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What are the specific applications of anhydrous aluminum sulfate in the paper industry?
Release time：
2022-07-07
Anhydrous aluminum sulfate (Al₂(SO₄)₃) has several key applications in the papermaking industry, primarily based on its properties as a flocculant, sizing agent, retention aid, and pH adjuster. Specific applications are as follows:
I. As a Sizing Agent
Function: Sizing is a crucial step in papermaking that imparts liquid resistance to paper. Anhydrous aluminum sulfate is commonly used in acidic sizing systems.
Principle: When used with sizing agents such as rosin, aluminum sulfate hydrolyzes to form positively charged aluminum hydroxide colloids, which combine with negatively charged rosin particles and deposit on the fiber surface to form a waterproof film, improving the water resistance of the paper (such as printing paper and packaging paper).
Advantages: Low cost, mature technology, suitable for paper types with moderate sizing requirements, such as cultural paper and writing paper.
II. As a Flocculant
Function: Promotes the aggregation of fibers, fillers (such as kaolin and calcium carbonate), and additives, improving the dewatering performance and formation quality of the pulp.
Principle: Anhydrous aluminum sulfate dissociates into Al³⁺ when dissolved in water. This compresses the double layer and neutralizes the negative charge on the surface of the colloidal particles, causing small fibers and filler particles to aggregate into larger flocs, reducing fiber loss during papermaking.
Application Scenarios:
In the pulp preparation stage, it improves the retention rate of fibers and fillers (reducing raw material waste);
In wastewater treatment, it flocculates suspended solids in wastewater, reducing COD (chemical oxygen demand), and facilitating subsequent purification.
III. As a Retention and Drainage Aid
Function: Optimizes the dewatering efficiency of the pulp on the paper machine wire section, improving paper uniformity and production speed.
Principle: Through charge neutralization and bridging, it accelerates the sedimentation of fibers and fillers, reducing the loss of fine particles in white water, while also reducing the viscosity of the pulp, allowing water to filter out faster.
Typical Applications: In high-speed papermaking production of newsprint and tissue paper, it is used in conjunction with polymeric retention aids (such as polyacrylamide) to further enhance the effect.
IV. Adjusting Pulp pH
Function: Anhydrous aluminum sulfate hydrolyzes to be acidic (pH ≈ 3.0-5.0) and can be used to adjust the acidity or alkalinity of the pulp system to suit acidic sizing or specific process requirements.
Application Scenarios:
In acidic papermaking processes, it maintains an acidic environment to ensure the reaction efficiency of the sizing agent and fibers;
Some paper types (such as coated paper board) require pH control to optimize filler dispersibility and paper strength.
V. Auxiliary Role in Surface Sizing and Coating
Surface Sizing: A small amount is added to the surface sizing solution to enhance the binding force between the sizing material (such as starch) and the paper surface, improving the water resistance and printability of the paper.
Coating Processing: Used as an additive in coating formulations to adjust the viscosity and rheology of the coating, prevent pigment sedimentation, and improve the smoothness and gloss of coated paper.
VI. Precautions and Limitations
Acidic System Limitation: Anhydrous aluminum sulfate is only suitable for acidic papermaking processes (pH < 7) and is gradually being replaced by cationic starch and synthetic sizing agents in alkaline papermaking (an environmentally friendly trend).
Corrosiveness: Long-term use of acidic systems can lead to equipment corrosion and requires corrosion protection measures.
Retention and Strength Balance: Excessive addition may reduce the strength of the paper (such as tensile strength and folding endurance), and the dosage must be strictly controlled (usually 0.5%-2.0%, based on oven-dry pulp).
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