This paper examines how the gypsum-containing waste processing procedure influences the degree of rare-earth metal (REM) extraction from the waste. Gypsum-containing waste from the production of phosphoric acid was treated with aqueous solutions of various acids in the temperature range 25–140°C at solid : liquid ratios from 1 : 1 to 1 : 3. The results demonstrate that the largest REM extraction from phosphogypsum can be reached by treating with hydrochloric or nitric acid at the boiling point of the aqueous solution of the acid. Nitric acid is preferable because, after the acid treatment, the gypsum contains a large amount of nitrates, which can be used as agricultural fertilizers. Treatment of phosphogypsum at elevated temperatures and pressures leads to dissolution of a larger amount of gypsum, without increasing the REM yield. The highest economic efficiency of REM extraction from phosphogypsum is ensured by a solid : liquid ratio of about 1 ; 2, treatment temperature of 103–105°C, and cascade time no more than 10 min. [ABSTRACT FROM AUTHOR]

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