As is well known, disc refiners are major energy consumers in the production process of paper enterprises. Fully understanding and mastering knowledge about disc refiners is highly beneficial for their optimal use and energy conservation.
Types of Disc Refiners
The disc refiner is the main equipment for producing Refiner Mechanical Pulp (RMP) and Chemi-Mechanical Pulp (CMP). Main types include: single disc refiner, double disc refiner, and triple disc refiner.
A single disc refiner consists of one stationary disc and one rotating disc. The rotating disc on the drive shaft is driven by a single motor. Stock enters the refining zone through the center hole of the stationary disc. The rotational speed of the rotating disc is 1500-1800 rpm. The disc gap is adjusted via a hydraulic system or a gear motor.
A double disc refiner consists of two counter-rotating discs, each driven by its own motor. Feed is forced in by a twin-screw feeder. The disc gap can be accurately controlled using a Linear Variable Differential Transformer (LVDT).
A triple disc refiner consists of two stationary discs and one rotating disc in the middle. The rotating disc has two toothed surfaces on either side, forming two refining chambers with the two stationary discs. This design avoids issues with disc deflection even at high rotational speeds. Steam generated during refining can be discharged through two feed inlets and one outlet. The two axially linked stationary discs allow for gap adjustment and application of load to the rotating disc via a hydraulic system. This design eliminates the need for large thrust bearings.
Higher rotational speeds in a disc refiner increase the frequency at which the bars act upon the fibers. On the other hand, both increasing rotational speed and increasing disc diameter can enhance the production capacity of a single refiner. Therefore, there is a trend towards higher speeds and larger diameters for both single and double disc refiners. Refiners with maximum disc diameters of 2082 mm and motor power of 26,000 KW have been developed.
Increasing speed generates significant centrifugal force in the refiner, which can affect the normal distribution of stock between the discs and cause equipment stability issues. The triple disc refiner increases the refining area; without increasing speed or disc diameter, the refining area is doubled, thereby increasing production and improving pulp quality.
Refining Mechanism of Disc Refiners
The refining process involves three overlapping stages:
First, wood chips are broken down into rough fragments at the inlet of the refining zone.
Then, in the middle of the refining zone, the fragments are separated into individual fibers.
Finally, in the outer periphery of the refining zone, the fibers undergo refining between the disc plates.
The second stage is the most critical. Coarse fiber bundles are randomly oriented within the refining zone. Under the shear forces of the disc bars, they are broken down. Fiber bundles aligned parallel to the bars are separated into individual fibers, while those aligned perpendicular to the bars are ground into fragments. Disc refining can be considered a typical longitudinal refining method.
Process of Wood Chip Changes during Refining:
In the inner ring of the refining zone of a disc refiner, a considerable amount of coarse fiber bundles undergo recirculation; they flow back along the grooves of the stationary disc to the breakage zone, and then flow along the grooves of the rotating disc to the outer ring of the refining zone.
In the coarse refining zone, fibers are classified under the combined actions of shear and friction from the disc bars.
In the fine refining zone, fibers flow forward along the bars and grooves. Most of the work done on the fibers occurs here without significantly reducing fiber length. Pulp strength develops rapidly in this zone.
Three Zones of the Refining Process:
(1) Breakage Zone: During refining, wood chips first enter the breakage zone in the central part of the discs. This zone has the largest disc gap, with thick bars and fewer teeth. Here, wood chips are initially broken down into matchstick-like splinters under high temperature.
(2) Coarse Refining Zone: The disc gap gradually narrows from the inside out. The raw material has a longer residence time and is gradually ground into needle-like wood slivers. Through mutual friction and the action of the discs, they are separated into fiber bundles and some individual fibers.
(3) Fine Refining Zone: This area is located at the periphery of the discs, featuring more bars and narrower grooves. Fiber bundles and individual fibers coming from the initial refining zone are further separated and fibrillated here before leaving the disc refiner.
Main Factors Influencing Refiner Mechanical Pulp (RMP)
(1) Material and Chip Specifications
Different wood species used for RMP result in differences in physical properties, fiber morphology, and chemical composition, leading to corresponding changes in the properties of the produced pulp. For example, adding hardwood to softwood in TMP production reduces the long fiber fraction and decreases pulp strength.
(2) Refining Consistency
Refining consistency is a crucial parameter in RMP production, generally considered to be within the range of 20%-30%. When using multi-stage refining, the goal of the first stage is fiber separation. To minimize fiber cutting, separation relies mainly on inter-fiber friction, so the pulp consistency should be higher, generally between 25%-45%. The second stage aims primarily at developing strength, so the refining consistency should not be too high, typically between 20%-25%.
Refining consistency significantly impacts the strength properties, shive content, and freeness of RMP.
(3) Preheating Temperature and Time
Refining at 170°C, where lignin is sufficiently softened, causes fiber separation to occur in the lignin-rich middle lamella and primary wall. While fibers separate easily, the softened lignin adheres to the fibers. After the first-stage pressure refining, when the stock cools, the lignin condenses into a glassy coating, creating an obstacle for second-stage refining. This leads to high energy consumption, difficulty in promoting fibrillation, no drop in freeness, and反而 a decrease in brightness. Conversely, preheating and refining at 115°C, where most lignin is not fully softened and remains hard (i.e., refining below the lignin glass transition point), makes fibers susceptible to damage.
A critical refining condition is the treatment temperature, meaning the temperature for chip preheating and first-stage pressure refining, should not exceed the lignin glass transition temperature of 120-135°C. Pulp properties will be entirely different if refining occurs above or below this temperature range. Practical production proves that steaming for 1-2 minutes at 98-294 Kpa is sufficient. If pressure exceeds 294 Kpa, pulp brightness and opacity will decrease significantly, while energy consumption increases. If steam pressure is below 98 Kpa, the shive content will increase.
(4) Energy Consumption and Energy Distribution
Both fiber separation and the fibrillation of individual fibers require energy input. Refining quality is also determined by the energy consumed. Energy is primarily used for fiber separation and refining, with separation requiring less energy. Most energy is consumed in the refining that develops fiber strength. Production of lightweight coated paper may require an energy input of 11 MW for the first stage refiner and 6.8 MW for the second stage. A low-consistency refiner might have an energy input of 600-800 KW.
(5) Disc Gap
Three controllable parameters are very important in a disc refiner: pulp consistency, energy consumption, and disc gap. When maintaining one parameter, such as energy consumption, constant, increasing the refining consistency requires opening the gap. If consistency is fixed, then current (amperage) and gap interact and constrain each other; reducing the gap increases energy consumption.
(6) Plate Characteristics
Plate characteristics depend on the pattern, taper, and material. The pattern refers to bar length, quantity, thickness; groove depth, width, and distribution; the distribution of dams; bar arrangement and distribution features; and the plate taper. The refining area in the breakage, coarse refining, and fine refining zones all significantly impact refining production, quality, and energy consumption. Plate taper refers to the slope per unit radial bar length.
The purpose of designing a taper in the breakage zone of the plate is firstly to facilitate the entry of raw material, and secondly to avoid a sudden increase in mechanical energy.