The conventional technique for removing zinc from the spinning acid wastestream has been direct lime precipitation to ~pH 10, with no zinc recovery. The economics of this approach are compared to the American Enka zinc recycle process.
The economics of recovery are a very strong function of the amount of zinc used in the preparation of the yarn and the ratio of acid to zinc in the spinning bath. In manufacturing industrial yarns and tire cords, it is common to use 4.5 to 7.5 kg of zinc per 100 kg of yarn. This high concentration of zinc makes recovery extremely attractive. Textile yarns use less zinc, and although recovery is still the most economic solution, it offers less of a return. These two cases are presented as extremes, with many plants falling between the two values.
The use of two-stage precipitation combined with zinc recycle offers a saving of 2007 USD 498,000 over neutralization for a plant producing industrial yarns and a saving of USD 88,400 for textile yarns. Many plants produce a mix of the two and the results would therefore fall between these values. The costs associated with the more extensive sludge handling and storage in neutralization and precipitation only are not included. The cost of installing the complete neutralization and zinc recycle system would have negligible economic impact on the rayon industry, running from USD 1.14/100 kg profit to a USD 0.37/100 kg cost compared to selling prices of USD 86 to 100/100 kg of staple, USD 200 to 230/100 kg of tire yam, and USD 290 to 430/100 kg of filament. Zinc oxide manufacturers face the loss of the bulk of a 22.7 million kg (50 million lb/yr) market as this product is reused rather than wasted.14
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