The present invention provides a surface-treated silica filler for suppressing an increase in viscosity when added to a resin composition used for applications such as a semiconductor sealing material, and the resin composition containing the surface-treated silica filler. The surface-treated silica filler of the present invention is surface-treated with a basic substance having an acid dissociation constant (pKa) of its conjugate acid of 9.4 or more.

The present invention relates to a high purity synthetic fluorite (CaF.sub.2). The present invention further relates to a process and an apparatus for preparing said high purity synthetic fluorite (CaF.sub.2), classified as acid grade, starting from fluorosilicic acid H.sub.2SiF.sub.6 (FSA) and calcium carbonate (CaCO.sub.3). Finally, the present invention relates to the use of said high purity synthetic fluorite (CaF.sub.2) in the industrial production of hydrofluoric acid.

A process for preparing synthetic calcium fluoride (CaF.sub.2) (min 90% CaF.sub.2 by weight) from fluosilicic acid is provided. The processes comprises the steps of (a) reacting fluosilicic acid (H.sub.2SiF.sub.6) with ammonium hydroxide or ammonia in a first reactor so as to obtain a first slurry and filtering the first slurry so as to obtain a filtrate containing a solution of ammonium fluoride (b) precipitating the solution of ammonium fluoride with calcium in a second reactor so as to produce a second slurry containing calcium fluoride and (c) evolving the major part of ammonia from the second reactor and then scrubbing and returning said ammonia to the first reactor.

The present invention provides a surface-treated silica filler for suppressing an increase in viscosity when added to a resin composition used for applications such as a semiconductor sealing material, and the resin composition containing the surface-treated silica filler. The surface-treated silica filler of the present invention is surface-treated with a basic substance having an acid dissociation constant (pKa) of its conjugate acid of 9.4 or more.

The invention relates to a process for preparing synthetic calcium fluoride (CaF.sub.2) (min 90% CaF.sub.2 by weight) from fluosilicic acid, comprising the following steps: (a) reacting fluosilicic acid (H.sub.2SiF.sub.6) with ammonium hydroxide or ammonia in a first reactor so as to obtain a first slurry; filtering said first slurry so as to obtain a filtrate containing a solution of ammonium fluoride; (b) precipitating the solution of ammonium fluoride obtained as a filtrate in step (a) with calcium carbonate as a dry form or as a suspension at a concentration ranging from 10 to 80% by weight in a second reactor so as to produce a second slurry containing calcium fluoride and ammonium carbonate; filtering said second slurry so as to obtain a filter cake containing calcium fluoride and a filtrate containing a solution of ammonium carbonate; washing and drying said filter cake so as to obtain calcium fluoride and a filter cake washing containing a solution of ammonium carbonate; wherein a portion of the second slurry ranging from 10 to 70% is recycled to the second reactor (3) so as to enhance calcium fluoride crystallization; (c) evolving the major part of ammonia from the second reactor (3) in step (b) because of the partial decomposition of ammonium carbonate under reactor conditions and then scrubbing and returning said ammonia to the first reactor, and gathering and treating by distillation and condensation both ammonium carbonate solutions obtained as the filtrate and the filter cake washing in step (b) to recover liquid ammonia which is recycled to the first reactor (1).

The present invention relates to a high purity synthetic fluorite (CaF.sub.2). The present invention further relates to a process and an apparatus for preparing said high purity synthetic fluorite (CaF.sub.2), classified as acid grade, starting from fluorosilicic acid H.sub.2SiF.sub.6 (FSA) and calcium carbonate (CaCO.sub.3). Finally, the present invention relates to the use of said high purity synthetic fluorite (CaF.sub.2) in the industrial production of hydrofluoric acid.

Provided are an independent porous spherical silica such that the alkali metal content is reduced, the hardness of the particles is increased, and the number of shred particles is reduced; and a method of producing such a silica. In the porous spherical silica, the diameter (D50) of volume based particle size distribution is 2 m to 200 m, the ratio (D10/D90) of the diameter (D10) to the diameter (D90) is at least 0.3, the pore volume is 0.5 to 8 mL/g, the mode pore radius is 5 to 50 nm, the specific surface area is 50 to 400 m.sup.2/g, and the arithmetic mean value of compressive test forces when specimens are found to break obtained according to JIS Z8844 is 1.010.sup.1 to 2.010.sup.1 mN, the number of particles each having a circularity of at most 0.7 is at most 20, and the alkali metal content is at most 50 ppm.