Reasonable Use of Alkali and Acid Washing for Press Felts

In the paper making process, press felt cleanliness is crucial to dewatering efficiency, paper quality, and the life of the press felt itself. Press felt contamination is primarily caused by impurities such as fibers, fillers (such as calcium carbonate and kaolin), sizing agents, and resins that clog the press felt pores or coat its surface.

How to judge whether the press felt needs alkali washing

1. Deterioration in paper quality

• Increased appearance of translucent spots and holes: This is the most direct and common early sign. Soiled press felts dehydrate unevenly, causing the wet paper to dehydrate poorly at the soiled spots, resulting in thin spots or even holes in the subsequent drying process. These defects typically occur along the press felt’s grain.
• Dirty spots and marks: Contaminants (resin clumps, oil stains, and adhesives) on the press felt can transfer to the paper surface, forming visible spots or marks.
• Uneven moisture content across the paper sheet: Localized press felt blockage can cause excess moisture in that area of the paper, impacting drying efficiency and sheet flatness.
• Changes in paper sheet surface roughness and smoothness: Dirt can affect the press felt’s ability to support and transfer the paper, resulting in reduced surface quality.

2. Decreased dewatering efficiency:

• After pressing, the moisture content of the wet paper web increases: This is the most critical process indicator. Clogging of the press felt pores will severely hinder the transfer of water from the wet paper web to the press felt and ultimately to be removed by the vacuum box or press nip. An online moisture meter or laboratory test will show that the moisture content of the paper web at the press exit is significantly higher than normal.
• Abnormally high vacuum level: To overcome the dewatering resistance caused by press felt blockage, the vacuum box requires a higher vacuum level to achieve the desired dewatering effect. Monitor the vacuum pump current or vacuum gauge reading. If the current continues to rise and has been eliminated for other reasons (such as a damaged vacuum box panel or a defective seal), press felt blockage is likely the cause.
• Increased current in the press roll: High dewatering resistance increases the electromechanical load driving the press rolls, leading to increased current.
• Increased steam consumption in the dryer section: Inefficiency due to abnormal dewatering results in higher moisture content in the wet paper web entering the dryer section, requiring more steam to dry the paper, resulting in increased energy consumption.

3. Changes in the appearance and feel of press felts:

• Press felts lose their original color and become darker: Clean press felts show the weave pattern and color, but contamination gradually darkens the color.
• Blurred patterns: debris blocks the holes in the blanket and covers the fiber surface, making the weaving patterns on the blanket surface blurred or even disappear.
• Shiny or sticky surface: Contaminants such as resin and adhesives can make the press felt surface appear greasy or shiny, and may feel sticky to the touch when the machine is stopped.
• Localized lumps or compaction: Severe resin or adhesive deposits may form localized lumps on the press felt.
• Odor: Microbial growth can produce a rancid odor.

4. Operating Parameter Trends:

Continuously monitor historical trends in monitoring indicators (moisture content, vacuum level, current, and steam consumption). If these parameters show a gradual deterioration, after excluding other equipment or process adjustments, press felt contamination is a key concern.

5. Press Felt Age:

Based on factors such as paper grade, pulp (especially those containing a large amount of recycled fiber or mechanical pulp), machine speed, and press felt type, mills should accumulate experience with press felt cleaning cycles. If the cleaning cycle approaches or exceeds the typical cleaning cycle, even if some indicators remain acceptable, preventive maintenance cleaning should be considered. For example, a paper machine using a large amount of recycled pulp may have a press felt cleaning cycle as short as 3-5 days, while a paper machine producing cleaner pulp grades may have a press felt cleaning cycle of 7-14 days or even longer.

Alkali and Acid Washing:
Alkali washing primarily targets: resin, stickies (such as adhesives and hot melt adhesives in recycled pulp), organic precipitates (such as starch and synthetic adhesive residues), some microorganisms and their secretions, and oil-based contaminants.
Acid washing primarily targets: inorganic filler deposits (such as calcium carbonate and kaolin), calcium soaps, and metal salt deposits.

Situations to Distinguish:
Nature of Contaminant: If the press felt surface is sticky, shiny, or resinous, or if an odor is detected, an alkaline wash is indicated. If the
press felt is primarily chalky, then an acid wash is indicated.

Press felt Cleaning Results:
If dewatering efficiency does not improve or even deteriorates after an acid wash, it indicates that the acid wash has caused some resin to precipitate. Organic matter is likely the primary blockage, and an alkaline wash is indicated.

Press felt cleaning recommendations:
Many mills use an acid-alkaline or alkaline-acid cleaning sequence to ensure all types of deposits are removed from the press felt. Alkaline washing is often the first or most critical step, as it effectively addresses the most common organic blockages that have the greatest impact on dewatering. Sometimes, an alkaline wash alone can significantly improve the situation.

It is recommended to maintain records of relevant press felt operating parameters (moisture content, vacuum level, and current).

Regularly inspect the quality of the paper exiting the press section and the appearance of the press felt.

Develop a reasonable preventive press felt cleaning plan based on the paper grade and pulp characteristics. (Alkali and Acid Wash Cycles)
If an abnormality occurs, prioritize troubleshooting other equipment (vacuum chamber seals, roller surface, spray water, etc.). Once the problem is confirmed to be with the press felt, schedule cleaning.

Generally, when dehydration efficiency drops to the point where product quality is affected or energy consumption increases significantly, the press felt should undergo alkali or combined cleaning. Do not wait until the press felt pores are completely clogged, as this will result in poor cleaning results and may damage the press felt.