Technology for the production and application of calcium carbonate as a refined filler from alkali recovery sludge

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Technology for the Production and Application of Calcium Carbonate as a Refined Filler from Alkali Recovery Sludge – Author: Beijing Watermad Environmental Protection Technology Co., Ltd. I. Overview: Alkali recovery sludge resulting from the alkaline treatment of grass pulp is a waste product generated by paper mills. Due to its high content of impurities such as silica, it is difficult to calcine it for reuse as lime. At present, most paper mills simply transport this sludge for landfilling. This approach results in two problems: first, companies have to bear the costs associated with landfilling the sludge; second, the secondary pollution caused by landfilling leads to complaints from local residents and environmental protection authorities. Therefore, finding a solution to the issue of this sludge has always been a challenge for companies. Through years of extensive research and development, Beijing Watermad Environmental Protection Technology Company has successfully found a way to completely solve the problem of white sludge. This company refines the white sludge into filler-grade light calcium carbonate, which is then used in papermaking machines. The production process has been industrialized and specialized. The main advantage of this technology lies in its integration of high-quality alkali solution recovery with the scientific production of calcium carbonate products derived from white sludge. This technology was successfully put into use in October 2006 at the first calcium carbonate production line for refining white clay fillers, built by Shandong MCC Galaxy Paper Industry. After its commissioning, the system operated well, and the product quality met all the design specifications. It is used in the manufacturing process of medium- to high-grade paper for cultural purposes, thereby improving the quality of the paper produced. In December of the same year, the calcium carbonate project for refining filler from white clay at Henan Xinya Paper Industry, undertaken by the company, went into operation and passed the acceptance inspection successfully. The practical experience of the project to refine filler using alkali recovery from alkaline grass pulp has shown that the technology for refining calcium carbonate from white mud is fully capable of meeting the requirements of industrial production as well as papermaking processes. Both the quality of the calcium carbonate produced and the quality of the paper obtained after adding this filler meet or exceed the relevant standards. The success of this technology provides a new method for the comprehensive utilization of white sludge generated in the alkaline recovery process of grass pulp, offering significant social benefits as well as good economic benefits. The first phase of the 10,000-ton project for the refinement of calcium carbonate from white clay at Shandong MCC Galaxy Paper Group was completed in February; this project was carried out by Beijing Watermad Environmental Protection Technology Company. By December 2007, this project had produced nearly 7,000 tons of white mud light calcium carbonate (on a dry basis), as well as over 20,000 tons of paper; it also reduced the area required for landfilling by 6,000 cubic meters, equivalent to 20 mu of land, generating direct economic benefits of 2 million yuan. The refined calcium carbonate project at Henan Xinya Paper Industry Group Co., Ltd. was also undertaken by the same company; it was completed in March and inspected in April. During this period, 10,000 tons of white mud light calcium carbonate were produced, and talc, which was used as a filler in paper manufacturing, was completely replaced by refined calcium carbonate derived from white mud. II. Technical Introduction – Process Overview The production process for recovering and refining calcium carbonate from slaked lime involves two aspects: the optimization of the existing causticization process, and the process for refining calcium carbonate from slaked lime ； The optimization and modification of the causticization process involve adding steps such as lime grading, green liquor purification, serpentine treatment, green liquor filtration, and removal of excess ash to the existing process, as well as optimizing the conditions for causticization ； The refining process for refined calcium carbonate from white clay includes steps such as alkali removal, slag removal, washing and concentration adjustment, pH value adjustment and stabilization, antibacterial treatment, and homogenization. III. Technical Features: High-quality quicklime raw materials are used, and strict grading of the lime is implemented to ensure its quality, thereby enabling its proper use ； The combined action of various chemical additives enables the purification of green liquor through a process that truly results in clean green liquor ； The loss of alkali was reduced through the washing and filtration of chlorite ； The