Wet calcining of trona (sodium sesquicarbonate) and bicarbonate in a mixed solvent

Trona ore is used in large amounts for the production of soda ash. A key step in this process is the conversion of trona (sodium sesquicarbonate: Na₂CO₃·NaHCO₃·2H₂O) into soda (sodium carbonate anhydrate: Na₂CO₃). Currently, this conversion is done industrially by calcining of the raw ore in rotary calciners at ca. 120°C or higher (Natural Soda Ash - Occurrences, Processing, and Use, Van Nostrand Reinhold, New York, 1991, p. 267). Trona can however be converted at lower temperatures by using a "wet calcining" technique. In this technique, trona is contacted with an organic or mixed organic-aqueous solvent at a conversion temperature that depends on the water activity of the used solvent. In pure ethylene glycol this temperature can be as low as 55°C. The conversion by "wet calcining" occurs very similar to that in the regular dry calcining process via a solid phase conversion. The anhydrate crystals form directly from the solid trona. This produces pseudomorphs (J. Chem. Eng. Data 8(3) (1963) 301), i.e. agglomerates of fine anhydrate crystals (1-10 μm). At high temperatures, dense, finely pored agglomerates are formed, while the outer shape of the agglomerate retains the prism shape of the trona crystal. At low conversion temperatures, loosely packed or even unstable agglomerates are found.