Plant for the purification of spent phosphoric acid based etchants.
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.