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Treat precipitation reactions as reactive crystallizations

By Tim Frank, Wayne Fort and Chris Jones, The Dow Chemical Company

ChemicalProcessing.com

Crystallization concepts can provide crucial insights for improving precipitate filterability.

In disposing of phosphoric acid waste, simply neutralizing the waste with caustic and sending it to a wastewater treatment plant (WWTP) often is not an option because the resulting phosphate salts are soluble in water and the total amount of phosphate that can be discharged from the WWTP is strictly controlled for environmental reasons. Incineration methods are sometimes used, but they can be expensive and generally do not allow recovery of the phosphate content.

A number of years ago we implemented an alternative method involving reaction of phosphoric acid with hydrated or “slaked” lime (Ca(OH)₂ particles slurried in water) to precipitate calcium phosphate solids. The processing scheme involved adding slaked lime to a batch of phosphoric acid in a stirred tank, and filtration of calcium phosphate solids from the resulting slurry; we opted for batch operation as it fit well into the specialty chemicals plant where the phosphoric acid waste was generated. Because calcium phosphate solids are only sparingly soluble in water at neutral pH, the filtrate from this process could be sent to the WWTP. The low product solubility not only allowed removal of more than 95% of the phosphate present in the feed, but also made the task of building large, filterable particles challenging. This article explains why and illustrates the importance of understanding and controlling solute supersaturation levels in a crystallization operation.

Chemistry and product solubility
The mix of calcium phosphate reaction products generated depends upon the ion speciation — that is, the type of ions present in solution, whether Ca²⁺, H₂PO₄^-, HPO₄^2-, or PO₄^3-, among others. This depends upon pH and also upon whether the products of the various possible calcium phosphate reactions are fairly soluble and able to undergo further reaction in solution, or only sparingly soluble. A review of the literature [1, 2] indicates that hydroxyapatite, Ca₁₀ (PO₄)₆(OH)₂, is the primary product obtained at alkaline conditions, while dicalcium phosphate dihydrate, CaHPO₄•2H₂O, is the primary product at pH 2-4 and temperatures below 80°C. We focused our efforts on crystallizing CaHPO₄•2H₂O.