Determining the reject rate in waste paper pulping is a critical quality control parameter for paper mills and recycling facilities seeking to optimize raw material utilization. This technical article provides a comprehensive overview of the testing methodologies, sample preparation protocols, and calculation standards used to assess the impurity content in recovered paper. By accurately measuring the coarse reject rate, manufacturers can enhance production efficiency, reduce equipment wear, and ensure the stability of the pulping process.
Technical Overview
The reject rate in waste paper pulping, often referred to as the coarse reject rate, represents the percentage of materials that cannot be dissociated into fibers during the slushing process and are subsequently separated by screening. This metric is a vital indicator of the quality of recovered paper grades, such as OCC (Old Corrugated Containers) and mixed paper. A high reject rate indicates a significant presence of non-fibrous contaminants like plastics, staples, sand, and adhesives, or the use of poorly fibrillated fibers. Efficient testing of this parameter is essential for evaluating the economic value of raw materials and for adjusting the pulping consistency and screen plate design in the stock preparation line.
Testing Samples
The selection of samples is the first step in ensuring accurate detection results. Samples must be representative of the entire batch of waste paper being evaluated.
Common Waste Paper Grades
- Corrugated Containers (OCC): Comprises used corrugated boxes, which are a major source of fiber for packaging papers.
- Mixed Paper: A mixture of various paper types and qualities, often containing higher levels of contaminants.
- Old Newspapers (ONP): Used newspapers, requiring careful separation of ink and contaminants.
- High-Grade Deinked Stock: Office waste and printing papers where low reject rates are expected.
Sampling Requirements
According to standards such as ISO 7213 or equivalent national standards, sampling should be random and sufficient in quantity. Typically, a gross sample is collected from multiple bales or loose piles to form a composite sample, ensuring that variations within the batch are captured. The sample must be stored in moisture-proof bags to prevent changes in moisture content before testing.
Testing Items
The primary objective of the test is to quantify the debris and unslushed materials. The specific items analyzed during the process include:
- Total Reject Rate: The total mass of materials retained on the screen mesh relative to the oven-dry mass of the original sample.
- Non-fibrous Rejects: Impurities such as plastics, metals, glass, and synthetic textiles that are physically separated.
- Fibrous Rejects: Unbeaten fiber bundles, shives, and undispersed paper fragments resulting from insufficient pulping time or low mechanical action.
- Moisture Content: Determined simultaneously to calculate results on a dry weight basis.
Testing Methods
The detection of the reject rate involves a standardized laboratory simulation of the industrial pulping and screening process. The following methodology outlines the typical procedure based on relevant ISO or TAPPI standards.
Sample Preparation
A specific mass of the waste paper sample is weighed and conditioned. If the sample contains moisture outside the standard range, it must be adjusted or accounted for in the calculations. The sample is then manually sorted to remove large, obvious foreign objects if the standard requires, or processed as-is to simulate total input.
Pulping Process
The sample is placed in a laboratory pulper (or disintegrator) with a measured amount of water. The pulping consistency is typically set between 4% and 12%, depending on the test standard. The pulper runs for a specified duration at a controlled temperature (usually 20°C to 30°C) to defiber the paper. The goal is to achieve complete defibration without excessively breaking down contaminants (to simulate actual production conditions).
Screening and Washing
The slurry is then transferred to a laboratory screening device equipped with a specific screen plate (e.g., slotted screen or hole screen, typically 0.15mm to 2.0mm depending on the grade). The accepts (fiber that passes through) are separated from the rejects (material retained on the screen). The retained material is washed to remove any adhering usable fibers.
Drying and Calculation
The rejects are collected, dried in an oven at 105°C, and weighed. The moisture content of the original sample is determined separately. The reject rate is calculated using the formula:
Reject Rate (%) = (Dry mass of rejects / Dry mass of original sample) × 100
Results can be further broken down by manually sorting the dried rejects into categories like plastics, metals, and heavy solids to provide a detailed impurity profile.
Applications
The data obtained from reject rate testing is utilized across various stages of the paper manufacturing and recycling supply chain:
- Raw Material Procurement: Mills use this data to price waste paper bales, paying less for high-reject loads that increase processing costs.
- Process Optimization: Results help process engineers adjust pulper consistency, rotor speed, and screening slot velocities to maximize fiber recovery.
- Quality Assurance: Ensures that the incoming raw material meets the specifications required for the final paper product quality.
- Equipment Protection: Identifying high levels of heavy debris (like metals) helps prevent damage to expensive pulping and screening equipment.
FAQ
What is the difference between the reject rate and the ash content?
The reject rate measures macroscopic contaminants and undispersed materials that are physically screened out, while ash content measures inorganic fillers and residues remaining after combustion at 575°C. Reject rate relates to physical impurities, whereas ash content relates to chemical composition and filler loading.
How does pulping time affect the test results?
Extended pulping times generally reduce the reject rate by breaking down fibrous bundles and dispersing some adhesives (stickies). However, excessive pulping may also break down plastic films into smaller particles that pass through the screen, artificially lowering the reject rate but potentially causing issues later in the paper machine. Therefore, adhering to standardized pulping durations is critical for comparability.
Why is temperature control important during the test?
Temperature affects the hydration of fibers and the dispersion of adhesives. High temperatures can melt hot melts or waxes, allowing them to pass through the screen as micro-stickies, altering the reject measurement. Standard testing is usually conducted at ambient temperature to ensure consistency.