Effective Synthesis of Artificial Zeolite from Coal Fly Ash

Transcription

1 Effective Synthesis of Artificial Zeolite from Coal Fly Ash Introduction Large quantities of coal are used in power plants around the world every year. The disposal of the huge amount of coal fly ash (CFA) generated from the combustion of coal poses a serious environmental problem. Using the state of Japan as an example, 1.5 million tons of coal ash generated from power and general industries is disposed of without reuse every year. Because of the shortage of landfill sites and tighter environmental regulation, new ways of utilizing the CFA are required. Recently, several authors have converted zeolites from CFA [1-6], termed artificial zeolites. Zeolites are very useful materials for a wide range of applications such as ion exchange, molecular sieves, adsorbents and catalysis. Therefore converting CFA into zeolites not only alleviates the disposal problem but also turns an otherwise waste material into a useful one. However, there are some technical and economical problems on the synthesis of the artificial zeolites. Processing time of the process usually exceeds 24 h [3], and difficulty in filtration arises because of by-products as water glass. Overabundant ratio of alkaline metal hydroxide solution and CFA (8-12 ml/g) is needed since the reaction rate for the conversion depends on the concentration of the alkaline metal hydroxide solution. Because of the above problems, the cost of the artificial zeolite is much higher than that of natural zeolite of 40 to 120 yen/kg (0.5 to 1.5 AU$/kg). New zeolite production process is proposed by KEM Corporation [7]. The characteristics of the process are followings. 1. Processing time is shortened by increase in temperature with a high-pressure kneader. 2. Ratio of the alkaline metal hydroxide solution and CFA is minimized as 0.1 to 1.5 ml/g. 3. Preventing the reduction of reaction rate by maintaining high alkali concentration with removing the water. Further advantage is needlessness of the costs of filtration and wastewater treatment by the water removal. This article reports the examination of suitable experimental condition with a test unit having volume of 5 L/batch and development of pilot plant having volume of 600 L/batch. Experimental Raw materials for artificial zeolite are CFA and sodium hydroxide (NaOH) solution. Composition of the CFA is shown in Table 1. Test unit used was 5 L/batch of high-pressure (to 1.0 MPa) kneader equipped with oil heater. Temperature of the sample was estimated from pressure inside the kneader using saturated vapor pressure of water. Steam was removed from the sample during experiment

2 using steam exhaust line. Experimental conditions used for the test unit are followings. 1. Pressures; 0.2, 0.4 and 0.7 MPa-G 2. Ratio of alkaline solution/cfa (liquid/solid ratio); 0.7 to 1.23 ml/g 3. Concentration of NaOH; 1.5 to 3.5 N (Normal) Operational condition for pilot plant was decided based on the results obtained from the test unit. 1 kg of the CFA and known amount of NaOH solution were introduced into the test unit. The sample was heated from room temperature to a desired temperature for a period of 1 to 1.5 h. The temperature and the pressure were held for 0.5 h and then water was removed from steam exhaust line with keeping its pressure for 2 to 3 h. All of the water was removed from the sample during the operation. Dried artificial zeolite was obtained after the cooling the sample. Obtained zeolite was subjected to XRD (X-ray diffraction) and CEC (cation exchangeable capacity) measurements. Photograph of pilot plant is shown in Fig.1. The plant is batch-type kneader having capacity of 600 L. The kneader has two shafts with kneading wings. Heated oil (210 to 220ºC) is circulated around the kneader for heating the sample. Maximum pressure of operation is 0.38 MPa-G, which is corresponding to steam vapor pressure of 153ºC. Conventional high-pressure kneaders usually have trouble around the shaft because sample penetrates sealing of the shaft. In this process, sample penetrated may grind the shaft or cause overload of motor. The pilot plant equips sealing box to avoid this trouble. The pressure of the box is slightly higher than inside the kneader with supplying air, which prevents penetration of sample. For operation of pilot plant, 130 kg of the CFA and a known amount of NaOH solution were introduced into the kneader. The sample is heated with the heated oil to a desired temperature. When the temperature inside the kneader reached to boiling point of water at the pressure, water was removed from the kneader through the steam exhaust line with maintaining the pressure. Dried artificial zeolite is obtained after the cooling the sample. Time periods for increasing temperature and removing water are 2 and 4 h, respectively. Obtained zeolite is also subjected to XRD and CEC measurements. Results (1) Results of test unit The characteristics and experimental conditions of zeolites obtained with the test unit are listed in Table 2. At a pressure of experiments of 0.4 MPa-G, CEC of the zeolite increase with increasing the liquid/solid ratio and the NaOH concentration. Water is fundamental to produce zeolite since liquid act as transport medium of precursor of zeolite. In case 4 (low liquid/solid ratio), most of water was removed at the early stage of the reaction, and reaction rate extremely decreased. Therefore, initial liquid/solid ratio should be greater than 0.88 ml/g. This ratio may change with the agitation efficiency.

