Removing silicates from

[aquatrade]

Hello!
Please help us. We serve one client, which has steam generator (6 tons of steam/hour, 10 bar, 120-150C). Raw water is from borehole, then it passes through deironing filters (24", Pyrolox - Filter AG), reverse osmosis, 2-stage ion exchanging softeners and steam deaerator. Also system has antiscalent dosing before osmosis and alkaline phosphates dosing to deaerator.
Approximately three months ago our client made prophylactic service of steam generator and found out glass-like scurfs in it. Analysis showed that it was silicates. In old raw water analysis the level of silicates was very-very low, the same - in new ones.
This week we've met with them, they said, that other plants in their area had the same problem, and even in one plant their steam generator exploded due to overheating! So, we have some geological changing, which cause unstable water components with occasional silicates emissions.
So, we have 2 main problems.
1) How is it possible to remove these scurfs? They are very inert and their examples do not react with hot and cold water, weak solutions of acids and alkali. We know that silicates destroy by strong ant hot alkali solutions, but we afraid to damage the steam generator).
2) Due to water components changing, how it will be better to change water treatment system to protect the steam generator from silicates?
Additional problem is that the emissions of silicates are occasional, we can not exactly know either peak quantity of them in raw water, or longitude of emission, or even the probability and characteristics of future emissions.
Thank You
Sincerely Yours Anton S. Tyuin
Russia, Nizhny Novgorod

 

[ashtree]

When you say that silicates are very very low: How low do you mean?

You obviously have some iron present in the feed water but is there also any aluminium either naturally occurring or added in the form of a coagulant. At pHs below 7.0 the presence of very low levels of iron and aluminium have been known to cause precipitation of insoluble forms of silica even though they are below the theoretical saturation threshold.

It is most likely that operation at a different pH will improve the silica solubility enough to reduce the problem. Without knowing the chemistry of your water it would be most likely that you need to operate above 7.8 pH if you are not already doing so.

Regards
Ashtree
"Any water can be made potable if you filter it through enough money"

 

[aquatrade]

Hello, ashtree, thank You for the replying!
Here I attach the analysis (it is in Russian, so I'll try to translate it) of raw water from 21.12.2016.
And maybe You can tell, what to do with scurfs in the boiler, how to remove them and not to destroy the boiler at the same time?
1st column - the name of parameter and measure unit, 2nd - name of standart (here are the standarts for drinking, not boiling water - no matter, just miss this and last columns), 3rd - results of analysis, 4th - fluctuation of measurement, 5th - normal standart for drinking water in Russia
Smell at 20C/60C - 1/1 score
Coloration - less than 1 degree
Turbidity (kaolin) - 0.58 mg/l
Fe - 0.46 mg/l
Mn - 0.08 mg/l
pH - 6.3 mg/l
Hardness total - 7.0 mg-equiv/l
Total salts content - 398 mg/l
Chlorides - 21.0 mg/l
Sulfates - 8.6 mg/l
Nitrogen (ammonium) - 0.27 mg/l
Nitrates - less than 0.5 mg/l
Nitrites - 0.02 mg/l
COD (using potassium permanganate method) - 2.8 mg-oxygen/l
Silicates - 0.06 mg/l
Sincerely Yours
Anton S. Tyuin

 

[aquatrade]

Also, unfortunately, I haven't data about aluminium now, we'll take it.
About pH - we dose alkali phosphating solutions (I don't know exactly how to explain properly... in Russia it is sold as Aminat KO-4, solution for antiscaling and corrosion inhibiting in boilers, here is the site -

http://www.travers.su/products/indu...hemical-regimes-of-steam-boilers/aminat-ko-4/

but it is in Russian). It increases the level of pH, though we should try to increase it much more than now - 11-11.5 today's level.

 

[aquatrade]

And once more - I found English catalog with description of this chemistry - Aminat KO-4. Attached, page 21.

 

[bimr]

You need to replace the 2-stage ion exchanging softeners with demineralizers to eliminate future silica.

Have a the boiler water chemical supplier recommend a chemical program to eliminate the scrufs.

 

[aquatrade]

Thank You!
And what about anionic ion exchange module? It will cost cheaper than demineralizer. Will it remove silica? Or they are in not-ionic form?

 

[bimr]

You should be using a strong base anion unit.

Some silica is colloidal. The colloidal silica should not pass through the RO process.

 

[aquatrade]

Thank You bimr!
We thought about it. As we know from practice, colloidal something (silica, as example) quickly kills the membrane's surface and causes expensive membranes changing, so it must be removed before RO (coagulation, filtration etc.). And can You recommend any anionic exchange resin, which specializes in ionic silica removing?
Sincerely Yours
Anton S Tyuin

 

[bimr]

There are some resins target to colloidal silica.

I would prefer to coagulation and filtration to remove the colloidal silica.

Ultrafiltration is another method and is easier to operate.

 

[Jack_Aqucell]

I think what bimr said would solve the current problem and you may find out why it happened following ashtree's advice.
An ultrafiltration device could be placed before RO process, or you can place it just before the steam generator.
The difference depends on which step coagulation is utilized.