Abstract: Using hydrated magnesium chloride&nbsp; and ammonia to produce magnesium hydrate is a ‘clean’ project in rational-using lake/sea magnesium resources.But magnesium hydrate precipitate is a flocculent colloid and difficult to be filtered and separated,so the operating process and equipment selection are required highly.In order to speed up the deposition of magnesium hydrate,the effect of adding seed crystal and flocculant has been researched and analyzed.The adding quantity of seed crystal and flocculant was determined by deposition velocity of magnesium hydrate,filtering time of product,and washing time after filtering during the experiment.The results showed that the optimal adding of quantity seed crystal was 10% of magnesium hydrate product with the grain size less than 45 μm.The optimal adding quantity of flocculant is&nbsp; 0.3 mg &nbsp;of PAM to 1 g&nbsp; Mg(OH)<SUB>2</SUB>.