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Doubts about Sodium Metabisulfite and its reaction with water

Doubts about Sodium Metabisulfite and its reaction with water

Doubts about Sodium Metabisulfite and its reaction with water

Hi everyone,

I'm currently working on a mining project for a gold mine that involves the usage of Sodium Metabisulfite.

They will mix Sodium Metabisulfite with water to create a first solution at 30% in a make up tank (preparation tank + mixer/stirrer) and after that will be deposited in a storage tank of 12 hours autonomy (it's a batch system), this solution will be dosing to the process not before pass through a post-dilution system (static mixer + water) that will make final solution has 10% of concentration.

The enviromental conditions are:

Humidity is around 70-90%
Temperature range: 1°C - 15°c
Altitude: 4000 m.a.l.s

My doubts are relating of how to handle properly this chemical powder and its solution.

NOTE: This is the basic scheme of the process

Sodium Metabisulfite contains 65% of SO2 available.

When Sodium MBS get in contact with air becomes oxidize (loss part of its SO2 available)
Due to its hygroscopic properties this powder absorve the moisture contains in the air quickly.

The water they will use is demineralized.

The reaction of Sodium MBS and water is rule by the following:

Na2S2O5 + H2O ------> 2NaHSO3

NaHSO3 = Sodium Bisulfite.

However after reading about this chemical it looks like a certain amount of SO2 is released.
The amount of SO2 released depends on temperature, ph, and the % concentration.

So my questions are:

- How hygroscopic is this powder?
- The oxidation process when SMBS get in contact with air, how fast is it?
- Sodium MBS + demineralized water release SO2?
- How much amount of this gas is released?
- How can I control the releasing of SO2?
- Control the Ph level and the solution temperature would prevent detachment of SO2?
- In order to avoid oxidation I read that inject dry air or N2 to the powder before mixing with water would help, but how can I know the amount of Dry air/N2 has to be injected?
- I also read that at low temperatures (just like this case), sodium metabisulfite forms hydrates with 6 and 7 moles of water. Canthis affect the reactivity of the powder?
Look this figure for reference

Sodium Bisulfite solution at 30%

This will be contained in a closed tank

- This can also release SO2?
- If Sodium Bisulfite solution get in contact with O2 becomes oxidize (to sodium sulfate), but how fast is this oxidation?
- If there a way to eliminate the O2 inside of the tank?
- What would be the best way to evacuate the produced gas outside of the tank?
- Due to I don't know how much gas will be given off, do you considere a scrubber is required?
- Is there another way to extract this gas from the tank?

I have more questions but these ones are the most important.

It will be very helpful for me if someone can clear my doubts up.

Thanks in advance.

RE: Doubts about Sodium Metabisulfite and its reaction with water

low Ph will favor the formation of SO2.
Remember that it isn't S2O5, it is SO2 + SO3.
Purge with nitrogen to keep the moisture low (dew point), I would expect that you want dew point below 0C (though I don't have a reference for this).
This is very hygroscopic and the reaction will be fast, almost instantaneous. It wants to joint with 5 or 6 moles of water to form the stable hydride.
At the same time keep the pH elevated.
There should be plenty of references since this chemical is used in food sanitation and for dechlorination of treated water.

= = = = = = = = = = = = = = = = = = = =
Plymouth Tube

RE: Doubts about Sodium Metabisulfite and its reaction with water


We are india based Sodium Metabisulphite and Sodium bisulphite manufacturer. Basically you are making Sodium Bisulphite solution (30% concentration) by mixing SMBS and Water. Below are the answers of your questions.

(1) This material is highly hygroscopic. Due to this nature SO2 gas decomposes. Purity of SMBS goes down to 2%-5%. So, all you need is high density inner liner in bags and make sure it should be air tight. Then there won't be issue of lumps for at least 2 months.

(2) If you add SMBS in water, SO2 won't release in atmosphere. So do not worry about it.

(3) Once the solution of 30% is made, there won't be decomposition, because S02 is in SBS solution.
(4) you can also store this SBS solution in PP-FRP / HDPE Tank for a year. There will not be any purity difference. Yes, there will be odour of SO2 in solution but won't be added in the atmosphere.
(5) You can install AIR POLLUTION SYSTEM (SO2 scrubber) with Mixing tank, if you feel SO2 gas while mixing SMBS with Water.

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