Published
by Tamsyn Grewar, Senior Scientist at Mintek
We're looking for ways of cleaning up acid mine drainage, one of the issues we are facing is that our process produces sulphides that need to be removed downstream.
Does anyone know of a relatively cheap/simple way of removing sulphides from water?
We don't necessarily want to add expensive oxidants and the chemistry seems a bit complicated as the sulphides seem to revert to sulphate under certain conditions which would then mean we need to remove the sulphates too.
We have experience in South Africa with our system which can deliver the solution we believe you seek. Please see the attached Brief and our website at www.earthsustainingsciences.com
Good day, Tamsyn. If you have the full details, ie. volumes to be treated, standard you want met after treatment and so forth, please contact me on james@nuwaterglobal.com I can run a complete mass balance for you and advise exactly whats needed. Do you have budget allocated for this project as yet? NuWater has extensive mining experience treating such streams as you have to either discharge limits specification or even for re use in process. Let me know. Contact number is +27 82 803 1010
If you SO3 concentration is less than 208 mg/lit then oxidising them to SO4 gives you no problem as the limit for SO4 is 250 mg/lit. Oxidising is the only clean and sure way of getting rid of SO3 without any undesired by-products.
Of course it all depends on your water flowrate.
There is a wide choice of oxidants but I would personally use ozone as any residual O3 reverts naturally to O2 within a certain time limit.
Hi Wayne, so is CHEMISTRY outmoded ???? There is a lot of discussion about biotechnology lately so it has become like the cell phones, everybody has got one !!!!!
We use specific microbes capable converting sulfides into elemental sulfur. Our main application is in sewer lines and wastewater treatment plant to prevent H2S formation but I am sure these would also could help you if operating conditions would be favorable for bacterial development. Sent me an e-mail on r.wagenveld@qmes.nl and I can forward additional info.
There is a nice, cheap and natural way to do it, using naturally occurring sulphur oxidising bacteria (SOB). I have attached a link to a paper on the subject.
It basically just uses long lengths of pipe with a bio-film and uses no energy and was very effective. In fact it was the Global Winner for the Design Projects Category of the 2014 International Water Association (IWA) Global Project Innovation Awards Competition with the award presented in Lisbon. I know the design engineer very well and can put you in touch if interested.
Hi Tamsyn - ozone is an excellent method of sulphide removal/reduction. Made on site, natural treatment, provides additional treatment as well. We have used it on large and small scale water/WW issues with great success. Happy to chat further
The biological oxidation of hydrogen sulphide by aerobic Thiobacillus -like bacteria has been described. The hydrogen sulphide is oxidised into sulphur particles which are in the submicron range. The colloidal properties of these sulphur particles are compared with those of a standard LaMer sulphur sol. The biologically produced sulphur particles are composed of a core of elemental sulphur covered by a layer of natural charged polymers, presumably proteins. The polymer layer renders the particles hydrophilic. Colloidal stability can be attributed mainly to steric repulsion. Although the electrokinetic charge is always negative with varying pH, the point of zero charge is found at pH 5.8. This indicates that the polymeric molecules are oriented with their negative charges to the bulk solution. An expanded-bed reactor was developed in order to stimulate the aggregation of the sulphur particles into large, well-settleable sulphur flocs with a diameter of about 3 mm.
Rakesh, only Group 1 & 2 metals form soluble sulfides (probably do not occur in acid mine drainage), the rest are insoluble. I am assuming that the sulfide moiety we are mainly dealing with is dissolved H2S. This can be precipitated or immobilized as Iron sulfide or free sulfur as long as the acidity has been neutralized with lime or limestone. The acid insoluble "heavy metal sulfides" are valuable & maybe worth recovery. I do not think it is a good idea to degas as H2S is so toxic.
Under the right conditions, you can precipitate native sulfur which will need to be periodically harvested or it could revert back to sulfate. See the Eh-pH diagram for sulfur. However, as suggested below, iron can be used to sequester sulfur as a metal monosulfide. The problem is, treatment with this method in perpetuity is unsustainable. Have you looked at bactericides for ARD source control? The method was pioneered 30 years ago and has sadly fallen off our remediation radar. I'm trying to reverse that trend. Good luck.
Metal sulfide are insoluble, so I presume that you are using sulfide to precipitate the metals and now you have dissolved sulfide left in the water. What is the pH since sulfide can be neutral as dissolved hydrogen sulfide gas or as sulfide ion?
if the pH is low, which means that most of the sulfide is neutral, then we can degas the hydrogen sulfide gas
You would have to replace the iron ore every so often. The iron sulfide could be regenerated by roasting but the SO2 + SO3 generated would have to be trapped as H2SO4 or CaSO4. Another way would be to add sodium bisulfite: SO2 + 2H2S = 2H2O + 2S
You have limited choices, you can precipitate it (lime or some other chemistry) or you can oxidize it and filter it (more chemicals). All removal options produce a waste that needs to be dealt with. If you look into IX or electro/capacitive DI you still have a waste stream to deal with with a concentrated mineral content. The IX/Cap-DI will also give you a bunch of other inorganic components to deal with as will precipitation. No easy answer.
The efficiency of biological processes using organisms (bacteria) depends on local circumstances. This means you would have to control and monitor (24x7) environmental circumstances, such as temperature, pH, pollutants. If you are using "ACID mine drainage" water, bacteria may be unsuitable.
A more expensive, but more RELIABLE method, could be using reversed osmosis. This does require a lot of energy (electricity) and it gives you pure water (other salts are also removed). It does not need much space, even for large capacity. Both small and large devices are available. In Holland these machines are used in commercial horticulture to remove surplus salts from water, so the water can be recycled for growing plants. To save budget consider buying a second hand reversed osmosis system. Contact lenntech.nl for more info.
Note: The amount of sulphates on earth is limited; it is being overexploited. Therefore, sulphates may offer you a source of income (in future). This may help you to develop a solid financial plan.