the-box engineering can lead to better ways of doing things. I recommend that you read the paper if you can get a copy.

Carbon Dioxide for pH Control in Mineral Processing

D. Tahija; Hecla Greens Creek Mining, Juneau, AK

Carbonic acid prepared on-site from carbon dioxide gas is widely used in water treatment and increasingly in other industrial applications. It has not seen extensive use in mineral processing but is an attractive alternative to sulphuric acid for pH control, with particular interest in the need to counteract high flotation circuit pH levels caused by cemented backfill. The fundamental chemistry of carbonate solutions as applied to flotation and flocculation circuits is described as well as plant-scale applications. Planned use of carbon dioxide for pH control at Hecla’s Greens Creek mill will be outlined.

Obersturmbann
1 year ago
Obersturmbann 1 year ago

Sorry to disappoint you but the Russians were testing gases in flotation in the 1950s and carbon dioxide was one of them.

We had a look at carbon dioxide in our research group at BOC Gases in the 1990s and confirmed some advantages the Russians reported. Has a good buffering capacity which is important where a fixed pH is required (e.g. certain non-sulphide flotation separations).

Soda ash is often used in the flotation of non-sulphides such as scheelite.

If you have a source of carbon dioxide (e.g. nearby gas well or off-gas from a power generator) would be cheaper than using soda ash.

But great to hear about a gaseous application - at least carbon dioxide doesn't load the solution with cations such as sodium.

Tony Verdeschi
1 year ago
Tony Verdeschi 1 year ago

I am aware that this isn't a new thing, the presenter from Hecla also pointed this out and acknowledged that the concept was pioneered a number of years ago > specifically referenced the work in Australia.

I'm not sure about the delivery method that was used in the past but they way these guys are using a venturi-style arrangement to get the gas into solution and then will dose the solution to the floats which is going to be interesting. Did BOC do this or direct sparge?

Zander Barcalow
1 year ago
Zander Barcalow 1 year ago

Was there any mention about safety concerns with over dosing of CO2?

Obersturmbann
1 year ago
Obersturmbann 1 year ago

Only 350mm over two days so not too bad. As always with dissolving gases, it is all about surface area and making small bubbles and minimising coalescence - however with a highly soluble gas such as carbon dioxide the addition methods can be a lot cruder than say with the less soluble gases such as oxygen and nitrogen.

A venturi is an excellent method to dose a soluble gas and control on pH. I also meant add in my previous comments that soda ash is commonly used in nickel sulphide flotation.

A great way to deal with carbon dioxide emissions!

Amar
1 year ago
Amar 1 year ago

Good R&D and repeated research on such rare technologies need to be commercialized.

This will reduce Carbon foot prints. Congratulations to all who tried CO2 in flotation.

Tony Verdeschi
1 year ago
Tony Verdeschi 1 year ago

I don't recall any specific mention in the presentation (and didn't see any in the paper) as the focus was more on the chemistry. The paper does note that simply sparing CO2 into the slurry would be quiet inefficient with a lot vented off (I'd imagine this would be a safety concerned in an enclosed area). It was emphasized that the carbon dioxide dosing system (storage and mixing into water) was an existing technology supplied by a specialist group who deal with CO2 systems > so I'd imagine these issues have been carefully considered and addressed.

I did meet one of the guys from the CO2 equipment specialists, I can't recall the company name at present, though they were based in Georgia, and used this setup commonly in water treatment plants.

Ace Levy
1 year ago
Ace Levy 1 year ago

You should take a look at the use of CO2 for pH control in Cu-Mo separation. Helps avoid risk of dangerous lower pH excursions when using NaHS.

Obersturmbann
1 year ago
Obersturmbann 1 year ago

The only problem is that it may well cause this 'dangerous lower pH excursion' - the pH becomes more acidic with increasing dissolved CO2 concentration, which is not good for NaHS which becomes gaseous H2S in increasing quantities as the pH falls below 5.

Besides the toxicity (gaseous H2S), the amount of S2-, which is the effective species that displaces the collector from the copper sulphide surfaces, decreases to vanishingly ineffective quantities.

Ace Levy
1 year ago
Ace Levy 1 year ago

The target pH for the Cu-Mo separation I mentioned is in the range of 8.5.

Coming from the bulk (collective) flotation of Cu minerals and moly at pH 10+, the idea is to lower pH which can greatly improve the separation performance of moly from copper minerals, etc.

Using CO2 instead of H2SO4 reduces the risk of low pH excursions with results you mention. This has been commercially practiced for some time. Happy to provide additional details.

Obersturmbann
1 year ago
Obersturmbann 1 year ago

Not being used as the primary pH control (which would be lime of course during the roughing-scavenging stages). CO2 would be a very useful way to control the pH in this range.

It is much easier and generally much safer to handle gases than reagents like sulphuric acid.

I would appreciate some additional details - since at pH 8.5, the primary speciation is HS-, which does not as readily displace adsorbed collector as S2- - however obviously does so satisfactorily in practice.

Helena Russell
1 year ago
Helena Russell 1 year ago

My comments sorry for the delay!

Using lime and then CO2 is possible getting scaling issues (carbonates) and there are not too much problems using sulphuric acid. I agree is better handling CO2 than sulphuric.

Also, The CO2 addition is too much better in conditioner tanks than pipelines. The H2S begins to 9.0 pH or lower. I have to much experience in safety-production in Cu-Mo separation. I have the equilibrium diagram for H2S and HS- vs. pH

Sturmbann
1 year ago
Sturmbann 1 year ago

Thanks for your generous comments on my SME presentation. I only saw this discussion a couple of days ago so my apologies for not responding sooner.

SME informs me that they captured the session my presentation was in and plan to post the session, among others, for general viewing by SME membership. I'll probably also post the paper and my slide deck on my personal website at some point.

We got the system installed and began limited operation with it a couple of weeks ago. We are still just using it as a replacement for sulphuric acid, until we are confident that all our operators are comfortable with its use. In the weeks to come we will begin exploring lower pH targets.

To date, carbon dioxide has worked well for us and certainly performs better than sulphuric acid. We can use as much of it as we need and its pH control effects are both more consistent and quicker than those of sulphuric. We have seen some process effects described in my paper and presentation but it would be premature to comment on them yet.

Our extensive system of carbon dioxide monitors has not shown any indoor atmospheric concentrations approaching our low-alarm level (8 hours PEL) let alone our high-alarm point. Like most mill buildings, our is well ventilated to get rid of heat from the various equipment motors so any process off-gas finds its way outside right away. We used sniffers to identify leaks for repair prior to start-up so the piping and mixing panels are not losing any.

We will likely put together another paper/presentation or two, reporting on operational results, over the next several months. One at the 2016 SME meeting in Phoenix is most likely, with another possible at the CMP in Ottawa.

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