In the dewatering treatment of papermaking sludge, the selection of sludge dewatering chemicals needs to address the following core difficulties and technical challenges, which should be comprehensively considered in combination with the characteristics of the sludge, the mechanism of action of the chemicals and the treatment process:
First, the characteristics of sludge are complex, and it has high requirements for the adaptability of chemicals
High moisture content and fiber interwoven structure
The initial moisture content of papermaking sludge can reach over 99%, and it remains above 95% after concentration. Moreover, it contains a large amount of fibers (such as short fibers accounting for more than 2.5%), which can easily form a network structure to encapsulate water. Therefore, sludge dewatering agents need to have strong penetrating power to destroy the bound water.
The difficulty lies in that traditional inorganic flocculants (such as PAC) are hard to penetrate the fiber network and need to be combined with the adsorption bridging effect of cationic polyacrylamide (CPAM), but the cost is relatively high.
High ash content and interference from inorganic substances
The ash content of the sludge is 50% to 70%, and it contains inorganic particles such as calcite and talc, which may neutralize the charge of the reagent or form a dense layer to hinder dewatering.
The difficulty lies in the need for the agent to have both electro-neutralization and steric hindrance effects to prevent inorganic substances from interfering with the formation of flocs.
The neutral pH is in contradiction with the salt content
The pH of the sludge is approximately 7.2, but its salt content is relatively high (possibly due to the introduction of the bleaching process), which affects the solubility of the reagent and the flocculation effect.
The difficulty lies in choosing agents with strong salt resistance (such as high-molecular-weight CPAM), or adjusting the ionic strength through a compound formula.
Second, it is difficult to match the mechanism of action of the agent with that of the sludge
The dilemma of choosing inorganic vs. organic flocculants
Inorganic flocculants (such as polyaluminium chloride) : They are low in cost, but the flocs are loose and prone to rupture under high pressure, resulting in a relatively high moisture content after dewatering (for example, the moisture content after dewatering by a belt filter press is 75%-85%).
Organic flocculants (such as CPAM) : They require less dosage and form compact flocs, but their cost is 2 to 3 times that of the former, and the residual monomers may pollute the environment.
The difficulty lies in determining the most cost-effective combination through small-scale trials. For instance, inorganic + organic compound agents (PAC+CPAM) can balance cost and effectiveness.
The compatibility of coagulant AIDS with sludge components
Lime, fly ash and other coagulant AIDS can enhance the structure of flocs, but the fibers in papermaking sludge may adsorb the coagulant aid particles, reducing their effective concentration.
The difficulty lies in precisely controlling the dosage of the coagulant aid to prevent excessive use from clogging the filter cloth or causing a rise in the moisture content of the mud cake.
Iii. Constraints of Treatment Processes and Equipment on Chemicals
The type of equipment affects the selection of chemicals
Belt filter press: It is necessary for the chemicals to quickly form large flocs to prevent the filter belt from clogging. CPAM is commonly used.
High-pressure dryer (such as 6-8MPa) : The agent needs to withstand high pressure to prevent flocs from breaking, and anti-shear additives (such as silicic acid) need to be added.
The difficulty lies in the fact that the effect of the same agent varies greatly among different devices, and targeted tests are required.
The synergistic requirements of chemicals in the dehydration stage
Gravity zone: Rapid flocculation is required to prevent the loss of fine particles.
Pressurized zone: Chemicals are needed to enhance the release of bound water (such as Fenton reagent oxidation to break EPS).
Sludge dewatering is divided into gravity dewatering → pressure dewatering → pressurized dewatering. The chemicals required for each stage should be optimized respectively:
The difficulty lies in the complex multi-stage drug coordination, which requires dynamic adjustment of the dosing point and dosage.
Iv. The Challenge of Balancing Environmental Protection and Cost
The conflict between chemical residue and sludge resource utilization
Cationic PAM and other agents may remain in the mud cake, affecting incineration or utilization in building materials (such as brick-making and cement production).
The difficulty lies in choosing biodegradable or low-toxicity agents (such as modified natural polymer products), or adding a mud cake washing process.
Long-term operating cost optimization
The cost of chemicals accounts for 30% to 40% of the operating expenses for dehydration, and it is necessary to balance the initial effect with the long-term economy.
The difficulty lies in that although some low-cost chemicals (such as iron salts) have a low initial cost, the increase in sludge leads to higher transportation/disposal costs, and a full life cycle cost analysis is required.
V. Suggestions for Selection and Optimization Strategies
Experiment-driven
The types of chemicals were screened through small-scale sludge tests (such as beaker experiments), and the dewatering performance was evaluated by combining the sludge specific resistance (SRF) and capillary suction time (CST) indicators.
Compound formula development
In response to the characteristics of papermaking sludge, a ternary system of "inorganic flocculant + organic flocculant + coagulant aid" has been developed, such as PAC+CPAM+ lime, which takes into account both the strength of flocs and shear resistance.
Equipment adaptation adjustment
Adjust the molecular weight of the reagent according to the type of filter press: Use low molecular weight CPAM (5 -8 million) for belt presses, and high molecular weight (10 -12 million) for plate and frame presses.
Dynamic regulation mechanism
Establish a sludge characteristic database, and adjust the dosage ratio of chemicals in real time based on parameters such as sludge moisture content, ash content, and fiber content to avoid excessive reliance on empirical values.
Conclusion
The selection of dewatering agents for papermaking sludge needs to break through the triangular contradiction of "complex sludge characteristics - single agent effect - insufficient process matching". Through refined experiments, compound formula development and process collaborative optimization, a balance between dewatering efficiency, cost and environmental benefits can be achieved. In the future, intelligent dosing systems can be explored, integrating online monitoring and AI algorithms to dynamically adjust the drug formula and further enhance the accuracy of drug selection.