Causticization- treated green liquor is subjected to fine filtration, ensuring that no factor that could affect its quality is overlooked ； The ash removal process for white clay is scientifically integrated with the causticization process ； The impurity removal process is integrated into the causticization and refining stages, with solid impurities in the white clay being removed step by step through several stages of separation ； The active alkalis that are difficult to remove from white clay can be treated chemically using desalkalizing agents, which helps improve the quality of the white clay and saves water ； While ensuring proper adjustment and stabilization of the pH value, anti-corrosion and sterilization treatment is applied to the white clay calcium carbonate ； To obtain calcium carbonate particles with better uniformity, high-intensity dispersion and homogenization treatments were applied to the white clay. In summary, this technology enables the efficient production of light calcium carbonate based on the operating conditions in the causticization plant. It allows for the production of high-quality light calcium carbonate sludge that meets the required standards, while simultaneously ensuring stable and optimized alkali recovery. Moreover, it meets the requirements of paper machine operations during paper manufacturing, resulting in high-quality paper. IV. Technical Production and Application, Product Quality Indicators and Characteristics: Brightness: over 88% ； Calcium carbonate content: over 90% ； Free base (on a CaO basis): 0.05% or less ； pH value: around 8.5-9.5 ； Average particle size: 4-7µm ； The maximum particle size shall not exceed 15µm ； Residue: less than 0.005% on a 125µm test sieve, less than 0.30% on a 45µm test sieve ； Dust level: It is not allowed to exceed 0.4 m2 ； Total sedimentation volume: greater than 2.8/g (emulsion) ； Concentration of the finished emulsion: 18%-20%. Product inspection test sheet: The particle size distribution of the white mud calcium carbonate product is shown in the figure below. 2. Effect of substitution: 100% white clay was used as a substitute for commercial calcium carbonate and added during the production process on high-end cultural paper machines. The test results for the paper produced in this way are as follows: Compared with paper produced using commercial calcium carbonate, this substitution had the following effects on the paper’s properties: —— The thickness of the paper increased; with the same ash content, the thickness was about 5% higher than before. ——The opacity has been increased; with the same ash content, the opacity is more than 5% higher than before. ——It has no adverse effect on whiteness. ——Parameters such as break length, fold resistance, and surface strength have improved slightly. ——The amount of filler used has been reduced from 32% to around 28% at present, and the retention rate of the filler has increased by more than 8%; the white clay calcium carbonate filler exhibits a strong adhesive capacity toward fibers. ——To achieve the same surface absorption value, the amount of AKD used increases significantly, by about 25%-50%. V. Benefit Analysis At present, the \"Comprehensive Work Plan for Energy Conservation and Emission Reduction\" issued by the State Council, formulated by the National Development and Reform Commission in conjunction with relevant departments, outlines the goals, tasks, and overall requirements for achieving energy conservation and emission reduction in China by 2010. The Plan states that the comprehensive utilization rate of industrial solid waste during the 11th Five-Year Plan period must reach over 60%. The white mud refined calcium carbonate technology provides a good guide for paper and pulp manufacturing enterprises to achieve this goal. Refined fillers made from white clay are used in projects related to the addition of fillers to paper sheets. By introducing new technologies for refining white clay into calcium carbonate, as well as carrying out technical upgrades to the causticization process, it is possible to produce calcium carbonate suitable for use as a filler in paper sheets. This not only increases the company’s profit margins but also reduces the loss of alkali during the causticization process, thereby lowering the cost per unit of product in terms of alkali recovery. The enterprise not only addresses environmental protection issues but also achieves a net increase in economic benefits of 4 million yuan per year (taking an annual production of 20,000 tons of completely dry white clay as an example). VI. Conclusion: The use of alkali method for the recovery of calcium carbonate from grass pulp ash, by employing Beijing Watermad Environmental Protection Technology Co., Ltd.’s new technology for refining calcium carbonate from such ash to produce products for use as fillers in paper, has proven to be feasible. The successful application of this method helps to protect the environment, reduce pollution, turn waste into valuable resources, and save resources; it brings about good economic, environmental, and social benefits