To this end, the solution obtained in stage e, which has a resistivity of 1000 ohm.cm, is adjusted to a pH-value of around 6.8, after which the ionic strength of the solution is adjusted to 10- 2 by adding a salt, such as calcium chloride, but preferably a zinc salt, such as zinc acetate or zinc chloride, after which the solution is left at 0??? C in order to complete precipitation.