The waste paper pulp is made of various fiber mixture, including mechanical pulp fiber and chemical pulp fiber, the paddings, coloring matters, pigment, and some ink particle produced in the printing process. With the development of printing and coating, the chemical property of waste paper becomes complex.
How to make paper pulp from waste paper? The waste paper pulp production line can be generally divided into five steps, such as cooking, screening, washing, deinking and bleaching. The common paper pulper machines of waste paper pulp plant include paper pulper, pressure screen, disc filter, and other pulp making machines.
Pulp bleaching is a necessary link in the waste paper pulp processing. Why? After the screening, cleaning, washing and deinking, the waste paper pulp is yellowed and dingy, which is caused by the mechanical wood pulp of waste paper and alkaline deinking. Besides that, most of the impurities are removed before bleaching, but some impurities still exist in the pulp and affect the color and bleaching property.
The data shows that bleaching is not available for pigment, and coloring matters are also immune to oxidation bleaching. So the bleaching process of waste paper pulp is decided by the real situation.
CNBM is a paper pulping machines manufacturer, our products include pulp digester, vacuum drum washer, twin roll press, disc filter, pressure screen, etc. We also provide pulp solution for wood, wheat straw, bamboo, bagasse, reed, jute, waste paper. Today, we’ll analyze how to bleach the waste paper pulp.
About waste paper
According to the standard of the ISRI, there are 51 types of waste paper and 31 types of special waste paper. The widely recycled waste paper includes old newspaper, old magazine, mixed office waste paper, old books, etc. The old newspaper, old magazines, and mixed office waste paper take the biggest share of all waste papers, and mixed office waste paper is cheapest among all kinds of waste paper.
Bleaching process of waste paper pulp
Generally, the paper and pulp mill adopts pulp chloridizator to bleach the waste paper pulp, if the chlorinity of bleaching agent is 7%, the bleaching time is about 2h. As for the whiteness, take the hypochlorite as an example, single stage or two stage bleaching can reach 60%-78%, multiple stage bleaching can reach 75%-85%.
- For newspaper edge made from mechanical pulp and chemical pulp or the deinked old newspaper, hydrogen peroxide bleaching or sodium sulfite bleaching is applicable. The bleaching agents change the chromophoric group into colorless group, and improve the whiteness of paper pulp. Besides, the bleaching agent is good for the saponification and reduces the ink, and permeates into the fibers, also good for removing Ink particles.
- For mixed office waste paper, firstly classify the waste papers according to ink marks quantity and degreed of oil pollution, and use different pulp processing to deink and bleach.
- The whiteness of printing paper pulp is decided by the whiteness of printing paper. The printing ink is hard to remove and bleach by the common bleaching method, the bleaching process and deinking process should be combined.
The method for improving pulp whiteness
- To strength washing and screen tiny fiber
Some waste paper pulp is difficult to deink and bleach, which has close connection with tiny fibers of pulp, the analytics of whitewater suspended solids screening prove that. It is necessary to sacrifice part of fines retention for increase the waste paper pulp.
- Bleach pulp by the length of fibers
The illustrated magazine waste paper pulp is centrifugalized, and produces two groups of long fiber and short fiber, the proportion of two groups is 52% and 48%, and whiteness is 69% and 64%. The long fiber pulp can be used to manufacture paper with 80% whiteness.
- enzymatic pretreatment before bleaching
The test shows that add enzymatic in the deinking process can improve the bleaching. The whiteness of different deinked pulp as follows:
| Project | Enzymatic | Fatty acid soap | Synthetic versatile dispersant |
| After repulping | 44.0 | 44.0 | 44.0 |
| After flotation | 52.6 | 52.6 | 51.3 |
| After peroxide bleaching | 62.5 | 62.5 | 59.6 |
| H2O2% | 1.0 | 2.0 | 2.0 |
Enzymatic deinking pulp has good drain ability, tearing strength and tensile strength. In addition to, it also has advantages of reducing energy consumption and wastewater pollution.
- Ozone and hydrogen peroxide bleaching
TCF is easy to bleach the pulp into higher whiteness. Use the ozone and hydrogen bleaching also achieve same goal. Ozone is good at bleaching but reduces the degree of polymerization of cellulose, so the dosage should be under control.
