Overview
The Bauer screening method is a widely used standard method for pulp fiber classification, the core of which is the step-by-step screening of pulp suspensions through a series of screens of different pore sizes, thereby separating the fibers into different components by length and morphological characteristics. The method is based on the principles of fluid dynamics and screen interception and is operated under standard conditions such as specific concentrations, flow rates, and screening times to ensure comparability and repeatability of results. The screening process usually uses a special screener, the instrument drives the fibers through the screen through the circulating water flow, and the fibers that are not intercepted by the screen enter the next stage of the screen with the water flow, and finally achieve grading.
Fiber grading
Fiber grading depends on the interaction between fiber length, diameter, flexibility and morphology and screen pore size. As the pulp suspension flows through the screen, shorter fibers or fine fiber components (often called fine fibers) easily pass through the screen pore size, while longer fibers are intercepted on the screen. The grading efficiency is influenced by various factors, including the suspension concentration of the fiber, the velocity of the water flow, the frequency of screen vibration (if applicable), and the physical properties of the fiber itself. From a hydrodynamic point of view, the probability of fibers passing through the screen can be described by the following relationship:
P = f(L/d, Re, C)
P is the fiber passage probability, L is the fiber length, d is the screen pore size, Re is the Reynolds number, and C is the fiber concentration. This relationship shows that fiber length to pore size ratio (L/d) is a key parameter that determines the screening effect.
Workflow
Standard screeners typically contain multiple screen units, arranged in series by aperture from large to small. During operation, the pulp sample is pumped into the screening system after dilution and flows through the screens in turn. The fiber residues collected by each screen are dried and weighed, and the mass fraction of the fibers of each screen grade can be calculated. Key control parameters include:
| Screen pore size sequence | Common is 28 mesh to 200 mesh (corresponding pore diameter approx. 0.595mm to 0.075mm) |
| Slurry concentration | It is usually controlled in the range of 0.1% to 0.5% |
| Water flow velocity | It needs to be stable, and the standard value is about 10L/min |
| Screening time | Set according to standard methods, e.g. 15 minutes |
| temperature conditions | Room temperature or standard temperature (e.g. 20°C) |
The standardization of these parameters is the basis for ensuring the consistency of screening results, and different national or industry standards may have slight adjustments to specific values.
Application and Significance
The fiber distribution data obtained after screening can be used to evaluate the processability and paper quality of the pulp. Long fiber components typically contribute to the strength properties of the paper, while fine fibers affect the smoothness, opacity, and printability of the paper. By analyzing the proportion of fibers in each sieve grade, the optimization of the pulping process can be guided, such as the degree of pulping, screening efficiency or adjustment of raw material ratio. In addition, fiber grading data can also be used for product quality control and research and development to provide a basis for paper performance prediction.
Notes:
Although the Bauer screening method is widely used, its limitations need to be noted. For example, the morphological characteristics of fibers (e.g., crimping, filamentation) may affect the screening accuracy; Very high or low fiber concentrations can lead to clogged screens or inadequate grading. Therefore, standard procedures should be strictly followed during operation, and instruments should be calibrated regularly. At the same time, the screening results should be comprehensively interpreted in combination with other fiber analysis methods (such as optical morphology analysis) to obtain more comprehensive fiber characteristic information.
Cited Literature
TAPPI T233 standard: Fiber length grading method
ISO 16065-2: Determination of the length distribution of pulp fibers
"Pulp and Paper Analysis and Testing", China Light Industry Press