Thursday, 12 July 2007

ashes to soda ash

Bang goes my hypothetical reed bed plans... soda ash is toxic to aquatic life! Pretty obvious really, considering its multiple uses as cleaner. According to the Fine Chem Trading's MSDS the toxicity comes from "a damaging action due to shift in pH". So all dye rinse water that contains soda ash fix must go down the mains drain for treatment and not be used for watering the garden (OK, mine isn't an aquatic garden but it does rain-a-lot in Cornwall and, well, it might eventually filter through to the village stream.) Also, here on Bodmin Moor the soil is acidic - Ansac's MSDS indicates that soda ash can react violently with strong acids with, uh oh, none other than carbon dioxide (and heat) evolving. Maybe the moor isn't acidic enough for that, but I'll stick to the mains!

Na2CO3. Sodium carbonate. It was the chemistry that got me into researching soda ash. More specifically the carbon aspect. What does happen to it – errr, it doesn't go into the atmosphere hopefully? And, well, where does soda ash come from? Is it natural-ish or entirely man-made?

Before soda ash, potash (potassium carbonate) was the alkali equivalent. It was extracted from wood fire ash with water; later soda ash was found in the ashes of certain salt marsh plants, also in kelp ashes. This is really exciting news - I could make my own soda ash, maybe with strandline kelp! Ashes from a beach barbie kelp fire!

But, as it seems with everything, more questions emerge. Would producing my own kelp-derived (soda) ashes release more carbon into the environment than the commercially produced ones? Would the amount of strandline kelp I'd need deplete this mini-marine ecosystem faster than it could be replenished? Would the amount of calorific energy I'd need to get to (probably) Crackington Haven to gather kelp and stoke a fire outweigh the oil-based energy required to bulk manufacture and ship the soda ash from factory to my house via the supplier?

I'm not going to try to answer these! Just keep in mind that soda ash can be derived from kelp ashes. As an aside, I was once told Procion dyes' chemical structure is derived from kelp's.

Anyway plant-ash derived soda ash soon couldn't keep up with C17 European demand (or plant "replacement" couldn't?), and physicists began researching artificial production. Frenchman Fresnel discovered in 1811 that carbon dioxide bubbled through brine containing ammonia created sodium bicarbonate (NaHCO3) and from this sodium carbonate is derived. The process was commercialised in 1864 by Belgian Solvay. Using ammonia doesn't sound too pleasant but most is recovered and recycled. Carbon dioxide and water are also are recoverable and re-used. Calcium chloride, the main by-product, is sold as road salt (hold on... I thought mined rock salt provided that), and the only raw inputs are salt, limestone and some form of thermal energy.

The process involved:

CaCO3 goes to CO2 + CaO
calcium carbonate goes to carbon dioxide + calcium oxide

NaCl + CO2 + NH3 + H2O goes to NaHCO3 + NH4Cl
sodium chloride + carbon dioxide + ammonia + water goes to sodium bicarbonate plus ammonium chloride

2 NaHCO3 goes to Na2CO3 + H2O +CO2
2 sodium bicarbonate goes to sodium carbonate + water + carbon dioxide

2 NH4Cl + CaO goes to 2 NH3 + CaCl2 + H2O
2 ammonium chloride + calcium oxide goes to 2 ammonia + calcium chloride + water

The Solvay Process Wiki explains this chemistry more fully, and Solvay SA's soda ash website informs about all stages in the process. For instance, their salt is extracted from underground salt mines through solution mining; coke is used to fire the kilns that decompose mined limestone into lime and carbon dioxide; ammonia and carbon dioxide are recovered for re-use in a distillation process...

The Solvay process predominates production worldwide, except in North America where trona-derived production has replaced it - mainly because the production process is cheaper. Trona is a naturally-occurring mineral found in non-marine evaporated deposits. The US has the world's largest resource in Wyoming, with enough trona to supply the world for 1300 years. Apparently. A batik artist wouldn't use much of that but it's still a finite global resource if mined faster than replaced. The Wyoming trona was laid down 50 million years ago as a large lake in the Green River Basin evaporated.

Trona is Na3HCO3CO3.2H2O and needs a bit of work to bring it down to Na2CO3. The Solvay SA website explains the trona process well, clearly demonstrating the simpler procedure compared to the Solvay process.

I have to admit to being a bit bamboozled by all this chemistry... and what it means for me as a simple artist so am going to post this now and come back when I've dug and thought deeper.

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