Water balance improvement in an effluent treatment process for sulphate removal

A method of improving water balance in a process for treating a waste water stream (12) comprising a sulphate solution, which method includes the steps of:

• (a) contacting the water stream (12) with hydrated lime (14) to precipitate solids (18) therefrom and to produce a saturated calcium sulphate-containing solution (20),
• (b) producing an ettringite-containing slurry (26) by adding lime (24) to an amorphous aluminium trihydroxide-containing suspension (50) and an entrained calcium sulphate solution (20),
• (c) adding acid (40) and aluminium sulphate (41) to the ettringite-containing slurry (26) to form a saturated calcium sulphate-containing slurry (42) in which amorphous aluminium trihydroxide and gypsum are suspended,
• (d) producing a diluted saturated calcium sulphate-containing slurry (45) by using the saturated calcium sulphate-containing solution (20) to dilute the saturated calcium sulphate-containing slurry (42), and
• (e) subjecting the diluted saturated calcium sulphate-containing slurry (45) to a solid-solid separation step to produce said amorphous aluminium trihydroxide-containing suspension (50) and the entrained saturated calcium sulphate solution (20), for use in step (b);
• wherein said method is performed without a dewatering step of the amorphous aluminum trihydroxide-containing suspension following the solid-solid separation step.