Method for deinking recycled paper using a low flow elongated open top flotation vessel

A method for producing pulp for papermaking from waste printed paper which comprises:

• (1) mixing the waste printed paper in a pulping apparatus with a repulping solution which comprises water and an ink particle agglomerating agent under conditions which are sufficient to disintegrate the printed paper and provide an initial pulp slurry which includes water, wood fibers, relatively small ink particles of about or less than the size of the wood fibers and relatively large ink particle agglomerates substantially larger than the wood fibers;
• (2) screening the pulp slurry to separate relatively large ink particle agglomerates therefrom and produce a screened pulp slurry which is passed to a pulp feed tank;
• (3) conducting the screened pulp slurry from the pulp feed tank through a first cleaning stage including:
• (a) conducting the screened pulp slurry through a first cyclone cleaner to separate ink particles from the pulp slurry to produce a first cyclone cleaner accepts pulp slurry including water, wood fibers and a reduced amount of relatively small ink particles as compared to the initial pulp slurry, and a first cyclone cleaner rejects pulp slurry including water, wood fibers and relatively small ink particles,
• (b) introducing gas bubbles into the first cyclone cleaner accepts pulp slurry,
• (c) conducting the first cyclone cleaner accepts pulp slurry at a consistency of about or less than 1% through one or more first elongate upright vessels over a residence time of 10 to 60 seconds from an entrance port to an exit port thereof, wherein the entrance and exit ports of the elongate upright vessels are vertically spaced apart and located adjacent upper and lower ends of the one or more vessels, the flow rate and flow conditions of the pulp slurry and dilution water through the one or more vessels are selected to maintain an upper surface of the slurry in the one or more vessels adjacent their upper end and to enable gas bubbles in the slurry to rise to the upper surface, the gas bubbles having ink particles associated therewith so as to generate a froth on the upper surface containing ink particles and ink particle agglomerates elevated to the surface in association with the gas bubbles,
• (d) removing the froth from the upper surface, and
• (e) conducting the cleaned pulp slurry from the exit port of the one or more first elongate upright vessels for further processing into pulp for papermaking;
• (4) conducting the first cyclone cleaner rejects pulp slurry through a second cleaning stage producing a second cyclone cleaner accepts pulp slurry which is conducted back to the pulp feed tank and a second cyclone cleaner rejects pulp slurry which is conducted to a third cleaning stage, the second cleaning stage including:
• (a) introducing gas bubbles into the first cyclone cleaner rejects pulp slurry,
• (b) conducting the first cyclone cleaner rejects pulp slurry at a consistency of about or less than 1% through one or more second elongate upright vessels from an entrance port to an exit port thereof,
• (c) removing the froth from the upper surface,
• (d) conducting the cleaned pulp slurry from the exit port of the one or more second elongate upright vessels through a second cyclone cleaner to separate ink particles from the pulp slurry,
• (e) producing a second cyclone cleaner accepts pulp slurry including water, wood fibers and a reduced amount of relatively small ink particles as compared to the first cyclone cleaner rejects pulp slurry,
• (f) producing a second cyclone cleaner rejects pulp slurry including water, wood fibers and relatively small ink particles, and
• (g) conducting the second cyclone cleaner accepts pulp slurry back to the pulp feed tank;
• and
• (5) conducting the second cyclone cleaner rejects pulp through a third cleaning stage producing a third cyclone cleaner accepts pulp slurry which is conducted back to the first or second cleaning stage and a third cyclone cleaner rejects pulp slurry, the third cleaning stage including:
• (a) introducing gas bubbles into the second cyclone cleaner rejects pulp slurry,
• (b) conducting the second cyclone cleaner rejects pulp slurry at a consistency of about or less than 1% through one or more third elongate upright vessels over a residence time of 10 to 60 seconds from an entrance port to an exit port thereof, wherein the entrance and exit ports of the elongate upright one or more third vessels are vertically spaced apart and located adjacent upper and lower ends of the one or more third vessels, the flow rate and flow conditions of the pulp slurry and dilution water through the one or more third vessels are selected to maintain an upper surface of the slurry in the one or more third vessels adjacent their upper end at atmospheric pressure and to enable gas bubbles in the slurry to rise to the upper surface, the gas bubbles having ink particles associated therewith so as to generate a froth on the upper surface containing ink particles and ink particle agglomerates elevated to the surface in association with the gas bubbles,
• (c) removing the froth from the upper surface,
• (d) conducting the cleaned pulp slurry from,the exit port of the one or more third elongate upright vessels through a third cyclone cleaner to separate ink particles from the pulp slurry, to produce a third cyclone cleaner accepts pulp slurry including water, wood fibers and a reduced amount of relatively small ink particles as compared to the second cyclone cleaner rejects pulp slurry, and a third cyclone cleaner rejects pulp slurry including water, wood fibers and relatively small ink particles, and
• (e) conducting the third cyclone cleaner accepts pulp slurry back to the first or second cleaning stage.