High consistency re-pulping method, apparatus and absorbent products incorporating recycled fiber
A method of pulping wastepaper includes providing a pulping vessel with a rotor and at least one flow sensor adapted to measure slurry flow within the pulping vessel. The pulping vessel is charged with wastepaper and water to form a slurry, the amounts of wastepaper and water being present such that the slurry has consistency in the range of from 10% to 30%. The wastepaper charge in the pulping vessel is pulped at a pre-selected power level while monitoring poloidal slurry flow in the pulping vessel; and from time-to-time, water may be added to the pulping vessel when the poloidal flow falls below a predetermined lower threshold flow value in order to reduce viscosity and thereby restore poloidal flow within the pulping vessel. Doppler velocimetry is a preferred method of monitoring pulp flow. In a particularly preferred construction the rotor has a variable power drive. The inventive method is especially useful for making high quality, high brightness furnishes from wastepaper which varies from batch to batch.
1. A method of pulping wastepaper comprising:
(a) providing a pulping vessel with a rotor having a power drive and at least one flow sensor adapted to measure slurry flow within the pulping vessel;
(b) charging the pulping vessel with wastepaper and water to form a wastepaper slurry, the amounts of wastepaper and water being present such that the slurry has consistency in the range of from 10% to 30%;
(c) pulping the wastepaper slurry in the pulping vessel while monitoring poloidal slurry flow in the pulping vessel; and
(d) maintaining poloidal slurry flow in the pulper within a range of from 0.05 ft/sec to 0.6 ft/sec during pulping,
wherein the poloidal slurry flow is maintained at or above a lower threshold slurry flow value of 0.05 ft/sec for at least 60% of pulping duration.
2. The method according to claim 1 , wherein the pulping vessel is provided with an ultrasonic Doppler flow sensor.
3. The method according to claim 1 , wherein the poloidal slurry flow is maintained at or above the lower threshold slurry flow value for at least 75% of pulping duration.
4. The method according to claim 3 , wherein poloidal slurry flow is maintained at or above the lower threshold slurry flow value for at least 75% of pulping duration by (i) adding additional water, (ii) adjusting power input to the wastepaper slurry, (iii) adjusting rotor speed, or a combination of one or more of (i), (ii) and (iii).
5. The method according to claim 1 , wherein the lower threshold slurry flow value is 0.1 ft/sec.
6. The method according to claim 5 , wherein the target level of energy input to the wastepaper charge is within the range of from 5 kwh/MT to 20 kwh/MT.
7. The method according to claim 1 , further comprising selecting a target level of energy input to the wastepaper charge and continuing pulping of the wastepaper charge until the target level of energy input to the wastepaper charge is achieved.
8. The method according to claim 1 , wherein the poloidal slurry flow in the pulper is maintained within the range of from about 0.1 ft/sec to about 0.4 ft/sec during pulping.
9. The method according to claim 8 , wherein the poloidal slurry flow in the pulper is maintained within the range of from about 0.15 ft/sec to about 0.25 ft/sec during pulping.
10. The method according to claim 1 , wherein power input to the power drive is maintained within the range of from about 700 kw to about 1500 kw during pulping.
11. The method according to claim 1 , wherein the rotor RPM is maintained within the range of from about 60 to about 180 during pulping.
12. The method according to claim 11 , wherein the rotor RPM is maintained within the range of from about of from about 75 to about 140 during pulping.
13. The method according to claim 1 , wherein the ratio of power input to the power drive to rotor RPM is maintained within the range of from about 6 kw/RPM to about 10 kw/RPM during pulping.
14. The method according to claim 1 , wherein the method includes pulping the wastepaper charge in the pulping vessel while monitoring (i) poloidal slurry flow in the pulping vessel; (ii) power input to the power drive; and (iii) rotor RPM; and adding additional water to the pulping vessel during pulping of the wastepaper charge to adjust measured levels of one or more of (i) poloidal slurry flow, (ii) power input to the power drive, and (iii) rotor RPM toward a corresponding target level.
15. The method according to claim 1 , wherein the method includes pulping the wastepaper charge in the pulping vessel while monitoring (i) the ratio of power input to the power drive/rotor RPM and (ii) poloidal slurry flow in the pulping vessel; and adding additional water to the pulping vessel during pulping of the wastepaper charge to adjust measured levels of one or more of (i) the ratio of power input to the power drive/rotor RPM and (ii) poloidal slurry flow toward a corresponding target level.
16. The method according to claim 1 , wherein the method includes pulping the wastepaper charge in the pulping vessel while monitoring poloidal slurry flow in the pulping vessel; and discontinuing pulping of the wastepaper charge after an onset of instability in poloidal slurry flow.
17. The method according to claim 16 , wherein pulping is discontinued within 2 minutes after the onset of instability of poloidal slurry flow.
18. The method according to claim 1 , wherein the method includes pulping the wastepaper charge in the pulping vessel while monitoring poloidal slurry flow in the pulping vessel; and adding additional water to the pulping vessel to adjust measured levels of poloidal slurry flow toward a target level.
19. The method according to claim 1 , wherein the method includes pulping the wastepaper charge in the pulping vessel while monitoring poloidal slurry flow in the pulping vessel; and operating the pulping vessel including adding additional water to the pulping vessel while maintaining poloidal slurry flow within the range of from about 0.1 ft/sec to about 0.4 ft/sec.
20. A method of making absorbent sheet comprising:
(a) obtaining papermaking fiber from wastepaper by way of pulping wastepaper according to the method of claim 1 ;
(b) recovering papermaking fiber from the pulping vessel, optionally further processing the recovered fiber by de-inking and/or bleaching the recovered fiber; and
(c) incorporating the recovered papermaking fiber into an absorbent sheet.