GENERAL RESEARCH Utilization of Nitric Acid Wastes from Bleaching Earth Production La ´ szlo ´ Ko ´ tai,* Be ´ la Kazinczy, Istva ´ n Ga ´ cs, Kla ´ ra Szentmiha ´ lyi, A Ä gnes Keszler, and Krisztina Sza ´ sz Institute of Chemistry, Chemical Research Center, Hungarian Academy of Sciences, P.O. Box 17, H-1525 Budapest, Hungary Nitric acid activation of bentonite has been studied with the aim of harmonizing bleaching earth and ammonium nitrate fertilizer production, where the latter is obtained from waste acid via neutralization with ammonia. In this paper, a mathematical model is described for composition measurement of the waste acid formed during the activation of the bentonite. On the basis of the results obtained from the model, the amount of concentrated nitric acid necessary for refreshing the recycled acidic solution can easily be estimated. The amounts of the products formed during ammoniacal neutralization, such as ammonium nitrate, hydroxides of hydrolizable metals, and nitrates of nonhydrolizable metals, as well as the amount of activated bentonite, can also be calculated. It is established that, with an increase in the recycling number, the activation process becomes more economical via decreasing energy and material requirements. Introduction During the production of bleaching earth from min- eral bentonites by hydrochloric or sulfuric acid, consid- erable amounts of acidic wastewater form. 1-5 The wastewater contains polyvalent cations [e.g., aluminum, magnesium, calcium, or iron(III)] originally present in the bentonite. 1-5 Although, various methods have been developed for the utilization of these wastewaters, the cost of neutralization and transformation to other products is high. 6-8 Moreover, the environmental impact due to chloride, sulfate, and free acid means a strong limit to practical applicability of the treated waste. Recently, a method for producing bleaching earth from mineral bentonite by means of nitric acid have been developed. 9 In this process, the neutralization of the waste acid is carried out with ammonia, similarly to the ammonium nitrate manufacturing process. 10 Thus, the waste acid is utilized as a raw material for producing an ammonium nitrate fertilizer. In this way, the environmental problems that had previously arisen could be eliminated without a significant change in the quality of the activated bentonite produced. The optimal nitric acid concentration in the treatment of the bentonite is about 30% (w/w), 9 whereas am- monium nitrate production requires a concentration of 45-65%. 8 If dilute waste nitric acid were used in the ammonium nitrate fertilizer production process, the cost of solvent (water) evaporation from the dilute am- monium nitrate solution would be high. Therefore, it was essential to develop a mathematical model capable of estimating both the waste acid composition and the amount of concentrated nitric acid necessary for re- freshing the recycled dilute acidic solution. In this work, the results of our study directed to the harmonization of the activation and neutralization steps (in order to achieve optimum bleaching earth and ammonium nitrate production) is presented. The per- formance of the activated bentonite is illustrated by its bleaching capacity, which also was investigated as a function of the recycling steps, whereas characteriza- tions via surface acidity, IR spectra, and surface area will be addressed in a following paper. Experimental Section Materials and Methods. The bentonite was ob- tained from Bentonite Mine, Pa ´ pa, Hungary, and the nitric acid (67%) was purchased from Nitrogen Works, Pe ´tfu ¨ rdo ¨, Hungary. The nitric acid was diluted 1.5-fold v/v with distilled water. Technical-grade vaseline (com- mercial product of MOL Rt., Nyı ´rbogda ´ ny, Hungary) was used for testing the bleaching earth. The elemental composition of the bentonite was determined by means of an Atom Scan 25 ICP spec- trometer (Thermo Jarrel Ash). In the case of silicon, a gravimetric method was applied. UV-vis measurements were performed with a Unicam UV4 UV/VIS spectro- photometer. Sample Treatment for Elemental Analysis. (a) Powdered bentonite sample (1 g) was ground with 2.5 g of a mixture containing sodium carbonate (88% w/w) and boric acid (12% w/w). The ground sample was covered with an additional 1 g of the mixture, and it was heated to 1050 °C for 20 min. The bulk sample was then dissolved in a minimal amount of 1:1 (diluted with water, by volume) HCl, and the solution obtained was evaporated to dryness. This procedure was repeated twice. Following this, the sample was dried at 135 °C to constant mass, 10 cm 3 of concentrated HCl was added, and the resulting mixture was boiled for 5 min. * Corresponding author. E-mail: kotail@cric.chemres.hu. Fax:+36 13257554. 3920 Ind. Eng. Chem. Res. 2000, 39, 3920-3925 10.1021/ie990650f CCC: $19.00 © 2000 American Chemical Society Published on Web 09/14/2000