Dewatering: Thickening, Filtering, CCD, Water Treatment & Tailings Disposal

Dewatering: Thickening, Filtering, CCD, Water Treatment & Tailings Disposal 2017-04-04T06:57:46+00:00
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Treatment of a low TDS acid water with Hydrated Lime (9 replies)

Ace Levy
2 years ago
Ace Levy 2 years ago

In a Acid water treatment System with hydrated lime, if the amount of Sulfates (SO4(2-)), ranges between 60 and 120 mg/L, Fe ranges between 3-5 mg/L, and Aluminum ranges 2-5 mg/L, Fe can be eliminated as Fe(OH)3, and Aluminum as Gibbsite (Al(OH)3)), but sulfates does not reach the saturation state to form Gypsum....How is possible to eliminate this amount of sulfates? Or definitively it remains in the neutralized solution?

Oberstorm
2 years ago
Oberstorm 2 years ago

Are you sure you need to remove those levels of SO4? Would be ultra filtration (such as reverse osmosis) or sulfate reduction. You could look at removal with Ba, because the Ksp of BaSO4 is so much lower than for CaSO4. But there are cost considerations and TDS to consider, and there are water-quality limits on Ba in some jurisdictions, so you have to consider the details of treatment and reagent safety. Well, you do with lime, too, of course.

Maya Rothman
2 years ago
Maya Rothman 2 years ago

We remove 97% of the Fe and Al to oxides/hydroxides by oxidation, and then remove the rest with most of the sulphates using bacteria. The sulphates are mostly converted to carbonates which are typically used to cover the acid forming rock to reduce further sulphate and acidity generation. Using this method one doesn't generally need to add lime or other chemicals (organic carbon is added for the bacteria). This process saves a huge amount of money. The level of sulphates removed can be varied. Typically we remove down to 30 to 50 mg/L to keep the hydraulic retention time (capital cost) to a minimum.

Bill Rico
2 years ago
Bill Rico 2 years ago

For low TDS water, ion exchange will have the lowest Capital cost and give the highest water recovery. There will however, be an operating cost in regenerate chemicals, but these may be no more than the power costs of membrane systems.

OberstGruppen
2 years ago
OberstGruppen 2 years ago

I'm assuming you have not heard of a novel technology called Eutectic Freeze Crystallization? You can read this paper for more information:

http://www.sciencedirect.com/science/article/pii/S0011916410006284

We are able to remove sulphates in the form of sulphate salt(s) and reduce the concentration significantly together with the volumes of the brine. The freezing process reduces the temperature and thus changes the saturation limit of the solution. Also, as the solution becomes more concentrated (by producing ice) the solution becomes more saturated with respect to salts and they crystallize out of solution. The process is also able to produce more than one pure salt by operating different eutectic temperatures.

John Koenig
2 years ago
John Koenig 2 years ago

I agree that without a bioreactor then ion exchange or treatment using Ba as mentioned are possible options. I modified your PHREEQC file to show the effect of adding BaCl2 on dissolved SO4, you can see the potential effecting.

Oberstorm
2 years ago
Oberstorm 2 years ago

How you approach the problem probably depends on some very practical matters: What is the flow rate you need to treat? Must it be continuous, or could it be managed in batch operations? What are the logistical constraints (e.g., power supply and its operational availability)? What are your target levels, and what are the consequences of not meeting them? Are there some special circumstances, such as high (or low) transportation costs for reagents and waste, or seasonal loss of transport capacity (i.e. road, cyclonic flooding).

Ace Levy
2 years ago
Ace Levy 2 years ago

In this moment this is the level of sulfates, due to that the existing tunnel where comes the acid water, is in standby (no exploitation-no exploration works). Probably when drilling and exploitation works will start again, this level of sulfates will increase and in the current treatment system (with hydrated lime) will be removed as Gypsum.

Is really of concern this level of sulfates? Which are the risks associated with this amount of sulfates in the discharge?

The trouble is that currently there is a system to treat this water with hydrated lime, so is hard to implement another process. In this point I have another question, how can I estimate the retention time to remove all Fe and Al?

Oberstorm
2 years ago
Oberstorm 2 years ago

In North America, to my knowledge, there is only one sub-jurisdiction that would be much troubled by the range you site, and so most of us would sing hymns of thanksgiving that we did not have to treat SO4. The one jurisdiction is the US state of Minnesota, where the State sets an in-stream standard of 10 mg/L to protect wild-rice (in reaches of streams where wild-rice germinates). Although I am not at all expert in the regulatory issue, I take it that a waste-load allocation approach is used to establish site-specific loading rates for sources.

You are wise to look down the road to what the potential water-quality conditions may be. Note in doing this that the often-cited value of 1470 mg SO4/L at gypsum solubility is for a system that is simplified to consider that molality and activity are equivalent (strictly, "infinite" dilution). You should assume for most ARD systems that the ionic strength will be sufficient and the presence of additional components such that the activity of free SO42- will be a fraction of the total analytical SO4, and so whereas you may be at Gypsum solubility, the observed analytical value of SO4 will be substantially higher. Especially if there is significant Mg is solution, the SO4 concentration can be far above 1500 mg/L. [Several porphyry coppers where I have worked have waste streams with >10,000 mg/L SO4 although the solutions are saturated with gypsum (and one can easily identify the newly formed gypsum in solid samples.]

Maya Rothman
2 years ago
Maya Rothman 2 years ago

Thanks for your reply. At the time of writing I was not aware it was an existing system. Yes, if you have an existing system it's hard to convince people to invest in capital expenditure. I am not sure about your country, but here in Australia the imperative to the mining industry is to reduce operating cost due to the lack of profit margin caused by the low prices. In this respect the biological system typically pays for itself in approx 2 to 3 years when it replaces lime addition, after which operating costs are significantly reduced as no imported chemicals are required.

Regarding aeration, we have found there are too many variables to consider, so the best approach is to trail the water to determine the most appropriate aeration/oxidation system.

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