3 Alkali concentration is important to dissolve Si and Al in the CFA. Furthermore the alkaline solution is supply source of Na + for zeolite. For all experiments, ratio of Na + used is appreciably high and is greater than mol/mol. If this ratio is realized without removing water, alkali concentration decreases from 3.5 mol/l to 0.89 mol/l. Initial concentration of alkaline should be high since the rate of diffusion of Na + into bone structure of zeolite depends on the concentration of Na +. In this process, water was continuously removed from sample during operation. This can prevent lowering of Na + concentration. However surplus of alkaline should be held down considering cost of zeolite production. At operation pressure of 0.4 MPa-G, appropriate alkaline concentration and ratio of liquid/solid are 2.5 to 3.5 mol/l and 0.88 to 1.10 ml/g, respectively. From the results of cases 2, 8 and 9, CEC of the obtained zeolite increase with increasing operation pressure. This implies that higher temperature of operation accelerates the rate of zeolite production. Accelerated zeolite production rate can minimize operation time, which is directly concerned with processing ability of this batch process. (2) Results of pilot plant Properties of zeolites obtained with test unit, pilot plant and conventional method were compared and listed in Table 3. For conventional method, liquid/solid ratio, operation temperature, and time period of operation were 8 ml/g, 100ºC, and 24 h, respectively. Furthermore, water was not removed from the sample during operation. After the operation, obtained slurry was filtrated and dried to obtain artificial zeolite. Alkali concentration used in these experiments was 3.5 N. Comparison of the results obtained with test unit and with pilot plant (Cases 8, 2, 10 and 11) implies that artificial zeolite produced with the pilot plant has greater CEC than that produced with the test unit. This result comes from high agitation efficiency of the pilot plant. In compassion of cases 10, 11 and 12, zeolite produced with the pilot plant is not inferior to that produced with the conventional method. This process can produce artificial zeolite with low cost. Figure 2 illustrates the XRD profiles of the original CFA and artificial zeolites. XRD profile of CFA is composed of peaks of quartz and mullite. Intensities of these peaks decrease and peaks caused by generation of zeolite increase with treatment of zeolite production. These peaks are attributed to GIS-type zeolite. Intensity of these peaks obtained with the pilot plant is higher than that obtained with the test unit. Higher intensity of these peaks means that more zeolite was produced. Penta-Ocean Construction Co., Ltd performed investigation of application of obtained zeolite and cost estimation of the zeolite production. Obtained zeolite can be used for adsorption of heavy metal and ammonia and for growth promotion of vegetables. Assuming the commercial plant having capacity of 3000 ton/year, sales price of the artificial zeolite is estimated as 80 yen/kg (1 AU$/kg).

4 References 1. Chang H-L, Shih W-H. A general method for the conversion of fly ash into zeolites as ion exchangers for Cesium. Ind. Eng. Chem. Res. 1998;37: Hollman GG, Steenbruggen G, Janssen-Jurkovicova. A two-step process for the synthesis of zeolites from coal fly ash. Fuel 1999;78: Mouhtaris Th, Charistos D, Kantiranis N, Filippidis A, Kassoli-Fournaraki A, Tsirambidis A. GIS-type zeolite synthesis from Greek lignite sulphocalcic fly ashes promoted by NaOH solutions. Micropor. Mesopor. Mat. 2003;61: Querol X, Umana JC, Plana F, Alastuey A, Lopez-Soler A, Medinaceli A, Valero A, Domingo MJ, Garcia-Rojo E. Synthesis of zeolites from fly ash in a pilot plant scale. Examples of potential environmental applications. International Ash Utilization Symposium Andres JM, Ferrer P, Querol X, Plana F, Umana JC. Zeolitisation of coal fly ashes using microwaves. Process optimization. International Ash Utilization Symposium Maruyama N, Yamamoto H, Shibata J. Mechanism of zeolite synthesis from coal fly ash by alkali hydrothermal reaction. Int. J. Miner. Process 2002;64: Katayama Y. US patent No. 6,599,494, 2003, Katayama Y, AU patent No /99.

5 Table 1. Composition of raw material SiO 2 Al 2 O 3 CaO SiO 2 /Al 2 O 3 Sample wt% wt% wt% [wt/wt] CFA Table 2. Characteristics and experimental conditions of zeolites obtained with the test unit Experimental Results Condition Solid/Liquid Alkali Pressure CEC Sodium Sodium Case ratio [ml/g] concentration [mol/l] [MPa-G] [cmol/kg] addition [mol/kg-ash] utilized [mol/mol]

6 Table 3. Comparison of characteristics of zeolite obtained with various apparatus Experimental Results Condition Apparatus Solid/Liquid Pressure CEC Case ratio [ml/g] [MPa-G] [cmol/kg] Test unit Pilot plant Conventional* *Temperature: 100ºC, Reaction time: 24 hours Figure 1. Photograph of pilot plant

7 Figure 2. XRD patterns Q: Quartz [SiO 2 ], M: Mullite [3Al 2 O 3 2SiO 2 ], G: Gismondite [Na 5.7 Al 5.7 Si 10.3 O 32 12H 2 O]

8 Attachment Photographs of test plant

9 Sample inlet Oil heater Cooling tower NaOH tank NaOH tank Kneader Sample outlet

10 Kneader MPa Sealing box Sealing box Schematic diagram Screw Blade Flow of sample Kneader inside Kneading shafts

11 Ash silo Container Introduction of coal ash Product

12 SEM photographs of original CFA and zeolite (This CFA is not same as CFA used in this manuscript)