Pulp concentration
According to the concentration of the pulp, pulping can be classified into low-concentration pulping, medium-concentration pulping and high-concentration pulping. Generally, when the pulp concentration is below 10%, it is called low-concentration pulping; when it is between 10% and 20%, it is called medium-concentration pulping; and when it is 20% to 30% or higher, it is called high-concentration pulping. The concentration of the pulp has a significant impact on the pulping quality and ultimately affects the uniformity of the paper's formation on the polyester forming fabric. The characteristics are described as follows.
1. Low concentration pulping
The concentration of the pulping process is determined by the type of paper, the pulping method, and the performance of the pulping equipment. Generally speaking, for the batch pulping trough-type pulper, the pulping concentration for making viscous pulp is 6% to 8%, and for making free-flowing pulp, it is 3% to 5%; however, for the continuous pulping mill, the pulping concentration for making viscous pulp is 4% to 6%, and for making free-flowing pulp, it is 2% to 4%. By appropriately increasing the pulping concentration, the amount of pulp entering between the rotating disc and the fixed disc (or between the flying knife and the bottom knife) increases, and the pressure each fiber bears decreases accordingly, thereby reducing the cutting effect of the fibers and promoting the squeezing and kneading effect between the fibers, which is beneficial for the dispersion, swelling and fine fiberization of the fibers. Therefore, increasing the pulping concentration is suitable for making viscous pulp, and this type of pulp dries more slowly, which has specific requirements for the subsequent dehydration and shaping process on the polyester forming fabric. On the contrary, reducing the pulping concentration is beneficial for fiber cutting and is suitable for making free-flowing pulp.
Increasing the pulp concentration can enhance the pulp production volume and reduce the power consumption per ton of pulp, thereby lowering the production cost. However, increasing the pulp concentration is often limited by the conditions of the pulping equipment. For instance, the maximum concentration that a tubular pulping machine can ensure for the uniform circulation of the pulp within the circulating tank is less than 10%; continuous pulping equipment has higher requirements for the throughput and concentration stability of the pulp, and is restricted by the supply system, rarely handling a concentration exceeding 6%. Since lower concentrations are less economically viable, increasing the pulp concentration is beneficial for increasing efficiency and reducing costs. Under the conditions permitted by the process and equipment, according to the quality requirements of the paper type being produced, the pulp concentration should be increased as much as possible. For example, when producing high-strength packaging paper, using a slightly higher pulp concentration can improve the fiber bonding force and enable the formed paper to achieve better wet paper sheet strength on the polyester forming fabric.
For specialty papers that require the use of high-end "polyester forming fabric" to ensure the utmost uniformity and surface properties of the paper, the control accuracy of the pulping concentration is even more stringent. Ultimately, the optimized pulping process must be compatible with the characteristics of the "polyester forming fabric" in order to ensure the stability and efficiency throughout the entire process from pulping to forming.
