Diatomite filter aids were prepared using raw diatomite and various fluxes. The effects of composition, amount and granularity of the flux on the micro-structure and filtration performance of the diatomite filter aids were studied. Filter aids with high flow rate and filtrate clarity were successfully obtained when 7 wt. % of Na2CO3 flux was employed. It was shown that opal in the diatomite was converted to cristobalite in the presence of flux at 1000 degrees C, being similar to the formative process of silicate glass. The additive alkali ions from the flux are believed to partially break Si-O-Si chains of the opal at a high temperature and then recombine the broken structure to a hexatomic ring mesh structure during the cooling process, which is significative to control the phase composition for preparation of diatomite filter aids. (C) 2014 Elsevier B.V. All rights reserved.