Leaching Process: Aluminium Hydroxide Precipitation

[zorro]

In the Leaching process of Aluminium, alumina present in bauxite dissolves forming a soluble sodium aluminate. The impurites are filtered off. The filtrate is diluted and a little freshly precipitated aluminium hydroxide is added which causes the precipitaion of aluminium hydroxide.

How does it cause the precipitation of aluminium hydroxide? According to Le Chatelier's Principle backward reaction will occur consuming aluminium hydroxide. What is the actual reason?

 

[alxm]

The solution becomes over-saturated because the leaching is done at a high temperature, which is then lowered. Adding some solid aluminum hydroxide creates nucleation points for the precipitation.

(While we're on the subject, it's worth noting that those filtered-off impurities just spilled out all over in Hungary)

 

[zorro]

The solution becomes over-saturated because the leaching is done at a high temperature, which is then lowered. Adding some solid aluminum hydroxide creates nucleation points for the precipitation.

The solution becomes oversaturated with what? Aluminium Hydroxide?
Please elaborate 'nucleation points'.