Rationale
A screener is an instrument that grades particles or fibers in size based on the principle of physical screening. In pulp fiber analysis, fiber suspensions are separated under specific conditions, such as water flow or mechanical vibration, through a series of standard screens with different pore sizes. The length distribution of the fibers can be calculated based on the retention rate of fibers on different screens based on fiber length or thickness. Common screening technologies mainly include hydrostatic sedimentation and dynamic water flow type, the core of which is the synergy between fluid mechanics and screen to achieve effective classification of fiber groups.
Correlation between fiber morphology parameters and pulp properties
The physical properties of pulp are closely related to the morphological characteristics of its fibers. Key morphological parameters include fiber length, width, thickness, and fine fiber content. Longer fibers typically provide higher paper tensile strength and tear because they form more and stronger bonds between fibers. Fiber coarseness affects the looseness and opacity of the paper. Fine fibers (usually the components that pass through a 200-mesh screen) significantly affect the smoothness, opacity, and printability of the paper, but too much can reduce water filtration performance and strength. Together, these parameters determine the suitability and quality class of the final paper product.
The function of fiber length distribution is often expressed as the relationship between cumulative retention rate R(l) and fiber length l. A common way to characterize is to fit using the following models:
R(l) = 100 * exp(-(l/λ)k)
where λ is the characteristic length and k is the distribution width parameter. This model helps to quantify the homogeneity of fiber populations.
Sieve analysis provides a standardized method for assessing the length distribution of pulp fibers. The typical operation process is: first, disperse the dry slurry sample into a uniform suspension; It is then passed through a set of screens with decreasing aperture from top to bottom (e.g., from 28 mesh to 200 mesh or finer); Sifting at standardized water flow velocity and time; Finally, the fibers on each screen are collected and dried, and their mass fraction is weighed and calculated.
The data obtained through this process can directly generate fiber length distribution curves and cumulative distribution curves. The core metrics of interest in the analysis include the weight average fiber length, the length-weighted average fiber length, and the percentage of fine fiber components (usually the part that passes through the 200-mesh screen). These data provide a quantitative basis for optimizing the beating process, the ratio of different slurries, and the prediction of final paper performance.
The fiber length distribution data provided by the screener is the key bridge between the pulp and paper process and product performance. In the pulping or pulping process, the screening results can intuitively reflect the degree of fiber cutting and fine fibrosis, and guide the adjustment of beating intensity and time to achieve the target fiber morphology. The difference in the screening curves of different sources of pulp (such as softwood pulp and hardwood pulp) in the slurry ratio can help engineers scientifically design mixing ratios to meet product strength, looseness and other requirements in an economical way. In addition, for quality control of recycled fibers, screening analysis can effectively assess the degradation of fibers after multiple reuses.
Technical highlights
To ensure comparability and accuracy of screening results, strict standard methods are followed. Key points include: representative sampling and standard release of slurry samples, water quality and temperature control of screening water, calibration and maintenance of standard screens, and precise control of screening time and water flow rate. Different standard systems (e.g., ISO, TAPPI, GB) may have specific specifics, but the core principles are the same. Laboratories need to regularly calibrate instruments using standard samples to monitor the stability of the system.
The following table compares two types of fiber components that are commonly of interest in screening analysis and their general effects:
| Long fiber components (retained in thicker screens) | It mainly contributes to the mechanical strength of the paper, such as tensile strength and tear level. |
| Fine fiber components (through fine screens) | Too much can reduce water filtration by affecting the smoothness, opacity, and printing performance of the paper. |
Epilogue
As a fundamental tool for fiber morphology analysis, the length distribution information provided by the screener is the cornerstone of understanding and regulating pulp performance. Through standardized operation and accurate data interpretation, it can effectively serve the optimization of pulping process, product quality control and the development of new products. With the development of technologies such as image analysis, screening continues to play an important role in industry and scientific research due to its cost-effectiveness, good repeatability, and rich experience in correlation with paper performance. Understanding the physical implications behind screening data and combining it with other analytical techniques is an important way to improve the science and technology of papermaking.