Screening and cleaning decide how smoothly a paper machine runs by keeping the stock free of contaminants that cause breaks, defects, and high energy use. In this guide, we map the Top 10 Screening and Cleaning Systems in Paper Stock Prep so both beginners and seasoned engineers can follow a clear path. You will learn what each system removes, how it works, and when to use it. The aim is practical understanding, not only theory. With simple language, structured sections, and tips that connect equipment choices to quality, yield, and uptime, you can apply these ideas in mills of every size.
#1 Coarse screening and detrashing
Coarse screening is the first line of defense after pulping. Scalper screens, drum screens, and knotters remove large contaminants such as plastics, wire, ragger tails, stick bundles, and uncooked knots that could damage downstream equipment. Keep consistencies matched to equipment design so fiber mats carry rejects without blinding the surface. Install junk traps and stone traps ahead of pressure screens to protect baskets from impact. Stable flow, even feed consistency, and correct open area determine capture efficiency. Trend differential pressure and reject solids online to catch plugging early. When coarse systems work well, fine screens see fewer shocks and the machine has fewer sheet defects.
#2 Pressure screens with slotted baskets
Pressure screens remove shives, stickies, and long fibers above a chosen width by directing stock through slotted or holed baskets under controlled turbulence. Modern slotted baskets with narrow apertures and low wear metallurgy raise cleanliness while reducing plugging. Select rotor geometry to balance shear that keeps slots open with gentle conditions that preserve fiber length and bonding potential. Control feed consistency, accept flow, and reject ratio to hold reject thickening in range. Continuous monitoring of vibration, power, and pulse amplitude reveals early wear or imbalance. Right sized screen areas across stages prevent bottlenecks and keep quality steady across grade changes.
#3 Multistage screening and fractionation
A single pass rarely meets aggressive cleanliness targets, so mills use primary, secondary, and tertiary screens with controlled recirculation. Fractionation splits the fiber stream so coarse fractions see tighter screening while fines bypass high shear zones to protect strength. Cascading accepts forward and routing rejects countercurrent saves fiber without overloading stages. Balance mass, ash, and stickies across loops to avoid concentrating fillers where slots plug. Use valves and flow transmitters to hold constant reject rates and stabilize residence time. A short mill trial that measures dirt count, stickies, and fiber loss at each stage will quickly optimize the loop.
#4 High density forward cleaners
High density cleaners use centrifugal force to remove heavy contaminants such as sand, glass, stones, paper clips, and metal fragments. The conical body accelerates stock so dense particles migrate to the outer wall and exit as rejects, while accepts leave through an inner outlet. Select tip sizes and operating pressure to match particle size, density, and furnish. Install automatic purge controls and reject thickeners to minimize fiber loss from the reject leg. Place cleaners early to protect downstream screens, refiners, and pumps. Regular nozzle inspections, solids sampling, and wear checks sustain efficiency and keep abrasive loads from reaching the paper machine.
#5 Reverse and light weight cleaners
Light contaminants such as plastics, hot melts, waxes, foams, and trapped air behave differently under centrifugal fields. Reverse or lightweight cleaners create a flow pattern that pulls low density particles toward a separate reject outlet while clean stock exits centrally. Operating pressure, cone geometry, temperature, and air management are key to stability. Because these contaminants deform, moderate heat reduces smearing that would carry them to accepts. Stage cleaners in parallel with measured split ratios to improve removal while conserving energy. Routine mass balance of accepts, rejects, and fiber loss confirms that the system removes the target load without sacrificing yield.
#6 Flotation deinking for hydrophobic particles
In recycled fiber lines, flotation cells remove ink and hydrophobic specks by attaching bubbles to particles and lifting them into a stable foam. Proper surfactant chemistry, air rate, and residence time set removal efficiency and brightness gain for a given furnish. Balance froth depth, reject flow, and launders to capture fine ink without excessive fiber carryover or foam collapse. Pre screening prevents large stickies from destabilizing the froth and improves bubble attachment. Temperature and consistency control further enhance kinetics and lower energy. Modern cells with froth cameras, torque monitoring, and automatic chemical control deliver consistent cleanliness through furnish swings and start ups.
#7 Washing and dilution thickening
Washing removes dissolved solids, fines, and residual chemicals that elevate dirt counts and deposit risk. Disc filters, drum washers, and fourdrinier style washers exchange liquor by dilution and displacement across multiple stages. Set dilution factors to the lowest level that still meets cleanliness and brightness goals to conserve water and steam. Countercurrent flows reduce fresh water demand while pushing contaminants toward the waste stream. Good filtrate zoning and seal management keep carryover from re entering clean loops. Track conductivity, ash, anionic trash, and brightness to verify removal, and link those metrics to break frequency and size loss.
#8 Hot screening and dispersion for stickies control
Thermoplastic contaminants soften with heat, which lets mills screen and disperse them more effectively. Hot screening at elevated temperature improves slot passage for fibers while rejecting tacky agglomerates that cause picking and holes. Dispersion units apply mechanical energy at controlled temperature to break residual specks into sub visible sizes that no longer print through. Detackifiers and optimized pH reduce redeposition on wires and felts. The key is balancing energy, temperature, and residence time so fiber damage and hornification do not rise. Monitor dirt count, speck size distribution, stickies tack, and sheet faults to confirm measurable quality gains.
#9 Approach flow cleaning and machine screening
Final cleaning occurs just before the headbox, where stability matters most. Machine screens with very narrow slots remove last shives and agglomerates without cutting fibers or creating pulses. Deaeration tanks and vacuum elements strip entrained air that would cause pinholes, streaks, and basis weight variation. If required, compact forward cleaners in the approach flow catch heavy grit that escaped earlier stages. Pressure control, pulse dampening, and constant consistency valves protect headbox flow uniformity and formation. A clean, bubble free approach system gives the paper machine smoother starts, better retention, and fewer web breaks at speed.
#10 Saveall and white water clarification
Even with strong upstream screening, fines and fibers escape in white water. Saveall systems such as microscreens and dissolved air flotation units recover usable fiber while lowering suspended solids that would recirculate contaminants. Returning recovered fiber to the correct chest protects strength and keeps ash in balance. Coagulant control, screen mesh selection, and proper backwash timing are important to avoid blinding. Clean water loops improve washer and screen performance upstream, creating a virtuous cycle of cleanliness, yield, and uptime. Regular audits of recovery efficiency and solids in clarified water reduce raw material cost and improve system stability.