Process for enhanced deposition and retention of particulate filler on papermaking fibers

A process for the in situ deposition, in a reactor, of particulate filler onto papermaking fibers, the steps of:

• in separate vessels,
• preparing a first slurry containing papermaking fibers,
• preparing a second slurry containing calcium hydroxide or the equivalent thereof,
• and thereafter,
• combining said first and second slurries with a gaseous carbon dioxide precipitant in a flowing stream, under conditions whereby shear is imparted to said gaseous precipitant,
• to form in situ calcium carbonate on at least the external surfaces of the fibers,
• wherein the order of introduction of said first and second slurries and said precipitant is selected from one of (a) premixing of said first and second slurries prior to their introduction as a single flowing stream into said flowing stream, (b) introducing said first slurry into said flowing stream at a most upstream location, introducing said precipitant to said flowing stream at a downstream location from the location of the introduction of said first slurry to said flowing stream, and introducing said second slurry to said flowing stream at a downstream location from the location of the introduction of said precipitant to and introducing a further portion of said second slurry to said flowing stream at a downstream location from the location of the introduction of said further portion of said precipitant to said flowing stream, or (d) introducing at least a portion of said second slurry or a portion of said gaseous precipitant to said flowing stream at a first location downstream of the location of the introduction of said first slurry to said flowing stream, introducing at a second location downstream of said first location at least a further portion of that one of said second slurry or said gaseous precipitant which was not introduced to said flowing stream at said first location.