estaban tamizando
-they/you were sieving
Imperfect progressive ellos/ellas/ustedes conjugation of tamizar.

tamizar

Se habían extraído cantidades considerables de grava que se estaban tamizando y concentrando en una instalación de lavado bien estructurada situada cerca de la mina.
Significant quantities of gravel had been extracted which were in the process of being sieved and concentrated in a well-structured washing facility located close to the mine.
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