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During the manufacture of recycled paper, paper sludge is discharged as an industrial waste. Over 3 million tonnes of sludge are discharged per year in Japan, and approximately 8 and 2 million tonnes are discharged in the United States and the United Kingdom, respectively [1, 2] with these amounts increasing annually. To reduce its volume, the sludge is incinerated, and paper sludge ash is produced. Although paper sludge ash is used as a cement additive and as a material for artificial aggregates [3, , a large amount of the ash is disposed of by dumping. It has become increasingly difficult to secure sufficient land capacity for waste disposal and it is therefore essential to develop new techniques of ash utilization for further recycling. The conversion of ash into zeolite has recently been investigated . In this process, aluminosilicate, which is the main component of the ash, is transformed into zeolites by reaction with alkalis. Because of their unique pore structures and ion exchange properties, zeolites, including those synthesized from paper sludge ash, are used in water purification and to improve soil characteristics [2, . Calcite, which is used in paper as a filler, has resulted in recent ash contents high in calcium (in the form of anorthite and gehlenite). It has been reported that calcium-containing ash can be converted into zeolites, but that the product has a low cation exchange capacity (CEC) . Moreover, it has also been reported that calcium compounds in the ash prevent the separation of the product slurry into solid and liquid fractions during filtration and sedimentation . The reduction of calcium content, therefore, is an important issue in the production of desirable zeolite materials. To control the calcium content for zeolite synthesis, we propose a calcium reduction process through pre-treatment of the paper sludge ash by acid leaching. Many researchers have reported on the synthesis of zeolites from coal fly ash by hydrothermal activation methods, such as classic alkali activation , alkali fusion , and two-stage synthesis . In previous studies, we leached paper sludge ash with HCl to reduce its Ca content and converted the leached ash into products with high CEC, including zeolite-P and zeolite-A . Zeolite syntheses from raw and leached ash were compared to establish the reaction mechanism . Acid leaching is simple, has low energy consumption as it requires no prior heating, fusion, or addition of other reagents and two dangerous waste solutions from acid leaching (acid) and zeolitization (alkali) can be simultaneously neutralized by mixing with one other. In addition, the leached Ca-solution can be used to convert Nato Ca-zeolite, which is more environmentally friendly . Little information is available, however, on the pre-treatment of raw ashes by acid leaching with acid solution other than HCl. Furthermore, the leach time required to synthesize zeolitic materials (which is important for the equipment design) has not been established. In this study, Ca removal from paper sludge ash via acid leaching was investigated to synthesize a zeolitic material with high CEC. We compared the leaching behavior of three different acid solutions, with additional aims of reducing the synthesis time as compared with previous studies [14] and obtaining a product with high CEC.

Removal of Ca from Paper Sludge Ash by Acid Leaching and Synthesis of High Cation Exchange Capacity Zeolite Material