Recycling Phosphoric acid based etchants
Crystallization recovers valuable raw materials
Orthophosphoric acid in water is the common form for most solutions
labeled phosphoric acid. Phosphoric acid is used in the semiconductor industry,
the flat panel display industry (LCD screens) and in other electronics
industries as etchant medium in the production of photoresist treated circuit
plates. The circuits are becoming increasingly smaller and the circuitry
remaining on the etched panels is typically measured in nanometers (1,000,000
nm = 1mm). In these applications, the presence of minute amounts of impurities
can significantly impact the quality of the produced chips and circuit boards.
High purity phosphoric acid is required for various other uses, as
well: in food products (additive E338) it provides a sharp tangy flavor and is
far less expensive than natural citric acid, dental use to etch tooth surfaces
to improve adhesion of bridges and fillings and even cosmetics to control pH in
skin care products.
Phosphoric acid based etchants are used
specifically in the removal of aluminum, aluminum based alloys and silicon
nitride (Si3N4)
During the etching process a significant amount of
impurities will be dissolved into the acid etchant solution (typically a
combination of stronger acids and buffers). The spent etchant from such
applications contains the periodic table of various elements including: Na, Mg,
Al, Si, K, Ca, Cr, Mn, Fe, Ni, Cu, Zn, Ag, As, Sr, Mo, Cd, Sn, Sb, Au, Pb and
Bi. A significant amount of acid has to be discharged from such process and
replaced by fresh acid in order to avoid accumulation of these impurities.
Since the tolerable levels are remarkably low and high purity acid is
relatively expensive it is therefore economically and ecologically attractive
to recover such spent acid.
Distillation is not effective in the
separation of these impurities and therefore the spent etchants have been
typically used for fertilizer or animal feed additives. Crystallization has
been shown to be an efficient method to recover the phosphoric acid spent
etchants. Operating with purification ratios of 100-300 per step purification
can be completed in a single step depending on the required use and feed stock.
GEA Messo PT can provide systems and engineering for the recovery
of the raw acids from spent etchants.